ISOLATION AND IDENTIFICATION OF NEW RAPAMYCIN DIHYDRODIOL METABOLITESFROM DEXAMETHASONE-INDUCED RAT-LIVER MICROSOMES

1. Rapamycin is metabolically transformed in rat liver microsomes to 3 ,4- and 5,6-dihydrodiol metabolites under the influence of the cytochr ome P-150 mixed function oxygenase system. These metabolites were prod uced from dexamethasone-induced as well as from non-induced rat liver microsomes. The comparison of the :on spray mass spectra of the 5,6-di hydrodiol with the 3,4-dihydrodiol of rapamycin shows clearly that dih ydrodiols were formed in two distinct positions of rapamycin. 2. FAB m ass spectra as well as electrospray mass spectra of two additional pea ks isolated from the same chromatographic run confirm the presence of a 3,4-dihydrodiol metabolite of rapamycin as also strongly suggested b y UV spectra. Hplc reinjection of each individual peak always resulted in chromatograms showing a combination of the same three peaks and th erefore are to be considered as tautomers of the 3,4-dihydrodiol of ra pamycin. 3. These tautomeric conformations were found to have no immun osuppressive potency, most probably due to important structural and st ereochemical modifications of the rapamycin binding domain to the bind ing protein (FKBP-12) and/or to important metabolic structural modific ations of rapamycin effector domain.