Sitagliptin With Pembro in RCC and Melanoma

This study is testing whether adding the drug sitagliptin to the standard immunotherapy pembrolizumab is safe and may help people with advanced melanoma or advanced renal cell carcinoma (kidney cancer) whose cancer has stopped responding to prior PD-1 or PD-L1 immunotherapy. The study has two parts. In the first part, small groups of participants will receive different doses of sitagliptin along with a fixed dose of pembrolizumab. This helps researchers find the highest dose of sitagliptin that can be given safely. In the second part, more participants will receive the safest dose to see how well the drug combination works against their cancer.