- Ipilimumab is a drug used to treat melanoma that cannot be treated surgically. It targets a molecule found on T-cells in the human immune system. Blocking these molecules on the T-cells might allow the cells to help destroy melanoma cells more effectively. This drug has also been studied in other cancers such as prostate cancer and lung cancer, but not yet in Merkel cell carcinoma (MCC). Researchers think therapy like ipilimumab that enhances the immune system may be effective against MCC. They want to study how safe the drug is and its effect on the immune system and tumors.