Pharmacological modification of multi‐drug resistance (mdr) in vitro detected by a novel fluorometric microculture cytotoxicity assay. Reversal of resistance and selective cytotoxic actions of cyclosporin a and verapamil on mdr leukemia t‐cells

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Abstract
A novel fluorometric microculture cytotoxicity assay (FMCA), based on measurements of fluorescein diacetate (FDA) hydrolysis and DNA staining by Hoechst 33342, was used for drug sensitivity testing and detection of resistance reversal in acute lymphoblastic leukemia (ALL) cell lines. The 72‐hr assay was found to be sensitive, reproducible and linearly related to the number of viable cells within a broad range of cell concentrations. At clinically achievable drug concentrations, the calcium channel blocker Verapamil (ver) and the immunosuppressant Cyclosporin A (csA) were found to partly reverse acquired Vincristine (vcr) resistance in multidrug resistant (MDR) T‐ALL LI00 cells with little or no effect on the drug‐sensitive parental LO cell line. By combining the fluorometric indices, we found that low concentrations of csA were growth‐inhibitory, whereas higher concentrations (>IOμg/ml) were progressively cytotoxic for drug‐sensitive LO cells. In MDR LlOO cells, on the other hand, csA produced significant cell kill even at low drug concentrations. Ver had no effects on sensitive LO cells but showed considerable cytotoxic action towards MDR LIOO cells. There was no apparent relationship between drug reversal of vcr resistance and the cytotoxic actions of the drug per se since the calcium channel blocker diltiazem (dil) significantly potentiated the actions of vcr on MDR LIOO cells without being more toxic to these cells (compared to vcr‐sensitive LO cells).