X0508 4 2025-01-15 0 0001442836 Mersana Therapeutics, Inc. MRSN 0001645755 Carvajal Alejandra C/O MERSANA THERAPEUTICS, INC. 840 MEMORIAL DRIVE CAMBRIDGE MA 02139 0 1 0 0 SVP, Chief Legal Officer 1 Common Stock 2025-01-15 4 M 0 12500 0 A 83392 D Common Stock 2025-01-16 4 S 0 5096 0.58 D 78296 D Restricted Stock Unit 2025-01-15 4 M 0 12500 0 D Common Stock 12500 37500 D Stock Option (Right to Buy) 0.64 2025-01-15 4 A 0 243750 0 A 2035-01-14 Common Stock 243750 243750 D Restricted Stock Unit 2025-01-15 4 A 0 54167 0 A Common Stock 54167 54167 D Represents shares of common stock received upon vesting of a portion of the restricted stock units ("RSUs") awarded to the Reporting Person on January 15, 2024. Represents the sale of shares of common stock to satisfy the Reporting Person's tax withholding obligations in connection with the service-based vesting and settlement of the RSUs pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person on May 10, 2023. The sale was effected through an automatic "sell to cover" transaction that did not represent a discretionary trade by the Reporting Person. The price reported is a weighted average sales price. The shares were sold in multiple transactions at prices ranging from $0.56 to $0.58, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in this footnote. Each RSU represents the contingent right to receive one share of common stock of the Issuer. 25% of the total number of RSUs granted vested on January 15, 2025, and the remainder shall vest thereafter in equal annual installments over the next three years. The option shall vest in equal quarterly installments over four years from the date of grant. The RSUs shall vest in equal annual installments over four years from the date of grant. /s/ Alejandra Carvajal 2025-01-17