The in vitro activity, radioimmunoassay cross-reactivity, and molecular weight of thirteen rabbit cyclosporine metabolites.

Thirteen metabolites of cyclosporine were isolated from the bile of rabbits receiving intravenous cyclosporine. The molecular weights of these metabolites were determined by fast atom bombardment mass spectrometry. These molecular weights were consistent with hydroxylated, N-demethylated, and carboxylated metabolites of cyclosporine as described previously. The in vitro activities of the metabolites were established using mitogen-stimulated lymphocyte proliferation assays. Only the two monohydroxylated metabolites were found to have significant activity, this being between 5 and 10% of that of the parent drug. The metabolites were also compared with cyclosporine in two commercial radioimmunoassay kits. The metabolites were found to cross-react with the parent drug in amounts ranging from 20 to 100%, with the least polar metabolites cross-reacting the most strongly. It is concluded that the cross-reacting metabolites measured by the presently available radioimmunoassays for cyclosporine probably do not represent significant additional immunosuppressive activity in vivo.