Variation of benzbromarone elimination in man--a population study

Pharmacogenetic considerations in the treatment of gout

Gout is one of the most common forms of arthritis and the prevalence is increasing. Management comprises rapid and effective control of the inflammation in acute gout and sustained urate lowering in the long term. Improving the outcomes for cheaper old drugs and for the increasing number of new, more expensive agents is an important clinical goal. The role of pharmacogenetics in predicting response and adverse events to gout therapies is of considerable interest. Currently, prospective screening is employed to detect HLA-B*5801 carriage and glucose-6-phosphate dehydrogenase deficiency, to minimize occurrence of allopurinol hypersensitivity and pegloticase-related hemolytic anemia. In the future it is likely that other genetic markers of drug response will make the transition to clinical practice to further improve the efficacy and safety of gout therapies. In this review, we will examine the potential clinical relevance of specific genetic variants in the management of gout.

Frequency of CYP2C9 polymorphisms in Polynesian people and potential relevance to management of gout with benzbromarone

OBJECTIVES

Gout is a major health problem in Polynesians and allopurinol, the drug of choice for the management gout, appears to be less effective in Polynesian patients. The uricosuric drug benzbromarone is an alternative treatment but CYP2C9 poor metabolisers (PMs) may be at a heightened risk of benzbromarone-induced hepatotoxicity. The objectives of this study were to determine the frequency of the PM alleles CYP2C9*2 and CYP2C9*3 in New Zealand (NZ) Caucasian and Polynesian gout cohorts; and then to test for novel CYP2C9 polymorphisms in Polynesians.

METHODS

Eight hundred and fifty-two Caucasians (537 controls, 315 gout patients) and 1072 Māori and Pacific Island (Polynesian) people (620 controls, 452 gout patients) were genotyped for CYP2C9*2 and CYP2C9*3. Forty Polynesians were screened for novel CYP2C9 polymorphisms using whole genome sequencing.

RESULTS

Frequency of CYP2C9 PM alleles was significantly higher in Caucasians compared to Polynesians (CYP2C9*2: 13.5% versus 3.1%; CYP2C9*3: 5.5% versus 1.6%, P<1.2E-11). Within Polynesians, CYP2C9 PM alleles were rarer in Western Polynesians (Samoa, Tonga) than Eastern Polynesians (NZ and Cook Island Maori; CYP2C9*2: 0.6% versus 2.5%; CYP2C9*3: 0.4% versus 2.0%; P<0.03). A total of 152 SNPs were found by sequencing. None of these variants were predicted by in silico analysis to significantly impact on CYP2C9 expression or activity.

CONCLUSION

Prospective CYP2C9 genotyping of Caucasian gout patients may be warranted for benzbromarone, whereas the low frequencies of CYP2C9 PM alleles in Polynesians suggests that the CYP2C9 polymorphism may be of little or no relevance to benzbromarone prescribing in this population.

Refractory gout attack

Refractory gout attack is an uncommon problem, since gout flares are usually self-limited. This clinical condition is characterized by serum uric acid higher than 6 mg/Dl or continuous manifestations of recurrent flares, chronic arthritis, and increased tophi. We report in this paper a 69-year-old man with a polyarticular and protracted gout attack, despite usual treatment and low urate sera levels. In order to manage this problem, we reviewed gout pathophysiology and developed a therapeutic solution based on benzbromarone pharmacokinetics. We also review herein new options for gout treatment that could be used in similar cases.

Sequential metabolism and bioactivation of the hepatotoxin benzbromarone: formation of glutathione adducts from a catechol intermediate

Benzbromarone (BBR) is a uricosuric agent that has been used as a treatment for chronic gout. Although never approved in the United States, BBR was recently withdrawn from European markets due to several clinical cases linking the drug to an idiosyncratic hepatotoxicity that is sometimes fatal. We report here a possible mechanism of toxicity that involves the bioactivation of BBR through sequential hydroxylation of the benzofuran ring to a catechol, which can then be further oxidized to a reactive quinone intermediate capable of adducting protein. NADPH-supplemented human liver microsomes generated a single metabolite that was identified as 6-OH BBR by comparison with the synthesized chemical standard. CYP2C9 was the major recombinant enzyme capable of catalyzing the formation of 6-OH BBR, although CYP2C19 also showed a lower degree of activity. Further oxidation of either 6-OH BBR or 5-OH BBR by human liver microsomes resulted in the formation of a dihydroxy metabolite with identical chromatographic and mass spectral properties. This product of sequential metabolism of BBR was identified as the catechol, 5,6-dihydroxybenzbromarone. Incubation of the catechol with liver microsomes, in the presence of glutathione, resulted in the formation of two glutathione adducts that could derive from a single ortho-quinone intermediate. Isoform profiling with recombinant human P450s suggested that CYP2C9 is primarily responsible for the formation of this reactive quinone intermediate.

Metabolism of benzbromarone in man: structures of new oxidative metabolites, 6-hydroxy- and 1'-oxo-benzbromarone, and the enantioselective formation and elimination of 1'-hydroxybenzbromarone

1. The uricosuric drug benzbromarone is extensively metabolized in man and two main metabolites are formed: the previously characterized 1'-hydroxybenzbromarone (metabolite M1) and an arylhydroxybenzbromarone (metabolite M2) of unknown structure. A dimethyl derivative was isolated from urine after methylation and was characterized by gas chromatography-mass spectrometry (g.l.c.-m.s.) and high resolution nuclear magnetic resonance spectroscopy as 4''-O-methyl-6-methoxybenzbromarone; the structure of M2 therefore is 6-hydroxybenzbromarone. 2. A minor metabolite was similarly characterized as 1'-oxobenzbromarone by comparison with authentic synthetic samples and is a product of biodegradation and not an artifact derived from the in vitro oxidation of 1'-hydroxybenzbromarone. Further minor metabolites were detected and were provisionally characterized by g.l.c.-m.s. after derivatization and include: 2'-hydroxybenzbromarone (an isomer of 1'-hydroxybenzbromarone); 1',6-dihydroxybenzbromarone; dihydroxy-aryl-benzbromarone; and two structure isomers of 6-hydroxybenzbromarone. Debrominated metabolites were not detectable. 3. Benzbromarone is hydroxylated in vivo at the prochiral centre C1' to 1'-hydroxybenzbromarone; analysis of 1'-hydroxybenzbromarone from plasma and urine extracts by h.p.l.c. using a chiral column revealed that two peaks were eluted which showed a mean enantiomeric ratio of 2.1 for plasma and 7.3 for urine; these data demonstrate that the formation and elimination of this metabolite is enantioselective; the absolute configuration of the 1'-chiral centre is presently unknown.

Rapid and slow benzbromarone elimination phenotypes in man: benzbromarone and metabolite profiles

Following oral administration of the uricosuric drug benzbromarone two major metabolites appear in the circulation. 1'-hydroxy-benzbromarone (M1), and a second product (M2) of unknown structure. The plasma concentrations of the parent drug and of M1 and M2 have now been compared in two different elimination phenotypes. 10 subjects who eliminated the drug rapidly (S1-10) and one individual (S11) whose elimination capacity was impaired, presumably due to genetic variation (S11). The AUC (0-96) of the parent drug in S11 was 145 micrograms.ml-1 h. and in the other individuals it averaged 18.3 (11.4-24.5) micrograms.ml-1 h. The plasma elimination half life of benzbromarone was 3.34 (1.77-5.24) h in the rapid eliminators, and 13.08 h in the subject with the elimination defect. The mean plasma elimination half life of the metabolites in S1-10 amounted to 20.1 (11.9-41.2) h for M1, and 17.2 (12.9-30.7) h for M2. In S11 the plasma elimination half life of M1 was prolonged to 76.6 h, and of M2 to 75.4 h. Thus, the elimination defect in S11 was not restricted to the parent drug, but it also involved the two major metabolites M1 and M2. This might be a consequence of a hepatic enzyme deficiency, or be due to impairment of drug excretion.