Scientists find immune drug that could halt skin cancer’s deadly spread

Pembrolizumab Trial Results Show Reduced Risk of Cancer Spread

The randomized phase 3 STAMP trial is the largest study so far to evaluate pembrolizumab as adjuvant therapy for Merkel cell carcinoma after tumor removal. After two years, 73% of patients who received pembrolizumab remained free of cancer, compared with 66% in the group that did not receive the drug. Although this difference was not statistically significant, those treated with pembrolizumab had a 42% lower chance of developing distant metastases, meaning cancer that spread to areas such as the liver, lungs, or bones.

Expert View: "Much-Needed Good News" for Patients

"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," said 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. "This is much-needed good news for people who are living with the highly aggressive cancer that is Merkel cell carcinoma," Dr. Mehnert added.

Understanding Merkel Cell Carcinoma

Merkel cell carcinoma is a fast-developing cancer that starts in the skin's touch-sensing cells and often appears as a firm, painless lump on areas frequently exposed to sunlight. Although skin cancer is the most common type of cancer in the United States, Merkel cell carcinoma -- also known as neuroendocrine carcinoma of the skin -- is extremely rare, affecting no more than three in one million people each year. It is highly aggressive, with fewer than half of patients surviving five years after diagnosis.

Large-Scale Collaboration Enables Rare Cancer Study

The STAMP trial (NCT03712605) enrolled 293 patients between 2018 and 2023, all of whom had undergone surgery to remove their tumors. Of these, 147 patients were randomly assigned to receive pembrolizumab infusions following surgery, while 146 were monitored without additional drug treatment. Some participants also received radiation therapy as recommended by their doctors.

Because Merkel cell carcinoma is so uncommon, Dr. Mehnert noted that national collaboration was crucial to make the study possible. With support from the National Cancer Institute (NCI), part of the National Institutes of Health (NIH), ECOG-ACRIN conducted the trial through the NCI's National Clinical Trials Network, involving more than 500 hospitals and cancer centers across the country.

How Pembrolizumab Works

Pembrolizumab belongs to a class of immunotherapies known as PD-1 inhibitors. It works by blocking the PD-1 receptor, a protein cancer cells use to hide from immune system attack. By interrupting this process, pembrolizumab helps immune cells recognize and destroy cancer cells more effectively. The U.S. Food and Drug Administration has already approved pembrolizumab (brand name KEYTRUDA) to treat recurrent locally advanced or metastatic Merkel cell carcinoma.

Overall survival, the other co-primary endpoint of the trial, continues to be followed and will be reported at a later date.

Dr. Mehnert will present the trial at the European Society for Medical Oncology (ESMO) 2025 Congress on Monday, October 20 in Berlin, Germany.