CAMBRIDGE, Mass., Dec. 11, 2015 -- 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.  

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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.

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