Isolation, identification, and biological activities of oxidative metabolites of FK506, a potent immunosuppressive macrolide lactone.

To characterize structures and biological activities of FK506 metabolites, FK506 was incubated with liver microsomes prepared from phenobarbital-treated rats in the presence of NADPH generating system under aerobic condition. Oxidative metabolites formed in the reaction medium were isolated and identified. Purified samples were analyzed by HPLC, mass spectrometry, and NMR spectroscopy. M-I, M-II, and M-III were the O-demethylated metabolites at the 13-, 31-, and 15-positions of FK506, respectively, and M-IV was the monohydroxylated metabolite at the 12-position. M-I was the dominant metabolite in this reaction system. M-II and M-III retained the tetrahydropyrane ring in their structures like FK506, but M-I and M-IV had rearranged structures in which the tetrahydropyrane ring was changed to a tetrahydrofuran ring. Measuring the immunosuppressive activity in the mouse mixed lymphocyte reaction system, IC50 values for M-I, M-II, M-III, M-IV, and FK506 were 1.65, 0.23, > 127, 5.52, and 0.15 nM, respectively. Reactivity of the metabolites with mouse anti-FK506 monoclonal antibody was studied and immunocross-reactivity of M-I, M-II, M-III, and M-IV with the antibody were nil, 109.0, 90.5, and 8.8% of FK506, respectively. These results indicate that rat hepatic microsomes oxidatively metabolize FK506 to four metabolites, and some of them exhibit pharmacological activity.