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Mersana Therapeutics' Lead Fleximer(R) Immunoconjugate Demonstrates Potent Activity in Low HER2-Expressing Tumor Models

– XMT-1522 Designed to Expand Patient Population Treated With HER2-Targeted Therapy –

– Complete Tumor Regressions Observed in HER2 1+ and 2+ Cancer Models Refractory to Current Anti-HER2 Therapies –

– IND Anticipated Q4 2015 –

CAMBRIDGE, Mass., April 17, 2015 (GLOBE NEWSWIRE) -- Mersana Therapeutics, Inc. today announced that preclinical data for its lead product candidate, XMT-1522, demonstrated significant anti-cancer activity in low HER2-expressing tumor models refractory to currently available therapies, as well as HER2-amplified tumor models in combination with trastuzumab-based therapies. These data will be presented in a late-breaking poster at the American Association for Cancer Research (AACR) Annual Meeting on April 21 in Philadelphia, PA.

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. The conjugate, optimized for payload delivery, utilizes a novel HER2-targeted antibody, which binds to a different epitope than existing anti-HER2 antibodies.

"Current HER2-targeted therapies are effective in treating HER2-positive cancers, but only address roughly 20 percent of patients with breast or gastric cancer," said Donald A. Bergstrom, M.D., Ph.D., Chief Medical Officer of Mersana. "Our preclinical data suggest that XMT-1522 has the potential to greatly expand the number of patients who may benefit from HER2-targeted therapies, because the compound provides efficient drug delivery in cancers where there are as few as 10,000 HER2 receptors, where other therapies are inactive."

Single doses of 1 mg/kg or 0.67 mg/kg of XMT-1522 showed complete regression in low HER2-expressing breast and gastric cancer models, where ado-trastuzumab emtansine was inactive at doses of 10 mg/kg and above. In HER2-driven tumor models, XMT-1522 showed synergistic efficacy in combination with widely used anti-HER2 therapies trastuzumab and pertuzumab. XMT-1522 demonstrated an excellent pharmacokinetic profile and was well tolerated in non-human primates at therapeutic doses.

"XMT-1522, our lead clinical immunoconjugate candidate, exemplifies the significant advantages of Mersana's Fleximer platform. It's the first in a portfolio of targeted therapies we are working on to address unmet needs in cancer," said Anna Protopapas, President and Chief Executive Officer of Mersana.

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.

CONTACT: Media Contact 6 Degrees Tony Plohoros tplohoros@6degreespr.com (908) 940-0135Source:Mersana Therapeutics, Inc.