REVERSAL AGENT INHIBITION OF THE MULTIDRUG-RESISTANCE PUMP IN HUMAN LEUKEMIC LYMPHOBLASTS

Multidrug resistant cancer cells of the MDR-1 phenotype utilize an ATP -dependent pump to excrete toxic drugs. Rhodamine 123 (R123) is a fluo rescent substrate of the MDR pump. An assay for the ATP-dependent init ial efflux of R123 from CEM/VLB(100) human leukemic lymphoblasts has b een developed. The MDR-1 cells were treated with a reversal agent and preloaded with 40.0 nM R123 in buffer at 30 degrees C that contained s odium azide and 2-deoxyglucose. The cells were rinsed with cold buffer and resuspended in L-glutamine/glucose solution at 23 degrees C. The cell suspension was passed through a filter and R123 in the filtrate w as detected at 2-s intervals by fluorescence. Efflux of R123 was inhib ited by the reversal agents amiodarone, cyclosporin A, Roll-2933 (DMDP ), quinidine, and the optical isomers of propranolol. The MDR pump is stereospecific for the (R)-diastereomer quinidine; however, the (S)-di astereomer quinine is a relatively weak inhibitor of the pump. Cyclosp orin A was the most potent inhibitor tested against the efflux of R123 by the MDR pump.