Mycophenolic acid: measurement and relationship to pharmacologic effects

Validated high-performance liquid chromatographic (HPLC) methods have been developed for the reliable measurement in plasma of mycophenolate mofetil (MMF), mycophenolic acid (MPA), its pharmacologically active metabolite, and mycophenolic acid glucuronide (MPAG), the inactive and primary metabolite of MPA. Using these validated HPLC methods, the pharmacokinetic behavior of MMF has been characterized in renal transplant patients as part of double-blind randomized multicenter clinical trials. The analytical performance characteristics of the HPLC methods are described. Based on investigations of the metabolism of MMF in animals, normal volunteers, and renal transplant patients, it has been established that MMF is rapidly converted to MPA. MPAG is the primary urinary excretion product derived from MPA. In vitro binding studies have revealed that MPA is extensively bound to plasma proteins and that human serum albumin is the primary binding protein. Pharmacokinetic studies have revealed the following: (1) MPA exhibits enterohepatic circulation in humans as a result of biliary excretion of MPAG followed by MPA production in the gastrointestinal tract and the appearance of secondary MPA peaks in plasma; (2) chronic renal impairment produces little change in the clearance of MPA but a marked decrease in plasma MPAG clearance, with a consequent substantial increase in the circulating MPAG concentration; and (3) pharmacokinetic/pharmacodynamic (PK/PD) studies have revealed a good correlation between drug exposure (AUC) and the probability of rejection. The implications of the PK/PD studies for the development of TDM practices are discussed.