KRIBB3, a novel microtubule inhibitor, induces mitotic arrest and apoptosis in human cancer cells

KRIBB3 (5-(5-ethyl-2-hydroxy-4-methoxyphenyl)-4-(4-methoxyphenyl) isoxazole) inhibited cancer cell growth in vitro and in vivo. Flow cytometry studies showed that KRIBB3 caused cell cycle arrest at the G2/M phase and subsequent apoptosis. This was confirmed as accumulation of Cyclin B1 and cleavage of poly(ADP-ribose) polymerase (PARP) were detected. While transient inhibition by KRIBB3 led to reversible mitotic arrest, prolonged exposure to KRIBB3-induced apoptosis. Co-immunoprecipitation experiments showed that KRIBB3 initially induced association of inhibitory Mad2 with p55CDC (mammalian homologue of CDC20), an activator of APC/C (anaphase-promoting complex/cyclosome), suggesting that the mitotic spindle checkpoint was activated by KRIBB3. However, the level of this inhibitory complex of Mad2 with p55CDC was gradually decreased 24 h after KRIBB3 treatment, and was hardly detectable after 48 h, indicating some slipping of the mitotic checkpoint. Consistent with these observations, KRIBB3 activated the mitotic spindle checkpoint by disrupting the microtubule cytoskeleton. KRIBB3 was proven to be a tubulin inhibitor using in vitro polymerization assays and in vivo indirect immunofluorescence staining. The temporal pattern of Bax activation by KRIBB3 was similar to PARP cleavage, suggesting that Bax is a mediator of KRIBB3-dependent apoptosis. Furthermore, when KRIBB3 was administered intraperitoneally into nude mice at 50 mg/kg or 100 mg/kg, it inhibited 49.5 or 70.3% of tumor growth, respectively. These results suggest that KRIBB3 is a good drug candidate for cancer therapy.