Mersana’s fleximer immunoconjugate shows potent activity in low HER2-expressing tumor models

XMT-1522 is based on the company’s Fleximer immunoconjugate technology that carries an average of 15 proprietary auristatin payload molecules.

Optimized for payload delivery, the conjugate uses a new HER2-targeted antibody, which binds to a different epitope than existing anti-HER2 antibodies.

According to the data, XMT-1522 showed 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.

Mersana chief medical officer Dr Donald Bergstrom said: "Current HER2-targeted therapies are effective in treating HER2-positive cancers, but only address roughly 20 percent of patients with breast or gastric cancer.

"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."

The company said that single doses of 1mg/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.

Mersana president and chief executive officer Anna Protopapas said: "It’s the first in a portfolio of targeted therapies we are working on to address unmet needs in cancer."

The company said that an investigational new drug (IND) application is anticipated in the fourth quarter of 2015.