CAY10503 is a proapoptotic, antiproliferative compound that is able to arrest cell cycle progression in the G0-G1 phase. CAY10503 inhibits the growth of HL60, multidrug resistant HL60R, and K562 cells with IC50 values of 7.0, 23, and 20 µM, respectively. Accumulation of HL60 cells in G0-G1 occurred within eight hours following treatment with 50 µM CAY10503, whereas 10 µM CAY10503 required 96 hours for cell cycle arrest to appear. CAY10503 also induces differentiation of HL60 cells into the following positive cells: CD14 (monocytic marker) as well as CD11b and CD11c (granulocytic markers). Approximately 60% of HL60 cells exposed to 10 µM CAY10503 expressed these markers within 72 hours.