Study of MT-8421 as Monotherapy and in Combination With Nivolumab in Patients With Selected Advanced Solid Cancer Types

Inclusion Criteria: * Histologically confirmed, unresectable, locally advanced, or metastatic melanoma, HCC, NSCLC, RCC, MSI-H/dMMR malignancies, urothelial carcinoma, esophageal squamous cell carcinoma, mesothelioma, SCCHN, or cervical carcinoma not amenable to standard treatment, or standard treatment is not available, or in the Investigator's opinion, the standard treatment would not be in the patient's best interest. Part A only: Evaluable only or measurable disease according to RECIST v1.1 Part B only: At least 1 measurable tumor lesion according to RECIST v1.1 * Prior treatment must include a PD-1 or programmed cell death - ligand 1 (PD-L1) inhibitor. Prior treatment with a CTLA-4 inhibitor is not required. * Eastern Cooperate Oncology Group (ECOG) performance score of 0 or 1 * Adequate bone marrow function, as determined by: * Absolute neutrophil count (ANC) ≥ 1500/ μL (should not have received growth factors within 2 weeks prior to screening) * Platelet count ≥ 75,000/ μL * Hemoglobin ≥ 8.0 g/dL (no red blood cell transfusion within 2 weeks of study treatment start is allowed) * Adequate renal function, based on estimated creatinine clearance (eCrCl) ≥ 50 mL/min, calculated by the Cockcroft-Gault equation. * Adequate hepatic function, as determined by: * Total bilirubin ≤ 1.5x upper limit of normal (ULN) or ≤ 2x ULN direct bilirubin for patients with Gilbert's Syndrome * Aspartate transaminase (AST) ≤ 3x ULN (or ≤ 5x ULN if liver metastasis or HCC). *…