P-glycoprotein-independent mechanism of resistance to VP-16 in multidrug-resistant tumor cell lines: pharmacokinetic and photoaffinity labeling studies.

The interaction of etoposide (VP-16), Vinca alkaloids, and verapamil with the P-glycoprotein (P-gp) was studied in human breast (MCF-7) and Chinese hamster lung (DC3F) cell lines and the corresponding multidrug-resistant MCF-7/ADR and DC3F/ADX tumor cell lines, selected for resistance to Adriamycin and actinomycin D, respectively, and overexpressing P-gp. Verapamil (10 microM) markedly reversed resistance to vincristine (11-fold in DC3F/ADX and 125-fold in MCF-7/ADR; 1-hr exposure), but it had a very modest effect on resistance to VP-16 (3- to 4-fold; 1-hr exposure). Resistant cells accumulated 2- to 4-fold less VP-16 and vincristine than the parental cell lines. Verapamil (10 microM) significantly increased accumulation and retention of vincristine, but not of VP-16, in resistant cell lines. Photoaffinity labeling of resistant cell lines with radioactive analogs of verapamil [N(p-azido-3-125I-salicyl)-N'-beta-aminoethylverapamil (NASVP)] and vinblastine[N-(p-azido-3-125I-salicyl)-N'-beta-aminoethylvindesine (NASV)] showed distinctly labeled P-gp bands in both resistant cell lines, compared with wild-type cells. Excess nonradioactive vinblastine or verapamil effectively competed with the P-gp photolabeling by either NASVP or NASV, with IC50 levels of 0.6 and 10 microM, respectively. In contrast, nonradioactive VP-16 was 100- to 500-fold less potent than vinblastine in competing with P-gp photolabeling, suggesting that VP-16 has significantly lower affinity for P-gp than Vinca alkaloids have. Taken together, our data indicate that P-gp glycoprotein by itself may not be important in the transport/efflux of VP-16 and, thus, in the mechanism of resistance to VP-16 in these cells.