Merrimack Pharmaceuticals Stops Mid-Stage Breast Cancer Trial

Shares of Merrimack Pharmaceuticals Inc. (NASDAQ: MACK) dropped on Wednesday after the company reported that it would stop a mid-stage cancer study. Specifically the company announced that, following a recent independent Data and Safety Monitoring Board (DSMB) recommendation and subsequent futility analysis, Merrimack has decided to stop the Phase 2 HERMIONE study of MM-302 in HER2-positive metastatic breast cancer patients.

Note that these are also patients who had previously been treated with trastuzumab (Herceptin), pertuzumab (Perjeta) and ado-trastuzumab emtansine (T-DM1, Kadcyla).

The decision to stop the trial was made following the DSMB’s opinion that continuing would be unlikely to demonstrate benefit over the comparator treatments. Following this recommendation, a futility assessment was performed that confirmed the DSMB’s opinion. Both the treatment and control arms were found to have shorter than expected median progression free survival.

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There were no new or unexpected safety concerns. Patients currently enrolled in the trial may choose to continue on their assigned treatment based on discussion with their study physician. Also the company now expects to provide further details about MM-302, as well as the results of its full pipeline review, in January.

Istvan Molnar, M.D., Vice President of Clinical Development at Merrimack, commented:

Late line HER2-positive breast cancer is very difficult to treat, especially in this new and previously unstudied group of patients who appear to experience rapid cancer progression following treatment with trastuzumab, pertuzumab and ado-trastuzumab emtansine. 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.

Shares of Merrimack were down about 18% at $4.41 on Wednesday, with a consensus analyst price target of $9.67 and a 52-week trading range of $4.35 to $9.02.

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