Anna Protopapas - 06 Jan 2026 Form 4 Insider Report for Mersana Therapeutics, Inc. (MRSN)

Reporting relationship: Director
Issuer ticker: MRSN
Reported as of: 06 Jan 2026
Net transaction value: $0
SEC form: 4
Accepted by SEC: 06 Jan 2026, 16:42:17 UTC
Prior SEC filing: 20 Jun 2025
Next SEC filing: 18 Jun 2026
Source filing: View source filing
Reporting owner signature: /s/ Anna Protopapas

Key filing fact

Anna Protopapas filed Form 4 for Mersana Therapeutics, Inc. (MRSN) on 06 Jan 2026.

Key facts

This page summarizes Anna Protopapas's Form 4 filing for Mersana Therapeutics, Inc. (MRSN).
20 reported transactions and 17 derivative rows are listed below.
Accepted by SEC: 06 Jan 2026, 16:42.

Change

Previous filing in this sequence was filed on 20 Jun 2025.
Current net transaction value: $0.

Research use

This tells you what this filing adds before you inspect full transaction and derivative tables.
You can trace every row back to the original SEC filing document.

Evidence

Filed on Form 4

Ownership activity is grounded in SEC Form 4 disclosures.

View source filing

Reporting Owners (1)

CIK 0001347582 Primary reporting owner

Protopapas Anna

Relationship: Director
Address: C/O MERSANA THERAPEUTICS, INC., 840 MEMORIAL DRIVE, CAMBRIDGE
Signature: /s/ Anna Protopapas
Signature date: 06 Jan 2026

Reported non-derivative transactions

Shares, units, or other non-derivative securities reported in this filing.

MRSN transaction

Common Stock

Disposition pursuant to a tender of shares in a change of control transaction

Transaction value
Shares: -6,721
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Footnotes: F1, F2

MRSN transaction

Common Stock

Disposition pursuant to a tender of shares in a change of control transaction

Transaction value
Shares: -2,890
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: See footnote
Footnotes: F1, F2, F5

MRSN transaction

Common Stock

Disposition pursuant to a tender of shares in a change of control transaction

Transaction value
Shares: -9,609
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: See footnote
Footnotes: F1, F2, F6

Reported derivative securities

Options, warrants, convertible securities, or similar derivative positions disclosed in the filing.

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -2,460
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 2,460
Exercise price: $9.08
Footnotes: F3

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -2,500
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 2,500
Exercise price: $174.50
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -480
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 480
Exercise price: $355.75
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -7,519
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 7,519
Exercise price: $355.75
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -542
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 542
Exercise price: $87.75
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -8,137
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 8,137
Exercise price: $87.75
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -1
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 1
Exercise price: $87.75
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -18
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 18
Exercise price: $87.75
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -988
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 988
Exercise price: $154.00
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -11,761
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 11,761
Exercise price: $154.00
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -665
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 665
Exercise price: $33.00
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -9,043
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 9,043
Exercise price: $157.00
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -11,250
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 11,250
Exercise price: $151.50
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -2,200
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 2,200
Exercise price: $56.50
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -1,131
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 1,131
Exercise price: $102.50
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -5,227
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 5,227
Exercise price: $102.50
Footnotes: F4

MRSN transaction Derivative

Stock Option (right to buy)

Disposed to Issuer

Transaction value
Shares: -166
Change %: -100%
Price
Shares after: 0
Date: 06 Jan 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 166
Exercise price: $174.50
Footnotes: F4

^* marks a reported price that did not pass the local price check.

Additional SEC filing notes

Filing notes and footnotes

Section 16 status

Anna Protopapas is no longer subject to Section 16 filing requirements. Form 4 or Form 5 obligations may still apply in specific circumstances.

Explanation of responses 6 footnotes

Footnote F1

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"),

Footnote F2

(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.

Footnote F3

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.

Footnote F4

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.

Footnote F5

These Shares were held by the Kinney/Protopapas Family Irrevocable Trust (the "Kinney/Protopapas Trust") for the benefit of the Reporting Person's children. The Reporting Person's spouse is a co-trustee of the Kinney/Protopapas Trust.

Footnote F6

These Shares were held by the Anna Protopapas Irrevocable Trust (the "Protopapas Trust") for the benefit of the Reporting Person's spouse and children. The Reporting Person's spouse is the trustee of the Protopapas Trust.