Mycophenolic acid glucuronidation and its inhibition by non-steroidal anti-inflammatory drugs in human liver and kidney

Objective: The aims of this investigation were to study the glucuronidation of mycophenolic acid (MPA) in human liver and kidney and to search for a c ompound that inhibits MPA glucuronidation among the non-steroidal anti-infl ammatory drugs (NSAIDs). Methods: A sensitive and reproducible radiometric assay was developed to measure the rate of MPA glucuronidation in human liv er and kidney microsomes. The assay employed uridine 5'-diphosphate-[U-C-14 ]-glucuronic acid (UDPGA) and MPA-glucuronide was isolated by TLC. The fina l concentrations of UDPGA and MPA necessary were 1 mM (liver), and MPA conc entration was 0.5 mM (kidney). The inhibition of MPA glucuronidation was st udied with 18 NSAIDs and tacrolimus. Results: Glucuronosyl transferase activity followed Michaelis-Menten kineti cs and the K-m (mean +/- SD; mM) was 0.31 +/- 0.06 (liver; n = 5) and 0.28 +/- 0.07 (kidney; n = 5; P = 0.555); the V-max (mean +/- SD; nmol/mg per mi nute) was 5.2 +/- 1.4(liver; n = 5) and 10.5 +/- 1.2 (kidney; n = 5; P = 0. 0005). The MPA glucuronidation rates (mean+/-SD; nmol/min/mg) were 3.3 +/- 0.9 (liver; n = 10) and 7.8 +/- 1.5 (kidney; n = 10; P = 0.0002). The rate of MPA glucuronidation ranged between 2.0 and 5.1 nmol/mg per minute with a 2.5-fold variation (liver) and between 5.7 and 9.8 nmol/mg per minute with a 1.7-fold variation (kidney). The inhibition study was performed in liver and revealed that the percentage of control ranged from 8% +/- 3% (niflumi c acid) to 119% +/- 16% (Ketoralac). The inhibition curves for MPA glucuron idation rate were determined with the four most effective inhibitors: niflu mic acid, flufenamic acid, mefenamic acid and diflunisal. Their IC50 estima tes (muM) were 8 +/- 1, 19 +/- 9, 63 +/- 8 and 109 +/- 15, respectively (li ver), and 8 +/- 2, 13 +/- 2, 49 +/- 4 and 122 +/- 18, respectively (kidney) . The IC50 estimate for niflumic acid was eightfold lower than the peak pla sma levels after a single oral dose of 250 mg of this drug. Conclusion: The human liver and kidney are important sites of MPA glucuroni dation. MPA glucuronidation was inhibited to various extents by different N SAIDs and the four most effective inhibitors were niflumic acid, flufenamic acid, mefenamic acid and diflunisal. These drugs have similar molecular st ructures consisting of two aromatic rings bearing a carboxylic group.