A large clinical trial has shown that the immunotherapy drug pembrolizumab may help prevent the deadly spread of Merkel cell carcinoma, a rare and aggressive skin cancer, after surgery. While it did not significantly reduce overall recurrence rates, the treatment lowered the risk of distant metastases by 42%. The findings offer hope for patients facing this fast-developing disease.
The phase 3 STAMP trial, conducted by the ECOG-ACRIN Cancer Research Group, evaluated pembrolizumab as an adjuvant therapy for Merkel cell carcinoma following tumor removal. This randomized study, the largest of its kind, enrolled 293 patients between 2018 and 2023 across more than 500 hospitals and cancer centers in the United States, supported by the National Cancer Institute.
Patients were divided into two groups: 147 received pembrolizumab infusions, while 146 were monitored without the drug. Some participants also underwent radiation therapy as recommended. After two years, 73% of the pembrolizumab group remained cancer-free, compared to 66% in the observation group. Although this difference was not statistically significant for overall recurrence, the treatment group had a 42% lower risk of distant metastases, such as to the liver, lungs, or bones.
Lead investigator Janice M. Mehnert, MD, co-chair of the ECOG-ACRIN Melanoma Committee and director of Melanoma and Cutaneous Medical Oncology at NYU Langone Health's Perlmutter Cancer Center, highlighted the results' importance. "The STAMP trial provides the first evidence that immunotherapy with pembrolizumab after surgery may help people with Merkel cell carcinoma by preventing their cancer from returning in organs considered distant from the site of the original disease," she said. "This is much-needed good news for people who are living with the highly aggressive cancer that is Merkel cell carcinoma."
Merkel cell carcinoma, also known as neuroendocrine carcinoma of the skin, arises in touch-sensing cells and often presents as a firm, painless lump on sun-exposed areas. It affects no more than three in one million people annually in the United States and has a poor prognosis, with fewer than half of patients surviving five years post-diagnosis. Pembrolizumab, a PD-1 inhibitor branded as KEYTRUDA, blocks a protein that helps cancer cells evade the immune system and is already FDA-approved for advanced cases of the disease.
Overall survival data from the trial is still being followed and will be reported later. Dr. Mehnert presented the findings at the European Society for Medical Oncology Congress on October 20, 2025, in Berlin, Germany.