Brefeldin A (BFA) can induce a wide variety of human cancer cells to differentiation and apoptosis and is in development as an anticancer agent. To elucidate structural requirements for cytotoxicity and induction of differentiation and apoptosis, BFA was structurally modified into various derivatives including 4-epi-BFA in this study. Their inducing activities of apoptosis were evaluated with their cytotoxicities, DNA fragmentation and morphological changes in human colon cancer cell HCT 116. The cytotoxicity of 4-epi-BFA (TX-1923) (IC50 = 60 microM) was 300 times lower than that of BFA (IC50 = 0.2 microM). Furthermore, 4-epi-BFA induced DNA fragmentation and apoptotic morphological changes at much higher concentrations (70 and 50 microM, respectively) than BFA (0.11 and 0.36 microM, respectively). These results indicated that the configuration of 4-hydroxyl group of brefeldin A plays a key role in the cytotoxicity and induction of apoptosis. On the other hand, 7-O-acetyl-BFA, 4-O-acetyl-BFA, and 4,7-di-O-acetyl-BFA exhibited potential activities in cytotoxicity and inducibility of apoptosis. We suggested that the structural determinants for BFA include the moiety of the Michael acceptor, the conformational rigidity of the 13-membered ring, and the configuration of 4-hydroxyl group.