More than a year into an international clinical trial of the immunotherapy drug nivolumab (anti–PD–1), researchers report that more than half of a small group of patients with an aggressive and rare form of skin cancer called Merkel cell carcinoma (MCC) have responded to the drug.

Preliminary results from the clinical trial including 25 patients with MCC show eight complete responses, meaning elimination of all detectable tumors lasting at least 30 days, and eight partial responses, meaning approximately 50 percent reduction in tumors, as of the data analysis in February 2017. The patients were part of a large clinical trial underway at more than 30 hospitals in the U.S. and internationally studying the use of nivolumab in patients with a variety of virus–linked cancers.

“We saw responses in patients with advanced MCC who received previous chemotherapy for their disease, as well as those who were receiving nivolumab as their first treatment,” says Suzanne Topalian, MD, professor of surgery and oncology at the Johns Hopkins Bloomberg~Kimmel Institute and the lead investigator for the clinical trial. She noted that the researchers spotted responses in patients’ tumors as early as eight weeks after the start of treatment with nivolumab.

The effects of treatment were long lasting in most patients, and median rates of overall and disease–free survival have not yet been reached among these patients. There were no deaths among the 25 patients after a median follow–up period of 51 weeks, and 20 of them experienced side effects that included fatigue, diarrhea and itching. Most side effects were mild.

Topalian and her colleagues previously found similar responses rates among patients with advanced MCC using the immunotherapy drug pembrolizumab (anti–PD–1). The related immunotherapy drug avelumab (anti–PD–L1) was recently approved by the U.S. Food and Drug Administration for patients with advanced MCC. The study was presented as Abstract #CT074 at the the American Association for Cancer Research Annual Meeting.