Stability of Mycophenolic Acid and Mycophenolic Acid Glucuronide in Human Plasma

Abstract

Mycophenolic acid (MPA), the active metabolite of mycophenolate mofetil (MMF), is being used increasingly in immunosuppressant therapy after solid organ transplantation (1)(2)(3). Its immunosuppressive activity is thought to reside in the inhibition of inosine monophosphate dehydrogenase (IMPDH), leading to a suppression of purine nucleotide synthesis in lymphocytes, thereby suppressing cell mitosis (3)(4). MPA is converted in the liver mainly to the 7- O -glucuronide metabolite, mycophenolic acid glucuronide (MPAG), which does not have immunosuppressant activity (3)(5). During MMF therapy, MPAG peak concentrations observed in plasma of patients (5–600 mg/L) can be up to 300-fold higher than those of MPA (0.1–50 mg/L). Whether drug monitoring of MPA or MPAG is required to improve the therapeutic efficacy or to minimize adverse side effects is still under investigation (3)(6). For monitoring of MPA concentrations in human plasma, several HPLC procedures as well as one immunoassay (Emit; Behring-Syva) are available (3)(6). In the present investigation we addressed the question as to whether MPA and MPAG are stable in human plasma because it is possible that deglucuronidation of MPAG might take place in vitro during shipping and sample storage, thus leading to false high MPA concentrations. For this purpose, we added three different MPA and MPAG concentrations to three different pools of human plasma and followed both MPA and MPAG plasma concentrations for 7 days at three different storage temperatures. MPA and MPAG concentrations were measured using an HPLC method developed recently in our laboratory (7). For this study, MPA, MPAG, and the carboxy butoxy ether of MPA (MPAC) were a gift of Hoffmann-La Roche (Grenzach-Wyhlen, Germany). Sodium tungstate dihydrate, potassium dihydrogen phosphate, sodium hydroxide, phosphoric acid, and perchloric acid were from Merck. Acetonitrile (HPLC grade) was obtained from S.T. Baker …