RT Journal Article SR Electronic T1 AN INVESTIGATION OF HUMAN AND RAT LIVER MICROSOMAL MYCOPHENOLIC ACID GLUCURONIDATION: EVIDENCE FOR A PRINCIPAL ROLE OF UGT1A ENZYMES AND SPECIES DIFFERENCES IN UGT1A SPECIFICITY JF Drug Metabolism and Disposition JO Drug Metab Dispos FD American Society for Pharmacology and Experimental Therapeutics SP 1513 OP 1520 DO 10.1124/dmd.105.004663 VO 33 IS 10 A1 Kristini K. Miles A1 Stephan T. Stern A1 Philip C. Smith A1 Fay K. Kessler A1 Shazia Ali A1 Joseph K. Ritter YR 2005 UL http://dmd.aspetjournals.org/content/33/10/1513.abstract AB Mycophenolic acid (MPA; 1,3-dihydro-4-hydroxy-6-methoxy-7-methyl-3-oxo-5-isobenzylfuranyl)-4-methyl-4-hexenoate), the active metabolite of the immunosuppressant prodrug, mycophenolate mofetil, undergoes glucuronidation to its 7-O-glucuronide as a primary route of metabolism. Because differences in glucuronidation may influence the efficacy and/or toxicity of MPA, we investigated the MPA UDP-glucuronosyltransferase (UGT) activities of human liver microsomes (HLMs) and rat liver microsomes with the goal of identifying UGTs responsible for MPA catalysis. HLMs (n = 23) exhibited higher average MPA glucuronidation rates (14.7 versus 6.0 nmol/mg/min, respectively, p < 0.001) and higher apparent affinity for MPA (Km = 0.082 mM versus 0.20 mM, p < 0.001) compared with rat liver microsomes. MPA UGT activities were reduced >80% in liver microsomes from Gunn rats. To identify the active enzymes, human and rat UGT1A enzymes were screened for MPA-glucuronidating activity. UGT1A9 was the only human liver-expressed UGT1A enzyme with significant activity and exhibited both high affinity (Km = 0.077 mM) and high activity (Vmax = 28 nmol · min-1 · mg-1). Spearman correlation analyses revealed a stronger relationship between HLM MPA UGT activities and 1A9-like content (r2 = 0.79) relative to 1A1 (r2 = 0.20), 1A4-like (r2 = 0.22), and 1A6 (r2 = 0.41) protein. A different profile was observed for rat with three active liver-expressed UGT1A enzymes: 1A1 (medium affinity/capacity), 1A6 (low affinity/medium capacity), and 1A7 (high affinity/capacity). Our data suggest that UGT1A enzymes are the major contributors to hepatic MPA metabolism in both species, but 1A9 is dominant in human, whereas 1A1 and 1A7 are likely the principal mediators in control rat liver. This information should be useful for interpretation of MPA pharmacokinetic and toxicity data in clinical and animal studies. The American Society for Pharmacology and Experimental Therapeutics