ROLES OF PEPTIDYL-PROLYL CIS-TRANS ISOMERASE AND CALCINEURIN IN THE MECHANISMS OF ANTIMALARIAL ACTION OF CYCLOSPORINE-A, FK506, AND RAPAMYCIN

The immunosuppressive peptide cyclosporin A inhibits the growth of malaria parasites in vitro and in vivo, but little is known about its mechanism of antimalarial action. The immunosuppressive action of cyclosporin A is believed to result from binding of the drug to cyclophilins (intracellular peptidyl-prolyl cis-trans isomerases), and inhibition of the protein phosphatase calcineurin by the cyclosporin A-cyclophilin complex. Two immunosuppressive macrolides, FK506 and rapamycin, bind to a distinct isomerase, FKBP12, and the FK506-FKBP complex also inhibits calcineurin. Calcineurin itself is apparently involved in signal transduction between the T-cell membrane and nucleus, and its inhibition blocks T-cell activation. Rapamycin inhibits a later step in T-cell proliferation. Peptidyl-prolyl cis-trans isomerase activity was detected in extracts of Plasmodium falciparum. It was completely inhibited by concentrations of cyclosporin A above 0.1 mu M, but not by FK506 or rapamycin, and probably represented one or more cyclophilins. Comparison of the antimalarial and anti-isomerase activities of a series of cyclosporin analogues failed to reveal a correlation between the two properties. Cyclosporin A and its more active 8'-oxymethyl-dihydro-derivative, in combination with the cyclophilin-containing P. falciparum extract, inhibited the protein phosphatase activity of bovine calcineurin. Therefore inhibition of a putative P. falciparum calcineurin by a complex of CsA and cyclophilin might be responsible for the antimalarial action of the drug. The most active cyclosporin, however, was a 3'- keto-derivative of cyclosporin D (SDZ PSC-833) which inhibited P. falciparum growth with a 50% inhibitory concentration (IC50) of 0.032 mu M (compared with 0.30 mu M for cyclosporin A), but was a poor inhibitor of the parasite isomerase. 3'-Keto-cyclosporin D has negligible immunosuppressive activity, but it strongly inhibits the P-glycoprotein of multi-drug resistant mammalian tumour cells. FK506 and rapamycin were also active antimalarials (IC50 of 1.9 and 2.6 mu M, respectively) but in the absence of detectable FKBP in P. falciparum extracts, their mechanisms of antimalarial action remain unclear.