Isolation and identification of a C39 demethylated metabolite of rapamycin from pig liver microsomes and evaluation of its immunosuppressive activity.

We studied in vitro metabolism of rapamycin using pig liver microsomes. After extraction of the metabolites from the incubation medium, the crude metabolite extract was submitted to normal and subsequently to reversed-phase HPLC chromatography. We describe in the current study a metabolite of retention time 23.2 min collected from reversed-phase HPLC and identified by fast atom bombardment mass spectrometry (MS) and electrospray MS-MS as a C39 demethylated rapamycin metabolite. In vitro immunosuppressive activity of this metabolite, determined by the mixed lymphocyte reaction, was negligible compared with that of the parent compound. The decrease of in vitro immunosuppressive activity compared with the parent compound is likely to be attributed to important structural modifications of the rapamycin binding region to the FK-506 binding protein.