Lactulose to Improve Gut Health in Cancer Patients Receiving Immunotherapy

Inclusion Criteria: * Inclusion Criteria Phase I * Patients must have a histologically confirmed malignancy that is to receive exclusively ICIs (no chemotherapy or RT in combination) per standard of care during the time in which lactulose will be administered. This includes, but is not limited to, melanoma, cutaneous squamous cell carcinoma, non-small cell lung cancer, mesothelioma, head and neck squamous cell carcinoma, classical Hodgkin lymphoma, primary mediastinal large B-cell lymphoma, urothelial cancer, MSI/MMRd cancer, cancers with high tumor mutational burden (\>=10 muts/mB), esophageal cancer, hepatocellular carcinoma, and renal cell carcinoma. Anti-PD-1 or anti-PD-L-1 therapy combinations with anti-LAG-3 or anti-CTLA-4 combinations are permitted. * Age ≥18 years. Because no dosing or adverse event data are currently available on the use of lactulose in combination with ICIs in patients \<18 years of age, children are excluded from this study. * ECOG performance status ≤2 (Karnofsky ≥60%, see Appendix A). * Patients must be able to ingest liquids. * Inclusion Criteria Phase II * Patients must have a histologically confirmed cutaneous or mucosal melanoma. Uveal melanoma is excluded. * Measurable disease per RECIST 1.1.32 * Documented primary or acquired resistance to anti-PD-1 and anti-CTLA4 therapy per SITC guidelines.21 Primary resistance is defined as disease progression after a minimum of 6 weeks of therapy, provided the patient has received at least two full cyc…