ENHANCEMENT OF CISPLATIN-INDUCED CYTOTOXICITY BY 7-HYDROXYSTAUROSPORINE (UCN-01), A NEW G(2)-CHECKPOINT INHIBITOR

DNA-damaging agents arrest cell cycle progression at either G(1) or G( 2). A variety of agents such as caffeine have been shown to abrogate t he DNA damage-dependent G(2) checkpoint and enhance cytotoxicity. Unfo rtunately, this strategy has not enhanced therapeutic activity because adequate concentrations of these modulators are not tolerated in vivo , Here, using Chinese hamster ovary cell lines, we show that the poten t protein kinase inhibitor 7-hydroxystaurosporine (UCN-01) abrogates t he G(2) arrest induced by the DNA-damaging agent cisplatin, UCN-01 not only was effective at inducing mitosis when added to G(2)-arrested ce lls but also prevented cells from arresting in G(2) when added to S-ph ase cells. Furthermore, UCN-01 did not cause premature mitosis of S-ph ase cells; rather, the cells progressed to G(2) before undergoing mito sis, These effects were observed at noncytotoxic concentrations of UCN -01 that alone had no effect on cell cycle passage, Furthermore, the s ame concentrations of UCN-01 that resulted in abrogation of the cispla tin-induced G(2) arrest also enhanced cisplatin-induced cytotoxicity, as determined by a colony formation assay, UCN-01 enhanced cisplatin c ytotoxicity up to 60-fold and reduced by S-fold the concentration of c isplatin required to kill 90% of the cells, The concentrations of UCN- 01 required for this enhancement have been shown to be well tolerated in animal models, suggesting that this combination may represent an ef fective strategy for enhancing cisplatin-based chemotherapeutic regime ns.