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Mersana Therapeutics' Lead Immunoconjugate Continues to Demonstrate Complete Tumor Regressions in Multiple HER2-Expressing Breast Cancer Tumor Models

Data validate the potential of XMT-1522 to expand number of patients who may benefit from HER2-targeted therapies

IND anticipated mid-2016

CAMBRIDGE, Mass., Dec. 11, 2015 (GLOBE NEWSWIRE) -- Mersana Therapeutics, Inc. today announced positive preclinical data for its lead product candidate, XMT-1522, which demonstrated significant anti-cancer activity in both HER2-positive and HER2 low-expressing tumor models refractory to currently available therapies. The new data were presented today during a poster session at the 2015 San Antonio Breast Cancer Symposium in San Antonio, Texas.

XMT-1522 is a novel HER2-targeting therapy based on Mersana's Fleximer® immunoconjugate technology that carries an average of 15 proprietary auristatin payload molecules per antibody. The conjugate, optimized for payload delivery, utilizes a novel HER2-targeted antibody, which binds to a different epitope than existing anti-HER2 antibodies.

The study evaluated XMT-1522 in models of HER2 low-expressing (IHC 1+/2+) advanced breast cancer, and HER2-positive breast cancer insensitive to ado-trastuzumab emtansine (T-DM1) and other HER2-targeted therapies.

"These results demonstrate XMT-1522's potential to expand the population of breast cancer patients for whom HER2-targeted therapy is appropriate, from the 20 percent currently diagnosed with HER2-positive breast cancer to the full range of patients with HER2 expression," said Donald A. Bergstrom, M.D., Ph.D., Chief Medical Officer of Mersana. "The data strongly support moving XMT-1522 into clinical trials with breast cancer patients who have both HER2-positive and HER2 low-expressing tumors."

The study showed significant efficacy of XMT-1522 in all eight xenograft models representative of the target patient populations. In three HER2-positive models, XMT-1522 dosed at 1 mg/kg or 3 mg/kg induced durable complete tumor regressions, while currently available HER2-targeted therapies (T-DM1 or lapatinib/gemcitabine) were inactive. In five patient-derived xenograft models of HER2 low-expressing breast cancer, a 1 mg/kg or 3 mg/kg XMT-1522 dose led to complete tumor regression in three of the five models, with tumor stasis in the remaining two models. All models achieving complete regression were refractory to gemcitabine, a standard agent for advanced breast cancer patients with HER2-negative disease. The doses of XMT-1522 associated with tumor regression in these models have previously been shown to be well-tolerated in non-human primate safety studies.

"We continue to be pleased with the progress of XMT-1522 toward an anticipated Investigational New Drug Application in mid-2016," said Anna Protopapas, President and Chief Executive Officer of Mersana. "XMT-1522 holds significant promise for breast cancer patients and demonstrates the ability of the Fleximer platform to expand the patient populations addressable with immunoconjugates."

About Mersana Therapeutics

Mersana Therapeutics is advancing a proprietary pipeline of targeted oncology therapeutics leveraging its game-changing Fleximer® immunoconjugate technology. Mersana's first product candidate XMT-1522 has the potential to address significant unmet needs and improve patient outcomes in multiple oncology indications. Fleximer-based immunoconjugate molecules have been shown to have superior efficacy, including with targets previously considered not amenable to antibody-drug conjugate approaches. Mersana has collaborations utilizing Fleximer technology with Takeda, Merck KGaA, and Asana BioSciences. For more information, please visit www.mersana.com.

CONTACT: Media Contact 6 Degrees Tony Plohoros tplohoros@6degreespr.com (908) 591-2839 Investors Contact Stern Investor Relations, Inc. Jesse Baumgartner Jesse@sternir.com (212) 362-1200

Source:Mersana Therapeutics, Inc.

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