Identification of a Pharmacologically Active Metabolite of Mycophenolic Acid in Plasma of Transplant Recipients Treated with Mycophenolate Mofetil

Abstract

Mycophenolic acid (MPA), the active moiety of mycophenolate mofetil (MMF), is an antiproliferative agent that acts by inhibition of inosine monophosphate dehydrogenase type II (IMPDH-II), a key enzyme in the de novo purine biosynthetic pathway (1)(2). Several studies have documented that MMF is effective in the treatment of refractory rejection in renal, heart, and liver transplant recipients (2). The major pathway for elimination of MPA involves glucuronidation (3) at the phenolic hydroxyl group to form mycophenolate 7- O -glucuronide (7-O-MPAG). Modification of this phenolic hydroxy residue leads to a loss of pharmacological activity toward IMPDH-II (4)(5). Most studies on the pharmacokinetics of MPA have utilized HPLC procedures (6)(7) to measure both MPA and MPAG. Recently, the first immunoassay became available for the quantification of MPA (Emit-MPA, Dade Behring). 7-O- MPAG does not cross-react with this assay. Comparison of plasma MPA concentrations from clinical samples determined with HPLC showed an overestimation in relation to those obtained with the Emit of up to 100%, with an average of 35% in a group of 37 kidney recipients, which accounts for a mean overestimation of 20% for the calculated areas under the concentration-time curve (8). Through a modification of our HPLC procedure (7), we were able to identify two putative MPA metabolites, M-1 and M-2, from the plasma of transplant recipients, of which M-2 was found to cross-react in the immunoassay (8). Recently, we showed that in clinical samples from heart, kidney, and liver recipients, the relative amounts of M-2 correlate with the bias between MPA values determined with HPLC and the immunoassay (9). In pharmacokinetic studies, it was shown that the areas under the concentration-time curve for both M-1 and M-2 can account for∼10% of those of MPA, whereas M-2 can be present in predose trough …