Mathematical modeling of the in vitro hepatic disposition of mycophenolic acid and its glucuronide in sandwich-cultured human hepatocytes.

In recent years, it has become increasingly important to test the safety of circulating metabolites of novel drugs as part of drug discovery and development programs. Accordingly, it is essential to develop suitable methods for identifying the major metabolites and their disposition in animal species and in humans. Mycophenolic acid (MPA), a selective inosine-5'-monophosphate dehydrogenase (IMPDH) inhibitor, is metabolized by glucuronidation and enterohepatic circulation of MPA-glucuronides is an important factor in the continuous systemic exposure of MPA. In humans, about 90% of the administered MPA dose is finally excreted as MPA phenyl-glucuronide (MPAG) in urine. Notably, the plasma concentration of MPAG is much higher than that of MPA. These factors suggest that, after its formation in hepatocytes, MPAG is excreted into bile and is also transported across the basolateral membrane to enter the circulation. In the present study, we performed metabolic/hepatobiliary transport studies of MPA and MPAG using sandwich-cultured human hepatocytes (SCHH) and constructed mathematical models of their hepatic disposition. We also performed vesicular transport studies to identify which human multidrug resistance-associated proteins (MRPs) are involved in the transport of MPAG from hepatocytes. MPAG was a preferred substrate for the biliary excretion transporter MRP2 and the hepatic basolateral transporters MRP3 and MRP4 in conventional and metabolic/hepatobiliary transport studies using SCHH and vesicular transport studies using human MRP-expressing membrane vesicles. The resulting mathematical model suggested that the basolateral transport plays an important role in the hepatic disposition of MPAG formed in hepatocytes. Our findings suggest that mathematical modeling of metabolic/hepatobiliary transport studies using SCH will provide useful information for determining the fate of metabolites formed in hepatocytes.