Inactivation of CYP3A4 by Benzbromarone in Human Liver Microsomes

BACKGROUND

Benzbromarone is a uricosuric drug in current clinical use that can cause serious hepatotoxicity. Chemically reactive and/or cytotoxic metabolites of benzbromarone have been identified; however there is a lack of available information on their role in benzbromarone hepatotoxicity. The reactive metabolites of some hepatotoxic drugs are known to covalently bind, or alternatively are targeted, to specific cytochrome P450 (P450) enzymes, a process that is often described as mechanism-based inhibition.

OBJECTIVE

We examined whether benzbromarone causes a mechanism-based inhibition of human P450 enzymes.

METHOD

Microsomes from human livers were preincubated with benzbromarone and NADPH, followed by evaluation of CYP2C9 and CYP3A4 activities.

RESULTS

Benzbromarone metabolism resulted in inhibition of CYP3A4 but not CYP2C9 in a time-dependent manner. Confirmation of pseudo-first order kinetics of inhibition, a requirement for NADPH, and a lack of protection by scavengers suggested that benzbromarone is a mechanism-based CYP3A4 inhibitor.

CONCLUSION

Modification of the P450 enzyme by the reactive metabolite is a common trait of drugs that induce idiosyncratic hepatotoxicity, and might provide a speculative, mechanistic model for the rare occurrences of this type of drug toxicity.

Lebensbedrohliche Nebenwirkungen der Gichtbehandlung

Die Behandlung der Gicht mit Allopurinol kann schwere unerwünschte Wirkungen zur Folge haben. Neben dem häufigen Hautausschlag unter Allopurinol kann ein selteneres Hypersensitivitäts-Syndrom mit Fieber, Hautexanthem (bis zum Lyell-Syndrom), Hepatopathie, Eosinophilie, Niereninsuffizienz und Vaskulitis lebensbedrohlich sein. Die Inzidenz dieser schweren Nebenwirkung kann wahrscheinlich mit einer der Nierenfunktion angepassten Reduktion der Dosis von Allopurinol gesenkt werden. Da Azathioprin und Mercaptopurin über die Xanthinoxidase, die durch Allopurinol gehemmt wird, metabolisiert werden, kann es bei gleichzeitiger Verabreichung zu lebensbedrohlichen Neutropenien kommen, wenn die Dosis des Allopurinols nicht um zirka 75% reduziert wird. Das urikosurisch wirkende Benzbromaron wurde kürzlich wegen schwerer Leberschädigungen vom Markt genommen.

Effects of endotoxin-induced fever and probenecid on disposition of enrofloxacin and its metabolite ciprofloxacin after intravascular administration of enrofloxacin in goats

Pharmacokinetics of enrofloxacin and its active metabolite ciprofloxacin were investigated in normal, febrile and probenecid-treated adult goats after single intravenous (i.v.) administration of enrofloxacin (5 mg/kg). Pharmacokinetic evaluation of the plasma concentration-time data of enrofloxacin and ciprofloxacin was performed using two- and one-compartment open models, respectively. Plasma enrofloxacin concentrations were significantly higher in febrile (0.75-7 h) and probenecid-treated (5-7 h) goats than in normal goats. The sum of enrofloxacin and ciprofloxacin concentrations in plasma > or =0.1 microg /mL was maintained up to 7 and 8 h in normal and febrile or probenecid-treated goats, respectively. The t1/2beta, AUC, MRT and ClB of enrofloxacin in normal animals were determined to be 1.14 h, 6.71 microg .h/mL, 1.5 h and 807 mL/h/kg, respectively. The fraction of enrofloxacin metabolized to ciprofloxacin was 28.8%. The Cmax., t1/2beta, AUC and MRT of ciprofloxacin in normal goats were 0.45 microg /mL, 1.79 h, 1.84 microg .h/mL and 3.34 h, respectively. As compared with normal goats, the values of t1/2beta (1.83 h), AUC (11.68 microg ? h/mL) and MRT (2.13 h) of enrofloxacin were significantly higher, whereas its ClB (430 mL/h/kg) and metabolite conversion to ciprofloxacin (8.5%) were lower in febrile goats. The Cmax. (0.18 microg /mL) and AUC (0.99 microg .h/mL) of ciprofloxacin were significantly decreased, whereas its t1/2beta (2.75 h) and MRT (4.58 h) were prolonged in febrile than in normal goats. Concomitant administration of probenecid (40 mg/kg, i.v.) with enrofloxacin did not significantly alter any of the pharmacokinetic variables of either enrofloxacin or ciprofloxacin in goats.

Chemical structure and toxicity of diuretics in isolated hepatocytes

The effects of several diuretics, including tienilic acid and indacrinone, on isolated rat heaptocytes were examined. Addition of tienilic acid and indacrinone at 1 mM to a suspension of freshly isolated cells caused dose-dependent loss of cell viability as judged by the LDH-latency test. Survey of 19 structurally related compounds revealed that the extent of cell injury and chemical structure were correlated, and an intense adverse effect was attributed to the 2-thienylcarbonyl moiety. Several other factors influencing cell viability are also disclosed. Further study revealed that tienilic acid and indacrinone were toxic to the primary culture of hepatocytes at a lower dose than that a freshly isolated hepatocytes. Thus, an isolated hepatocyte system can be used to select compounds displaying low hepatotoxicity, as for example is needed when screening diuretics.

Toxicity of uricosuric diuretics in rat hepatocyte culture

Several aryloxyacetic acid diuretics have shown hepatotoxicity in humans, yet there continues to be interest in developing these compounds because of the uricosuric properties of some of them. This study was designed to test the utility of the hepatocyte monolayer culture as a model for studying these compounds. In addition, an attempt was made to define the structural components that are common to hepatotoxicity. Ticrynafen, indacrinone, ethacrynic acid and A-49816, an investigational compound, were found to be toxic in hepatocyte cultures; thus, with the exception of indacrinone, paralleling the experience in humans. The toxic compounds share a ketodichlorophenoxyacetic acid chemical structure. A-56234, an investigational uricosuric, was also found to be toxic in cultures but has not been demonstrated to be hepatotoxic in humans in limited clinical experience. It does not possess the ketodichlorophenoxyacetic acid structure proper but may be metabolized to a closely related structure. Furosemide, which does not have the ketodichlorophenoxyacetic acid structure, was not toxic in hepatocyte cultures and has not been hepatotoxic in humans. Thus, the structure common to the toxic compounds is ketodichlorophenoxyacetic acid or a closely related compound. The hepatocyte monolayer system appears to be a good model for demonstrating toxicity and, perhaps, for predicting toxicity of new compounds under development.