Mersana Therapeutics Reports Positive Interim Data from the Expansion Portion of the XMT-1536 Phase 1 Study - Achieved 35% objective response rate, including 10% complete response rate, and 80% disease control rate among

Mersana Therapeutics Reports Positive

Interim Data from the Expansion Portion of the

XMT-1536 Phase 1 Study

CAMBRIDGE, Mass., May 27, 2020 --

Mersana Therapeutics, Inc. (NASDAQ:MRSN), a clinical-stage biopharmaceutical company focused on discovering and developing a pipeline

of antibody-drug conjugates (ADCs) targeting cancers in areas of high unmet medical need, today reported interim safety, tolerability

and efficacy data from the ongoing expansion portion of the Phase 1 study evaluating XMT-1536, its first-in-class ADC candidate

targeting NaPi2b, in patients with ovarian cancer and non-small cell lung (NSCLC) adenocarcinoma. The

Company will host a conference call and webcast today, Wednesday, May 27, 2020, at 8:00 a.m. ET during which

investigator Debra L. Richardson, MD, Associate Professor of Gynecologic Oncology at

the Stephenson Cancer Center at the University of Oklahoma Health Sciences Center and the Sarah Cannon

Research Institute and members of the Mersana executive team will present and discuss these data. These data will also be

presented in a poster session at the American Society of Clinical Oncology 2020 Virtual Scientific Program on Friday, May 29, 2020

starting at 8:00 a.m. ET.

data demonstrate not only that XMT-1536, our first-in-class Dolaflexin ADC targeting NaPi2b, can deliver confirmed complete responses,

partial responses and durable stable disease in platinum-resistant ovarian cancer, but also that these responses can deepen over

time in a patient population with poor prognosis and limited treatment options," said Anna Protopapas, President and Chief

Executive Officer of Mersana Therapeutics. "XMT-1536 continues to demonstrate that it is generally well tolerated, without

the dose-limiting toxicities of other ADC platforms such as severe neutropenia, neuropathy and ocular toxicity. These are encouraging

signals as we look forward to reporting more mature data in the second half of the year and continuing to advance XMT-1536 for

both platinum-resistant ovarian cancer and NSCLC adenocarcinoma patients."

The expansion portion of the Phase 1 study

is enrolling patients with platinum-resistant ovarian cancer, fallopian tube or primary peritoneal cancer who have received up

to three lines of prior therapy and in some cases four lines of prior therapy regardless of platinum status as well as patients

with NSCLC adenocarcinoma who had received prior treatment with platinum-based therapy and immunotherapy or targeted agents. With

a data cutoff of May 1, 2020 these data include 34 patients total, 27 with ovarian cancer and seven with NSCLC adenocarcinoma.

Patients with ovarian cancer had a median of three prior lines of treatment (range 1-5), and patients with NSCLC adenocarcinoma

had a median of two lines of therapy (range 1-3). Fifteen of the patients were dosed at 36 mg/m2, and 19 patients

were dosed at 43 mg/m2 every four weeks. Key findings include:

*7 patients not evaluable: 1 withdrew consent (Lower

NaPi2b Expression); 1 with unrelated SAE leading to discontinuation and death (Lower NaPi2b Expression); 5 have not yet received

**uPR=1 patient with unconfirmed PR; confirmatory

scan pending at the time of data cut

O Higher NaPi2b Expression: defined in

dose escalation as at / above lowest H-score at which response observed ( 110)

OO Lower NaPi2b Expression: defined in

dose escalation as below the lowest H-score at which response observed (<110)

Conference Call Details

Mersana Therapeutics will host a conference

call and webcast today at 8:00 a.m. ET during which investigator Debra L. Richardson, MD, Associate Professor of Gynecologic Oncology

at the Stephenson Cancer Center at the University of Oklahoma Health Sciences Center and the Sarah Cannon Research Institute and

members of the Mersana executive team will present and discuss these data. To access the call, please dial 877-303-9226 (domestic)

or 409-981-0870 (international) and provide the Conference ID 7785868. A live webcast of the presentation will be available on

the Investors & Media section of the Mersana website at www.mersana.com.

About Mersana Therapeutics

Mersana Therapeutics is a clinical-stage

biopharmaceutical company using its differentiated and proprietary ADC platforms to rapidly develop novel ADCs with optimal efficacy,

safety and tolerability to meaningfully improve the lives of people fighting cancer. Mersana's lead product candidate, XMT-1536,

is in the expansion portion of a Phase 1 proof-of-concept clinical study in patients with ovarian cancer and NSCLC adenocarcinoma.

XMT-1592, Mersana's second ADC product candidate targeting NaPi2b-expressing tumors, was created using Mersana's customizable

and homogeneous Dolasynthen platform and is in the dose escalation portion of a Phase 1 proof-of-concept clinical study. The Company's

early stage programs include a B7-H4 targeting ADC, as well as a STING-agonist ADC developed using the Company's Immunosynthen

platform. In addition, multiple partners are using Mersana's Dolaflexin platform to advance their ADC pipelines.

Forward-Looking Statements

This press release 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 strategy and

the design, progression and timing of its clinical trials and expectations regarding future clinical results based on data

achieved to date. Forward-looking statements generally can be identified by terms such as "aims,"

"anticipates," "believes," "contemplates," "continues," "could,"

"estimates," "expects," "goal," "intends," "may," "on

track," "plans," "possible," "potential," "predicts,"

"projects," "seeks," "should," "target," "will,"

"would" or similar expressions and the negatives of those terms. Forward-looking statements represent

management's beliefs and assumptions only as of the date of this press release. The Company's operations involve

risks and uncertainties, many of which are outside its control, and any one of which, or combination of which, could

materially affect its results of operations and whether the forward-looking statements ultimately prove to be correct.

Factors that may materially affect the Company's results of operations and whether these forward-looking statements

prove to be correct include, among other things, that preclinical testing or early clinical results may not be predictive of

the results or success of ongoing or later clinical trials, and that the development and testing of the Company's

product candidates will take longer and/or cost more than planned, as well as those listed in the Company's Annual

Report on Form 10-K filed with the Securities and Exchange Commission ("SEC") on February 28, 2020, the

Company's Quarterly Report on Form 10-Q filed with the SEC on May 8, 2020 and subsequent SEC filings. In addition,

while we expect that the COVID-19 pandemic might adversely affect the Company's preclinical and clinical development

efforts, business operations and financial results, the extent of the impact on the Company's operations and the value

of and market for the Company's common stock will depend on future developments that are highly uncertain and cannot be

predicted with confidence at this time, such as the ultimate duration of the pandemic, travel restrictions, quarantines,

physical distancing and business closure requirements in the U.S. and in other countries, and the effectiveness of actions

taken globally to contain and treat the disease. Except as required by law, the Company assumes no obligation to update these

forward-looking statements publicly, or to update the reasons actual results could differ materially from those anticipated

in the forward-looking statements, even if new information becomes available in the future.

Investor & Media Contact