Utility of the HRN™ (hepatic cytochrome P450 reductase null) mice for investigating mechanisms of liver toxicity of carboxylic-acid-containing drugs

Many carboxylic-acid-containing drugs cause liver injury in humans. Examples include fenclozic acid (FA), which was withdrawn due to jaundice observed in clinical trials, and diclofenac (DFC) which remains widely prescribed despite being associated with liver damage. To explore whether these toxicities could be due to metabolic bioactivation mediated by cytochrome P450 (CYP) or conjugative enzymes, covalent binding (CVB) assays were done using liver microsomal incubations from wild-type and hepatic cytochrome P450 reductase null (HRN™) mice, which are deficient in CYP activity. High levels of CYP-mediated CVB of [14C]-FA and [14C]-DFC were observed in wild-type microsomes, but not in HRN™ microsomes. No UDPGAmediated CVB was detected in microsomes incubated with [14C]-FA. Wild-type and HRN™ mice were orally administered DFC or FA orally for 7days. At 100 mg/kg, FA caused a significant (p<0.05) time-dependent increase in plasma alanine amino transferase (ALT) in wild-type but not HRN™ mice. Aberrant liver histopathology and liver clinical chemistry were evident in HRNTM mice and treatment with DFC and FA “normalised” the elevated ALT levels. These data demonstrate that FA undergoes CYP mediated bioactivation and that HRNTM mice are well suited to investigations of metabolism, but not of liver toxicity, due to impaired liver function.