Staurosporine derivatives reverse multidrug resistance without correlation with their protein kinase inhibitory activities.

Staurosporine, a potent inhibitor of protein kinases, has been reported to increase the accumulation of vincristine in multidrug-resistant (MDR) tumor cells and to bind P-glycoprotein, which works as an effluxpump in the plasma membrane of MDRcells1}. We have recently reported that N-ethoxycarbonyl-7-oxostaurosporine (NA-382) which has a low but selective inhibitory activity on protein kinase C, potently inhibits the function of P-glycoprotein2). Since P-glycoprotein is phosphorylated by protein kinase C3), it is important to discover whether the drug accumulation by these compounds is concerned with the inhibition of protein kinase C or not. We investigated in this study the effects of several staurosporine derivatives on protein kinase C, protein kinase A, and MDR. Staurosporine derivatives used in this study are shown in Scheme 1. 7-Oxostaurosporine was synthesized from staurosporine in our laboratory by the known method4), and the other derivatives NA-381, NA-382, NA-478 and NA-460 were also synthesized from staurosporine by simple methods5). At first, the inhibitory activities of these staurosporine derivatives on protein kinase A and protein kinase C were measured by the method described in a previous report2). As shownin Table 1, 7-oxostaurosporine was muchless inhibitory than staurosporine. Although NA-382 selectively inhibited protein kinase C as reported previously2*, another iV-ethoxycarbonyl compound (NA-38 1 ) had weak inhibitory activity, and iV-benzoxycarbonyl compounds (NA-478 and NA-460) had negligible inhibitory activity on both kinases. Next, the effects of these derivatives on the intracellular accumulation of vinblastine (VBL) in adriamycin-resistant P388 cells (P388/ADR), which had the MDRphenotype, were studied. The effects were shown by the concentration necessary for a 5-fold increase of intracellular VBL (C5) in Table 1. The results indicate that the conversion of the lactam moiety of staurosporine to the imide (7-oxostaurosporine) decreased the effect on the drug accumulation, but N-substitutions on the tetrahydropyran ring of these