title: Effect of verapamil on the uptake and efflux of etoposide (VP16) in both sensitive and resistant cancer cells.
abstract: The effect of calcium antagonist verapamil on the uptake and efflux of Etoposide (VP16), a semi-synthetic derivative of podophylotoxin and a broad spectrum antineoplastic agent, has been investigated and compared in sensitive (UM-UC-2) and resistant (UM-UC-9) human bladder cancer cells, and L1210 leukemia cells, by using both radioisotope (3[H]-VP16) liquid scintillation and high performance liquid chromatography assay with electrochemical detection. The uptake of VP16 was rapid in all three cell lines, showing an initial rapid linear phase followed by a second slower phase, but at steady state the ratios of intracellular to extracellular VP16 concentrations were only 0.004-0.006. No significant difference in drug uptake was observed in sensitive UM-UC-2 and resistant UM-UC-9 cells at all concentrations studied. Verapamil at a concentration of 10 microM enhanced the intracellular VP-16 levels in all sensitive and resistant cell lines. The increments were 21.5% for UM-UC-2, 11.8% for UM-UC-9, and 31.0% for L1210 cells after 30 minutes incubation with 1 microM VP16. A slower efflux of VP16 was observed in verapamil treated cells in all three cell lines. There was a small increase in the nonexchangeable components in verapamil treated cells, although only 5-10% of VP16 was retained. No peak other than that of VP16 was detected in the HPLC chromatogram of extracts from both cell pellet and influx or efflux medium.
authors.affiliation: College of Pharmacy, University of Michigan, Ann Arbor 48104.
authors.forename: M R
authors.initials: MR
authors.lastname: Feng
authors.forename: M
authors.initials: M
authors.lastname: Liebert
authors.forename: G
authors.initials: G
authors.lastname: Wedemeyer
authors.forename: H B
authors.initials: HB
authors.lastname: Grossman
authors.forename: W R
authors.initials: WR
authors.lastname: Mancini
authors.forename: M
authors.initials: M
authors.lastname: Williams
authors.forename: J G
authors.initials: JG
authors.lastname: Wagner
mesh_terms: D000818:Animals; D002295:Carcinoma, Transitional Cell / Q000378:metabolism* / Q000473:pathology; D003470:Culture Media; D004351:Drug Resistance; D005047:Etoposide / Q000493:pharmacokinetics*; D005947:Glucose / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D007939:Leukemia L1210 / Q000378:metabolism*; D051379:Mice; D014407:Tumor Cells, Cultured; D001749:Urinary Bladder Neoplasms / Q000378:metabolism* / Q000473:pathology; D014700:Verapamil / Q000494:pharmacology*
chemical_list: D003470:Culture Media; D005047:Etoposide; D014700:Verapamil; D005947:Glucose