Merger terms reshape Mersana (NASDAQ: MRSN) director and fund holdings

Rhea-AI Filing Summary

Mersana Therapeutics, Inc. director Andrew Hack reported changes in his holdings tied to the company’s acquisition by Day One Biopharmaceuticals. Under the merger agreement, each share of Mersana common stock tendered before the offer deadline was exchanged for $25.00 in cash per share plus one contingent value right, which may pay up to an additional $30.25 in cash per CVR if specified milestones are met. As of January 6, 2026, when the merger became effective and Mersana became a wholly owned subsidiary of Day One, Hack’s direct holding of 578 common shares and indirect interest in 346,545 shares held by Bain Capital Life Sciences entities were reported as affected by this exchange. Multiple stock options to buy Mersana common stock were also reported as disposed of with zero options remaining following the merger-related treatment of in-the-money and out-of-the-money awards.

Insights

Equity compensation and M&A analyst

Form 4 shows Mersana’s cash-and-CVR sale closing and related cleanup of director equity awards.

This filing ties an individual director’s equity position to the closing of Mersana’s sale to Day One Biopharmaceuticals. Common shares tendered into the offer are exchanged for $25.00 in cash per share plus one contingent value right for potential additional cash payments up to $30.25 per CVR, according to the merger terms.

The derivative table shows a series of stock options reported as disposed with zero derivative securities remaining, consistent with the merger provisions for both cash-out and out-of-the-money options. Indirect holdings of 346,545 common shares are attributed to Bain Capital Life Sciences entities, with the director’s interest limited to his pecuniary stake as described.

This is primarily a mechanical reflection of the already-agreed acquisition and equity award treatment rather than a discretionary trading decision. Subsequent company disclosures outside this filing would frame how often, and under what conditions, CVR milestones might be reached.

SEC Form 4

FORM 4

UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934
or Section 30(h) of the Investment Company Act of 1940

OMB APPROVAL

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X

Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan for the purchase or sale of equity securities of the issuer that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

1. Name and Address of Reporting Person^*

Hack Andrew A. F.

(Last) (First) (Middle)

C/O BAIN CAPITAL LIFE SCIENCES INVESTORS

200 CLARENDON STREET

(Street)

BOSTON MA 02116

(City) (State) (Zip)

2. Issuer Name and Ticker or Trading Symbol
Mersana Therapeutics, Inc. [ MRSN ]

5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

X	Director		10% Owner
	Officer (give title below)		Other (specify below)

3. Date of Earliest Transaction (Month/Day/Year)
01/06/2026

4. If Amendment, Date of Original Filed (Month/Day/Year)

6. Individual or Joint/Group Filing (Check Applicable Line)

X	Form filed by One Reporting Person
	Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	3. Transaction Code (Instr. 8)		4. Securities Acquired (A) or Disposed Of (D) (Instr. 3, 4 and 5)			5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4)	6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
			Code	V	Amount	(A) or (D)	Price
Common Stock	01/06/2026		U(1)(2)		578	D	(1)(2)	0	D
Common Stock	01/06/2026		U(1)(2)		346,545	D	(1)(2)	0	I	See Footnote(5)

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 3)	2. Conversion or Exercise Price of Derivative Security	3. Transaction Date (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	4. Transaction Code (Instr. 8)		5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5)		6. Date Exercisable and Expiration Date (Month/Day/Year)		7. Title and Amount of Securities Underlying Derivative Security (Instr. 3 and 4)		8. Price of Derivative Security (Instr. 5)	9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)	10. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares
Stock Option (right to buy)	$9.0775	01/06/2026		D			2,460	(3)	(3)	Common Stock	2,460	(3)	0	D
Stock Option (right to buy)	$125	01/06/2026		D			2,088	(4)	(4)	Common Stock	2,088	(4)	0	D
Stock Option (right to buy)	$306.75	01/06/2026		D			400	(4)	(4)	Common Stock	400	(4)	0	D
Stock Option (right to buy)	$107.75	01/06/2026		D			1,000	(4)	(4)	Common Stock	1,000	(4)	0	D
Stock Option (right to buy)	$572.5	01/06/2026		D			368	(4)	(4)	Common Stock	368	(4)	0	D
Stock Option (right to buy)	$527.25	01/06/2026		D			118	(4)	(4)	Common Stock	118	(4)	0	D
Stock Option (right to buy)	$372.5	01/06/2026		D			1,002	(4)	(4)	Common Stock	1,002	(4)	0	D
Stock Option (right to buy)	$161	01/06/2026		D			439	(4)	(4)	Common Stock	439	(4)	0	D
Stock Option (right to buy)	$90.5	01/06/2026		D			1,800	(4)	(4)	Common Stock	1,800	(4)	0	D
Stock Option (right to buy)	$216	01/06/2026		D			734	(4)	(4)	Common Stock	734	(4)	0	D
Stock Option (right to buy)	$56.5	01/06/2026		D			2,200	(4)	(4)	Common Stock	2,200	(4)	0	D

Explanation of Responses:

1. Pursuant to the terms of that certain Agreement and Plan of Merger (the "Merger Agreement"), by and among the Issuer, Day One Biopharmaceuticals, Inc. ("Parent") and Parent's direct wholly-owned subsidiary, Emerald Merger Sub, Inc. ("Purchaser"), dated as of November 12, 2025, the shares (the "Shares") of common stock, par value $0.0001 per share, of the Issuer (the "Common Stock") that were tendered to Purchaser prior to the expiration time of the Offer were exchanged for: (i) $25.00 per Share, net to the stockholder in cash, without interest and less any applicable tax withholding (the "Upfront Cash Consideration"), plus (ii) one non-tradeable contingent value right per Share (each, a "CVR"),

2. (continued from footnote 1) which represents the right to receive certain contingent milestone payments of up to an aggregate of $30.25 per CVR in cash, without interest and less any applicable tax withholding, upon the achievement of certain specified milestones in accordance with the terms and subject to the conditions of a contingent value rights agreement entered into by Parent and the rights agent (the Upfront Cash Consideration plus one CVR, collectively, the "Offer Price"). After completion of the tender offer, pursuant to the terms of the Merger Agreement, Purchaser merged with and into the Issuer, effective as of January 6, 2026 (the "Effective Time"), with the Issuer continuing as the surviving corporation and a wholly owned subsidiary of Parent.

3. Pursuant to the terms of the Merger Agreement, effective as of immediately prior to the Effective Time, each option to purchase shares of Common Stock ("Issuer Option") that was outstanding and unexercised as of immediately prior to the Effective Time and had an exercise price per Share that was less than the Upfront Cash Consideration (each, a "Cash-Out Option"), became fully vested and was automatically cancelled and converted into the right to receive, without interest and subject to deduction for any required tax withholding, (i) the Offer Price minus (ii) the exercise price payable per share of Common Stock underlying such Cash-Out Option.

4. Pursuant to the terms of the Merger Agreement, effective as of 10 business days prior to the closing of the Merger (the "Acceleration Date"), each Issuer Option then outstanding and unexercised that had an exercise price per Share that was equal to or greater than the Upfront Cash Consideration (each, an "OTM Option"), became fully vested and exercisable up to and through the closing of regular trading on the Nasdaq Stock Market on the fifth business day following the Acceleration Date (the "Last Exercise Date"). OTM Options not exercised on or prior to the Last Exercise Date were cancelled and ceased to exist as of the Effective Time, and no consideration was delivered in exchange for such OTM Option.

5. Represents shares of Common Stock held directly by Bain Capital Life Sciences Fund II, L.P. ("BCLS II"), BCIP Life Sciences Associates, LP ("BCIPLS") and BCLS II Investco, LP ("BCLS II Investco" and, together with BCLS II and BCIPLS, the "Bain Capital Life Sciences Entities"). Bain Capital Life Sciences Investors, LLC ("BCLSI") (i) is the ultimate general partner of each of BCLS II and BCLS II Investco and (ii) governs the investment strategy and decision-making process with respect to investments held by BCIPLS. The Reporting Person is a Partner of BCLSI. As a result, the Reporting Person may be deemed to share voting and dispositive power with respect to the shares held by the Bain Capital Life Sciences Entities. The Reporting Person disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein.

	/s/ Andrew Hack	01/06/2026
	** Signature of Reporting Person	Date
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FAQ

What insider activity did Mersana (MRSN) director Andrew Hack report on this Form 4?

The Form 4 reports that Andrew Hack, a director of Mersana Therapeutics, reflected merger-related changes to his holdings. These include the treatment of his Mersana common stock and the cancellation or payoff of various stock options in connection with the company’s acquisition by Day One Biopharmaceuticals.

What consideration did Mersana (MRSN) shareholders receive in the Day One Biopharmaceuticals merger?

Each share of Mersana common stock tendered in the offer was exchanged for $25.00 in cash per share, plus one non-tradeable contingent value right per share. Each CVR may pay up to an additional $30.25 in cash if specified milestones are achieved.

How were Mersana (MRSN) stock options treated in the merger described in this Form 4?

Options with an exercise price below the $25.00 upfront cash amount became fully vested and were cancelled in exchange for the offer price minus the exercise price per share. Options with an exercise price equal to or above $25.00 became fully vested and exercisable for a limited period and, if not exercised by the last exercise date, were cancelled at the merger effective time with no consideration.

What happened to Andrew Hack’s Mersana stock options according to the filing?

The filing lists multiple stock options (rights to buy Mersana common stock) with transaction code D on January 6, 2026, and shows zero derivative securities remaining after the reported transactions. This reflects the merger-driven cancellation or settlement of his options under the terms of the merger agreement.

How many Mersana (MRSN) shares were indirectly associated with Bain Capital Life Sciences entities?

The Form 4 shows an indirect position of 346,545 shares of common stock attributed to Bain Capital Life Sciences Fund II, BCIP Life Sciences Associates, and BCLS II Investco. The reporting person, as a partner of Bain Capital Life Sciences Investors, may be deemed to share voting and dispositive power but disclaims beneficial ownership beyond his pecuniary interest.

Did Mersana Therapeutics remain independent after the transactions in this Form 4?

No. After completion of the tender offer, Emerald Merger Sub merged with and into Mersana Therapeutics, and as of January 6, 2026, Mersana continued as the surviving corporation and became a wholly owned subsidiary of Day One Biopharmaceuticals.