X0306 4 2022-06-09 0 0001442836 Mersana Therapeutics, Inc. MRSN 0001657434 Hack Andrew A. F. C/O BAIN CAPITAL LIFE SCIENCES INVESTORS 200 CLARENDON STREET BOSTON MA 02116 1 0 0 0 Stock Option (right to buy) 3.62 2022-06-09 4 A 0 45000 0.00 A 2032-06-08 Common Stock 45000 45000 D The option vests in full on the earlier of the first anniversary of the date of grant or the date of the 2023 Annual Meeting of Stockholders of Mersana Therapeutics, Inc., subject to the reporting person's continued service on such vesting date. /s/ Andrew Hack 2022-06-10