ISOLATION AND IDENTIFICATION OF A C-39 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 r eversed-phase HPLC chromatography. We describe in the current study a metabolite of retention time 23.2 min collected from reversed-phase HP LC and identified by fast atom bombardment mass spectrometry (MS) and electrospray MS-MS as a C-39 demethylated rapamycin metabolite. In vit ro immunosuppressive activity of this metabolite, determined by the mi xed lymphocyte reaction, was negligible compared with that of the pare nt compound. The decrease of in vitro immunosuppressive activity compa red with the parent compound is likely to be attributed to important s tructural modifications of the rapamycin binding region to the FK-506 binding protein.