This study is designed to assess the safety, tolerability and engraftment (cumulative relative abundance) of MET-4 strains when given in combination with immune checkpoint inhibitors (ICIs). There will be a safety cohort (group A) of 5 subjects which will receive MET-4 in addition to standard of care (SOC) ICI. After the safety cohort, 40 patients will be enrolled in group B which will be randomized to MET4 with SOC ICI vs. control group with SOC ICI only. Group C will enroll 20 patients who have already started on SOC ICI and have had first unconfirmed progression of disease and expected to continue with standard ICI treatment. These patients will be randomized to continue receiving standard ICI alone, or SOC ICI with MET4.

Human associated microorganisms (the microbiota) inhabit virtually all surfaces of the human body. The gut is densely colonized by the microbiota which aids in the digestion. Animal and human observational and experimental evidence show a link between gut microbiota and the activation, regulation and function of the immune system. Pre-clinical studies in mouse models have linked the gut microbiota to efficacy of anticancer therapies. Microbial Ecosystem Therapeutics (MET) is a new treatment approach developed as an alternative to fecal transplantation. MET consists of a defined mixture of pure live cultures of intestinal bacteria isolated from a stool sample of a healthy donor.

This study is designed to assess the safety, tolerability and engraftment (cumulative relative abundance) of MET-4 strains when given in combination with immune checkpoint inhibitors (ICIs).

Inclusion Criteria:

• Signed written and voluntary informed consent
• Age >=18 years, male or female
• Histologically or cytological documented locally-advanced or metastatic solid malignancy which is incurable.
• Group A: Is already on treatment with monotherapy anti-PD-1 or PD-L1 immune checkpoint inhibitor, not in the context of a therapeutic clinical trial.

Group B: Is intended to start on treatment with monotherapy anti-PD-1 or PD-L1 immune checkpoint inhibitors as considered appropriate by treatment physician, and not in the context of a therapeutic clinical trial.

Group C: Is already on treatment with monotherapy anti-PD-1 or PD-L1 immune checkpoint inhibitor, not in the context of a therapeutic clinical trial with first unconfirmed PD on evaluation scan per investigator's assessment.

• Be willing to provide 10-15 unstained slides of archival tissue sample. Subjects who decline or have not sufficient archived tissue samples may still enroll if all other criteria are eligible.
• Be willing and able to provide stool and blood specimen for analyses at protocol specified time points.
• Have measurable disease based on RECIST 1.1
• Performance status of 0-2 on the Eastern Cooperative Oncology Group (ECOG) performance scale.
• Prior therapy with any immunotherapy allowed.
• Not pregnant for females of child bearing potential as indicated by negative serum or urine pregnancy test within 72 hours of study start.

Exclusion Criteria:

• Subjects unable to swallow orally administered medications or any subjects with gastrointestinal disorders likely to interfere with absorption (e.g. bowel obstruction, short gut syndrome, blind loop syndrome, ileostomy etc). Subjects with colostomies may be enrolled.
• Any condition that, in the opinion of the Investigator, would interfere with subject safety, or evaluation of the collected specimen and interpretation of study result.
• Pregnant or planning to get pregnant in the next 6 months.