title: P-glycoprotein-independent mechanism of resistance to VP-16 in multidrug-resistant tumor cell lines: pharmacokinetic and photoaffinity labeling studies.
abstract: 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.
authors.affiliation: Division of Cancer Treatment, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: P M
authors.initials: PM
authors.lastname: Politi
authors.forename: S T
authors.initials: ST
authors.lastname: Arnold
authors.forename: R L
authors.initials: RL
authors.lastname: Felsted
authors.forename: B K
authors.initials: BK
authors.lastname: Sinha
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000345:Affinity Labels; D000818:Animals; D001386:Azides; D001667:Binding, Competitive; D001943:Breast Neoplasms / Q000188:drug therapy*; D006224:Cricetinae; D003412:Cricetulus; D003609:Dactinomycin / Q000494:pharmacology; D004317:Doxorubicin / Q000494:pharmacology; D004351:Drug Resistance; D005047:Etoposide / Q000493:pharmacokinetics*; D006801:Humans; D008562:Membrane Glycoproteins / Q000493:pharmacokinetics*; D009363:Neoplasm Proteins / Q000493:pharmacokinetics*; D014407:Tumor Cells, Cultured; D014700:Verapamil / Q000493:pharmacokinetics; D014747:Vinblastine / Q000493:pharmacokinetics; D014751:Vindesine / Q000031:analogs & derivatives
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000345:Affinity Labels; D001386:Azides; D008562:Membrane Glycoproteins; D009363:Neoplasm Proteins; C048695:N-(4-azido-3-iodosalicyl)-N'-beta-aminoethylvindesine; D003609:Dactinomycin; D014747:Vinblastine; D005047:Etoposide; D004317:Doxorubicin; D014700:Verapamil; D014751:Vindesine