Pembrolizumab TX-naive Distant Mets Melanoma and Use of (C11-AMT) PET at Baseline as Imaging Biomarker

18F-fluorodeoxy glucose (FDG) positron emission tomography (PET) with intravenous (IV) contrast images are obtained before and at the end study treatment (pembrolizumab infusion).

11C-methyl-L-tryptophan (C11-AMT) positron emission tomography (PET) images are obtained before study treatment (pembrolizumab infusion).

200 mg Pembrolizumab IV administered over 30 minutes on day 1. Repeat every 3 weeks for 4 cycles, until progression, or subject withdrawal for other reasons

Computerized tomography (CT) images are taken without IV contrast before the treatment (pembrolizumab infusion) and with IV contrast 3 months after the treatment start.