ASTX660 is a synthetic small molecule dual antagonist of cellular inhibitor of apoptosis protein (cIAP) 1 and X-linked inhibitor of apoptosis protein (XIAP) that has been shown to have potent proapoptotic and tumor growth inhibitory activity in nonclinical models. ASTX660 has not been previously evaluated in human subjects. The Phase 1 portion of the study will determine the MTD, RP2D, and recommended dosing regimen. The Phase 2 portion will evaluate activity in selected tumor types.