METABOLISM OF THE NEW IMMUNOSUPPRESSOR CYCLOSPORINE-G BY HUMAN LIVER CYTOCHROMES P450

Cyclosporin G is a new immunosuppressor structurally similar to cyclos porin A. Although this drug is pharmacologically as active as cyclospo rin A, it is less toxic, in particular at the kidney level. The aim of this work was to identify the enzyme system(s) involved in the oxidat ive metabolism of cyclosporin G in man: (1) in a bank of human liver m icrosomes (n = 22), cyclosporin G oxidase activity correlated signific antly with cyclosporin A oxidase activity (P < 0.0001) and with the le vel of CYP3A4 (P < 0.002), determined by immunoblot; (2) specific inhi bitors of CYP3A4, troleandomycin, and ketoconazole, inhibited cyclospo rin G oxidase activity by more than 80%; (3) antiCYP3A4 antibodies spe cifically inhibited this activity by nearly 90%; (4) cyclosporin A was a competitive inhibitor of cyclosporin G oxidase and vice versa; (5). Among a battery of cDNA-expressed CYPs, only CYP3A4 was able to gener ate detectable amounts of metabolites of cyclosporin G and cyclosporin A with a turnover number close to that calculated from experiments wi th liver microsomes; (6) in human hepatocytes in culture, pretreatment of cells with rifampicin and phenobarbital, 2 inducers of CYP3A4, pro duced a great increase in cyclosporin G oxidase activity, while P-naph thoflavone, an inducer of CYP1As, did not. We conclude that CYP3A4 is the major enzyme involved in the oxidative metabolism of cyclosporin G in human liver.