HUMAN MULTI-DRUG-RESISTANT CANCER-CELLS EXHIBIT A HIGH DEGREE OF SELECTIVITY FOR STEREOISOMERS OF VERAPAMIL AND QUINIDINE

An in vitro cell proliferation assay was developed to measure the capacity of substances to overcome multi‐drug resistance (MDR). The assay is a modification of the MTT (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide) method. The inclusion of cell titration curves for each concentration of the resistance modifier (RM) allows the IC50 of the RM to be calculated and provides empirical correction of the cell survival curves for the effect of the RM when it is combined with a standard cytotoxic drug, vincristine. The resistance modification index (RMI) is defined as the ratio of the IC50 of vincristine obtained in control cultures divided by that measured in the presence of RM and is linearly related to the dose of RM. The RMI0.1, the RMI at a one‐tenth the IC50 of the RM, provides a relative comparison between the activities of different RMs at non‐toxic doses. The results obtained using the MDR cell line, KB‐8‐5, show that I‐(‐)‐verapamil is approximately 4 times more active than d‐(+)‐verapamil in modifying MDR. The racemic mixture has an intermediate activity. A similar comparison between the epimers quinidine and quinine shows that, at equimolar doses, quinine has a higher RMI but, because it is more toxic, the RMI0.1 is about one‐half of that of quinidine. These results demonstrate the importance of comparing the resistance‐modifying activities of different compounds at doses relative to their own toxicity. Copyright © 1990 Wiley‐Liss, Inc., A Wiley Company