Abstract

Previously, a series of 21 N10-substituted phenoxazines were examined for reversing vinca alkaloid resistance against MDR KBChR-8-5 and GC3/cl cells. Within the series, there are compounds that inhibit efflux (verapamil-like activity), whereas others markedly increased vinca alkaloid accumulation without having detectable inhibitory activity of the efflux component. It has been shown that MDR modulators that inhibit photoaffinity labeling of P-gp were generally the most potent MDR reversers. To show whether this observation is true, P-gp rich membrane fractions from KB-V1 cells were isolated and the interaction of [3H] azidopine with membrane fractions in the presence of 25, 50 and 100 mM concentration of each of the twenty N10 -substituted phenoxazines was undertaken and the extent of competition was compared to a standard modulator, verapamil. Examination of the competition data showed that only two modulators 4 and 6 exhibited the maximum competition (>50%) and the remaining modulators were found to exhibit the inhibition of the photolabeling by less than 45%. However, modulators 12, 14 and 19 failed to compete for azidopine 1abeling. Within the series of compounds examined, the competition of phenoxazines for [3H] azidopine binding to P-gp follows the order: propyl>butyl>acetyl series. It has been found that, from among the compounds examined, three of them interact strongly (>50%), six marginally (<45%) and remaining failed to interact with P-gp, indicating that there may be multiple mechanisms for MDR.