title: Relationship of VP-16 to the classical multidrug resistance phenotype.
abstract: The classical multidrug resistance (MDR) phenotype is characterized by cross-resistance between a number of chemically unrelated drugs due to an increased efflux across the plasma membrane via a P-glycoprotein-mediated mechanism. The epipodophyllotoxin derivatives etoposide (VP-16) and teniposide (VM-26) are usually included among the drugs recognized by this MDR phenotype, and the MDR EHR2/DNR cell line is greater than 50-fold cross-resistant to VP-16. The steady-state accumulation of VP-16 in EHR2/DNR cells is only half that of wild-type EHR2 cells, and deprivation of energy by sodium azide surprisingly increased accumulation to a similar extent in both sublines. Efflux was rapid (halflife of 32-35 s) and similar in both sublines, while initial influx was markedly lower in the resistant cells. The temperature coefficients over 10 degrees C for VP-16 in- and efflux indicated passive transport in both sublines. In agreement with this finding, up to 10-fold molar excess (50 microM) VM-26 had no effect on VP-16 accumulation in MDR cells. VP-16 at a 100-fold molar excess inhibited azidopine photoaffinity labeling of P-glycoprotein by only 30% and vincristine binding to plasma membrane vesicles from EHR/DNR cells by 45%. However, VP-16 itself did not differentially bind to plasma membrane vesicles from EHR2 and EHR2/DNR cells. Finally, neither VP-16 accumulation nor cytotoxicity in EHR2/DNR cells were increased to the same degree as for daunorubicin and vincristine by verapamil, and the modulation was similar in wild-type and resistant cells. Thus, although VP-16 may be a substrate for P-glycoprotein, its other transport characteristics such as rapid diffusion and sensitivity to membrane perturbation in wild-type cells lessen any effect of P-glycoprotein-mediated efflux, resulting in a lack of differential modulation by verapamil. These results may be considered when planning clinical trials involving MDR modulators and epipodophyllotoxin derivatives.
authors.affiliation: Department of Pathology, Sundby Hospital, Copenhagen, Denmark.
authors.forename: M
authors.initials: M
authors.lastname: Sehested
authors.forename: E
authors.initials: E
authors.lastname: Friche
authors.forename: P B
authors.initials: PB
authors.lastname: Jensen
authors.forename: E J
authors.initials: EJ
authors.lastname: Demant
mesh_terms: D000345:Affinity Labels; D000818:Animals; D002250:Carbon Radioisotopes; D002286:Carcinoma, Ehrlich Tumor / Q000188:drug therapy / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism; D003630:Daunorubicin / Q000494:pharmacology; D004351:Drug Resistance / Q000235:genetics*; D005047:Etoposide / Q000378:metabolism / Q000493:pharmacokinetics* / Q000494:pharmacology; D010641:Phenotype; D011136:Polysorbates / Q000494:pharmacology; D014407:Tumor Cells, Cultured; D014700:Verapamil / Q000494:pharmacology
chemical_list: D000345:Affinity Labels; D002250:Carbon Radioisotopes; D011136:Polysorbates; D005047:Etoposide; D014700:Verapamil; D003630:Daunorubicin