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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2018	Apr. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Trading Symbol	UMRX
Entity Registrant Name	UNUM THERAPEUTICS INC.
Entity Central Index Key	0001622229
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		29,839,086

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 25,704	$ 28,270
Marketable securities	6,696	12,691
Accounts receivable	1,601	830
Prepaid expenses and other current assets	1,176	513
Restricted cash	50	75
Total current assets	35,227	42,379
Property and equipment, net	3,902	4,108
Restricted cash	1,255	1,255
Deferred offering costs	2,612	1,373
Other assets	419
Total assets	43,415	49,115
Current liabilities:
Accounts payable	3,083	1,346
Accrued expenses and other current liabilities	2,358	2,953
Deferred revenue	17,519	6,891
Total current liabilities	22,960	11,190
Deferred rent	902	906
Deferred revenue, net of current portion	3,655	8,714
Total liabilities	27,517	20,810
Commitments and contingencies (Note 11)
Redeemable convertible preferred stock (Series A and B), $0.001 par value; 20,791,407 shares authorized at March 31, 2018 and December 31, 2017; 20,771,850 shares issued and outstanding at March 31, 2018 and December 31, 2017; liquidation preference of $77,297 at March 31, 2018 and December 31, 2017	77,167	77,151
Stockholders' deficit:
Common stock, $0.001 par value; 60,040,000 shares authorized at March 31, 2018 and December 31, 2017; 10,208,058 shares and 10,201,690 shares issued and outstanding at March 31, 2018 and December 31, 2017, respectively	10	10
Additional paid-in capital	2,990	2,499
Accumulated other comprehensive loss	(7)	(16)
Accumulated deficit	(64,262)	(51,339)
Total stockholders' deficit	(61,269)	(48,846)
Total liabilities, redeemable convertible preferred stock and stockholders' deficit	$ 43,415	$ 49,115

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 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	20,791,407	20,791,407
Redeemable convertible preferred stock (Series A and B), Shares issued	20,771,850	20,771,850
Redeemable convertible preferred stock (Series A and B), Shares outstanding	20,771,850	20,771,850
Redeemable convertible preferred stock (Series A and B), Liquidation preference	$ 77,297	$ 77,297
Common stock, Par value	$ 0.001	$ 0.001
Common stock, Shares authorized	60,040,000	60,040,000
Common stock, Shares issued	10,208,058	10,201,690
Common stock, Shares outstanding	10,208,058	10,201,690

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Collaboration revenue	$ 2,220	$ 1,827
Operating expenses:
Research and development	8,142	6,952
General and administrative	1,064	944
Total operating expenses	9,206	7,896
Loss from operations	(6,986)	(6,069)
Other income (expense):
Interest income	81	90
Other income, net	170	40
Total other income, net	251	130
Net loss	(6,735)	(5,939)
Accretion of redeemable convertible preferred stock to redemption value	(16)	(16)
Net loss attributable to common stockholders	$ (6,751)	$ (5,955)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.66)	$ (0.58)
Weighted average common shares outstanding, basic and diluted	10,204,591	10,190,228
Comprehensive loss:
Net loss	$ (6,735)	$ (5,939)
Other comprehensive income (loss):
Unrealized gains (losses) on marketable securities, net of tax of $0	9	(12)
Total other comprehensive income (loss)	9	(12)
Comprehensive loss	$ (6,726)	$ (5,951)

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Unrealized gains (losses) on marketable securities, net of tax	$ 0	$ 0

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (6,735)	$ (5,939)	$ 25,500
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	314	278
Stock-based compensation expense	479	300
Premiums paid on marketable securities		(14)
Net amortization of premiums on marketable securities	4	2
Non-cash interest expense	5	4
Changes in operating assets and liabilities:
Accounts receivable	(771)	41
Prepaid expenses and other current assets	(668)	(185)
Other assets	(419)
Accounts payable	1,341	(61)
Accrued expenses and other current liabilities	(908)	(761)
Deferred rent	(4)	8
Deferred revenue	(619)	(941)
Net cash used in operating activities	(7,981)	(7,268)
Cash flows from investing activities:
Purchases of property and equipment	(75)	(320)
Purchases of marketable securities		(4,500)
Maturities and sales of marketable securities	6,000	4,000
Net cash provided by (used in) investing activities	5,925	(820)
Cash flows from financing activities:
Proceeds from issuance of common stock upon stock option exercises	28
Payments of initial public offering costs	(563)
Net cash used in financing activities	(535)
Net decrease in cash, cash equivalents and restricted cash	(2,591)	(8,088)
Cash, cash equivalents and restricted cash at beginning of period	29,600	42,576	42,576
Cash, cash equivalents and restricted cash at end of period	27,009	34,488	$ 29,600
Supplemental disclosure of noncash investing and financing information:
Purchases of property and equipment included in accounts payable	108
Deferred offering costs included in accounts payable and accrued expenses	956
Accretion of redeemable convertible preferred stock to redemption value	$ 16	$ 16

Nature of the Business and Basis of Presentation	3 Months Ended
Nature of the Business and Basis of Presentation	Mar. 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 technology, called antibody-coupled T cell receptor (“ACTR”), is 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. 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 7). 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 $64.4 million after deducting underwriting discounts and commissions but before deducting 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”). The shares of common stock sold to Seattle Genetics in the concurrent private placement are not registered under the Securities Act of 1933, as amended, and are subject to a 180-day lock-up agreement with the underwriters of the IPO. Upon closing of the IPO, the Company’s outstanding redeemable convertible preferred stock automatically converted into shares of common stock (see Note 7). 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 $6.7 million for the three months ended March 31, 2018 and $25.5 million for the year ended December 31, 2017. As of March 31, 2018, the Company had an accumulated deficit of $64.3 million. The Company expects to continue to generate operating losses in the foreseeable future. As of May 14, 2018, the issuance date of the interim consolidated financial statements, the Company expects that its cash, cash equivalents and marketable securities, including net proceeds it received from the completion of its IPO and concurrent private placement in April 2018, will be sufficient to fund its operating expenses and capital expenditure requirements through at least 12 months from the issuance date of the interim consolidated financial statements, without considering available borrowings under the Company’s loan and security agreement. 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

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Unaudited Interim Financial Information

The consolidated balance sheet at December 31, 2017 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited consolidated financial statements as of March 31, 2018 and for the three months ended March 31, 2018 and 2017 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in the financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2017 included in the Company’s Registration Statement on Form S-1, File No. 333-223414 on file with the SEC. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair statement of the Company’s financial position as of March 31, 2018 and results of operations and cash flows for the three months ended March 31, 2018 and 2017 have been made. The Company’s results of operations for the three months ended March 31, 2018 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2018.

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.

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. Cash equivalents consisted of money market funds at March 31, 2018 and December 31, 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 assets and liabilities.

Marketable Securities

The Company’s marketable securities are classified as available-for-sale and are carried at fair value, with the unrealized gains and losses 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 has classified redeemable convertible preferred stock outside of stockholders’ equity (deficit) because the shares contain certain redemption features that are not solely within the control of the Company. The carrying values of the redeemable convertible preferred stock are accreted to their respective redemption values from the date of issuance through the earliest date of redemption.

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 5), 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. At March 31, 2018, the Company had current deferred revenue and deferred revenue, net of current portion of $17.5 million and $3.7 million, respectively, related to its collaboration. The Company recognized revenue of $2.2 million during three months ended March 31, 2018 from the deferred revenue balance at January 1, 2018. The Company recognizes deferred revenue by first allocating from the beginning deferred revenue balance to the extent 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.

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. At March 31, 2018, the Company has not capitalized any costs to obtain its contract.

Recently Adopted Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (“FASB”) issued 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 5) 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 $6.2 million 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 will recognize 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 will include 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 will account 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 will 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 Sheets


	At March 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Deferred revenue, current portion	$	17,519		$	7,222		$	10,297
Deferred revenue, net of current portion	$	3,655		$	7,323		$	(3,668	)
Accumulated deficit	$	(64,262	)	$	(57,633	)	$	(6,629	)

Consolidated Statements of Operations and Comprehensive Loss


	Three Months Ended March 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Collaboration revenue	$	2,220		$	2,661		$	(441	)
Net loss	$	(6,735	)	$	(6,294	)	$	(441	)
Net loss attributable to common stockholders	$	(6,751	)	$	(6,310	)	$	(441	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.66	)	$	(0.62	)	$	(0.04	)
Comprehensive loss	$	(6,726	)	$	(6,285	)	$	(441	)

Consolidated Statements of Cash Flows


	Three Months Ended March 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Net loss	$	(6,735	)	$	(6,294	)	$	(441	)
Change in deferred revenue	$	(619	)	$	(1,060	)	$	441

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 unaudited consolidated statements of cash flows. The inclusion of restricted cash increased the beginning and ending balances of the unaudited consolidated statements of cash flows by $1.3 million for the three months ended March 31, 2018 and 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. The Company is currently evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements.

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 is currently evaluating the impact that the adoption of ASU 2017-11 will have on its consolidated financial statements.

Fair Value of Financial Assets and Liabilities

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

3. Fair Value of Financial Assets and Liabilities

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


	March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury notes (due within one year)	$	4,003	$	—	$	(5	)	$	3,998
U.S. government agency bonds (due within one year)		2,700		—		(2	)		2,698

	$	6,703	$	—	$	(7	)	$	6,696


	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury 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 information about the Company’s assets that are measured at fair value on a recurring basis (in thousands):


	Fair Value Measurements at March 31, 2018 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	—	$	22,293	$	—	$	22,293
Marketable securities:
U.S. Treasury notes		3,998		—		—		3,998
U.S. government agency bonds		—		2,698		—		2,698

	$	3,998	$	24,991	$	—	$	28,989


	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

During the three months ended March 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

Accrued Expenses and Other Current Liabilities

3 Months Ended

Mar. 31, 2018

Payables and Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

4. Accrued Expenses and Other Current Liabilities

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


	March 31, 2018		December 31, 2017
Accrued employee compensation and benefits	$	357	$	1,315
Accrued professional fees		964		980
Accrued external research and development expense		806		478
Accrued other		231		180

	$	2,358	$	2,953

Collaboration Agreement	3 Months Ended
Collaboration Agreement	Mar. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration Agreement	5. 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. Under the collaboration agreement, the Company recognized revenue of $2.2 million and $1.8 million for the three months ended March 31, 2018 and 2017, respectively, related to research and clinical development activities performed. As of March 31, 2018 and December 31, 2017, deferred revenue of $21.2 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 March 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 $76.0 million, which is expected to be recognized as revenue through December 31, 2022.

Loan and Security Agreement	3 Months Ended
Loan and Security Agreement	Mar. 31, 2018
Debt Disclosure [Abstract]
Loan and Security Agreement	6. Loan and Security Agreement 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. In connection with the Loan Agreement, the Company agreed to enter into warrant agreements with PWB 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 Agreement, divided by the applicable exercise price. No amounts have been borrowed as term loans under the Loan Agreement as of March 31, 2018. Borrowings under the Loan Agreement are collateralized by substantially all of the Company’s assets, except for its intellectual property. Under the Loan 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 Agreement. Events of default under the Loan 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

3 Months Ended

Mar. 31, 2018

Equity [Abstract]

Redeemable Convertible Preferred Stock

7. Redeemable Convertible Preferred Stock

The Company 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.

As of each balance sheet date, the Preferred Stock consisted of the following (in thousands, except share amounts):


	As of March 31, 2018
	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	$	12,271	$	12,297		7,832,001
Series B preferred stock		8,494,131		8,474,574		64,896		65,000		5,397,361

		20,791,407		20,771,850	$	77,167	$	77,297		13,229,362


	As of December 31, 2017
	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 are entitled to vote, together with the holders of common stock, on matters submitted to stockholders for a vote. The holders of Preferred Stock are 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, are entitled to elect two directors of the Company. The holders of Series B preferred stock, voting exclusively and as a separate class, are entitled to elect one director of the Company.

Conversion

Each share of Preferred Stock is convertible at the option of the holder at any time after the date of issuance. Each share of Preferred Stock will be 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 will be 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 will be 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 is determined by dividing the Original Issue Price of each series by the Conversion Price of each series. The Original Issue Price is $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 are entitled to receive noncumulative dividends if and when declared by the Company’s board of directors. The Company may 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 receive, or simultaneously receive, 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 is 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 is 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 declares, pays or sets 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 shall be calculated based upon the dividend on the class or series of capital stock that would result in the highest Preferred Stock dividend. Stockholders are not entitled to any accruing dividends. Through March 31, 2018 and December 31, 2017, no dividends were 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 will receive, 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 receive 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 are not sufficient to permit payment to the holders of Preferred Stock in the full amount to which they are entitled, the assets available for distribution will be 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 shall be 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, elect otherwise, a Liquidating Event shall include 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 are 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.

Common Stock	3 Months Ended
Common Stock	Mar. 31, 2018
Equity [Abstract]
Common Stock	8. Common Stock 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.

Stock-Based Compensation

3 Months Ended

Mar. 31, 2018

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

Stock-Based Compensation

9. Stock-Based Compensation

2015 Stock Incentive Plan

The Company’s 2015 Stock Incentive Plan (the “2015 Plan”) provides 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 is 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 may 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 are 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 may be issued under the 2015 Plan was 4,144,876 shares as of March 31, 2018. Upon effectiveness of the Company’s 2018 Stock Option and Incentive Plan, the (“2018 Plan”) in March 2018, the remaining 1,041,700 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 awards 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. The Company’s board of directors values 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 1,041,700 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 March 31, 2018, 3,065,350 shares remained available for future issuance under the 2018 Plan.

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.

Stock Option Issuances

During the three months ended March 31, 2018, the Company granted options to employees, consultants and directors for the purchase of 777,071 shares of common stock with exercise prices of $12.00 per share and a weighted average grant-date fair value of $6.28 per share.

Stock-Based Compensation

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


	Three Months Ended March 31,
	2018		2017
Research and development expenses	$	401	$	260
General and administrative expenses		78		40

Total	$	479	$	300

As of March 31, 2018, total unrecognized compensation cost related to the unvested stock-based awards was $9.8 million, which is expected to be recognized over a weighted average period of 3.0 years.

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	10. 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. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”) which allows the Company to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. Since the Act was passed late in the fourth quarter of 2017, and ongoing guidance and accounting interpretation are expected over the next 12 months, the Company considers the accounting for the effect of the TCJA to be provisional in accordance with SAB 118 at March 31, 2018. *Income Taxes* During the three months ended March 31, 2018 and 2017, the Company recorded no income tax benefits for the net operating losses incurred or for the research and development tax credits generated in each year due to its uncertainty of realizing a benefit from those items. The Company has provided a valuation allowance for the full amounts of its net deferred tax assets because, at March 31, 2018 and December 31, 2017, it was more likely than not that any future benefit from deductible temporary differences and net operating loss and tax credit carryforwards would not be realized. As of March 31, 2018 and December 31, 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 2014. No federal or state tax audits are currently in process.

Commitments and Contingencies

3 Months Ended

Mar. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

11. 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.

The following table summarizes the future minimum payments due under the operating leases as of March 31, 2018 (in thousands):


Year Ending December 31,
2018 (nine months ending December 31)	$	1,378
2019		1,878
2020		1,933
2021		1,989
2022		2,046
Thereafter		689

	$	9,913

Rent expense for the three months ended March 31, 2018 and 2017 was $0.4 million and $0.4 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 12-month sublease agreement with a tenant for approximately 5,000 square feet of general office and laboratory space at its headquarters. The Company recognized $0.2 million and less than $0.1 million received under the subleases as other income in the consolidated statements of operations and comprehensive loss for the three months ended March 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 $4.2 million as of March 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 March 31, 2018, the Company had non-cancelable minimum purchase commitments under contract manufacturing agreements for payments totaling $0.9 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 March 31, 2018 or December 31, 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

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Net Loss Per Share

12. 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):


	Three Months Ended March 31,
	2018			2017
Numerator:
Net loss	$	(6,735	)	$	(5,939	)
Accretion of redeemable convertible preferred stock to redemption value		(16	)		(16	)

Net loss attributable to common stockholders	$	(6,751	)	$	(5,955	)

Denominator:
Weighted average common shares outstanding, basic and diluted		10,204,591			10,190,228

Net loss per share attributable to common stockholders, basic and diluted	$	(0.66	)	$	(0.58	)

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:


	March 31,
	2018		2017
Redeemable convertible preferred shares (as converted to common stock)		13,229,362		13,229,362
Stock options to purchase common stock		3,873,879		2,549,808

		17,103,241		15,779,170

Defined contribution plan	3 Months Ended
Defined contribution plan	Mar. 31, 2018
Retirement Benefits [Abstract]
Defined contribution plan	13. 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 three months ended March 31, 2018 or 2017.

Subsequent Events	3 Months Ended
Subsequent Events	Mar. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	14. Subsequent Events 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 $64.4 million after deducting underwriting discounts and commissions but before deducting offering costs. On April 3, 2018, concurrent with the IPO, the Company completed the sale of $5.0 million of shares of common stock at the public offering price of $12.00 per share, or 416,666 shares, to Seattle Genetics in a concurrent private placement. These shares are not registered under the Securities Act of 1933, as amended, and are subject to a 180-day lock-up agreement with the underwriters of the IPO. The concurrent private placement was subject to certain closing conditions. The Company received the full proceeds from the sale and did not pay any underwriting discounts or commissions with respect to the shares of common stock that were sold in the concurrent private placement. Upon closing of the IPO on April 3, 2018, the Company’s outstanding redeemable convertible preferred stock automatically converted into shares of common stock (see Note 7). Upon conversion of the redeemable convertible preferred stock, the Company reclassified the carrying value of the Preferred Stock to common stock and additional paid-in capital. On April 25, 2018, in connection with the IPO, the Company issued and sold an additional 215,000 shares of its common stock at the IPO price of $12.00 per share, less underwriting discounts and commissions, pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock, resulting in additional net proceeds to the Company of approximately $2.4 million after deducting underwriting discounts and commissions.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Unaudited Interim Financial Information

Unaudited Interim Financial Information

Concentrations of Credit Risk and of Significant Suppliers

Concentrations of Credit Risk and of Significant Suppliers

Cash Equivalents

Cash Equivalents

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

Collaboration Agreements

Collaboration Agreements

Revenue Recognition of Collaboration Agreements

Revenue Recognition of Collaboration Agreements

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 Sheets


	At March 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Deferred revenue, current portion	$	17,519		$	7,222		$	10,297
Deferred revenue, net of current portion	$	3,655		$	7,323		$	(3,668	)
Accumulated deficit	$	(64,262	)	$	(57,633	)	$	(6,629	)

Consolidated Statements of Operations and Comprehensive Loss


	Three Months Ended March 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Collaboration revenue	$	2,220		$	2,661		$	(441	)
Net loss	$	(6,735	)	$	(6,294	)	$	(441	)
Net loss attributable to common stockholders	$	(6,751	)	$	(6,310	)	$	(441	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.66	)	$	(0.62	)	$	(0.04	)
Comprehensive loss	$	(6,726	)	$	(6,285	)	$	(441	)

Consolidated Statements of Cash Flows


	Three Months Ended March 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Net loss	$	(6,735	)	$	(6,294	)	$	(441	)
Change in deferred revenue	$	(619	)	$	(1,060	)	$	441

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

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 Sheets


	At March 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Deferred revenue, current portion	$	17,519		$	7,222		$	10,297
Deferred revenue, net of current portion	$	3,655		$	7,323		$	(3,668	)
Accumulated deficit	$	(64,262	)	$	(57,633	)	$	(6,629	)

Consolidated Statements of Operations and Comprehensive Loss


	Three Months Ended March 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Collaboration revenue	$	2,220		$	2,661		$	(441	)
Net loss	$	(6,735	)	$	(6,294	)	$	(441	)
Net loss attributable to common stockholders	$	(6,751	)	$	(6,310	)	$	(441	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.66	)	$	(0.62	)	$	(0.04	)
Comprehensive loss	$	(6,726	)	$	(6,285	)	$	(441	)

Consolidated Statements of Cash Flows


	Three Months Ended March 31, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Net loss	$	(6,735	)	$	(6,294	)	$	(441	)
Change in deferred revenue	$	(619	)	$	(1,060	)	$	441

Fair Value of Financial Assets and Liabilities (Tables)

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Marketable Securities by Type

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


	March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury notes (due within one year)	$	4,003	$	—	$	(5	)	$	3,998
U.S. government agency bonds (due within one year)		2,700		—		(2	)		2,698

	$	6,703	$	—	$	(7	)	$	6,696


	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury 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 Assets Measured at Fair Value on Recurring Basis

The following tables present information about the Company’s assets that are measured at fair value on a recurring basis (in thousands):


	Fair Value Measurements at March 31, 2018 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	—	$	22,293	$	—	$	22,293
Marketable securities:
U.S. Treasury notes		3,998		—		—		3,998
U.S. government agency bonds		—		2,698		—		2,698

	$	3,998	$	24,991	$	—	$	28,989


	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

Accrued Expenses and Other Current Liabilities (Tables)

3 Months Ended

Mar. 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):


	March 31, 2018		December 31, 2017
Accrued employee compensation and benefits	$	357	$	1,315
Accrued professional fees		964		980
Accrued external research and development expense		806		478
Accrued other		231		180

	$	2,358	$	2,953

Redeemable Convertible Preferred Stock (Tables)

3 Months Ended

Mar. 31, 2018

Equity [Abstract]

Summary of Preferred Stock

As of each balance sheet date, the Preferred Stock consisted of the following (in thousands, except share amounts):


	As of March 31, 2018
	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	$	12,271	$	12,297		7,832,001
Series B preferred stock		8,494,131		8,474,574		64,896		65,000		5,397,361

		20,791,407		20,771,850	$	77,167	$	77,297		13,229,362


	As of December 31, 2017
	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)

3 Months Ended

Mar. 31, 2018

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

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):


	Three Months Ended March 31,
	2018		2017
Research and development expenses	$	401	$	260
General and administrative expenses		78		40

Total	$	479	$	300

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Summary of Future Minimum Payments Due under Operating Leases

The following table summarizes the future minimum payments due under the operating leases as of March 31, 2018 (in thousands):


Year Ending December 31,
2018 (nine months ending December 31)	$	1,378
2019		1,878
2020		1,933
2021		1,989
2022		2,046
Thereafter		689

	$	9,913

Net Loss Per Share (Tables)

3 Months Ended

Mar. 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):


	Three Months Ended March 31,
	2018			2017
Numerator:
Net loss	$	(6,735	)	$	(5,939	)
Accretion of redeemable convertible preferred stock to redemption value		(16	)		(16	)

Net loss attributable to common stockholders	$	(6,751	)	$	(5,955	)

Denominator:
Weighted average common shares outstanding, basic and diluted		10,204,591			10,190,228

Net loss per share attributable to common stockholders, basic and diluted	$	(0.66	)	$	(0.58	)

Summary of Potential Dilutive Securities


	March 31,
	2018		2017
Redeemable convertible preferred shares (as converted to common stock)		13,229,362		13,229,362
Stock options to purchase common stock		3,873,879		2,549,808

		17,103,241		15,779,170

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	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	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 loss				$ (6,735)	$ (5,939)	$ 25,500
Accumulated deficit				$ (64,262)		$ (51,339)
Funding term of borrowings for operating expense and capital expenditure requirement				12 months
Subsequent Event [Member] \| 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
Public offering price, per share \| $ / shares	$ 12.00	$ 12.00
Net proceeds from initial public offering	$ 2,400	$ 64,400
Subsequent Event [Member] \| Common Stock [Member] \| Private Placement [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Public offering price, per share \| $ / shares		$ 12.00
Sale of stock, consideration received on transaction		$ 5,000
Sale of stock, number of shares issued in transaction \| shares		416,666
Period of lock-up agreement		180 days

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Deferred revenue	$ 17,519,000		$ 6,891,000
Deferred revenue, net of current portion	3,655,000		$ 8,714,000
Revenue recognized	$ 2,200,000
New accounting pronouncement revenue recognition timing	The Company recognized revenue of $2.2 million during three months ended March 31, 2018 from the deferred revenue balance at January 1, 2018. The Company recognizes deferred revenue by first allocating from the beginning deferred revenue balance to the extent 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
Capitalized contract costs	$ 0
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
ASU 2016-18 [Member]
Significant Accounting Policies [Line Items]
Increase in restricted cash	$ 1,300,000	$ 1,300,000
ASU 2014-09 [Member]
Significant Accounting Policies [Line Items]
Deferred revenue	17,519,000
Deferred revenue, net of current portion	3,655,000
ASU 2014-09 [Member] \| Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Significant Accounting Policies [Line Items]
Adjustment due to application of the new revenue standards	$ 6,200,000

Summary of Significant Accounting Policies - Summary of Cumulative Effect to Consolidated Balance Sheet upon Adoption of New Revenue Standard (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	$ (64,262)	$ (51,339)
ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	(64,262)
Scenario, Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current and net of current portion		15,605
Accumulated deficit		(51,339)
Effect of Change [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	$ (6,629)
Effect of Change [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		(6,188)
Scenario As Restated [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)

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
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current portion	$ 17,519		$ 6,891
Deferred revenue, net of current portion	3,655		8,714
Accumulated deficit	(64,262)		(51,339)
Collaboration revenue	2,220	$ 1,827
Net loss	(6,735)	(5,939)	25,500
Net loss attributable to common stockholders	$ (6,751)	$ (5,955)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.66)	$ (0.58)
Comprehensive loss	$ (6,726)	$ (5,951)
Net loss	(6,735)	(5,939)	25,500
Change in deferred revenue	(619)	$ (941)
Effect of Change [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current portion	10,297
Deferred revenue, net of current portion	(3,668)
Accumulated deficit	(6,629)
Collaboration revenue	(441)
Net loss	(441)
Net loss attributable to common stockholders	$ (441)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.04)
Comprehensive loss	$ (441)
Net loss	(441)
Change in deferred revenue	441
ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current portion	17,519
Deferred revenue, net of current portion	3,655
Accumulated deficit	(64,262)
Collaboration revenue	2,220
Net loss	(6,735)
Net loss attributable to common stockholders	$ (6,751)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.66)
Comprehensive loss	$ (6,726)
Net loss	(6,735)
Change in deferred revenue	(619)
ASU 2014-09 [Member] \| Effect of Change [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit			$ (6,188)
Under Topic 605 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current portion	7,222
Deferred revenue, net of current portion	7,323
Accumulated deficit	(57,633)
Collaboration revenue	2,661
Net loss	(6,294)
Net loss attributable to common stockholders	$ (6,310)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.62)
Comprehensive loss	$ (6,285)
Net loss	(6,294)
Change in deferred revenue	$ (1,060)

Fair Value of Financial Assets and Liabilities - Schedule of Marketable Securities by Type (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,703	$ 12,707
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7)	(16)
Fair Value	6,696	12,691
U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,003	5,007
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(5)	(10)
Fair Value	3,998	4,997
U.S. Government Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,700	7,700
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(6)
Fair Value	$ 2,698	$ 7,694

Fair Value of Financial Assets and Liabilities - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 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	$ 28,989	$ 36,887
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	22,293	24,196
U.S. Treasury Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	3,998	4,997
U.S. Government Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	2,698	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	3,998	4,997
Level 1 [Member] \| U.S. Treasury Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	3,998	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	24,991	31,890
Level 2 [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	22,293	24,196
Level 2 [Member] \| U.S. Government Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	$ 2,698	$ 7,694

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	Mar. 31, 2018	Mar. 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

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued employee compensation and benefits	$ 357	$ 1,315
Accrued professional fees	964	980
Accrued external research and development expense	806	478
Accrued other	231	180
Total	$ 2,358	$ 2,953

Collaboration Agreement - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue related to research and clinical development activities	$ 2,220	$ 1,827
Remaining performance obligation related to two product candidates	76,000
ASU 2014-09 [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue related to research and clinical development activities	2,220
ASU 2014-09 [Member] \| Cumulative Effect of Changes in Accounting Principle [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue			$ 6,200
Seattle Genetics [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue related to research and clinical development activities	2,200	$ 1,800
Deferred revenue	$ 21,200			$ 15,600

Loan and Security Agreement - Additional Information (Detail) - Loan And Security Agreement [Member] - Pacific Western Bank [Member]	3 Months Ended
	Mar. 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)		3 Months Ended
	Mar. 16, 2018	Mar. 31, 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 are entitled to vote, together with the holders of common stock, on matters submitted to stockholders for a vote. The holders of Preferred Stock are 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, are entitled to elect two directors of the Company. The holders of Series B preferred stock, voting exclusively and as a separate class, are entitled to elect one director of the Company.
Conversion, description		Each share of Preferred Stock is convertible at the option of the holder at any time after the date of issuance. Each share of Preferred Stock will be 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 will be 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 will be 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 may 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 receive, or simultaneously receive, 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 is 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 is 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 declares, pays or sets 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 shall be calculated based upon the dividend on the class or series of capital stock that would result in the highest Preferred Stock dividend. Stockholders are 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 are 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	Mar. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Preferred Stock Authorized	20,791,407	20,791,407
Preferred Stock Issued	20,771,850	20,771,850
Preferred Stock Outstanding	20,771,850	20,771,850
Carrying Value	$ 77,167	$ 77,151
Liquidation Preference	$ 77,297	$ 77,297
Common Stock Issuable Upon Conversion	13,229,362	13,229,362
Series A preferred stock [Member]
Temporary Equity [Line Items]
Preferred Stock Authorized	12,297,276	12,297,276
Preferred Stock Issued	12,297,276	12,297,276
Preferred Stock Outstanding	12,297,276	12,297,276
Carrying Value	$ 12,271	$ 12,267
Liquidation Preference	$ 12,297	$ 12,297
Common Stock Issuable Upon Conversion	7,832,001	7,832,001
Series B preferred stock [Member]
Temporary Equity [Line Items]
Preferred Stock Authorized	8,494,131	8,494,131
Preferred Stock Issued	8,474,574	8,474,574
Preferred Stock Outstanding	8,474,574	8,474,574
Carrying Value	$ 64,896	$ 64,884
Liquidation Preference	$ 65,000	$ 65,000
Common Stock Issuable Upon Conversion	5,397,361	5,397,361

Common Stock - Additional Information (Detail)	3 Months Ended
Common Stock - Additional Information (Detail)	Mar. 31, 2018
Equity [Abstract]
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.

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Mar. 16, 2018	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option grants		777,071
Exercise price		$ 12.00
Weighted average grant date fair value		$ 6.28
Total unrecognized stock-based compensation expense		$ 9.8
Period for recognition of unrecognized expense		3 years
Two Thousand Fifteen 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 shares initially reserved for issuance		4,144,876
Shares available for future issuance		1,041,700
Two Thousand Eighteen Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares initially reserved for issuance	2,800,721
Shares available for future issuance		3,065,350
Percentage applied to the outstanding shares as annual increase in the number of shares authorized for issuance	4.00%
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] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option grants	500,000
Percentage of shares of common stock available for issuance	1.00%

Stock-Based Compensation - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share Based Compensation Expense	$ 479	$ 300
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share Based Compensation Expense	401	260
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share Based Compensation Expense	$ 78	$ 40

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
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
Income tax benefits	0	$ 0
Unrecognized tax benefits	$ 0		$ 0

Commitments and Contingencies - Additional Information (Detail) $ in Millions	1 Months Ended		3 Months Ended
	Dec. 31, 2017 ft²	Jan. 31, 2017 ft²	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2018 SGD ($)
Commitments And Contingencies Disclosure [Line Items]
Lease expiration date			Apr. 30, 2023
Rent expense			$ 400,000	$ 400,000
Sublease term	12 months	12 months
Area of subleased property \| ft²	5,000
Sublease rent recognized			200,000
Non-cancelable minimum purchase commitments			900,000
License Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
License maintenance fees on eighth anniversary and each year thereafter			100,000
Aggregate milestone payments			$ 4,200,000
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
Sublease rent recognized				$ 100,000
Maximum [Member] \| License Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
License maintenance fees for each of first seven years			$ 100,000
Aggregate milestone payments					$ 5.5
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 Payments Due under Operating Leases (Detail) $ in Thousands	Mar. 31, 2018 USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018 (nine months ending December 31)	$ 1,378
2019	1,878
2020	1,933
2021	1,989
2022	2,046
Thereafter	689
Operating Leases, Future Minimum Payments Due, Total	$ 9,913

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
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Earnings Per Share [Abstract]
Net loss	$ (6,735)	$ (5,939)	$ 25,500
Accretion of redeemable convertible preferred stock to redemption value	(16)	(16)
Net loss attributable to common stockholders	$ (6,751)	$ (5,955)
Weighted average common shares outstanding, basic and diluted	10,204,591	10,190,228
Net loss per share attributable to common stockholders, basic and diluted	$ (0.66)	$ (0.58)

Net Loss Per Share - Summary of Potential Dilutive Securities (Detail) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 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	17,103,241	15,779,170
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	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	3,873,879	2,549,808

Retirement Plan - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Retirement Benefits [Abstract]
Defined contribution plan, matching amount	$ 0	$ 0

Subsequent Events - Additional information (Detail) - Subsequent Event [Member] - Common Stock [Member] - USD ($) $ / shares in Units, $ in Millions	Apr. 25, 2018	Apr. 03, 2018
IPO [Member]
Subsequent Event [Line Items]
Shares of common stock issued and sold	215,000	5,770,000
Public offering price, per share	$ 12.00	$ 12.00
Net proceeds from initial public offering	$ 2.4	$ 64.4
Private Placement [Member]
Subsequent Event [Line Items]
Public offering price, per share		$ 12.00
Sale of stock, consideration received on transaction		$ 5.0
Sale of stock, number of shares issued in transaction		416,666
Period of lock-up agreement		180 days