KRT-232 in Subjects With Relapsed or Refractory Small Cell Lung Cancer

Inclusion Criteria: * Histologically or cytologically confirmed diagnosis of SCLC documented as TP53WT * Disease must be measurable per RECIST Version 1.1 * Evidence of radiographic progression during or after at least one prior platinum-containing therapy with no curative therapy available. Subjects who have received only one prior line of therapy must not be candidates for platinum-based regimens at relapse. * Subjects must have received a checkpoint inhibitor (PD-1 or PD-L1) unless contraindicated if checkpoint inhibitors are approved and available. * ECOG ≤ 2 Exclusion Criteria: * Symptomatic or uncontrolled central nervous system (CNS) metastases. * Prior treatment with MDM2 inhibitors * Chemotherapy, immune therapy, cytokine therapy, or any investigational therapy within 14 days prior to the first dose of study treatment * Grade 2 or higher QTc prolongation (\> 480 milliseconds per NCI-CTCAE criteria, version 5.0) * History of major organ transplant