Antagonistic interplay between antimitotic and G(1)-S arresting agents observed in experimental combination therapy

Paclitaxel is a naturally occurring antimitotic agent that has been shown t o stabilize microtubules, induce mitotic arrest, and ultimately induce apop totic cell death. The favorable clinical activity of paclitaxel has prompte d considerable interest in combining paclitaxel with numerous other antineo plastic agents. Our previous studies have suggested 5-fluorouracil (5-FU), an antineoplastic agent that usually arrests tumor cells at the G(1)-S phas e of the cell cycle, in combination with paclitaxel significantly represses paclitaxel-induced mitotic arrest and apoptosis. In the present study, we have extended this investigation to include several other antimitotic agent s (vinblastine, colchicine, and nocodazole) in various combination schedule s with the G(1)-S arresting agents 5-FU and hydroxyurea (HU). We found 5-FU , as well as HU, could significantly interfere with the overall cytotoxicit y as compared with treatment with antimitotic agents alone. It appeared tha t 5-FU or HU severely limited the antimitotic agents' cytotoxic effects on both mitotic arrest and apoptosis. No combination of a G(1)-S arresting age nt with an antimitotic agent in any schedule produced an antitumor effect g reater than that of the antimitotic agent alone. In addition, biochemical e xamination revealed that 5-FU and HU blocked the antimitotic agent-induced increase of p21(WAF1/CIPI) protein levels, as well as prevented the hyperph osphorylation of the bcl-2 and c-raf-1 proteins. These findings suggest tha t careful considerations may be necessary when combining antineoplastic age nts that exert their cytotoxic action at different phases of the cell cycle .