Immunosuppressors and reversion of multidrug-resistance.

Drug resistance is the major reason for failure of cancer therapy. When one drug elicits a response in tumour cells resulting in resistance to a large variety of chemically unrelated drugs, this is called multidrug-resistance (MDR). ATP-binding cassette (ABC) transporters contribute to drug resistance via ATP-dependent drug efflux. P-glycoprotein (Pgp) encoded by MDR1 gene, confers resistance to certain anticancer agents. The development of agents able to modulate MDR mediated by Pgp and ABC transporters remained a major goal for the past 10 years. Immunosuppressors, cyclosporin A (CSA) in particular, were shown to modulate Pgp activity in laboratory models and entered very early into clinical trials for reversal of MDR. The proof of reversing activity of CSA was found in phase II studies with myeloma and acute leukaemia. In phase III studies, the results were less convincing regarding the response rate, progression-free survival and overall survival were detected in advanced refractory myeloma. The non-immunosuppressive derivative PSC833 was then extensively studied. This compound shows 10-fold higher potency in reversal of MDR mediated by Pgp. Results from clinical trials with this modulator are still emerging and the notable finding was the need to reduce the dose of anticancer agent used in combination with it. Other effects of CSA and PSC833 on MDR have been described. These two molecules have been shown to have an action on the metabolism of ceramide which stands as second messenger of anticancer agents-induced apoptosis. PSC833 stimulates de novo ceramide synthesis and enhances cell death induced by anticancer agents, such as camptothecins and anthracyclines. In addition, ceramide glycosylation and storage in some cell lines have been described to play a crucial role in resistance to anticancer drugs. CSA is able to inhibit ceramide glucosylation and modulate MDR phenotype. The emergence of other modulators with several ABC protein targets like VX710 are of clinical interest in malignancies expressing several efflux pumps.