R-5780-01 In Combination With PD-1 Checkpoint Inhibitors (Checkpoint Protein on Immune Cells Called T Cells) in Patients With Solid Tumors

Inclusion Criteria: * At least 18 years of age to 80 * Ability to provide written informed consent * Unresectable stage III or stage IV melanoma, basal cell carcinoma, or squamous cell carcinoma as per the American Joint Committee on Cancer 2017 Guidelines (8th Edition) regardless of BRAF mutation status and other solid tumors. * Refractory to anti-PD (checkpoint Protein on Immune Cells called T cells)-1/L1 therapy per RECIST v1.1 defined as subject who has disease progression after receiving at least two complete cycles of ICI (immune checkpoint inhibitors) therapy or disease progression 6 months from initiation of ICI (immune checkpoint inhibitors) therapy while still on active therapy. * Life expectancy of greater than 3 months * Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2 * Subjects must have evaluable disease by CT (computer tomography) or MRI (magnet resonance imaging) per RECIST 1.1 criteria or clinically apparent disease that the investigator can follow for response. Exclusion Criteria: * Any serious medical condition or laboratory abnormality or psychiatric condition or any other significant or unstable concurrent medical illness (in the opinion of the Investigator) would preclude protocol adherence or would make the safety of the study drug difficult to assess * Treatment with systemic broad-spectrum antibiotics. * No active viral infections. * Coexisting severe chronic diseases other than cancer (autoimmunity, inflammatory diseases) * Secondary gastrointestinal motility disorders * History of solid organ transplant or bone marrow transplant * Prior CAR-T (chimeric antigen receptor) or allogeneic cellular therapy * Ongoing systemic immunosuppressive therapy, with the exclusion of prednisone (10 mg/day) * Concurrent therapy with any other investigational agent, vaccine, or device * Pregnant or breastfeeding or planning to conceive or father a child during the trial period * Subjects with untreated brain metastasis. Treated brain metastasis are permitted if stable * More than 4 prior systemic therapies * Other cancer medications during treatment period are not permitted * Enrollment in other clinical trials.