0001628280-24-036864 | 10-Q | XBRL Viewer

Stock-based compensation (Tables)

6 Months Ended

Jun. 30, 2024

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, 2024	10,902,845	$	7.42
Granted	2,678,525	$	2.91
Exercised	(14,223)	$	3.25
Cancelled/forfeited	(924,795)	$	7.23
Outstanding at June 30, 2024	12,642,352	$	6.48

Exercisable at June 30, 2024	7,121,556	$	8.16

Schedule of Restricted Stock Activity

A summary of the RSU activity is as follows:


			Number of Shares
Unvested at January 1, 2024			3,834,108
Granted			2,360,949
Vested			(754,153)
Forfeited(a)			(500,054)
Unvested at June 30, 2024			4,940,850

(a) Includes 14,467 rescinded RSUs.

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 June 30,				Six Months Ended June 30,
(in thousands)	2024		2023		2024		2023
Stock options	$	2,498	$	4,288	$	5,095	$	8,507
Restricted stock units and other stock awards	1,813		2,060		3,762		3,953
Employee stock purchase plan	114		295		228		590
Stock-based compensation expense included in total operating expenses	$	4,425	$	6,643	$	9,085	$	13,050

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 June 30,				Six Months Ended June 30,
(in thousands)	2024		2023		2024		2023
Research and development	$	2,362	$	3,396	$	4,890	$	6,728
General and administrative	2,063		3,247		4,195		6,322
Stock-based compensation expense included in total operating expenses	$	4,425	$	6,643	$	9,085	$	13,050

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 June 30,				Six Months Ended June 30,
	2024		2023		2024		2023
Risk-free interest rate	4.4	%	3.7	%	4.0	%	3.6	%
Expected dividend yield	—	%	—	%	—	%	—	%
Expected term (years)	5.49		5.84		5.98		6.04
Expected stock price volatility	118	%	100	%	116	%	99	%

Fair value measurements

6 Months Ended

Jun. 30, 2024

Fair Value Disclosures [Abstract]

Fair value measurements

The following table presents information about the Company's assets measured at fair value on a recurring basis and indicates the level within fair value hierarchy of the valuation techniques utilized to determine such value.


	June 30, 2024
(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	$	34,586	$	34,586	$	—	$	—


Marketable securities
U.S. treasury securities	$	103,744	$	103,744	$	—	$	—


	December 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	$	90,649	$	90,649	$	—	$	—
U.S. treasury securities	24,889		24,889		—		—
	$	115,538	$	115,538	$	—	$	—
Marketable securities
U.S. treasury securities	$	29,548	$	29,548	$	—	$	—
U.S. government agency securities	4,975		—		4,975		—
	$	34,523	$	29,548	$	4,975	$	—

There were no changes in valuation techniques or transfers between fair value measurement levels during the six months ended June 30, 2024 or during the year ended December 31, 2023.

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 consist 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 payable and accrued expenses approximate their fair values due to their short-term nature.

As of June 30, 2024 and December 31, 2023, the carrying value of the Company’s outstanding borrowing under the New Credit Facility (as defined in Note 7, Debt) 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.

Cover Page - shares	6 Months Ended
Cover Page - shares	Jun. 30, 2024	Aug. 09, 2024
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2024
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		122,675,051
Entity Central Index Key	0001442836
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q2
Amendment Flag	false

Debt - Narrative (Details) - New Credit Facility - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Oct. 29, 2021	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Debt
Interest expense		$ 1.0	$ 1.0	$ 1.9	$ 1.9
Line of Credit
Debt
Proceeds from line of credit	$ 25.0
Long-term line of credit		$ 25.0		$ 25.0

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482949/835-30-55-8

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(f))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-3

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 69B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481568/470-20-55-69B

Reference 6: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 69C
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481568/470-20-55-69C

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 69E
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481568/470-20-55-69E

Reference 8: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 69F
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481568/470-20-55-69F

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 11: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1D
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1D

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1D
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1D

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1D
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1D

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1E
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1E

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1E
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1E

Reference 20: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1E
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1E

Reference 21: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1F
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1F

Reference 22: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1F
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1F

Reference 23: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1F
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1F

Reference 24: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1F
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1F

Reference 25: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1I
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1I

Reference 26: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482925/835-30-45-2

Reference 27: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482900/835-30-50-1

+ Details

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References + Details

Accrued expenses (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Accrued Liabilities, Current [Abstract]
Accrued payroll and related expenses	$ 7,810	$ 8,807
Accrued research, development and non-clinical expenses	3,039	3,090
Accrued manufacturing expenses	2,950	2,566
Accrued clinical expenses	2,813	5,063
Accrued professional fees	817	936
Accrued restructuring expenses	0	1,047
Accrued other	353	389
Total	$ 17,782	$ 21,898

Accrued expenses

6 Months Ended

Jun. 30, 2024

Accrued Liabilities, Current [Abstract]

Accrued expenses

Accrued expenses consisted of the following as of June 30, 2024 and December 31, 2023:


(in thousands)	June 30, 2024		December 31, 2023
Accrued payroll and related expenses	$	7,810	$	8,807
Accrued research, development and non-clinical expenses	3,039		3,090
Accrued manufacturing expenses	2,950		2,566
Accrued clinical expenses	2,813		5,063
Accrued professional fees	817		936
Accrued restructuring expenses	—		1,047
Accrued other	353		389
	$	17,782	$	21,898

Stock-based compensation

6 Months Ended

Jun. 30, 2024

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 Company's 2017 Stock Incentive Plan (the "2017 Plan"), as described below.

In June 2017, the Company’s stockholders approved the 2017 Plan. Under the 2017 Plan, shares of common stock could be granted to the Company's employees, officers, directors, consultants and advisors in the form of options, 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, 2024, the number of shares of common stock issuable under the 2017 Plan was increased by 4,828,469 shares. During the six months ended June 30, 2024, the Company granted an aggregate of 4,969,274 RSUs and options to purchase shares of common stock to employees and non-employee directors under the 2017 Plan. As of June 30, 2024, there were 3,167,915 shares available for future issuance under the 2017 Plan.

Under the 2017 Plan, with respect to both incentive stock options and nonqualified stock options, the exercise price per share will not be less than the fair market value of the common stock on the date of grant and the vesting period for options granted to employees is generally four years. In accordance with the Company's non-employee director compensation policy, as in effect from time to time, options granted to non-employee directors in lieu of cash retainer fees earned are fully vested upon grant, options granted to non-employee directors upon initial election to the board of directors vest over three years, and options granted to non-employee directors on the date of each of annual meeting of stockholders vest over one year. Options granted under the 2017 Plan expire no later than 10 years from the date of grant. Options under the 2007 Plan were granted at an exercise price established by the Board (or an authorized committee thereof) that was not less than the fair market value of the underlying common stock on the date of grant and subject to such vesting provisions determined by the Board (or an authorized committee thereof). The Board may accelerate vesting or otherwise adjust the terms of granted options in the case of a merger, consolidation, dissolution, or liquidation of the Company.

Inducement awards

From time to time, the Company grants to its employees, upon approval by the Board or an authorized committee thereof, options to purchase shares of common stock and/or RSUs as an inducement to employment in accordance with Nasdaq Listing Rule 5635(c)(4). Prior to February 2022, only options to purchase shares of common stock were granted as inducement awards, and they were granted outside of an existing equity incentive plan. These options are subject to terms substantially the same as the 2017 Plan.

In February 2022, the Board adopted the Company's 2022 Inducement Stock Incentive Plan (the "Inducement Plan"), which provides for the grant of nonstatutory options, stock appreciation rights, restricted stock, RSUs and other stock-based awards, with respect to an aggregate of 2,000,000 shares of the Company's common stock (subject to adjustment as provided in the Inducement Plan). During the six months ended June 30, 2024, the Company granted an aggregate of 70,200 RSUs and options to purchase shares of common stock to newly hired employees under the Inducement Plan. As of June 30, 2024, there were 1,224,852 shares available for future issuance under the Inducement Plan.

As of June 30, 2024, there were options to purchase 422,343 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, 2024	10,902,845	$	7.42
Granted	2,678,525	$	2.91
Exercised	(14,223)	$	3.25
Cancelled/forfeited	(924,795)	$	7.23
Outstanding at June 30, 2024	12,642,352	$	6.48

Exercisable at June 30, 2024	7,121,556	$	8.16

The weighted-average grant date fair value of options granted during the six months ended June 30, 2024 and 2023 was $2.50 and $4.88 per share, respectively. The aggregate intrinsic value of options exercised during the six months ended June 30, 2024 and 2023 was de minimis and $0.3 million, respectively. The aggregate intrinsic value represents the difference between the exercise price and the selling price received by option holders upon the exercise of stock options during the period.

Cash received from the exercise of stock options was de minimis and $0.4 million, respectively, for the six months ended June 30, 2024 and 2023.

Restricted stock units and other awards

The Company periodically issues RSUs with a service condition to certain officers and other employees that typically vest between one year and four years from the grant date. In accordance with its non-employee director compensation policy, as in effect from time to time, in 2023, the Company issued RSUs with a service condition to non-employee directors that generally vested one year from the date of grant, and the Company may also issue shares of common stock in lieu of cash retainer fees earned to certain non-employee directors, which shares are fully vested upon grant.

A summary of the RSU activity is as follows:


			Number of Shares
Unvested at January 1, 2024			3,834,108
Granted			2,360,949
Vested			(754,153)
Forfeited(a)			(500,054)
Unvested at June 30, 2024			4,940,850

(a) Includes 14,467 rescinded RSUs.

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, 2024. The Company issued 153,729 and 291,260 shares, respectively, under the 2017 ESPP during the six months ended June 30, 2024 and 2023. As of June 30, 2024, there were 660,554 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 June 30,				Six Months Ended June 30,
(in thousands)	2024		2023		2024		2023
Stock options	$	2,498	$	4,288	$	5,095	$	8,507
Restricted stock units and other stock awards	1,813		2,060		3,762		3,953
Employee stock purchase plan	114		295		228		590
Stock-based compensation expense included in total operating expenses	$	4,425	$	6,643	$	9,085	$	13,050

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 June 30,				Six Months Ended June 30,
(in thousands)	2024		2023		2024		2023
Research and development	$	2,362	$	3,396	$	4,890	$	6,728
General and administrative	2,063		3,247		4,195		6,322
Stock-based compensation expense included in total operating expenses	$	4,425	$	6,643	$	9,085	$	13,050

As of June 30, 2024, there was $14.6 million and $13.4 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 1.8 years and 2.8 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 June 30,				Six Months Ended June 30,
	2024		2023		2024		2023
Risk-free interest rate	4.4	%	3.7	%	4.0	%	3.6	%
Expected dividend yield	—	%	—	%	—	%	—	%
Expected term (years)	5.49		5.84		5.98		6.04
Expected stock price volatility	118	%	100	%	116	%	99	%

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.

Collaboration agreements

6 Months Ended

Jun. 30, 2024

Organization, Consolidation and Presentation of Financial Statements [Abstract]

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 and six months ended June 30, 2024, the Company recorded collaboration revenue of a de minimis amount related to its efforts under the GSK agreement. During the three and six months ended June 30, 2023, the Company recorded collaboration revenue of $0.6 million and $1.3 million, respectively, related to its efforts under the GSK Agreement. As of June 30, 2024 and December 31, 2023, the Company had recorded $94.5 million and $94.6 million, respectively, in deferred revenue related to the unsatisfied performance obligations under the GSK Agreement. This deferred revenue will be recognized over the remaining performance period and classified as current or noncurrent on the consolidated balance sheets based upon the expected timing of satisfaction of the performance obligations.

Johnson & Johnson

In February 2022, the Company entered into a research collaboration and license agreement with Janssen Biotech Inc. ("Johnson & Johnson" and such agreement, as amended on July 14, 2023 and September 25, 2023, the "Johnson & Johnson Agreement") focused on the research, development and commercialization of novel ADCs for three oncology targets by leveraging Mersana’s ADC expertise and Dolasynthen platform with Johnson & Johnson’s proprietary antibodies. Upon execution of the Johnson & Johnson Agreement, the Company received a non-refundable upfront payment of $40.0 million. Johnson & Johnson may select up to three targets and may substitute each target once prior to a substitution deadline. Johnson & Johnson is not required to pay a fee for its first substitution right, but must pay a one-time fee for access to the subsequent substitution rights following its exercise of its second substitution right. During the year ended December 31, 2023, Johnson & Johnson exercised its first substitution right for a certain target.

Pursuant to mutually agreed research and chemistry, manufacturing and controls ("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 Johnson & Johnson. The Company estimates that its activities under the research plans for the targets will be performed into 2025.

Johnson & Johnson is required to pay for the Company's CMC activities at agreed-upon rates. Unless earlier terminated, the Johnson & Johnson Agreement will expire upon the expiration of the last royalty term for a product under the Johnson & Johnson Agreement.

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

Accounting Analysis

The Company assessed the Johnson & Johnson Agreement in accordance with ASC 606 and concluded that the contract counter party, Johnson & Johnson, is a customer. The Company identified the following seven material performance obligations under the Johnson & Johnson Agreement: (i) exclusive Johnson & Johnson Licenses and research activities for each of the three designated targets, (ii) CMC activities for each of the three designated targets and (iii) the first target substitution right.

The Company determined that the consideration for CMC activities represents variable consideration. CMC activities for one of the three designated targets have been initiated. The Company has elected to apply the right to invoice practical expedient in accordance with ASC 606 related to the CMC activities under the Johnson & Johnson Agreement. As such, the Company will recognize revenue related to the CMC activities when the services are performed over the corresponding CMC plan for a given target.

As of June 30, 2024, the revised total transaction price for the Johnson & Johnson Agreement was $48.0 million based on the reassessment of the constraint of certain development milestones and the remaining risks associated with the development required to achieve the milestones. As of June 30, 2024, the Company considered these milestones to be fully constrained as achievement was outside the control of the Company.

The Company is recognizing revenue related to the Johnson & Johnson Licenses and research services performance obligation over the estimated period of the research services using a proportional performance model. The Company measures proportional performance based on the costs incurred relative to the total costs expected to be incurred.

In February 2024, the Company arranged to provide additional CMC activities to Johnson & Johnson (the "2024 CMC Arrangement"). These CMC activities are a single combined performance obligation. As of June 30, 2024, the transaction price was $5.0 million based on the total costs expected to be incurred to develop and manufacture drug substance. The Company is recognizing revenue related to the performance obligation over the estimated period of the CMC activities using a proportional performance model. During the three months ended June 30, 2024, the Company charged Johnson & Johnson $1.6 million pursuant to the 2024 CMC Arrangement, which amount has been recorded as an accounts receivable on the consolidated balance sheet. As of June 30, 2024, no revenue has been recognized under the 2024 CMC Arrangement.

During the three months ended June 30, 2024 and 2023 and the six months ended June 30, 2024 and 2023 the Company recorded collaboration revenue of $1.8 million, $7.5 million, $7.9 million, and $9.1 million, respectively, related to its performance obligations under the Johnson & Johnson Agreement. During the three months ended June 30, 2023, the Company recognized revenue of $2.0 million related to achievement of a development milestone under the Johnson & Johnson Agreement for which performance obligations were previously completed. As of June 30, 2024 and December 31, 2023, the Company had recorded $5.4 million and $10.4 million, respectively, in deferred revenue related to the Johnson & Johnson Agreement and the 2024 CMC Arrangement that will be recognized over the remaining performance period and classified as current on the consolidated balance sheets based upon the expected timing of satisfaction of respective performance obligations.

Merck KGaA

Immunosynthen Platform Agreement

In December 2022, the Company entered into a research collaboration and license agreement with Ares Trading S.A., a wholly owned subsidiary of Merck KGaA, Darmstadt, Germany (Merck KGaA and/or its affiliate, as applicable, "Merck KGaA" and such agreement, the "2022 Merck KGaA Agreement"), focused on the research, development and commercialization of novel ADCs for up to two specific target antigens by leveraging Mersana’s ADC expertise and Immunosynthen platform with Merck KGaA’s proprietary antibodies. In connection with the 2022 Merck KGaA Agreement, the Company received a non-refundable upfront payment of $30.0 million. Pursuant to the 2022 Merck KGaA Agreement, the Company granted Merck KGaA two exclusive, non-transferable, worldwide licenses - a research license and a commercialization license (together, the "Merck KGaA Licenses").

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 Merck KGaA. The Company estimates that its activities under the research plans for the targets will be performed into 2025.

The Company's CMC activities will be compensated by Merck KGaA 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.

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

Accounting Analysis

The Company assessed the 2022 Merck KGaA Agreement in accordance with ASC 606 and concluded that the contract counter party, Merck KGaA, is a customer. The Company identified the following four material performance obligations under the 2022 Merck KGaA Agreement: (i) exclusive Merck KGaA Licenses and research activities for each of the two designated targets and (ii) CMC activities for each of the two designated targets.

The Company is recognizing revenue related to the Merck KGaA Licenses and research services performance obligation over the estimated period of the research services using a proportional performance model. The Company measures proportional performance based on the costs incurred relative to the total costs expected to be incurred.

During the three months ended June 30, 2024 and 2023 and the six months ended June 30, 2024 and 2023 the Company recorded collaboration revenue of a de minimis amount, $2.5 million, $3.2 million, and $5.6 million, respectively, related to its efforts under the 2022 Merck KGaA Agreement. As of June 30, 2024 and December 31, 2023, the Company had recorded $17.0 million and $20.2 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.

Summary of Contract Assets and Liabilities

The following table presents changes in the balances of the Company's contract liabilities:


(in thousands)	Balance at Beginning of Period		Additions		Deductions		Balance at End of Period
Six months ended June 30, 2024

Contract liabilities:





Total deferred revenue	$	125,314	$	2,940	$	11,338	$	116,916

Six months ended June 30, 2023
Contract liabilities:

Total deferred revenue	$	147,653	$	808	$	13,764	$	134,697

The Company did not record any contract assets associated with its collaboration agreements as of June 30, 2024 and 2023.

During the three and six months ended June 30, 2024 and 2023, the Company recognized the following revenues as a result of changes in the contract liability balances in the respective periods:


	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2024		2023		2024		2023
Revenue recognized in the period from:




Amounts included in the contract liability at the beginning of the period	$	2,293	$	8,497	$	11,231	$	13,607

Other Revenue

The Company has provided limited services for a collaborator, Asana BioSciences, LLC ("Asana Biosciences"), pursuant to a 2012 research, development and license agreement (the "Asana Biosciences Agreement"). The Company did not recognize revenue related to these services during the three and six months ended June 30, 2024 and 2023. During the six months ended June 30, 2023, the Company recognized revenue of $2.5 million related to achievement of a development milestone under the Asana Biosciences Agreement for which performance obligations were previously completed.

Fair value measurements (Tables)

6 Months Ended

Jun. 30, 2024

Fair Value Disclosures [Abstract]

Schedule of Fair Value, Assets Measured on Recurring Basis


	June 30, 2024
(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	$	34,586	$	34,586	$	—	$	—


Marketable securities
U.S. treasury securities	$	103,744	$	103,744	$	—	$	—


	December 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	$	90,649	$	90,649	$	—	$	—
U.S. treasury securities	24,889		24,889		—		—
	$	115,538	$	115,538	$	—	$	—
Marketable securities
U.S. treasury securities	$	29,548	$	29,548	$	—	$	—
U.S. government agency securities	4,975		—		4,975		—
	$	34,523	$	29,548	$	4,975	$	—

Cash, cash equivalents, and short-term marketable securities - Debt Securities, Available-for-Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		$ 34,512
Gross Unrealized Gains		13
Gross Unrealized Losses		(2)
Fair Value	$ 103,744	34,523
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	103,815	29,535
Gross Unrealized Gains	1	13
Gross Unrealized Losses	(72)	0
Fair Value	$ 103,744	29,548
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		4,977
Gross Unrealized Gains		0
Gross Unrealized Losses		(2)
Fair Value		$ 4,975

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (aa)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

+ Details

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	At Market Equity Offering Program	Common Stock	Common Stock At Market Equity Offering Program	Additional Paid-in Capital	Additional Paid-in Capital At Market Equity Offering Program	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at beginning of period (in shares) at Dec. 31, 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 from at-the-market transactions, net of issuance costs (in shares)				3,535,093
Issuance of common stock from at-the-market transactions, net of issuance costs		$ 21,795				$ 21,795
Exercise of stock options (in shares)			8,826
Exercise of stock options	34				34
Vesting of restricted stock units and other stock awards (in shares)			372,291
Stock-based compensation expense	6,407				6,407
Other comprehensive gain (loss)	164						164
Net loss	(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)
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
Net loss	(110,470)
Balance at end of period (in shares) at Jun. 30, 2023			120,459,232
Balance at end of period at Jun. 30, 2023	89,800		$ 12		854,998		(49)	(765,161)
Balance at beginning of period (in shares) at Mar. 31, 2023			109,061,074
Balance at beginning of period at Mar. 31, 2023	64,294		$ 11		775,125		12	(710,854)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock from at-the-market transactions, net of issuance costs (in shares)				10,929,438
Issuance of common stock from at-the-market transactions, net of issuance costs		71,875		$ 1		71,874
Exercise of stock options (in shares)			88,770
Exercise of stock options	393				393
Vesting of restricted stock units and other stock awards (in shares)			88,690
Purchase of common stock under ESPP (in shares)			291,260
Purchase of common stock under ESPP	963				963
Stock-based compensation expense	6,643				6,643
Other comprehensive gain (loss)	(61)						(61)
Net loss	(54,307)							(54,307)
Balance at end of period (in shares) at Jun. 30, 2023			120,459,232
Balance at end of period at Jun. 30, 2023	$ 89,800		$ 12		854,998		(49)	(765,161)
Balance at beginning of period (in shares) at Dec. 31, 2023	120,711,745		120,711,745
Balance at beginning of period at Dec. 31, 2023	$ 36,904		$ 12		863,242		11	(826,361)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock from at-the-market transactions, net of issuance costs (in shares)				1,041,201
Issuance of common stock from at-the-market transactions, net of issuance costs		$ 5,780				$ 5,780
Exercise of stock options (in shares)			12,117
Exercise of stock options	48				48
Vesting of restricted stock units and other stock awards (in shares)			594,067
Stock-based compensation expense	4,660				4,660
Other comprehensive gain (loss)	(101)						(101)
Net loss	(19,306)							(19,306)
Balance at end of period (in shares) at Mar. 31, 2024			122,359,130
Balance at end of period at Mar. 31, 2024	$ 27,985		$ 12		873,730		(90)	(845,667)
Balance at beginning of period (in shares) at Dec. 31, 2023	120,711,745		120,711,745
Balance at beginning of period at Dec. 31, 2023	$ 36,904		$ 12		863,242		11	(826,361)
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)	14,223
Net loss	$ (43,574)
Balance at end of period (in shares) at Jun. 30, 2024	122,675,051		122,675,051
Balance at end of period at Jun. 30, 2024	$ 8,427		$ 12		878,421		(71)	(869,935)
Balance at beginning of period (in shares) at Mar. 31, 2024			122,359,130
Balance at beginning of period at Mar. 31, 2024	27,985		$ 12		873,730		(90)	(845,667)
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)			2,106
Exercise of stock options	8				8
Vesting of restricted stock units and other stock awards (in shares)			160,086
Purchase of common stock under ESPP (in shares)			153,729
Purchase of common stock under ESPP	258				258
Stock-based compensation expense	4,425				4,425
Other comprehensive gain (loss)	19						19
Net loss	$ (24,268)							(24,268)
Balance at end of period (in shares) at Jun. 30, 2024	122,675,051		122,675,051
Balance at end of period at Jun. 30, 2024	$ 8,427		$ 12		$ 878,421		$ (71)	$ (869,935)

Collaboration agreements - GlaxoSmithKline (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Aug. 31, 2022 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Aug. 06, 2022 performanceObligation
Collaboration agreements
Collaboration revenue		$ 2,293	$ 10,654	$ 11,538	$ 18,456
Deferred revenue		116,916	134,697	116,916	134,697	$ 125,314	$ 147,653
GSK Agreement
Collaboration agreements
Upfront payment received	$ 100,000
Collaboration arrangement, potential option exercise receivable	$ 90,000
Performance obligations identified \| performanceObligation								2
Collaboration revenue		0	$ 600	0	$ 1,300
Deferred revenue		$ 94,500		$ 94,500		$ 94,600

- 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
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 41
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-41

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 270
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482964/270-10-50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (ee)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-32

Reference 4: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-32

Reference 5: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-32

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 924
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 11.L)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479941/924-10-S99-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-5

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 30
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-30

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-42

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-22

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 40
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-40

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-22

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-4

+ Details

Cash, cash equivalents, and short-term marketable securities - Narrative (Details)	6 Months Ended
	Jun. 30, 2024 USD ($) security	Jun. 30, 2023 USD ($)
Investments, Debt and Equity Securities [Abstract]
Number of securities in unrealized loss position \| security	13
Value of securities in an unrealized loss position \| $	$ 89,100,000
Securities in unrealized loss position greater than 12 months \| security	0
Charges for credit-related impairments \| $	$ 0	$ 0

Stock-based compensation - Restricted Stock Units and Other Awards (Details)	6 Months Ended
	Jun. 30, 2024 shares
Restricted stock units
Restricted stock units
Unvested at beginning of period (in shares)	3,834,108
Granted (in shares)	2,360,949
Vested (in shares)	(754,153)
Forfeited (in shares)	(500,054)
Unvested at end of period (in shares)	4,940,850
Rescinded (in shares)	500,054
Rescinded restricted stock units
Restricted stock units
Forfeited (in shares)	(14,467)
Rescinded (in shares)	14,467

Nature of business and basis of presentation	6 Months Ended
Nature of business and basis of presentation	Jun. 30, 2024
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. (the "Company") is a clinical-stage biopharmaceutical company focused on developing antibody-drug conjugates ("ADCs") that offer a clinically meaningful benefit for cancer patients with significant unmet need. The Company’s next-generation ADC platforms include Dolasynthen, which delivers a proprietary auristatin payload, and Immunosynthen, which delivers a proprietary stimulator of interferon genes ("STING") agonist payload. The Company is investigating XMT-1660, a B7-H4-directed Dolasynthen ADC, in a Phase 1 clinical trial enrolling patients with solid tumors, including in breast, endometrial and ovarian cancers. The Company initiated a Phase 1 clinical trial to investigate XMT-2056, an Immunosynthen STING-agonist ADC that is designed to target a novel epitope of human epidermal growth factor receptor 2 ("HER2"), in January 2023, enrolling previously treated patients with advanced/recurrent solid tumors expressing HER2, including breast, gastric, colorectal and non-small cell lung cancers. In March 2023, following a voluntary suspension of this clinical trial by the Company, this clinical trial was placed on clinical hold by the U.S. Food and Drug Administration ("FDA"), and the FDA lifted this clinical hold in October 2023. The Company is currently enrolling patients in its Phase 1 clinical trial of XMT-2056. The Company also has two additional earlier stage preclinical candidates, XMT-2068 and XMT-2175, that leverage the Company's Immunosynthen platform. In July 2023, the Company announced top-line data from its Phase 2 UPLIFT clinical trial of upifitamab rilsodotin ("UpRi"), which did not meet its primary endpoint. In connection with this announcement, on July 27, 2023, the Company further announced that its primary focus moving forward would be on advancing product candidates and collaborations utilizing its next-generation ADC platforms, Dolasynthen and Immunosynthen. As a result, the Company wound down its UpRi-related development activities and its regulatory and commercial readiness efforts and terminated its UPGRADE-A and Phase 3 UP-NEXT clinical trials of UpRi, on which the FDA had placed a partial clinical hold in June 2023. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, the need for additional capital, risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval and reimbursement for any drug product candidate that it may identify and develop, the need to successfully commercialize and gain market acceptance of its product candidates, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development of technological innovations by competitors, reliance on third party manufacturers and the ability to transition from pilot-scale production to large-scale manufacturing of products. The Company has incurred cumulative net losses since inception. For the three months ended June 30, 2024, the net loss was $24.3 million, compared to $54.3 million in the three months ended June 30, 2023. For the six months ended June 30, 2024, the net loss was $43.6 million, compared to $110.5 million in the six months ended June 30, 2023. The Company expects to continue to incur operating losses for at least the next several years. As of June 30, 2024, the Company had an accumulated deficit of $869.9 million. The future success of the Company is dependent upon, 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, 2023 and the notes thereto, included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023, filed with the SEC on February 28, 2024. 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 June 30, 2024, the results of its operations for the three and six months ended June 30, 2024 and 2023, the statements of stockholders’ equity for the three and six months ended June 30, 2024 and 2023 and statements of cash flows for the six months ended June 30, 2024 and 2023. Such adjustments are of a normal and recurring nature. The results for the three and six months ended June 30, 2024 are not necessarily indicative of the results for the year ending December 31, 2024, or for any future period. Certain items in the prior period financial statements have been reclassified to conform to current period presentation.

Commitments	6 Months Ended
Commitments	Jun. 30, 2024
Commitments and Contingencies Disclosure [Abstract]
Commitments	Commitments License agreements During the three and six months ended June 30, 2024 and 2023, the Company did not record research and development expense related to non-refundable license payments or development milestones.

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Income Statement [Abstract]
Collaboration revenue	$ 2,293	$ 10,654	$ 11,538	$ 18,456
Operating expenses:
Research and development	17,245	48,968	35,931	96,243
General and administrative	10,503	18,187	22,063	36,515
Total operating expenses	27,748	67,155	57,994	132,758
Other income (expense):
Interest income	2,170	3,219	4,867	5,840
Interest expense	(983)	(1,025)	(1,985)	(2,008)
Total other income, net	1,187	2,194	2,882	3,832
Net loss	(24,268)	(54,307)	(43,574)	(110,470)
Unrealized gain (loss) on marketable securities	19	(61)	(82)	103
Comprehensive loss	(24,249)	(54,368)	(43,656)	(110,367)
Net loss attributable to common stockholders — basic	(24,268)	(54,307)	(43,574)	(110,470)
Net loss attributable to common stockholders — diluted	$ (24,268)	$ (54,307)	$ (43,574)	$ (110,470)
Net loss per share attributable to common stockholders — basic (in dollars per share)	$ (0.20)	$ (0.47)	$ (0.36)	$ (0.99)
Net loss per share attributable to common stockholders — diluted (in dollars per share)	$ (0.20)	$ (0.47)	$ (0.36)	$ (0.99)
Weighted-average number of shares of common stock used in net loss per share attributable to common stockholders — basic (in shares)	122,440,124	115,608,156	121,932,540	111,583,765
Weighted-average number of shares of common stock used in net loss per share attributable to common stockholders — diluted (in shares)	122,440,124	115,608,156	121,932,540	111,583,765

Collaboration agreements - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Contract liabilities
Balance at Beginning of Period			$ 125,314	$ 147,653
Additions			2,940	808
Deductions			11,338	13,764
Balance at End of Period	$ 116,916	$ 134,697	116,916	134,697
Amounts included in the contract liability at the beginning of the period	$ 2,293	$ 8,497	$ 11,231	$ 13,607

Debt

6 Months Ended

Jun. 30, 2024

Debt Disclosure [Abstract]

Debt

On October 29, 2021, the Company entered into a loan and security agreement (as amended on February 17, 2022, October 17, 2022, December 27, 2022, and March 23, 2023, 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. 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 June 30, 2024. As of June 30, 2024, no additional borrowing amounts were available to the Company under the New Credit Facility.

Refer to Note 8, Debt, in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023, filed with the SEC on February 28, 2024, for more information regarding the New Credit Facility. As of June 30, 2024, the Company was in compliance with all covenants under the New Credit Facility. There were no events of default under the New Credit Facility as of June 30, 2024.

The following is a summary of obligations under the New Credit Facility as of June 30, 2024:


(in thousands)	June 30, 2024
Total debt	$	25,000
Less: Current portion of long-term debt	(8,333)
Total debt, net of current portion	16,667
Debt financing costs, net of accretion	(194)
Accretion related to final payment	576
Long-term debt, net	$	17,049

Interest expense related to the New Credit Facility for each of the three months ended June 30, 2024 and 2023 was $1.0 million and for each of the six months ended June 30, 2024 and 2023 was $1.9 million.

- 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481544/470-10-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481544/470-10-50-6

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477092/405-40-50-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477092/405-40-50-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477092/405-40-50-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477092/405-40-50-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477092/405-40-50-1

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 470
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org/470/tableOfContent

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482925/835-30-45-2

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1C
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1C

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1C
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1C

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1C
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1C

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1E
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1E

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1I
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1I

Reference 20: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1I
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1I

Reference 21: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1I
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1I

+ Details

Net loss per share

6 Months Ended

Jun. 30, 2024

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 and Six Months Ended June 30, 2024	Three and Six Months ended June 30, 2023
Stock options	12,642,352	12,359,185
Unvested restricted stock units	4,940,850	3,671,153
Warrants	—	22,590
	17,583,202	16,052,928

Nature of business and basis of presentation (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2024 USD ($) candidate	Mar. 31, 2024 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Jun. 30, 2024 USD ($) candidate	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of additional preclinical candidates \| candidate	2				2
Net loss	$ (24,268)	$ (19,306)	$ (54,307)	$ (56,163)	$ (43,574)	$ (110,470)
Accumulated deficit	$ (869,935)				$ (869,935)		$ (826,361)

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483443/250-10-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483443/250-10-50-9

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 60
-Name Accounting Standards Codification
-Section 65
-Paragraph 1
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476176/805-60-65-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 323
-Name Accounting Standards Codification
-Section 65
-Paragraph 2
-Subparagraph (g)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478666/740-323-65-2

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(20))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(ii))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 323
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481687/323-10-50-3

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 28
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-28

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482765/220-10-50-6

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483443/250-10-50-3

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483443/250-10-50-1

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 65
-Paragraph 1
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480175/815-40-65-1

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483443/250-10-50-8

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483443/250-10-50-11

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483443/250-10-50-11

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483443/250-10-50-4

Reference 17: http://www.xbrl.org/2003/role/exampleRef
-Topic 946
-SubTopic 830
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479168/946-830-55-10

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section 45
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479105/946-220-45-7

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(18))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1

Reference 20: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(9))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 21: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-09(1)(d))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-3

Reference 22: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1A
-Subparagraph (SX 210.13-01(a)(4)(i))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480097/470-10-S99-1A

Reference 23: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1A
-Subparagraph (SX 210.13-01(a)(4)(ii))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480097/470-10-S99-1A

Reference 24: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1A
-Subparagraph (SX 210.13-01(a)(4)(iii)(A))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480097/470-10-S99-1A

Reference 25: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1A
-Subparagraph (SX 210.13-01(a)(4)(iv))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480097/470-10-S99-1A

Reference 26: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1A
-Subparagraph (SX 210.13-01(a)(5))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480097/470-10-S99-1A

Reference 27: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1B
-Subparagraph (SX 210.13-02(a)(4)(i))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480097/470-10-S99-1B

Reference 28: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1B
-Subparagraph (SX 210.13-02(a)(4)(iii)(A))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480097/470-10-S99-1B

Reference 29: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1B
-Subparagraph (SX 210.13-02(a)(4)(iii)(B))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480097/470-10-S99-1B

Reference 30: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1B
-Subparagraph (SX 210.13-02(a)(4)(iv))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480097/470-10-S99-1B

Reference 31: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1B
-Subparagraph (SX 210.13-02(a)(5))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480097/470-10-S99-1B

Reference 32: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 60B
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482689/260-10-45-60B

Reference 33: http://www.xbrl.org/2003/role/disclosureRef
-Topic 205
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483499/205-20-50-7

Reference 34: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

Reference 35: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1A
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-1A

Reference 36: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1B
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-1B

Reference 37: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(22))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

+ Details

- Definition

Amount of accumulated undistributed earnings (deficit).

+ References + Details

Accrued expenses (Tables)

6 Months Ended

Jun. 30, 2024

Accrued Liabilities, Current [Abstract]

Schedule of Accrued Expenses

Accrued expenses consisted of the following as of June 30, 2024 and December 31, 2023:


(in thousands)	June 30, 2024		December 31, 2023
Accrued payroll and related expenses	$	7,810	$	8,807
Accrued research, development and non-clinical expenses	3,039		3,090
Accrued manufacturing expenses	2,950		2,566
Accrued clinical expenses	2,813		5,063
Accrued professional fees	817		936
Accrued restructuring expenses	—		1,047
Accrued other	353		389
	$	17,782	$	21,898

Net loss per share (Details) - shares	3 Months Ended		6 Months Ended
Net loss per share (Details) - shares	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Anti-dilutive securities
Anti-dilutive securities (in shares)	17,583,202	16,052,928	17,583,202	16,052,928
Stock options
Anti-dilutive securities
Anti-dilutive securities (in shares)	12,642,352	12,359,185	12,642,352	12,359,185
Unvested restricted stock units
Anti-dilutive securities
Anti-dilutive securities (in shares)	4,940,850	3,671,153	4,940,850	3,671,153
Warrants
Anti-dilutive securities
Anti-dilutive securities (in shares)	0	22,590	0	22,590

Debt - Schedule of Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Debt Disclosure [Abstract]
Total debt	$ 25,000
Less: Current portion of long-term debt	(8,333)	$ (2,083)
Total debt, net of current portion	16,667
Debt financing costs, net of accretion	(194)
Accretion related to final payment	576
Total debt, net of current portion	$ 17,049	$ 23,148

Fair value measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Assets, Fair Value Disclosure [Abstract]
Marketable securities	$ 103,744	$ 34,523
U.S. treasury securities
Assets, Fair Value Disclosure [Abstract]
Marketable securities	103,744	29,548
U.S. government agency securities
Assets, Fair Value Disclosure [Abstract]
Marketable securities		4,975
Recurring basis
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		115,538
Marketable securities		34,523
Recurring basis \| Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	34,586	90,649
Recurring basis \| U.S. treasury securities
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		24,889
Marketable securities	103,744	29,548
Recurring basis \| U.S. government agency securities
Assets, Fair Value Disclosure [Abstract]
Marketable securities		4,975
Recurring basis \| Quoted Prices in Active Markets (Level 1)
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		115,538
Marketable securities		29,548
Recurring basis \| Quoted Prices in Active Markets (Level 1) \| Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	34,586	90,649
Recurring basis \| Quoted Prices in Active Markets (Level 1) \| U.S. treasury securities
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		24,889
Marketable securities	103,744	29,548
Recurring basis \| Quoted Prices in Active Markets (Level 1) \| U.S. government agency securities
Assets, Fair Value Disclosure [Abstract]
Marketable securities		0
Recurring basis \| Significant Other Observable Inputs (Level 2)
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		0
Marketable securities		4,975
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
Marketable securities	0	0
Recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. government agency securities
Assets, Fair Value Disclosure [Abstract]
Marketable securities		4,975
Recurring basis \| Significant Unobservable Inputs (Level 3)
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		0
Marketable securities		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
Marketable securities	$ 0	0
Recurring basis \| Significant Unobservable Inputs (Level 3) \| U.S. government agency securities
Assets, Fair Value Disclosure [Abstract]
Marketable securities		$ 0

Stock-based compensation - Fair Value Assumptions (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Risk-free interest rate	4.40%	3.70%	4.00%	3.60%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Expected term (years)	5 years 5 months 26 days	5 years 10 months 2 days	5 years 11 months 23 days	6 years 14 days
Expected stock price volatility	118.00%	100.00%	116.00%	99.00%

Debt (Tables)

6 Months Ended

Jun. 30, 2024

Debt Disclosure [Abstract]

Schedule of Debt

The following is a summary of obligations under the New Credit Facility as of June 30, 2024:


(in thousands)	June 30, 2024
Total debt	$	25,000
Less: Current portion of long-term debt	(8,333)
Total debt, net of current portion	16,667
Debt financing costs, net of accretion	(194)
Accretion related to final payment	576
Long-term debt, net	$	17,049

Stock-based compensation - Stock Based Compensation Expense by Award Type (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Stock options
Stock-based compensation expense included in total operating expenses	$ 4,425	$ 6,643	$ 9,085	$ 13,050
Stock options
Stock options
Stock-based compensation expense included in total operating expenses	2,498	4,288	5,095	8,507
Restricted stock units and other stock awards
Stock options
Stock-based compensation expense included in total operating expenses	1,813	2,060	3,762	3,953
Employee stock purchase plan
Stock options
Stock-based compensation expense included in total operating expenses	$ 114	$ 295	$ 228	$ 590

Net loss per share (Tables)

6 Months Ended

Jun. 30, 2024

Earnings Per Share [Abstract]

Schedule of Outstanding Potentially Dilutive Securities


	Three and Six Months Ended June 30, 2024	Three and Six Months ended June 30, 2023
Stock options	12,642,352	12,359,185
Unvested restricted stock units	4,940,850	3,671,153
Warrants	—	22,590
	17,583,202	16,052,928

Collaboration agreements - Johnson & Johnson (Details)	1 Months Ended	3 Months Ended		6 Months Ended
Collaboration agreements - Johnson & Johnson (Details)	Feb. 28, 2022 USD ($) performanceObligation target	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Collaboration agreements
Collaboration revenue		$ 2,293,000	$ 10,654,000	$ 11,538,000	$ 18,456,000
Deferred revenue		116,916,000	134,697,000	116,916,000	134,697,000	$ 125,314,000	$ 147,653,000
Johnson & Johnson Agreement
Collaboration agreements
Number of targets designated \| target	3
Upfront payment received	$ 40,000,000
Performance obligations identified \| performanceObligation	7
Revised transaction price		48,000,000		48,000,000
Johnson & Johnson Agreement \| Performance obligations
Collaboration agreements
Collaboration revenue		1,800,000	7,500,000	7,900,000	$ 9,100,000
Johnson & Johnson Agreement \| Development milestone achieved
Collaboration agreements
Collaboration revenue			$ 2,000,000
Johnson and Johnson 2024 CMC Arrangement
Collaboration agreements
Revised transaction price		5,000,000		5,000,000
Receivable from customer		1,600,000		1,600,000
Collaboration revenue				0
Johnson and Johnson Agreement and Johnson and Johnson 2024 CMC Arrangement
Collaboration agreements
Deferred revenue		$ 5,400,000		$ 5,400,000		$ 10,400,000

Stockholders' equity (Tables)

6 Months Ended

Jun. 30, 2024

Stockholders' Equity Note [Abstract]

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


	June 30, 2024	December 31, 2023
Stock options	12,642,352	10,902,845
Restricted stock units	4,940,850	3,834,108


	17,583,202	14,736,953

- 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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(27))
-SubTopic 10
-Topic 210
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480008/505-10-S99-1

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-3

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-4

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-5

Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481142/505-10-45-2

Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-10

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-8

Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section S99
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480244/480-10-S99-1

Reference 12: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-6

Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-7

+ Details

Collaboration agreements (Tables)

6 Months Ended

Jun. 30, 2024

Organization, Consolidation and Presentation of Financial Statements [Abstract]

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:


(in thousands)	Balance at Beginning of Period		Additions		Deductions		Balance at End of Period
Six months ended June 30, 2024

Contract liabilities:





Total deferred revenue	$	125,314	$	2,940	$	11,338	$	116,916

Six months ended June 30, 2023
Contract liabilities:

Total deferred revenue	$	147,653	$	808	$	13,764	$	134,697

During the three and six months ended June 30, 2024 and 2023, the Company recognized the following revenues as a result of changes in the contract liability balances in the respective periods:


	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2024		2023		2024		2023
Revenue recognized in the period from:




Amounts included in the contract liability at the beginning of the period	$	2,293	$	8,497	$	11,231	$	13,607

Cash, cash equivalents, and short-term marketable securities (Tables)

6 Months Ended

Jun. 30, 2024

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 June 30, 2024 and 2023.


	Six Months Ended June 30, 2024				Six Months Ended June 30, 2023
(in thousands)	Beginning of period		End of period		Beginning of period		End of period
Cash and cash equivalents	$	174,561	$	58,998	$	128,885	$	128,732
Restricted cash included in other assets, noncurrent	478		478		478		478
Total cash, cash equivalents and restricted cash per statement of cash flows	$	175,039	$	59,476	$	129,363	$	129,210

Schedule of Debt Securities, Available-for-Sale

The following tables summarize the Company's marketable securities held at June 30, 2024 and December 31, 2023.


	June 30, 2024
(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable securities
U.S. treasury securities	$	103,815	$	1	$	(72)	$	103,744


	December 31, 2023
(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable securities
U.S. treasury securities	$	29,535	$	13	$	—	$	29,548
U.S. government agency securities	4,977		—		(2)		4,975
Total	$	34,512	$	13	$	(2)	$	34,523

- Definition

Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-9

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (aa)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 9: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 10: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 11: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 12: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

+ Details

Condensed Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023
At Market Equity Offering Program
Stock issuance costs	$ 185	$ 1,524	$ 558

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2024	Dec. 31, 2023
Stockholders' equity:
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	25,000,000	25,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	350,000,000	350,000,000
Common stock, shares issued (in shares)	122,675,051	120,711,745
Common stock, shares outstanding (in shares)	122,675,051	120,711,745

Pay vs Performance Disclosure - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Thousands	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023
Pay vs Performance Disclosure
Net loss	$ (24,268)	$ (19,306)	$ (54,307)	$ (56,163)	$ (43,574)	$ (110,470)

- Definition

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

+ References

+ Details

Stockholders' equity

6 Months Ended

Jun. 30, 2024

Stockholders' Equity Note [Abstract]

Stockholders’ equity

Preferred stock

As of June 30, 2024, 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 February 2022, the Company established an 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, 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 six months ended June 30, 2024 and 2023, the Company sold 1,041,201 and 14,208,145 shares of common stock under the November 2022 ATM, respectively, resulting in net proceeds of $5.8 million and $92.2 million, respectively. As of June 30, 2024, approximately $50.0 million remained unsold and available for sale under the November 2022 ATM.

In February 2024, the Company established an additional ATM equity offering program (the "February 2024 ATM"), pursuant to which it is able to offer and sell up to $100.0 million of its common stock from time to time at prevailing market prices. As of June 30, 2024, $100.0 million remained unsold and available for sale under the February 2024 ATM.

Common stock

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 June 30, 2024 and December 31, 2023, there were 17,583,202 and 14,736,953 shares, respectively, of common stock reserved for the exercise of outstanding stock options and restricted stock units ("RSUs").


	June 30, 2024	December 31, 2023
Stock options	12,642,352	10,902,845
Restricted stock units	4,940,850	3,834,108


	17,583,202	14,736,953

- Definition

The entire disclosure for equity.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-14

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 235
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477968/946-235-50-2

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 235
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477968/946-235-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478448/946-505-50-6

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480237/815-40-50-6

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480008/505-10-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(e)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org/505/tableOfContent

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-14

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-14

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-16

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-18

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-18

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-18

+ Details

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Current assets:
Cash and cash equivalents	$ 58,998	$ 174,561
Short-term marketable securities	103,744	34,523
Accounts receivable	1,570	0
Prepaid expenses and other current assets	4,651	4,973
Total current assets	168,963	214,057
Property and equipment, net	3,005	3,831
Operating lease right-of-use assets	6,182	7,694
Other assets, noncurrent	978	478
Total assets	179,128	226,060
Current liabilities:
Accounts payable	3,013	7,319
Accrued expenses	17,782	21,898
Deferred revenue	29,345	28,147
Operating lease liabilities	3,654	3,252
Short-term debt	8,333	2,083
Other current liabilities	57	938
Total current liabilities	62,184	63,637
Operating lease liabilities, noncurrent	3,271	5,149
Total debt, net of current portion	17,049	23,148
Deferred revenue, noncurrent	87,571	97,167
Other liabilities, noncurrent	626	55
Total liabilities	170,701	189,156
Commitments (Note 11)
Stockholders' equity:
Preferred stock, $0.0001 par value; 25,000,000 shares authorized; 0 shares issued and outstanding at June 30, 2024 and December 31, 2023, respectively	0	0
Common stock, $0.0001 par value; 350,000,000 shares authorized; 122,675,051 and 120,711,745 shares issued and outstanding at June 30, 2024 and December 31, 2023, respectively	12	12
Additional paid-in capital	878,421	863,242
Accumulated other comprehensive (loss) income	(71)	11
Accumulated deficit	(869,935)	(826,361)
Total stockholders’ equity	8,427	36,904
Total liabilities and stockholders’ equity	$ 179,128	$ 226,060

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		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Stock options
Share-based compensation expense	$ 4,425	$ 6,643	$ 9,085	$ 13,050
Research and development
Stock options
Share-based compensation expense	2,362	3,396	4,890	6,728
General and administrative
Stock options
Share-based compensation expense	$ 2,063	$ 3,247	$ 4,195	$ 6,322

Stock-based compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 01, 2024	Jun. 30, 2017	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Feb. 28, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares outstanding (in shares)			12,642,352		12,642,352		10,902,845
Weighted-average grant date fair value of options granted (in dollars per share)					$ 2.50	$ 4.88
Intrinsic vale of options exercised					$ 0	$ 300
Cash received from the exercise of stock options					$ 56	$ 427
Expected dividend yield			0.00%	0.00%	0.00%	0.00%
Stock options
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Unrecognized stock compensation cost, option			$ 14,600		$ 14,600
Weighted-average amortization period of unrecognized stock compensation cost					1 year 9 months 18 days
Restricted stock units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Unrecognized stock compensation cost			$ 13,400		$ 13,400
Weighted-average amortization period of unrecognized stock compensation cost					2 years 9 months 18 days
Restricted stock units \| Director
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting period					1 year
Restricted stock units \| Minimum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting period					1 year
Restricted stock units \| Maximum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting period					4 years
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)	4,828,469
Number of options granted in period (in shares)					4,969,274
Number of shares available for future issuance (in shares)			3,167,915		3,167,915
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
2017 Plan \| Options granted upon initial election \| Director
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting period					3 years
2017 Plan \| Options granted at annual meeting \| Director
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting period					1 year
Inducement award program
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of options granted in period (in shares)					70,200
Number of shares available for future issuance (in shares)			1,224,852		1,224,852
Number of shares authorized (in shares)								2,000,000
Inducement award prior to inducement award program
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares outstanding (in shares)			422,343		422,343
2017 ESPP \| Stock options
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Purchase of common stock under ESPP (in shares)					153,729	291,260
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)			660,554		660,554

Cash, cash equivalents, and short-term marketable securities

6 Months Ended

Jun. 30, 2024

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 June 30, 2024 and 2023.


	Six Months Ended June 30, 2024				Six Months Ended June 30, 2023
(in thousands)	Beginning of period		End of period		Beginning of period		End of period
Cash and cash equivalents	$	174,561	$	58,998	$	128,885	$	128,732
Restricted cash included in other assets, noncurrent	478		478		478		478
Total cash, cash equivalents and restricted cash per statement of cash flows	$	175,039	$	59,476	$	129,363	$	129,210

The restricted cash is held as a security deposit for a standby letter of credit related to the Company's facility lease.

Marketable securities

The following tables summarize the Company's marketable securities held at June 30, 2024 and December 31, 2023.


	June 30, 2024
(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable securities
U.S. treasury securities	$	103,815	$	1	$	(72)	$	103,744


	December 31, 2023
(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Marketable securities
U.S. treasury securities	$	29,535	$	13	$	—	$	29,548
U.S. government agency securities	4,977		—		(2)		4,975
Total	$	34,512	$	13	$	(2)	$	34,523

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 six months ended June 30, 2024 and 2023, and as a result, the Company did not reclassify any amounts out of accumulated comprehensive loss.

As of June 30, 2024, the Company's debt security portfolio consisted of 13 securities that were in an unrealized loss position and had an aggregate fair value of $89.1 million. There were no securities in an unrealized loss position for greater than 12 months as of June 30, 2024. 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 any debt securities in its portfolio for the six months ended June 30, 2024 and 2023.

Summary of significant accounting policies	6 Months Ended
Summary of significant accounting policies	Jun. 30, 2024
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 and six months ended June 30, 2024 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, 2023 filed with the SEC on February 28, 2024. 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, treasury securities, commercial paper and government agency securities, which are highly liquid and have strong credit ratings. These investments are subject to minimal credit and market risks. Cash and cash equivalents are stated at cost, which approximates market value. Recently Issued Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies, and the Company adopts such pronouncements as of the specified effective date. Unless otherwise discussed below, the Company does not believe that the adoption of recently issued standards has had or may have a material impact on the Company's condensed consolidated financial statements or disclosures. In November 2023, the FASB issued Accounting Standard Update, or ASU, 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures, which is intended to improve reportable segment disclosure requirements, primarily through additional disclosures about significant segment expenses. The standard is effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024, with early adoption permitted. The amendments should be applied retrospectively to all prior periods presented in the financial statements. The Company is currently evaluating the impact ASU 2023-07 may have on the Company's consolidated financial statements. In December 2023, the FASB, issued ASU 2023-09, Improvements to Income Tax Disclosures, which requires entities to disclose disaggregated information about their effective tax rate reconciliation as well as expanded information on income taxes paid by jurisdiction. The disclosure requirements will be applied on a prospective basis, with the option to apply them retrospectively. The standard is effective for fiscal years beginning after December 15, 2024, with early adoption permitted. The Company is currently evaluating the impact ASU 2023-09 may have on the Company's consolidated financial statements.

Summary of significant accounting policies (Policies)	6 Months Ended
Summary of significant accounting policies (Policies)	Jun. 30, 2024
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 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.
Concentration of Credit Risk and Off-balance Sheet Risk	Concentration of Credit Risk and Off-balance Sheet Risk Financial instruments that potentially subject the Company to concentrations of credit risk primarily consist of cash equivalents and marketable securities. Under its investment policy, the Company limits amounts invested in such securities by credit rating, maturity, industry group, investment type and issuer, except for securities issued by the U.S. government. The Company does not believe that it is subject to any significant concentrations of credit risk from these financial instruments. The Company has no financial instruments with off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements.
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, treasury securities, commercial paper and government agency securities, which are highly liquid and have strong credit ratings. These investments are subject to minimal credit and market risks. Cash and cash equivalents are stated at cost, which approximates market value.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies, and the Company adopts such pronouncements as of the specified effective date. Unless otherwise discussed below, the Company does not believe that the adoption of recently issued standards has had or may have a material impact on the Company's condensed consolidated financial statements or disclosures. In November 2023, the FASB issued Accounting Standard Update, or ASU, 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures, which is intended to improve reportable segment disclosure requirements, primarily through additional disclosures about significant segment expenses. The standard is effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024, with early adoption permitted. The amendments should be applied retrospectively to all prior periods presented in the financial statements. The Company is currently evaluating the impact ASU 2023-07 may have on the Company's consolidated financial statements. In December 2023, the FASB, issued ASU 2023-09, Improvements to Income Tax Disclosures, which requires entities to disclose disaggregated information about their effective tax rate reconciliation as well as expanded information on income taxes paid by jurisdiction. The disclosure requirements will be applied on a prospective basis, with the option to apply them retrospectively. The standard is effective for fiscal years beginning after December 15, 2024, with early adoption permitted. The Company is currently evaluating the impact ASU 2023-09 may have on the Company's consolidated financial statements.
Net Loss per share	Net loss per share Basic net loss per share of common stock is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period, without further consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock and potentially dilutive securities outstanding for the period determined using the treasury stock method. For purposes of the diluted net loss per share calculation, stock options, unvested RSUs and warrants to purchase common stock are considered to be potentially dilutive securities, but are excluded from the calculation of diluted net loss per share because their effect would be anti-dilutive and therefore, basic and diluted net loss per share were the same for all periods presented.

Stockholders' equity - Narrative (Details) $ in Thousands	3 Months Ended	6 Months Ended
Stockholders' equity - Narrative (Details) $ in Thousands	Mar. 31, 2023 USD ($) shares	Jun. 30, 2024 USD ($) vote shares	Jun. 30, 2023 USD ($) shares	Feb. 29, 2024 USD ($)	Dec. 31, 2023 shares	Nov. 30, 2022 USD ($)	Feb. 28, 2022 USD ($)
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares) \| shares		25,000,000			25,000,000
Preferred stock, shares issued (in shares) \| shares		0			0
Net proceeds from public offering of common stock		$ 5,780	$ 93,620
Number of shares reserved for future issuance (in shares) \| shares		17,583,202			14,736,953
Common Stock
Class of Stock [Line Items]
Number of votes per common share \| vote		1
At-the-market Equity Offering Program (February 2022 ATM)
Class of Stock [Line Items]
Amount of shares authorized to be offered and sold							$ 100,000
Net proceeds from public offering of common stock	$ 1,600
At-the-market Equity Offering Program (February 2022 ATM) \| Common Stock
Class of Stock [Line Items]
Issuance of common stock from at-the-market transactions, net of issuance costs (in shares) \| shares	256,386
At-the-market Equity Offering Program (November 2022 ATM)
Class of Stock [Line Items]
Amount of shares authorized to be offered and sold						$ 150,000
Net proceeds from public offering of common stock		$ 5,800	$ 92,200
At-the-market Equity Offering Program (November 2022 ATM) \| Common Stock
Class of Stock [Line Items]
Issuance of common stock from at-the-market transactions, net of issuance costs (in shares) \| shares		1,041,201	14,208,145
Remaining unsold and available amount		$ 50,000
At Market Equity Offering Program (February 2024 ATM)
Class of Stock [Line Items]
Amount of shares authorized to be offered and sold				$ 100,000
At Market Equity Offering Program (February 2024 ATM) \| Common Stock
Class of Stock [Line Items]
Remaining unsold and available amount		$ 100,000

Stock-based compensation - Activity Under Stock Option Plan (Details) - $ / shares	6 Months Ended
	Jun. 30, 2024
Number of Shares
Outstanding at beginning of period (in shares)	10,902,845
Granted (in shares)	2,678,525
Exercised (in shares)	(14,223)
Cancelled/forfeited (in shares)	(924,795)
Outstanding at end of period (in shares)	12,642,352
Exercisable, at end of period (in shares)	7,121,556
Weighted- Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 7.42
Granted (in dollars per share)	2.91
Exercised (in dollars per share)	3.25
Cancelled/forfeited (in dollars per share)	7.23
Outstanding at end of period (in dollars per share)	6.48
Exercisable, at end of period (in dollars per share)	$ 8.16

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023
Cash flows from operating activities
Net loss	$ (43,574)	$ (110,470)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	826	643
Net amortization of premiums and discounts on marketable securities	(2,402)	(3,191)
Stock-based compensation	9,085	13,050
Non-cash operating lease expense	1,512	1,344
Other non-cash items	153	319
Changes in operating assets and liabilities:
Accounts receivable	(1,570)	30,000
Prepaid expenses and other current assets	321	(1,992)
Other assets	(500)	0
Accounts payable	(4,174)	3,337
Accrued expenses	(4,116)	(9,502)
Operating lease liabilities	(1,476)	(1,421)
Deferred revenue	(8,597)	(12,956)
Net cash used in operating activities	(54,512)	(90,839)
Cash flows from investing activities
Maturities of marketable securities	40,000	119,000
Purchase of marketable securities	(106,901)	(121,733)
Purchase of property and equipment	(132)	(1,310)
Net cash used in investing activities	(67,033)	(4,043)
Cash flows from financing activities
Net proceeds from at-the-market facilities	5,780	93,620
Proceeds from exercise of stock options	56	427
Proceeds from purchases of common stock under ESPP	258	963
Payment of debt issuance costs	0	(150)
Payments under finance lease obligations	(112)	(131)
Net cash provided by financing activities	5,982	94,729
Decrease in cash, cash equivalents and restricted cash	(115,563)	(153)
Cash, cash equivalents and restricted cash, beginning of period	175,039	129,363
Cash, cash equivalents and restricted cash, end of period	59,476	129,210
Supplemental cash flow information:
Purchases of property and equipment in accounts payable and accrued expenses	0	367
Common stock issuance costs in accounts payable and accrued expenses	0	81
Cash paid for interest	$ 1,751	$ 1,637

Summary of significant accounting policies - Narrative (Details)	6 Months Ended
	Jun. 30, 2024 segment
Accounting Policies [Abstract]
Number of operating segments	1

Insider Trading Arrangements	3 Months Ended
Insider Trading Arrangements	Jun. 30, 2024
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Cash, cash equivalents, and short-term marketable securities - Schedule of Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023
Restricted Cash [Roll Forward]
Cash and cash equivalents at beginning of period	$ 174,561	$ 128,732	$ 128,885
Restricted cash, beginning of period	478	478	478
Cash, cash equivalents and restricted cash, beginning of period	175,039	129,210	129,363
Cash and cash equivalents at end of period	58,998	174,561	128,732
Restricted cash, end of period	478	478	478
Cash, cash equivalents and restricted cash, end of period	$ 59,476	$ 175,039	$ 129,210

Collaboration agreements - Other Revenue (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Collaboration agreements
Collaboration revenue	$ 2,293,000	$ 10,654,000	$ 11,538,000	$ 18,456,000
Asana BioSciences \| Limited services
Collaboration agreements
Collaboration revenue	$ 0	$ 0	$ 0	0
Asana BioSciences \| Development milestone achieved
Collaboration agreements
Collaboration revenue				$ 2,500,000

- Definition

+ References

+ Details

Commitments (Details) - USD ($)	3 Months Ended		6 Months Ended
Commitments (Details) - USD ($)	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Commitments and Contingencies Disclosure [Abstract]
Research and development expense	$ 0	$ 0	$ 0	$ 0

Collaboration agreements - Merck Immunosythen Agreement (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Dec. 31, 2022 USD ($) performanceObligation target license	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Collaboration agreements
Collaboration revenue		$ 2,293	$ 10,654	$ 11,538	$ 18,456
Deferred revenue	$ 147,653	116,916	134,697	116,916	134,697	$ 125,314
2022 Merck Research Collaboration & License Agreement
Collaboration agreements
Number of targets designated \| target	2
Upfront payment received	$ 30,000
Number of licenses \| license	2
Performance obligations identified \| performanceObligation	4
Collaboration revenue		0	$ 2,500	3,200	$ 5,600
Deferred revenue		$ 17,000		$ 17,000		$ 20,200

- Definition

+ References

+ Details

Stockholders' equity - Common Stock (Details) - shares	Jun. 30, 2024	Dec. 31, 2023
Common stock
Number of shares reserved for future issuance (in shares)	17,583,202	14,736,953
Stock options
Common stock
Number of shares reserved for future issuance (in shares)	12,642,352	10,902,845
Restricted stock units
Common stock
Number of shares reserved for future issuance (in shares)	4,940,850	3,834,108