Melanoma Checkpoint and Gut Microbiome Alteration With Microbiome Intervention

This is a Phase 1b, multicenter, randomized, placebo-controlled, blinded study in adult participants with anti-PD-1 therapy naïve, unresectable or metastatic melanoma to evaluate the safety and tolerability of SER-401, or matching placebo in combination with anti-PD-1 therapy (nivolumab). Prior to initiating microbiome study intervention and nivolumab, participants will undergo an antibiotic or antibiotic placebo treatment lead-in to prime the gut microbiome for engraftment of the oral microbiome study intervention. Study intervention groups will be assessed for safety, changes in the microbiome, changes in the percentage of tumoral CD8 T cells, and antitumor activity. Participants must have measurable disease that can be biopsied and consent to baseline and on-treatment biopsies, as well as stool and blood biomarker collection throughout the study.