Human multi‐drug‐resistant cancer cells exhibit a high degree of selectivity for stereoisomers of verapamil and quinidine

Abstract

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 RMI_0.1, 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 RMI_0.1 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.