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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Feb. 28, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	UMRX
Entity Registrant Name	UNUM THERAPEUTICS INC.
Entity Central Index Key	0001622229
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		30,090,862
Entity Public Float			$ 208.4

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 55,671	$ 28,270
Marketable securities	22,923	12,691
Accounts receivable	1,668	830
Prepaid expenses and other current assets	740	513
Restricted cash		75
Total current assets	81,002	42,379
Property and equipment, net	3,251	4,108
Restricted cash	1,255	1,255
Deferred offering costs		1,373
Other assets	419
Total assets	85,927	49,115
Current liabilities:
Accounts payable	1,519	1,346
Accrued expenses and other current liabilities	5,477	2,953
Deferred revenue	17,949	6,891
Total current liabilities	24,945	11,190
Deferred rent	748	906
Deferred revenue, net of current portion		8,714
Total liabilities	25,693	20,810
Commitments and contingencies (Note 12)
Redeemable convertible preferred stock (Series A and B), $0.001 par value; no shares and 20,791,407 shares authorized at December 31, 2018 and 2017, respectively; no shares and 20,771,850 shares issued and outstanding at December 31, 2018 and 2017, respectively		77,151
Stockholders' equity (deficit):
Preferred stock, $0.001 par value; 10,000,000 shares and no shares authorized at December 31, 2018 and 2017, respectively; no shares issued or outstanding
Common stock, $0.001 par value; 150,000,000 shares and 60,040,000 shares authorized at December 31, 2018 and 2017, respectively; 30,057,970 shares and 10,201,690 shares issued and outstanding at December 31, 2018 and 2017, respectively	30	10
Additional paid-in capital	152,275	2,499
Accumulated other comprehensive loss	(12)	(16)
Accumulated deficit	(92,059)	(51,339)
Total stockholders' equity (deficit)	60,234	(48,846)
Total liabilities, redeemable convertible preferred stock and stockholders' equity (deficit)	$ 85,927	$ 49,115

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Redeemable convertible preferred stock (Series A and B), Par value	$ 0.001	$ 0.001
Redeemable convertible preferred stock (Series A and B), Shares authorized	0	20,791,407
Redeemable convertible preferred stock (Series A and B), Shares issued	0	20,771,850
Redeemable convertible preferred stock (Series A and B), Shares outstanding	0	20,771,850
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, Par value	$ 0.001	$ 0.001
Common stock, Shares authorized	150,000,000	60,040,000
Common stock, Shares issued	30,057,970	10,201,690
Common stock, Shares outstanding	30,057,970	10,201,690

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Collaboration revenue	$ 9,734	$ 8,360
Operating expenses:
Research and development	38,285	29,832
General and administrative	7,454	4,680
Total operating expenses	45,739	34,512
Loss from operations	(36,005)	(26,152)
Other income (expense):
Interest income	1,153	386
Other income, net	320	274
Total other income, net	1,473	660
Net loss	(34,532)	(25,492)
Accretion of redeemable convertible preferred stock to redemption value	(16)	(65)
Net loss attributable to common stockholders	$ (34,548)	$ (25,557)
Net loss per share attributable to common stockholders, basic and diluted	$ (1.39)	$ (2.51)
Weighted average common shares outstanding, basic and diluted	24,895,670	10,191,807
Comprehensive loss:
Net loss	$ (34,532)	$ (25,492)
Other comprehensive income (loss):
Unrealized gains on marketable securities, net of tax of $0	4	8
Total other comprehensive income (loss)	4	8
Comprehensive loss	$ (34,528)	$ (25,484)

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Unrealized gains (losses) on marketable securities, tax	$ 0	$ 0

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock [Member]	IPO [Member]	Private Placement [Member]	Common Stock [Member]	Common Stock [Member] IPO [Member]	Common Stock [Member] Private Placement [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] IPO [Member]	Additional Paid-in Capital [Member] Private Placement [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balances at Dec. 31, 2016	$ (24,698)	$ 77,086			$ 10			$ 1,163			$ (24)	$ (25,847)
Beginning Balances, Shares at Dec. 31, 2016		20,771,850			10,190,228
Issuance of common stock upon exercise of stock options	60							60
Issuance of common stock upon exercise of stock options, Shares					11,462
Stock-based compensation expense	1,341							1,341
Unrealized gains on marketable securities	8										8
Accretion of redeemable convertible preferred stock to redemption value	(65)	$ 65						(65)
Net loss	(25,492)											(25,492)
Ending Balances at Dec. 31, 2017	$ (48,846)	$ 77,151			$ 10			2,499			(16)	(51,339)
Ending Balances, Shares at Dec. 31, 2017	10,201,690	20,771,850			10,201,690
Adjustment to retained earnings for change in accounting policy	$ (6,188)											(6,188)
Issuance of common stock upon exercise of stock options	$ 609							609
Issuance of common stock upon exercise of stock options, Shares	225,252				225,252
Stock-based compensation expense	$ 3,088							3,088
Unrealized gains on marketable securities	4										4
Accretion of redeemable convertible preferred stock to redemption value	(16)	$ 16						(16)
Conversion of redeemable convertible preferred stock to common stock	77,167	$ (77,167)			$ 13			77,154
Conversion of redeemable convertible preferred stock to common stock, Shares		(20,771,850)			13,229,362
Issuance of common stock sold in initial public offering, net of underwriting discounts, commissions and offering costs			$ 63,948	$ 5,000		$ 6	$ 1		$ 63,942	$ 4,999
Issuance of common stock sold in initial public offering, net of underwriting discounts, commissions and offering costs, Shares						5,985,000	416,666
Net loss	(34,532)											(34,532)
Ending Balances at Dec. 31, 2018	$ 60,234				$ 30			$ 152,275			$ (12)	$ (92,059)
Ending Balances, Shares at Dec. 31, 2018	30,057,970				30,057,970

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (34,532)	$ (25,492)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	3,088	1,341
Depreciation and amortization expense	1,321	1,171
Premiums paid on marketable securities		(13)
Net amortization (accretion) of premiums (discounts) on marketable securities	(246)	17
Loss on disposal of fixed assets	10
Non-cash interest expense	23	20
Changes in operating assets and liabilities:
Accounts receivable	(838)	98
Prepaid expenses and other current assets	(250)	(237)
Other assets	(419)
Accounts payable	367	(31)
Accrued expenses and other current liabilities	2,883	1,168
Deferred rent	(52)	(2)
Deferred revenue	(3,844)	(3,875)
Net cash used in operating activities	(32,489)	(25,835)
Cash flows from investing activities:
Purchases of property and equipment	(549)	(912)
Purchases of marketable securities	(47,682)	(6,500)
Maturities and sales of marketable securities	37,700	21,000
Net cash provided by (used in) investing activities	(10,531)	13,588
Cash flows from financing activities:
Proceeds from initial public offering, net of underwriting discounts and commissions	66,793
Proceeds from private placement concurrent with initial public offering	5,000
Proceeds from issuance of common stock upon stock option exercises	609	60
Payments of initial public offering costs	(2,056)	(789)
Net cash provided by (used in) financing activities	70,346	(729)
Net increase (decrease) in cash, cash equivalents and restricted cash	27,326	(12,976)
Cash, cash equivalents and restricted cash at beginning of period	29,600	42,576
Cash, cash equivalents and restricted cash at end of period	56,926	29,600
Supplemental disclosure of noncash investing and financing information:
Conversion of convertible redeemable preferred stock into common stock	77,154
Purchases of property and equipment included in accounts payable		75
Deferred offering costs included in accounts payable and accrued expenses		584
Accretion of redeemable convertible preferred stock to redemption value	$ 16	$ 65

Nature of the Business and Basis of Presentation	12 Months Ended
Nature of the Business and Basis of Presentation	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Unum Therapeutics Inc. (“Unum” or the “Company”) is a clinical-stage biopharmaceutical company focused on the development and commercialization of novel immunotherapy products designed to harness the power of a patient’s immune system to cure cancer. The Company’s proprietary technologies include Antibody-Coupled T cell Receptor (“ACTR”), a universal, engineered cell therapy that is intended to be used in combination with a wide range of tumor-specific antibodies to target different tumor types, and Bolt-On Chimeric Receptor (“BOXR”), a novel approach to engineered T cell therapy designed specifically for solid tumor applications. Unum was incorporated in March 2014 under the laws of the State of Delaware. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. On March 16, 2018, the Company effected a one-for-1.5701314513884 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s redeemable convertible preferred stock (see Note 8). Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the redeemable convertible preferred stock conversion ratios. On April 3, 2018, the Company completed an initial public offering (“IPO”) of its common stock and issued and sold 5,770,000 shares of common stock at a public offering price of $12.00 per share, resulting in net proceeds of $61.5 million after deducting underwriting discounts and commissions and other offering costs. In addition, Seattle Genetics, Inc. (“Seattle Genetics”) purchased from the Company, concurrently with the IPO in a private placement, $5.0 million of shares of common stock at a price per share equal to the initial public offering price, or 416,666 shares (the “concurrent private placement”). Upon closing of the IPO, the Company’s outstanding redeemable convertible preferred stock automatically converted into shares of common stock (see Note 8). Upon conversion of the redeemable convertible preferred stock, the Company reclassified the carrying value of the redeemable convertible preferred stock to common stock and additional paid-in capital. On April 25, 2018, the Company issued and sold an additional 215,000 shares of its common stock at the IPO price of $12.00 per share pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock, resulting in additional net proceeds of $2.4 million after deducting underwriting discounts and commissions. The accompanying consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Since inception, the Company has funded its operations with the sales of redeemable convertible preferred stock, payments received in connection with a collaboration agreement, and most recently, with proceeds from the IPO and concurrent private placement completed in April 2018. The Company has incurred recurring losses since inception, including net losses attributable to the Company of $34.5 million for the year ended December 31, 2018. As of December 31, 2018, the Company had an accumulated deficit of $92.1 million. The Company expects to continue to generate operating losses in the foreseeable future. As of March 28, 2019, the issuance date of the consolidated financial statements, the Company expects that its cash, cash equivalents and marketable securities will be sufficient to fund its operating expenses and capital expenditure requirements through at least 12 months from the issuance date of the consolidated financial statements, without considering available borrowings under the Company’s loan and security agreement. The Company will ultimately need to seek additional funding through equity offerings, debt financings, collaborations, licensing arrangements and other marketing and distribution arrangements, partnerships, joint ventures, combinations or divestitures of one or more of its businesses. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into collaborative arrangements or divest its assets. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. Arrangements with collaborators or others may require the Company to relinquish rights to certain of its technologies or product candidates. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate its research and development programs or commercialization efforts, which could adversely affect its business prospects. The Company’s consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. All intercompany accounts and transactions have been eliminated in consolidation.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual of research and development expenses and the valuation of stock-based awards. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions.

Concentrations of Credit Risk and of Significant Suppliers

Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company maintains most of its cash and cash equivalents at three accredited financial institutions. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships.

The Company is dependent on third-party vendors for its product candidates. In particular, the Company relies, and expects to continue to rely, on a small number of vendors to manufacture supplies and process its product candidates for its development programs. These programs could be adversely affected by a significant interruption in the manufacturing process.

Deferred Offering Costs

The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. Should the planned equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expenses in the consolidated statement of operations and comprehensive loss.

Debt Issuance Costs

The Company capitalizes certain legal and other third-party fees that are directly associated with obtaining access to capital under credit facilities. Debt issuance costs incurred in connection with obtaining access to capital are recorded in prepaid expenses and other current assets and are amortized over the availability period or term of the credit facility. Debt issuance costs related to a recognized debt liability are recorded as a direct reduction of the carrying amount of the debt liability.

Cash Equivalents

The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents.

Property and Equipment

Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows:


		Estimated Useful Life
Laboratory equipment		5 years
Computer equipment and software		3 years
Furniture and fixtures		5 years
Leasehold improvements		Shorter of life of lease or 10 years

Costs for capital assets not yet placed into service are capitalized as construction-in-progress and depreciated in accordance with the above guidelines once placed into service. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred.

Impairment of Long-Lived Assets

Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized in loss from operations when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. The Company did not record any impairment losses on long-lived assets during the years ended December 31, 2018 or 2017.

Fair Value Measurements

Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

•

Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts receivable, accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities.

Marketable Securities

The Company’s marketable securities, consisting of debt securities, are classified as available-for-sale and are reported at fair value. Unrealized gains and losses on available-for-sale debt securities are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Realized gains and losses and declines in value determined to be other than temporary are based on the specific identification method and are included as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss. The Company classifies its marketable securities with maturities beyond one year as short-term, based on their highly liquid nature and because such marketable securities are available for current operations.

The Company evaluates its marketable securities with unrealized losses for other-than-temporary impairment. When assessing marketable securities for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.

Classification and Accretion of Redeemable Convertible Preferred Stock

The Company classified redeemable convertible preferred stock outside of stockholders’ equity (deficit) because the shares contained certain redemption features that were not solely within the control of the Company. The carrying values of the redeemable convertible preferred stock were accreted to their respective redemption values from the date of issuance through the earliest date of redemption.

Segment Information

The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is the development and commercialization of immunotherapy products for cancer. All of the Company’s tangible assets are held in the United States.

Collaboration Agreements

The Company follows the accounting guidance for collaboration agreements, which requires that certain transactions between the Company and collaborators be recorded in its consolidated statements of operations and comprehensive loss on either a gross basis or net basis, depending on the characteristics of the collaborative relationship, and requires enhanced disclosure of collaborative relationships. The Company evaluates its collaboration agreements for proper classification in its consolidated statements of operations and comprehensive loss based on the nature of the underlying activity. If payments to and from collaborative partners are not within the scope of other authoritative accounting literature, the consolidated statements of operations classification for the payments is based on a reasonable, rational analogy to authoritative accounting literature that is applied in a consistent manner. When the Company has concluded that it has a customer relationship with one of its collaborators, such as that with Seattle Genetics (see Note 6), the Company follows the guidance in Accounting Standards Codification (“ASC”) Topic 606, Revenue From Contracts With Customers (“ASC 606”).

Revenue Recognition of Collaboration Agreements

On January 1, 2018, the Company adopted the new revenue standard, discussed below under the heading “Recently Adopted Accounting Pronouncements”, which amended revenue recognition principles and provides a single, comprehensive set of criteria for revenue recognition within and across all industries. The new revenue standard provides a five-step framework whereby revenue is recognized when control of promised goods or services is transferred to a customer at an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To determine revenue recognition for arrangements that the Company determines are within the scope of the new revenue standard, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies a performance obligation. The Company only applies the five-step model to contracts when collectability of the consideration to which the Company is entitled in exchange for the goods or services it transfers to the customer is determined to be probable.

At contract inception, once the contract is determined to be within the scope of the new revenue standard, the Company assesses whether the goods or services promised within each contract are distinct and, therefore, represent a separate performance obligation. Goods and services that are determined not to be distinct are combined with other promised goods and services until a distinct bundle is identified. In determining whether goods or services are distinct, management evaluates certain criteria, including whether (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (capable of being distinct) and (ii) the good or service is separately identifiable from other goods or services in the contract (distinct in the context of the contract).

At the inception of an arrangement that includes options for a customer to purchase additional services or products at agreed upon prices in the future, the Company evaluates whether each option provides a material right. An option that provides a material right will be accounted for as a separate performance obligation.

The Company then determines the transaction price, which is the amount of consideration it expects to be entitled from a customer in exchange for the promised goods or services, for each performance obligation and recognizes the associated revenue as each performance obligation is satisfied. The Company’s estimate of the transaction price for each contract includes all variable consideration to which it expects to be entitled. Variable consideration includes payments in the form of collaboration payments, regulatory milestone payments, commercial milestone payments, and royalty payments. For collaboration, regulatory milestone, and commercial milestone payments the Company evaluates whether it is probable that the consideration associated with each milestone will not be subject to a significant reversal in the cumulative amount of revenue recognized. Amounts that meet this threshold are included in the transaction price using the most likely amount method, whereas amounts that do not meet this threshold are considered constrained and excluded from the transaction price until they meet this threshold. At the end of each subsequent reporting period, the Company re-evaluates the probability of a significant reversal of the cumulative revenue recognized for its milestones, and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis. The Company excludes sales-based royalties until the sale occurs.

The new revenue standard requires the Company to allocate the arrangement consideration on a relative standalone selling price basis for each performance obligation after determining the transaction price of the contract and identifying the performance obligations to which that amount should be allocated. The relative standalone selling price is defined in the new revenue standard as the price at which an entity would sell a promised good or service separately to a customer. If other observable transactions in which the Company has sold the same performance obligation separately are not available, the Company is required to estimate the standalone selling price of each performance obligation. Key assumptions to determine the standalone selling price may include forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success. A performance obligation is satisfied and revenue is recognized when “control” of the promised good or service is transferred, either over time or at a point in time, to the customer. A customer obtains control of a good or service if it has the ability to (1) direct its use and (2) obtain substantially all of the remaining benefits from it.

If a contract should be accounted for as a combined performance obligation, the Company determines the period over which the performance obligations will be performed and revenue will be recognized. The Company will recognize revenue using the cost-to-cost method, which it believes best depicts the transfer of control to the customer. Under the cost-to-cost method, the extent of progress towards completion is measured based on the ratio of actual costs incurred to the total estimated costs expected upon satisfying the identified performance obligation. Under this method, revenue will be recorded as a percentage of the estimated transaction price based on the extent of progress towards completion. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company is expected to complete its performance obligations under an arrangement. The estimate of the Company’s measure of progress and estimate of variable consideration to be included in the transaction price will be updated at each reporting date as a change in estimate. The amount of transaction price allocated to the satisfied portion of the performance obligation, based on the Company’s measure of progress, will be recognized immediately on a cumulative catch-up basis, resulting in an adjustment to revenue in the period of change. The amount related to the unsatisfied portion will be recognized as that portion is satisfied over time.

Amounts received prior to satisfying the revenue recognition criteria listed above are recorded as deferred revenue in the accompanying consolidated balance sheets. Amounts expected to be recognized as revenue within 12 months of the balance sheet date are classified as current deferred revenue. Amounts not expected to be recognized as revenue within the following 12 months of the balance sheet date are classified as deferred revenue, net of current portion. The Company recognizes deferred revenue by first allocating from the beginning deferred revenue balance to the extent that the beginning deferred revenue balance exceeds the revenue to be recognized. Billings during the period are added to the deferred revenue balance to be recognized in future periods. To the extent that the beginning deferred revenue balance is less than revenue to be recognized during the period, billings during the period are allocated to revenue. In the event that a collaboration agreement was to be terminated and the Company had no further performance obligations, the Company would recognize as revenue any portion of the upfront payment and other payments that had not previously been recorded as revenue and were classified as deferred revenue at the date of such termination. The Company recognized revenue of $9.7 million during the year ended December 31, 2018 from the deferred revenue balance at January 1, 2018. At December 31, 2018, the Company had deferred revenue of $17.9 million related to its collaboration.

Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. The Company expenses incremental costs of obtaining a contract as and when incurred if the expected amortization period of the asset that the Company would have recognized is one year or less or the amount is immaterial. The Company has not capitalized any costs to obtain its contract.

Research and Development Costs

Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries, stock-based compensation and benefits, facilities costs and laboratory supplies, depreciation, manufacturing expenses and external costs of outside vendors engaged to conduct preclinical development activities and clinical trials as well as the cost of licensing technology. Research and development costs include costs for the development of product candidates that the Company is jointly developing with Seattle Genetics and for which it receives reimbursement as specified in the agreement.

Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed.

Research Contract Costs and Accruals

The Company has entered into various research and development contracts with companies both inside and outside of the United States. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.

Patent Costs

All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.

Stock-Based Compensation

The Company measures all stock-based awards granted to employees and directors based on the fair value on the date of grant using the Black-Scholes option-pricing model. Compensation expense of those awards is recognized over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues awards with only service-based vesting conditions and records the expense for these awards using the straight-line method. Forfeitures are accounted for as they occur.

For stock-based awards granted to non-employee consultants, compensation expense is recognized over the period during which services are rendered by such consultants until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company’s common shares and updated assumption inputs in the Black-Scholes option-pricing model.

The Company classifies stock-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.

Comprehensive Loss

Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2018 and 2017, the Company’s only element of other comprehensive loss was unrealized gains (losses) on marketable securities.

Net Income (Loss) per Share

Prior to the closing of its IPO, the Company followed the two-class method when computing net income (loss) per share, as the Company had issued shares that met the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. The Company’s redeemable convertible preferred stock contractually entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reported a net loss, such losses were not allocated to such participating securities, and as a result, basic and diluted net loss per share were the same.

Subsequent to the closing of its IPO, basic net income (loss) per common share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) per common share is computed by dividing net income (loss) by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock options. Accordingly, in periods in which the Company reported a net loss, dilutive common shares were not assumed to have been issued as their affect was anti-dilutive, and as a result, diluted net loss per common share was the same as basic net loss per common share.

Income Taxes

The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies.

The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

Recently Adopted Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes existing revenue recognition guidance under GAAP. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The standard outlines a five-step process to achieve this principle, and requires companies to use more judgment and make more estimates than under the previous guidance. The Company determined that these judgments and estimates include identifying performance obligations in the customer contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which delayed the effective date of ASU 2014-09 such that the standard became effective for public entities for annual periods beginning after December 15, 2017 and for interim periods within those fiscal years. The FASB subsequently issued amendments to ASU No. 2014-09 that had the same effective dates and transition requirements as ASU 2014-09, all of which collectively are herein referred to as “ASC 606”.

On January 1, 2018, the Company adopted the new revenue standard by applying the modified retrospective method to its collaboration agreement with Seattle Genetics (see Note 6) which was not completed as of January 1, 2018. As a result, while reporting periods beginning on the Company’s adoption of the new revenue standard are presented under the new revenue standard, prior period amounts have not been adjusted and continue to be presented under the revenue standard in effect prior to January 1, 2018.

The following table summarizes the cumulative effect to the Company’s consolidated balance sheet upon the adoption of the new revenue standard on January 1, 2018 (in thousands):


	Balance at December 31, 2017			Adjustments			Balance at January 1, 2018
Deferred revenue, current and net of current portion	$	15,605		$	6,188		$	21,793
Accumulated deficit	$	(51,339	)	$	(6,188	)	$	(57,527	)

The adjustment is the result of the application of the new revenue standard regarding how entities should measure progress in satisfying performance obligations and the contract’s transaction price. Under ASC 606, the Company recognizes revenue using the cost-to-cost method, which it believes best depicts the transfer of control to the customer. In contrast, under the previous revenue standard, the Company recognized revenue on a straight-line basis over the estimated period of performance. In addition, under ASC 606, the estimated transaction price includes variable consideration for payments expected to be earned for preclinical research and clinical development activities through Phase I, which, under the previous standard, the Company was precluded from including in the estimated transaction price until such payments were determinable and due.

The Company accounts for the license, research and development services, and steering committee services under ASC 606 as a single performance obligation under the collaboration agreement, just as it accounted for those items as a single unit of accounting under the previous standard. The options held by Seattle Genetics are expected to continue to be accounted for separately as they do not represent material rights based on the criteria of ASC 606. Further, ASC 606 does not have an impact on the Company’s current accounting for milestone or royalty payments.

In accordance with the new revenue standard requirements, the following tables summarize the impact of adoption on the Company’s consolidated balance sheets, consolidated statements of operations and comprehensive loss, and consolidated statements of cash flows (in thousands):

Consolidated Balance Sheet


	At December 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Deferred revenue, current portion	$	17,949		$	8,110		$	9,839
Deferred revenue, net of current portion	$	—		$	2,128		$	(2,128	)
Accumulated deficit	$	(92,059	)	$	(84,348	)	$	(7,711	)

Consolidated Statements of Operations and Comprehensive Loss


	Year Ended December 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Collaboration revenue	$	9,734		$	11,257		$	(1,523	)
Net loss	$	(34,532	)	$	(33,009	)	$	(1,523	)
Net loss attributable to common stockholders	$	(34,548	)	$	(33,025	)	$	(1,523	)
Net loss per share attributable to common stockholders, basic and diluted	$	(1.39	)	$	(1.33	)	$	(0.06	)
Comprehensive loss	$	(34,528	)	$	(33,005	)	$	(1,523	)

Consolidated Statement of Cash Flows


	Year Ended December 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Net loss	$	(34,532	)	$	(33,009	)	$	(1,523	)
Change in deferred revenue	$	(3,844	)	$	(5,367	)	$	1,523

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”), to address diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. The Company adopted the standard retrospectively for all periods presented on the required effective date of January 1, 2018, and its adoption had no impact on the Company’s financial position, results of operations or cash flows.

In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows, Restricted Cash, requiring restricted cash and restricted cash equivalents to be included with cash and cash equivalents on the statement of cash flows when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The Company adopted this standard during the first quarter of 2018. Restricted cash is now included as a component of cash, cash equivalents, and restricted cash on the Company’s consolidated statements of cash flows. The inclusion of restricted cash increased the beginning and ending balances of the consolidated statement of cash flows by $1.3 million for the year ended December 31, 2017.

In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”), which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The Company adopted the standard prospectively beginning on the required effective date of January 1, 2018, and its adoption did not have a material impact on the Company’s financial position, results of operations or cash flows.

Recently Issued Accounting Pronouncements

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease, respectively. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. The guidance is effective for public entities for annual reporting periods beginning after December 15, 2018 and for interim periods within those fiscal years, and early adoption is permitted. ASU 2016-02 initially required adoption using a modified retrospective approach, under which all years presented in the financial statements would be prepared under the revised guidance. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842), which added an optional transition method under which financial statements may be prepared under the revised guidance for the year of adoption, but not for prior years. Under the latter method, entities will recognize a cumulative catch-up adjustment to the opening balance of retained earnings in the period of adoption. The Company plans to adopt the new leasing standard on January 1, 2019, using a modified retrospective transition approach to be applied to leases existing as of, or entered into after, January 1, 2019. The Company will apply the “package of practical expedients”, which permits the Company not to reassess under the new standards for prior conclusions about lease identification, lease classification and initial direct costs. Upon adoption of the new leasing standards, the Company expects to recognize a lease liability of approximately $7.5 million and a related right-of-use asset of approximately $6.6 million on its consolidated balance sheet with the difference being due to the elimination of previously reported deferred rent. The Company does not expect the adoption of the standard to have a material impact on its results of operations or cash flows.

In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception (“ASU 2017-11”). Part I applies to entities that issue financial instruments such as warrants, convertible debt or convertible preferred stock that contain down-round features. Part II replaces the indefinite deferral for certain mandatorily redeemable noncontrolling interests and mandatorily redeemable financial instruments of nonpublic entities contained within ASC Topic 480 with a scope exception and does not impact the accounting for these mandatorily redeemable instruments. ASU 2017-11 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. The Company plans to adopt ASU No. 2017-11 on January 1, 2019 and does not expect the adoption of this guidance to have a material impact on its consolidated financial statements.

In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). ASU 2018-07 is intended to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The Company does not expect the adoption of this guidance to have a material impact on its consolidated financial statements.

Marketable Securities and Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Marketable Securities and Fair Value Measurements

3. Marketable Securities and Fair Value Measurements

Marketable securities by security type consisted of the following (in thousands):


	December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury bills and notes (due within one year)	$	22,935	$	—	$	(12	)	$	22,923

	$	22,935	$	—	$	(12	)	$	22,923


	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury bills and notes (due within one year)	$	5,007	$	—	$	(10	)	$	4,997
U.S. government agency bonds (due within one year)		7,700		—		(6	)		7,694

	$	12,707	$	—	$	(16	)	$	12,691

The following tables present the Company’s fair value hierarchy for its cash equivalents and marketable securities, which are measured at fair value on a recurring basis (in thousands):


	Fair Value Measurements at December 31, 2018 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	—	$	52,100	$	—	$	52,100
Marketable securities:
U.S. Treasury bills and notes		22,923		—		—		22,923

	$	22,923	$	52,100	$	—	$	75,023


	Fair Value Measurements at December 31, 2017 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	—	$	24,196	$	—	$	24,196
Marketable securities:
U.S. Treasury notes		4,997		—		—		4,997
U.S. government agency bonds		—		7,694		—		7,694

	$	4,997	$	31,890	$	—	$	36,887

U.S. Treasury bills and notes were valued based on Level 1 inputs. Money market funds and U.S. government agency bonds were valued by the Company using quoted prices in active markets for similar securities, which represent a Level 2 measurement within the fair value hierarchy.

During the years ended December 31, 2018 and 2017, there were no transfers between Level 1, Level 2 and Level 3.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment, Net

12 Months Ended

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

4. Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):


	December 31,
	2018			2017
Laboratory equipment	$	5,801		$	5,327
Computer equipment and software		203			218
Furniture and fixtures		317			317
Leasehold improvements		426			426

		6,747			6,288
Less: Accumulated depreciation and amortization		(3,496	)		(2,180	)

	$	3,251		$	4,108

Depreciation and amortization expense was $1.3 million and $1.2 million for the years ended December 31, 2018 and 2017, respectively.

Accrued Expenses and Other Current Liabilities

12 Months Ended

Dec. 31, 2018

Payables and Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

5. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	December 31,
	2018		2017
Accrued employee compensation and benefits	$	1,599	$	1,315
Accrued external research and development expense		1,799		478
Accrued professional fees		400		980
Other		1,679		180

	$	5,477	$	2,953

Collaboration Agreement	12 Months Ended
Collaboration Agreement	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration Agreement	6. Collaboration Agreement The Company has a collaboration agreement with Seattle Genetics, entered into in 2015, whereby the parties agreed to jointly develop two product candidates incorporating the Company’s ACTR platform and Seattle Genetics’ antibodies. Under the collaboration agreement, the Company conducts preclinical research and clinical development activities related to the two specified product candidates through Phase I clinical development, and Seattle Genetics provides the funding for those activities. Seattle Genetics will continue development activities of the two specified product candidates in collaboration with the Company unless it exercises one of its two options to opt-out from further development and commercialization activities for each of the two product candidates during specified periods subsequent to Phase I clinical development. In addition, the Company has an option to opt-out from further development and commercialization activities for each of the two product candidates, exercisable during a specified period subsequent to Phase II clinical development. If neither party exercises its options to opt-out from further development and commercialization activities for each product candidate, the parties will work together to co-develop and fund each product candidate after Phase I clinical development and Seattle Genetics will pay the Company specified collaboration and milestone payments upon the occurrence of specified events related to each product candidate. As of December 31, 2018, the Company was eligible to receive future collaboration and milestone payments under the collaboration agreement of up to an aggregate of $400.0 million across the two active product candidates, consisting of $100.0 million of aggregate collaboration payments, $100.0 million of aggregate regulatory milestone payments and $200.0 million of aggregate commercial milestone payments. The individual collaboration payments are payable upon the occurrence of specified clinical development events and range up to $30.0 million per product candidate. The individual regulatory milestone payments are payable upon the first regulatory approval of each product in the United States and the first regulatory approval of each product in specified territories outside the United States and range up to $35.0 million per product. The individual commercial milestone payments are payable upon the achievement of specified aggregate annual net sales for each product and range up to $60.0 million per product. In the event that a party exercises its option to opt-out from further development and commercialization of a product candidate, the parties will negotiate in good faith the payment obligations of the continuing party to the opt-out party for that product candidate. Unless either party exercises its right to opt-out from further development and commercialization activities, the Company and Seattle Genetics will co-commercialize and share profits and losses equally on any co-developed products in the United States. Seattle Genetics will retain exclusive commercial rights outside of the United States and is obligated to pay the Company tiered royalties ranging in the high single-digit to mid-teens percentages based on net sales outside of the United States. The royalties are payable on a product-by-product basis and may be reduced in specified circumstances. Seattle Genetics will purchase ACTR T cells from the Company on a cost-plus basis for its commercial supply outside of the United States. Unless earlier terminated, the collaboration agreement will expire on a product-by-product basis in the United States on the date on which neither party is researching, developing or commercializing such product. Outside of the United States, the collaboration agreement will expire on a product-by-product and country-by-country basis at the end of the applicable royalty term for such product in such country. The royalty term will be in effect beginning at the first commercial sale of a product and ending upon the later to incur of (i) expiration of the last valid claim within any patent right that the Company or Seattle Genetics has that would be infringed by the manufacture, use, sale, offer for sale, or importation of such product in such country, (ii) the end of any regulatory exclusivity periods that apply to the manufacture, use, sale, offer for sale, or importation of such product in such country, or (iii) ten years from the first commercial sale of such product in such country. The Company evaluated whether the performance obligations under this agreement, including the license, research and development services, steering committee participation, and manufacturing services should be accounted for as a single unit or multiple units of accounting. Because of the risk associated with obtaining approval for commercial sale in the Seattle Genetics territories, manufacturing services associated with commercial supply were considered a contingent deliverable and will be accounted for if and when performed. The Company accounts for the license, research and development services, and steering committee services under ASC 606 as a single performance obligation under the collaboration agreement. The Company determined that the transaction price includes the $25.0 million upfront payment and the total payments to be earned for preclinical research and clinical development activities. The total transaction price is being recognized as revenue over the performance period using the cost-to-cost method, which the Company believes best depicts the transfer of control to the customer. As payments from Seattle Genetics are earned related to the Company’s preclinical research and clinical development activities through Phase I clinical development, the Company recognizes as revenue the portion of the payments equal to the percentage of the cost of completed research and development activities to the cost of the total estimated research and development activities. Any future milestone payments will be recognized, along with the other arrangement consideration, over the remaining estimated period of performance, if any, beginning at the time a milestone payment is earned, with a cumulative catch up being recognized for the completed portion of the estimated research costs. At the inception of the arrangement, the Company evaluated the separate options held by Seattle Genetics (i) to expand the collaboration to include a third product candidate upon payment of an additional fee and (ii) to continue development activities beyond Phase I clinical development activities and determined that each option was substantive. Each option represents a separate buying decision by Seattle Genetics, is not essential to the functionality of the current deliverables, and was not offered at a substantially discounted price. As each option was deemed to be substantive, the item underlying the option was not considered to be a deliverable at the inception of the arrangement and the incremental fees associated with each option were not included in the initial arrangement consideration. These options will be accounted for as separate units of accounting when, and if, such options are exercised by Seattle Genetics. Under the collaboration agreement, the Company recognized revenue of $9.7 million and $8.4 million for the years ended December 31, 2018 and 2017, respectively. As of December 31, 2018 and 2017, deferred revenue of $17.9 million and $15.6 million, respectively, was recorded related to this agreement. As noted in Note 2 above, deferred revenue was increased by $6.2 million as of January 1, 2018 upon adoption of ASC 606. As of December 31, 2018, the aggregate amount of the transaction price allocated to the remaining performance obligation for preclinical research and clinical development activities related to the two specified product candidates through Phase I is estimated to be approximately $54.1 million, which is expected to be recognized as revenue through December 31, 2022.

Loan and Security Agreement	12 Months Ended
Loan and Security Agreement	Dec. 31, 2018
Debt Disclosure [Abstract]
Loan and Security Agreement	7. Loan and Security Agreement In January 2017, the Company entered into a loan and security agreement with a lender, the (“Loan Agreement”), which provides for term loan borrowings of up to $15.0 million through January 19, 2019. Borrowings under the loan and security agreement bear interest at a variable annual rate equal to the greater of (i) the prime rate plus 0.25% or (ii) 3.75%, and are payable over an interest-only period until January 19, 2019, followed by a 24-month period of equal monthly payments of principal and interest. All amounts outstanding as of the maturity date of January 19, 2021 become immediately due and payable. In connection with the loan and security agreement, the Company agreed to enter into warrant agreements with the lender pursuant to which warrants will be issued to purchase a number of shares of the Company’s capital stock equal to 1% of the amount of each term loan borrowing under the loan and security agreement, divided by the applicable exercise price. No amounts have been borrowed as term loans under the loan and security agreement as of December 31, 2018. In January 2019, the Company amended the Loan Agreement to extend the available borrowing date, the interest-only period and the repayment period (see Note 16). Borrowings under the loan and security agreement are collateralized by substantially all of the Company’s assets, except for its intellectual property. Under the loan and security agreement, the Company has agreed to affirmative and negative covenants to which it will remain subject until maturity. These covenants include limitations on the Company’s ability to incur additional indebtedness and engage in certain fundamental business transactions, such as mergers or acquisitions of other businesses. There are no financial covenants associated with the loan and security agreement. Events of default under the loan and security agreement include failure to make payments when due, insolvency events, failure to comply with covenants and material adverse effects with respect to the Company.

Redeemable Convertible Preferred Stock

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Redeemable Convertible Preferred Stock

8. Redeemable Convertible Preferred Stock

The Company had issued Series A redeemable convertible preferred stock (the “Series A preferred stock”) and Series B redeemable convertible preferred stock (the “Series B preferred stock”). The Series A preferred stock and the Series B preferred stock are collectively referred to as the “Preferred Stock”. The Preferred Stock converted to shares of common stock on a 1:1.5701314513884 basis upon the closing of the IPO on April 3, 2018.

As of December 31, 2017, the Preferred Stock consisted of the following (in thousands, except share amounts):


	Preferred Stock Authorized		Preferred Stock Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series A preferred stock		12,297,276		12,297,276	$	12,267	$	12,297		7,832,001
Series B preferred stock		8,494,131		8,474,574		64,884		65,000		5,397,361

		20,791,407		20,771,850	$	77,151	$	77,297		13,229,362

Prior to the closing of the IPO, the holders of the Preferred Stock had the following rights and preferences:

Voting

The holders of Preferred Stock were entitled to vote, together with the holders of common stock, on matters submitted to stockholders for a vote. The holders of Preferred Stock were entitled to the number of votes equal to the number of common shares into which each such share of Preferred Stock could convert. In addition, the holders of Series A preferred stock, voting exclusively and as a separate class, were entitled to elect two directors of the Company. The holders of Series B preferred stock, voting exclusively and as a separate class, were entitled to elect one director of the Company.

Conversion

Each share of Preferred Stock was convertible at the option of the holder at any time after the date of issuance. Each share of Preferred Stock would have automatically converted into shares of common stock at the applicable conversion ratio then in effect upon the closing of a firm commitment public offering with at least $50.0 million of gross proceeds to the Company. Shares of Series A preferred stock would have automatically converted into shares of common stock at the applicable conversion ratio then in effect upon written consent of the holders of at least 65% of the then-outstanding shares of Series A preferred stock. Shares of Series B preferred stock would have automatically converted into shares of common stock at the applicable conversion ratio then in effect upon written consent of the holders of at least a majority of the then-outstanding shares of Series B preferred stock.

The conversion ratio of each series of Preferred Stock was determined by dividing the Original Issue Price of each series by the Conversion Price of each series. The Original Issue Price was $1.00 per share for Series A preferred stock and $7.67 per share for Series B preferred stock. The Conversion Price at issuance was $1.570131 per share for Series A preferred stock and $12.042908 per share for Series B preferred stock, subject to appropriate adjustment in the event of any stock split, stock dividend, combination or other similar recapitalization and other adjustments as set forth in the Company’s certificate of incorporation, as amended and restated.

Dividends

The holders of Preferred Stock were entitled to receive noncumulative dividends if and when declared by the Company’s board of directors. The Company could not declare, pay or set aside any dividends on shares of any other series of capital stock of the Company, other than dividends on common stock payable in common stock, unless the holders of the Series A and Series B preferred stock first received, or simultaneously received, a dividend on each outstanding share of Series A and Series B preferred stock in an amount at least equal to the greater of (i) $0.08 per share in the case of Series A preferred stock and $0.61 per share in the case of Series B preferred stock, each subject to appropriate adjustment in the event of any stock split, stock dividend, combination or other similar recapitalization with respect to such shares, and (ii) (A) in the case of a dividend on common stock or any class or series of stock that was convertible into common stock, that dividend per share of Preferred Stock as would equal the product of (1) the dividend payable on each share of such class or series determined, if applicable, as if all shares of such class or series had been converted into common stock and (2) the number of shares of common stock issuable upon conversion of each share of Preferred Stock, or (B) in the case of a dividend on any class or series that was not convertible into common stock, at a rate per share of Preferred Stock determined by (1) dividing the amount of the dividend payable on each share of such class or series of capital stock by the Original Issue Price of such class or series of capital stock (subject to appropriate adjustment in the event of any stock dividend, stock split, combination of or other similar recapitalization affecting such shares) and (2) multiplying such fraction by an amount equal to the Original Issue Price of each series of Preferred Stock. If the Company declared, paid or set aside, on the same date, a dividend on shares of more than one class or series of capital stock of the Company, the dividend payable to the holders of the Preferred Stock would have been calculated based upon the dividend on the class or series of capital stock that would have resulted in the highest Preferred Stock dividend. Stockholders were not entitled to any accruing dividends. No dividends had been declared or paid.

Liquidation

In the event of any voluntary or involuntary liquidation, dissolution or winding-up of the Company or Liquidating Event (as described below), the holders of shares of Preferred Stock would have received, in preference to the common stockholders, an amount equal to the greater of (i) the Original Issue Price per share of the respective share of Preferred Stock, plus all dividends declared but unpaid on such shares, or (ii) the amount the holders would have received if the Preferred Stock were converted into common stock prior to such liquidation event. In the event that the assets available for distribution to the Company’s stockholders had not been sufficient to permit payment to the holders of Preferred Stock in the full amount to which they were entitled, the assets available for distribution would have been distributed on a pro rata basis among the holders of the Series A and Series B preferred stock. After the payment of all preferential amounts to the holders of the Preferred Stock then, to the extent available, the remaining assets available for distribution would have been distributed among the holders of the common stock ratably based on the number of shares of common stock held by each holder.

Unless the holders of at least two-thirds of the then-outstanding shares of Preferred Stock, voting together as a single class on an as-converted basis, elected otherwise, a Liquidating Event would have included a merger or consolidation (other than one in which stockholders of the Company own a majority by voting power of the outstanding shares of the surviving or acquiring corporation) or a sale, lease, transfer, exclusive license or other disposition of all or substantially all of the assets of the Company.

Redemption

At any time on or after June 10, 2020, shares of each of the Series A and Series B preferred stock were subject to mandatory redemption by the Company in three equal annual installments beginning 60 days after receipt of a notice of redemption from the holders of at least two-thirds of the combined voting power of the holders of outstanding shares of Series A and Series B preferred stock, voting together as a single class, in an amount equal to the Original Issue Price per share of each series of Preferred Stock plus any dividends declared but unpaid thereon.

Equity	12 Months Ended
Equity	Dec. 31, 2018
Equity [Abstract]
Equity	9. Equity Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are not entitled to receive dividends, unless declared by the board of directors. On March 16, 2018, the Company effected a one-for-1.5701314513884 reverse stock split of its issued and outstanding shares of common stock. Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split. On April 3, 2018, the Company completed an IPO of its common stock and issued and sold 5,770,000 shares of common stock at a public offering price of $12.00 per share, resulting in net proceeds of $61.5 million after deducting underwriting discounts and commissions and other offering costs. In addition, Seattle Genetics purchased from the Company, concurrently with the IPO in a private placement, $5.0 million of shares of common stock at a price per share equal to the initial public offering price, or 416,666 shares. Upon closing of the IPO, the Company’s authorized shares of common stock were increased to 150,000,000 shares. The Company also authorized 10,000,000 shares of undesignated preferred stock. On April 25, 2018, the Company issued and sold an additional 215,000 shares of its common stock at the IPO price of $12.00 per share pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock, resulting in additional net proceeds of $2.4 million after deducting underwriting discounts and commissions.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Compensation

10. Stock-Based Compensation

2015 Stock Incentive Plan

The Company’s 2015 Stock Incentive Plan (the “2015 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2015 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The board of directors could also delegate to one or more officers of the Company the power to grant awards to employees and certain officers of the Company. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted under the 2015 Plan with service-based vesting conditions generally vest over four years and expire after ten years. The total number of shares of common stock that could have been issued under the 2015 Plan was 4,144,876 shares. Upon effectiveness of the Company’s 2018 Stock Option and Incentive Plan, the (“2018 Plan”) in March 2018, the remaining shares available under the 2015 Plan became available for issuance under the 2018 Plan and no future issuance will be made under the 2015 Plan. Additionally, outstanding options under the 2015 Plan that are expired, terminated, surrendered or canceled without having been fully exercised will be available for future issuance under the 2018 Plan.

The exercise price for stock options granted is not less than the fair value of common shares as determined by the board of directors as of the date of grant. Prior to the Company’s IPO, the Company’s board of directors valued the Company’s common stock, taking into consideration its most recently available valuation of common stock performed by third parties as well as additional factors which may have changed since the date of the most recent contemporaneous valuation through the date of grant.

2018 Stock Option and Incentive Plan

On March 16, 2018, the Company’s stockholders approved the 2018 Plan, which became effective on March 27, 2018. The 2018 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, unrestricted stock awards, cash-based awards and dividend equivalent rights. The number of shares initially reserved for issuance under the 2018 Plan is 2,800,721 plus the shares of common stock remaining available for issuance under the 2015 Plan. The number of shares reserved shall be cumulatively increased on January 1, 2019 and each January 1 thereafter by 4% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or a lesser number of shares determined by the Company’s board of directors. The shares of common stock underlying any awards that are forfeited, canceled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, repurchased or are otherwise terminated by the Company under the 2018 Plan or the 2015 Plan will be added back to the shares of common stock available for issuance under the 2018 Plan. As of December 31, 2018, 2,793,738 shares remained available for future issuance under the 2018 Plan. The number of authorized shares reserved for issuance under the 2018 Plan was increased by 1,202,319 shares effective as of January 1, 2019.

2018 Employee Stock Purchase Plan

On March 16, 2018, the Company’s stockholders approved the 2018 Employee Stock Purchase Plan (the “ESPP”), which became effective on March 28, 2018. A total of 314,000 shares of common stock were reserved for issuance under this plan. In addition, the number of shares of common stock that may be issued under the ESPP will automatically increase on January 1, 2019, and each January 1 thereafter through January 1, 2027, by the least of (i) 500,000 shares of common stock, (ii) 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or (iii) such lesser number of shares as determined by the ESPP administrator. The number of authorized shares reserved for issuance under the ESPP was increased by 300,580 shares effective as of January 1, 2019.

Stock Option Valuation

The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically had been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock options granted to employees and directors:


	Year Ended December 31,
	2018			2017
Risk-free interest rate		2.64	%		1.81	%
Expected volatility		67.40	%		66.77	%
Expected dividend yield		—			—
Expected life (in years)		6.07			6.06

The following table summarizes the Company’s option activity since December 31, 2017:


	Number of Shares			Weighted Average Exercise Price		Weighted Average Contractual Term		Aggregate Intrinsic Value
						(in years)		(in thousands)
Outstanding as of December 31, 2017		3,156,939		$	4.01
Granted		929,231			12.00
Exercised		(225,252	)		2.71
Forfeited		(198,936	)		7.54

Outstanding as of December 31, 2018		3,661,982		$	5.92		6.2	$	5,019

Vested and expected to vest as of December 31, 2018		3,661,982		$	5.92		6.2	$	5,019

Options exercisable as of December 31, 2018		2,086,919		$	2.89		5.0	$	4,873

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had strike prices lower than the fair value of the Company’s common stock.

The aggregate intrinsic value of options exercised during the years ended December 31, 2018 and 2017 was $1.6 million and less than $0.1 million, respectively.

The weighted average grant-date fair value of awards granted during the years ended December 31, 2018 and 2017 was $6.50 per share and $5.39 per share, respectively.

As of December 31, 2018, there were outstanding unvested service-based stock options held by non-employees for the purchase of 18,566 shares of common stock.

The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):


	Year Ended December 31,
	2018		2017
Research and development expenses	$	2,184	$	1,159
General and administrative expenses		904		182

Total	$	3,088	$	1,341

As of December 31, 2018, total unrecognized compensation cost related to the unvested stock-based awards was $8.6 million, which is expected to be recognized over a weighted average period of 2.6 years.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

11. Income Taxes

2017 U.S. Tax Reform

On December 22, 2017, the Tax Cuts and Jobs Act (the “TCJA”) was signed into United States law. The TCJA includes a number of changes to existing tax law, including, among other things, a permanent reduction in the federal corporate income tax rate from 34% to 21%, effective as of January 1, 2018, as well as limitation of the deduction for net operating losses to 80% of annual taxable income and elimination of net operating loss carrybacks, in each case, for losses arising in taxable years beginning after December 31, 2017 (though any such net operating losses may be carried forward indefinitely). The tax rate change resulted in (i) a reduction in the gross amount of the Company’s deferred tax assets recorded as of December 31, 2017, without an impact on the net amount of its deferred tax assets, which are recorded with a full valuation allowance, and (ii) no income tax expense or benefit being recognized as of the enactment date of the TCJA. The Company finalized its accounting for the income tax effects of TCJA during 2018, with no adjustment to the provisional amounts previously recorded.

Income Taxes

During the years ended December 31, 2018 and 2017, the Company recorded no current or deferred income tax benefits for the net operating losses or research and development tax credits generated in each year due to its uncertainty of realizing a benefit from those items. The Company had no foreign operating losses.

A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows:


	Year Ended December 31,
	2018			2017
Federal statutory income tax rate		(21.0	)%		(34.0	)%
State taxes, net of federal benefit		(6.2	)		(5.1	)
Federal and state research and development tax credits		(4.2	)		(7.6	)
Federal research and development tax credit add-back		—			2.3
Nondeductible items		0.7			1.3
2017 Tax Acts and Jobs Cut		—			21.9
Increase in deferred tax asset valuation allowance		30.7			21.2

Effective income tax rate		0.0	%		0.0	%

Net deferred tax assets as of December 31, 2018 and 2017 consisted of the following (in thousands):


	December 31,
	2018			2017
Deferred tax assets:
Net operating loss carryforwards		19,189		$	8,211
Research and development and investment tax credits		4,891			3,467
Deferred revenue		3,294			3,693
Accrued expenses		437			491
Capitalized start-up costs		93			102
Capitalized research and development expense		60			73
Other		933			574

Total deferred tax assets		28,897			16,611
Valuation allowance		(28,897	)		(16,611	)

Net deferred tax assets	$	—		$	—

As of December 31, 2018, the Company had U.S. federal and state net operating loss carryforwards of $69.8 million and $71.7 million, respectively, which may be available to offset future income tax liabilities and begin to expire in 2035. The 2018 federal net operating loss of $39.6 million is available to be carried forward indefinitely but can only offset 80% of taxable income per year. As of December 31, 2018, the Company also had U.S. federal and state research and development tax credit carryforwards of $4.0 million and $0.9 million, respectively, which may be available to offset future income tax liabilities and begin to expire in 2034 and 2030, respectively. As of December 31, 2018, the Company has Massachusetts investment tax credits of $0.2 million which generally have a 3 year carryover period.

Utilization of the U.S. federal and state net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986, and corresponding provisions of state law, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain stockholders or public groups in the stock of a corporation by more than 50% over a three-year period. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382, at any time since inception, utilization of the federal and state net operating loss carryforwards or research and development tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and development tax credit carryforwards before utilization. Further, until a study is completed by the Company and any limitation is known, no amounts are being presented as an uncertain tax position.

The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2018 and 2017. Management reevaluates the positive and negative evidence at each reporting period.

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2018 and 2017 related primarily to the increase in net operating loss carryforwards and research and development tax credit carryforwards, partially offset in 2017 by a decrease in deferred tax assets resulting from the decreased federal corporate tax rate, and were as follows (in thousands):


	Year Ended December 31,
	2018		2017
Valuation allowance as of beginning of year	$	16,611	$	11,208
Decreases recorded as benefit to income tax provision		—		(5,575	)
Increases recorded to income tax provision		12,286		10,978

Valuation allowance as of end of year	$	28,897	$	16,611

As of December 31, 2018 and 2017, the Company had not recorded any amounts for unrecognized tax benefits. The Company files income tax returns in the U.S. and Massachusetts. The statute of limitations for assessment by the Internal Revenue Service and Massachusetts tax authorities remains open for all years since 2015. The Company’s tax attributes related to years prior to 2015 can still be adjusted under audit. No federal or state tax audits are currently in process.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

12. Commitments and Contingencies

Operating Leases

The Company leases its facility under a non-cancelable operating lease that expires in April 2023. Under the terms of the lease, the Company secured a $1.3 million letter of credit as security for its leased facility. The underlying cash securing this letter of credit has been classified as non-current restricted cash in the accompanying consolidated balance sheets. The lease includes annual rent escalations, which are accrued, such that rent expense is recognized on a straight-line basis over the terms of occupancy.

Future minimum lease payments under the operating lease as of December 31, 2018 are as follows (in thousands):


Year Ending December 31,
2019		1,878
2020		1,933
2021		1,989
2022		2,046
2023		689

	$	8,535

Rent expense for the years ended December 31, 2018 and 2017 was $1.8 million and $1.8 million, respectively.

In January 2017, the Company entered into a 12-month sublease agreement with a tenant for up to 2,500 square feet of general office and laboratory space at its headquarters. In December 2017, the Company entered into a new sublease agreement with a tenant for approximately 5,000 square feet of general office and laboratory space at its headquarters. The sublease ended in June 2018. The Company recognized $0.3 million and $0.3 million received under the subleases as other income in the consolidated statements of operations and comprehensive loss for the years ended December 31, 2018 and 2017, respectively.

License Agreement

Under its license agreement with National University of Singapore and St. Jude Children’s Research Hospital, Inc. (collectively the “Licensors”) entered into in 2014, the Company is obligated to pay license maintenance fees on each anniversary of the effective date of the agreement that escalate from less than $0.1 million for each of the first seven years to $0.1 million on the eighth anniversary and each year thereafter. The Company is also obligated to make aggregate milestone payments of up to 5.5 million Singapore dollars (equivalent to approximately $4.0 million as of December 31, 2018) upon the achievement of specified clinical and regulatory milestones and to pay tiered royalties ranging in the low single-digit percentages on annual net sales of licensed products sold by the Company or its sublicensees. The royalties are payable on a product-by-product and country-by-country basis and may be reduced in specified circumstances. Additionally, under certain circumstances, the Company is obligated to pay the Licensors a percentage of amounts received from sublicensees.

The license agreement will expire on a country-by-country basis until the last to expire of the patents and patent applications covering such licensed product or service. The Licensors may terminate the license agreement within 60 days after written notice in the event of a breach of contract. The Licensors may also terminate the agreement upon written notice in the event of the Company’s bankruptcy, liquidation, or insolvency. In addition, the Company has the right to terminate this agreement in its entirety at will upon 90 days’ advance written notice to the Licensors. However, if the Company has commenced the commercialization of licensed products, the Company can only terminate at will if it ceases all development and commercialization of licensed products.

Manufacturing Commitment

As of December 31, 2018, the Company had non-cancelable minimum purchase commitments under contract manufacturing agreements for payments totaling $1.5 million over the following 12 months.

Indemnification Agreements

In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its executive officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2018 or 2017.

Legal Proceedings

The Company is not currently party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to such legal proceedings.

Net Loss per Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss per Share

13. Net Loss per Share

Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts):


	Year Ended December 31,
	2018			2017
Numerator:
Net loss	$	(34,532	)	$	(25,492	)
Accretion of redeemable convertible preferred stock to redemption value		(16	)		(65	)

Net loss attributable to common stockholders	$	(34,548	)	$	(25,557	)

Denominator:
Weighted average common shares outstanding, basic and diluted		24,895,670			10,191,807

Net loss per share attributable to common stockholders, basic and diluted	$	(1.39	)	$	(2.51	)

The Company’s potential dilutive securities have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:


	December 31,
	2018		2017
Redeemable convertible preferred shares (as converted to common stock)		—		13,229,362
Stock options to purchase common stock		3,661,982		3,156,939

		3,661,982		16,386,301

Retirement Plan	12 Months Ended
Retirement Plan	Dec. 31, 2018
Retirement Benefits [Abstract]
Retirement Plan	14. Retirement Plan The Company has a defined-contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pre-tax basis. As currently established, the Company is not required to make and to date has not made any contributions to the 401(k) Plan. The Company did not make any matching contributions during the years ended December 31, 2018 or 2017.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

15. Selected Quarterly Financial Data (Unaudited)

The following information has been derived from unaudited financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information (in thousands except per share data):


	Mar 31, 2017			Jun 30, 2017			Sep 30, 2017			Dec 31, 2017			Mar 31, 2018			Jun 30, 2018			Sep 30, 2018			Dec 31, 2018
Statements of Operations Data:
Revenue	$	1,827		$	2,079		$	2,331		$	2,123		$	2,220		$	1,666		$	2,043		$	3,805
Loss from operations		(6,069	)		(6,033	)		(7,170	)		(6,880	)		(6,986	)		(9,439	)		(10,576	)		(9,004	)
Net loss		(5,939	)		(5,863	)		(7,000	)		(6,690	)		(6,735	)		(9,023	)		(10,168	)		(8,606	)
Net loss attributable to common stockholders		(5,955	)		(5,880	)		(7,016	)		(6,706	)		(6,751	)		(9,023	)		(10,168	)		(8,606	)
Basic and diluted net loss attributable to common stockholders per share:	$	(0.58	)	$	(0.58	)	$	(0.69	)	$	(0.66	)	$	(0.66	)	$	(0.31	)	$	(0.34	)	$	(0.29	)

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	16. Subsequent Events In January 2019, the Company amended the Loan Agreement (see Note 7) to extend the available date for borrowings from January 19, 2019 to June 30, 2019 and extend the interest only period from January 19, 2019 to June 30, 2020, with the possibility of further extension to March 31, 2021 if certain equity financing considerations are met. Additionally, the loan repayment period will be over a 24-month period following the end of the interest-only period.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Use of Estimates

Use of Estimates

Concentrations of Credit Risk and of Significant Suppliers

Concentrations of Credit Risk and of Significant Suppliers

Deferred Offering Costs

Deferred Offering Costs

Debt Issuance Costs

Debt Issuance Costs

Cash Equivalents

Cash Equivalents

The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents.

Property and Equipment

Property and Equipment


		Estimated Useful Life
Laboratory equipment		5 years
Computer equipment and software		3 years
Furniture and fixtures		5 years
Leasehold improvements		Shorter of life of lease or 10 years

Impairment of Long-Lived Assets

Impairment of Long-Lived Assets

Fair Value Measurements

Fair Value Measurements

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Marketable Securities

Marketable Securities

Classification and Accretion of Redeemable Convertible Preferred Stock

Classification and Accretion of Redeemable Convertible Preferred Stock

Segment Information

Segment Information

Collaboration Agreements

Collaboration Agreements

Revenue Recognition of Collaboration Agreements

Revenue Recognition of Collaboration Agreements

Research and Development Costs

Research and Development Costs

Research Contract Costs and Accruals

Research Contract Costs and Accruals

Patent Costs

Patent Costs

Stock-Based Compensation

Stock-Based Compensation

Comprehensive Loss

Comprehensive Loss

Net Income (Loss) per Share

Net Income (Loss) per Share

Income Taxes

Income Taxes

Recently Adopted and Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements

The following table summarizes the cumulative effect to the Company’s consolidated balance sheet upon the adoption of the new revenue standard on January 1, 2018 (in thousands):


	Balance at December 31, 2017			Adjustments			Balance at January 1, 2018
Deferred revenue, current and net of current portion	$	15,605		$	6,188		$	21,793
Accumulated deficit	$	(51,339	)	$	(6,188	)	$	(57,527	)

Consolidated Balance Sheet


	At December 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Deferred revenue, current portion	$	17,949		$	8,110		$	9,839
Deferred revenue, net of current portion	$	—		$	2,128		$	(2,128	)
Accumulated deficit	$	(92,059	)	$	(84,348	)	$	(7,711	)

Consolidated Statements of Operations and Comprehensive Loss


	Year Ended December 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Collaboration revenue	$	9,734		$	11,257		$	(1,523	)
Net loss	$	(34,532	)	$	(33,009	)	$	(1,523	)
Net loss attributable to common stockholders	$	(34,548	)	$	(33,025	)	$	(1,523	)
Net loss per share attributable to common stockholders, basic and diluted	$	(1.39	)	$	(1.33	)	$	(0.06	)
Comprehensive loss	$	(34,528	)	$	(33,005	)	$	(1,523	)

Consolidated Statement of Cash Flows


	Year Ended December 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Net loss	$	(34,532	)	$	(33,009	)	$	(1,523	)
Change in deferred revenue	$	(3,844	)	$	(5,367	)	$	1,523

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Estimated Useful Life


		Estimated Useful Life
Laboratory equipment		5 years
Computer equipment and software		3 years
Furniture and fixtures		5 years
Leasehold improvements		Shorter of life of lease or 10 years

ASU 2014-09 [Member]

Summary of Cumulative Effect to Consolidated Balance Sheet upon Adoption of New Revenue Standard

The following table summarizes the cumulative effect to the Company’s consolidated balance sheet upon the adoption of the new revenue standard on January 1, 2018 (in thousands):


	Balance at December 31, 2017			Adjustments			Balance at January 1, 2018
Deferred revenue, current and net of current portion	$	15,605		$	6,188		$	21,793
Accumulated deficit	$	(51,339	)	$	(6,188	)	$	(57,527	)

Summary of Impact of Adoption of New Revenue Standard Requirements on Company's Financial Statements

Consolidated Balance Sheet


	At December 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Deferred revenue, current portion	$	17,949		$	8,110		$	9,839
Deferred revenue, net of current portion	$	—		$	2,128		$	(2,128	)
Accumulated deficit	$	(92,059	)	$	(84,348	)	$	(7,711	)

Consolidated Statements of Operations and Comprehensive Loss


	Year Ended December 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Collaboration revenue	$	9,734		$	11,257		$	(1,523	)
Net loss	$	(34,532	)	$	(33,009	)	$	(1,523	)
Net loss attributable to common stockholders	$	(34,548	)	$	(33,025	)	$	(1,523	)
Net loss per share attributable to common stockholders, basic and diluted	$	(1.39	)	$	(1.33	)	$	(0.06	)
Comprehensive loss	$	(34,528	)	$	(33,005	)	$	(1,523	)

Consolidated Statement of Cash Flows


	Year Ended December 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Net loss	$	(34,532	)	$	(33,009	)	$	(1,523	)
Change in deferred revenue	$	(3,844	)	$	(5,367	)	$	1,523

Marketable Securities and Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Marketable Securities by Type

Marketable securities by security type consisted of the following (in thousands):


	December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury bills and notes (due within one year)	$	22,935	$	—	$	(12	)	$	22,923

	$	22,935	$	—	$	(12	)	$	22,923


	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury bills and notes (due within one year)	$	5,007	$	—	$	(10	)	$	4,997
U.S. government agency bonds (due within one year)		7,700		—		(6	)		7,694

	$	12,707	$	—	$	(16	)	$	12,691

Schedule of Cash Equivalents and Marketable Securities Measured at Fair Value on Recurring Basis

The following tables present the Company’s fair value hierarchy for its cash equivalents and marketable securities, which are measured at fair value on a recurring basis (in thousands):


	Fair Value Measurements at December 31, 2018 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	—	$	52,100	$	—	$	52,100
Marketable securities:
U.S. Treasury bills and notes		22,923		—		—		22,923

	$	22,923	$	52,100	$	—	$	75,023


	Fair Value Measurements at December 31, 2017 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	—	$	24,196	$	—	$	24,196
Marketable securities:
U.S. Treasury notes		4,997		—		—		4,997
U.S. government agency bonds		—		7,694		—		7,694

	$	4,997	$	31,890	$	—	$	36,887

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):


	December 31,
	2018			2017
Laboratory equipment	$	5,801		$	5,327
Computer equipment and software		203			218
Furniture and fixtures		317			317
Leasehold improvements		426			426

		6,747			6,288
Less: Accumulated depreciation and amortization		(3,496	)		(2,180	)

	$	3,251		$	4,108

Accrued Expenses and Other Current Liabilities (Tables)

12 Months Ended

Dec. 31, 2018

Payables and Accruals [Abstract]

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	December 31,
	2018		2017
Accrued employee compensation and benefits	$	1,599	$	1,315
Accrued external research and development expense		1,799		478
Accrued professional fees		400		980
Other		1,679		180

	$	5,477	$	2,953

Redeemable Convertible Preferred Stock (Tables)

12 Months Ended

Dec. 31, 2018

Temporary Equity Disclosure [Abstract]

Summary of Preferred Stock

As of December 31, 2017, the Preferred Stock consisted of the following (in thousands, except share amounts):


	Preferred Stock Authorized		Preferred Stock Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series A preferred stock		12,297,276		12,297,276	$	12,267	$	12,297		7,832,001
Series B preferred stock		8,494,131		8,474,574		64,884		65,000		5,397,361

		20,791,407		20,771,850	$	77,151	$	77,297		13,229,362

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Fair Value Option Granted


	Year Ended December 31,
	2018			2017
Risk-free interest rate		2.64	%		1.81	%
Expected volatility		67.40	%		66.77	%
Expected dividend yield		—			—
Expected life (in years)		6.07			6.06

Schedule of Common Stock Option Activity

The following table summarizes the Company’s option activity since December 31, 2017:


	Number of Shares			Weighted Average Exercise Price		Weighted Average Contractual Term		Aggregate Intrinsic Value
						(in years)		(in thousands)
Outstanding as of December 31, 2017		3,156,939		$	4.01
Granted		929,231			12.00
Exercised		(225,252	)		2.71
Forfeited		(198,936	)		7.54

Outstanding as of December 31, 2018		3,661,982		$	5.92		6.2	$	5,019

Vested and expected to vest as of December 31, 2018		3,661,982		$	5.92		6.2	$	5,019

Options exercisable as of December 31, 2018		2,086,919		$	2.89		5.0	$	4,873

Schedule of Stock Based Compensation Expense

The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):


	Year Ended December 31,
	2018		2017
Research and development expenses	$	2,184	$	1,159
General and administrative expenses		904		182

Total	$	3,088	$	1,341

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Effective Income Tax Rate Reconciliation

A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows:


	Year Ended December 31,
	2018			2017
Federal statutory income tax rate		(21.0	)%		(34.0	)%
State taxes, net of federal benefit		(6.2	)		(5.1	)
Federal and state research and development tax credits		(4.2	)		(7.6	)
Federal research and development tax credit add-back		—			2.3
Nondeductible items		0.7			1.3
2017 Tax Acts and Jobs Cut		—			21.9
Increase in deferred tax asset valuation allowance		30.7			21.2

Effective income tax rate		0.0	%		0.0	%

Schedule of Deferred Tax Assets

Net deferred tax assets as of December 31, 2018 and 2017 consisted of the following (in thousands):


	December 31,
	2018			2017
Deferred tax assets:
Net operating loss carryforwards		19,189		$	8,211
Research and development and investment tax credits		4,891			3,467
Deferred revenue		3,294			3,693
Accrued expenses		437			491
Capitalized start-up costs		93			102
Capitalized research and development expense		60			73
Other		933			574

Total deferred tax assets		28,897			16,611
Valuation allowance		(28,897	)		(16,611	)

Net deferred tax assets	$	—		$	—

Changes in Valuation Allowance for Deferred Tax Assets


	Year Ended December 31,
	2018		2017
Valuation allowance as of beginning of year	$	16,611	$	11,208
Decreases recorded as benefit to income tax provision		—		(5,575	)
Increases recorded to income tax provision		12,286		10,978

Valuation allowance as of end of year	$	28,897	$	16,611

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Summary of Future Minimum Lease Payments under Operating Lease

Future minimum lease payments under the operating lease as of December 31, 2018 are as follows (in thousands):


Year Ending December 31,
2019		1,878
2020		1,933
2021		1,989
2022		2,046
2023		689

	$	8,535

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Schedule of Earnings Per Share Basic and Diluted

Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts):


	Year Ended December 31,
	2018			2017
Numerator:
Net loss	$	(34,532	)	$	(25,492	)
Accretion of redeemable convertible preferred stock to redemption value		(16	)		(65	)

Net loss attributable to common stockholders	$	(34,548	)	$	(25,557	)

Denominator:
Weighted average common shares outstanding, basic and diluted		24,895,670			10,191,807

Net loss per share attributable to common stockholders, basic and diluted	$	(1.39	)	$	(2.51	)

Summary of Potential Dilutive Securities


	December 31,
	2018		2017
Redeemable convertible preferred shares (as converted to common stock)		—		13,229,362
Stock options to purchase common stock		3,661,982		3,156,939

		3,661,982		16,386,301

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Summary of Quarterly Financial Data


	Mar 31, 2017			Jun 30, 2017			Sep 30, 2017			Dec 31, 2017			Mar 31, 2018			Jun 30, 2018			Sep 30, 2018			Dec 31, 2018
Statements of Operations Data:
Revenue	$	1,827		$	2,079		$	2,331		$	2,123		$	2,220		$	1,666		$	2,043		$	3,805
Loss from operations		(6,069	)		(6,033	)		(7,170	)		(6,880	)		(6,986	)		(9,439	)		(10,576	)		(9,004	)
Net loss		(5,939	)		(5,863	)		(7,000	)		(6,690	)		(6,735	)		(9,023	)		(10,168	)		(8,606	)
Net loss attributable to common stockholders		(5,955	)		(5,880	)		(7,016	)		(6,706	)		(6,751	)		(9,023	)		(10,168	)		(8,606	)
Basic and diluted net loss attributable to common stockholders per share:	$	(0.58	)	$	(0.58	)	$	(0.69	)	$	(0.66	)	$	(0.66	)	$	(0.31	)	$	(0.34	)	$	(0.29	)

Nature of the Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands				3 Months Ended								12 Months Ended
	Apr. 25, 2018 USD ($) $ / shares shares	Apr. 03, 2018 USD ($) $ / shares shares	Mar. 16, 2018	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Reverse stock split description			One-for-1.5701314513884 reverse stock split
Reverse stock split ratio			0.636889
Net proceeds from initial public offering												$ 66,793
Net loss				$ (8,606)	$ (10,168)	$ (9,023)	$ (6,735)	$ (6,690)	$ (7,000)	$ (5,863)	$ (5,939)	(34,532)	$ (25,492)
Accumulated deficit				$ (92,059)				$ (51,339)				$ (92,059)	$ (51,339)
Funding term of borrowings for operating expense and capital expenditure requirement												12 months
Common Stock [Member] \| IPO [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares of common stock issued and sold \| shares	215,000	5,770,000										5,985,000
Public offering price, per share \| $ / shares	$ 12.00	$ 12.00
Net proceeds from initial public offering	$ 2,400	$ 61,500
Common Stock [Member] \| Private Placement [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares of common stock issued and sold \| shares		416,666										416,666
Sale of stock, consideration received on transaction		$ 5,000

Summary of Significant Accounting Policies - Schedule of Estimated Useful Life (Detail)	12 Months Ended
	Dec. 31, 2018
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Computer equipment and software	5 years
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Computer equipment and software	3 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Computer equipment and software	5 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Leasehold improvements	Shorter of life of lease or 10 years

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Jan. 01, 2019	Dec. 31, 2017	Dec. 31, 2016
Significant Accounting Policies [Line Items]
Impairment losses on long-lived assets	$ 0
Revenue recognized	9,700,000
Deferred revenue	17,900,000
Capitalized contract costs	0
Cash, cash equivalents and restricted cash	$ 56,926,000		$ 29,600,000	$ 42,576,000
ASU 2016-18 [Member]
Significant Accounting Policies [Line Items]
Cash, cash equivalents and restricted cash			$ 1,300,000
Accounting Standards Update 2016-02 [Member] \| Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Lease liability expected to be recognized		$ 7,500,000
Right of use assets		$ 6,600,000
Maximum [Member]
Significant Accounting Policies [Line Items]
Expenses incremental costs expiration period	1 year
Period of Transfer of promised goods or services to customer	1 year

Summary of Significant Accounting Policies - Summary of Cumulative Effect to Consolidated Balance Sheet upon Adoption of New Revenue Standard (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current and net of current portion			$ 15,605
Accumulated deficit	$ (92,059)		$ (51,339)
ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current and net of current portion		$ 21,793
Accumulated deficit		(57,527)
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] \| ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current and net of current portion		6,188
Accumulated deficit	$ (7,711)	$ (6,188)

Summary of Significant Accounting Policies - Summary of Impact of Adoption of New Revenue Standard Requirements on Company's Financial Statements (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current portion	$ 17,949								$ 17,949
Accumulated deficit	(92,059)				$ (51,339)				(92,059)	$ (51,339)
Collaboration revenue	3,805	$ 2,043	$ 1,666	$ 2,220	2,123	$ 2,331	$ 2,079	$ 1,827	9,734	8,360
Net loss	(8,606)	(10,168)	(9,023)	(6,735)	(6,690)	(7,000)	(5,863)	(5,939)	(34,532)	(25,492)
Net loss attributable to common stockholders	$ (8,606)	$ (10,168)	$ (9,023)	$ (6,751)	$ (6,706)	$ (7,016)	$ (5,880)	$ (5,955)	$ (34,548)	$ (25,557)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.29)	$ (0.34)	$ (0.31)	$ (0.66)	$ (0.66)	$ (0.69)	$ (0.58)	$ (0.58)	$ (1.39)	$ (2.51)
Comprehensive loss									$ (34,528)	$ (25,484)
Net loss									(34,532)	(25,492)
Change in deferred revenue									(3,844)	$ (3,875)
ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit											$ (57,527)
Calculated under Revenue Guidance in Effect before Topic 606 [Member] \| ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current portion	$ 8,110								8,110
Deferred revenue, net of current portion	2,128								2,128
Accumulated deficit	(84,348)								(84,348)
Collaboration revenue									11,257
Net loss									(33,009)
Net loss attributable to common stockholders									$ (33,025)
Net loss per share attributable to common stockholders, basic and diluted									$ (1.33)
Comprehensive loss									$ (33,005)
Net loss									(33,009)
Change in deferred revenue									(5,367)
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] \| ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current portion	9,839								9,839
Deferred revenue, net of current portion	(2,128)								(2,128)
Accumulated deficit	$ (7,711)								(7,711)		$ (6,188)
Collaboration revenue									(1,523)
Net loss									(1,523)
Net loss attributable to common stockholders									$ (1,523)
Net loss per share attributable to common stockholders, basic and diluted									$ (0.06)
Comprehensive loss									$ (1,523)
Net loss									(1,523)
Change in deferred revenue									$ 1,523

Marketable Securities and Fair Value Measurements - Schedule of Marketable Securities by Type (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 22,935	$ 12,707
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(12)	(16)
Fair Value	22,923	12,691
U.S. Treasury Bills and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,935	5,007
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(12)	(10)
Fair Value	$ 22,923	4,997
U.S. Government Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		7,700
Gross Unrealized Gains		0
Gross Unrealized Losses		(6)
Fair Value		$ 7,694

Marketable Securities and Fair Value Measurements - Schedule of Cash Equivalents and Marketable Securities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	$ 75,023	$ 36,887
Money Market Funds [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	52,100	24,196
U.S. Treasury Bills and Notes [Member] \| Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	22,923	4,997
U.S. Government Agency Bonds [Member] \| Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis		7,694
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	22,923	4,997
Level 1 [Member] \| U.S. Treasury Bills and Notes [Member] \| Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	22,923	4,997
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	52,100	31,890
Level 2 [Member] \| Money Market Funds [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	$ 52,100	24,196
Level 2 [Member] \| U.S. Government Agency Bonds [Member] \| Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis		$ 7,694

Marketable Securities and Fair Value Measurements - Additional Information (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Fair value asset, transfers between Level 1, Level 2 and Level 3, amount	$ 0	$ 0

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 6,747	$ 6,288
Less: Accumulated depreciation and amortization	(3,496)	(2,180)
Property and equipment, net	3,251	4,108
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	5,801	5,327
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	203	218
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	317	317
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 426	$ 426

Property Plant Equipment Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipments [Abstract]
Depreciation and amortization expense	$ 1,321	$ 1,171

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued employee compensation and benefits	$ 1,599	$ 1,315
Accrued external research and development expense	1,799	478
Accrued professional fees	400	980
Other	1,679	180
Total	$ 5,477	$ 2,953

Collaboration Agreement - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue related to research and clinical development activities	$ 3,805	$ 2,043	$ 1,666	$ 2,220	$ 2,123	$ 2,331	$ 2,079	$ 1,827	$ 9,734	$ 8,360
Deferred revenue	17,900								17,900
Increase in deferred revenue					15,605					15,605
Collaborative Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future collaboration and milestone payments									100,000
Regulatory Milestone Payments [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future collaboration and milestone payments									100,000
Commercial Milestone Payments [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future collaboration and milestone payments									200,000
Preclinical Research And Clinical Development [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative Arrangement Upfront Payment recognized									25,000
Seattle Genetics [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue related to research and clinical development activities									9,700	8,400
Deferred revenue	17,900				$ 15,600				17,900	$ 15,600
Remaining performance obligation related to two product candidates	$ 54,100								54,100
Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future collaboration and milestone payments									400,000
Maximum [Member] \| Regulatory Milestone Payments [Member] \| First Regulatory Approval [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future collaboration and milestone payments									35,000
Maximum [Member] \| Commercial Milestone Payments [Member] \| Annual Net Sales Achievement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future collaboration and milestone payments									60,000
Maximum [Member] \| Clinical Development Events [Member] \| Collaborative Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future collaboration and milestone payments									$ 30,000
ASU 2014-09 [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in deferred revenue											$ 21,793
ASU 2014-09 [Member] \| Cumulative Effect of Changes in Accounting Principle [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in deferred revenue											$ 6,200

Loan and Security Agreement - Additional Information (Detail) - Loan and Security Agreement [Member] - Pacific Western Bank [Member]	12 Months Ended
	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Warrant to be issued to purchase number of shares capital stock equal to the amount of each term loan borrowing percentage	1.00%
Term Loan [Member]
Debt Instrument [Line Items]
Maximum term loan borrowings	$ 15,000,000
Interest rate during period	3.75%
Term loan payment terms	In January 2017, the Company entered into a loan and security agreement (the “Loan Agreement”) with Pacific Western Bank (“PWB”), which provides for term loan borrowings of up to $15.0 million through January 19, 2019. Borrowings under the Loan Agreement bear interest at a variable annual rate equal to the greater of (i) the prime rate plus 0.25% or (ii) 3.75%, and are payable over an interest-only period until January 19, 2019, followed by a 24-month period of equal monthly payments of principal and interest. All amounts outstanding as of the maturity date of January 19, 2021 become immediately due and payable.
Outstanding amount maturity date	Jan. 19, 2021
Term loans borrowed	$ 0
Term Loan [Member] \| Prime Rate [Member]
Debt Instrument [Line Items]
Annual variable interest rate	0.25%

Redeemable Convertible Preferred Stock - Additional Information (Detail)			9 Months Ended
	Apr. 03, 2018	Mar. 16, 2018	Sep. 30, 2018 USD ($) $ / shares
Temporary Equity [Line Items]
Reverse stock split description		One-for-1.5701314513884 reverse stock split
Reverse stock split ratio		0.636889
Voting, description			The holders of Preferred Stock were entitled to vote, together with the holders of common stock, on matters submitted to stockholders for a vote. The holders of Preferred Stock were entitled to the number of votes equal to the number of common shares into which each such share of Preferred Stock could convert. In addition, the holders of Series A preferred stock, voting exclusively and as a separate class, were entitled to elect two directors of the Company. The holders of Series B preferred stock, voting exclusively and as a separate class, were entitled to elect one director of the Company.
Conversion, description			Each share of Preferred Stock was convertible at the option of the holder at any time after the date of issuance. Each share of Preferred Stock would have automatically converted into shares of common stock at the applicable conversion ratio then in effect upon the closing of a firm commitment public offering with at least $50.0 million of gross proceeds to the Company. Shares of Series A preferred stock would have automatically converted into shares of common stock at the applicable conversion ratio then in effect upon written consent of the holders of at least 65% of the then-outstanding shares of Series A preferred stock. Shares of Series B preferred stock would have automatically converted into shares of common stock at the applicable conversion ratio then in effect upon written consent of the holders of at least a majority of the then-outstanding shares of Series B preferred stock.
Minimum gross proceeds from Initial Public Offering for automatic conversion \| $			$ 50,000,000
Percentage eligibility of conversion, upon outstanding shares held			65.00%
Preferred dividend description			The Company could not declare, pay or set aside any dividends on shares of any other series of capital stock of the Company, other than dividends on common stock payable in common stock, unless the holders of the Series A and Series B preferred stock first received, or simultaneously received, a dividend on each outstanding share of Series A and Series B preferred stock in an amount at least equal to the greater of (i) $0.08 per share in the case of Series A preferred stock and $0.61 per share in the case of Series B preferred stock, each subject to appropriate adjustment in the event of any stock split, stock dividend, combination or other similar recapitalization with respect to such shares, and (ii) (A) in the case of a dividend on common stock or any class or series of stock that was convertible into common stock, that dividend per share of Preferred Stock as would equal the product of (1) the dividend payable on each share of such class or series determined, if applicable, as if all shares of such class or series had been converted into common stock and (2) the number of shares of common stock issuable upon conversion of each share of Preferred Stock, or (B) in the case of a dividend on any class or series that was not convertible into common stock, at a rate per share of Preferred Stock determined by (1) dividing the amount of the dividend payable on each share of such class or series of capital stock by the Original Issue Price of such class or series of capital stock (subject to appropriate adjustment in the event of any stock dividend, stock split, combination of or other similar recapitalization affecting such shares) and (2) multiplying such fraction by an amount equal to the Original Issue Price of each series of Preferred Stock. If the Company declared, paid or set aside, on the same date, a dividend on shares of more than one class or series of capital stock of the Company, the dividend payable to the holders of the Preferred Stock would have been calculated based upon the dividend on the class or series of capital stock that would have resulted in the highest Preferred Stock dividend. Stockholders were not entitled to any accruing dividends.
Dividend payable \| $			$ 0
Redemption, description			At any time on or after June 10, 2020, shares of each of the Series A and Series B preferred stock were subject to mandatory redemption by the Company in three equal annual installments beginning 60 days after receipt of a notice of redemption from the holders of at least two-thirds of the combined voting power of the holders of outstanding shares of Series A and Series B preferred stock, voting together as a single class, in an amount equal to the Original Issue Price per share of each series of Preferred Stock plus any dividends declared but unpaid thereon.
Preferred Stock [Member]
Temporary Equity [Line Items]
Reverse stock split description	1:1.5701314513884 basis upon the closing of the IPO
Reverse stock split ratio	0.63688935
Series A preferred stock [Member]
Temporary Equity [Line Items]
Original Issue Price, per share			$ 1.00
Conversion Price at issuance			1.570131
Series A preferred stock [Member] \| Minimum [Member]
Temporary Equity [Line Items]
Dividend Rate			0.08
Series B preferred stock [Member]
Temporary Equity [Line Items]
Original Issue Price, per share			7.67
Conversion Price at issuance			12.042908
Series B preferred stock [Member] \| Minimum [Member]
Temporary Equity [Line Items]
Dividend Rate			$ 0.61

Redeemable Convertible Preferred Stock - Summary of Preferred Stock (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Preferred Stock Authorized	0	20,791,407
Preferred Stock Issued	0	20,771,850
Preferred Stock Outstanding	0	20,771,850
Carrying Value		$ 77,151
Liquidation Preference		$ 77,297
Common Stock Issuable Upon Conversion		13,229,362
Series A preferred stock [Member]
Temporary Equity [Line Items]
Preferred Stock Authorized		12,297,276
Preferred Stock Issued		12,297,276
Preferred Stock Outstanding		12,297,276
Carrying Value		$ 12,267
Liquidation Preference		$ 12,297
Common Stock Issuable Upon Conversion		7,832,001
Series B preferred stock [Member]
Temporary Equity [Line Items]
Preferred Stock Authorized		8,494,131
Preferred Stock Issued		8,474,574
Preferred Stock Outstanding		8,474,574
Carrying Value		$ 64,884
Liquidation Preference		$ 65,000
Common Stock Issuable Upon Conversion		5,397,361

Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands				12 Months Ended
	Apr. 25, 2018 USD ($) $ / shares shares	Apr. 03, 2018 USD ($) $ / shares shares	Mar. 16, 2018	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 shares
Class of Stock [Line Items]
Common stock voting right				Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company's stockholders.
Reverse stock split description			One-for-1.5701314513884 reverse stock split
Reverse stock split ratio			0.636889
Net proceeds from initial public offering \| $				$ 66,793
Common stock, Shares authorized		150,000,000		150,000,000	60,040,000
Preferred stock, shares authorized		10,000,000		10,000,000	0
Common Stock [Member] \| IPO [Member]
Class of Stock [Line Items]
Shares of common stock issued and sold	215,000	5,770,000		5,985,000
Public offering price, per share \| $ / shares	$ 12.00	$ 12.00
Net proceeds from initial public offering \| $	$ 2,400	$ 61,500
Common Stock [Member] \| Private Placement [Member]
Class of Stock [Line Items]
Shares of common stock issued and sold		416,666		416,666
Sale of stock, consideration received on transaction \| $		$ 5,000

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended	12 Months Ended
	Jan. 01, 2019	Mar. 16, 2018	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of options exercised				$ 1.6
Weighted average grant date fair value				$ 6.50	$ 5.39
Total unrecognized stock-based compensation expense				$ 8.6
Period for recognition of unrecognized expense				2 years 7 months 6 days
Non Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding unvested service based stock options				18,566
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of options exercised					$ 0.1
2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of stock option				4 years
Expiration period of stock option				10 years
Number of increases in authorized shares				4,144,876
2018 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of increases in authorized shares		2,800,721
Percentage applied to the outstanding shares as annual increase in the number of shares authorized for issuance		4.00%
Shares available for future issuance				2,793,738
2018 Stock Option and Incentive Plan [Member] \| Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increased in authorized shares reserved for issuance	1,202,319
Two Thousand Eighteen Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance		314,000
Two Thousand Eighteen Employee Stock Purchase Plan [Member] \| Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increased in authorized shares reserved for issuance	300,580
Scenario, Forecast [Member] \| Two Thousand Eighteen Employee Stock Purchase Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of increases in authorized shares	500,000		500,000
Percentage of shares of common stock available for issuance			1.00%

Stock-Based Compensation - Schedule of Fair Value Option Granted (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Stock Options/Shares Outstanding, Weighted-Average Exercise Price, and Additional Disclosures [Abstract]
Risk-free interest rate	2.64%	1.81%
Expected volatility	67.40%	66.77%
Expected dividend yield	0.00%	0.00%
Expected life (in years)	6 years 25 days	6 years 21 days

Stock-Based Compensation - Schedule of Common Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of Options Outstanding, Beginning balance \| shares	3,156,939
Number of Options, Granted \| shares	929,231
Number of Options, Exercised \| shares	(225,252)
Number of Options, Forfeited \| shares	(198,936)
Number of Options Outstanding, Ending balance \| shares	3,661,982
Number of Options, Vested and expected to vest \| shares	3,661,982
Number of Options, Exercisable \| shares	2,086,919
Weighted-Average Exercise Price, Outstanding, Beginning balance \| $ / shares	$ 4.01
Weighted-Average Exercise Price, Granted \| $ / shares	12.00
Weighted-Average Exercise Price, Exercised \| $ / shares	2.71
Weighted-Average Exercise Price, Forfeited \| $ / shares	7.54
Weighted-Average Exercise Price, Outstanding, Ending balance \| $ / shares	5.92
Weighted-Average Exercise Price, Vested and expected to vest \| $ / shares	5.92
Weighted-Average Exercise Price, Exercisable \| $ / shares	$ 2.89
Weighted-Average Contractual Term, Outstanding, Ending balance	6 years 2 months 12 days
Weighted-Average Contractual Term, Vested and expected to vest	6 years 2 months 12 days
Weighted-Average Contractual Term, Exercisable	5 years
Aggregate intrinsic value Outstanding \| $	$ 5,019
Aggregate intrinsic value Vested and expected to vest \| $	5,019
Aggregate intrinsic value Exercisable \| $	$ 4,873

Stock-Based Compensation - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share Based Compensation Expense	$ 3,088	$ 1,341
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share Based Compensation Expense	2,184	1,159
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share Based Compensation Expense	$ 904	$ 182

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Federal corporate income tax rate	21.00%	34.00%
Percentage of net operating loss carry forward deductible from current year taxable income	80.00%
Income tax expense or benefit recognized	$ 0
Current income tax benefits	0	$ 0
Deferred income tax benefits	$ 0	0
Operating losses carried forward, expiration date	2035
Operating loss carryforwards, federal	$ 39,600,000
Investment tax credits	$ 200,000
Tax credit carryover period	3 years
Unrecognized tax benefits	$ 0	$ 0
U.S. federal [Member]
Income Tax Disclosure [Line Items]
Net operating losses carryforwards	69,800,000
Research and development tax credits carryforwards	$ 4,000,000
Tax credit carryforwards, expiration year	2034
State [Member]
Income Tax Disclosure [Line Items]
Net operating losses carryforwards	$ 71,700,000
Research and development tax credits carryforwards	$ 900,000
Tax credit carryforwards, expiration year	2030

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax rate	(21.00%)	(34.00%)
State taxes, net of federal benefit	(6.20%)	(5.10%)
Federal and state research and development tax credits	(4.20%)	(7.60%)
Federal research and development tax credit add-back		2.30%
Nondeductible items	0.70%	1.30%
2017 Tax Acts and Jobs Cut		21.90%
Increase in deferred tax asset valuation allowance	30.70%	21.20%
Effective income tax rate	(0.00%)	(0.00%)

Income Taxes - Schedule of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 19,189	$ 8,211
Research and development and investment tax credits	4,891	3,467
Deferred revenue	3,294	3,693
Accrued expenses	437	491
Capitalized start-upcosts	93	102
Capitalized research and development expense	60	73
Other	933	574
Total deferred tax assets	28,897	16,611
Valuation allowance	(28,897)	(16,611)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Changes in Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Valuation allowance as of beginning of year	$ 16,611	$ 11,208
Decreases recorded as benefit to income tax provision		(5,575)
Increases recorded to income tax provision	12,286	10,978
Valuation allowance as of end of year	$ 28,897	$ 16,611

Commitments and Contingencies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2018 SGD ($)	Jan. 31, 2017 ft²
Commitments And Contingencies Disclosure [Line Items]
Lease expiration date	Apr. 30, 2023
Rent expense	$ 1,800,000	$ 1,800,000
Sublease term				12 months
Area of subleased property \| ft²		5,000
Sublease rent recognized	300,000	$ 300,000
Non-cancelable minimum purchase commitments	1,500,000
License Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
License maintenance fees on eighth anniversary and each year thereafter	$ 100,000
License agreement expiration terms	The license agreement will expire on a country-by-country basis until the last to expire of the patents and patent applications covering such licensed product or service. The Licensors may terminate the license agreement within 60 days after written notice in the event of a breach of contract. The Licensors may also terminate the agreement upon written notice in the event of the Company’s bankruptcy, liquidation, or insolvency. In addition, the Company has the right to terminate this agreement in its entirety at will upon 90 days’ advance written notice to the Licensors. However, if the Company has commenced the commercialization of licensed products, the Company can only terminate at will if it ceases all development and commercialization of licensed products.
Maximum [Member]
Commitments And Contingencies Disclosure [Line Items]
Area of subleased property \| ft²				2,500
Maximum [Member] \| License Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
License maintenance fees for each of first seven years	$ 100,000
Contingent contractual obligation	4,000,000		$ 5,500,000
Non-current restricted cash [Member] \| Letter of Credit [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease security	$ 1,300,000

Commitments and Contingencies - Summary of Future Minimum Lease Payments under Operating Lease (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	$ 1,878
2020	1,933
2021	1,989
2022	2,046
2023	689
Operating Leases, Future Minimum Payments Due, Total	$ 8,535

Net Loss Per Share - Schedule of Earnings Per Share Basic and Diluted (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net loss	$ (8,606)	$ (10,168)	$ (9,023)	$ (6,735)	$ (6,690)	$ (7,000)	$ (5,863)	$ (5,939)	$ (34,532)	$ (25,492)
Accretion of redeemable convertible preferred stock to redemption value									(16)	(65)
Net loss attributable to common stockholders	$ (8,606)	$ (10,168)	$ (9,023)	$ (6,751)	$ (6,706)	$ (7,016)	$ (5,880)	$ (5,955)	$ (34,548)	$ (25,557)
Weighted average common shares outstanding, basic and diluted									24,895,670	10,191,807
Net loss per share attributable to common stockholders, basic and diluted	$ (0.29)	$ (0.34)	$ (0.31)	$ (0.66)	$ (0.66)	$ (0.69)	$ (0.58)	$ (0.58)	$ (1.39)	$ (2.51)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss Per Share - Summary of Potential Dilutive Securities (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential dilutive securities excluded from computation of diluted net loss per common share	3,661,982	16,386,301
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential dilutive securities excluded from computation of diluted net loss per common share		13,229,362
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential dilutive securities excluded from computation of diluted net loss per common share	3,661,982	3,156,939

Retirement Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Defined contribution plan, matching amount	$ 0	$ 0

Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 3,805	$ 2,043	$ 1,666	$ 2,220	$ 2,123	$ 2,331	$ 2,079	$ 1,827	$ 9,734	$ 8,360
Loss from operations	(9,004)	(10,576)	(9,439)	(6,986)	(6,880)	(7,170)	(6,033)	(6,069)	(36,005)	(26,152)
Net loss	(8,606)	(10,168)	(9,023)	(6,735)	(6,690)	(7,000)	(5,863)	(5,939)	(34,532)	(25,492)
Net loss attributable to common stockholders	$ (8,606)	$ (10,168)	$ (9,023)	$ (6,751)	$ (6,706)	$ (7,016)	$ (5,880)	$ (5,955)	$ (34,548)	$ (25,557)
Basic and diluted net loss attributable to common stockholders per share:	$ (0.29)	$ (0.34)	$ (0.31)	$ (0.66)	$ (0.66)	$ (0.69)	$ (0.58)	$ (0.58)	$ (1.39)	$ (2.51)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Subsequent Event - Additional Information (Detail)	Jan. 01, 2019
Subsequent Event [Member] \| Term Loan [Member] \| Loan and Security Agreement [Member]
Subsequent Event [Line Items]
Term loan payment terms	In January 2019, the Company amended the Loan Agreement (see Note 7) to extend the available date for borrowings from January 19, 2019 to June 30, 2019 and extend the interest only period from January 19, 2019 to June 30, 2020, with the possibility of further extension to March 31, 2021 if certain equity financing considerations are met. Additionally, the loan repayment period will be over a 24-month period following the end of the interest-only period.