Metabolism of cyclosporin G in the mouse, rat, and dog

Cyclosporin G (CsG; Sandoz compound OG 37-325) is a cyclic undecapeptide with potent, immunosuppressive activity and is currently in clinical testing for prevention of transplanted solid organ rejection. Although structurally similar to cyclosporin A (CsA), results in animals suggest that CsG has a reduced potential for nephrotoxicity when compared with CsA, while retaining equivalent therapeutic efficacy. In the present study, the major metabolic pathways of CsG in the mouse, rat, and dog were investigated using radiolabeled drug substance to determine if interspecies differences in metabolism exist. The results indicated that the major metabolic pathways in these animal species are similar to those previously reported for CsA, including oxidative modifications at amino acids 1, 4, and 9, and concomitant cyclization of amino acid 1 in two of these metabolites. Moreover, the seven major CsG metabolites (designated GM19, GM1c9, GM4N9, GM1, GM9, GM1c, and GM4N) observed in animal excreta and/or blood were identical to those identified in humans. The major circulating metabolite in blood was GM9 (9-hydroxylated CsG) in all species. In addition, numerous unidentified minor metabolites were observed. Renal excretion was a minor elimination pathway, with the majority of drug-related material excreted via the fecal route. In conclusion, CsG was found to proceed through the same metabolic pathways in three animal species and humans, and that species differences in metabolism were primarily because of differences in the relative importance of the pathways observed.