Peripheral T Cell Determinants of Response and Resistance to Pembrolizumab in Melanoma

Primary Objective: To understand how the systemic immune profile (T cell activation and expansion in TME) changes in response to pembrolizumab therapy in patients with advanced melanoma on pembrolizumab monotherapy. Exploratory Objectives : I. To correlate the peripheral T cell profiles with the objective response rate (ORR) at 24 weeks in patients with advanced melanoma on pembrolizumab monotherapy. II. To correlate the peripheral T cell profiles with progression free survival (PFS) in patients with advanced melanoma on pembrolizumab monotherapy. III. To correlate the peripheral T cell profiles with overall survival (OS) in patients with advanced melanoma on pembrolizumab monotherapy. IV. To correlate the peripheral T cell profiles with toxicity profile. V. Transcriptional and phenotypic features of tumor directed T cells in blood using a combination of phenotypic markers derived from COMET and cite-seq. Outline: Participants will have blood drawn and tumor biopsied. Participants will be followed for 6 months from time of treatment initiation. After 6 months, participants do not need to be followed but standard of care scans and survival status can be assessed for up to 5 years.