Cyclosporin A: antiparasite drug, modulator of the host-parasite relationship and immunosuppressant.

Cyclosporin A (CsA), a cyclic undecapeptide with powerful properties of immunosuppression, acts on parasitic infections in laboratory animals in various ways. The outcome of drug administration in vivo varies with timing of treatment relative to infection, route of administration, dose and number of treatments applied. CsA is clearly antiparasitic against malaria, schistosomes, adult tapeworms, metacestodes and filarial nematodes. By contrast, it acts as an immunomodulator against trypanosomes and Giardia, by exacerbating infection; in the case of Leishmania spp. the drug acts variously. In some other infections CsA acts both as an antiparasite drug and as an immunosuppressant (Toxoplasma, avian coccidiosis and gastrointestinal nematodes). This range of activities is reviewed and possible modes of action discussed in the light of emerging data on in vitro drug activity and on putative receptor binding. The potential value of a non-immunosuppressive analogue of CsA in the control of parasitic infections of humans and domestic animals is considered but this paper lays particular stress on the seminal role of CsA as a laboratory tool.