Cellular and Biochemical Characterization of Thioxanthenes for Reversal of Multidrug Resistance in Human and Murine Cell Lines1

We have previously shown that phenothiazines sensitize multidrug resistant (MDR) cells to chemotherapeutic drugs in a manner related to specific structural features, and have identified structurally related thioxanthenes with increased anti-MDR activity. We have now studied the structure-activity relationships of 16 thioxanthenes in the human breast cancer line MCF-7 Adr". rra/is-Thioxanthene stereoisomers were 2to 7-fold more potent than cu-thioxanthenes for antagonizing MDR. The most potent thioxanthenes possessed a halogenated tricyclic ring con nected by a 3-carbon alkyl bridge to a piperazinyl or piperadinyl side group. The chemosensitizing effects of the lead compound, rra/u-flupenthixol, its stereoisomer cis-flupenthixol, its phenothiazine homologue fluphenazine, and the calcium channel blocker verapamil, were further studied in a series of sensitive and MDR cell lines. rrans-Flupenthixol caused a greater reversal of cellular resistance to doxorubicin, vinblastine, vincristine, and colchicine in MCF-7 Adr", KB-V1, and P388/DOX MDR cells than the other chemosensitizers. In particular, frani-flupenthixol was 2to 3-fold more potent for reversing MDR than equimolar concen trations of verapamil. Furthermore, fra/u-flupenthixol fully reversed resistance to doxorubicin, vincristine, and colchicine in MDR MCF-7 and NIH 3T3 cells transfected with the md>\ gene. None of these agents altered MDR in a non-P-glycoprotein expressing MCF-7 cell line selected with mitoxantrone, nor in any of the parental cell lines. The stereoselective antagonism of the flupenthixol isomers on several putative cellular targets was studied to explore the mechanism of their chemosensitizing activity. 1-/.Vand fra/H-flupenthixol were equally active inhibitors of protein kinase C and calmodulin. Both dsand fra/u-flupenthixol were also potent inhibitors of |'H|a/idopine binding to P-glycoprotein. The apparent lack of clinical toxicity of rra/u-flupenthixol makes it an attrac tive drug for possible use in the modulation of tumor resistance in vivo if appropriate tissue concentrations can be achieved.