Effect of duration of exposure to verapamil on vincristine activity against multidrug-resistant human leukemic cell lines.
Verapamil sensitizes multidrug-resistant cell lines to various heterocyclic anticancer drugs by inhibition of energy-dependent release of drug, presumably by interaction with membrane glycoproteins involved in drug efflux. This work assessed verapamil sensitization of human multidrug-resistant lymphocytic and myeloid leukemic cell lines (CEM/VLB100, HL-60/AR) to vincristine during exposures of short duration (4 h). When cells were transferred to drug-free medium immediately after simultaneous 4-h exposures to vincristine and verapamil, the antiproliferative activity of vincristine was not altered in CEM/VLB100 cells and was only moderately increased in HL-60/AR cells. In contrast, when cells were transferred to verapamil-containing medium, vincristine activity was greatly increased against both CEM/VLB100 and HL-60/AR cells. Verapamil enhanced accumulation and inhibited release of [3H]vincristine by CEM/VLB100 and HL-60/AR cells, indicating that the sensitization was due to an increase in cell-associated vincristine after transfer of cells to vincristine-free medium. Slot blot analysis of cellular RNA with the pMDR1 probe revealed high levels of expression of the mdr1 gene in CEM/VLB100 cells but no detectable expression in HL-60/AR cells. Consistent with this finding, polypeptides (Mr 170,000 to 180,000) that were recognized by a monoclonal antibody (C219) against P-glycoprotein were greatly overexpressed in CEM/VLB100 cells, but were expressed at low levels, if at all, in HL-60/AR cells. These results demonstrate the importance of duration of exposure to verapamil in reversing multidrug resistance, not only in cells that overexpress P-glycoprotein but also in cells, such as HL-60/AR, that express little, if any, P-glycoprotein.
Cass
C E
CE
McEachern Laboratory, Department of Biochemistry, University of Alberta, Edmonton, Canada.
Janowska-Wieczorek
A
A
Lynch
M A
MA
Sheinin
H
H
Hindenburg
A A
AA
Beck
W T
WT
D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D004351:Drug Resistance; D006801:Humans; D007700:Kinetics; D007945:Leukemia, Lymphoid; D015473:Leukemia, Promyelocytic, Acute; D014407:Tumor Cells, Cultured / Q000166:cytology / Q000187:drug effects*; D014700:Verapamil / Q000494:pharmacology*; D014750:Vincristine / Q000494:pharmacology*
D014750:Vincristine; D014700:Verapamil