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