CHARACTERIZATION OF 2-CHLORO-N-10-SUBSTITUTED PHENOXAZINES FOR REVERSING MULTIDRUG-RESISTANCE IN CANCER-CELLS

Twenty-one 2-chloro-N-10-substituted phenoxazines have been synthesize d and characterized as potential modulators of multidrug resistance (M DR). Many of the compounds, at a concentration of 100 mu M, enhanced a ccumulation of vinblastine (VLB) in drug-resistant KB8-5 cells to a gr eater extent than the same concentration of verapamil (VRP). However, the effects on VLB accumulation were specific, because these derivativ es had little activity in the parental drug-sensitive line KB3-1. The compounds slowed the efflux of VLB from KB8-5 cells, suggesting that t he chlorophenoxazines, like VRP, can inhibit P-glycoprotein (P-gp)-med iated efflux of VLB from this cell line. Two of the chlorophenoxazine derivatives, and also VRP, were able to stimulate the vanadate-sensiti ve ATPase activity attributable to P-gp in membranes isolated from MDR 1 baculovirus-infected Sf9 cells. This result suggests that these modu lators exert their effect by directly interacting with P-gp. Apart fro m the parent unsubstituted molecule, 2-chlorophenoxazine, there was a good correlation between log(10)P and the ability of the compounds to enhance VLB accumulation in KB8-5. This suggests that lipophilicity of a modulator is important, but is not the sole determinant of potency. Within this series of compounds, the optimal structural features for MDR modulation include a hydrophobic phenoxazine ring with a -Cl atom in the C-2 position and a tertiary amine group four carbons from the t ricyclic ring. Many of the agents at the IC10 concentration completely reversed the 37-fold VLB resistance in KB8-5 cells. The most active a gents in KB8-5 were able to partially reverse VLB resistance in an MDR colon carcinoma cell line GC(3)/c1 and completely reversed the 86-fol d VLB resistance in the MDR1-overexpressing breast carcinoma cell line BC19/3. These same agents could only partially sensitize BC19/3 cells to taxol and doxorubicin, suggesting that the chlorophenoxazine deriv atives show some specificity for modulating VLB resistance.