Comparison of cyclosporin A and SDZ PSC833 as multidrug-resistance modulators in a daunorubicin-resistant Ehrlich ascites tumor.

Recent studies by Boesch et al. have demonstrated that a nonimmunosuppressive cyclosporin analog, SDZ PSC 833 (an analog of cyclosporin D), is an active multidrug-resistance modifier that is at least 10 times more potent than cyclosporin A. In vitro accumulation and cytotoxicity experiments using daunorubicin (DNR) and vincristine (VCR) under the influence of SDZ PSC 833 and cyclosporin A were performed in wild-type (EHR2) and the corresponding highly DNR-resistant (about 80-fold) Ehrlich ascites tumor cells (EHR2/DNR+). In accumulation experiments, both SDZ PSC 833 and cyclosporin A were found to reverse the multidrug-resistant (MDR) phenotype, but to the same degree at equimolar concentrations. Thus, in EHR2/DNR+ cells, both cyclosporins at 5 micrograms/ml enhanced DNR and VCR accumulation to sensitive levels, but only a negligible effect on DNR accumulation in the drug-sensitive cells was seen. In the clonogenic assay, the cytotoxicity of the two modulators was equal. The lethal dose for 50% of the cell population (LD50) was approx. 7 micrograms/ml for both compounds, and no toxicity was observed at concentrations below 2 micrograms/ml. At nontoxic doses, both cyclosporins effectively increased the cytotoxicity of DNR and VCR in a concentration-dependent manner. The dose-response curves were nearly identical and did not demonstrate differences in modulator potency. These data permit the conclusion that cyclosporin A and SDZ PSC 833 do raise the intracellular accumulation of DNR and VCR to the same levels and that SDZ PSC 833 does not potentiate cytotoxicity better than cyclosporin A in EHR2/DNR+ cells. However, since the new compound is nonimmunosuppressive and causes less organ toxicity, clinical studies of its MDR modulating effect seem highly relevant.