0001628280-23-016643 | 10-Q | XBRL Viewer

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Cover Page - shares	3 Months Ended
Cover Page - shares	Mar. 31, 2023	May 05, 2023
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2023
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 Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		114,385,810
Entity Central Index Key	0001442836
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q1
Amendment Flag	false

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 122,825	$ 128,885
Short-term marketable securities	151,094	151,827
Accounts receivable	0	30,000
Prepaid expenses and other current assets	7,888	8,507
Total current assets	281,807	319,219
Property and equipment, net	3,994	3,985
Operating lease right-of-use assets	9,806	10,475
Other assets, noncurrent	588	661
Total assets	296,195	334,340
Current liabilities:
Accounts payable	18,156	13,951
Accrued expenses	32,588	43,184
Deferred revenue	28,769	30,610
Operating lease liabilities	2,904	2,798
Other current liabilities	985	990
Total current liabilities	83,402	91,533
Operating lease liabilities, noncurrent	7,773	8,575
Long-term debt, net	25,004	24,929
Deferred revenue, noncurrent	115,582	117,043
Other liabilities, noncurrent	140	203
Total liabilities	231,901	242,283
Commitments (Note 11)
Stockholders' equity:
Preferred stock, $0.0001 par value; 25,000,000 shares authorized; 0 shares issued and outstanding at March 31, 2023 and December 31, 2022, respectively	0	0
Common stock, $0.0001 par value; 350,000,000 shares authorized; 109,061,074 and 105,144,864 shares issued and outstanding at March 31, 2023 and December 31, 2022, respectively	11	11
Additional paid-in capital	775,125	746,889
Accumulated other comprehensive income (loss)	12	(152)
Accumulated deficit	(710,854)	(654,691)
Total stockholders’ equity	64,294	92,057
Total liabilities and stockholders’ equity	$ 296,195	$ 334,340

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 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
Common stock, shares issued (in shares)	109,061,074	105,144,864
Common stock, shares outstanding (in shares)	109,061,074	105,144,864

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Income Statement [Abstract]
Collaboration revenue	$ 7,802	$ 2,036
Operating expenses:
Research and development	47,275	35,806
General and administrative	18,328	12,782
Total operating expenses	65,603	48,588
Other income (expense):
Interest income	2,621	18
Interest expense	(983)	(724)
Total other income (expense), net	1,638	(706)
Net loss	(56,163)	(47,258)
Unrealized gain on marketable securities	164	0
Comprehensive loss	(55,999)	(47,258)
Net loss attributable to common stockholders — basic	(56,163)	(47,258)
Net loss attributable to common stockholders — diluted	$ (56,163)	$ (47,258)
Net loss per share attributable to common stockholders — basic (in dollars per share)	$ (0.52)	$ (0.59)
Net loss per share attributable to common stockholders — diluted (in dollars per share)	$ (0.52)	$ (0.59)
Weighted-average number of shares of common stock used in net loss per share attributable to common stockholders — basic (in shares)	107,514,655	79,928,591
Weighted-average number of shares of common stock used in net loss per share attributable to common stockholders — diluted (in shares)	107,514,655	79,928,591

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at beginning of period (in shares) at Dec. 31, 2021		73,709,056
Balance at beginning of period at Dec. 31, 2021	$ 121,741	$ 7	$ 572,213	$ 0	$ (450,479)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under public offering, net of issuance costs (in shares)		13,169,903
Issuance of common stock under public offering, net of issuance costs	60,462	$ 2	60,460
Exercise of stock options (in shares)		26,951
Exercise of stock options	96		96
Vesting of restricted stock units, net of employee tax obligations (in shares)		167,174
Vesting of restricted stock units	0
Stock-based compensation expense	5,485		5,485
Net (loss) income	(47,258)				(47,258)
Balance at end of period (in shares) at Mar. 31, 2022		87,073,084
Balance at end of period at Mar. 31, 2022	140,526	$ 9	638,254	0	(497,737)
Balance at beginning of period (in shares) at Dec. 31, 2022		105,144,864
Balance at beginning of period at Dec. 31, 2022	92,057	$ 11	746,889	(152)	(654,691)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under public offering, net of issuance costs (in shares)		3,535,093
Issuance of common stock under public offering, net of issuance costs	$ 21,795		21,795
Exercise of stock options (in shares)	8,826	8,826
Exercise of stock options	$ 34		34
Vesting of restricted stock units, net of employee tax obligations (in shares)		372,291
Vesting of restricted stock units	0
Stock-based compensation expense	6,407		6,407
Other comprehensive gain	164			164
Net (loss) income	(56,163)				(56,163)
Balance at end of period (in shares) at Mar. 31, 2023		109,061,074
Balance at end of period at Mar. 31, 2023	$ 64,294	$ 11	$ 775,125	$ 12	$ (710,854)

Condensed Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
At market equity offering program
Stock issuance costs	$ 558	$ 1,322

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Cash flows from operating activities
Net loss	$ (56,163)	$ (47,258)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	319	205
Net amortization of premiums and discounts on marketable securities	(1,411)	0
Stock-based compensation	6,407	5,485
Other non-cash items	152	194
Changes in operating assets and liabilities:
Accounts receivable	30,000	0
Prepaid expenses and other current assets	620	(462)
Accounts payable	4,728	(2,524)
Accrued expenses	(10,327)	(2,159)
Operating lease right-of-use assets	669	889
Operating lease liabilities	(697)	(579)
Deferred revenue	(3,302)	38,254
Net cash used in operating activities	(29,005)	(7,955)
Cash flows from investing activities
Maturities of marketable securities	66,000	0
Purchase of marketable securities	(63,693)	0
Purchase of property and equipment	(911)	(329)
Net cash provided by (used in) investing activities	1,396	(329)
Cash flows from financing activities
Net proceeds from at-the-market facilities	21,730	60,374
Proceeds from exercise of stock options	34	96
Payment of debt issuance costs	(150)	0
Payments under finance lease obligations	(65)	(76)
Net cash provided by financing activities	21,549	60,394
(Decrease) increase in cash, cash equivalents and restricted cash	(6,060)	52,110
Cash, cash equivalents and restricted cash, beginning of period	129,363	178,425
Cash, cash equivalents and restricted cash, end of period	123,303	230,535
Supplemental disclosures of non-cash activities:
Purchases of property and equipment in accounts payable and accrued expenses	169	457
Common stock issuance costs in accounts payable and accrued expenses	66	0
Cash paid for interest	$ 791	$ 531

Nature of business and basis of presentation	3 Months Ended
Nature of business and basis of presentation	Mar. 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 has leveraged over 20 years of industry learning in the ADC field to develop three proprietary and differentiated platforms that enable it to develop ADCs that are designed to have improved efficacy, safety and tolerability relative to existing ADCs and other approved therapies. The Company’s platforms include Dolaflexin and Dolasynthen, each of which deliver the novel and proprietary auristatin DolaLock payload, as well as Immunosynthen, which delivers the novel stimulator of interferon genes ("STING") agonist ImmunoLock payload. The Company's lead product candidate, upifitamab rilsodotin ("UpRi"), is a first-in-class Dolaflexin ADC targeting NaPi2b, an antigen broadly expressed in ovarian cancer and other cancers. The Company is currently evaluating UpRi in platinum-resistant ovarian cancer in a single-arm registrational trial, referred to as UPLIFT. The Company is also conducting a placebo-controlled Phase 3 clinical trial, referred to as UP-NEXT, to investigate UpRi as a monotherapy maintenance treatment following treatment with platinum doublets in recurrent platinum-sensitive ovarian cancer. Additionally, the Company is conducting a Phase 1 combination trial, referred to as UPGRADE-A. UPGRADE-A is exploring the combination of UpRi with carboplatin, a standard platinum chemotherapy broadly used in the treatment of platinum-sensitive ovarian cancer. The Company is also 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 voluntary suspension by the Company, this clinical trial of XMT-2056 was placed on clinical hold by the U.S. Food and Drug Administration. The Company also has two additional earlier stage preclinical candidates, XMT-2068 and XMT-2175, that leverage the Company's Immunosynthen platform. 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. For the three months ended March 31, 2023, the net loss was $56.2 million, compared to $47.3 million in the three months ended March 31, 2022. The Company expects to continue to incur operating losses for at least the next several years. As of March 31, 2023, the Company had an accumulated deficit of $710.9 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 Quarterly Report on Form 10-Q. 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 Company’s unaudited condensed 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"). Certain information and footnote disclosures normally included in annual financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2022 and the notes thereto, included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022, filed with the SEC on February 28, 2023. The unaudited condensed consolidated financial statements have been prepared on the same basis as the audited financial statements. In the opinion of the Company’s management, the accompanying unaudited condensed consolidated financial statements contain all adjustments that are necessary to present fairly the Company’s financial position as of March 31, 2023, the results of its operations for the three months ended March 31, 2023 and 2022, the statements of stockholders’ equity for the three months ended March 31, 2023 and 2022 and statements of cash flows for the three months ended March 31, 2023 and 2022. Such adjustments are of a normal and recurring nature. The results for the three months ended March 31, 2023 are not necessarily indicative of the results for the year ending December 31, 2023, or for any future period.

Summary of significant accounting policies	3 Months Ended
Summary of significant accounting policies	Mar. 31, 2023
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of significant accounting policies Principles of Consolidation The accompanying unaudited condensed 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 unaudited condensed 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. Summary of Accounting Policies The significant accounting policies used in preparation of these condensed consolidated financial statements for the three months ended March 31, 2023 are consistent with those discussed in Note 2, Summary of Significant Accounting Policies, in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022. 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. 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. 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. The Company determined that these investments are subject to minimal credit and market risks. Cash and cash equivalents are stated at cost, which approximates market value. Recently Issued Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that the Company adopts as of the specified effective date. Unless otherwise discussed below, the Company does not believe that the adoption of recently issued standards have or may have a material impact on the Company's condensed consolidated financial statements or disclosures.

Collaboration agreements

3 Months Ended

Mar. 31, 2023

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.

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 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 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. The Company 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 three months ended March 31, 2023, the Company recorded collaboration revenue of $0.7 million related to its efforts under the GSK Agreement. As of March 31, 2023 and December 31, 2022 the Company had recorded $97.3 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.

Janssen

In February 2022, the Company entered into a research collaboration and license agreement with Janssen Biotech Inc. ("Janssen" and such agreement, the "Janssen 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 Janssen’s proprietary antibodies. Upon execution of the Janssen Agreement, the Company received a non-refundable upfront payment of $40.0 million from Janssen. Janssen may select up to three targets and may substitute each target once prior to a substitution deadline. Janssen 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.

Pursuant to mutually agreed research and CMC plans, the Company will 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 Janssen. The Company estimates that its activities under the research plans for the targets will be performed through 2024.

The Company's CMC activities will be compensated by Janssen at agreed upon rates. Unless earlier terminated, the Janssen Agreement will expire upon the expiration of the last royalty term for a product under the Janssen Agreement.

Janssen may request that the Company perform clinical manufacturing services under a separate clinical supply agreement. Janssen may also request that the Company perform a technology transfer of bioconjugation and manufacturing process technology, at Janssen's cost, at an agreed upon rate.

Accounting Analysis

The Company assessed the Janssen Agreement in accordance with ASC 606 and concluded that the contract counter party, Janssen, is a customer. The Company identified the following seven material performance obligations under the Janssen Agreement: (i) exclusive Janssen 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 determined that the consideration for CMC activities represents variable consideration. The Company has not included potential cost reimbursements within the transaction price as no CMC activities for any of the three targets have been initiated. The Company elected to apply the Right to Invoice practical expedient under ASC 606. As such, the Company will recognize revenue related to the CMC activities when the services are performed.

The Company is recognizing revenue related to the Janssen 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.

The Company recognizes revenue related to the first target substitution right over time in congruence with the Janssen Licenses and research activities, upon the exercise of the option. If the first target substitution option is not exercised, the Company will recognize the entirety of the revenue in the period when the option expires.

During the three months ended March 31, 2023 and March 31, 2022, the Company recorded collaboration revenue of $1.5 million and $1.7 million, respectively, related to its performance obligations under the Janssen Agreement. As of March 31, 2023 and December 31, 2022, the Company had recorded $16.2 million and $15.8 million, respectively, in deferred revenue related to the Janssen 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 and affiliates

Immunosynthen Platform Agreement

In December 2022, the Company entered into a research collaboration and license agreement with Ares Trading S.A. ("MRKDG" and such agreement, the "2022 Merck KGaA Agreement"), a wholly owned subsidiary of Merck KGaA, Darmstadt, Germany, 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 MRKDG’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 MRKDG two exclusive, non-transferable, worldwide licenses - a research license and a commercialization license (together, the "MRKDG Licenses").

Pursuant to mutually agreed research and CMC plans, the Company will 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 MRKDG. 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 MRKDG at agreed upon rates. 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.

MRKDG may request that the Company perform clinical manufacturing services under a separate clinical supply agreement. MRKDG may also request that the Company perform a technology transfer of bioconjugation technology, at MRKDG'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, MRKDG, is a customer. The Company identified the following four material performance obligations under the 2022 Merck KGaA Agreement: (i) exclusive MRKDG Licenses and research activities for each of the two designated targets and (ii) CMC activities for each of the two designated targets.

The Company is recognizing revenue related to the MRKDG 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 three months ended March 31, 2023, the Company recorded collaboration revenue of $3.1 million related to its efforts under the 2022 Merck KGaA Agreement. As of March 31, 2023 and December 31, 2022, the Company had recorded $26.9 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 (the "2014 Merck KGaA Agreement"). Upon the execution of the 2014 Merck KGaA Agreement, Merck KGaA paid the Company a non-refundable technology access fee of $12.0 million for the right to develop ADCs directed to six exclusive targets over a specified period of time. No additional fees are due when a target is designated and the commercial license to the target is granted. Merck KGaA will be responsible for the product development and marketing of any products resulting from this collaboration.

Under the terms of the 2014 Merck KGaA Agreement, the Company and Merck KGaA develop research plans to evaluate Merck KGaA's antibodies as ADCs incorporating the Company's technology. The Company receives reimbursement for its efforts under the research plans. The goal of the research plans is to provide Merck KGaA with sufficient information to formally nominate a development candidate and begin IND-enabling studies.

All six targets were designated prior to 2018. The Company has previously received $3.0 million related to development milestones under the 2014 Merck KGaA Agreement. There have been no additional milestone payments during the three months ended March 31, 2023 or 2022.

In May 2018, the Company entered into a Supply Agreement with Merck KGaA (the "2018 Merck KGaA Supply Agreement"). Under the terms of the 2018 Merck KGaA Supply Agreement, the Company will provide Merck KGaA preclinical non-good manufacturing practice ("non-GMP") ADC drug substance and clinical good manufacturing practice ("GMP") drug substance for use in clinical trials associated with one of the antibodies designated under the 2014 Merck KGaA Agreement. The Company receives fees for its efforts under the 2018 Merck KGaA Supply Agreement and reimbursement equal to the supply cost. The Company may also enter into future supply agreements to provide clinical supply material should Merck KGaA pursue clinical development of any other candidates nominated under the 2014 Merck KGaA Agreement.

Accounting Analysis

The Company concluded that Merck KGaA is a customer and accounted for the 2014 Merck KGaA Agreement in accordance with ASC 606. The Company 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 is recognizing revenue related to the exclusive license and research and development services performance obligation over the estimated period of the research and development 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. To the extent that the Company receives fees for the research services as they are performed, these amounts are recorded as deferred revenue. Revenue related to future technological improvements and joint research committee services will be recognized ratably over the performance period (which in the case of the joint research committee services approximate the time and cost incurred each period), which are 10 and 5 years, respectively. The Company is continuing to reassess the estimated remaining term at each subsequent reporting period.

As of March 31, 2023, the Company has completed its research service obligations associated with four of the six designated targets and the joint research committee services. Collaboration revenue recognized during the three months ended March 31, 2023 and 2022 was immaterial. There was no collaboration revenue or corresponding research and development expense recognized during the three months ended March 31, 2023 and 2022 related to the 2018 Merck KGaA Supply Agreement.

As of March 31, 2023 and 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, that will be recognized over the remaining performance period.

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
Three months ended March 31, 2023

Contract liabilities:





Total deferred revenue	$	147,653	$	—	$	3,302	$	144,351

Three months ended March 31, 2022
Contract liabilities:

Total deferred revenue	$	3,944	$	40,000	$	1,746	$	42,198

The Company did not record any contract assets associated with its collaboration agreements as of March 31, 2023 and March 31, 2022.

During the three months ended March 31, 2023 and 2022, the Company recognized the following revenues as a result of changes in the contract liability balances in the respective periods:


	Three months ended March 31,
	2023		2022
Revenue recognized in the period from:




Amounts included in the contract liability at the beginning of the period	$	5,110	$	11

Other Revenue

The Company has provided limited services for a collaborator, Asana BioSciences, LLC ("Asana Biosciences"). The Company did not recognize revenue related to these services during the three months ended March 31, 2023. During the three months ended March 31, 2022 the Company recognized revenue of $0.3 million related to these services. During the three months ended March 31, 2023 the Company recognized revenue of $2.5 million related to achievement of a development milestone under the research, development and license agreement for which performance obligations were previously completed.

Fair value measurements

3 Months Ended

Mar. 31, 2023

Fair Value Disclosures [Abstract]

Fair value measurements

Fair value measurementsThe 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.


	March 31, 2023
(in thousands)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents
Money market funds	$	51,394	$	51,394	$	—	$	—
U.S. treasury securities	9,972		9,972		—		—
	$	61,366	$	61,366	$	—	$	—
Marketable securities
U.S. treasury securities	$	77,095	$	77,095	$	—	$	—
U.S. government agency securities	73,999		—		73,999		—
	$	151,094	$	77,095	$	73,999	$	—


	December 31, 2022
(in thousands)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
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 three months ended March 31, 2023 or during the year ended December 31, 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 March 31, 2023 and December 31, 2022, the carrying value of the Company’s outstanding borrowing under the New Credit Facility (as defined in Note 7) 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 7, Debt.

Cash, cash equivalents, and short-term marketable securities

3 Months Ended

Mar. 31, 2023

Investments, Debt and Equity Securities [Abstract]

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 March 31, 2023 and 2022.


	Three Months Ended March 31, 2023				Three Months Ended March 31, 2022
(in thousands)	Beginning of period		End of period		Beginning of period		End of period
Cash and cash equivalents	$	128,885	$	122,825	$	177,947	$	230,057
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	$	123,303	$	178,425	$	230,535

Marketable securities

The following tables summarize the Company's marketable securities held at March 31, 2023 and December 31, 2022.


	March 31, 2023
(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable securities
U.S. treasury securities	$	77,104	$	30	$	(39)	$	77,095
U.S. government agency securities	73,981		33		(15)		73,999
Total	$	151,085	$	63	$	(54)	$	151,094


	December 31, 2022
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
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 recognized on the sale of marketable securities during the three months ended March 31, 2023, and, as a result, the Company did not reclassify any amounts out of accumulated comprehensive loss.

As of March 31, 2023, the Company's debt security portfolio consisted of 7 securities that were in an unrealized loss position and had an aggregate fair value of $42.4 million. There were no securities in an unrealized loss position for greater than 12 months as of March 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. As a result, the Company did not record any charges for credit-related impairments for its marketable debt securities for the three months ended March 31, 2023.

Accrued expenses

3 Months Ended

Mar. 31, 2023

Accrued Liabilities, Current [Abstract]

Accrued expenses

Accrued expenses consisted of the following as of March 31, 2023 and December 31, 2022:


(in thousands)	March 31, 2023		December 31, 2022
Accrued clinical expenses	$	13,414	$	14,822
Accrued manufacturing expenses	7,801		11,536
Accrued payroll and related expenses	4,754		11,558
Accrued research and non-clinical expenses	3,488		2,767
Accrued professional fees	2,726		1,865
Accrued other	405		636
	$	32,588	$	43,184

Debt

3 Months Ended

Mar. 31, 2023

Debt Disclosure [Abstract]

Debt

On October 29, 2021, the Company entered into a loan and security agreement (the "New Credit Facility") with Silicon Valley Bank ("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. Pursuant to the New Credit Facility, as amended on February 17, 2022, October 17, 2022, December 27, 2022, and March 23, 2023, the Company can borrow term loans in an aggregate amount of $100.0 million, which includes (i) $40.0 million available at the option of the Company in up to four principal advances through June 30, 2023, (ii) an additional $40.0 million in one principal advance, if the Company reaches certain development milestone events through September 30, 2023, and (iii) an additional tranche of $20.0 million, subject to conditional approval from the Lenders. The New Credit Facility 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 has drawn $25.0 million under the New Credit Facility as of March 31, 2023.

Refer to Note 8, Debt, in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022 for more information regarding the New Credit Facility. As of March 31, 2023, the Company was in compliance with all covenants under the New Credit Facility. There are no events of default under the New Credit Facility as of March 31, 2023.

The following is a summary of obligations under the New Credit Facility as of March 31, 2023:


(in thousands)	March 31, 2023
Total debt	$	25,000
Less: Current portion of long-term debt	—
Total debt, net of current portion	25,000
Debt financing costs, net of accretion	(302)
Accretion related to final payment	306
Long-term debt, net	$	25,004

Interest expense related to the New Credit Facility for the three months ended March 31, 2023 and 2022 was $0.9 million and $0.7 million, respectively.

- Definition

The entire disclosure for information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, own-share lending arrangements and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.

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+ Details

Stockholders' equity

3 Months Ended

Mar. 31, 2023

Stockholders' Equity Note [Abstract]

Stockholders’ equity

Preferred stock

As of March 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 three months ended March 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 March 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 three months ended March 31, 2022, the Company sold 1,429,693 shares of common stock under the February 2022 ATM, resulting in net proceeds of $5.8 million. During the three months ended March 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 March 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 three months ended March 31, 2023, the Company sold 3,278,707 shares of common stock under the November 2022 ATM, resulting in net proceeds of $20.2 million. As of March 31, 2023, approximately $129.3 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 129,491 shares of common stock. The warrants have 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 March 31, 2023, there were warrants to purchase 22,590 shares of common stock outstanding. During the three months ended March 31, 2023, there were no exercises of warrants in exchange for common stock.

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 March 31, 2023 and December 31, 2022, there were 15,769,504 and 11,944,664, respectively, shares of common stock reserved for the exercise of outstanding stock options, restricted stock units ("RSUs") and warrants.


	March 31, 2023	December 31, 2022
Stock options	12,299,527	10,051,283
Restricted stock units	3,447,387	1,870,791
Warrants	22,590	22,590

	15,769,504	11,944,664

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

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+ Details

Stock-based compensation

3 Months Ended

Mar. 31, 2023

Share-Based Payment Arrangement [Abstract]

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 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, 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 three months ended March 31, 2023, the Company granted 4,317,336 RSUs and options to purchase shares of common stock to employees under the 2017 Plan. As of March 31, 2023, there were 1,738,539 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. 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 were granted, 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 three months ended March 31, 2023, the Company granted 236,870 RSUs and options to purchase shares of common stock to newly hired employees under the Inducement Plan. As of March 31, 2023, there were 1,105,305 shares available for future issuance under the Inducement Plan.

As of March 31, 2023, there were options to purchase 757,500 shares of common stock outstanding which were granted as inducement awards prior to the establishment of the Inducement Plan.

Stock option activity

A summary of stock option activity is as follows:


	Number of Shares	Weighted- Average Exercise Price
Outstanding at January 1, 2023	10,051,283	$	9.84
Granted	2,530,740	$	6.07
Exercised	(8,826)	$	3.80
Cancelled	(273,670)	$	10.08
Outstanding at March 31, 2023	12,299,527	$	9.07

Exercisable at March 31, 2023	5,902,352	$	9.51

The weighted-average grant date fair value of options granted during the three months ended March 31, 2023 and 2022 was $6.06 and $4.39 per share, respectively. The total intrinsic value of options exercised during the three months ended March 31, 2023 and 2022 was immaterial. 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 immaterial for the three months ended March 31, 2023 and 2022.

Restricted stock units

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.

A summary of the RSU activity is as follows:


			Number of Shares
Unvested at January 1, 2023			1,870,791
Granted			2,023,466
Vested			(372,291)
Forfeited			(74,579)
Unvested at March 31, 2023			3,447,387

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 number of shares of common stock issuable under the 2017 ESPP was increased by 450,000 on January 1, 2023. The Company did not issue any shares under the 2017 ESPP for the three months ended March 31, 2023 or 2022. As of March 31, 2023, there were 745,791 shares available for issuance under the 2017 ESPP.

Stock-based compensation expense

The Company uses the provisions of ASC 718, Stock Compensation, 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 condensed consolidated statements of operations and comprehensive loss:


	Three Months Ended March 31,
(in thousands)	2023		2022
Stock options	$	4,219	$	4,118
Restricted stock units	1,893		1,209
Employee stock purchase plan	295		158
Stock-based compensation expense included in total operating expenses	$	6,407	$	5,485

The following table presents stock-based compensation expense as reflected in the Company’s condensed consolidated statements of operations and comprehensive loss:


	Three Months Ended March 31,
(in thousands)	2023		2022
Research and development	$	3,332	$	2,933
General and administrative	3,075		2,552
Stock-based compensation expense included in total operating expenses	$	6,407	$	5,485

As of March 31, 2023, there was $36.7 million and $21.4 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 2.2 years and 3.2 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:


	Three Months Ended March 31,
	2023		2022
Risk-free interest rate	3.6	%	1.6	%
Expected dividend yield	—	%	—	%
Expected term (years)	6.07		6.04
Expected stock price volatility	99	%	87	%

Expected volatility for the Company’s common stock is determined based on its historical volatility. 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

3 Months Ended

Mar. 31, 2023

Earnings Per Share [Abstract]

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):


	Three months ended March 31, 2023	Three months ended March 31, 2022
Stock options	12,299,527	10,028,741
Unvested restricted stock units	3,447,387	1,456,440
Warrants	22,590	39,474
	15,769,504	11,524,655

Commitments	3 Months Ended
Commitments	Mar. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments	Commitments License agreements During the three months ended March 31, 2023 and the Company did not record research and development expense related to non-refundable license payments. During the three months ended March 31, 2022, the Company recorded research and development expense related to non-refundable license payments of $1.5 million. During the three months ended March 31, 2023 and 2022, the Company did not record research and development expense related to development milestones.

Summary of significant accounting policies (Policies)	3 Months Ended
Summary of significant accounting policies (Policies)	Mar. 31, 2023
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The accompanying unaudited condensed 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 unaudited condensed 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.
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.
Concentration of Credit Risk and Off-balance Sheet Risk	Concentration of Credit Risk and Off-balance Sheet RiskFinancial 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.
Concentration of Credit Risk and Off-balance Sheet Risk	Concentration of Credit Risk and Off-balance Sheet RiskFinancial 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.
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. The Company determined that these investments are subject to minimal credit and market risks. Cash and cash equivalents are stated at cost, which approximates market value.
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 that the Company adopts as of the specified effective date. Unless otherwise discussed below, the Company does not believe that the adoption of recently issued standards have or may have a material impact on the Company's condensed consolidated financial statements or disclosures.
Net Loss per Share	Net loss per shareBasic 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.

Collaboration agreements (Tables)

3 Months Ended

Mar. 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
Three months ended March 31, 2023

Contract liabilities:





Total deferred revenue	$	147,653	$	—	$	3,302	$	144,351

Three months ended March 31, 2022
Contract liabilities:

Total deferred revenue	$	3,944	$	40,000	$	1,746	$	42,198

During the three months ended March 31, 2023 and 2022, the Company recognized the following revenues as a result of changes in the contract liability balances in the respective periods:


	Three months ended March 31,
	2023		2022
Revenue recognized in the period from:




Amounts included in the contract liability at the beginning of the period	$	5,110	$	11

Fair value measurements (Tables)

3 Months Ended

Mar. 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.


	March 31, 2023
(in thousands)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents
Money market funds	$	51,394	$	51,394	$	—	$	—
U.S. treasury securities	9,972		9,972		—		—
	$	61,366	$	61,366	$	—	$	—
Marketable securities
U.S. treasury securities	$	77,095	$	77,095	$	—	$	—
U.S. government agency securities	73,999		—		73,999		—
	$	151,094	$	77,095	$	73,999	$	—


	December 31, 2022
(in thousands)	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
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 short-term marketable securities (Tables)

3 Months Ended

Mar. 31, 2023

Investments, Debt and Equity Securities [Abstract]

Schedule of Restricted Cash

The following table summarizes the Company's cash, cash equivalents, and restricted cash as of March 31, 2023 and 2022.


	Three Months Ended March 31, 2023				Three Months Ended March 31, 2022
(in thousands)	Beginning of period		End of period		Beginning of period		End of period
Cash and cash equivalents	$	128,885	$	122,825	$	177,947	$	230,057
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	$	123,303	$	178,425	$	230,535

Debt Securities, Available-for-Sale

The following tables summarize the Company's marketable securities held at March 31, 2023 and December 31, 2022.


	March 31, 2023
(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable securities
U.S. treasury securities	$	77,104	$	30	$	(39)	$	77,095
U.S. government agency securities	73,981		33		(15)		73,999
Total	$	151,085	$	63	$	(54)	$	151,094


	December 31, 2022
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
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

Accrued expenses (Tables)

3 Months Ended

Mar. 31, 2023

Accrued Liabilities, Current [Abstract]

Schedule of Accrued Expenses

Accrued expenses consisted of the following as of March 31, 2023 and December 31, 2022:


(in thousands)	March 31, 2023		December 31, 2022
Accrued clinical expenses	$	13,414	$	14,822
Accrued manufacturing expenses	7,801		11,536
Accrued payroll and related expenses	4,754		11,558
Accrued research and non-clinical expenses	3,488		2,767
Accrued professional fees	2,726		1,865
Accrued other	405		636
	$	32,588	$	43,184

Debt (Tables)

3 Months Ended

Mar. 31, 2023

Debt Disclosure [Abstract]

Schedule of Debt

The following is a summary of obligations under the New Credit Facility as of March 31, 2023:


(in thousands)	March 31, 2023
Total debt	$	25,000
Less: Current portion of long-term debt	—
Total debt, net of current portion	25,000
Debt financing costs, net of accretion	(302)
Accretion related to final payment	306
Long-term debt, net	$	25,004

Stockholders' equity (Tables)

3 Months Ended

Mar. 31, 2023

Stockholders' Equity Note [Abstract]

Schedule for Number of Common Stock Reserved for Exercise of Outstanding Stock Options and Warrants


	March 31, 2023	December 31, 2022
Stock options	12,299,527	10,051,283
Restricted stock units	3,447,387	1,870,791
Warrants	22,590	22,590

	15,769,504	11,944,664

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

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+ Details

Stock-based compensation (Tables)

3 Months Ended

Mar. 31, 2023

Share-Based Payment Arrangement [Abstract]

Schedule of Stock Option Activity Under Plan

A summary of stock option activity is as follows:


	Number of Shares	Weighted- Average Exercise Price
Outstanding at January 1, 2023	10,051,283	$	9.84
Granted	2,530,740	$	6.07
Exercised	(8,826)	$	3.80
Cancelled	(273,670)	$	10.08
Outstanding at March 31, 2023	12,299,527	$	9.07

Exercisable at March 31, 2023	5,902,352	$	9.51

Schedule of Restricted Stock Activity

A summary of the RSU activity is as follows:


			Number of Shares
Unvested at January 1, 2023			1,870,791
Granted			2,023,466
Vested			(372,291)
Forfeited			(74,579)
Unvested at March 31, 2023			3,447,387

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 condensed consolidated statements of operations and comprehensive loss:


	Three Months Ended March 31,
(in thousands)	2023		2022
Stock options	$	4,219	$	4,118
Restricted stock units	1,893		1,209
Employee stock purchase plan	295		158
Stock-based compensation expense included in total operating expenses	$	6,407	$	5,485

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 condensed consolidated statements of operations and comprehensive loss:


	Three Months Ended March 31,
(in thousands)	2023		2022
Research and development	$	3,332	$	2,933
General and administrative	3,075		2,552
Stock-based compensation expense included in total operating expenses	$	6,407	$	5,485

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:


	Three Months Ended March 31,
	2023		2022
Risk-free interest rate	3.6	%	1.6	%
Expected dividend yield	—	%	—	%
Expected term (years)	6.07		6.04
Expected stock price volatility	99	%	87	%

Net loss per share (Tables)

3 Months Ended

Mar. 31, 2023

Earnings Per Share [Abstract]

Schedule of Outstanding Potentially Dilutive Securities


	Three months ended March 31, 2023	Three months ended March 31, 2022
Stock options	12,299,527	10,028,741
Unvested restricted stock units	3,447,387	1,456,440
Warrants	22,590	39,474
	15,769,504	11,524,655

Nature of business and basis of presentation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Net loss	$ (56,163)	$ (47,258)
Accumulated deficit	$ (710,854)		$ (654,691)

- Definition

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

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-URI https://asc.fasb.org/extlink&oid=126975872&loc=SL124442526-122756

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-Paragraph 28
-Subparagraph (f)
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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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Reference 35: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-SubTopic 10
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-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e8933-108599

+ Details

Summary of significant accounting policies - Narrative (Details)	3 Months Ended
	Mar. 31, 2023 segment
Cash and Cash Equivalents [Abstract]
Number of operating segments	1

Collaboration agreements - GlaxoSmithKline (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Aug. 31, 2022	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022	Dec. 31, 2021
Collaboration agreements
Collaboration revenue		$ 7,802	$ 2,036
Deferred revenue		144,351	$ 42,198	$ 147,653	$ 3,944
GSK Agreement
Collaboration agreements
Upfront payment received	$ 100,000
Collaboration arrangement, potential option exercise receivable	$ 90,000
Collaboration revenue		700
Deferred revenue		$ 97,300		$ 98,000

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
-Topic 606
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-Paragraph 4
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=126920106&loc=SL49130543-203045

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 924
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-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 11.L)
-URI https://asc.fasb.org/extlink&oid=6472922&loc=d3e499488-122856

+ Details

Collaboration agreements - Janssen Biotech (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Feb. 28, 2022 USD ($) target	Mar. 31, 2023 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Collaboration agreements
Collaboration revenue		$ 7,802	$ 2,036
Deferred revenue		144,351	42,198	$ 147,653	$ 3,944
Janssen Agreement
Collaboration agreements
Number of targets designated \| target	3
Upfront payment received	$ 40,000
Collaboration revenue		1,500	$ 1,700
Deferred revenue		$ 16,200		$ 15,800

- Definition

+ References

+ Details

Collaboration agreements - Merck Immunosythen Agreement (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Feb. 28, 2023 USD ($)	Mar. 31, 2023 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2022 USD ($) target performanceObligation	Dec. 31, 2021 USD ($)
Collaboration agreements
Collaboration revenue		$ 7,802	$ 2,036
Deferred revenue		144,351	$ 42,198	$ 147,653	$ 3,944
2022 Merck Research Collaboration & License Agreement
Collaboration agreements
Number of targets designated \| target				2
Upfront payment received	$ 30,000
Performance obligations identified \| performanceObligation				4
Collaboration revenue		3,100
Deferred revenue		$ 26,900		$ 30,000

- Definition

+ References

+ Details

Collaboration agreements - Merck Dolaflexin Agreement (Details)	1 Months Ended	3 Months Ended		60 Months Ended
	Jun. 30, 2014 USD ($) target	Mar. 31, 2023 USD ($) target	Mar. 31, 2022 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2017 target
Collaboration agreements
Collaboration revenue		$ 7,802,000	$ 2,036,000
Deferred revenue		$ 144,351,000	42,198,000		$ 147,653,000	$ 3,944,000
2014 Merck KGaA
Collaboration agreements
Upfront payment received	$ 12,000,000
Number of targets designated \| target	6	6					6
Amount of additional fees receivable when target is designated and commercial license to target is granted	$ 0
Development milestones		$ 0	0	$ 3,000,000
Number of targets with satisfied performance obligations \| target		4
2018 Merck KGaA
Collaboration agreements
Collaboration revenue		$ 0	$ 0
2014 & 2018 Collaboration and Commercial License Agreement and Supply Agreement Merck KGaA
Collaboration agreements
Deferred revenue		$ 3,900,000			$ 3,900,000
Rights to Future Technological Improvements \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-04-01 \| 2014 Merck KGaA
Collaboration agreements
Expected recognition period		10 years
Joint Research Committee Services \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-04-01 \| 2014 Merck KGaA
Collaboration agreements
Expected recognition period		5 years

Collaboration agreements - Contract Assets and Liabilities (Details) - USD ($)	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Contract liabilities
Balance at Beginning of Period	$ 147,653,000	$ 3,944,000
Additions	0	40,000,000
Deductions	3,302,000	1,746,000
Balance at End of Period	144,351,000	42,198,000
Amounts included in the contract liability at the beginning of the period	5,110,000	11,000
Janssen Agreement
Contract liabilities
Balance at Beginning of Period	15,800,000
Balance at End of Period	16,200,000
Contract assets	0	0
GSK Agreement
Contract liabilities
Balance at Beginning of Period	98,000,000
Balance at End of Period	97,300,000
Contract assets	0	0
2022 Merck Research Collaboration & License Agreement
Contract liabilities
Balance at Beginning of Period	30,000,000
Balance at End of Period	26,900,000
Contract assets	0	0
2014 & 2018 Collaboration and Commercial License Agreement and Supply Agreement Merck KGaA
Contract liabilities
Balance at Beginning of Period	3,900,000
Balance at End of Period	3,900,000
Contract assets	$ 0	$ 0

Collaboration agreements - Other Revenue (Details) - USD ($)	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Collaboration agreements
Collaboration revenue	$ 7,802,000	$ 2,036,000
Asana BioSciences \| Limited services
Collaboration agreements
Collaboration revenue	0	$ 300,000
Asana BioSciences \| Development Milestone Achieved
Collaboration agreements
Collaboration revenue	$ 2,500,000

- Definition

+ References

+ Details

Fair value measurements - Fair Value, Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Assets, Fair Value Disclosure [Abstract]
Short-term marketable securities	$ 151,094	$ 151,827
U.S. treasury securities
Assets, Fair Value Disclosure [Abstract]
Short-term marketable securities	77,095	107,810
U.S. government agency securities
Assets, Fair Value Disclosure [Abstract]
Short-term marketable securities	73,999	44,017
Recurring basis
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	61,366	60,464
Short-term marketable securities	151,094	151,827
Recurring basis \| Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	51,394	50,471
Recurring basis \| U.S. treasury securities
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	9,972
Short-term marketable securities	77,095	107,810
Recurring basis \| U.S. government agency securities
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		9,993
Short-term marketable securities	73,999	44,017
Recurring basis \| Quoted Prices in Active Markets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	61,366	50,471
Short-term marketable securities	77,095	107,810
Recurring basis \| Quoted Prices in Active Markets (Level 1) \| Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	51,394	50,471
Recurring basis \| Quoted Prices in Active Markets (Level 1) \| U.S. treasury securities
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	9,972
Short-term marketable securities	77,095	107,810
Recurring basis \| Quoted Prices in Active Markets (Level 1) \| U.S. government agency securities
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		0
Short-term marketable securities	0	0
Recurring basis \| Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0	9,993
Short-term marketable securities	73,999	44,017
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0	0
Recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. treasury securities
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0
Short-term marketable securities	0	0
Recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. government agency securities
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		9,993
Short-term marketable securities	73,999	44,017
Recurring basis \| Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0	0
Short-term marketable securities	0	0
Recurring basis \| Significant Unobservable Inputs (Level 3) \| Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0	0
Recurring basis \| Significant Unobservable Inputs (Level 3) \| U.S. treasury securities
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0
Short-term marketable securities	0	0
Recurring basis \| Significant Unobservable Inputs (Level 3) \| U.S. government agency securities
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		0
Short-term marketable securities	$ 0	$ 0

Cash, cash equivalents, and short-term marketable securities - Schedule of Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022
Investments, Debt and Equity Securities [Abstract]
Cash and cash equivalents at beginning of period	$ 128,885	$ 230,057	$ 177,947
Cash and cash equivalents at end of period	122,825	128,885	230,057
Restricted cash, beginning of period	478	478	478
Restricted cash, end of period	478	478	478
Cash, cash equivalents and restricted cash, beginning of period	129,363	230,535	178,425
Cash, cash equivalents and restricted cash, end of period	$ 123,303	$ 129,363	$ 230,535

Cash, cash equivalents, and short-term marketable securities - Debt Securities, Available-for-Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 151,085	$ 151,980
Gross Unrealized Gains	63	31
Gross Unrealized Losses	(54)	(184)
Fair Value	151,094	151,827
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	77,104	107,964
Gross Unrealized Gains	30	7
Gross Unrealized Losses	(39)	(161)
Fair Value	77,095	107,810
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	73,981	44,016
Gross Unrealized Gains	33	24
Gross Unrealized Losses	(15)	(23)
Fair Value	$ 73,999	$ 44,017

Cash, cash equivalents, and short-term marketable securities - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities in unrealized loss position \| security	7
Value of securities in an unrealized loss position \| $	$ 42.4
Securities in unrealized loss position greater than 12 months \| security	0
Charges for credit-related impairments \| $	$ 0.0

Accrued expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Accrued Liabilities, Current [Abstract]
Accrued manufacturing expenses	$ 7,801	$ 11,536
Accrued clinical expenses	13,414	14,822
Accrued research and non-clinical expenses	3,488	2,767
Accrued payroll and related expenses	4,754	11,558
Accrued professional fees	2,726	1,865
Accrued other	405	636
Total	$ 32,588	$ 43,184

Debt - Narrative (Details)		3 Months Ended
Debt - Narrative (Details)	Oct. 29, 2021 USD ($) advance	Mar. 31, 2023 USD ($)	Mar. 31, 2022 USD ($)	Mar. 23, 2023 USD ($)
New Credit Facility
Debt
Interest expense		$ 900,000	$ 700,000
New Credit Facility \| Line of Credit
Debt
Maximum borrowing capacity	$ 100,000,000
Principal advances \| advance	4
Proceeds from line of credit	$ 25,000,000
Long-term line of credit				$ 25,000,000
New Credit Facility, Tranche One \| Line of Credit
Debt
Maximum borrowing capacity	40,000,000
New Credit Facility, Tranche Two \| Line of Credit
Debt
Maximum borrowing capacity	40,000,000
New Credit Facility, Tranche Three \| Line of Credit
Debt
Maximum borrowing capacity	40,000,000
New Credit Facility, Tranche Four \| Line of Credit
Debt
Maximum borrowing capacity	40,000,000
New Credit Facility, Tranche Five \| Line of Credit
Debt
Maximum borrowing capacity	$ 40,000,000
Principal advances \| advance	1
New Credit Facility, Tranche Six \| Line of Credit
Debt
Maximum borrowing capacity	$ 20,000,000

Debt - Schedule of Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Debt Disclosure [Abstract]
Total debt	$ 25,000
Less: Current portion of long-term debt	0
Total debt, net of current portion	25,000
Debt financing costs, net of accretion	(302)
Accretion related to final payment	306
Long-term debt, net	$ 25,004	$ 24,929

- Definition

Amount, excluding unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References + Details

Stockholders' equity - Narrative (Details)	3 Months Ended		12 Months Ended
Stockholders' equity - Narrative (Details)	Mar. 31, 2023 USD ($) vote $ / shares shares	Mar. 31, 2022 USD ($) shares	Dec. 31, 2013 $ / shares shares	Dec. 31, 2022 $ / shares 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
Net proceeds from public offering of common stock \| $	$ 21,730,000	$ 60,374,000
Number of shares into which warrant issued during period may be converted (in shares)			129,491
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)	22,590
Exercise of common stock warrant in exchange for common stock (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	175,000,000
Number of shares reserved for future issuance (in shares)	15,769,504			11,944,664
Common Stock
Class of Stock [Line Items]
Issuance of common stock under public offering, net of issuance costs (in shares)	3,535,093	13,169,903
Number of votes per common share \| vote	1
2020 ATM
Class of Stock [Line Items]
Amount of shares authorized to be offered and sold \| $									$ 100,000,000
Net proceeds from public offering of common stock \| $		$ 54,800,000
2020 ATM \| Common Stock
Class of Stock [Line Items]
Issuance of common stock under public offering, net of issuance costs (in shares)		11,740,210
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
Net proceeds from public offering of common stock \| $	$ 1,600,000	$ 5,800,000
At Market Equity Offering Program (February 2022 ATM) \| Common Stock
Class of Stock [Line Items]
Issuance of common stock under public offering, net of issuance costs (in shares)	256,386	1,429,693
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
Net proceeds from public offering of common stock \| $	$ 20,200,000
At Market Equity Offering Program (November 2022 ATM) \| Common Stock
Class of Stock [Line Items]
Issuance of common stock under public offering, net of issuance costs (in shares)	3,278,707
Remaining unsold and available amount \| $	$ 129,300,000

Stockholders' equity - Common stock (Details) - shares	Mar. 31, 2023	Dec. 31, 2022
Common stock
Number of shares reserved for future issuance (in shares)	15,769,504	11,944,664
Stock options
Common stock
Number of shares reserved for future issuance (in shares)	12,299,527	10,051,283
Restricted stock units
Common stock
Number of shares reserved for future issuance (in shares)	3,447,387	1,870,791
Warrants
Common stock
Number of shares reserved for future issuance (in shares)	22,590	22,590

Stock-based compensation - Plans Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended
	Jan. 01, 2023	Jun. 30, 2017	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022	Feb. 28, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares outstanding (in shares)			12,299,527		10,051,283
Weighted-average grant date fair value of options granted (in dollars per share)			$ 6.06	$ 4.39
Cash received from the exercise of stock options			$ 34	$ 96
Expected dividend yield			0.00%	0.00%
Stock options
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Unrecognized stock compensation cost			$ 36,700
Weighted-average amortization period of unrecognized stock compensation cost			2 years 2 months 12 days
Restricted stock units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Unrecognized stock compensation cost			$ 21,400
Weighted-average amortization period of unrecognized stock compensation cost			3 years 2 months 12 days
Restricted stock units \| Maximum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting period			4 years
Restricted stock units \| Minimum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting period			1 year
2017 Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Cumulative annual increase in number of shares issuable		4.00%
Additional shares authorized (in shares)	4,205,794
Number of shares available for future issuance (in shares)			1,738,539
Number of options granted in period (in shares)			4,317,336
2017 Plan \| Maximum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expiration period			10 years
2017 Plan \| Stock options
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting period			4 years
Inducement award program
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized (in shares)						2,000,000
Number of shares available for future issuance (in shares)			1,105,305
Number of options granted in period (in shares)			236,870
Inducement award prior to inducement award program
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares outstanding (in shares)			757,500
2017 ESPP \| Employee stock purchase plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Additional shares authorized (in shares)	450,000
Number of shares available for future issuance (in shares)			745,791
Purchase of common stock under ESPP (in shares)			0	0

Stock-based compensation - Activity Under Stock Option Plan (Details)	3 Months Ended
	Mar. 31, 2023 $ / shares shares
Number of Shares
Outstanding at beginning of period (in shares) \| shares	10,051,283
Granted (in shares) \| shares	2,530,740
Exercised (in shares) \| shares	(8,826)
Cancelled (in shares) \| shares	(273,670)
Outstanding at end of period (in shares) \| shares	12,299,527
Options exercisable, at end of period (in shares) \| shares	5,902,352
Weighted- Average Exercise Price
Outstanding at beginning of period (in dollars per share) \| $ / shares	$ 9.84
Granted (in dollars per share) \| $ / shares	6.07
Exercised (in dollars per share) \| $ / shares	3.80
Cancelled (in dollars per share) \| $ / shares	10.08
Outstanding at end of period (in dollars per share) \| $ / shares	9.07
Options exercisable, at end of period (in dollars per share) \| $ / shares	$ 9.51

Stock-based compensation - Restricted Stock Units (Details) - Restricted stock units	3 Months Ended
	Mar. 31, 2023 shares
Restricted stock units
Unvested at beginning of period (in shares)	1,870,791
Granted (in shares)	2,023,466
Vested (in shares)	(372,291)
Forfeited (in shares)	(74,579)
Unvested at end of period (in shares)	3,447,387

Stock-based compensation - Stock Based Compensation Expense by Award Type (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Stock options
Stock-based compensation expense included in total operating expenses	$ 6,407	$ 5,485
Stock options
Stock options
Stock-based compensation expense included in total operating expenses	4,219	4,118
Restricted stock units
Stock options
Stock-based compensation expense included in total operating expenses	1,893	1,209
Employee stock purchase plan
Stock options
Stock-based compensation expense included in total operating expenses	$ 295	$ 158

Stock-based compensation - Schedule of Stock-Based Compensation Expense as Reflected in the Company's Condensed Consolidated Statements of Operations and Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Stock options
Share-based compensation expense	$ 6,407	$ 5,485
Research and development
Stock options
Share-based compensation expense	3,332	2,933
General and administrative
Stock options
Share-based compensation expense	$ 3,075	$ 2,552

Stock-based compensation - Fair Value Assumptions (Details)	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Risk-free interest rate	3.60%	1.60%
Expected dividend yield	0.00%	0.00%
Expected term (years)	6 years 25 days	6 years 14 days
Expected stock price volatility	99.00%	87.00%

Net loss per share (Details) - shares	3 Months Ended
Net loss per share (Details) - shares	Mar. 31, 2023	Mar. 31, 2022
Anti-dilutive securities
Anti-dilutive securities (in shares)	15,769,504	11,524,655
Stock options
Anti-dilutive securities
Anti-dilutive securities (in shares)	12,299,527	10,028,741
Unvested restricted stock units
Anti-dilutive securities
Anti-dilutive securities (in shares)	3,447,387	1,456,440
Warrants
Anti-dilutive securities
Anti-dilutive securities (in shares)	22,590	39,474

Commitments (Details) - USD ($) $ in Millions	3 Months Ended
Commitments (Details) - USD ($) $ in Millions	Mar. 31, 2023	Mar. 31, 2022
License Agreement, Upfront Payments
Commitments
Research and development expense	$ 0.0	$ 1.5
License Agreement, Milestone Payments
Commitments
Research and development expense	$ 0.0	$ 0.0