A phase Ib clinical trial to evaluate the efficacy and tolerability of pembrolizumab in patients with metastatic triple-negative breast cancer showed the drug was well tolerated and yielded durable responses, according to data presented at the 2014 San Antonio Breast Cancer Symposium.

“We observed that 18.5% of patients with PD-L1-expressing triple-negative breast tumors had a response to therapy,” said Rita Nanda, MD, assistant professor of medicine and associate director of the Breast Medical Oncology Program at the University of Chicago. “Pembrolizumab therapy was well tolerated, with a manageable side effect profile.”

For those patients who responded to therapy, responses were quite durable, and median duration of response was not yet reached at the time of presentation.

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“Currently, there are no approved targeted therapies to treat metastatic triple-negative breast cancer (TNBC), and the median survival for patients with this disease is approximately one year,” Nanda added. “The promising activity of pembrolizumab seen in PD-L1-expressing triple-negative breast cancer is very exciting, and certainly worthy of further investigation.”

Nanda and colleagues recruited 32 patients to this multicenter, nonrandomized trial. Participants were ages 29 to 72 years, and had metastatic TNBC that had either relapsed after treatment for early stage disease or progressed on therapy for advanced disease. All patients received intravenous infusions of 10 mg/kg of pembrolizumab every 2 weeks.

Primary end points of this study were to determine the safety, tolerability, and antitumor activity of pembrolizumab.

Five patients experienced at least one grade 3 to grade 5 drug-related adverse event, with one patient experiencing a grade 5 event.

Upon central review, of the 27 patients with measurable disease for best overall response, one patient had a complete response (CR), four had a partial response (PR), seven had stable disease, and 12 had progressive disease. In addition, three patients discontinued therapy prior to the first postbaseline scan due to clinical progression. At the time of this presentation, one patient with a CR and two patients with a PR continue to be treated with pembrolizumab.

This study was funded by Merck, known as MSD outside the United States and Canada. Nanda declares no conflicts of interest.