Differential effectiveness of a range of novel drug-resistance modulators, relative to verapamil, in influencing vinblastine or teniposide cytotoxicity in human lymphoblastoid CCRF-CEM sublines expressing classic or atypical multidrug resistance.

A series of five potential modulators of resistance were tested for their relative ability, as compared with verapamil, to sensitize CEM lymphoblastoid leukemia drug-resistant tumor sublines expressing either the classic or the atypical multidrug-resistance (MDR) phenotype to vinblastine or teniposide. Maximal non-cytotoxic concentrations of each modulator were tested and sensitization induces (SIs) were derived by comparing the drug concentration required to inhibit growth by 50% in their presence or absence. Like verapamil (10 microM) itself, three of the other modulators tested, namely, S9788 (4 microM), flunarizine (20 microM) and quinidine (30 microM), resulted in 2- to 3-fold sensitization of vinblastine against the parental CEM cells, and comparable effects were noted in the CEM/VM-1 cells, which were not cross-resistant to vinblastine. In contrast, cyclosporin A (0.5 microM) and B859-35 (2 microM) did not enhance vinblastine growth inhibition in these lines. However, the greatest sensitization with all the modulators was noted in the classic MDR VBL1000 cells, with SIs ranging from 40- to 350-fold, except for cyclosporin A, which proved ineffective at the concentration tested (SI, 2.6). The greatest extent of differential sensitization of these VBL1000 tumor cells occurred with quinidine or B859-35, which proved significantly more effective than verapamil alone. Combinations of modulators resulted in additive effects, with B859-35 plus cyclosporin A proving superior to B859-35 plus verapamil. In contrast, none of these compounds proved effective as a sensitizer to teniposide. The growth-inhibitory effects of this drug were not modified significantly in either the 92-fold teniposide-resistant VM-1 cells or in the parental cells. Addition of verapamil itself also failed to modulate teniposide growth inhibition in the VBL1000 cells, which express significant cross-resistance to this drug (36-fold). However, SI values of 3- to 5-fold were obtained using quinidine or B859-35. These results serve (a) to emphasise the need to monitor the effects of modulators not only on drug-resistant cells but also on their drug-sensitive counterparts so as to ensure differential sensitization such that normal sensitive tissues are not likely to be adversely influenced and (b) to highlight the observation that the extent of modulation differs depending not only on the antitumor drug used but also on the mechanism of drug resistance expressed. This in vitro model system appears to provide a useful screening system for resistance modulators and certainly could be used in attempts to identify alternative agents that may influence teniposide sensitivity in these drug-resistant sublines.