SDZ PSC 833, the cyclosporine A analogue and multidrug resistance modulator, activates ceramide synthesis and increases vinblastine sensitivity in drug-sensitive and drug-resistant cancer cells

Resistance to chemotherapy is the major cause of cancer treatment failure. Insight into the mechanism of action of agents that modulate multidrug resi stance (MDR) is instrumental for the design of more effective treatment mod alities. Here we show, using KB-V-1 MDR human epidermoid carcinoma cells an d [H-3]palmitic acid as metabolic tracer, that the MDR modulator SDZ PSC 83 3 (PSC 833) activates ceramide synthesis. In a short time course experiment , ceramide was generated as early as 15 min (40% increase) after the additi on of PSC 833 (5.0 mu M), and by 3 h, [H-3]ceramide was >3-fold that of con trol cells. A 24-h dose-response experiment showed that at 1.0 and 10 mu M PSC 833, ceramide levels were 2.5- and 13.6-fold higher, respectively, than in untreated cells. Concomitant with the increase in cellular ceramide was a progressive decrease in cell survival, suggesting that ceramide elicited a cytotoxic response. Analysis of DNA in cells treated with PSC 833 showed oligonucleosomal DNA fragmentation, characteristic of apoptosis. The inclu sion of fumonisin B-1, a ceramide synthase inhibitor, blocked PSC 833-induc ed ceramide generation. Assessment of ceramide mass by TLC lipid charring c onfirmed that PSC 833 markedly enhanced ceramide synthesis, not only in KB- V-1 cells but also in wild-type KB-3-1 cells. The capacity of PSC 833 to re verse drug resistance was demonstrated with vinblastine. Whereas each agent at a concentration of 1.0 mu M reduced cell survival by similar to 20%, wh en PSC 833 and vinblastine were coadministered, cell viability fell to zero . In parallel experiments measuring ceramide metabolism it was shown that t he PSC 833/vinblastine combination synergistically increased cellular ceram ide levels. Vinblastine toxicity, also intensified by PSC 833 in wild-type KB-3-1 cells, was as well accompanied by enhanced ceramide formation. These data demonstrate that PSC 833 has mechanisms of action in addition to P-gl ycoprotein chemotherapy efflux pumping.