Mersana Reports Preliminary Phase 1 Results of XMT-1001 in Patients with Advanced...

Mersana Reports Preliminary Phase 1 Results of XMT-1001 in Patients with
Advanced Solid Tumors

- Results Presented at American Society of Clinical Oncology (ASCO) Meeting
Show Fleximer(R) Conjugated Drug Is Well Tolerated with Favorable
Pharmacokinetics and Prolonged Stable Disease in Refractory Tumors -

CAMBRIDGE, Mass., June 1 /PRNewswire/ -- Mersana Therapeutics presented
preliminary results of a Phase 1 clinical trial for its lead development
candidate, XMT-1001, in a poster session at the 2009 Annual Meeting of the
American Society of Clinical Oncology (ASCO) in Orlando. XMT-1001 is a
broad-spectrum cytotoxic, based on camptothecin (CPT), conjugated to Mersana's
biodegradable polymer platform, Fleximer(R).

Ten of 37 evaluable patients dosed to date demonstrated evidence of stable
disease and seven patients had prolonged stable disease for at least 12 weeks.
The study has also demonstrated that XMT-1001 can be safely given to patients.
Toxicities such as hemorrhagic cystitis and serious diarrhea were not observed
in this preliminary assessment. In addition, a favorable pharmacokinetic
profile was observed. Full text of the abstract can be viewed online at the
ASCO website at www.abstract.asco.org.

"We are encouraged by the favorable safety and pharmacokinetic profile
demonstrated by XMT-1001 thus far," said Julie Olson, Chief Executive Officer
of Mersana. "These preliminary results build on our preclinical studies,
which showed that XMT-1001 has greater efficacy in human tumor xenograft
models than comparable doses of irinotecan, an agent with a similar mechanism
of action as camptothecin and that is approved to treat patients with colon
cancer. We look forward to advancing XMT-1001 into a Phase 2 trial in a solid
tumor indication."

About the XMT-1001 Preliminary Study
The Phase 1 trial is an open label, dose escalation study of XMT-1001
administered as an IV infusion once every three weeks in patients with
advanced solid tumors. The objectives of the study are to determine the
maximum tolerated dose (MTD) as well as to assess safety and pharmacokinetics.
The maximum tolerated dose is not yet defined and accrual to the study
continues.

ASCO Poster Information

"A Phase I study of the safety and pharmacokinetics (PK) of XMT-1001 given as
an intravenous (IV) infusion once every three weeks to patients with advanced
solid tumors."

By E. Sausville, L. Garbo, G. J. Weiss, S. Anthony, D. Shkolny, A. V.
Yurkovetskiy, C. Bethune, R. J. Fram; University of Maryland, Baltimore, MD;
New York Oncology Hematology, Albany, NY; TGen Clinical Research Services,
Scottsdale, AZ; Mersana Therapeutics, Inc., Cambridge, MA; Covance
Bioanalytical, Madison, WI

Session: Developmental Therapeutics: Cytotoxic Chemotherapy
Type: General Poster Session
Time: Saturday May 30, 2009, 8:00 AM to 12:00 PM
Location: Level 2, West Hall C

About Fleximer(R)

The key component of Mersana's platform is Fleximer(R), a novel, biodegradable
and bio-inert polymer that can be chemically linked to small molecules and
biologics. Fleximer(R) technology improves the therapeutic index of existing
compounds by uniquely combining biodegradability with "biological stealth"
properties, making Fleximer(R) materials and their conjugates long-circulating
and non-immunotoxic. Fleximer(R) molecules are characterized by solubility in
water, with stability in common manufacturing procedures.

About Mersana Therapeutics, Inc.
Mersana Therapeutics employs its biodegradable polymer platform (Fleximer(R))
to create new and better medicines. We are advancing our own clinical-stage
pipeline of novel compounds with the potential to address multiple oncology
indications. We also leverage the versatility of Fleximer through partnerships
to overcome the safety, efficacy, and delivery challenges of nucleic acids,
biologics, and small molecules in numerous therapeutic areas.

Mersana is headquartered in Cambridge, MA and its investors include Fidelity
Biosciences, ProQuest Investments, Rho Ventures, Harris & Harris Group and
PureTech Ventures. For more information, visit www.mersana.com.

Fleximer(R) and Mersana(R) are registered trademarks of Mersana Therapeutics,
Inc.

SOURCE Mersana Therapeutics, Inc.

Kathryn Morris of KMorrisPR, +1-845-635-9828, kathryn@kmorrispr.com