Identification of glucoside and carboxyl-linked glucuronide conjugates of mycophenolic acid in plasma of transplant recipients treated with mycophenolate mofetil

1 Mycophenolic acid (MPA), is primarily metabolized in the liver to 7-O-MPA -beta-glucuronide (MPAG). Using RP-h.p.l.c. we observed three further MPA m etabolites, M-1, M-2, M-3, in plasma of transplant recipients on MMF therap y. To obtain information on the structure and source of these metabolites: (A) h.p.l.c. fractions containing either metabolite or MPA were collected a nd analysed by tandem mass spectrometry; (B) the metabolism of MPA was stud ied in human liver microsomes in the presence of UDP-glucuronic acid, UDP-g lucose or NADPH; (C) hydrolysis of metabolites was investigated using beta- glucosidase, beta-glucuronidase or NaOH; (D) cross-reactivity of each metab olite was tested in an immunoassay for MPA (EMIT). 2 Mass spectrometry of M-1, M-2, MPA and MPAG in the negative ion mode reve aled molecular ions of m/z 481, m/z 495, m/z 319 and m/z 495 respectively. 3 Incubation of microsomes with MPA and UDP-glucose produced M-1, with MPA and UDP-glucuronic acid MPAG and M-2 were formed, while with MPA and NADPH, M3 was observed. 4 beta-Glucosidase hydrolysed M-1 completely. beta-Glucuronidase treatment led to a complete disappearance of MPAG whereas the amount of M-2 was reduc ed by approximately 30%. Only M-2 was labile to alkaline treatment. 5 M-2 and MPA but not M-1 and MPAG cross-reacted in the EMIT assay. 6 These results suggest that: (i) M-1 is the 7-OH glucose conjugate of MPA; (ii) M-2 is the acyl glucuronide conjugate of MPA; (iii) M-3 is derived fr om the hepatic CYP450 system.