15-Desmethyl FK-506 and 15,31-desmethyl FK-506 from human liver microsomes: isolation, identification (by fast atom bombardment mass spectrometry and NMR), and evaluation of in vitro immunosuppressive activity.

15-Desmethyl FK-506 and 15,31-desmethyl FK-506 metabolites extracted from incubated human liver microsomes were separated by normal and reversed-phase HPLC. The metabolites were identified by fast atom bombardment/mass spectrometry and nuclear magnetic resonance spectroscopy. The in vitro 50% inhibitory concentration, tested against bidirectional mixed lymphocyte reactions, was 325 mg/L for 15,31-desmethyl FK-506 and 4 mg/L for 15-desmethyl FK-506, indicating that these products of FK-506 demethylation retain in vitro immunosuppressive activity.