Mersana Therapeutics (MRSN) 10-K 28 Feb 24 2023 FY Annual report Financial data

Cover Page

Cover Page - USD ($)	12 Months Ended
	Dec. 31, 2023	Feb. 23, 2024	Jun. 30, 2023
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2023
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-38129
Entity Registrant Name	Mersana Therapeutics, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	04-3562403
Entity Address, Address Line One	840 Memorial Drive
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	498-0020
Title of 12(b) Security	Common Stock, $0.0001 par value
Trading Symbol	MRSN
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity Public Float			$ 363,809,750
Entity Common Stock, Shares Outstanding		121,303,007
Documents Incorporated by Reference	Portions of the registrant’s definitive proxy statement that will be filed for the 2024 Annual Meeting of Stockholders within 120 days of the end of the registrant’s fiscal year ended December 31, 2023 are incorporated by reference in Part III of this Annual Report on Form 10-K to the extent stated herein.
Entity Central Index Key	0001442836
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
Amendment Flag	false

Audit Information

Audit Information	12 Months Ended
	Dec. 31, 2023
Auditor [Abstract]
Auditor Name	Ernst & Young LLP
Auditor Location	Boston, Massachusetts
Auditor Firm ID	42

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 174,561	$ 128,885
Short-term marketable securities	34,523	151,827
Accounts receivable	0	30,000
Prepaid expenses and other current assets	4,973	8,507
Total current assets	214,057	319,219
Property and equipment, net	3,831	3,985
Operating lease right-of-use assets	7,694	10,475
Other assets, noncurrent	478	661
Total assets	226,060	334,340
Current liabilities:
Accounts payable	7,319	13,951
Accrued expenses	21,898	43,184
Deferred revenue	28,147	30,610
Operating lease liabilities	3,252	2,798
Short-term debt	2,083	0
Other current liabilities	938	990
Total current liabilities	63,637	91,533
Operating lease liabilities, noncurrent	5,149	8,575
Long-term debt, net	23,148	24,929
Deferred revenue, noncurrent	97,167	117,043
Other liabilities, noncurrent	55	203
Total liabilities	189,156	242,283
Commitments (Note 15)
Stockholders' equity
Preferred stock, $0.0001 par value; 25,000,000 shares authorized; 0 shares issued and outstanding at December 31, 2023 and December 31, 2022, respectively	0	0
Common stock, $0.0001 par value; 350,000,000 shares authorized; 120,711,745 and 105,144,864 shares issued and outstanding at December 31, 2023 and December 31, 2022, respectively	12	11
Additional paid-in capital	863,242	746,889
Accumulated other comprehensive income (loss)	11	(152)
Accumulated deficit	(826,361)	(654,691)
Total stockholders’ equity	36,904	92,057
Total liabilities and stockholders’ equity	$ 226,060	$ 334,340
Common stock, shares authorized (in shares)	350,000,000	350,000,000

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2023	Dec. 31, 2022
Stockholders' equity
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	25,000,000	25,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	350,000,000	350,000,000
Common stock, shares issued (in shares)	120,711,745	105,144,864
Common stock, shares outstanding (in shares)	120,711,745	105,144,864

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Statement [Abstract]
Collaboration revenue	$ 36,855	$ 26,581	$ 43
Operating expenses:
Research and development	148,269	173,385	132,013
General and administrative	59,543	56,963	36,888
Restructuring expenses	8,713	0	0
Total operating expenses	216,525	230,348	168,901
Other income (expense):
Interest income	12,073	2,883	65
Interest expense	(4,073)	(3,328)	(1,267)
Total other income (expense), net	8,000	(445)	(1,202)
Net loss	(171,670)	(204,212)	(170,060)
Other comprehensive loss:
Unrealized gain (loss) on marketable securities	163	(152)	0
Comprehensive loss	(171,507)	(204,364)	(170,060)
Net loss attributable to common stockholders - basic	(171,670)	(204,212)	(170,060)
Net loss attributable to common stockholders - diluted	$ (171,670)	$ (204,212)	$ (170,060)
Net loss per share attributable to common stockholders — basic (in dollars per share)	$ (1.48)	$ (2.18)	$ (2.41)
Net loss per share attributable to common stockholders — diluted (in dollars per share)	$ (1.48)	$ (2.18)	$ (2.41)
Weighted-average number of shares of common stock used in net loss per share attributable to common stockholders — basic (in shares)	116,112,891	93,654,243	70,580,949
Weighted-average number of shares of common stock used in net loss per share attributable to common stockholders — diluted (in shares)	116,112,891	93,654,243	70,580,949

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	At Market Equity Offering Program	Common Stock	Common Stock At Market Equity Offering Program	Additional Paid-in Capital	Additional Paid-in Capital At Market Equity Offering Program	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at beginning of period (in shares) at Dec. 31, 2020			68,841,288
Balance at beginning of period at Dec. 31, 2020	$ 228,087		$ 7		$ 508,499		$ 0	$ (280,419)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock from at-the-market transactions, net of issuance costs (in shares)				3,961,074
Issuance of common stock from at-the-market transactions, net of issuance costs		$ 43,087				$ 43,087
Exercise of stock options (in shares)			421,381
Exercise of stock options	1,837				1,837
Vesting of restricted stock units, net of employee tax obligation (in shares)			407,060
Vesting of restricted stock units	(259)				(259)
Purchase of common stock under ESPP (in shares)			78,253
Purchase of common stock under ESPP	640				640
Stock-based compensation expense	18,409				18,409
Net loss	(170,060)							(170,060)
Balance at end of period (in shares) at Dec. 31, 2021			73,709,056
Balance at end of period at Dec. 31, 2021	121,741		$ 7		572,213		0	(450,479)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock from at-the-market transactions, net of issuance costs (in shares)				30,497,875
Issuance of common stock from at-the-market transactions, net of issuance costs		150,762		$ 4		150,758
Exercise of stock options (in shares)			414,914
Exercise of stock options	1,331				1,331
Exercise of common stock warrant (in shares)			16,654
Vesting of restricted stock units, net of employee tax obligation (in shares)			235,591
Purchase of common stock under ESPP (in shares)			270,774
Purchase of common stock under ESPP	1,065				1,065
Stock-based compensation expense	21,522				21,522
Other comprehensive income (loss)	(152)						(152)
Net loss	(204,212)							(204,212)
Balance at end of period (in shares) at Dec. 31, 2022			105,144,864
Balance at end of period at Dec. 31, 2022	$ 92,057		$ 11		746,889		(152)	(654,691)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock from at-the-market transactions, net of issuance costs (in shares)				14,464,531
Issuance of common stock from at-the-market transactions, net of issuance costs		$ 93,670		$ 1		$ 93,669
Exercise of stock options (in shares)	102,596		102,596
Exercise of stock options	$ 427				427
Vesting of restricted stock units, net of employee tax obligation (in shares)			618,246
Purchase of common stock under ESPP (in shares)			381,508
Purchase of common stock under ESPP	1,121				1,121
Stock-based compensation expense	21,136				21,136
Other comprehensive income (loss)	163						163
Net loss	(171,670)							(171,670)
Balance at end of period (in shares) at Dec. 31, 2023			120,711,745
Balance at end of period at Dec. 31, 2023	$ 36,904		$ 12		$ 863,242		$ 11	$ (826,361)

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
At Market Equity Offering Program
Issuance costs	$ 2,082	$ 3,476	$ 988

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities
Net loss	$ (171,670)	$ (204,212)	$ (170,060)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	1,517	927	855
Net amortization of premiums and discounts on marketable securities	(4,569)	(1,462)	0
Stock-based compensation	21,136	21,522	18,409
Non-cash operating lease expense	2,780	2,777	1,829
Other non-cash items	687	763	723
Changes in operating assets and liabilities:
Accounts receivable	30,000	(30,000)	0
Prepaid expenses and other current assets	3,534	3,863	(2,734)
Other assets	0	0	(718)
Accounts payable	(6,080)	947	483
Accrued expenses	(20,935)	13,594	12,570
Operating lease liabilities	(3,143)	(2,539)	(1,827)
Deferred revenue	(22,139)	143,709	(43)
Other liabilities	0	748	525
Net cash used in operating activities	(168,882)	(49,363)	(139,988)
Cash flows from investing activities
Maturities of marketable securities	277,970	97,000	0
Purchase of marketable securities	(155,919)	(247,519)	0
Purchase of property and equipment	(2,168)	(2,197)	(648)
Net cash provided by (used in) investing activities	119,883	(152,716)	(648)
Cash flows from financing activities
Proceeds from exercise of stock options	427	1,331	1,837
Proceeds from purchases of common stock under ESPP	1,121	1,065	640
Payment of employee tax obligations related to vesting of restricted stock units	0	0	(259)
Proceeds from issuance of debt, net of issuance costs	(150)	0	24,042
Repayment of debt	0	0	(5,486)
Payments under finance lease obligations	(262)	(272)	(215)
Net cash provided by financing activities	94,675	153,017	63,646
Increase (decrease) in cash, cash equivalents and restricted cash	45,676	(49,062)	(76,990)
Cash, cash equivalents and restricted cash, beginning of period	129,363	178,425	255,415
Cash, cash equivalents and restricted cash, end of period	175,039	129,363	178,425
Supplemental disclosures of non-cash activities:
Purchases of property and equipment in accounts payable and accrued expenses	132	753	0
Debt financing costs in accrued expenses	0	150	0
Common stock issuance costs in accounts payable and accrued expenses	0	131	0
Cash paid for interest	3,380	2,463	429
Right-of-use assets obtained in exchange for operating lease liabilities	0	298	3,783
Right-of-use assets obtained in exchange for financing lease liabilities	0	0	609
At Market Equity Offering Program
Cash flows from financing activities
Net proceeds from at-the-market facilities	$ 93,539	$ 150,893	$ 43,087

Nature of business and basis of

Nature of business and basis of presentation	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of business and basis of presentation	Nature of business and basis of presentation Mersana Therapeutics, Inc. is a clinical-stage biopharmaceutical company focused on developing antibody-drug conjugates ("ADCs") that offer a clinically meaningful benefit for cancer patients with significant unmet need. The Company’s next-generation ADC platforms include Dolasynthen, which delivers a proprietary auristatin payload, and Immunosynthen, which delivers a proprietary stimulator of interferon genes ("STING") agonist payload. The Company is investigating XMT-1660, a B7-H4-directed Dolasynthen ADC, in a Phase 1 clinical trial enrolling patients with solid tumors, including in breast, endometrial and ovarian cancers. The Company initiated a Phase 1 clinical trial to investigate XMT-2056, an Immunosynthen STING-agonist ADC that is designed to target a novel epitope of human epidermal growth factor receptor 2 ("HER2"), in January 2023, enrolling previously treated patients with advanced/recurrent solid tumors expressing HER2, including breast, gastric, colorectal and non-small cell lung cancers. In March 2023, following a voluntary suspension of this clinical trial by the Company, this clinical trial was placed on clinical hold by the U.S. Food and Drug Administration ("FDA"), and the FDA lifted this clinical hold in October 2023. The Company also has two additional earlier stage preclinical candidates, XMT-2068 and XMT-2175, that leverage the Company's Immunosynthen platform. In July 2023, the Company announced top-line data from its Phase 2 UPLIFT clinical trial of upifitamab rilsodotin ("UpRi"), which did not meet its primary endpoint. In connection with this announcement, on July 27, 2023, the Company further announced that its primary focus moving forward would be on advancing product candidates and collaborations utilizing its next-generation ADC platforms, Dolasynthen and Immunosynthen. As a result, the Company wound down its UpRi-related development activities and its regulatory and commercial readiness efforts and terminated its UPGRADE-A and Phase 3 UP-NEXT clinical trials of UpRi, on which the FDA had placed a partial clinical hold in June 2023. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, the need for additional capital, risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval and reimbursement for any drug product candidate that it may identify and develop, the need to successfully commercialize and gain market acceptance of its product candidates, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development of technological innovations by competitors, reliance on third party manufacturers and the ability to transition from pilot-scale production to large-scale manufacturing of products. The Company has incurred cumulative net losses since inception. The Company’s net loss was $171.7 million, $204.2 million and $170.1 million for the years ended December 31, 2023, 2022 and 2021, respectively. The Company expects to continue to incur operating losses for at least the next several years. As of December 31, 2023, the Company had an accumulated deficit of $826.4 million. The future success of the Company is dependent on, among other factors, its ability to identify and develop its product candidates and ultimately upon its ability to attain profitable operations. The Company has devoted substantially all of its financial resources and efforts to research and development and general and administrative expense to support such research and development. Net losses and negative operating cash flows have had, and will continue to have, an adverse effect on the Company’s stockholders’ equity and working capital. The Company believes that its currently available funds will be sufficient to fund the Company’s operations through at least the next twelve months from the issuance of this Annual Report on Form 10-K. Management’s belief with respect to its ability to fund operations is based on estimates that are subject to risks and uncertainties. If actual results are different from management’s estimates, the Company may need to seek additional funding. The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States ("U.S. GAAP") and the rules and regulations of the Securities and Exchange Commission ("SEC"). Any reference in these notes to applicable guidance is meant to refer to the authoritative U.S. GAAP as found in the Accounting Standards Codification ("ASC") and Accounting Standards Updates ("ASU") of the Financial Accounting Standards Board ("FASB").

Summary of significant accounti

Summary of significant accounting policies	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of significant accounting policies Principles of Consolidation The accompanying consolidated financial statements include those of the Company and its wholly owned subsidiary, Mersana Securities Corp. All intercompany balances and transactions have been eliminated. Use of Estimates The preparation of the Company's consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, equity, revenue, expenses and related disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenue and expenses during the reporting period. On an ongoing basis, the Company's management evaluates its estimates which include, but are not limited to, management's judgments with respect to the identification of performance obligations and standalone selling prices of those performance obligations within its revenue arrangements, accrued preclinical, manufacturing and clinical expenses, valuation of stock-based awards and income taxes. Actual results could differ from those estimates. Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision-maker, or decision making group, in deciding how to allocate resources and assess performance. The Company views its operations and manages its business as a single operating segment, which is the business of discovering and developing ADCs. Research and Development Research and development costs are expensed as incurred and include: • employee-related expenses, including salaries, bonuses, benefits, travel and stock-based compensation expense; • fees and expenses incurred under agreements with contract research organizations, investigative sites and other entities in connection with the conduct of preclinical studies, clinical trials and related services; • the cost of acquiring, developing and manufacturing ADC product candidates, clinical trial materials and other research and development materials; • fees and costs related to regulatory filings and activities; • costs associated with collaboration agreements and license fees and milestone payments related to license agreements; • costs associated with developing a NaPi2b diagnostic biomarker; • facilities, depreciation and other expenses, which include direct and allocated expenses for rent, utilities, maintenance of facilities, insurance and other supplies; and • other costs associated with clinical, preclinical, discovery and other research activities. Costs for certain development activities, such as preclinical studies, clinical trials and manufacturing development activities, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, and information provided to the Company by its vendors on their actual costs incurred or level of effort expended. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as prepaid or accrued preclinical, manufacturing and clinical expenses. Revenue Recognition The Company enters into collaboration agreements which are within the scope of ASC 606, Revenue from Contracts with Customers ("ASC 606"), under which the Company licenses rights to its technology and certain of the Company’s product candidates and performs research and development services for third parties. The terms of these arrangements typically include payment of one or more of the following: non-refundable, up-front fees; reimbursement of research and development costs; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of contract(s) with a customer; (ii) determination of whether the promised goods or services are performance obligations; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration to which it is entitled in exchange for the goods or services it transfers to the customer. The promised good or services in the Company’s arrangements typically consist of license rights to the Company’s intellectual property and research and development services. The Company also has optional additional items in contracts, which are considered marketing offers and are accounted for as separate contracts with the customer if such option is elected by the customer, unless the option provides a material right which would not be provided without entering into the contract. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer. Promised goods or services are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources or (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on their own and the availability of the required expertise. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include both fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration and at each reporting period, the Company evaluates the amount of potential payment and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected value method to estimate the amount expected to be received based on which method better predicts the amount of consideration to which the Company will be entitled. If it is probable that a significant revenue reversal would not occur, the variable consideration is included in the transaction price. The Company assessed each of its revenue generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in any of its arrangements because: (a) the promised consideration approximates the cash selling price of the promised goods and services; and (b) timing of payment approximates the transfer of goods and services and performance is over a relatively short period of time within the context of the entire term of the contract. The Company’s contracts often include development and regulatory milestone payments. At contract inception and at each reporting period, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not included in the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect license, collaboration and other revenues and earnings in the period of adjustment. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of the Company’s collaboration arrangements. The Company allocates the transaction price based on the estimated standalone selling price of the underlying performance obligations or in the case of certain variable consideration to one or more performance obligations. The Company must develop assumptions that require judgment to determine the standalone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the standalone selling price, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs to complete the respective performance obligation. Certain variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated to each performance obligation are consistent with the amounts the Company would expect to receive for each performance obligation. For performance obligations consisting of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, up-front fees. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company will recognize revenue from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. The Company receives payments from its customers based on billing schedules established in each contract. Such billings generally have 30-day terms. Up-front payments and fees are recorded as a contract liability (deferred revenue) upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the right to consideration is unconditional and only the passage of time is required before payment is due. If the right to consideration is subject to a condition other than the passage of time, then the amount is recorded as a contract asset until the right to payment becomes unconditional. In accordance with ASC 606, the Company presents contract assets and contract liabilities on a net basis by customer contract. Collaborative Arrangements The Company records the elements of its collaboration agreements that represent joint operating activities in accordance with ASC 808, Collaborative Arrangements ("ASC 808"). Accordingly, the elements of the collaboration agreements that represent activities in which both parties are active participants and to which both parties are exposed to the significant risks and rewards that are dependent on the commercial success of the activities, are recorded as collaborative arrangements. The Company also considers the guidance in ASC 606 by analogy in determining the appropriate treatment for the transactions between the Company and its collaborators and the transactions between the Company and third parties. Generally, the classification of transactions under the collaborative arrangements is determined based on the nature and contractual terms of the arrangement along with the nature of the operations of the participants. To the extent revenue is generated from a collaboration, the Company will recognize its share of the net sales on a gross basis if it is deemed to be the principal in the transactions with customers, or on a net basis if it is instead deemed to be the agent in the transactions with customers, consistent with the guidance in ASC 606. Fair Value Measurements Fair value is defined as the price that would be received upon sale of an asset or paid to transfer a liability between market participants at measurement dates. ASC 820, Fair Value Measurement , establishes a three-level valuation hierarchy for instruments measured at fair value. The hierarchy is based on the transparency of inputs to the valuation of an asset or liability as of the measurement date. The three levels are defined as follows: Level 1—Inputs to the valuation methodology are quoted prices (unadjusted) for identical assets or liabilities in active markets. Level 2—Inputs to the valuation methodology include quoted prices for similar assets and liabilities in active markets, and inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the financial instrument. Level 3—Inputs to the valuation methodology are unobservable and significant to the fair value measurement. Marketable Securities The Company’s investment strategy is focused on capital preservation. The Company invests in instruments that meet the credit quality standards outlined in the Company’s investment policy. Short-term marketable securities consist of investments in debt securities with maturities greater than three months and less than one year from the balance sheet date. The Company classifies all of its marketable securities as available-for-sale. Accordingly, these investments are recorded at fair value. Fair value is determined based on quoted market prices. Amortization and accretion of discounts and premiums are recorded as interest income within other income (expense), net. Realized gains and losses are included in other income (expense), net. The Company assesses its available-for-sale debt securities under the available-for-sale debt security impairment model in ASC 326, Financial Instruments - Credit Losses , as of each reporting date in order to determine if a portion of any decline in fair value below carrying value recognized on its available-for-sale debt securities is the result of a credit loss. The Company records credit losses in the consolidated statements of operations and comprehensive loss as a component of other income (expense), net, which is limited to the difference between the fair value and the amortized cost of the security. To date, the Company has not recorded any credit losses on its available-for-sale debt securities. Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity, or a remaining maturity at the time of purchase, of three months or less to be cash equivalents. The Company invests excess cash primarily in money market funds, commercial paper and government agency securities, which are highly liquid and have strong credit ratings. These investments are subject to minimal credit and market risks. Cash and cash equivalents are stated at cost, which approximates market value. Accounting for Stock-based Compensation The Company accounts for its stock-based compensation in accordance with ASC 718, Compensation—Stock Compensation ("ASC 718"). ASC 718 requires all stock-based payments to employees, directors and non-employees to be recognized as expense in the statements of operations based on their grant date fair values. The Company estimates the fair value of options granted using the Black-Scholes option pricing model. The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. The expected stock price volatility is calculated based on a period of time commensurate with the expected term assumption. Historically, due to the lack of a public market for the Company's common stock prior to completion of its initial public offering and a lack of company-specific historical and implied volatility data, the Company based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The computation of expected volatility was based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. During 2022, the Company began to estimate its volatility by using a blend of its stock price history for the length of time it has market data for its stock and the historical volatility of similar public companies for the expected term of each grant. During 2023, the Company began to estimate its volatility solely using its stock price history. The Company uses the simplified method as prescribed by the SEC Staff Accounting Bulletin No. 107, Share-Based Payment , to calculate the expected term for options granted to employees as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. For option grants with an expected term for which sufficient stock price history for the Company exists, expected stock price volatility is calculated using the average of volatilities for the period of the expected term prior to the grant date. For options granted to non-employees, the Company utilizes the contractual term of the option arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to do so. The Company determines the fair value of each restricted stock unit ("RSU") at its grant date based on the closing market price of the Company’s common stock on that date or, if the date of grant is not a day on which the Company's primary trading market was open, the immediately preceding trading day. For stock-based compensation subject to service-based vesting conditions, the Company recognizes stock-based compensation expense equal to the grant date fair value of stock-based compensation on a straight-line basis over the requisite service period. The Company records forfeitures as a cumulative adjustment in the period in which they occur. Property and Equipment Property and equipment is stated at cost, less accumulated depreciation. Depreciation is computed using the straight-line method over the estimated useful life of each asset as follows: Computer equipment, office equipment and software 3 years Laboratory equipment 5 years Leasehold improvements Shorter of useful life or life of lease Upon retirement or sale, the cost of the disposed assets and the related accumulated depreciation are eliminated from the balance sheet, and related gains or losses are reflected in the statements of operations and comprehensive loss. There were no material sales of assets during the years ended December 31, 2023, 2022 and 2021. The Company reviews its long-lived assets for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. If the Company performs an impairment review to evaluate an asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the asset to its carrying value. If the carrying amount of the asset exceeds its estimated undiscounted future net cash flows, the Company recognizes an impairment charge in the amount by which the carrying amount of the asset exceeds the fair value of the asset. The Company did not recognize impairment charges during the years ended December 31, 2023, 2022 and 2021. Leases Consistent with ASC 842, Leases , the Company determines if an arrangement is a lease at inception. Operating leases are included in right-of-use lease assets ("ROU assets"), current portion of lease obligations and long-term lease obligations on the Company’s consolidated balance sheets. Assets subject to finance leases are included in property and equipment, and the related lease obligation is included in other current liabilities and other long-term liabilities on the Company’s consolidated balance sheets. Lease assets are tested for impairment in the same manner as long-lived assets used in operations. Lease expense for operating leases is recognized on a straight-line basis over the lease term as an operating expense while expense for financing leases is recognized as depreciation expense and interest expense using the effective interest method. The Company has elected the short-term lease recognition exemption for short-term leases, which allows the Company not to recognize lease liabilities and ROU assets on the consolidated balance sheets for leases with an original term of twelve months or less. ROU assets represent the Company’s right to use an underlying asset for the lease term, and lease obligations represent the Company’s obligation to make lease payments arising from the lease. Operating lease liabilities and their corresponding ROU assets are initially recorded based on the present value of lease payments over the expected remaining lease term. When determining the lease term, the Company includes options to extend or terminate the lease when it is reasonably certain that the option will be exercised. Certain adjustments to the ROU asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate to discount lease payments. The incremental borrowing rate reflects the fixed rate at which the Company could borrow, on a collateralized basis, the amount of the lease payments in the same currency, for a similar term, in a similar economic environment. The Company accounts for lease agreements with lease and non-lease components separately. Patent Costs The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the accompanying consolidated statements of operations and comprehensive loss. Income Taxes The Company accounts for income taxes in accordance with ASC 740, Accounting for Income Taxes ("ASC 740"), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. The Company determines its deferred tax assets and liabilities based on differences between financial reporting and tax bases of assets and liabilities, which are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Valuation allowances are provided if, based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company recognizes the effect of income tax positions only if those positions are more likely than not to be sustained. Recognized income tax positions are measured at the largest amount that is more likely than not to be realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. Comprehensive Income (Loss) Comprehensive income (loss) comprises net loss and other comprehensive loss. For the years ended December 31, 2023 and 2022, other comprehensive income (loss) consisted of changes in unrealized income and loss on marketable securities. For the year ended December 31, 2021, comprehensive loss equaled net loss. Concentration of Credit Risk and Off-balance Sheet Risk Financial instruments that potentially subject the Company to concentrations of credit risk primarily consist of cash equivalents and marketable securities. Under its investment policy, the Company limits amounts invested in such securities by credit rating, maturity, industry group, investment type and issuer, except for securities issued by the U.S. government. The Company does not believe that it is subject to any significant concentrations of credit risk from these financial instruments. The Company has no financial instruments with off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Recently Issued Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard-setting bodies, and the Company adopts such pronouncements as of the specified effective date. Unless otherwise discussed below, the Company does not believe that the adoption of recently issued standards has had or may have a material impact on the Company's consolidated financial statements or disclosures. In November 2023, the FASB issued Accounting Standard Update 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures , which is intended to improve reportable segment disclosure requirements, primarily through additional disclosures about significant segment expenses. The standard is effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024, with early adoption permitted. The amendments should be applied retrospectively to all prior periods presented in the financial statements. The Company is currently evaluating the disclosure requirements related to the new standard. In December 2023, the FASB, issued Accounting Standard Update 2023-09, Improvements to Income Tax Disclosures , which requires entities to disclose disaggregated information about their effective tax rate reconciliation as well as expanded information on income taxes paid by jurisdiction. The disclosure requirements will be applied on a prospective basis, with the option to apply them retrospectively. The standard is effective for fiscal years beginning after December 15, 2024, with early adoption permitted. The Company is currently evaluating the disclosure requirements related to the new standard.

Collaboration agreements

Collaboration agreements	12 Months Ended
	Dec. 31, 2023
Collaboration agreements
Collaboration agreements	Collaboration agreements GSK On August 6, 2022, the Company entered into a Collaboration, Option and License Agreement (the "GSK Agreement") with GlaxoSmithKline Intellectual Property (No. 4) Limited ("GSK"), pursuant to which the Company granted GSK an exclusive option to obtain an exclusive license (the “Option”) to co-develop and to commercialize products containing XMT-2056 (the "Licensed Products"), exercisable within a specified time period (the “Option Period”) after the Company delivers to GSK data resulting from completion of dose escalation with enrichment for breast cancer patients in a Phase 1 single-agent clinical trial of XMT-2056. GSK’s exercise of the Option may require clearance under the Hart-Scott-Rodino Antitrust Improvements Act of 1976 (“HSR Clearance” and GSK’s exercise of the Option following any applicable HSR Clearance, the “GSK Option Exercise”). Prior to the GSK Option Exercise, the Company will lead and will be responsible for the costs of manufacturing, research, and early clinical development related to its XMT-2056 program. After the GSK Option Exercise, if any, GSK has the right to elect to manufacture XMT-2056, and GSK and the Company will co-develop XMT-2056 aimed at the approval of Licensed Product(s) in the United States and the European Union, with GSK being responsible for the majority of the development costs. GSK will be responsible for all development costs aimed solely at gaining approval outside the United States and European Union. Pursuant to the GSK Agreement, following the GSK Option Exercise and subject to certain exceptions and specified payment obligations, the Company’s aggregate shared development costs are capped at a fixed amount, with any amounts in excess to be borne by GSK unless and until the Company exercises its option to receive (or bear) a specified share of U.S. profits (or losses) for any Licensed Products (“Profit Share Election”). The excess development costs will accrue interest as specified in the GSK Agreement and will later either be repaid by the Company or offset against future regulatory and sales milestones or royalty payments that may become due to the Company. If the Company exercises its Profit Share Election, the cap on the Company’s share of development costs shall no longer apply, and the Company must pay any then-outstanding excess plus accrued interest costs. Additionally, if the Company exercises its Profit Share Election, it may also simultaneously elect to co-promote any Licensed Products in the United States. Pursuant to the GSK Agreement, GSK paid the Company a non-refundable, upfront fee of $100.0 million in August 2022. Following the GSK Option Exercise, if any, GSK is obligated to pay the Company an option exercise payment of $90.0 million (the "Option Payment"). The Company is eligible to receive future development, regulatory, and commercial milestone payments up to approximately $1.3 billion and, if the Company does not exercise its Profit Share Election, tiered royalties up to the mid-twenty percent range based on global sales of Licensed Products. Included in the aggregate milestone payments amount is $30 million that the Company is eligible to earn upon the satisfaction of early clinical development milestones that may occur prior to the GSK Option Exercise. If the Company exercises its Profit Share Election, the Company will be eligible to receive reduced development, regulatory, and commercial milestone payments and reduced royalty rates on sales outside of the United States. Whether or not the Company exercises its Profit Share Election, GSK will be responsible for certain milestone payments or royalties due to specified third parties with which the Company currently has agreements that relate to the XMT-2056 program. The GSK Agreement will terminate at the end of the Option Period if GSK does not exercise its Option. In the event of the GSK Option Exercise, unless earlier terminated, the GSK Agreement will continue in effect until the date on which the royalty term and all payment obligations with respect to all Licensed Products in all countries have expired. Accounting Analysis The Company assessed the GSK Agreement in accordance with ASC 606 and concluded that the contract counterparty, GSK, is a customer. The Company identified the following two material performance obligations under the GSK Agreement: (i) development activities, including manufacturing, research and early clinical development activities, necessary to deliver the package of data, information and materials specified in the GSK agreement (the "Development Activities") and (ii) the Option to co-develop and to commercialize Licensed Products (the "License Option"). The Company concluded that the Development Activities are one distinct performance obligation, as the underlying activities are not distinguishable in the context of the contract and are inputs to an integrated development program that will generate data and information providing value to GSK in determining whether to exercise the Option. The License Option is considered a material right as the value of the license exceeds the Option Payment, and is therefore a distinct performance obligation. In accordance with ASC 606, the Company determined that the initial transaction price under the GSK Agreement equals $100.0 million, consisting of the upfront, non-refundable and non-creditable payment paid by GSK. None of the early clinical development milestones that may occur prior to the GSK Option Exercise have been included in the initial transaction price, as all milestone amounts were fully constrained. As part of its evaluation of the constraint, the Company considered numerous factors, including stage of development and the remaining risks associated with the development required to achieve the milestones, as well as whether the achievement of the milestones is outside the control of the Company or GSK. The GSK Option payment is excluded from the initial transaction price at contract inception along with any future development, regulatory, and commercial milestone payments (including royalties) following the GSK Option Exercise. Consistent with the allocation objective under ASC 606, the Company allocated the $100.0 million fixed upfront payment in the transaction price to the Development Activities and the License Option based on each performance obligation’s relative standalone selling price. The standalone selling price for the Development Activities was calculated using a cost-plus margin approach for the estimated pre-option development timeline. For the standalone selling price of the License Option, the Company utilized an income-based approach which included the following key assumptions: post-option development timeline and costs, revenue forecast, discount rates and probabilities of technical and regulatory success. The Company is recognizing revenue related to the Development Activities performance obligation over the estimated period of the pre-option development using a proportional performance model as the underlying activities are performed. Th e C ompany measures proportional performance based on the costs incurred relative to the total costs expected to be incurred. The Company deferred revenue recognition related to the License Option. If the License Option is exercised and GSK obtains an exclusive license, the Company will recognize revenue as it fulfills its obligations under the GSK Agreement. If the Option is not exercised, the Company will recognize the entirety of the revenue in the period when the Option expires. During the years ended December 31, 2023 and 2022, the Company recorded collaboration revenue of $3.4 million and $2.0 million, respectively, related to its efforts under the GSK Agreement. As of December 31, 2023 and 2022 , the Company had recorded $94.6 million and $98.0 million, respectively, in deferred revenue related to the unsatisfied performance obligations under the GSK Agreement. This deferred revenue will be recognized over the remaining performance period and classified as current or noncurrent on the consolidated balance sheets based upon the expected timing of satisfaction of the performance obligations. Johnson & Johnson In February 2022, the Company entered into a research collaboration and license agreement with Janssen Biotech Inc. ("Johnson & Johnson" and such agreement, as amended on July 14, 2023 and September 25, 2023, the "Johnson & Johnson Agreement") focused on the research, development and commercialization of novel ADCs for three oncology targets by leveraging Mersana’s ADC expertise and Dolasynthen platform with Johnson & Johnson’s proprietary antibodies. Upon execution of the Johnson & Johnson Agreement, the Company received a non-refundable upfront payment of $40.0 million from Johnson & Johnson. Pursuant to the Johnson & Johnson Agreement, the Company granted Johnson & Johnson two exclusive, nontransferable, worldwide licenses - a research license and a commercialization license (together, the "Johnson & Johnson Licenses"). The research license that forms a part of the Johnson & Johnson Licenses provides Johnson & Johnson, on a target-by-target basis, rights under the Company’s technology and the Company’s interest in the technology developed jointly through the collaboration solely to conduct Johnson & Johnson’s activities under the research and Chemistry, Manufacturing and Controls ("CMC") plans with respect to each target. The commercialization license that forms a part of the Johnson & Johnson Licenses is a royalty-bearing license granted on a target-by-target basis under the Company’s technology and the Company’s interest in the technology developed jointly through the collaboration to develop, manufacture, commercialize and otherwise exploit licensed ADCs and any licensed products containing licensed ADCs directed toward a target. Johnson & Johnson may select up to three targets and may substitute each target once prior to a substitution deadline. Johnson & Johnson is not required to pay a fee for its first substitution right, but must pay a one-time fee for access to the subsequent substitution rights following its exercise of its second substitution right. During the year ended December 31, 2023, Johnson & Johnson exercised its first substitution right for a certain target. Pursuant to mutually agreed research and CMC plans, the Company agreed to perform bioconjugation, production development, preclinical manufacturing, and certain related research and preclinical development activities, in order to progress the targets through investigational new drug application ("IND") submission for further development, manufacture and commercialization by Johnson & Johnson. Johnson & Johnson will have sole responsibility for IND-enabling studies, IND submission, clinical development, regulatory activities and commercialization of the licensed ADCs. Both the Company and Johnson & Johnson will have equal representation on a Joint Research Committee and Joint Manufacturing Committee to oversee the research and CMC activities. The Company estimates that its activities under the research plans for the targets will be performed into 2025. Johnson & Johnson is required to pay for the Company's CMC activities at agreed upon rates. Assuming successful development and commercialization of all three targets by Johnson & Johnson, the Company is eligible to receive up to $505 million in development and regulatory milestones and $530 million in sales milestones, as well as tiered mid single-digit to low double-digit royalties on aggregate net sales of the ADC products. Unless earlier terminated, the Johnson & Johnson Agreement will expire upon the expiration of the last royalty term for a product under the Johnson & Johnson Agreement. The Johnson & Johnson Agreement contains customary provisions for termination by either party, including in the event of breach of the Johnson & Johnson Agreement, subject to cure, by Johnson & Johnson for convenience and by the Company upon a challenge of the licensed patents, and customary provisions regarding the effects of termination. Johnson & Johnson may request that the Company perform clinical manufacturing services under a separate clinical supply agreement. Johnson & Johnson may also request that the Company perform a technology transfer of bioconjugation and manufacturing process technology, at Johnson & Johnson's cost, at an agreed upon rate. Accounting Analysis The Company assessed the Johnson & Johnson Agreement in accordance with ASC 606 and concluded that the contract counter party, Johnson & Johnson, is a customer. The Company identified the following seven material performance obligations under the Johnson & Johnson Agreement: (i) exclusive Johnson & Johnson Licenses and research activities for each of the three designated targets, (ii) CMC activities for each of the three designated targets and (iii) the first target substitution right. The Company concluded that the Johnson & Johnson Licenses and research activities are one combined performance obligation for each target as the Johnson & Johnson Licenses are not capable of being distinct from the research activities given their proprietary nature. The CMC activities are considered a distinct performance obligation for each target as the activities could be performed by a third-party provider. The first target substitution right is considered a material right as there is no option exercise fee and, as such, is a distinct performance obligation. In accordance with ASC 606, the Company determined that the initial transaction price under the Johnson & Johnson Agreement equals $40.0 million, consisting of the upfront, non-refundable and non-creditable payment. None of the development and the regulatory milestones were included in the initial transaction price, as all milestone amounts were fully constrained. As part of its evaluation of the constraint, the Company considered numerous factors, including stage of development and the remaining risks associated with the development required to achieve the milestones, as well as whether the achievement of the milestones is outside the control of the Company or Johnson & Johnson. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as such milestones were determined to relate predominantly to the license granted to Johnson & Johnson and therefore have also been excluded from the transaction price. As of December 31, 2023, the revised total transaction price for the Johnson & Johnson Agreement was $48.0 million. During 2023, the Company revised the estimated transaction price by $6.0 million based on the reassessment of the constraint of certain development milestones and the remaining risks associated with the development required to achieve the milestones. The Company determined that the consideration for CMC activities represents variable consideration. CMC activities for one of the three designated targets have been initiated. The Company has elected to apply the Right to Invoice practical expedient under ASC 606 related to the CMC activities. As such, the Company will recognize revenue related to the CMC activities when the services are performed over the corresponding CMC plan for a given target. Consistent with the allocation objective under ASC 606, the Company allocated the total transaction price to the Johnson & Johnson Licenses and research activities and first substitution right based on each performance obligation’s relative standalone selling price. Each of the standalone selling prices for the Johnson & Johnson Licenses and research activities and for the first substitution right were estimated utilizing an income approach, along with the likelihood of exercise for the substitution right and included the following key assumptions: the development timeline, revenue forecast, discount rate and probabilities of technical and regulatory success. Due to Johnson & Johnson's exercise of its first substitution right, the transaction price related to that performance obligation has been reallocated to the Johnson & Johnson Licenses and research activities for the three designated targets. The Company is recognizing revenue related to the Johnson & Johnson Licenses and research services performance obligation over the estimated period of the research services using a proportional performance model. The Company measures proportional performance based on the costs incurred relative to the total costs expected to be incurred. During the years ended December 31, 2023 and 2022, the Company recorded collaboration revenue of $16.6 million and $24.2 million, respectively, related to its performance obligations under the Johnson & Johnson Agreement. As of December 31, 2023 and 2022, the Company had recorded $10.4 million and $15.8 million, respectively, in deferred revenue related to the Johnson & Johnson Agreement that will be recognized over the remaining performance period and classified as current or noncurrent on the consolidated balance sheets based upon the expected timing of satisfaction of respective performance obligations. Merck KGaA Immunosynthen Platform Agreement In December 2022, the Company entered into a research collaboration and license agreement with Ares Trading S.A., a wholly owned subsidiary of Merck KGaA, Darmstadt, Germany (Merck KGaA and/or its affiliate, as applicable, "Merck KGaA" and such agreement, the "2022 Merck KGaA Agreement"), focused on the research, development and commercialization of novel ADCs for up to two specific target antigens by leveraging Mersana’s ADC expertise and Immunosynthen platform with Merck KGaA’s proprietary antibodies. In connection with the 2022 Merck KGaA Agreement, the Company received a non-refundable upfront payment of $30.0 million. Pursuant to the 2022 Merck KGaA Agreement, the Company granted Merck KGaA two exclusive, non-transferable, worldwide licenses - a research license and a commercialization license (together, the "Merck KGaA Licenses"). The research license that forms a part of the Merck KGaA Licenses provides Merck KGaA, on a target-by-target basis, rights under the Company’s technology and the Company’s interest in the technology developed jointly through the collaboration solely to conduct Merck KGaA’s activities under the research and CMC plans with respect to each target. The commercialization license that forms a part of the Merck KGaA Licenses is a royalty-bearing license granted on a target-by-target basis under the Company’s technology and the Company’s interest in the technology developed jointly through the collaboration to develop, manufacture, commercialize and otherwise exploit licensed ADCs and any licensed products containing licensed ADCs directed toward a target. Pursuant to mutually agreed research and CMC plans, the Company agreed to perform bioconjugation, production development, preclinical manufacturing, and certain related research and preclinical development activities, in order to progress the targets through IND (or foreign equivalent) submission for further development, manufacture and commercialization by Merck KGaA. Merck KGaA will have sole responsibility for IND-enabling studies, IND submission, clinical development, regulatory activities and commercialization of the licensed ADCs. Both the Company and Merck KGaA will have equal representation on a Joint Research Committee and Joint Manufacturing Committee to oversee the research and CMC activities. The Company estimates that its activities under the research plans for the targets will be performed through 2026. The Company's CMC activities will be compensated by Merck KGaA at agreed upon rates. Assuming successful development and commercialization of the two targets by Merck KGaA, the Company is eligible to receive up to $200 million in development and regulatory milestones and $600 million in sales milestones as well as tiered single-digit to low double-digit royalties on aggregate net sales of the ADC products. Unless earlier terminated, the 2022 Merck KGaA Agreement will expire upon the expiration of the last royalty term for a product under the 2022 Merck KGaA Agreement. The 2022 Merck KGaA Agreement contains customary provisions for termination by either party, including in the event of breach of the 2022 Merck KGaA Agreement, subject to cure, by Merck KGaA for convenience and by the Company upon a challenge of the licensed patents, and customary provisions regarding the effects of termination. Merck KGaA may request that the Company perform clinical manufacturing services under a separate clinical supply agreement. Merck KGaA may also request that the Company perform a technology transfer of bioconjugation technology, at Merck KGaA's cost, at an agreed upon rate. Accounting Analysis The Company assessed the 2022 Merck KGaA Agreement in accordance with ASC 606 and concluded that the contract counter party, Merck KGaA, is a customer. The Company identified the following four material performance obligations under the 2022 Merck KGaA Agreement: (i) exclusive Merck KGaA Licenses and research activities for each of the two designated targets and (ii) CMC activities for each of the two designated targets. The Company concluded that the Merck KGaA Licenses and research activities are one combined performance obligation for each target as the Merck KGaA Licenses are not capable of being distinct from the research activities given their proprietary nature. The CMC activities are considered a distinct performance obligation for each target as the activities could be performed by a third-party provider. In accordance with ASC 606, the Company determined that the initial transaction price under the 2022 Merck KGaA Agreement equals $32.0 million, consisting of the $30.0 million upfront, non-refundable and non-creditable fee and certain near-term discovery milestones. The $30.0 million upfront fee was not received by the Company as of December 31, 2022, and was recorded as an accounts receivable with a corresponding deferred revenue liability for the year ended December 31, 2022. The Company subsequently received this payment in February 2023. The development and the regulatory milestones not included in the transaction price were constrained. As part of its evaluation of the constraint, the Company considered numerous factors, including stage of development and the remaining risks associated with the development required to achieve the milestones, as well as whether the achievement of the milestones is outside the control of the Company or Merck KGaA. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as such milestones were determined to relate predominantly to the license granted to Merck KGaA and therefore have also been excluded from the transaction price. The Company determined that the consideration for CMC activities represents variable consideration. The Company has elected to apply the Right to Invoice practical expedient under ASC 606 related to the CMC activities. As such, the Company will recognize revenue related to the CMC activities when the services are performed over the corresponding CMC plan for a given target. CMC activities for the targets have not yet been initiated. Consistent with the allocation objective under ASC 606, the Company allocated the $32.0 million estimated transaction price to the Merck KGaA Licenses and research activities based on each performance obligation’s relative standalone selling price. Each of the standalone selling prices for the Merck KGaA Licenses and research activities were estimated utilizing an adjusted market assessment approach, which was established based on comparable transactions. The Company is recognizing revenue related to the Merck KGaA Licenses and research services performance obligation over the estimated period of the research services using a proportional performance model. The Company measures proportional performance based on the costs incurred relative to the total costs expected to be incurred. During the year ended December 31, 2023, the Company recorded collaboration revenue of $10.7 million related to its efforts under the 2022 Merck KGaA Agreement. The Company did not record collaboration revenue related to the 2022 Merck KGaA Agreement during the year ended December 31, 2022. As of December 31, 2023 and 2022, the Company had recorded $20.2 million and $30.0 million, respectively, in deferred revenue related to the unsatisfied performance obligations under the 2022 Merck KGaA Agreement. This deferred revenue will be recognized over the remaining performance period and classified as current or noncurrent on the consolidated balance sheets based upon the expected timing of satisfaction of respective performance obligations. Dolaflexin Platform Agreement In June 2014, the Company entered into a collaboration and commercial license agreement with Merck KGaA, Darmstadt Germany (as amended from time to time, the "2014 Merck KGaA Agreement"). The 2014 Merck KGaA Agreement provided Merck KGaA with the right to develop ADCs directed to six exclusive targets. In May 2018, the Company entered into a Supply Agreement with Merck KGaA, Darmstadt, Germany (as amended from time to time, the "2018 Merck KGaA Supply Agreement"). Under the terms of the 2018 Merck KGaA Supply Agreement, the Company could provide Merck KGaA preclinical non-good manufacturing practice ADC drug substance and clinical good manufacturing practice drug substance for use in clinical trials associated with one of the antibodies designated under the 2014 Merck KGaA Agreement. In the fourth quarter of 2023, the Company and Merck KGaA mutually agreed to terminate the 2014 Merck KGaA Agreement and the 2018 Merck KGaA Supply Agreement. Accounting Analysis The 2014 Merck KGaA Agreement and 2018 Merck KGaA Supply Agreement were terminated during the fourth quarter of 2023. As there are no further performance obligations, the Company recognized $3.7 million of revenue related to the termination of the 2014 Merck KGaA Agreement and the 2018 Merck KGaA Supply Agreement during the year ended December 31, 2023. Prior to the termination of the agreements, the Company had identified the following performance obligations under the 2014 Merck KGaA Agreement: (i) exclusive license and research services for six designated targets, (ii) rights to future technological improvements and (iii) participation of project team leaders and providing joint research committee services. The Company had concluded that each license for a designated target was not distinct from the research services performed related to the designated target as Merck KGaA could not obtain the benefit of the license without the related research services. Each license for a designated target and the related services performance obligation was considered distinct from every other license for a designated target and related services performance obligation as each research plan was pursued independent of every other research plan for other designated targets. Collaboration revenue recognized related to the 2014 Merck KGaA Agreement and the 2018 Merck KGaA Supply Agreement during the year ended December 31, 2023 was $3.7 million. Collaboration revenue recognized related to the 2014 Merck KGaA Agreement and the 2018 Merck KGaA Supply Agreement during the years ended December 31, 2022 and 2021 was immaterial. As of December 31, 2023, the Company did not have deferred revenue related to the 2014 Merck KGaA Agreement and 2018 Merck KGaA Supply Agreement. As of December 31, 2022, the Company had recorded $3.9 million in deferred revenue related to the 2014 Merck KGaA Agreement and 2018 Merck KGaA Supply Agreement, in the aggregate. Summary of Contract Assets and Liabilities The following table presents changes in the balances of the Company's contract liabilities: Balance at Beginning of Period Additions Deductions Balance at End of Period Year ended December 31, 2023 Contract liabilities: Total deferred revenue $ 147,653 $ 5,517 $ 27,856 $ 125,314 Year ended December 31, 2022 Contract liabilities: Total deferred revenue $ 3,944 $ 170,000 $ 26,291 $ 147,653 The Company did not record any contract assets associated with its collaboration agreements as of December 31, 2023 and December 31, 2022. During the years ended December 31, 2023 and 2022 the Company recognized the following revenues as a result of changes in the contract liability balances in the respective periods: Year ended December 31, 2023 2022 Revenue recognized in the period from: Amounts included in the contract liability at the beginning of the period $ 27,870 $ 43 Other Revenue

Fair value measurements

Fair value measurements	12 Months Ended
	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Fair value measurements	Fair value measurements The following table presents information about the Company's assets measured at fair value on a recurring basis and indicates the level within fair value hierarchy of the valuation techniques utilized to determine such value. December 31, 2023 (in thousands) Total Quoted Prices Significant Significant Cash equivalents Money market funds $ 90,649 $ 90,649 $ — $ — U.S. treasury securities 24,889 24,889 — — $ 115,538 $ 115,538 $ — $ — Marketable securities U.S. treasury securities $ 29,548 $ 29,548 $ — $ — U.S government agency securities 4,975 — 4,975 — $ 34,523 $ 29,548 $ 4,975 $ — December 31, 2022 (in thousands) Total Quoted Prices Significant Significant Cash equivalents Money market funds $ 50,471 $ 50,471 $ — $ — U.S. government agency securities 9,993 — 9,993 — $ 60,464 $ 50,471 $ 9,993 $ — Marketable securities U.S. treasury securities $ 107,810 $ 107,810 $ — $ — U.S. government agency securities 44,017 — 44,017 — $ 151,827 $ 107,810 $ 44,017 $ — There were no changes in valuation techniques or transfers between fair value measurement levels during the years ended December 31, 2023 and 2022. Investments classified as Level 1 within the valuation hierarchy generally consist of U.S. treasury securities and money market funds, as the fair value is readily determinable based on active daily markets for identical securities. Investments classified as Level 2 within the valuation hierarchy generally consists of U.S. government agency securities, as the fair value is readily determinable based on active daily markets for similar securities and other observable inputs. The Company estimates the fair values of investments by taking into consideration valuations obtained from third-party pricing sources. The carrying amounts reflected in the consolidated balance sheets for prepaid expenses and other current assets, accounts receivable, accounts payable and accrued expenses approximate their fair values due to their short-term nature. As of December 31, 2023 and 2022, the carrying value of the Company’s outstanding borrowing under the New Credit Facility (as defined in Note 8) approximated fair value (a Level 2 fair value measurement), reflecting interest rates currently available to the Company. The New Credit Facility is discussed in more detail in Note 8, Debt .

Cash, cash equivalents, and sho

Cash, cash equivalents, and short-term marketable securities	12 Months Ended
	Dec. 31, 2023
Cash and Cash Equivalents [Abstract]
Cash, cash equivalents, and short-term marketable securities	Cash, cash equivalents, and short-term marketable securities Cash and cash equivalents The following table summarizes the Company's cash, cash equivalents, and restricted cash as of December 31, 2023 and 2022. (in thousands) Year Ended Year Ended December 31, 2022 Beginning End Beginning End Cash and cash equivalents $ 128,885 $ 174,561 $ 177,947 $ 128,885 Restricted cash included in other assets, noncurrent 478 478 478 478 Total cash, cash equivalents and restricted cash per statement of cash flows $ 129,363 $ 175,039 $ 178,425 $ 129,363 Marketable securities The following table summarizes the Company's marketable securities held at December 31, 2023 and 2022. December 31, 2023 (in thousands) Amortized Gross Gross Fair Marketable securities U.S. treasury securities $ 29,535 $ 13 $ — $ 29,548 U.S. government agency securities 4,977 — (2) 4,975 $ 34,512 $ 13 $ (2) $ 34,523 December 31, 2022 (in thousands) Amortized Gross Gross Fair Marketable securities U.S. treasury securities $ 107,964 $ 7 $ (161) $ 107,810 U.S. government agency securities 44,016 24 (23) 44,017 Total $ 151,980 $ 31 $ (184) $ 151,827 All of the Company's marketable securities are due within one year or less. The Company did not realize any gains or losses on the sale of marketable securities during the years ended December 31, 2023 and 2022, and, as a result, the Company did not reclassify any amounts out of accumulated comprehensive loss. As of December 31, 2023, the Company's debt security portfolio consisted of one security that was in an unrealized loss position and had an aggregate fair value of $5.0 million. There were no securities in an unrealized loss position for greater than 12 months as of December 31, 2023. The unrealized losses on the Company's marketable securities were caused by market interest rate increases. The Company has the intent and ability to hold such securities until recovery. The Company did not record any charges for credit-related impairments for its marketable securities during the years ended December 31, 2023 and 2022.

Property and equipment

Property and equipment	12 Months Ended
	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Property and equipment	Property and equipment Property and equipment consists of the following as of December 31, 2023 and 2022: (in thousands) December 31, December 31, Laboratory equipment $ 8,246 $ 7,960 Leasehold improvements 2,206 1,943 Computers, software, and office equipment 2,449 1,824 Total property and equipment at cost 12,901 11,727 Less: Accumulated depreciation (9,070) (7,742) $ 3,831 $ 3,985 Depreciation expense for the years ended December 31, 2023, 2022 and 2021 was $1.5 million, $0.9 million and $0.9 million, respectively.

Accrued expenses

Accrued expenses	12 Months Ended
	Dec. 31, 2023
Accrued Liabilities, Current [Abstract]
Accrued expenses	Accrued expenses Accrued expenses consist of the following as of December 31, 2023 and 2022: (in thousands) December 31, December 31, Accrued payroll and related expenses $ 8,807 $ 11,558 Accrued clinical expenses 5,063 14,822 Accrued research and non-clinical expenses 3,090 2,767 Accrued manufacturing expenses 2,566 11,536 Accrued restructuring expenses 1,047 — Accrued professional fees 936 1,865 Accrued other 389 636 $ 21,898 $ 43,184

Debt

Debt	12 Months Ended
	Dec. 31, 2023
Debt Disclosure [Abstract]
Debt	Debt On May 8, 2019, the Company entered into a loan and security agreement (the "Prior Credit Facility") with Silicon Valley Bank ("former SVB"), pursuant to which the Company borrowed $5.0 million. The Prior Credit Facility accrued interest at a floating per annum rate equal to the greater of (i) 4.0% and (ii) 1.50% below the Prime Rate. The Prior Credit Facility had an interest-only period through August 31, 2020. On August 28, 2020, the Company entered into a second amendment (the "Second Amendment") to the Prior Credit Facility. Pursuant to the Second Amendment, the Company drew $5.2 million upon execution of the Second Amendment, the proceeds of which were used to repay the Company’s existing balance under the Prior Credit Facility and satisfy its obligations to former SVB. The Prior Credit Facility, as amended by the Second Amendment, accrued interest at a floating per annum rate equal to the greater of (i) 4.25% and (ii) 1.00% above the Prime Rate. On October 29, 2021, the Company entered into a loan and security agreement (the "New Credit Facility") with former SVB and Oxford Finance LLC ("Oxford" and, together with former SVB and the other lenders from time to time a party thereto, the "Lenders"). In March 2023, Silicon Valley Bridge Bank, N.A ("SVBB"), as successor in interest to former SVB, replaced former SVB as a Lender, and then Silicon Valley Bank, a division of First-Citizens Bank & Trust Company ("SVB"), which assumed all deposits and loans of SVBB, subsequently replaced SVBB as a Lender. The New Credit Facility as amended on February 17, 2022, October 17, 2022, December 27, 2022, and March 23, 2023, is secured by substantially all of the Company's personal property owned or later acquired, excluding intellectual property (but including the rights to payments and proceeds from intellectual property), and a negative pledge on intellectual property. The Company drew $25.0 million upon execution of the New Credit Facility, of which $5.5 million of the proceeds was used to repay the existing balance under the Prior Credit Facility and satisfy its obligations to SVB. Upon entering into the New Credit Facility, the Company terminated all commitments by SVB to extend further credit under the Prior Credit Facility and all guarantees and security interests granted by the Company to SVB under the Prior Credit Facility. As of December 31, 2023, no additional borrowing amounts were available to the Company under the New Credit Facility, as amended. The New Credit Facility bears interest at a floating per annum rate equal to the greater of (i) 8.50% and (ii) 5.25% above the Prime Rate. Interest is payable monthly in arrears on the first day of each month. The Company is obligated to make interest-only payments through November 1, 2024, followed by equal monthly principal payments and applicable interest through the maturity date of October 1, 2026 (the "Maturity Date"). If certain development milestones are met, then the interest-only period will be extended to November 1, 2025. The Company is also required to make a final payment to the Lenders equal to 4.25% of the principal amount of the term loans then extended to the Company. This final payment is accreted under the effective interest method over the life of each term loan. The term loans are secured by substantially all of the Company’s assets, except for its intellectual property which is subject to a negative pledge, and certain other customary exclusions. At the Company’s option, it may prepay the outstanding principal balance of any term loans in whole but not in part, subject to a prepayment fee of 1.0% of the term loans then extended to the Company if the prepayment occurs after the second anniversary of the funding date of such term loan but before the Maturity Date. The New Credit Facility includes customary affirmative and restrictive covenants applicable to the Company. Affirmative covenants include, among others, covenants requiring the Company to maintain its corporate existence and governmental approvals, deliver certain financial reports, maintain insurance coverage and satisfy certain requirements regarding deposit accounts. The restrictive covenants include, among others, requirements relating to the Company’s ability to transfer collateral, incur additional indebtedness, engage in mergers or acquisitions, pay dividends or make other distributions, make investments, create liens, sell assets and agree to a change in control, in each case subject to certain customary exceptions. The Company’s payment obligations under the New Credit Facility are subject to acceleration upon the occurrence of specified events of default, which include, but are not limited to, the occurrence of a material adverse change in the Company’s business, operations, or financial or other condition. Amounts outstanding upon the occurrence of an event of default are payable upon the Lenders' demand and shall accrue interest at an additional rate of 5.0% per annum of the past due amount outstanding. As of December 31, 2023, the Company was in compliance with all covenants under the New Credit Facility. As such, as of December 31, 2023, the classification of the loan balance as stated on the balance sheet was based on the timing of defined future payment obligations. The following is a summary of obligations under the New Credit Facility as of December 31, 2023: (in thousands) December 31, Total debt $ 25,000 Less: Current portion of long-term-debt (2,083) Total debt, net of current portion 22,917 Debt financing costs, net of accretion (237) Accretion related to final payment 468 Long-term debt, net $ 23,148 As of December 31, 2023, the estimated future principal payments due are as follows: (in thousands) 2024 $ 2,083 2025 12,500 2026 10,417 Total debt $ 25,000 Interest expense related to the New Credit Facility for the years ended December 31, 2023 and 2022 was $3.9 million and $3.2 million, respectively. The Company did not recognize any interest expense related to the New Credit Facility during the year ended December 31, 2021. Interest expense related to the Prior Credit Facility for the year ended December 31, 2021 was $0.8 million. The Company did not recognize any interest expense related to the Prior Credit Facility during the years ended December 31, 2023 and 2022.

Stockholders_ equity

Stockholders’ equity	12 Months Ended
	Dec. 31, 2023
Stockholders' Equity Note [Abstract]
Stockholders’ equity	Stockholders’ equity Preferred stock As of December 31, 2023, the Company had 25,000,000 shares of authorized preferred stock. No shares of preferred stock have been issued. At-the-market ("ATM") equity offering program In May 2020, the Company established an ATM equity offering program (the "2020 ATM"), pursuant to which it was able to offer and sell up to $100.0 million of its common stock from time to time at prevailing market prices. During the year ended December 31, 2021, the Company sold 3,961,074 shares of common stock, resulting in net proceeds of $43.1 million. During the year ended December 31, 2022, the Company sold 11,740,210 shares of common stock under the 2020 ATM, resulting in net proceeds of $54.8 million. As of December 31, 2022, the 2020 ATM had been fully utilized. In February 2022, the Company established a new ATM equity offering program (the "February 2022 ATM"), pursuant to which it was able to offer and sell up to $100.0 million of its common stock from time to time at prevailing market prices. During the year ended December 31, 2022, the Company sold 18,757,665 shares of common stock under the February 2022 ATM, resulting in net proceeds of $96.4 million. During the year ended December 31, 2023, the Company sold 256,386 shares of common stock under the February 2022 ATM, resulting in net proceeds of $1.6 million. As of December 31, 2023, the February 2022 ATM had been fully utilized. In November 2022, the Company established an additional ATM equity offering program (the "November 2022 ATM"), pursuant to which it is able to offer and sell up to $150.0 million of its common stock from time to time at prevailing market prices. During the year ended December 31, 2023, the Company sold 14,208,145 shares of common stock under the November 2022 ATM, resulting in net proceeds of $92.2 million. As of December 31, 2023, approximately $55.9 million remained unsold and available for sale under the November 2022 ATM. Warrants In connection with a 2013 Series A-1 Preferred Stock issuance, the Company granted to certain investors warrants to purchase shares of common stock. All such outstanding warrants expired pursuant to their terms on September 27, 2023. The warrants had a $0.05 per share exercise price and a contractual life of 10 years. The fair value of these warrants was recorded as a component of equity at the time of issuance. As of December 31, 2023, there were no warrants to purchase shares of common stock outstanding. During the year ended December 31, 2022, the Company issued 16,654 shares of common stock upon the net exercise of warrants. Common Stock At the Company's 2022 Annual Meeting of Stockholders on June 9, 2022, the Company's stockholders approved an amendment to the Company’s Fifth Amended and Restated Certificate of Incorporation to increase the number of authorized shares of common stock, $0.0001 par value per share, from 175,000,000 to 350,000,000. This increase became effective upon filing of a Certificate of Amendment with the Secretary of State of the State of Delaware on June 9, 2022. The holders of the common stock are entitled to one vote for each share held. Common stockholders are not entitled to receive dividends, unless declared by the Board of Directors of the Company (the "Board"). As of December 31, 2023 and 2022 there were 14,736,953 and 11,944,664, respectively, shares of common stock reserved for the exercise of outstanding stock options, restricted stock units ("RSUs") and warrants. December 31, December 31, Stock options 10,902,845 10,051,283 Restricted stock units 3,834,108 1,870,791 Warrants — 22,590 14,736,953 11,944,664

Stock-based compensation

Stock-based compensation	12 Months Ended
	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Stock-based compensation	Stock-based compensation Stock incentive plans Prior to its initial public offering, the Company granted stock options pursuant to the Company’s 2007 Stock Incentive Plan (the "2007 Plan"). The 2007 Plan expired in June 2017. Any cancellations or forfeitures of options granted under the 2007 Plan will increase the options available under the Company's 2017 Stock Incentive Plan (the "2017 Plan"), as described below. In June 2017 the Company’s stockholders approved the 2017 Plan. Under the 2017 Plan, shares of common stock could be granted to the Company's employees, officers, directors, consultants and advisors in the form of options, restricted stock units ("RSUs") or other stock-based awards. The number of shares of common stock issuable under the 2017 Plan will be cumulatively increased annually on January 1 by the lesser of (a) 4% of the outstanding shares on the immediately preceding December 31 or (b) such other amount specified by the Board. The terms of the awards are determined by the Board, subject to the provisions of the 2017 Plan. Any cancellations or forfeitures of options granted under the 2007 Plan, which expired in June 2017, would increase the number of shares that could be granted under the 2017 Plan. On January 1, 2023, the number of shares of common stock issuable under the 2017 Plan was increased by 4,205,794 shares. During the year ended December 31, 2023, the Company granted an aggregate of 7,211,153 RSUs, options to purchase shares of common stock, and shares of common stock to employees and non-employee directors under the 2017 Plan. As of December 31, 2023 there were 2,043,328 shares available for future issuance under the 2017 Plan. Under the 2017 Plan, both with respect to incentive stock options and nonqualified stock options, the exercise price per share will not be less than the fair market value of the common stock on the date of grant, and the vesting period for options granted to employees is generally four years. In accordance with the Company's non-employee director compensation policy, as in effect from time to time, options granted to non-employee directors in lieu of cash retainer fees earned are fully vested upon grant, options granted to non-employee directors upon initial election to the board of directors vest over three years, and options granted to non-employee directors on the date of each of annual meeting of stockholders vest over one year. Options granted under the 2017 Plan expire no later than 10 years from the date of grant. Options under the 2007 Plan were granted at an exercise price established by the Board (or an authorized committee thereof) that was not less than the fair market value of the underlying common stock on the date of grant and subject to such vesting provisions determined by the Board (or an authorized committee thereof). The Board may accelerate vesting or otherwise adjust the terms of granted options in the case of a merger, consolidation, dissolution, or liquidation of the Company. Inducement awards From time to time, the Company grants to its employees, upon approval by the Board or an authorized committee thereof, options to purchase shares of common stock and/or RSUs as an inducement to employment in accordance with Nasdaq Listing Rule 5635(c)(4). Prior to February 2022, only options to purchase shares of common stock were granted as inducement awards, and they were granted outside of an existing equity incentive plan. These options are subject to terms substantially the same as the 2017 Plan. In February 2022, the Board adopted the Company's 2022 Inducement Stock Incentive Plan (the "Inducement Plan"), which provides for the grant of nonstatutory options, stock appreciation rights, restricted stock, RSUs and other stock-based awards, with respect to an aggregate of 2,000,000 shares of the Company's common stock (subject to adjustment as provided in the Inducement Plan). During the year ended December 31, 2023, the Company granted an aggregate of 877,575 RSUs and options to purchase shares of common stock to newly hired employees under the Inducement Plan. As of December 31, 2023, there were 1,170,752 shares available for future issuance under the Inducement Plan. As of December 31, 2023 there were options to purchase 457,500 shares of common stock outstanding which were granted as inducement awards prior to establishment of the Inducement Plan. Stock option activity A summary of stock option activity is as follows: Number of Shares Weighted- Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value Outstanding at January 1, 2023 10,051,283 $ 9.84 7.2 $ 8,197 Granted 4,017,763 $ 4.76 Exercised (102,596) $ 4.25 Cancelled/forfeited (3,063,605) $ 12.00 Outstanding at December 31, 2023 10,902,845 $ 7.42 6.8 $ 1,786 Exercisable at December 31, 2023 6,499,540 $ 8.23 5.5 $ 711 The weighted-average grant date fair value of options granted during the years ended December 31, 2023, 2022 and 2021, was $3.82, $3.97 and $11.71 per share, respectively. The total intrinsic value of options exercised during the years ended December 31, 2023, 2022 and 2021, was $0.3 million, $1.5 million, and $4.3 million, respectively. The aggregate intrinsic value represents the difference between the exercise price and the selling price received by option holders upon the exercise of stock options during the period. Cash received from the exercise of stock options was $0.4 million, $1.3 million and $1.8 million for the years ended December 31, 2023, 2022 and 2021, respectively. Restricted stock units and other stock awards The Company periodically issues RSUs with a service condition to certain officers and other employees that typically vest between one year and four years from the grant date. In accordance with its non-employee director compensation policy, as in effect from time to time, the Company annually issues RSUs with a service condition to non-employee directors that typically vest one year from the date of grant, and the Company also issues shares of common stock in lieu of cash retainer fees earned to certain non-employee directors, which shares are fully vested upon grant. A summary of the RSU activity is as follows: Number Weighted-Average Aggregate Weighted-Average Unvested at January 1, 2023 1,870,791 1.5 $ 10,963 $ 8.55 Granted 4,025,544 — $ 4.42 Vested (572,825) — $ 8.83 Forfeited (1,489,402) — $ 6.25 Unvested at December 31, 2023 3,834,108 1.4 $ 8,895 $ 5.01 The total fair value of RSUs vested during the years ended December 31, 2023, 2022 and 2021 was $3.1 million, $1.5 million and $5.8 million respectively. Employee stock purchase plan During the year ended December 31, 2017, the Board adopted, and the Company's stockholders approved the 2017 employee stock purchase plan (the "2017 ESPP"). The Company initially reserved 225,000 shares of common stock for issuance under the 2017 ESPP, plus an annual increase, to be added as of January 1st of each year, equal to the least of (i) 450,000 shares of common stock; (ii) one percent of the number of shares of common stock outstanding as of the close of business on the immediately preceding December 31st; and (iii) the number of shares of common stock determined by the Board on or prior to such date for such year, up to maximum of 4,725,000 shares of common stock in the aggregate. The number of shares of common stock issuable under the 2017 ESPP was increased by 450,000 on January 1, 2023. During the years ended December 31, 2023 and 2022 the Company issued 381,508 and 270,774 shares, respectively, under the 2017 ESPP. As of December 31, 2023, there were 364,283 shares available for issuance under the 2017 ESPP. Stock-based compensation expense The Company uses the provisions of ASC 718 to account for all stock-based awards to employees and non-employees. Stock-based compensation expense is recognized over the requisite service period, which is generally the vesting period, using the straight-line method. The following table presents stock-based compensation expense by award type included within the Company’s consolidated statements of operations and comprehensive loss: Year ended December 31, (in thousands) 2023 2022 2021 Stock options $ 14,171 $ 15,814 $ 14,528 Restricted stock units and other stock awards 6,184 5,175 3,522 Employee stock purchase plan 781 533 359 Stock-based compensation expense included in total operating expenses $ 21,136 $ 21,522 $ 18,409 The following table presents stock-based compensation expense as reflected in the Company’s consolidated statements of operations and comprehensive loss: Year ended December 31, (in thousands) 2023 2022 2021 Research and development $ 11,043 $ 11,386 $ 9,984 General and administrative 10,093 10,136 8,425 Stock-based compensation expense included in total operating expenses $ 21,136 $ 21,522 $ 18,409 As of December 31, 2023, there was $14.8 million and $11.8 million of unrecognized stock-based compensation expense related to unvested stock options and unvested RSUs, respectively, that is expected to be recognized over a weighted average period of 1.6 years and 2.4 years, respectively. The fair value of each option award is estimated on the date of grant using the Black–Scholes option pricing model with the following weighted average assumptions: December 31, 2023 2022 2021 Risk-free interest rate 3.8 % 2.1 % 0.9 % Expected dividend yield — % — % — % Expected term (years) 6.05 5.99 6.06 Expected stock price volatility 103 % 88 % 82 % Expected volatility for the Company’s common stock is currently determined based on its historical volatility. See Note 2, Summary of significant accounting policies , for more information. The risk-free interest rate is based on the yield of U.S. Treasury securities consistent with the expected term of the option. No dividend yield was assumed as the Company has not historically and does not expect to pay dividends on its common stock. The expected term of the options granted is based on the use of the simplified method, in which the expected term is presumed to be the mid-point between the vesting date and the end of the contractual term. The fair value of RSUs is determined based on the closing price of the Company’s common stock on the date of grant.

Net loss per share

Net loss per share	12 Months Ended
	Dec. 31, 2023
Earnings Per Share [Abstract]
Net loss per share	Net loss per share Basic net loss per share of common stock is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period, without further consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock and potentially dilutive securities outstanding for the period determined using the treasury stock method. For purposes of the diluted net loss per share calculation, stock options, unvested RSUs and warrants to purchase common stock are considered to be potentially dilutive securities, but are excluded from the calculation of diluted net loss per share because their effect would be anti-dilutive and therefore, basic and diluted net loss per share were the same for all periods presented. The following table sets forth the outstanding potentially dilutive securities that have been excluded from the calculation of diluted net loss per share because to include them would be anti-dilutive (in common stock equivalent shares): Year ended December 31, 2023 2022 2021 Stock options 10,902,845 10,051,283 8,342,429 Unvested restricted stock units 3,834,108 1,870,791 817,609 Warrants — 22,590 39,474 14,736,953 11,944,664 9,199,512

Leases

Leases	12 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Leases	Leases The Company has an operating lease for its office and lab space in Cambridge, Massachusetts and operating and finance leases for certain equipment. In March 2020, the Company entered into the Seventh Amendment to the office and lab space lease ("the Office Lease") to extend the term of the lease through March 2026. The Company has an option to extend the lease term of the Office Lease for an additional five years. On April 5, 2021, the Company entered into an Eighth Amendment to the Office Lease, which granted the Company additional office space in its existing building for five years, beginning July 1, 2021, and committed the Company to lease payments of $5.0 million over that period (the "Expansion Lease"). Independent from the option under the Office Lease, the Company has an option to extend the lease term of the Expansion Lease for an additional five years. The Company’s exercise of the options to extend the lease terms of both the Office Lease and Expansion Lease were not considered reasonably certain as of December 31, 2023. The Expansion Agreement is a lease modification accounted for as a separate contract, because it expands the scope of the Office Lease and the additional lease payments are commensurate with market rents. The Company assessed the lease classification of the Expansion Lease as of the date of signing and determined that the Expansion Lease should be accounted for as an operating lease. The right-of-use asset and corresponding operating lease liability have been calculated based on the present value of lease payments over the lease term. The Company determined the appropriate incremental borrowing rate to utilize as a discount rate by using a synthetic credit rating which was estimated based on an analysis of outstanding debt of companies with similar credit and financial profiles. Since the operating lease is a net lease, as the non-lease components (i.e., common area maintenance) are paid separately from rent based on actual costs incurred, such non-lease components were not included in the ROU asset and liability and are reflected as an expense in the period incurred. The Company had a standby letter of credit agreement for the benefit of its landlord in the amount of $0.5 million in connection with the Office Lease and Expansion Lease as of December 31, 2023 and 2022. The Company has remaining finance lease terms of one year to five years for certain equipment, some of which include options to purchase at fair value. The components of lease expense included in research and development and general and administrative expenses in the statement of operations and comprehensive loss were as follows: Years ended December 31, (in thousands) 2023 2022 2021 Operating lease cost $ 3,838 $ 3,793 $ 3,502 Finance lease cost: Amortization of right-of-use assets $ 185 $ 194 $ 169 Interest on lease liabilities 16 28 28 $ 201 $ 222 $ 197 Supplemental balance sheet information related to leases was as follows: Year ended December 31, 2023 2022 Operating leases: Operating lease right-of-use assets $ 7,694 $ 10,475 Operating lease liabilities, current $ 3,252 $ 2,798 Operating lease liabilities $ 5,149 $ 8,575 Finance leases: Property and equipment, gross $ 1,038 $ 1,038 Property and equipment, accumulated depreciation $ (723) $ (539) Other liabilities, current $ 144 $ 240 Other liabilities $ 54 $ 203 Weighted-average remaining lease term: Operating leases 2.3 years 3.3 years Finance leases 1.1 years 2.0 years Weighted-average discount rate: Operating leases 10.8 % 10.8 % Finance leases 4.4 % 5.4 % Supplemental cash flow information related to leases was as follows: Year ended December 31, (in thousands) 2023 2022 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 4,029 $ 3,865 Operating cash flows from finance leases $ 16 $ 28 Financing cash flows from finance leases $ 262 $ 272 Future lease payments under non-cancellable leases as of December 31, 2023 were as follows: (in thousands) Operating leases Finance leases 2024 $ 3,975 $ 141 2025 4,187 48 2026 1,310 8 2027 and thereafter — — Total lease payments 9,472 197 Present value adjustment (1,069) — Present value of lease liabilities $ 8,403 $ 197
Leases	Leases The Company has an operating lease for its office and lab space in Cambridge, Massachusetts and operating and finance leases for certain equipment. In March 2020, the Company entered into the Seventh Amendment to the office and lab space lease ("the Office Lease") to extend the term of the lease through March 2026. The Company has an option to extend the lease term of the Office Lease for an additional five years. On April 5, 2021, the Company entered into an Eighth Amendment to the Office Lease, which granted the Company additional office space in its existing building for five years, beginning July 1, 2021, and committed the Company to lease payments of $5.0 million over that period (the "Expansion Lease"). Independent from the option under the Office Lease, the Company has an option to extend the lease term of the Expansion Lease for an additional five years. The Company’s exercise of the options to extend the lease terms of both the Office Lease and Expansion Lease were not considered reasonably certain as of December 31, 2023. The Expansion Agreement is a lease modification accounted for as a separate contract, because it expands the scope of the Office Lease and the additional lease payments are commensurate with market rents. The Company assessed the lease classification of the Expansion Lease as of the date of signing and determined that the Expansion Lease should be accounted for as an operating lease. The right-of-use asset and corresponding operating lease liability have been calculated based on the present value of lease payments over the lease term. The Company determined the appropriate incremental borrowing rate to utilize as a discount rate by using a synthetic credit rating which was estimated based on an analysis of outstanding debt of companies with similar credit and financial profiles. Since the operating lease is a net lease, as the non-lease components (i.e., common area maintenance) are paid separately from rent based on actual costs incurred, such non-lease components were not included in the ROU asset and liability and are reflected as an expense in the period incurred. The Company had a standby letter of credit agreement for the benefit of its landlord in the amount of $0.5 million in connection with the Office Lease and Expansion Lease as of December 31, 2023 and 2022. The Company has remaining finance lease terms of one year to five years for certain equipment, some of which include options to purchase at fair value. The components of lease expense included in research and development and general and administrative expenses in the statement of operations and comprehensive loss were as follows: Years ended December 31, (in thousands) 2023 2022 2021 Operating lease cost $ 3,838 $ 3,793 $ 3,502 Finance lease cost: Amortization of right-of-use assets $ 185 $ 194 $ 169 Interest on lease liabilities 16 28 28 $ 201 $ 222 $ 197 Supplemental balance sheet information related to leases was as follows: Year ended December 31, 2023 2022 Operating leases: Operating lease right-of-use assets $ 7,694 $ 10,475 Operating lease liabilities, current $ 3,252 $ 2,798 Operating lease liabilities $ 5,149 $ 8,575 Finance leases: Property and equipment, gross $ 1,038 $ 1,038 Property and equipment, accumulated depreciation $ (723) $ (539) Other liabilities, current $ 144 $ 240 Other liabilities $ 54 $ 203 Weighted-average remaining lease term: Operating leases 2.3 years 3.3 years Finance leases 1.1 years 2.0 years Weighted-average discount rate: Operating leases 10.8 % 10.8 % Finance leases 4.4 % 5.4 % Supplemental cash flow information related to leases was as follows: Year ended December 31, (in thousands) 2023 2022 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 4,029 $ 3,865 Operating cash flows from finance leases $ 16 $ 28 Financing cash flows from finance leases $ 262 $ 272 Future lease payments under non-cancellable leases as of December 31, 2023 were as follows: (in thousands) Operating leases Finance leases 2024 $ 3,975 $ 141 2025 4,187 48 2026 1,310 8 2027 and thereafter — — Total lease payments 9,472 197 Present value adjustment (1,069) — Present value of lease liabilities $ 8,403 $ 197

Income taxes

Income taxes	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Income taxes	Income taxes For the years ended December 31, 2023, 2022 and 2021, the Company recorded no income tax benefit for the net operating losses ("NOLs") incurred in each year, due to the Company’s operating losses and a full valuation allowance on deferred tax assets. A reconciliation of the effective tax rate for the years ended December 31, 2023 and 2022 is as follows: 2023 2022 Statutory US Federal Rate 21.0 % 21.0 % State taxes, net of federal benefit 5.5 % 5.7 % Permanent differences (0.1) % (0.1) % General business credits 4.0 % 4.4 % Stock compensation (1.9) % (1.4) % Change in valuation allowance (28.5) % (29.6) % — % — % Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s net deferred tax assets as of December 31, 2023 and 2022 are as follows: (in thousands) 2023 2022 Deferred tax assets: Net operating losses $ 126,798 $ 113,981 R&D capitalization 67,107 40,380 Tax credit carryforwards 28,409 21,429 Stock-based compensation 6,899 6,522 Accrued expenses 3,142 3,340 Lease liabilities 2,274 3,096 Capitalized licenses 2,725 3,077 Deferred revenue 3,688 1,062 Depreciation 353 437 Other 156 116 Total gross deferred tax assets 241,551 193,440 Valuation allowance (239,468) (190,588) Net deferred tax assets less valuation allowance 2,083 2,852 Deferred tax liabilities Right-of-use assets (2,083) (2,852) Total gross deferred tax liabilities (2,083) (2,852) Net deferred taxes $ — $ — The Company has incurred NOLs since inception. At December 31, 2023, the Company had federal and state NOL carryforwards of approximately $479.0 million and $414.8 million, respectively. Of the $479.0 million of federal NOL carryforwards, $34.1 million expire at various dates through 2037. The remaining $444.8 million of federal NOL carryforwards do not expire. The state NOL carryforwards expire at various dates through 2043. At December 31, 2023, the Company had federal and state research and development tax credit carryforwards of approximately $23.2 million and $6.8 million, respectively, which expire at various dates through 2043. As required by ASC 740, management of the Company has evaluated the evidence bearing upon the reliability of its deferred tax assets. Based on the weight of available evidence, both positive and negative, management has determined that it is more likely than not that the Company will not realize the benefits of all of these assets. Accordingly, the Company recorded a valuation allowance of $239.5 million and $190.6 million at December 31, 2023 and December 31, 2022, respectively. The valuation allowance increased by $48.9 million during the year ended December 31, 2023, primarily a result of the Company's current year net loss and credit generation. Utilization of the NOLs and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 due to ownership change limitations that have occurred previously or that could occur in the future in accordance with Section 382, as well as similar state provisions. These ownership changes may limit the amount of NOLs and research and development tax credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. If a change in control as defined by Section 382 has occurred at any time since the Company’s formation, utilization of its NOLs or research and development tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, which could then be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the NOLs or research and development tax carryforwards before their utilization. The Company has determined that ownership changes have occurred in the past and that certain NOLs and research and development tax credit carryforwards will be subject to limitation. The Company applies the accounting guidance in ASC 740 related to accounting for uncertainty in income taxes. The Company’s reserves related to taxes are based on a determination of whether, and how much of, a tax benefit taken by the Company in its tax filings or positions is more likely than not to be realized following resolution of any potential contingencies present related to the tax benefit. As of December 31, 2023 and 2022, the Company had no unrecognized tax benefits. The Company commissioned a multi-year research and development credit study in 2023, which resulted in an adjustment to the Company's cumulative deferred tax asset as of December 31, 2023. The change in the deferred tax asset is offset by a full valuation allowance. Interest and penalties related to uncertain tax positions would be classified as income tax expense in the accompanying statements of operations and comprehensive loss. As of December 31, 2023 and 2022, the Company had no accrued interest or penalties related to uncertain tax positions. The Company files income tax returns in the United States federal tax jurisdiction and eight state jurisdictions. The Company did not have any foreign operations during the years ended December 31, 2023, 2022 and 2021. The statute of limitations for assessment by the Internal Revenue Service and state tax authorities is closed for tax years prior to 2019, although carryforward attributes that were generated prior to tax year 2019 may still be adjusted upon examination to the extent utilized in a future period. There are no federal or state audits currently in progress.

Employee benefit plan

Employee benefit plan	12 Months Ended
	Dec. 31, 2023
Retirement Benefits [Abstract]
Employee benefit plan	Employee benefit plan The Company has a defined contribution plan established under Section 401(k) of the Code ("401(k) Plan"), which covers substantially all employees. Employees who have attained the age of 21 and have worked more than 1,000 hours are eligible to participate in the 401(k) Plan. Employees may contribute up to 95% of eligible pay on a pre–tax basis up to the federal annual limits. For the years ended December 2023, 2022, and 2021, the Company made matching contributions equal to 100% of the employee’s contributions, subject to a maximum of 4% of eligible compensation. For the years ended December 31, 2023, 2022 and 2021, the Company recorded expense of $1.3 million, $1.1 million and $0.8 million, respectively, related to its contribution to its 401(k) Plan.

Commitments

Commitments	12 Months Ended
	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments	Commitments License agreements During the year ended December 31, 2023, the Company did not record research and development expense related to non-refundable license payments. During the years ended December 31, 2022 and 2021, the Company recorded research and development expense related to non-refundable license payments of $1.5 million and $3.1 million, respectively. During the year ended December 31, 2023, the Company did not record research and development expense related to development milestones. During the years ended December 31, 2022 and 2021, the Company recorded research and development expense related to development milestones of $0.7 million and $2.1 million, respectively. The 2022 and 2021 development milestones were associated with XMT-1660 and UpRi, respectively. See Note 12, Leases, for the Company’s future obligations related to leases as of December 31, 2023.

Restructuring

Restructuring	12 Months Ended
	Dec. 31, 2023
Restructuring and Related Activities [Abstract]
Restructuring	Restructuring On July 27, 2023, the Company announced decisions to reprioritize its areas of focus and to discontinue its clinical development of UpRi following an evaluation of top-line data from the Company’s UPLIFT Phase 2 clinical trial of UpRi in patients with platinum-resistant ovarian cancer, which did not meet its primary endpoint. In connection with these decisions, on July 26, 2023 the Company’s board of directors approved certain expense reduction measures, including a reduction of approximately 50% of the Company’s then-current total employee base (the "Restructuring"). Affected employees were eligible to receive severance and benefit payments, notice pay and outplacement services in connection with the reduction. As of December 31, 2023, the Restructuring was substantially completed. Restructuring costs incurred are included within restructuring expenses on the consolidated statements of operations and comprehensive loss. The following table summarizes the charges incurred in connection with the Restructuring: (in thousands) Severance & Employee Related Costs Contract Termination and Other Costs Total Costs Cumulative costs to date $ 6,924 $ 1,789 $ 8,713 Costs incurred during the year ended December 31, 2023 $ 6,924 $ 1,789 $ 8,713 The following tables summarizes the charges incurred in connection with the Restructuring related to research and development activities and general and administrative activities: (in thousands) Year ended December 31, 2023 Research and development related $ 5,393 General and administrative related $ 3,320 Accrued restructuring costs, which are included in accrued expenses on the consolidated balance sheets, were as follows: (in thousands) Severance & Employee Related Costs Contract Termination Costs Total Costs Balance at December 31, 2022 $ — $ — $ — Additional expense 6,924 372 7,296 Cash payments (5,917) — (5,917) Other adjustments (252) (80) (332) Balance at December 31, 2023 $ 755 $ 292 $ 1,047

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pay vs Performance Disclosure
Net loss	$ (171,670)	$ (204,212)	$ (170,060)

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2023
Trading Arrangements, by Individual
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false
Martin Huber [Member]
Trading Arrangements, by Individual
Material Terms of Trading Arrangement		During the fourth quarter of 2023, on November 7, 2023, Martin Huber, M.D., our President and Chief Executive and a member of our board of directors, adopted a Rule 10b5-1 trading arrangement intended to qualify as an “eligible sell to cover transaction” (as described in Rule 10b5-1(c)(1)(ii)(D)(3) under the Exchange Act). This sell to cover arrangement applies to restricted stock units, or RSUs, granted to him from time to time, whether vesting is based on the passage of time and/or the achievement of performance goals (other than those RSUs which by their terms require the us to withhold shares for tax withholding obligations in connection with vesting and settlement). This arrangement provides for the automatic sale of shares of our common stock that would otherwise be issuable on each settlement date of a covered RSU in an amount necessary to satisfy the applicable withholding obligation, with the proceeds of the sale delivered to us in the satisfaction of the applicable withholding obligation. The number of shares that will be sold under this arrangement is not currently determinable as the number will vary based on the extent to which vesting conditions are satisfied, the market price our common stock at the time of settlement and the potential future grant of additional RSUs subject to this arrangement.
Name	Martin Huber
Title	M.D., our President and Chief Executive and a member of our board of directors
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	November 7, 2023

Summary of significant accoun_2

Summary of significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements include those of the Company and its wholly owned subsidiary, Mersana Securities Corp. All intercompany balances and transactions have been eliminated.
Use of Estimates	Use of Estimates The preparation of the Company's consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, equity, revenue, expenses and related disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenue and expenses during the reporting period. On an ongoing basis, the Company's management evaluates its estimates which include, but are not limited to, management's judgments with respect to the identification of performance obligations and standalone selling prices of those performance obligations within its revenue arrangements, accrued preclinical, manufacturing and clinical expenses, valuation of stock-based awards and income taxes. Actual results could differ from those estimates.
Segment Information	Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision-maker, or decision making group, in deciding how to allocate resources and assess performance. The Company views its operations and manages its business as a single operating segment, which is the business of discovering and developing ADCs.
Research and Development	Research and Development Research and development costs are expensed as incurred and include: • employee-related expenses, including salaries, bonuses, benefits, travel and stock-based compensation expense; • fees and expenses incurred under agreements with contract research organizations, investigative sites and other entities in connection with the conduct of preclinical studies, clinical trials and related services; • the cost of acquiring, developing and manufacturing ADC product candidates, clinical trial materials and other research and development materials; • fees and costs related to regulatory filings and activities; • costs associated with collaboration agreements and license fees and milestone payments related to license agreements; • costs associated with developing a NaPi2b diagnostic biomarker; • facilities, depreciation and other expenses, which include direct and allocated expenses for rent, utilities, maintenance of facilities, insurance and other supplies; and • other costs associated with clinical, preclinical, discovery and other research activities. Costs for certain development activities, such as preclinical studies, clinical trials and manufacturing development activities, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, and information provided to the Company by its vendors on their actual costs incurred or level of effort expended. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as prepaid or accrued preclinical, manufacturing and clinical expenses.
Revenue Recognition	Revenue Recognition The Company enters into collaboration agreements which are within the scope of ASC 606, Revenue from Contracts with Customers ("ASC 606"), under which the Company licenses rights to its technology and certain of the Company’s product candidates and performs research and development services for third parties. The terms of these arrangements typically include payment of one or more of the following: non-refundable, up-front fees; reimbursement of research and development costs; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of contract(s) with a customer; (ii) determination of whether the promised goods or services are performance obligations; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration to which it is entitled in exchange for the goods or services it transfers to the customer. The promised good or services in the Company’s arrangements typically consist of license rights to the Company’s intellectual property and research and development services. The Company also has optional additional items in contracts, which are considered marketing offers and are accounted for as separate contracts with the customer if such option is elected by the customer, unless the option provides a material right which would not be provided without entering into the contract. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer. Promised goods or services are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources or (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on their own and the availability of the required expertise. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include both fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration and at each reporting period, the Company evaluates the amount of potential payment and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected value method to estimate the amount expected to be received based on which method better predicts the amount of consideration to which the Company will be entitled. If it is probable that a significant revenue reversal would not occur, the variable consideration is included in the transaction price. The Company assessed each of its revenue generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in any of its arrangements because: (a) the promised consideration approximates the cash selling price of the promised goods and services; and (b) timing of payment approximates the transfer of goods and services and performance is over a relatively short period of time within the context of the entire term of the contract. The Company’s contracts often include development and regulatory milestone payments. At contract inception and at each reporting period, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not included in the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect license, collaboration and other revenues and earnings in the period of adjustment. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of the Company’s collaboration arrangements. The Company allocates the transaction price based on the estimated standalone selling price of the underlying performance obligations or in the case of certain variable consideration to one or more performance obligations. The Company must develop assumptions that require judgment to determine the standalone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the standalone selling price, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs to complete the respective performance obligation. Certain variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated to each performance obligation are consistent with the amounts the Company would expect to receive for each performance obligation. For performance obligations consisting of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, up-front fees. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company will recognize revenue from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. The Company receives payments from its customers based on billing schedules established in each contract. Such billings generally have 30-day terms. Up-front payments and fees are recorded as a contract liability (deferred revenue) upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the right to consideration is unconditional and only the passage of time is required before payment is due. If the right to consideration is subject to a condition other than the passage of time, then the amount is recorded as a contract asset until the right to payment becomes unconditional. In accordance with ASC 606, the Company presents contract assets and contract liabilities on a net basis by customer contract.
Collaborative Arrangements	Collaborative Arrangements The Company records the elements of its collaboration agreements that represent joint operating activities in accordance with ASC 808, Collaborative Arrangements ("ASC 808"). Accordingly, the elements of the collaboration agreements that represent activities in which both parties are active participants and to which both parties are exposed to the significant risks and rewards that are dependent on the commercial success of the activities, are recorded as collaborative arrangements. The Company also considers the guidance in ASC 606 by analogy in determining the appropriate treatment for the transactions between the Company and its collaborators and the transactions between the Company and third parties. Generally, the classification of transactions under the collaborative arrangements is determined based on the nature and contractual terms of the arrangement along with the nature of the operations of the participants. To the extent revenue is generated from a collaboration, the Company will recognize its share of the net sales on a gross basis if it is deemed to be the principal in the transactions with customers, or on a net basis if it is instead deemed to be the agent in the transactions with customers, consistent with the guidance in ASC 606.
Fair Value Measurements	Fair Value Measurements Fair value is defined as the price that would be received upon sale of an asset or paid to transfer a liability between market participants at measurement dates. ASC 820, Fair Value Measurement , establishes a three-level valuation hierarchy for instruments measured at fair value. The hierarchy is based on the transparency of inputs to the valuation of an asset or liability as of the measurement date. The three levels are defined as follows: Level 1—Inputs to the valuation methodology are quoted prices (unadjusted) for identical assets or liabilities in active markets. Level 2—Inputs to the valuation methodology include quoted prices for similar assets and liabilities in active markets, and inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the financial instrument. Level 3—Inputs to the valuation methodology are unobservable and significant to the fair value measurement.
Marketable Securities	Marketable Securities The Company’s investment strategy is focused on capital preservation. The Company invests in instruments that meet the credit quality standards outlined in the Company’s investment policy. Short-term marketable securities consist of investments in debt securities with maturities greater than three months and less than one year from the balance sheet date. The Company classifies all of its marketable securities as available-for-sale. Accordingly, these investments are recorded at fair value. Fair value is determined based on quoted market prices. Amortization and accretion of discounts and premiums are recorded as interest income within other income (expense), net. Realized gains and losses are included in other income (expense), net. The Company assesses its available-for-sale debt securities under the available-for-sale debt security impairment model in ASC 326, Financial Instruments - Credit Losses , as of each reporting date in order to determine if a portion of any decline in fair value below carrying value recognized on its available-for-sale debt securities is the result of a credit loss. The Company records credit losses in the consolidated statements of operations and comprehensive loss as a component of other income (expense), net, which is limited to the difference between the fair value and the amortized cost of the security. To date, the Company has not recorded any credit losses on its available-for-sale debt securities.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity, or a remaining maturity at the time of purchase, of three months or less to be cash equivalents. The Company invests excess cash primarily in money market funds, commercial paper and government agency securities, which are highly liquid and have strong credit ratings. These investments are subject to minimal credit and market risks. Cash and cash equivalents are stated at cost, which approximates market value.
Accounting for Stock-based Compensation	Accounting for Stock-based Compensation The Company accounts for its stock-based compensation in accordance with ASC 718, Compensation—Stock Compensation ("ASC 718"). ASC 718 requires all stock-based payments to employees, directors and non-employees to be recognized as expense in the statements of operations based on their grant date fair values. The Company estimates the fair value of options granted using the Black-Scholes option pricing model. The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. The expected stock price volatility is calculated based on a period of time commensurate with the expected term assumption. Historically, due to the lack of a public market for the Company's common stock prior to completion of its initial public offering and a lack of company-specific historical and implied volatility data, the Company based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The computation of expected volatility was based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. During 2022, the Company began to estimate its volatility by using a blend of its stock price history for the length of time it has market data for its stock and the historical volatility of similar public companies for the expected term of each grant. During 2023, the Company began to estimate its volatility solely using its stock price history. The Company uses the simplified method as prescribed by the SEC Staff Accounting Bulletin No. 107, Share-Based Payment , to calculate the expected term for options granted to employees as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. For option grants with an expected term for which sufficient stock price history for the Company exists, expected stock price volatility is calculated using the average of volatilities for the period of the expected term prior to the grant date. For options granted to non-employees, the Company utilizes the contractual term of the option arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to do so. The Company determines the fair value of each restricted stock unit ("RSU") at its grant date based on the closing market price of the Company’s common stock on that date or, if the date of grant is not a day on which the Company's primary trading market was open, the immediately preceding trading day. For stock-based compensation subject to service-based vesting conditions, the Company recognizes stock-based compensation expense equal to the grant date fair value of stock-based compensation on a straight-line basis over the requisite service period. The Company records forfeitures as a cumulative adjustment in the period in which they occur.
Property and Equipment	Property and Equipment Upon retirement or sale, the cost of the disposed assets and the related accumulated depreciation are eliminated from the balance sheet, and related gains or losses are reflected in the statements of operations and comprehensive loss. There were no material sales of assets during the years ended December 31, 2023, 2022 and 2021. The Company reviews its long-lived assets for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. If the Company performs an impairment review to evaluate an asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the asset to its carrying value. If the carrying amount of the asset exceeds its estimated undiscounted future net cash flows, the Company recognizes an impairment charge in the amount by which the carrying amount of the asset exceeds the fair value of the asset. The Company did not recognize impairment charges during the years ended December 31, 2023, 2022 and 2021.
Leases	Leases Consistent with ASC 842, Leases , the Company determines if an arrangement is a lease at inception. Operating leases are included in right-of-use lease assets ("ROU assets"), current portion of lease obligations and long-term lease obligations on the Company’s consolidated balance sheets. Assets subject to finance leases are included in property and equipment, and the related lease obligation is included in other current liabilities and other long-term liabilities on the Company’s consolidated balance sheets. Lease assets are tested for impairment in the same manner as long-lived assets used in operations. Lease expense for operating leases is recognized on a straight-line basis over the lease term as an operating expense while expense for financing leases is recognized as depreciation expense and interest expense using the effective interest method. The Company has elected the short-term lease recognition exemption for short-term leases, which allows the Company not to recognize lease liabilities and ROU assets on the consolidated balance sheets for leases with an original term of twelve months or less. ROU assets represent the Company’s right to use an underlying asset for the lease term, and lease obligations represent the Company’s obligation to make lease payments arising from the lease. Operating lease liabilities and their corresponding ROU assets are initially recorded based on the present value of lease payments over the expected remaining lease term. When determining the lease term, the Company includes options to extend or terminate the lease when it is reasonably certain that the option will be exercised. Certain adjustments to the ROU asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate to discount lease payments. The incremental borrowing rate reflects the fixed rate at which the Company could borrow, on a collateralized basis, the amount of the lease payments in the same currency, for a similar term, in a similar economic environment. The Company accounts for lease agreements with lease and non-lease components separately.
Patent Costs	Patent Costs The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the accompanying consolidated statements of operations and comprehensive loss.
Income Taxes	Income Taxes The Company accounts for income taxes in accordance with ASC 740, Accounting for Income Taxes ("ASC 740"), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. The Company determines its deferred tax assets and liabilities based on differences between financial reporting and tax bases of assets and liabilities, which are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Valuation allowances are provided if, based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company recognizes the effect of income tax positions only if those positions are more likely than not to be sustained. Recognized income tax positions are measured at the largest amount that is more likely than not to be realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) comprises net loss and other comprehensive loss. For the years ended December 31, 2023 and 2022, other comprehensive income (loss) consisted of changes in unrealized income and loss on marketable securities. For the year ended December 31, 2021, comprehensive loss equaled net loss.
Concentration of Credit Risk and Off-balance Sheet Risk	Concentration of Credit Risk and Off-balance Sheet Risk Financial instruments that potentially subject the Company to concentrations of credit risk primarily consist of cash equivalents and marketable securities. Under its investment policy, the Company limits amounts invested in such securities by credit rating, maturity, industry group, investment type and issuer, except for securities issued by the U.S. government. The Company does not believe that it is subject to any significant concentrations of credit risk from these financial instruments. The Company has no financial instruments with off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard-setting bodies, and the Company adopts such pronouncements as of the specified effective date. Unless otherwise discussed below, the Company does not believe that the adoption of recently issued standards has had or may have a material impact on the Company's consolidated financial statements or disclosures. In November 2023, the FASB issued Accounting Standard Update 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures , which is intended to improve reportable segment disclosure requirements, primarily through additional disclosures about significant segment expenses. The standard is effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024, with early adoption permitted. The amendments should be applied retrospectively to all prior periods presented in the financial statements. The Company is currently evaluating the disclosure requirements related to the new standard. In December 2023, the FASB, issued Accounting Standard Update 2023-09, Improvements to Income Tax Disclosures , which requires entities to disclose disaggregated information about their effective tax rate reconciliation as well as expanded information on income taxes paid by jurisdiction. The disclosure requirements will be applied on a prospective basis, with the option to apply them retrospectively. The standard is effective for fiscal years beginning after December 15, 2024, with early adoption permitted. The Company is currently evaluating the disclosure requirements related to the new standard.

Summary of significant accoun_3

Summary of significant accounting policies (Tables)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Schedule of Estimated Useful Life of Property and Equipment	Depreciation is computed using the straight-line method over the estimated useful life of each asset as follows: Computer equipment, office equipment and software 3 years Laboratory equipment 5 years Leasehold improvements Shorter of useful life or life of lease

Collaboration agreements (Table

Collaboration agreements (Tables)	12 Months Ended
	Dec. 31, 2023
Collaboration agreements
Schedule of Changes in the Balance of Our Contract Assets and Liabilities and Recognized Revenue	The following table presents changes in the balances of the Company's contract liabilities: Balance at Beginning of Period Additions Deductions Balance at End of Period Year ended December 31, 2023 Contract liabilities: Total deferred revenue $ 147,653 $ 5,517 $ 27,856 $ 125,314 Year ended December 31, 2022 Contract liabilities: Total deferred revenue $ 3,944 $ 170,000 $ 26,291 $ 147,653 During the years ended December 31, 2023 and 2022 the Company recognized the following revenues as a result of changes in the contract liability balances in the respective periods: Year ended December 31, 2023 2022 Revenue recognized in the period from: Amounts included in the contract liability at the beginning of the period $ 27,870 $ 43

Fair value measurements (Tables

Fair value measurements (Tables)	12 Months Ended
	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Fair Value, Assets Measured on Recurring Basis	The following table presents information about the Company's assets measured at fair value on a recurring basis and indicates the level within fair value hierarchy of the valuation techniques utilized to determine such value. December 31, 2023 (in thousands) Total Quoted Prices Significant Significant Cash equivalents Money market funds $ 90,649 $ 90,649 $ — $ — U.S. treasury securities 24,889 24,889 — — $ 115,538 $ 115,538 $ — $ — Marketable securities U.S. treasury securities $ 29,548 $ 29,548 $ — $ — U.S government agency securities 4,975 — 4,975 — $ 34,523 $ 29,548 $ 4,975 $ — December 31, 2022 (in thousands) Total Quoted Prices Significant Significant Cash equivalents Money market funds $ 50,471 $ 50,471 $ — $ — U.S. government agency securities 9,993 — 9,993 — $ 60,464 $ 50,471 $ 9,993 $ — Marketable securities U.S. treasury securities $ 107,810 $ 107,810 $ — $ — U.S. government agency securities 44,017 — 44,017 — $ 151,827 $ 107,810 $ 44,017 $ —

Cash, cash equivalents, and s_2

Cash, cash equivalents, and short-term marketable securities (Tables)	12 Months Ended
	Dec. 31, 2023
Cash and Cash Equivalents [Abstract]
Schedule of Restricted Cash	The following table summarizes the Company's cash, cash equivalents, and restricted cash as of December 31, 2023 and 2022. (in thousands) Year Ended Year Ended December 31, 2022 Beginning End Beginning End Cash and cash equivalents $ 128,885 $ 174,561 $ 177,947 $ 128,885 Restricted cash included in other assets, noncurrent 478 478 478 478 Total cash, cash equivalents and restricted cash per statement of cash flows $ 129,363 $ 175,039 $ 178,425 $ 129,363
Schedule of Marketable Securities	The following table summarizes the Company's marketable securities held at December 31, 2023 and 2022. December 31, 2023 (in thousands) Amortized Gross Gross Fair Marketable securities U.S. treasury securities $ 29,535 $ 13 $ — $ 29,548 U.S. government agency securities 4,977 — (2) 4,975 $ 34,512 $ 13 $ (2) $ 34,523 December 31, 2022 (in thousands) Amortized Gross Gross Fair Marketable securities U.S. treasury securities $ 107,964 $ 7 $ (161) $ 107,810 U.S. government agency securities 44,016 24 (23) 44,017 Total $ 151,980 $ 31 $ (184) $ 151,827

Property and equipment (Tables)

Property and equipment (Tables)	12 Months Ended
	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consists of the following as of December 31, 2023 and 2022: (in thousands) December 31, December 31, Laboratory equipment $ 8,246 $ 7,960 Leasehold improvements 2,206 1,943 Computers, software, and office equipment 2,449 1,824 Total property and equipment at cost 12,901 11,727 Less: Accumulated depreciation (9,070) (7,742) $ 3,831 $ 3,985

Accrued expenses (Tables)

Accrued expenses (Tables)	12 Months Ended
	Dec. 31, 2023
Accrued Liabilities, Current [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following as of December 31, 2023 and 2022: (in thousands) December 31, December 31, Accrued payroll and related expenses $ 8,807 $ 11,558 Accrued clinical expenses 5,063 14,822 Accrued research and non-clinical expenses 3,090 2,767 Accrued manufacturing expenses 2,566 11,536 Accrued restructuring expenses 1,047 — Accrued professional fees 936 1,865 Accrued other 389 636 $ 21,898 $ 43,184

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2023
Debt Disclosure [Abstract]
Schedule of Debt	The following is a summary of obligations under the New Credit Facility as of December 31, 2023: (in thousands) December 31, Total debt $ 25,000 Less: Current portion of long-term-debt (2,083) Total debt, net of current portion 22,917 Debt financing costs, net of accretion (237) Accretion related to final payment 468 Long-term debt, net $ 23,148
Schedule of Estimated Future Payments	As of December 31, 2023, the estimated future principal payments due are as follows: (in thousands) 2024 $ 2,083 2025 12,500 2026 10,417 Total debt $ 25,000

Stockholders_ equity (Tables)

Stockholders’ equity (Tables)	12 Months Ended
	Dec. 31, 2023
Stockholders' Equity Note [Abstract]
Schedule for number of common stock reserved for exercise of outstanding stock options and warrants	December 31, December 31, Stock options 10,902,845 10,051,283 Restricted stock units 3,834,108 1,870,791 Warrants — 22,590 14,736,953 11,944,664

Stock-based compensation (Table

Stock-based compensation (Tables)	12 Months Ended
	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Schedule of Stock Option Activity	A summary of stock option activity is as follows: Number of Shares Weighted- Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value Outstanding at January 1, 2023 10,051,283 $ 9.84 7.2 $ 8,197 Granted 4,017,763 $ 4.76 Exercised (102,596) $ 4.25 Cancelled/forfeited (3,063,605) $ 12.00 Outstanding at December 31, 2023 10,902,845 $ 7.42 6.8 $ 1,786 Exercisable at December 31, 2023 6,499,540 $ 8.23 5.5 $ 711
Schedule of Restricted Stock Unit Activity	A summary of the RSU activity is as follows: Number Weighted-Average Aggregate Weighted-Average Unvested at January 1, 2023 1,870,791 1.5 $ 10,963 $ 8.55 Granted 4,025,544 — $ 4.42 Vested (572,825) — $ 8.83 Forfeited (1,489,402) — $ 6.25 Unvested at December 31, 2023 3,834,108 1.4 $ 8,895 $ 5.01
Schedule of Stock-Based Compensation Expense by Award Type	The following table presents stock-based compensation expense by award type included within the Company’s consolidated statements of operations and comprehensive loss: Year ended December 31, (in thousands) 2023 2022 2021 Stock options $ 14,171 $ 15,814 $ 14,528 Restricted stock units and other stock awards 6,184 5,175 3,522 Employee stock purchase plan 781 533 359 Stock-based compensation expense included in total operating expenses $ 21,136 $ 21,522 $ 18,409
Schedule of Stock-Based Compensation Expense as Reflected in the Company’s Condensed Consolidated Statements of Operations and Comprehensive Loss	The following table presents stock-based compensation expense as reflected in the Company’s consolidated statements of operations and comprehensive loss: Year ended December 31, (in thousands) 2023 2022 2021 Research and development $ 11,043 $ 11,386 $ 9,984 General and administrative 10,093 10,136 8,425 Stock-based compensation expense included in total operating expenses $ 21,136 $ 21,522 $ 18,409
Schedule of Weighted Average Assumptions for Estimating Fair Value of Option Awards	The fair value of each option award is estimated on the date of grant using the Black–Scholes option pricing model with the following weighted average assumptions: December 31, 2023 2022 2021 Risk-free interest rate 3.8 % 2.1 % 0.9 % Expected dividend yield — % — % — % Expected term (years) 6.05 5.99 6.06 Expected stock price volatility 103 % 88 % 82 %

Net loss per share (Tables)

Net loss per share (Tables)	12 Months Ended
	Dec. 31, 2023
Earnings Per Share [Abstract]
Schedule of Outstanding Potentially Dilutive Securities	The following table sets forth the outstanding potentially dilutive securities that have been excluded from the calculation of diluted net loss per share because to include them would be anti-dilutive (in common stock equivalent shares): Year ended December 31, 2023 2022 2021 Stock options 10,902,845 10,051,283 8,342,429 Unvested restricted stock units 3,834,108 1,870,791 817,609 Warrants — 22,590 39,474 14,736,953 11,944,664 9,199,512

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Schedule of Components of Lease Expense	The components of lease expense included in research and development and general and administrative expenses in the statement of operations and comprehensive loss were as follows: Years ended December 31, (in thousands) 2023 2022 2021 Operating lease cost $ 3,838 $ 3,793 $ 3,502 Finance lease cost: Amortization of right-of-use assets $ 185 $ 194 $ 169 Interest on lease liabilities 16 28 28 $ 201 $ 222 $ 197
Schedule of Supplemental Balance Sheet Information Related to Leases	Supplemental balance sheet information related to leases was as follows: Year ended December 31, 2023 2022 Operating leases: Operating lease right-of-use assets $ 7,694 $ 10,475 Operating lease liabilities, current $ 3,252 $ 2,798 Operating lease liabilities $ 5,149 $ 8,575 Finance leases: Property and equipment, gross $ 1,038 $ 1,038 Property and equipment, accumulated depreciation $ (723) $ (539) Other liabilities, current $ 144 $ 240 Other liabilities $ 54 $ 203 Weighted-average remaining lease term: Operating leases 2.3 years 3.3 years Finance leases 1.1 years 2.0 years Weighted-average discount rate: Operating leases 10.8 % 10.8 % Finance leases 4.4 % 5.4 %
Schedule of Supplemental Cash Flow Information Related to Leases	Supplemental cash flow information related to leases was as follows: Year ended December 31, (in thousands) 2023 2022 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 4,029 $ 3,865 Operating cash flows from finance leases $ 16 $ 28 Financing cash flows from finance leases $ 262 $ 272
Schedule of Future Minimum Payments Under Operating Leases	Future lease payments under non-cancellable leases as of December 31, 2023 were as follows: (in thousands) Operating leases Finance leases 2024 $ 3,975 $ 141 2025 4,187 48 2026 1,310 8 2027 and thereafter — — Total lease payments 9,472 197 Present value adjustment (1,069) — Present value of lease liabilities $ 8,403 $ 197
Schedule of Future Minimum Payments on Finance Leases	Future lease payments under non-cancellable leases as of December 31, 2023 were as follows: (in thousands) Operating leases Finance leases 2024 $ 3,975 $ 141 2025 4,187 48 2026 1,310 8 2027 and thereafter — — Total lease payments 9,472 197 Present value adjustment (1,069) — Present value of lease liabilities $ 8,403 $ 197

Income taxes (Tables)

Income taxes (Tables)	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Schedule of Reconciliation of Income Tax Rates	A reconciliation of the effective tax rate for the years ended December 31, 2023 and 2022 is as follows: 2023 2022 Statutory US Federal Rate 21.0 % 21.0 % State taxes, net of federal benefit 5.5 % 5.7 % Permanent differences (0.1) % (0.1) % General business credits 4.0 % 4.4 % Stock compensation (1.9) % (1.4) % Change in valuation allowance (28.5) % (29.6) % — % — %
Schedule of Significant Components of Net Deferred Tax Assets	Significant components of the Company’s net deferred tax assets as of December 31, 2023 and 2022 are as follows: (in thousands) 2023 2022 Deferred tax assets: Net operating losses $ 126,798 $ 113,981 R&D capitalization 67,107 40,380 Tax credit carryforwards 28,409 21,429 Stock-based compensation 6,899 6,522 Accrued expenses 3,142 3,340 Lease liabilities 2,274 3,096 Capitalized licenses 2,725 3,077 Deferred revenue 3,688 1,062 Depreciation 353 437 Other 156 116 Total gross deferred tax assets 241,551 193,440 Valuation allowance (239,468) (190,588) Net deferred tax assets less valuation allowance 2,083 2,852 Deferred tax liabilities Right-of-use assets (2,083) (2,852) Total gross deferred tax liabilities (2,083) (2,852) Net deferred taxes $ — $ —

Restructuring (Tables)

Restructuring (Tables)	12 Months Ended
	Dec. 31, 2023
Restructuring and Related Activities [Abstract]
Schedule of Restructuring and Related Costs	The following table summarizes the charges incurred in connection with the Restructuring: (in thousands) Severance & Employee Related Costs Contract Termination and Other Costs Total Costs Cumulative costs to date $ 6,924 $ 1,789 $ 8,713 Costs incurred during the year ended December 31, 2023 $ 6,924 $ 1,789 $ 8,713 The following tables summarizes the charges incurred in connection with the Restructuring related to research and development activities and general and administrative activities: (in thousands) Year ended December 31, 2023 Research and development related $ 5,393 General and administrative related $ 3,320 Accrued restructuring costs, which are included in accrued expenses on the consolidated balance sheets, were as follows: (in thousands) Severance & Employee Related Costs Contract Termination Costs Total Costs Balance at December 31, 2022 $ — $ — $ — Additional expense 6,924 372 7,296 Cash payments (5,917) — (5,917) Other adjustments (252) (80) (332) Balance at December 31, 2023 $ 755 $ 292 $ 1,047

Nature of business and basis _2

Nature of business and basis of presentation (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) candidate	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of additional preclinical candidates | candidate	2
Net loss	$ 171,670	$ 204,212	$ 170,060
Accumulated deficit	$ 826,361	$ 654,691

Summary of significant accoun_4

Summary of significant accounting policies - Revenue Recognition (Details)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Billing term	30 days

Summary of significant accoun_5

Summary of significant accounting policies - Stock-Based Compensation (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accounting Policies [Abstract]
Expected dividend yield	0%	0%	0%

Summary of significant accoun_6

Summary of significant accounting policies - Property and Equipment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property, equipment and software, net
Asset impairment charges	$ 0	$ 0	$ 0
Computer equipment, office equipment and software
Property, equipment and software, net
Estimated useful life	3 years
Laboratory equipment
Property, equipment and software, net
Estimated useful life	5 years

Summary of significant accoun_7

Summary of significant accounting policies - Risks (Details)	12 Months Ended
	Dec. 31, 2023 USD ($)
Concentration of Credit Risk and Off-balance Sheet Risk
Financial instruments with off-balance sheet risk	$ 0

Collaboration agreements - Glax

Collaboration agreements - GlaxoSmithKline (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Aug. 31, 2022 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Aug. 06, 2022 USD ($) performanceObligation
Collaboration agreements
Collaboration revenue		$ 36,855	$ 26,581	$ 43
Deferred revenue		125,314	147,653	$ 3,944
GSK Agreement
Collaboration agreements
Upfront payment received	$ 100,000
Collaboration arrangement, potential option exercise receivable	90,000
Revised transaction price					$ 100,000
Performance obligations identified | performanceObligation					2
Collaboration revenue		3,400	2,000
Deferred revenue		$ 94,600	$ 98,000
GSK Agreement | Developmental, Commercial, And Regulatory
Collaboration agreements
Aggregate milestones	1,300,000
GSK Agreement | Early Clinical Development
Collaboration agreements
Aggregate milestones	$ 30,000

Collaboration agreements - John

Collaboration agreements - Johnson & Johnson (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2022 USD ($) target license performanceObligation	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Collaboration agreements
Collaboration revenue		$ 36,855	$ 26,581	$ 43
Deferred revenue		125,314	147,653	$ 3,944
Johnson & Johnson Agreement
Collaboration agreements
Number of targets designated | target	3
Upfront payment received	$ 40,000
Number of licenses | license	2
Performance obligations identified | performanceObligation	7
Revised transaction price	$ 40,000	48,000
Transaction price adjustment		6,000
Collaboration revenue		16,600	24,200
Deferred revenue		$ 10,400	$ 15,800
Johnson & Johnson Agreement | Development and Regulatory
Collaboration agreements
Aggregate milestones	505,000
Johnson & Johnson Agreement | Commercial
Collaboration agreements
Aggregate milestones	$ 530,000

Collaboration agreements - Merc

Collaboration agreements - Merck Immunosynthen Agreement (Details)	1 Months Ended	12 Months Ended
	Dec. 31, 2022 USD ($) license target performanceObligation	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($) license target performanceObligation	Dec. 31, 2021 USD ($)
Collaboration agreements
Collaboration revenue		$ 36,855,000	$ 26,581,000	$ 43,000
Deferred revenue	$ 147,653,000	125,314,000	$ 147,653,000	$ 3,944,000
2022 Merck KGaA Agreement
Collaboration agreements
Number of targets designated | target	2		2
Upfront payment received	$ 30,000,000
Number of licenses | license	2		2
Performance obligations identified | performanceObligation	4		4
Revised transaction price	$ 32,000,000		$ 32,000,000
Contract with customer, receivable, after allowance for credit loss	30,000,000		30,000,000
Collaboration revenue		10,700,000	0
Deferred revenue	30,000,000	$ 20,200,000	30,000,000
2022 Merck KGaA Agreement | Developmental And Regulatory
Collaboration agreements
Aggregate milestones	200,000,000		200,000,000
2022 Merck KGaA Agreement | Commercial
Collaboration agreements
Aggregate milestones	$ 600,000,000		$ 600,000,000

Collaboration agreements - Me_2

Collaboration agreements - Merck Dolaflexin Agreement (Details)	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($) target	Dec. 31, 2021 USD ($)	Sep. 30, 2023 target	Feb. 28, 2022 target
Collaboration agreements
Collaboration revenue	$ 36,855,000	$ 26,581,000	$ 43,000
Deferred revenue	125,314,000	147,653,000	$ 3,944,000
2014 Merck KGaA Agreement and 2018 Merck KGaA Supply Agreement
Collaboration agreements
Collaboration revenue	3,700,000
Deferred revenue	0	3,900,000
Contract assets	0	0
2014 Merck KGaA
Collaboration agreements
Number of targets designated | target				6
Johnson & Johnson Agreement
Collaboration agreements
Collaboration revenue	16,600,000	24,200,000
Number of targets designated | target					3
Deferred revenue	10,400,000	15,800,000
Contract assets	0	0
2022 Merck KGaA Agreement
Collaboration agreements
Collaboration revenue	10,700,000	$ 0
Number of targets designated | target		2
Deferred revenue	20,200,000	$ 30,000,000
Contract assets	0	0
GSK Agreement
Collaboration agreements
Collaboration revenue	3,400,000	2,000,000
Deferred revenue	94,600,000	98,000,000
Contract assets	$ 0	$ 0

Collaboration agreements - Summ

Collaboration agreements - Summary of Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Contract Liabilities [Roll Forward]
Balance at Beginning of Period	$ 147,653	$ 3,944
Additions	5,517	170,000
Deductions	27,856	26,291
Balance at End of Period	125,314	147,653
Amounts included in the contract liability at the beginning of the period	$ 27,870	$ 43

Collaboration agreements - Othe

Collaboration agreements - Other Revenue (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaboration agreements
Collaboration revenue	$ 36,855,000	$ 26,581,000	$ 43,000
Limited services | Asana BioSciences
Collaboration agreements
Collaboration revenue	0	$ 300,000	$ 0
Development Milestone Achieved | Asana BioSciences
Collaboration agreements
Collaboration revenue	$ 2,500,000

Fair value measurements (Detail

Fair value measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Fair value measurements
Marketable securities	$ 34,523	$ 151,827
U.S. treasury securities
Fair value measurements
Marketable securities	29,548	107,810
U.S government agency securities
Fair value measurements
Marketable securities	4,975	44,017
Recurring basis
Fair value measurements
Cash equivalents	115,538	60,464
Marketable securities	34,523	151,827
Recurring basis | Quoted Prices in Active Markets (Level 1)
Fair value measurements
Cash equivalents	115,538	50,471
Marketable securities	29,548	107,810
Recurring basis | Significant Other Observable Inputs (Level 2)
Fair value measurements
Cash equivalents	0	9,993
Marketable securities	4,975	44,017
Recurring basis | Significant Unobservable Inputs (Level 3)
Fair value measurements
Cash equivalents	0	0
Marketable securities	0	0
Recurring basis | Money market funds
Fair value measurements
Cash equivalents	90,649	50,471
Recurring basis | Money market funds | Quoted Prices in Active Markets (Level 1)
Fair value measurements
Cash equivalents	90,649	50,471
Recurring basis | Money market funds | Significant Other Observable Inputs (Level 2)
Fair value measurements
Cash equivalents	0	0
Recurring basis | Money market funds | Significant Unobservable Inputs (Level 3)
Fair value measurements
Cash equivalents	0	0
Recurring basis | U.S. treasury securities
Fair value measurements
Cash equivalents	24,889
Marketable securities	29,548	107,810
Recurring basis | U.S. treasury securities | Quoted Prices in Active Markets (Level 1)
Fair value measurements
Cash equivalents	24,889
Marketable securities	29,548	107,810
Recurring basis | U.S. treasury securities | Significant Other Observable Inputs (Level 2)
Fair value measurements
Cash equivalents	0
Marketable securities	0	0
Recurring basis | U.S. treasury securities | Significant Unobservable Inputs (Level 3)
Fair value measurements
Cash equivalents	0
Marketable securities	0	0
Recurring basis | U.S government agency securities
Fair value measurements
Cash equivalents		9,993
Marketable securities	4,975	44,017
Recurring basis | U.S government agency securities | Quoted Prices in Active Markets (Level 1)
Fair value measurements
Cash equivalents		0
Marketable securities	0	0
Recurring basis | U.S government agency securities | Significant Other Observable Inputs (Level 2)
Fair value measurements
Cash equivalents		9,993
Marketable securities	4,975	44,017
Recurring basis | U.S government agency securities | Significant Unobservable Inputs (Level 3)
Fair value measurements
Cash equivalents		0
Marketable securities	$ 0	$ 0

Cash, cash equivalents, and s_3

Cash, cash equivalents, and short-term marketable securities - Schedule of Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 174,561	$ 128,885	$ 177,947
Restricted cash included in other assets, noncurrent	478	478	478
Total cash, cash equivalents and restricted cash per statement of cash flows	$ 175,039	$ 129,363	$ 178,425	$ 255,415

Cash, cash equivalents, and s_4

Cash, cash equivalents, and short-term marketable securities - Debt Securities, Available-for-Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 34,512	$ 151,980
Gross Unrealized Gains	13	31
Gross Unrealized Losses	(2)	(184)
Short-term marketable securities	34,523	151,827
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	29,535	107,964
Gross Unrealized Gains	13	7
Gross Unrealized Losses	0	(161)
Short-term marketable securities	29,548	107,810
U.S government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,977	44,016
Gross Unrealized Gains	0	24
Gross Unrealized Losses	(2)	(23)
Short-term marketable securities	$ 4,975	$ 44,017

Cash, cash equivalents, and s_5

Cash, cash equivalents, and short-term marketable securities - Narrative (Details)	Dec. 31, 2023 USD ($) security	Dec. 31, 2022 USD ($)
Cash and Cash Equivalents [Abstract]
Number of securities in an unrealized loss position | security	1
Securities in an unrealized loss position, accumulated loss | $	$ 5,000,000
Securities in an unrealized loss position greater than 12 months | security	0
Charges for credit-related impairments | $	$ 0	$ 0

Property and equipment - Summar

Property and equipment - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, equipment and software, net
Total property and equipment at cost	$ 12,901	$ 11,727
Less: Accumulated depreciation	(9,070)	(7,742)
Property and equipment, net	3,831	3,985
Laboratory equipment
Property, equipment and software, net
Total property and equipment at cost	8,246	7,960
Leasehold improvements
Property, equipment and software, net
Total property and equipment at cost	2,206	1,943
Computers, software, and office equipment
Property, equipment and software, net
Total property and equipment at cost	$ 2,449	$ 1,824

Property and equipment - Narrat

Property and equipment - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 1.5	$ 0.9	$ 0.9

Accrued expenses (Details)

Accrued expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Accrued Liabilities, Current [Abstract]
Accrued payroll and related expenses	$ 8,807	$ 11,558
Accrued clinical expenses	5,063	14,822
Accrued research and non-clinical expenses	3,090	2,767
Accrued manufacturing expenses	2,566	11,536
Accrued restructuring expenses	1,047	0
Accrued professional fees	936	1,865
Accrued other	389	636
Total	$ 21,898	$ 43,184

Debt - Narrative (Details)

Debt - Narrative (Details) - USD ($)				12 Months Ended
	Oct. 29, 2021	Aug. 28, 2020	May 08, 2019	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Existing Credit Facility
Debt
Interest expense				$ 0	$ 0	$ 800,000
New Credit Facility
Debt
Interest expense				$ 3,900,000	$ 3,200,000
Line of credit | Existing Credit Facility
Debt
Proceeds from line of credit			$ 5,000,000
Line of credit | Existing Credit Facility | Minimum
Debt
Debt interest rate percentage			4%
Line of credit | Existing Credit Facility | Minimum | Prime rate
Debt
Spread (as a percent)			(1.50%)
Line of credit | Amended Credit Facility
Debt
Proceeds from line of credit		$ 5,200,000
Repayments of lines of credit	$ 5,500,000
Line of credit | Amended Credit Facility | Minimum
Debt
Debt interest rate percentage		4.25%
Line of credit | Amended Credit Facility | Minimum | Prime rate
Debt
Spread (as a percent)		1%
Line of credit | New Credit Facility
Debt
Proceeds from line of credit	$ 25,000,000
Final payment as percentage of original principal amount	4.25%
Additional rate in event of default	5%
Line of credit | New Credit Facility | Prepayment Occurs After October 29, 2023 but Before October 1, 2026
Debt
Prepayment fee percentage	1%
Line of credit | New Credit Facility | Minimum
Debt
Debt interest rate percentage	8.50%
Line of credit | New Credit Facility | Minimum | Prime rate
Debt
Spread (as a percent)	5.25%

Debt - Schedule of Debt (Detail

Debt - Schedule of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Disclosure [Abstract]
Total debt	$ 25,000
Less: Current portion of long-term-debt	(2,083)
Total debt, net of current portion	22,917
Debt financing costs, net of accretion	(237)
Accretion related to final payment	468
Long-term debt, net	$ 23,148	$ 24,929

Debt - Schedule of Estimated Fu

Debt - Schedule of Estimated Future Payments (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Estimated future principal payments
2024	$ 2,083
2025	12,500
2026	10,417
Total debt	$ 25,000

Stockholders_ equity - Narrativ

Stockholders’ equity - Narrative (Details)	12 Months Ended
	Dec. 31, 2023 USD ($) vote $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) shares	Dec. 31, 2013 $ / shares	Nov. 30, 2022 USD ($)	Jun. 09, 2022 $ / shares shares	Jun. 08, 2022 shares	Feb. 28, 2022 USD ($)	May 31, 2020 USD ($)
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)	25,000,000	25,000,000
Preferred stock, shares issued (in shares)	0	0
Warrant exercise price per share (in dollars per share) | $ / shares				$ 0.05
Contractual life of warrants				10 years
Number of shares of common stock into which warrants may be converted (in shares)	0
Common stock, par value (in dollars per share) | $ / shares	$ 0.0001	$ 0.0001				$ 0.0001
Common stock, shares authorized (in shares)	350,000,000	350,000,000				350,000,000	175,000,000
Number of shares reserved for future issuance (in shares)	14,736,953	11,944,664
Common Stock
Class of Stock [Line Items]
Exercise of common stock warrant (in shares)		16,654
Number of votes per common share | vote	1
Warrants and share-based payment awards
Class of Stock [Line Items]
Number of shares reserved for future issuance (in shares)	14,736,953	11,944,664
At Market Equity Offering Program (2020 ATM)
Class of Stock [Line Items]
Amount of shares authorized to be offered and sold | $									$ 100,000,000
Proceeds from issuance of common stock | $		$ 54,800,000	$ 43,100,000
At Market Equity Offering Program (2020 ATM) | Common Stock
Class of Stock [Line Items]
Number of shares sold (in shares)		11,740,210	3,961,074
At Market Equity Offering Program (February 2022 ATM)
Class of Stock [Line Items]
Amount of shares authorized to be offered and sold | $								$ 100,000,000
Proceeds from issuance of common stock | $	$ 1,600,000	$ 96,400,000
At Market Equity Offering Program (February 2022 ATM) | Common Stock
Class of Stock [Line Items]
Number of shares sold (in shares)	256,386	18,757,665
At Market Equity Offering Program (November 2022 ATM)
Class of Stock [Line Items]
Amount of shares authorized to be offered and sold | $					$ 150,000,000
Proceeds from issuance of common stock | $	$ 92,200,000
At Market Equity Offering Program (November 2022 ATM) | Common Stock
Class of Stock [Line Items]
Number of shares sold (in shares)	14,208,145
Stock issuance program, remaining unsold and available amount | $	$ 55,900,000

Stockholders_ equity - Common S

Stockholders’ equity - Common Stock (Details) - shares	Dec. 31, 2023	Dec. 31, 2022
Class of Stock [Line Items]
Number of shares reserved for future issuance (in shares)	14,736,953	11,944,664
Warrants and share-based payment awards
Class of Stock [Line Items]
Number of shares reserved for future issuance (in shares)	14,736,953	11,944,664
Share-based payments | Stock options
Class of Stock [Line Items]
Number of shares reserved for future issuance (in shares)	10,902,845	10,051,283
Share-based payments | Restricted stock units
Class of Stock [Line Items]
Number of shares reserved for future issuance (in shares)	3,834,108	1,870,791
Warrants
Class of Stock [Line Items]
Number of shares reserved for future issuance (in shares)	0	22,590

Stock-based compensation - Narr

Stock-based compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Jan. 01, 2023	Jun. 30, 2017	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2017	Feb. 28, 2022
Share-based awards
Weighted-average grant date fair value of options granted (in dollars per share)			$ 3.82	$ 3.97	$ 11.71
Intrinsic value of options exercised			$ 300	$ 1,500	$ 4,300
Cash received from the exercise of stock options (in dollars)			$ 427	$ 1,331	$ 1,837
Number of shares reserved for future issuance (in shares)			14,736,953	11,944,664
Expected dividend yield			0%	0%	0%
Stock options
Share-based awards
Unrecognized stock compensation cost, unvested stock options			$ 14,800
Weighted-average amortization period of unrecognized stock compensation cost			1 year 7 months 6 days
Unvested restricted stock units
Share-based awards
Fair value of RSUs vested			$ 3,100	$ 1,500	$ 5,800
Unrecognized stock compensation cost, unvested RSUs			$ 11,800
Weighted-average amortization period of unrecognized stock compensation cost			2 years 4 months 24 days
Unvested restricted stock units | Non-employee director
Share-based awards
Vesting period			1 year
Unvested restricted stock units | Minimum
Share-based awards
Vesting period			1 year
Unvested restricted stock units | Maximum
Share-based awards
Vesting period			4 years
2017 Plan
Share-based awards
Cumulative annual increase in number of shares issuable, percentage		4%
Additional shares authorized (in shares)	4,205,794
Grants in period (in shares)			7,211,153
Number of shares available for future issuance (in shares)			2,043,328
2017 Plan | Maximum
Share-based awards
Expiration period			10 years
2017 Plan | Stock options
Share-based awards
Vesting period			4 years
2017 Plan | Stock options | Board of Directors
Share-based awards
Vesting period			3 years
2017 Plan | Stock options | Non-employee director
Share-based awards
Vesting period			1 year
Inducement Award Program
Share-based awards
Grants in period (in shares)			877,575
Number of shares available for future issuance (in shares)			1,170,752
Number of shares authorized (in shares)							2,000,000
Prior to Inducement Award Program
Share-based awards
Number of shares authorized (in shares)			457,500
2017 ESPP | Employee stock purchase plan
Share-based awards
Additional shares authorized (in shares)	450,000
Number of shares available for future issuance (in shares)			364,283
Number of shares reserved for future issuance (in shares)						225,000
Purchase of common stock under ESPP (in shares)			381,508	270,774
2017 ESPP | Employee stock purchase plan | Common Stock
Share-based awards
Cumulative annual increase in number of shares issuable, percentage						1%
Maximum annual increase (in shares)						450,000
2017 ESPP | Employee stock purchase plan | Maximum
Share-based awards
Number of shares authorized (in shares)						4,725,000

Stock-based compensation - Acti

Stock-based compensation - Activity Under Stock Option Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Number of Shares
Outstanding at beginning of period (in shares)	10,051,283
Granted (in shares)	4,017,763
Exercised (in shares)	(102,596)
Cancelled/forfeited (in shares)	(3,063,605)
Outstanding at end of period (in shares)	10,902,845	10,051,283
Exercisable, at end of period (in shares)	6,499,540
Weighted- Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 9.84
Granted (in dollars per share)	4.76
Exercised (in dollars per share)	4.25
Cancelled/forfeited (in dollars per share)	12
Outstanding at end of period (in dollars per share)	7.42	$ 9.84
Exercisable, at end of period (in dollars per share)	$ 8.23
Additional Disclosures
Weighted average remaining contractual term, options outstanding	6 years 9 months 18 days	7 years 2 months 12 days
Weighted average remaining contractual term, options exercisable	5 years 6 months
Aggregate intrinsic value, options outstanding	$ 1,786	$ 8,197
Aggregate intrinsic value, options exercisable	$ 711

Stock-based compensation - Rest

Stock-based compensation - Restricted Stock Units and Other Stock Awards (Details) - Unvested restricted stock units - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Number of Shares
Unvested at beginning of period (in shares)	1,870,791
Granted (in shares)	4,025,544
Vested (in shares)	(572,825)
Forfeited (in shares)	(1,489,402)
Unvested at end of period (in shares)	3,834,108	1,870,791
Weighted Average Remaining Contractual Term and Aggregate Intrinsic Value
Weighted-average remaining contractual term	1 year 4 months 24 days	1 year 6 months
Aggregate intrinsic value (in dollars)	$ 8,895	$ 10,963
Weighted-Average Grant Date Fair Value
Unvested at beginning of period (in dollars per share)	$ 8.55
Granted (in dollars per share)	4.42
Vested (in dollars per share)	8.83
Forfeited (in dollars per share)	6.25
Unvested at end of period (in dollars per share)	$ 5.01	$ 8.55

Stock-based compensation - Stoc

Stock-based compensation - Stock Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Stock options
Stock-based compensation expense included in total operating expenses	$ 21,136	$ 21,522	$ 18,409
Research and development
Stock options
Stock-based compensation expense included in total operating expenses	11,043	11,386	9,984
General and administrative
Stock options
Stock-based compensation expense included in total operating expenses	10,093	10,136	8,425
Stock options
Stock options
Stock-based compensation expense included in total operating expenses	14,171	15,814	14,528
Restricted stock units and other stock awards
Stock options
Stock-based compensation expense included in total operating expenses	6,184	5,175	3,522
Employee stock purchase plan
Stock options
Stock-based compensation expense included in total operating expenses	$ 781	$ 533	$ 359

Stock-based compensation - Fair

Stock-based compensation - Fair Value Assumptions (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-Based Payment Arrangement [Abstract]
Risk-free interest rate	3.80%	2.10%	0.90%
Expected dividend yield	0%	0%	0%
Expected term (years)	6 years 18 days	5 years 11 months 26 days	6 years 21 days
Expected stock price volatility	103%	88%	82%

Net loss per share (Details)

Net loss per share (Details) - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Anti-dilutive securities
Anti-dilutive securities (in shares)	14,736,953	11,944,664	9,199,512
Stock options
Anti-dilutive securities
Anti-dilutive securities (in shares)	10,902,845	10,051,283	8,342,429
Unvested restricted stock units
Anti-dilutive securities
Anti-dilutive securities (in shares)	3,834,108	1,870,791	817,609
Warrants
Anti-dilutive securities
Anti-dilutive securities (in shares)	0	22,590	39,474

Leases - Narrative (Details)

Leases - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Apr. 05, 2021
Leases
Lease liability	$ 8,403
Letter of credit for security deposit	$ 500	$ 500
Minimum
Leases
Finance lease, remaining lease term	1 year
Maximum
Leases
Finance lease, remaining lease term	5 years
Office space, Cambridge, MA, Leased
Leases
Renewal term	5 years
Office Space Expansion Cambridge Ma
Leases
Renewal term			5 years
Operating lease term of contract			5 years
Lease liability			$ 5,000

Leases - Components of Lease Ex

Leases - Components of Lease Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
Operating lease cost	$ 3,838	$ 3,793	$ 3,502
Finance lease cost:
Amortization of right-of-use assets	185	194	169
Interest on lease liabilities	16	28	28
Finance lease cost	$ 201	$ 222	$ 197

Leases - Supplemental Balance S

Leases - Supplemental Balance Sheet Information (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Operating leases:
Operating lease right-of-use assets	$ 7,694	$ 10,475
Operating lease liabilities, current	3,252	2,798
Operating lease liabilities	5,149	8,575
Finance leases:
Property and equipment, gross	1,038	1,038
Property and equipment, accumulated depreciation	(723)	(539)
Other liabilities, current	144	240
Other liabilities	$ 54	$ 203
Weighted-average remaining lease term:
Operating leases	2 years 3 months 18 days	3 years 3 months 18 days
Finance leases	1 year 1 month 6 days	2 years
Weighted-average discount rate:
Operating leases	10.80%	10.80%
Finance leases	4.40%	5.40%
Finance lease, liability, current, statement of financial position	Other current liabilities	Other current liabilities
Finance lease, liability, noncurrent, statement of financial position	Other liabilities, noncurrent	Other liabilities, noncurrent

Leases - Supplemental Cash Flow

Leases - Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 4,029	$ 3,865
Operating cash flows from finance leases	16	28
Financing cash flows from finance leases	$ 262	$ 272	$ 215

Leases - Future Minimum Lease P

Leases - Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Operating leases
2024	$ 3,975
2025	4,187
2026	1,310
2027 and thereafter	0
Total lease payments	9,472
Present value adjustment	(1,069)
Present value of lease liabilities	8,403
Finance leases
2024	141
2025	48
2026	8
2027 and thereafter	0
Total lease payments	197
Present value adjustment	0
Present value of lease liabilities	$ 197

Income taxes - Narrative (Detai

Income taxes - Narrative (Details)	12 Months Ended
	Dec. 31, 2023 USD ($) jurisdiction audit	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Income Tax Examination [Line Items]
Income tax benefit	$ 0	$ 0	$ 0
Valuation allowance	239,468,000	190,588,000
Increase (decrease) in valuation allowance	48,900,000
Unrecognized tax benefits	0	0
Accrued interest or penalties related to uncertain tax position	$ 0	$ 0
Income taxes, number of state jurisdictions | jurisdiction	8
Number of federal or state audits in progress | audit	0
Federal
Income Tax Examination [Line Items]
Net operating loss carry forwards	$ 479,000,000
Net operating loss carryforwards subject to expiration	34,100,000
Net operating loss carryforwards not subject to expiration	444,800,000
Federal | Research and development
Income Tax Examination [Line Items]
Tax credit carryforwards	23,200,000
State
Income Tax Examination [Line Items]
Net operating loss carry forwards	414,800,000
State | Research and development
Income Tax Examination [Line Items]
Tax credit carryforwards	$ 6,800,000

Income taxes - Reconciliation (

Income taxes - Reconciliation (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory US Federal Rate	21%	21%
State taxes, net of federal benefit	5.50%	5.70%
Permanent differences	(0.10%)	(0.10%)
General business credits	4%	4.40%
Stock compensation	(1.90%)	(1.40%)
Change in valuation allowance	(28.50%)	(29.60%)
Income tax benefit	0%	0%

Income taxes - Components of De

Income taxes - Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Net operating losses	$ 126,798	$ 113,981
R&D capitalization	67,107	40,380
Tax credit carryforwards	28,409	21,429
Stock-based compensation	6,899	6,522
Accrued expenses	3,142	3,340
Lease liabilities	2,274	3,096
Capitalized licenses	2,725	3,077
Deferred revenue	3,688	1,062
Depreciation	353	437
Other	156	116
Total gross deferred tax assets	241,551	193,440
Valuation allowance	(239,468)	(190,588)
Net deferred tax assets less valuation allowance	2,083	2,852
Deferred tax liabilities
Right-of-use assets	(2,083)	(2,852)
Total gross deferred tax liabilities	(2,083)	(2,852)
Net deferred taxes	$ 0	$ 0

Employee benefit plan (Details)

Employee benefit plan (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) hour	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Retirement Benefits [Abstract]
Eligible age to participate	21 years
Minimum number of hours for eligibility | hour	1,000
Maximum contribution by employee (as a percent)	95%
Percentage employer matches of the employee's percentage contribution matched	100%	100%	100%
Employer matching contribution (as a percent)	4%	4%	4%
Employee benefit expenses | $	$ 1.3	$ 1.1	$ 0.8

Commitments - License agreement

Commitments - License agreements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
License agreement, upfront payments
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development expense	$ 0	$ 1,500,000	$ 3,100,000
License agreement, milestone payments
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development expense	$ 0	$ 700,000	$ 2,100,000

Restructuring - Narrative (Deta

Restructuring - Narrative (Details)	Jul. 26, 2023
Restructuring and Related Activities [Abstract]
Percentage of positions to be eliminated	0.50

Restructuring - Charges Incurre

Restructuring - Charges Incurred (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restructuring Cost and Reserve [Line Items]
Cumulative costs to date	$ 8,713
Restructuring expenses	8,713	$ 0	$ 0
Severance & Employee Related Costs
Restructuring Cost and Reserve [Line Items]
Cumulative costs to date	6,924
Restructuring expenses	6,924
Contract Termination and Other Costs
Restructuring Cost and Reserve [Line Items]
Cumulative costs to date	1,789
Restructuring expenses	$ 1,789

Restructuring - Restructuring R

Restructuring - Restructuring Related to Research and Development Activities and General and Administrative Activities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 8,713	$ 0	$ 0
Research and development
Restructuring Cost and Reserve [Line Items]
Restructuring charges	5,393
General and administrative
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 3,320

Restructuring - Accrued Restruc

Restructuring - Accrued Restructuring Costs (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)
Restructuring Reserve [Roll Forward]
Balance at December 31, 2022	$ 0
Additional expense	7,296
Cash payments	(5,917)
Other adjustments	(332)
Balance at December 31, 2023	1,047
Severance & Employee Related Costs
Restructuring Reserve [Roll Forward]
Balance at December 31, 2022	0
Additional expense	6,924
Cash payments	(5,917)
Other adjustments	(252)
Balance at December 31, 2023	755
Contract Termination and Other Costs
Restructuring Reserve [Roll Forward]
Balance at December 31, 2022	0
Additional expense	372
Cash payments	0
Other adjustments	(80)
Balance at December 31, 2023	$ 292