Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K (Mark One) ☒ ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31, 2019. OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from to Commission file number 001-38129 Mersana Therapeutics, Inc. (Exact name of registrant as specified in its charter) (State or Other Jurisdicti ...

THERAPEUTICS Accelerating ADC Innovation …because patients are waiting 38th Annual JP Morgan Healthcare Conference January 16, 2020 Legal Disclaimer This presentation contains "forward-looking" statements within the meaning of federal securities laws. These forward-looking statements are not statements of historical facts and are based on management's beliefs and assumptions and on information currently available to management. Forward-looking statements include information concerning the Company's business ...

Financial Data and Key Metrics Changes - The company ended Q3 2019 with $112 million in cash, cash equivalents, and marketable securities, having used net cash of $16.3 million in operations during the quarter [29] - Net loss for Q3 2019 was $16.8 million or $0.35 per share, compared to a net loss of $17.1 million or $0.74 per share for the same period in 2018 [31] - Collaboration revenue for Q3 2019 was approximately $0.8 million, down from $2.2 million in Q3 2018, primarily due to a decrease in services performed in support of partners' programs [30] Business Line Data and Key Metrics Changes - Research and development expenses for Q3 2019 were approximately $13.7 million, down from $15.2 million in Q3 2018, driven by a decrease in manufacturing costs for XMT-1536 and XMT-1522 [30] - General and administrative expenses remained flat at $4.4 million compared to the same period in 2018 [31] Market Data and Key Metrics Changes - The company noted substantial interest in pursuing accelerated registration strategies for XMT-1536 based on discussions with investigators and emerging profiles [28] - Data presented at ESMO indicated a potential paradigm shift in the standard of care for first-line ovarian cancer, increasing the need for new mechanisms of action in later lines and platinum-resistant patients [26] Company Strategy and Development Direction - Mersana is focused on achieving proof-of-concept in multiple data readouts in 2020, with plans to report data from the full dose escalation portion of the XMT-1536 study and early data from ovarian and lung cohorts [33] - The company aims to file an IND and enter the clinic with its next ADC candidate in the first half of 2020, while also strengthening its discovery pipeline [34] Management's Comments on Operating Environment and Future Outlook - Management expressed optimism about the continued safety and efficacy profile of XMT-1536, indicating that 2020 will be a data-rich year with multiple potential inflection points [10][33] - The company is well-capitalized to execute its plans through mid-2021, with a cash balance of $112 million [35] Other Important Information - The company is leveraging its Dolaflexin, Dolasynthen, and Immunosynthen platforms to build a robust discovery pipeline for mid- to long-term growth [12] - The ongoing expansion study for XMT-1536 is evaluating patients with platinum-resistant ovarian cancer and NSCLC adenocarcinoma, with strong investigator interest and rapid site initiation [16] Q&A Session Summary Question: What is the experience at the 30 and 36 mg/m² cohorts? - The experience has been positive, with patients staying on study for extended periods and efficacy observed in doses between 20 and up [43] Question: Are there any durable responses beyond 4 months? - Management pointed to the ASCO poster as the last data disclosure, with plans to disclose full dose escalation and expansion datasets in 2020 [45] Question: What are the thoughts on the diagnostic and utilization in the ongoing expansion cohort? - A robust diagnostic has been developed and validated, being used in expansion cohorts to collect data on tissue expression levels [46] Question: Is the pharmacokinetic profile dose proportional? - Yes, the pharmacokinetic profile is dose proportional, with no accumulation observed over the current dosing schedule [48] Question: What is the pace of enrollment in the expansion study? - Enrollment is going very well, with the number of sites initiated ahead of schedule due to investigator enthusiasm [57]

FORM 10-Q ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended September 30, 2019 Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission file number 001-38129 Mersana Therapeutics, Inc. (Exact name of registrant as specified in its charter) (State or other jurisdiction of (I.R.S. Employer incorporation ...

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended June 30, 2019 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission file number 001-38129 Mersana Therapeutics, Inc. (Exact name of registrant as specified in its charter) (State or other jurisdiction of (I.R.S. Employer incorporation or or ...

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10‑Q ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended March 31, 2019 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission file number 001-38129 Mersana Therapeutics, Inc. (Exact name of registrant as specified in its charter) (State or other jurisdiction of (I.R.S. Employer incorporation or o ...

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K (Mark One) ☒ ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31, 2018. OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from to Commission file number 001-38129 Mersana Therapeutics, Inc. (Exact name of registrant as specified in its charter) Delaware 04-3562403 (State ...