Pilot Study of Biomarkers of Response to Immune Checkpoint Blockade in Metastatic Melanoma

Inclusion Criteria: * Participants must have a metastatic melanoma diagnosis (stage IV) for which treatment with nivolumab or pembrolizumab, either alone or in combination with other therapies, is planned. No additional laboratory testing is needed for the imaging in this study apart from the standard laboratory testing routinely obtained for treatment with nivolumab or pembrolizumab, either alone or in combination with other therapies. * Participants must have at least 2 subcutaneous or lymph node melanoma metastases that are 2 cm in greatest diameter and are amenable to being biopsied in clinic without requiring image-guidance. * Participants must be able to provide informed consent * Women of childbearing potential must be willing to use effective contraception as discussed with their oncologist while participating in this study. Exclusion Criteria: * Not able to receive treatment with either nivolumab or pembrolizumab. * Women of child-bearing potential must have a negative serum or urine pregnancy test within 7 days of the first study FLT PET/CT and must not be breast feeding for the duration of study participation.