The data and safety monitoring board (DSMB) said it was unlikely for MM-302 to demonstrate benefit over comparable treatments, with both the treatment and control arms found to have shorter-than-expected median progression free survival.
A subsequent futility analysis confirmed DSMB's opinion. Merrimack said there were no new or unexpected safety concerns.
The company expects to provide additional details about MM-302, as well as the results of its full pipeline review, in January 2017.
Merrimack Pharmaceuticals vice president of clinical development Istvan Molnar said late line HER2-positive breast cancer is very difficult to treat, particularly in this new and previously unstudied group of patients who appear to experience rapid cancer progression after treatment with trastuzumab, pertuzumab and ado-trastuzumab emtansine.
Molnar added: "While we are disappointed with this outcome, we would like to thank the study Steering Committee, the investigators and, most importantly, the patients who participated in the Hermione trial. We will report our learnings from this study at a later date."
MM-302 specifically targets cancer cells overexpressing the HER2 receptor. As a liposomal encapsulation of doxorubicin, it is designed to allow for the selective uptake of drug into tumor cells and limit exposure to healthy tissues, such as those of the heart.
Image: Merrimack stops the phase 2 Hermione trial of MM-302 in HER2-positive metastatic breast cancer patients. Photo: courtesy of jk1991 at FreeDigitalPhotos.net.