CHARACTERIZATION OF ANTITOXOPLASMA ACTIVITY OF SDZ 215-918, A CYCLOSPORINE DERIVATIVE LACKING IMMUNOSUPPRESSIVE AND PEPTIDYL-PROLYL-ISOMERASE-INHIBITING ACTIVITY - POSSIBLE ROLE OF A P-GLYCOPROTEIN IN TOXOPLASMA PHYSIOLOGY

The immunosuppressive agent cyclosporin A (CsA) also possesses broad-s pectrum antimicrobial activity. Previous investigators have reported t hat the obligate intracellular protozoan Toxoplasma gondii is sensitiv e to CsA, We have measured the sensitivity of Toxoplasma to 26 CsA der ivatives that maintain only a subset of the parent compound's activity . We identified one compound, SDZ 215-918, that is a particularly pote nt inhibitor of parasite invasion and replication, with a 50% inhibito ry concentration of 0.45 mu g/ml, which is 10-fold lower than that of CsA, Kinetic studies demonstrate that activity has a rapid onset (half -life, less than or equal to 20 min) and is initially reversible, alth ough long-term exposure (> 24 h) to 5 mu g/ml is lethal; in contrast, this concentration had no effect on host cell protein synthesis or cel l division, SDZ 215-918 acts directly on the parasite, as demonstrated by inhibition of macromolecular synthesis in host-free extracellular parasites, Inhibition of invasion is due to a reduction in parasite mo tility. SDZ 215-918 does not bind to cyclophilins, the ubiquitous cycl osporin-binding proteins, but is a potent inhibitor of the mammalian P glycoprotein, a member of the ATP binding cassette transporter superf amily and the pump responsible for multidrug resistance in cancer and parasite cell lines. SDZ 215-918 blocks the efflux of rhodamine 123 fr om extracellular parasites, consistent with inhibition of a P glycopro tein-like pump, We suggest that a P glycoprotein or a related transpor ter plays a crucial role in the biology of Toxoplasma and may be a nov el target for antiparasitic compounds. Preliminary studies with animal s indicate that SDZ 215-918 inhibits parasite growth in vivo; its rela tionship to CsA may make it suitable fdr clinical development.