Effect of P-glycoprotein on flavopiridol sensitivity

Flavopiridol is the first potent inhibitor of cyclin-dependent kinases (CDK s) to enter clinical trials. Little is known about mechanisms of resistance to this agent. In order to determine whether P-glycoprotein (Pgp) might pl ay a role in flavopiridol resistance, we examined flavopiridol sensitivity in a pair of Chinese hamster ovary cell lines differing with respect to lev el of Pgp expression. The IC(50)s of flavopilidol in parental AuxB1 (lower Pgp) and colchicine-selected CH(R)C5 (higher Pgp) cells were 90.2 +/- 6.6 n M and 117 +/- 2.3 nM, respectively (P < 0.01), suggesting that Pgp might ha ve a modest effect on flavopiridol action. Consistent with this hypothesis, pretreatment with either quinidine or verapamil (inhibitors of Pgp-mediate d transport) sensitized CH(R)G5 cells to the antiproliferative effects of f lavopiridol. Because of concern that colony forming assays might not accura tely reflect cytotoxicity, we also examined flavopiridol-treated cells by t rypan blue staining and flow cytometry. These assays confirmed that flavopi ridol was less toxic to cells expressing higher levels of Pgp. Further expe riments revealed that flavopiridol inhibited the binding of [H-3]-azidopine to Pgp in isolated membrane vesicles. but only at high concentrations. Col lectively, these results identify flavopiridol as a weak substrate for Pgp. (C) 2001 Cancer Research Campaign.