Mersana Therapeutics Provides Business Update and Announces First Quarter 2025 Financial Results Updated clinical data from Emi-Le Phase 1 dose escalation and backfill cohorts presented today at ESMO Breast Cancer 2025 o

Mersana Therapeutics Provides Business Update

First Quarter 2025 Financial Results

Mass., May 15, 2025 - Mersana Therapeutics, Inc. (NASDAQ: MRSN), a clinical-stage biopharmaceutical company

focused on developing a pipeline of antibody-drug conjugates (ADCs) targeting cancers in areas of high unmet medical need, today provided

a business update and reported financial results for the first quarter ended March 31, 2025.

"Given recent positive front-line data reported from topo-1 ADC

registrational trials, we believe the post-topo-1 breast cancer patient population is poised to expand significantly. Our initial aim

is to develop Emi-Le to serve this high unmet need population," said Martin Huber, M.D., President and Chief Executive Officer of

Mersana Therapeutics. "To that end, we were pleased to present encouraging preliminary progression free survival and overall survival

data for Emi-Le among patients with post-topo-1 TNBC today at ESMO Breast Cancer 2025. Additionally, our team has made considerable progress

enrolling these patients in our dose expansion cohorts thus far in 2025, putting us on track for an initial expansion data readout later

Emiltatug Ledadotin (Emi-Le; XMT-1660)

Mersana has continued to advance the development of Emi-Le, the company's

B7-H4-directed Dolasynthen ADC.

Clinical Data Presented at ESMO Breast Cancer 2025 Today: Earlier this morning at the European Society for Medical Oncology

Breast Cancer 2025 Annual Congress (ESMO Breast Cancer 2025) in Munich, Germany, updated clinical data as of a March 8, 2025 data

cutoff from Emi-Le's Phase 1 dose escalation and backfill cohorts were presented in a mini oral session.

The presentation included clinical activity data among evaluable patients

(those with measurable disease at baseline and at least one post-baseline scan) across all tumor types (TNBC, hormone-receptor-positive,

human epidermal growth factor receptor 2 (HER2) negative breast cancer; ovarian cancer; endometrial cancer and adenoid cystic carcinoma

type 1) with B7-H4 high tumor expression (defined as a tumor proportion score of 70% or higher) who received intermediate Emi-Le doses

of 38.1 milligrams per meter squared (mg/m2) to 67.4 mg/m2 per cycle. For these patients, the confirmed objective

response rate (ORR) was 31% (8 of 26 patients). This is an increase from the 23% ORR (6 of 26 patients) previously reported based upon

a December 13, 2024 data cutoff.

The primary focus of the presentation was on TNBC patients enrolled

in dose escalation and backfill cohorts. Safety and tolerability data from these patients were consistent with previously reported data

with no new safety signals. Among evaluable patients with TNBC who received intermediate Emi-Le doses, ORR, preliminary progression free

survival (PFS) and preliminary overall survival (OS) data include the following:

* Mersana's ongoing expansion cohorts are enrolling TNBC patients

who have received 1 to 4 prior lines of treatment, including at least one topo-1 ADC, with a primary focus on patients with B7-H4 high

In the ASCENT Phase 3 clinical trial of sacituzumab govitecan, a topo-1

ADC, the ORR, median PFS and median OS for the standard-of-care single-agent chemotherapy control arm in topo-na ve relapsed/refractory

TNBC were approximately 5%, 7 weeks and 7 months, respectively.

"The performance of today's standard of care for patients

with TNBC who have previously been treated with a topo-1 ADC is poor," said Erika Hamilton, M.D., Director Breast Cancer Research,

Sarah Cannon Research Institute in Nashville, Tennessee, who presented these data at ESMO Breast Cancer 2025. "In light of this

significant unmet medical need and research indicating that B7-H4 tumor expression is a negative prognostic factor, the data presented

today are promising. I am looking forward to Emi-Le's continued development."

The ESMO Breast Cancer 2025 presentation can be accessed on the Publications

section of the Mersana website at www.mersana.com.

Update: Mersana continues to advance the dose expansion portion of its Phase 1 clinical trial of Emi-Le in patients with TNBC

who have received one to four prior lines of therapy, including at least one topo-1 ADC. In recent months, the company has made significant

progress in the enrollment of patients in its "Dose A" cohort, in which patients are receiving 67.4 mg/m2 of Emi-Le

every four weeks (Q4W).

Mersana also recently initiated enrollment in its "Dose B"

cohort. These patients are receiving a starting dose of 44.5 mg/m2 of Emi-Le on days 1 and 8 of the first four-week cycle followed

by 80 mg/m2 of Emi-Le Q4W.

The company plans to report initial clinical data from the expansion

portion of its Phase 1 clinical trial in the second half of 2025.

ASCO Presentation: There will be two presentations regarding Emi-Le at the American Society of Clinical Oncology (ASCO) 2025

Annual Meeting taking place May 30-June 3, 2025 at McCormick Place, Chicago, IL. The first is an oral presentation that

includes clinical data from the company's Phase 1 dose escalation and backfill cohorts across all enrolled tumor types based upon

a March 8, 2025 data cutoff. The second is a trial-in-progress poster presentation discussing the ongoing expansion portion of Mersana's

Phase 1 clinical trial of Emi-Le.

The dose escalation portion of Mersana's Phase 1 clinical trial

of XMT-2056, the company's lead Immunosynthen ADC candidate targeting a novel HER2 epitope, is ongoing. GSK plc has an exclusive

global license option to co-develop and commercialize XMT-2056. Mersana plans to continue enrolling patients in dose escalation and

expects to present initial clinical pharmacodynamic STING activation data for XMT-2056 in 2025.

Mersana continues to support its collaborations with both Johnson &

Johnson (Dolasynthen research collaboration) and Merck KGaA, Darmstadt, Germany (Immunosynthen research collaboration).

First Quarter 2025 Financial Results

Conference Call Reminder

Mersana will host a conference call today at 8:00 a.m. ET to discuss

business updates and its financial results for the first quarter of 2025. To access the call, please dial 833-255-2826 (domestic) or 412-317-0689

(international). A live webcast of the presentation will be available on the Investors & Media section of the Mersana website

at www.mersana.com, and a replay of the webcast will be available in the same location following the conference call for approximately

About Mersana Therapeutics

Therapeutics is a clinical-stage biopharmaceutical company focused on the development of novel antibody-drug conjugates (ADCs) and driven

by the knowledge that patients are waiting for new treatment options. The company has developed proprietary cytotoxic (Dolasynthen) and

immunostimulatory (Immunosynthen) ADC platforms that have generated a pipeline of wholly-owned and partnered product candidates with the

potential to treat a range of cancers. Its pipeline includes Emi-Le (emiltatug ledadotin; XMT-1660), a Dolasynthen ADC targeting B7-H4,

and XMT-2056, an Immunosynthen ADC targeting a novel epitope of human epidermal growth factor receptor 2 (HER2). Mersana routinely posts

information that may be useful to investors on the "Investors & Media" section of its website at www.mersana.com.

Forward-Looking Statements

This press release contains "forward-looking"

statements and information within the meaning of the Private Securities Litigation Reform Act of 1995. These statements may be identified

by words such as "aims," "anticipates," "believes," "could," "estimates,"

"expects," "forecasts," "goal," "intends," "may," "plans," "possible,"

"potential," "seeks," "will" and variations of these words or similar expressions, although not all

forward-looking statements contain these words. Forward-looking statements in this press release include, but are not limited to, statements

concerning Mersana's strategic priorities; its plans regarding the clinical development of Emi-Le and XMT-2056, including with respect

to the progress and design of the clinical trials of these product candidates; the potential clinical benefits of and opportunity for

Emi-Le; Mersana's planned data presentations, including with respect to data from the expansion portion of its Phase 1 clinical

trial of Emi-Le and to clinical pharmacodynamic STING activation data related to XMT-2056; Mersana's collaborations with third parties;

the development and potential of Mersana's product candidates, platforms, technology and pipeline of ADC candidates; and Mersana's

expected cash runway. Mersana may not actually achieve the plans, intentions or expectations disclosed in these forward-looking statements,

and you should not place undue reliance on these forward-looking statements. Actual results or events could differ materially from the

plans, intentions and expectations disclosed in these forward-looking statements as a result of various factors, including, among other

things, uncertainties inherent in research and development, in the advancement, progression and completion of clinical trials and in the

clinical development of Mersana's product candidates, including Emi-Le and XMT-2056; the risk that Mersana may face delays in patient

enrollment in its Phase 1 clinical trials of Emi-Le and XMT-2056; the risk that outcomes of preclinical studies may not be predictive

of clinical trial results; the risk that initial or interim results from a clinical trial may not be predictive of the final results of

the trial or the results of future trials; the risk that clinical trial data may not support regulatory applications or approvals; the

risk that Mersana may not realize the intended benefits of its platforms, technology and collaborations; the risk that Mersana's projections

regarding its expected cash runway are inaccurate or that the conduct of its business requires more cash than anticipated; the occurrence

of impediments to Mersana's ability to execute its planned strategic restructuring and reprioritization as and on the timeline currently

contemplated; the risk that restructuring costs and charges may be greater than anticipated; the risk that Mersana's restructuring

and reprioritization efforts may adversely affect its ability to retain skilled and motivated personnel and may be distracting to employees

and management; the risk that Mersana's restructuring efforts may negatively impact its business operations and reputation; the