Immunologic Determinants of Response to Pembrolizumab (MK-3475) in Advanced Melanoma (MK-3475-161/KEYNOTE-161)

In this study, participants with advanced melanoma will be treated with pembrolizumab (MK-3475) and their tumors and blood will be analyzed for changes related to pembrolizumab therapy. The primary hypotheses are that participants who respond to pembrolizumab have: 1. a higher fraction of cytotoxic tumor-infiltrating T-lymphocytes (FCT) at baseline compared to those who do not respond to pembrolizumab 2. a higher fold-increase in FCT compared to baseline than those who do not respond to pembrolizumab 3. a higher Average Specific Cytotoxic T-lymphocyte Frequency Ratio (ASCTFR) compared to those who do not respond to pembrolizumab