Inclusion Criteria:

          -  ECOG performance status of 0 to 1

          -  Histologically confirmed MCC. Disease must be measurable, with at least 1 measurable
             lesion by RECIST 1.1

          -  MCC expressing p53WT based on any CLIA or FDA approved test

          -  Patients must have failed (i.e., relapsed or were refractory to) treatment with at
             least one PD-1 inhibitor or PD-L1 inhibitor for MCC

          -  Fresh or archival tumor tissue must be submitted for biomarker assessment. Archival
             tissue samples must have been obtained from biopsy performed ≤ 2 years before the date
             of signing the informed consent for this study. Adequate hematological, hepatic, and
             renal function within 14 days prior to the first dose of KRT-232:

               1. Hematologic: ANC ≥1.0 × 109/L in the absence of growth factors during the prior 7
                  days; platelet count ≥100 × 109/L

               2. Hepatic: total bilirubin ≤2.0 times the upper limit of normal (ULN), unless
                  Gilbert's Syndrome; aspartate transaminase/serum glutamic oxaloacetic
                  transaminase (AST/SGOT) and alanine transaminase/serum glutamic pyruvic
                  transaminase (ALT/SGPT) ≤2.5 ULN

               3. Renal: estimated creatinine clearance >30 mL/min by Cockcroft Gault:

        Exclusion Criteria:

          -  Concurrent anticancer treatment such as chemotherapy, cytoreductive therapy, immune
             therapy, or cytokine therapy within 28 days or approximately 5 half-lives, whichever
             is shorter, prior to the first dose of KRT-232

          -  Radiation therapy within 2 weeks prior to the first dose of KRT-232

          -  Toxicity from prior radiation therapy that has not resolved to National Cancer
             Institute Common Terminology Criteria for Adverse Events (NCI CTCAE) Grade 0 or Grade
             1 (with the exception of Grade 2 alopecia)

          -  Participation in another interventional clinical trial within the past 4 weeks of the
             first dose of KRT-232

          -  Patients previously treated with MDM2 antagonist therapies or p53-directed therapies

          -  History of major organ transplant

          -  Patients with known central nervous system (CNS) metastases that are previously
             untreated