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

Mycophenolic acid (MPA), is primarily metabolized in the liver to 7‐O‐MPA‐β‐glucuronide (MPAG). Using RP‐h.p.l.c. we observed three further MPA metabolites, M‐1, M‐2, M‐3, in plasma of transplant recipients on MMF therapy. To obtain information on the structure and source of these metabolites: (A) h.p.l.c. fractions containing either metabolite or MPA were collected and analysed by tandem mass spectrometry; (B) the metabolism of MPA was studied in human liver microsomes in the presence of UDP‐glucuronic acid, UDP‐glucose or NADPH; (C) hydrolysis of metabolites was investigated using β‐glucosidase, β‐glucuronidase or NaOH; (D) cross‐reactivity of each metabolite was tested in an immunoassay for MPA (EMIT). Mass spectrometry of M‐1, M‐2, MPA and MPAG in the negative ion mode revealed molecular ions of m/z 481, m/z 495, m/z 319 and m/z 495 respectively. 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, M‐3 was observed. β‐Glucosidase hydrolysed M‐1 completely. β‐Glucuronidase treatment led to a complete disappearance of MPAG whereas the amount of M‐2 was reduced by approximately 30%. Only M‐2 was labile to alkaline treatment. M‐2 and MPA but not M‐1 and MPAG cross‐reacted in the EMIT assay. 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 from the hepatic CYP450 system. British Journal of Pharmacology (1999) 126, 1075–1082; doi:10.1038/sj.bjp.0702399