Abexinostat in Combination With Pembrolizumab in Patients With Advanced Solid Tumor Malignancies

Inclusion Criteria: * Patient \>= 18 years of age at the time of study enrollment. * Has histologically confirmed locally advanced or metastatic solid tumor malignancy with one of the following tumor types: * Urothelial carcinoma * Melanoma * Non-small cell lung cancer * Small cell lung cancer * Non-pulmonary squamous cell carcinoma * Head and neck squamous cell carcinoma * MSI-high solid tumor, with MSI-high status defined by microsatellite instability testing by Polymerase chain reaction (PCR), loss of mismatch repair proteins by Immunohistochemistry (IHC), or Clinical Laboratory Improvement Amendments (CLIA)-approved next generation sequencing test. * Gastric and gastro-esophageal junction adenocarcinoma * Merkel cell carcinoma * Thymic carcinoma * Mesothelioma * Measurable disease by RECIST 1.1 criteria. * Dose Expansion only: * Disease progression during or within 3 months of last dose of most recent line of prior anti-PD-1/PD-L1-based treatment with a pattern of progression as defined as one of the following: * Primary Resistance (Cohort A): Progressive disease as best response to prior treatment with anti-PD-1/PD-L1 treatment * Acquired Resistance (Cohort B): Stable disease, partial response or complete response to prior anti-PD-1/PD-L1 treatment with subsequent disease progression during or within 3 months following last dose of anti-PD-1/PD-L1 treatment * A tumor biopsy is mandatory unless * There is no safely accessible les…