RESTORATION OF TAXOL SENSITIVITY OF MULTIDRUG-RESISTANT CELLS BY THE CYCLOSPORINE SDZ PSC-833 AND THE CYCLOPEPTOLIDE SDZ-280-446

Background: Taxol, a promising agent for the treatment of cancer, has entered phase II clinical trials. Nevertheless, it belongs to the class of compounds that show impaired retention in multidrug-resistant cells expressing P-glycoprotein (Pgp), a drug efflux pump. Chemosensitizers like verapamil modulate multidrug resistance by interfering with the efflux action of Pgp and thus can decrease drug resistance or can restore drug sensitivity by restoring normal drug accumulation and distribution within the multidrug-resistant tumor cell. The two strongest, nearly equipotent chemosensitizers identified to date are the cyclosporine derivative SDZ PSC 833 and the semisynthetic cyclopeptolide SDZ 280-446. Purpose: This study was designed to investigate the capacities of verapamil, SDZ PSC 833, and SDZ 280-446 to decrease resistance of two multidrug-resistant cell lines to taxol. Methods: We studied in vitro the growth of two multidrug-resistant tumor cell lines displaying high resistance to taxol: multidrug-resistant Chinese hamster ovary cells and murine monocytic leukemia P388 cells. We determined the taxol concentration that produced 50% inhibition of cell growth (IC50) in the two multidrug-resistant cell lines and in the parent cell lines, in the presence of a range of chemosensitizer concentrations (0-30 muM). IC50 values were determined in the presence and in the absence of verapamil, SDZ PSC 833, or SDZ 280-446. Results: At nontoxic concentrations (0.3-1 muM), SDZ PSC 833 and SDZ 280-446 produced an almost complete reversal of the high taxol resistance of the multidrug-resistant tumor cells, whereas only partial restoration of sensitivity to taxol was achieved with verapamil. Conclusion: SDZ PSC 833 and SDZ 280-446 can restore the normal taxol sensitivity of highly resistant multidrug-resistant tumor cells. Implications: The combination of taxol with SDZ PSC 833 or SDZ 280-446 may be recommended for treatment of multidrug-resistant cancers.