Identification of a new metabolite of macrolide immunosuppressant, like rapamycin and SDZ RAD, using high performance liquid chromatography and electrospray tandem mass spectrometry

Previously unknown metabolites from the two macrolide immunosuppressants ra pamycin (sirolimus) and SDZ RAD [40-O-(2-hydroxyethyl)rapamycin] obtained a fter in vitro incubation with human Liver microsomes have been purified. St ructure elucidation was performed by nanoelectrospray ionization tandem mas s spectrometry applying low energy collision activated dissociation. This i onization method is, as shown here, a powerful tool to determine metabolic pathways by analysis of even low abundance products. Product ion spectra of the isolated metabolites indicate a new kind of biotransformation reaction for rapamycin and SDZ RAD. The proposed metabolic pathway starts with an e ster hydrolysis which leads to a ring-opened structure. A dehydration on C3 3-C34 and a supplementary hydrogenation at C33-C34 result in a structure si milar to the ring-opened isomer with an single bond at C33-C34. (J Am Soc M ass Spectrom 2000, 11, 516-525) (C) 2000 American Society for Mass Spectrom etry.