The antimalarial action of FK506 and rapamycin: evidence for a direct effect on FK506-binding protein PfFKBP35

FK506 and rapamycin (Rap) are immunosuppressive drugs that act principally on T-lymphocytes. The receptors for both drugs are FK506-binding proteins (FKBPs), but the molecular mechanisms of immunosuppression differ. An FK506-FKBP complex inhibits the protein phosphatase calcineurin, blocking a key step in T-cell activation, while the Rap -FKBP complex binds to the protein kinase target of rapamycin (TOR), which is involved in a subsequent signalling pathway. Both drugs, and certain non-immunosuppressive compounds related to FK506, have potent antimalarial activity. There is however conflicting evidence on the involvement of Plasmodium calcineurin in the action of FK506, and the parasite lacks an apparent TOR homologue. We therefore set out to establish whether inhibition of the Plasmodium falciparum FKBP PfFKBP35 itself might be responsible for the antimalarial effects of FK506 and Rap. Similarities in the antiparasitic actions of FK506 and Rap would constitute indirect evidence for this hypothesis. FK506 and Rap acted indistinguishably on: (i) specificity for different intra-erythrocytic stages in culture, (ii) kinetics of killing or irreversible growth arrest of parasites and (iii) interactions with other antimalarial agents. Furthermore, PfFKBP35's inhibitory effect on calcineurin was independent of FK506 under a range of conditions, suggesting that calcineurin is unlikely to be involved in the antimalarial action of FK506.