Cogent Biosciences (COGT) 10-Q 12 Nov 19 2019 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Nov. 08, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
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 Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		30,663,054
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity File Number	001-38443
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-5308248
Entity Address, Address Line One	200 Cambridge Park Drive
Entity Address, Address Line Two	Suite 3100
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02140
City Area Code	(617)
Local Phone Number	945-5576
Title of 12(b) Security	Common Stock, $0.001 Par Value
Trading Symbol	UMRX
Security Exchange Name	NASDAQ

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 45,882	$ 55,671
Marketable securities	0	22,923
Accounts receivable	880	1,668
Prepaid expenses and other current assets	1,216	740
Total current assets	47,978	81,002
Operating lease, right-of-use asset	5,634
Property and equipment, net	2,170	3,251
Restricted cash	1,255	1,255
Other assets	847	419
Total assets	57,884	85,927
Current liabilities:
Accounts payable	1,594	1,519
Accrued expenses and other current liabilities	6,538	5,477
Operating lease liability	1,581
Deferred revenue	14,717	17,949
Total current liabilities	24,430	24,945
Deferred rent		748
Operating lease liability, net of current portion	4,831
Total liabilities	29,261	25,693
Commitments and contingencies (Note 8)
Stockholders’ equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized; no shares issued or outstanding
Common stock, $0.001 par value; 150,000,000 shares authorized; 30,663,054 shares and 30,057,970 shares issued and outstanding at September 30, 2019 and December 31, 2018, respectively	30	30
Additional paid-in capital	154,782	152,275
Accumulated other comprehensive loss		(12)
Accumulated deficit	(126,189)	(92,059)
Total stockholders’ equity	28,623	60,234
Total liabilities and stockholders’ equity	$ 57,884	$ 85,927

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, Par value	$ 0.001	$ 0.001
Common stock, Shares authorized	150,000,000	150,000,000
Common stock, Shares issued	30,663,054	30,057,970
Common stock, Shares outstanding	30,663,054	30,057,970

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Collaboration revenue	$ 1,020	$ 2,043	$ 7,211	$ 5,929
Operating expenses:
Research and development	10,335	10,252	33,355	27,520
General and administrative	2,721	2,367	8,274	5,410
Total operating expenses	13,056	12,619	41,629	32,930
Loss from operations	(12,036)	(10,576)	(34,418)	(27,001)
Other income (expense):
Interest income	31	405	206	745
Other income, net	82	3	82	330
Total other income, net	113	408	288	1,075
Net loss	(11,923)	(10,168)	(34,130)	(25,926)
Accretion of redeemable convertible preferred stock to redemption value				(16)
Net loss attributable to common stockholders	$ (11,923)	$ (10,168)	$ (34,130)	$ (25,942)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.39)	$ (0.34)	$ (1.12)	$ (1.12)
Weighted average common shares outstanding, basic and diluted	30,661,125	29,879,476	30,418,752	23,169,348
Comprehensive loss:
Net loss	$ (11,923)	$ (10,168)	$ (34,130)	$ (25,926)
Other comprehensive income (loss):
Unrealized gains (losses) on marketable securities, net of tax of $0		(11)	12	(8)
Total other comprehensive income (loss)		(11)	12	(8)
Comprehensive loss	$ (11,923)	$ (10,179)	$ (34,118)	$ (25,934)

Consolidated Statements of Op_2

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Unrealized gains (losses) on marketable securities, tax	$ 0	$ 0	$ 0	$ 0

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	Total	IPO [Member]	Private Placement [Member]	Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member]IPO [Member]	Common Stock [Member]Private Placement [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]IPO [Member]	Additional Paid-in Capital [Member]Private Placement [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balances at Dec. 31, 2017	$ (48,846)			$ 77,151	$ 10			$ 2,499			$ (16)	$ (51,339)
Beginning Balances, Shares at Dec. 31, 2017				20,771,850	10,201,690
Issuance of common stock upon exercise of stock options	28							28
Issuance of common stock upon exercise of stock options, Shares					6,368
Stock-based compensation expense	479							479
Unrealized gains (losses) on marketable securities	9										9
Accretion of redeemable convertible preferred stock to redemption value				$ 16
Accretion of redeemable convertible preferred stock to redemption value	(16)							(16)
Net loss	(6,735)											(6,735)
Ending Balances at Mar. 31, 2018	(61,269)			$ 77,167	$ 10			2,990			(7)	(64,262)
Ending Balances, Shares at Mar. 31, 2018				20,771,850	10,208,058
Beginning Balances at Dec. 31, 2017	(48,846)			$ 77,151	$ 10			2,499			(16)	(51,339)
Beginning Balances, Shares at Dec. 31, 2017				20,771,850	10,201,690
Stock-based compensation expense	2,186
Unrealized gains (losses) on marketable securities	(8)
Net loss	(25,926)
Ending Balances at Sep. 30, 2018	67,531				$ 30			150,978			(24)	(83,453)
Ending Balances, Shares at Sep. 30, 2018					29,924,859
Beginning Balances at Dec. 31, 2017	(48,846)			$ 77,151	$ 10			2,499			(16)	(51,339)
Beginning Balances, Shares at Dec. 31, 2017				20,771,850	10,201,690
Net loss	(34,500)
Ending Balances at Dec. 31, 2018	60,234				$ 30			152,275			(12)	(92,059)
Ending Balances, Shares at Dec. 31, 2018					30,057,970
Adjustment to retained earnings for change in accounting policy	(6,188)											(6,188)
Beginning Balances at Mar. 31, 2018	(61,269)			$ 77,167	$ 10			2,990			(7)	(64,262)
Beginning Balances, Shares at Mar. 31, 2018				20,771,850	10,208,058
Conversion of redeemable convertible preferred stock to common stock	77,167			$ (77,167)	$ 13			77,154
Conversion of redeemable convertible preferred stock to common stock, Shares				(20,771,850)	13,229,362
Issuance of common stock sold in initial public offering, net of underwriting discounts, commissions and offering costs		$ 63,948	$ 5,000			$ 6	$ 1		$ 63,942	$ 4,999
Issuance of common stock sold in initial public offering, net of underwriting discounts, commissions and offering costs, Shares						5,985,000	416,666
Issuance of common stock upon exercise of stock options	11							11
Issuance of common stock upon exercise of stock options, Shares					28,162
Stock-based compensation expense	847							847
Unrealized gains (losses) on marketable securities	(6)										(6)
Net loss	(9,023)											(9,023)
Ending Balances at Jun. 30, 2018	76,675				$ 30			149,943			(13)	(73,285)
Ending Balances, Shares at Jun. 30, 2018					29,867,248
Issuance of common stock upon exercise of stock options	175							175
Issuance of common stock upon exercise of stock options, Shares					57,611
Stock-based compensation expense	860							860
Unrealized gains (losses) on marketable securities	(11)										(11)
Net loss	(10,168)											(10,168)
Ending Balances at Sep. 30, 2018	67,531				$ 30			150,978			(24)	(83,453)
Ending Balances, Shares at Sep. 30, 2018					29,924,859
Beginning Balances at Dec. 31, 2018	60,234				$ 30			152,275			(12)	(92,059)
Beginning Balances, Shares at Dec. 31, 2018					30,057,970
Issuance of common stock upon exercise of stock options	11							11
Issuance of common stock upon exercise of stock options, Shares					60,852
Stock-based compensation expense	726							726
Unrealized gains (losses) on marketable securities	10										10
Net loss	(11,691)											(11,691)
Ending Balances at Mar. 31, 2019	49,290				$ 30			153,012			(2)	(103,750)
Ending Balances, Shares at Mar. 31, 2019					30,118,822
Beginning Balances at Dec. 31, 2018	60,234				$ 30			152,275			(12)	(92,059)
Beginning Balances, Shares at Dec. 31, 2018					30,057,970
Stock-based compensation expense	2,399
Unrealized gains (losses) on marketable securities	12
Net loss	(34,130)
Ending Balances at Sep. 30, 2019	28,623				$ 30			154,782				(126,189)
Ending Balances, Shares at Sep. 30, 2019					30,663,054
Beginning Balances at Mar. 31, 2019	49,290				$ 30			153,012			(2)	(103,750)
Beginning Balances, Shares at Mar. 31, 2019					30,118,822
Issuance of common stock upon exercise of stock options	97							97
Issuance of common stock upon exercise of stock options, Shares					541,732
Stock-based compensation expense	885							885
Unrealized gains (losses) on marketable securities	2										$ 2
Net loss	(10,516)											(10,516)
Ending Balances at Jun. 30, 2019	39,758				$ 30			153,994				(114,266)
Ending Balances, Shares at Jun. 30, 2019					30,660,554
Issuance of common stock upon exercise of stock options, Shares					2,500
Stock-based compensation expense	788							788
Net loss	(11,923)											(11,923)
Ending Balances at Sep. 30, 2019	$ 28,623				$ 30			$ 154,782				$ (126,189)
Ending Balances, Shares at Sep. 30, 2019					30,663,054

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (34,130)	$ (25,926)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	979	969
Stock-based compensation expense	2,399	2,186
Realized losses (gains) on sales of marketable securities	2
Net amortization (accretion) of premiums (discounts) on marketable securities	(55)	(159)
Gain on sale of property and equipment	(82)
Non-cash interest expense		16
Changes in operating assets and liabilities:
Accounts receivable	788	(589)
Prepaid expenses and other current assets	(476)	(463)
Operating lease, right-of-use asset	1,016
Other assets	(428)	(419)
Accounts payable	75	(349)
Accrued expenses and other current liabilities	1,167	3,007
Deferred rent		(35)
Operating lease liability	(1,092)
Deferred revenue	(3,232)	(1,707)
Net cash provided by (used in) operating activities	(33,069)	(23,469)
Cash flows from investing activities:
Purchases of property and equipment	(20)	(549)
Proceeds from sale of property and equipment	204
Purchases of marketable securities		(47,682)
Maturities and sales of marketable securities	22,988	22,700
Net cash provided by (used in) investing activities	23,172	(25,531)
Cash flows from financing activities:
Proceeds from initial public offering, net of underwriting discounts and commissions		66,793
Proceeds from private placement concurrent with initial public offering		5,000
Proceeds from issuance of common stock upon stock option exercises	108	214
Payments of initial public offering costs		(2,056)
Net cash provided by (used in) financing activities	108	69,951
Net increase (decrease) in cash, cash equivalents and restricted cash	(9,789)	20,951
Cash, cash equivalents and restricted cash at beginning of period	56,926	29,600
Cash, cash equivalents and restricted cash at end of period	$ 47,137	50,551
Supplemental disclosure of noncash investing and financing information:
Conversion of redeemable convertible preferred stock into common stock		77,154
Accretion of redeemable convertible preferred stock to redemption value		$ 16

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2019
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 developing curative cell therapies for cancer. Unum’s novel proprietary platforms include Antibody-Coupled T cell Receptor (“ACTR”), an autologous engineered cellular therapy that combines the cell-killing ability of T cells and the tumor-targeting ability of co-administered antibodies to exert potent antitumor immune responses, and Bolt-On Chimeric Receptor (“BOXR”), designed to improve the functionality of engineered T cells by identifying and incorporating a “bolt-on” transgene to overcome resistance of the solid tumor microenvironment to T cell attack. 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 April 3, 2018, the Company completed an initial public offering (“IPO”) of its common stock and issued and sold 5,770,000 shares of common stock at a public offering price of $12.00 per share, resulting in net proceeds of $61.5 million after deducting underwriting discounts and commissions and other offering costs. In addition, Seattle Genetics, Inc. (“Seattle Genetics”) purchased from the Company, concurrently with the IPO in a private placement, $5.0 million of shares of common stock at a price per share equal to the initial public offering price, or 416,666 shares (the “concurrent private placement”). Upon closing of the IPO, the Company’s outstanding redeemable convertible preferred stock automatically converted into shares of common stock. 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. On April 1, 2019, the Company filed a shelf registration statement on Form S-3 with the SEC. The shelf registration statement allows the Company to sell from time-to-time up to $150 million of common stock, preferred stock, debt securities, warrants, or units comprised of any combination of these securities, for its own account in one or more offerings. The terms of any offering under the shelf registration statement Additionally, on April 1, 2019 and pursuant to the Form S-3, the Company entered into a Sales Agreement (the “Sales Agreement”) with Cowen and Company, LLC (“Cowen”), pursuant to which the Company may issue and sell, from time to time, shares of its common stock having an aggregate offering price of up to $50.0 million through Cowen as the sales agent. As of September 30, 2019, no shares have been sold under this Sales Agreement. 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. The Company has incurred recurring losses since inception, including net losses attributable to the Company of $34.1 million for the nine months ended September 30, 2019 and $34.5 million for the year ended December 31, 2018. As of September 30, 2019, the Company had an accumulated deficit of $126.2 million. The Company expects to continue to generate operating losses in the foreseeable future. As of the issuance date of the interim consolidated financial statements, the Company expects that its cash and cash equivalents 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. However, the Company will require additional funding to complete the critical activities planned to support ongoing research and development. The Company will need to seek additional funding through equity offerings, debt financings, collaborations, licensing arrangements and other marketing and distribution arrangements, partnerships, joint ventures, combinations or divestitures of one or more of its businesses. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into collaborative arrangements or divest its assets. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. Arrangements with collaborators or others may require the Company to relinquish rights to certain of its technologies or product candidates. If the Company is unable to obtain sufficient funding in a timely manner, the Company could be forced to delay, reduce or eliminate its research and development programs or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. 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 Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
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, 2018 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited consolidated financial statements as of September 30, 2019 and for the three and nine months ended September 30, 2019 and 2018 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, 2018 included in the Company’s Annual Report on Form 10-K, File No. 001-38443, 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. 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, consisting of debt securities, are classified as available-for-sale and are reported at fair value. Unrealized gains and losses on available-for-sale debt securities are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Realized gains and losses and declines in value determined to be other than temporary are based on the specific identification method and are included as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss. The Company classifies its marketable securities with maturities beyond one year as short-term, based on their highly liquid nature and because such marketable securities are available for current operations. The Company evaluates its marketable securities with unrealized losses for other-than-temporary impairment. When assessing marketable securities for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented. Leases The Company accounts for a contract as a lease when it has the right to control the asset for a period of time while obtaining substantially all of the assets’ economic benefits. The Company determines the initial classification and measurement of its operating right-of-use assets and operating lease liabilities at the lease commencement date, and thereafter if modified. The lease term includes any renewal options that the Company is reasonably assured to exercise. The Company’s policy is to not record leases with an original term of twelve months or less on its consolidated balance sheets. The Company’s only existing lease is for office space. The right-of-use asset represents the right to use the leased asset for the lease term. The lease liability represents the present value of the lease payments under the lease. The present value of lease payments is determined by using the interest rate implicit in the lease, if that rate is readily determinable; otherwise, the Company uses its estimated secured incremental borrowing rate for that lease term. Lease payments included in the measurement of the lease liability consist of the following: the fixed noncancelable lease payments, payments for optional renewal periods where it is reasonably certain the renewal period will be exercised, and payments for early termination options unless it is reasonably certain the lease will not be terminated early. Leases may contain rent escalation clauses and variable lease payments that require additional rental payments in later years of the term, including payments based on an index or inflation rate. Payments based on the change in an index or inflation rate, or payments based on a change in the Company’s portion of the operating expenses, including real estate taxes and insurance, are not included in the initial lease liability and are recorded as a period expense when incurred. The operating leases may include an option to renew the lease term for various renewal periods and/or to terminate the leases early. These options to exercise the renewal or early termination clauses in the Company’s operating leases were not reasonably certain of exercise as of the date of adoption and these have not been included in the determination of the initial lease liability or operating lease expense. Rent expense for operating leases is recognized on a straight-line basis over the reasonably assured lease term based on the total lease payments and is included in operating expense in the consolidated statements of operations and comprehensive loss. For finance leases, any interest expense is recognized using the effective interest method and is included within interest expense. The Company has no financing leases. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates, assumptions and judgments that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates, assumptions and judgments reflected in these condensed consolidated financial statements include, but are not limited to, revenue, the accrual of research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates. 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 Revenue Recognition for Collaboration Agreements The Company performs the following five steps to determine revenue recognition for arrangements that are within the scope of ASC 606: (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 ASC 606, 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. ASC 606 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 recognizes revenue for its collaboration agreement 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 is 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 is 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, is 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 is 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 September 30, 2019, the Company had deferred revenue of $14.7 million related to its collaboration and all such amounts are classified as current. The Company recognized revenue of $1.0 and $7.2 million during the three and nine months ended September 30, 2019, respectively, from the deferred revenue balance at December 31, 2018. The Company recognizes deferred revenue by first allocating from the beginning deferred revenue balance to the extent that the beginning deferred revenue balance exceeds the revenue to be recognized. Billings during the period are added to the deferred revenue balance to be recognized in future periods. To the extent that the beginning deferred revenue balance is less than revenue to be recognized during the period, billings during the period are allocated to revenue. In the event that a collaboration agreement was to be terminated and the Company had no further performance obligations, the Company would recognize as revenue any portion of the upfront payment and other payments that had not previously been recorded as revenue and were classified as deferred revenue at the date of such termination. 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 September 30, 2019 and December 31, 2018, the Company has not capitalized any costs to obtain its contract. Recently Adopted Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842) Upon adoption of the new leasing standards, the Company recognized a lease liability of $7.5 million and a related right-of-use asset of $6.7 million on its consolidated balance sheet with the difference being due to the elimination of previously reported deferred rent. The adoption of the standard did not have a material impact on the results of operations or cash flows. In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting Recently Issued Accounting Pronouncements In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement, (“ASU 2018-13”). The new standard removes certain disclosures, modifies certain disclosures and adds additional disclosures related to fair value measurement. The new standard will be effective beginning January 1, 2020 and early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2018-13 will have on its consolidated financial statements. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606

Marketable Securities and Fair

Marketable Securities and Fair Value of Financial Assets and Liabilities	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Marketable Securities and Fair Value of Financial Assets and Liabilities	3. Marketable Securities and Fair Value of Financial Assets and Liabilities Marketable securities by security type consisted of the following (in thousands): September 30, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. Treasury bills (due within one year) $ — $ — $ — $ — $ — $ — $ — $ — December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. Treasury bills and notes (due within one year) $ 22,935 $ — $ (12 ) $ 22,923 $ 22,935 $ — $ (12 ) $ 22,923 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 September 30, 2019 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ — $ 484 $ — $ 484 Marketable securities: U.S. Treasury bills — — — $ — $ — $ 484 $ — $ 484 Fair Value Measurements at December 31, 2018 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ — $ 52,100 $ — $ 52,100 Marketable securities: U.S. Treasury bills and notes 22,923 — — 22,923 $ 22,923 $ 52,100 $ — $ 75,023 During the three and nine months ended September 30, 2019 and 2018, there were no transfers between Level 1, Level 2 and Level 3.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2019
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): September 30, 2019 December 31, 2018 Accrued employee compensation and benefits $ 2,259 $ 1,599 Accrued external research and development expense 2,618 1,799 Accrued external manufacturing costs 588 1,015 Other 1,073 1,064 $ 6,538 $ 5,477

Collaboration Agreement

Collaboration Agreement	9 Months Ended
	Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaboration Agreement	5. Collaboration Agreement In 2015, the Company entered into a collaboration agreement with Seattle Genetics, whereby the parties agreed to jointly develop two product candidates incorporating the Company’s ACTR platform and Seattle Genetics’ antibodies (the “Collaboration Agreement”). Under the terms of the Collaboration Agreement, the Company conducts preclinical research and clinical development activities related to the two specified product candidates through Phase 1 clinical development, and Seattle Genetics provides the funding for those activities. As a result of the Collaboration Agreement, the Company recognized revenue of $1.0 million and $2.0 million for the three months ended September 30, 2019 and 2018, respectively, and $7.2 million and $5.9 million for the nine months ended September 30, 2019 and 2018 related to research and clinical development activities performed. As of September 30, 2019, deferred revenue of $14.7 million was recorded related to this agreement. As of September 30, 2019, 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 1 is estimated to be approximately $40.7 million, which is expected to be recognized as revenue through December 31, 2022. In November 2019, the Company and Seattle Genetics suspended further development under the Collaboration Agreement. The impact to the Collaboration Agreement is outlined further in Note 11, “Subsequent Events.”

Loan and Security Agreement

Loan and Security Agreement	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Loan and Security Agreement	6. Loan and Security Agreement The Company has a loan and security agreement (the “Loan Agreement”) with Pacific Western Bank (“PWB”), entered into in 2017, which provided 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 were 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 January 2019, the Company amended the Loan Agreement to extend the available date for borrowings from January 19, 2019 to June 30, 2019 and extend the interest only period from January 19, 2019 to June 30, 2020, with the possibility of further extension to March 31, 2021 if certain equity financing considerations are met. Additionally, the loan repayment period will be over a 24-month period following the end of the interest-only period. In June 2019, the Company further amended the Loan Agreement to extend the available date for borrowings from June 30, 2019 to June 30, 2020. On July 31, 2019, the Company amended the Loan Agreement 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 September 30, 2019. 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.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	7. Stock-Based Compensation 2018 Stock Option and Incentive Plan The Company’s 2018 Stock Option and Incentive Plan, (the “2018 Plan”), which became effective on March 27, 2018 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. Additionally, the shares of common stock that remained available for issuance under the previously outstanding 2015 Stock Incentive Plan (the “2015 Plan”) became available under the 2018 Plan. The number of shares reserved for the 2018 Plan will automatically increase on each January 1 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. The number of authorized shares reserved for issuance under the 2018 Plan was increased by 1,202,319 shares effective as of January 1, 2019. As of September 30, 2019, 2,372,248 shares remained available for future issuance under the 2018 Plan. 2018 Employee Stock Purchase Plan The Company’s 2018 Employee Stock Purchase Plan (the “ESPP”) became effective on March 28, 2018 at which time a total of 314,000 shares of common stock were reserved for issuance. In addition, the number of shares of common stock that may be issued under the ESPP will automatically increase on each January 1 through January 1, 2027, by the least of (i) 500,000 shares of common stock, (ii) 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or (iii) such lesser number of shares as determined by the ESPP administrator. The number of authorized shares reserved for issuance under the ESPP was increased by 300,580 shares effective as of January 1, 2019. The first six month offering period was initiated on July 1, 2019. As of September 30, 2019, no shares have been issued under the ESPP and 614,580 shares remain available for issuance. Stock Option Issuances During the nine months ended September 30, 2019, the Company granted service-based options to participants for the purchase of 1,665,418 shares of common stock with a weighted average grant-date fair value of $2.25 per share. During the nine months ended September 30, 2019, the Company granted options to certain employees for the purchase of 635,000 shares of common stock with a weighted average grant-date fair value of $2.46 per share that vest under a combination of performance-based and service-based vesting conditions if certain performance vesting criteria are achieved on or before March 31, 2020. As of September 30, 2019, the Company has not recorded stock-based compensation expense as the performance conditions have not been deemed to be probable of being achieved. 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 September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Research and development expenses $ 505 $ 629 $ 1,692 $ 1,637 General and administrative expenses 283 231 707 549 Total $ 788 $ 860 $ 2,399 $ 2,186

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. Commitments and Contingencies Operating Lease The Company leases office and laboratory space under a non-cancelable operating lease that expires in April 2023 with the Company’s option to extend for an additional five-year term. The lessee has the right to terminate the lease in the event of the inability to use the space due to substantial damage while the lessor has the right to terminate the lease for tenant’s default of lease financial obligations. Per the terms of the lease agreement, the Company does not have any residual value guarantees. This extension has not been considered in the determination of the lease liability as the Company is not obligated to exercise their option and it is not reasonably certain that the option will be exercised. The lease payments include fixed lease payments that escalate over the term of the lease on an annual basis. The Company’s real estate lease in Cambridge is a net lease, as the non-lease components (i.e. common area maintenance) are paid separately from rent based on actual costs incurred. Therefore, the non-lease component and related payments are not included in the right-of-use asset and liability and are reflected as an expense in the period incurred. The discount rate used in determining the lease liability represents the Company’s incremental borrowing rate as the rate implicit in the lease could not be readily determined. The elements of the lease expense were as follows (in thousands): Three Months Ended September 30, 2019 Nine Months Ended September 30, 2019 Lease cost Operating lease cost $ 443 $ 1,329 Variable lease cost (1) 254 835 Total lease cost $ 697 $ 2,164 Other information Operating cash flows used for operating leases $ 2,240 Remaining lease term 3.58 years Discount rate 6.25 % (1) The variable lease costs for the quarter ended September 30, 2019 include common area maintenance and other operating charges. The following table summarizes the future minimum payments due under the operating lease as of September 30, 2019 (in thousands): Year Ending December 31, 2019 $ 474 2020 1,933 2021 1,989 2022 2,046 2023 689 Total future minimum lease payments 7,131 Less: imputed interest 719 Total operating lease liability $ 6,412 Included in the consolidated balance sheet: Current operating lease liability $ 1,581 Operating lease liability, net of current portion 4,831 Total operating lease liability $ 6,412 As previously disclosed in our 2018 Annual Report on Form 10-K, future minimum lease payments under the operating lease as of December 31, 2018 were as follows (in thousands): Year Ending December 31, 2019 $ 1,878 2020 1,933 2021 1,989 2022 2,046 2023 689 $ 8,535 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. This is a refundable deposit and not a lease payment. This has been excluded from the undiscounted cash flows above. License Agreement Under its license agreement with National University of Singapore and St. Jude Children’s Research Hospital, Inc. (collectively the “Licensors”) entered into in 2014, the Company is obligated to pay license maintenance fees on each anniversary of the effective date of the agreement that escalate from less than $0.1 million for each of the first seven years to $0.1 million on the eighth anniversary and each year thereafter. The Company is also obligated to make aggregate milestone payments of up to 5.5 million Singapore dollars (equivalent to approximately $4.0 million as of September 30, 2019) 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 September 30, 2019, the Company had non-cancelable minimum purchase commitments under contract manufacturing agreements for payments totaling $1.6 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 September 30, 2019 or December 31, 2018. 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

Net Loss Per Share	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	9. 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 September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Numerator: Net loss $ (11,923 ) $ (10,168 ) $ (34,130 ) $ (25,926 ) Accretion of redeemable convertible preferred stock to redemption value — — — (16 ) Net loss attributable to common stockholders $ (11,923 ) $ (10,168 ) $ (34,130 ) $ (25,942 ) Denominator: Weighted average common shares outstanding, basic and diluted 30,661,125 29,879,476 30,418,752 23,169,348 Net loss per share attributable to common stockholders, basic and diluted $ (0.39 ) $ (0.34 ) $ (1.12 ) $ (1.12 ) 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. 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 above because including them would have had an anti-dilutive effect: September 30, 2019 2018 Redeemable convertible preferred shares (as converted to common stock) — — Stock options to purchase common stock 4,663,988 3,768,371 4,663,988 3,768,371

Retirement Plan

Retirement Plan	9 Months Ended
	Sep. 30, 2019
Compensation And Retirement Disclosure [Abstract]
Retirement Plan	10. 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 and nine months ended September 30, 2019 and 2018.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2019
Subsequent Events [Abstract]
Subsequent Events	11. Subsequent Events Collaboration Agreement with Seattle Genetics In November 2019, the Company and Seattle Genetics suspended further development under the Collaboration Agreement, including suspension of further dose-escalation of the ATTCK-17-01 Phase 1 clinical trial, and the Company is reviewing the next steps with this program. In accordance with ASC 606, the Company has concluded that the performance obligations were not modified during the three or nine months ended September 30, 2019, and that the Company will account for the suspension of research and development activities in the fourth quarter of the fiscal year ended December 31, 2019.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Unaudited Interim Financial Information	Unaudited Interim Financial Information The consolidated balance sheet at December 31, 2018 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited consolidated financial statements as of September 30, 2019 and for the three and nine months ended September 30, 2019 and 2018 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, 2018 included in the Company’s Annual Report on Form 10-K, File No. 001-38443,
Concentrations of Credit Risk and of Significant Suppliers	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	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.
Fair Value Measurements	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	Marketable Securities The Company’s marketable securities, consisting of debt securities, are classified as available-for-sale and are reported at fair value. Unrealized gains and losses on available-for-sale debt securities are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Realized gains and losses and declines in value determined to be other than temporary are based on the specific identification method and are included as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss. The Company classifies its marketable securities with maturities beyond one year as short-term, based on their highly liquid nature and because such marketable securities are available for current operations. The Company evaluates its marketable securities with unrealized losses for other-than-temporary impairment. When assessing marketable securities for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.
Leases	Leases The Company accounts for a contract as a lease when it has the right to control the asset for a period of time while obtaining substantially all of the assets’ economic benefits. The Company determines the initial classification and measurement of its operating right-of-use assets and operating lease liabilities at the lease commencement date, and thereafter if modified. The lease term includes any renewal options that the Company is reasonably assured to exercise. The Company’s policy is to not record leases with an original term of twelve months or less on its consolidated balance sheets. The Company’s only existing lease is for office space. The right-of-use asset represents the right to use the leased asset for the lease term. The lease liability represents the present value of the lease payments under the lease. The present value of lease payments is determined by using the interest rate implicit in the lease, if that rate is readily determinable; otherwise, the Company uses its estimated secured incremental borrowing rate for that lease term. Lease payments included in the measurement of the lease liability consist of the following: the fixed noncancelable lease payments, payments for optional renewal periods where it is reasonably certain the renewal period will be exercised, and payments for early termination options unless it is reasonably certain the lease will not be terminated early. Leases may contain rent escalation clauses and variable lease payments that require additional rental payments in later years of the term, including payments based on an index or inflation rate. Payments based on the change in an index or inflation rate, or payments based on a change in the Company’s portion of the operating expenses, including real estate taxes and insurance, are not included in the initial lease liability and are recorded as a period expense when incurred. The operating leases may include an option to renew the lease term for various renewal periods and/or to terminate the leases early. These options to exercise the renewal or early termination clauses in the Company’s operating leases were not reasonably certain of exercise as of the date of adoption and these have not been included in the determination of the initial lease liability or operating lease expense. Rent expense for operating leases is recognized on a straight-line basis over the reasonably assured lease term based on the total lease payments and is included in operating expense in the consolidated statements of operations and comprehensive loss. For finance leases, any interest expense is recognized using the effective interest method and is included within interest expense. The Company has no financing leases.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates, assumptions and judgments that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates, assumptions and judgments reflected in these condensed consolidated financial statements include, but are not limited to, revenue, the accrual of research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.
Collaboration Agreements	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
Revenue Recognition for Collaboration Agreements	Revenue Recognition for Collaboration Agreements The Company performs the following five steps to determine revenue recognition for arrangements that are within the scope of ASC 606: (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 ASC 606, 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. ASC 606 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 recognizes revenue for its collaboration agreement 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 is 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 is 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, is 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 is 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 September 30, 2019, the Company had deferred revenue of $14.7 million related to its collaboration and all such amounts are classified as current. The Company recognized revenue of $1.0 and $7.2 million during the three and nine months ended September 30, 2019, respectively, from the deferred revenue balance at December 31, 2018. The Company recognizes deferred revenue by first allocating from the beginning deferred revenue balance to the extent that the beginning deferred revenue balance exceeds the revenue to be recognized. Billings during the period are added to the deferred revenue balance to be recognized in future periods. To the extent that the beginning deferred revenue balance is less than revenue to be recognized during the period, billings during the period are allocated to revenue. In the event that a collaboration agreement was to be terminated and the Company had no further performance obligations, the Company would recognize as revenue any portion of the upfront payment and other payments that had not previously been recorded as revenue and were classified as deferred revenue at the date of such termination. 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 September 30, 2019 and December 31, 2018, the Company has not capitalized any costs to obtain its contract.
Recently Adopted and Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842) Upon adoption of the new leasing standards, the Company recognized a lease liability of $7.5 million and a related right-of-use asset of $6.7 million on its consolidated balance sheet with the difference being due to the elimination of previously reported deferred rent. The adoption of the standard did not have a material impact on the results of operations or cash flows. In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting Recently Issued Accounting Pronouncements In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement, (“ASU 2018-13”). The new standard removes certain disclosures, modifies certain disclosures and adds additional disclosures related to fair value measurement. The new standard will be effective beginning January 1, 2020 and early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2018-13 will have on its consolidated financial statements. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606

Marketable Securities and Fai_2

Marketable Securities and Fair Value of Financial Assets and Liabilities (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Schedule of Marketable Securities by Type	Marketable securities by security type consisted of the following (in thousands): September 30, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. Treasury bills (due within one year) $ — $ — $ — $ — $ — $ — $ — $ — December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. Treasury bills and notes (due within one year) $ 22,935 $ — $ (12 ) $ 22,923 $ 22,935 $ — $ (12 ) $ 22,923
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 September 30, 2019 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ — $ 484 $ — $ 484 Marketable securities: U.S. Treasury bills — — — $ — $ — $ 484 $ — $ 484 Fair Value Measurements at December 31, 2018 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ — $ 52,100 $ — $ 52,100 Marketable securities: U.S. Treasury bills and notes 22,923 — — 22,923 $ 22,923 $ 52,100 $ — $ 75,023

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2019
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): September 30, 2019 December 31, 2018 Accrued employee compensation and benefits $ 2,259 $ 1,599 Accrued external research and development expense 2,618 1,799 Accrued external manufacturing costs 588 1,015 Other 1,073 1,064 $ 6,538 $ 5,477

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased 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 September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Research and development expenses $ 505 $ 629 $ 1,692 $ 1,637 General and administrative expenses 283 231 707 549 Total $ 788 $ 860 $ 2,399 $ 2,186

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Elements of Lease Expense	The elements of the lease expense were as follows (in thousands): Three Months Ended September 30, 2019 Nine Months Ended September 30, 2019 Lease cost Operating lease cost $ 443 $ 1,329 Variable lease cost (1) 254 835 Total lease cost $ 697 $ 2,164 Other information Operating cash flows used for operating leases $ 2,240 Remaining lease term 3.58 years Discount rate 6.25 % (1) The variable lease costs for the quarter ended September 30, 2019 include common area maintenance and other operating charges.
Summary of Future Minimum Payments under Operating Lease	The following table summarizes the future minimum payments due under the operating lease as of September 30, 2019 (in thousands): Year Ending December 31, 2019 $ 474 2020 1,933 2021 1,989 2022 2,046 2023 689 Total future minimum lease payments 7,131 Less: imputed interest 719 Total operating lease liability $ 6,412 Included in the consolidated balance sheet: Current operating lease liability $ 1,581 Operating lease liability, net of current portion 4,831 Total operating lease liability $ 6,412
Summary of Future Minimum Lease Payments Due under Operating Lease	As previously disclosed in our 2018 Annual Report on Form 10-K, future minimum lease payments under the operating lease as of December 31, 2018 were as follows (in thousands): Year Ending December 31, 2019 $ 1,878 2020 1,933 2021 1,989 2022 2,046 2023 689 $ 8,535

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2019
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 September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Numerator: Net loss $ (11,923 ) $ (10,168 ) $ (34,130 ) $ (25,926 ) Accretion of redeemable convertible preferred stock to redemption value — — — (16 ) Net loss attributable to common stockholders $ (11,923 ) $ (10,168 ) $ (34,130 ) $ (25,942 ) Denominator: Weighted average common shares outstanding, basic and diluted 30,661,125 29,879,476 30,418,752 23,169,348 Net loss per share attributable to common stockholders, basic and diluted $ (0.39 ) $ (0.34 ) $ (1.12 ) $ (1.12 )
Summary of Potential Dilutive Securities	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. 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 above because including them would have had an anti-dilutive effect: September 30, 2019 2018 Redeemable convertible preferred shares (as converted to common stock) — — Stock options to purchase common stock 4,663,988 3,768,371 4,663,988 3,768,371

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Apr. 01, 2019	Apr. 25, 2018	Apr. 03, 2018	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Net proceeds from initial public offering											$ 66,793
Net loss				$ (11,923)	$ (10,516)	$ (11,691)	$ (10,168)	$ (9,023)	$ (6,735)	$ (34,130)	$ (25,926)	$ (34,500)
Accumulated deficit				$ (126,189)						$ (126,189)		$ (92,059)
Funding term of borrowings for operating expense and capital expenditure requirement										12 months
Cowen [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares of common stock issued and sold										0
Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds from issuance of equity and debt financing	$ 150,000
Maximum [Member] | Cowen [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Stock offering cost	$ 50,000
Common Stock [Member] | IPO [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares of common stock issued and sold		215,000	5,770,000					5,985,000
Public offering price, per share		$ 12	$ 12
Net proceeds from initial public offering		$ 2,400	$ 61,500
Common Stock [Member] | Private Placement [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares of common stock issued and sold			416,666					416,666
Sale of stock, consideration received on transaction			$ 5,000

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Significant Accounting Policies [Line Items]
Deferred revenue	$ 14,717,000	$ 14,717,000		$ 17,949,000
Revenue recognized	1,000,000	$ 7,200,000
New accounting pronouncement revenue recognition timing		The Company recognized revenue of $1.0 and $7.2 million during the three and nine months ended September 30, 2019, respectively, from the deferred revenue balance at December 31, 2018. The Company recognizes deferred revenue by first allocating from the beginning deferred revenue balance to the extent that the beginning deferred revenue balance exceeds the revenue to be recognized. Billings during the period are added to the deferred revenue balance to be recognized in future periods. To the extent that the beginning deferred revenue balance is less than revenue to be recognized during the period, billings during the period are allocated to revenue.
Capitalized contract costs	0	$ 0		$ 0
Lease liability	6,412,000	6,412,000
Operating lease, right-of-use asset	$ 5,634,000	$ 5,634,000
Accounting Standards Update 2016-02 [Member]
Significant Accounting Policies [Line Items]
Lease liability			$ 7,500,000
Operating lease, right-of-use asset			$ 6,700,000
Maximum [Member]
Significant Accounting Policies [Line Items]
Expenses incremental costs expiration period		1 year
Period of Transfer of promised goods or services to customer		1 year

Marketable Securities and Fai_3

Marketable Securities and Fair Value of Financial Assets and Liabilities - Schedule of Marketable Securities by Type (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 0	$ 22,935
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(12)
Fair Value	0	22,923
U.S. Treasury Bills and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	22,935
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(12)
Fair Value	$ 0	$ 22,923

Marketable Securities and Fai_4

Marketable Securities and Fair Value of Financial Assets and Liabilities - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	$ 484	$ 75,023
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis		22,923
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	484	52,100
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents Measured at Fair Value on Recurring Basis	484	52,100
Money Market Funds [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents Measured at Fair Value on Recurring Basis	$ 484	52,100
Marketable securities [Member] | U.S. Treasury Bills and Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis		22,923
Marketable securities [Member] | Level 1 | U.S. Treasury Bills and Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis		$ 22,923

Marketable Securities and Fai_5

Marketable Securities and Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	Sep. 30, 2019	Sep. 30, 2018
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 Cu_3

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued employee compensation and benefits	$ 2,259	$ 1,599
Accrued external research and development expense	2,618	1,799
Accrued external manufacturing costs	588	1,015
Other	1,073	1,064
Total	$ 6,538	$ 5,477

Collaboration Agreement - Addit

Collaboration Agreement - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue related to research and clinical development activities	$ 1,020	$ 2,043	$ 7,211	$ 5,929
Seattle Genetics [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue related to research and clinical development activities	1,000	$ 2,000	7,200	$ 5,900
Deferred revenue	14,700		14,700
Remaining performance obligation related to two product candidates	$ 40,700		$ 40,700

Loan and Security Agreement - A

Loan and Security Agreement - Additional Information (Detail) - Loan and Security Agreement [Member] - Pacific Western Bank [Member]	9 Months Ended
	Sep. 30, 2019USD ($)
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	The Company has a loan and security agreement (the “Loan Agreement”) with Pacific Western Bank (“PWB”), entered into in 2017, which provided 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 were 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 January 2019, the Company amended the Loan Agreement to extend the available date for borrowings from January 19, 2019 to June 30, 2019 and extend the interest only period from January 19, 2019 to June 30, 2020, with the possibility of further extension to March 31, 2021 if certain equity financing considerations are met. Additionally, the loan repayment period will be over a 24-month period following the end of the interest-only period. In June 2019, the Company further amended the Loan Agreement to extend the available date for borrowings from June 30, 2019 to June 30, 2020. On July 31, 2019, the Company amended the Loan Agreement to provide for changes to the primary depository requirements with PWB.
Outstanding amount maturity date	Jan. 19, 2021
Term of loan	24 months
Term loan frequency of periodic payment	monthly
Term loans borrowed	$ 0
Term Loan [Member] | Prime Rate [Member]
Debt Instrument [Line Items]
Annual variable interest rate	0.25%

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - $ / shares	Jan. 01, 2019	Mar. 28, 2018	Mar. 27, 2018	Sep. 30, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock share issued				30,663,054	30,057,970
Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option grants				635,000
Weighted average grant date fair value				$ 2.46
Service Based Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option grants				1,665,418
Weighted average grant date fair value				$ 2.25
2018 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of increases in authorized shares			2,800,721
Percentage applied to the outstanding shares as annual increase in the number of shares authorized for issuance			4.00%
Increased in authorized shares reserved for issuance	1,202,319
Shares available for future issuance				2,372,248
2018 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increased in authorized shares reserved for issuance	300,580
Shares available for future issuance				614,580
Shares reserved for future issuance		314,000
Common stock share issued				0
2018 Employee Stock Purchase Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of increases in authorized shares		500,000
Percentage of shares of common stock available for issuance		1.00%

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share Based Compensation Expense	$ 788	$ 860	$ 2,399	$ 2,186
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share Based Compensation Expense	505	629	1,692	1,637
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share Based Compensation Expense	$ 283	$ 231	$ 707	$ 549

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - 9 months ended Sep. 30, 2019	USD ($)	SGD ($)
Commitments And Contingencies Disclosure [Line Items]
Lease expiration date	Apr. 30, 2023
Operating Lease option to extend	The Company’s option to extend for an additional five-year term.
Non-cancelable minimum purchase commitments	$ 1,600,000
License Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
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.
Eighth Year Anniversary and Thereafter [Member] | License Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
License maintenance fees	$ 100,000
Maximum [Member] | License Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent contractual obligation	4,000,000	$ 5,500,000
Maximum [Member] | First Seven Year Anniversary [Member] | License Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
License maintenance fees	100,000
Non-current restricted cash [Member] | Letter of Credit [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease security	$ 1,300,000

Commitments and Contingencies_2

Commitments and Contingencies - Elements of Lease Expense (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019USD ($)		Sep. 30, 2019USD ($)
Leases, Operating [Abstract]
Operating lease cost	$ 443		$ 1,329
Variable lease cost	254	[1]	835	[1]
Total lease cost	697		$ 2,164
Operating cash flows used for operating leases	$ 2,240
Remaining lease term	3 years 6 months 29 days		3 years 6 months 29 days
Discount rate	6.25%		6.25%
[1]	The variable lease costs for the quarter ended September 30, 2019 include common area maintenance and other operating charges.

Commitments and Contingencies_3

Commitments and Contingencies - Summary of Future Minimum Payments under Operating Lease (Detail) $ in Thousands	Sep. 30, 2019USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	$ 474
2020	1,933
2021	1,989
2022	2,046
2023	689
Total future minimum lease payments	7,131
Less: imputed interest	719
Total operating lease liability	6,412
Current operating lease liability	1,581
Operating lease liability, net of current portion	4,831
Total operating lease liability	$ 6,412

Commitments and Contingencies_4

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

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Earnings Per Share Basic and Diluted (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Earnings Per Share [Abstract]
Net loss	$ (11,923)	$ (10,516)	$ (11,691)	$ (10,168)	$ (9,023)	$ (6,735)	$ (34,130)	$ (25,926)	$ (34,500)
Accretion of redeemable convertible preferred stock to redemption value								(16)
Net loss attributable to common stockholders	$ (11,923)			$ (10,168)			$ (34,130)	$ (25,942)
Weighted average common shares outstanding, basic and diluted	30,661,125			29,879,476			30,418,752	23,169,348
Net loss per share attributable to common stockholders, basic and diluted	$ (0.39)			$ (0.34)			$ (1.12)	$ (1.12)

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Potential Dilutive Securities (Detail) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential dilutive securities excluded from computation of diluted net loss per common share	4,663,988	3,768,371
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	4,663,988	3,768,371

Retirement Plan - Additional In

Retirement Plan - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Compensation And Retirement Disclosure [Abstract]
Defined contribution plan, matching amount	$ 0	$ 0	$ 0	$ 0