Diclofenac is a nonsteroidal anti-inflammatory drug bearing a carboxylic acid functional group. As a result, the metabolism of diclofenac in humans partitions between acyl glucuronidation and phenyl hydroxylation, with the former reaction catalyzed primarily by uridine 5'-diphosphoglucuronosyl transferase 2B7 while the latter is catalyzed by cytochrome P450 (CYP)2C9 and 3A4. Further hydroxylation of diclofenac glucuronide was shown to occur in vitro with recombinant CYP2C8, which may be of clinical significance in terms of defining major metabolic routes involved in the elimination of diclofenac in humans. The 4'-hydroxylation of the drug appears to represent a feature reaction for CYP2C9 catalysis, and this regioselective oxidation is presumably dictated by interactions of the carboxylate moiety of the substrate with a putative cationic residue of the enzyme. Several other residues of CYP2C9 were identified in studies with site-directed mutants that influence substrate binding affinity and specificity, including Arg97, Phe114, Asn289 and Ser286. The 5-hydroxylation of diclofenac is subject to CYP3A4 cooperativity elicited by quinidine. In this case, enhancement by quinidine of diclofenac metabolism in vitro was attributed to increases in the V(max) with little contribution from changes in the K(m) value. These cooperative interactions in recombinant systems, however, appeared to be influenced by enzyme host membranes of various cDNA-directed expressing CYP3A4. Nevertheless, the in vivo significance of CYP3A cooperativity was demonstrated in a pharmacokinetic study in monkeys, wherein the hepatic clearance of diclofenac increased 2-fold when quinidine was co-administered. Therapeutic use of diclofenac is associated with rare but sometimes fatal hepatotoxicity characterized by delayed onset of symptoms and lack of a clear dose-response relationship. The toxicity has consequently been categorized as metabolic idiosyncrasy. In this regard, the acyl glucuronide of the drug was demonstrated to be reactive and capable of covalent modification of cellular proteins, with covalent binding to liver proteins in rats depending on the activity of multidrug resistance protein 2, a hepatic canalicular transporter. One of the modified proteins was identified as dipeptidyl peptidase IV. Formation of protein adducts also was evident following the oxidative metabolism of diclofenac catalyzed by CYP enzymes. The reactive intermediates in this case were presumably diclofenac 1',4'- and 2,5-quinone imines, both of which were trapped by conjugation with glutathione and identified as glutathione adducts. These same glutathione adducts were detected in rats as well as in human hepatocytes treated with diclofenac, and a corresponding mercapturic acid derivative was identified in urine from patients administered the drug. It is conceivable that the acyl glucuronide and benzoquinone imines derived from diclofenac modify proteins covalently and thereby produce toxicity in susceptible patients via either direct disruption of critical cellular functions or elicitation of immunological responses.