The composition of the gut microbiome has been associated with response and the development of toxicities on immune checkpoint inhibitors (ICIs) in multiple tumor types. The aim of this study is to examine the gut microbiome composition in patients undergoing standard of care treatment for advanced/unresectable and/or metastatic solid tumors with ICIs. Fecal samples and peripheral blood samples will be collected to further characterize the diversity of gut bacteria and to study potential mechanisms by which gut bacteria impact the immune response.

This is a prospective study of gut microbial markers. Patients with histologically confirmed advanced/unresectable or metastatic solid tumors who are planned to initiate standard of care ICIs or are undergoing standard of care treatment with ICIs will be approached for participation in the study.

Inclusion Criteria:

• Sign written and voluntary informed consent
• Adult patients aged >=18, male or female
• Eastern Cooperative Group (ECOG) performance status 0-2
• Histologic diagnosis of an advanced/unresectable or metastatic solid tumor
• Measurable disease as per RECIST 1.1 criteria
• Be suitable for or receiving ICI treatment
• Prior immunotherapy allowed
• Be willing and able to provide fecal and blood specimens for analysis as per protocol

Exclusion Criteria:

• Subjects with a history of inflammatory bowel disease, chronic diarrhea or malabsorption syndromes and significant prior bowel resection as judged by the study investigator

Adult patients with advanced/unresectable or metastatic solid tumors either planned to start standard of care treatment with ICIs or already on treatment with ICIs.