Verinurad combined with febuxostat in Japanese adults with gout or asymptomatic hyperuricaemia: a phase 2a, open-label study.

Objectives: Verinurad (RDEA3170) is a high-affinity inhibitor of the URAT1 transporter in clinical development for treating gout and asymptomatic hyperuricaemia. The aim of this Phase 2a, randomized, open-label study was to investigate the multiple-dose pharmacodynamics, pharmacokinetics and safety of oral verinurad combined with febuxostat vs febuxostat alone and verinurad alone. Methods: Japanese male subjects aged 21-65 years with gout (n = 37) or asymptomatic hyperuricaemia (n = 35) and serum urate (sUA) ⩾8 mg/dl were randomized to febuxostat (10, 20, 40 mg) in combination with verinurad (2.5-10 mg), verinurad alone (2.5-15 mg), febuxostat alone (10, 20, 40 mg) or benzbromarone alone (50 mg). There were four treatment periods per cohort and each treatment period was 7 days. Study drugs were administered once-daily after breakfast. Plasma, serum and urine samples were measured at pre-set intervals on days -1, 7, 14, 21 and 28. Results: Verinurad combined with febuxostat decreased sUA in dose-dependent manner, providing greater sUA lowering than febuxostat alone at the same dose (P < 0.001). Urinary uric acid excretion rate was increased by verinurad, reduced by febuxostat and comparable to baseline for verinurad combined with febuxostat. Verinurad from 2.5 mg to 15 mg was well tolerated, with no withdrawals due to adverse events. Laboratory assessments showed no clinically meaningful changes during combination treatment. Conclusion: Verinurad combined with febuxostat decreased sUA dose-dependently while maintaining uric acid excretion similar to baseline. All dose combinations of verinurad and febuxostat were generally well tolerated. These data support continued investigation of oral verinurad in patients with gout. Trial registration: ClinicalTrials.gov, https://clinicaltrials.gov, NCT02317861.

Association of a reactive intermediate derived from 1',6-dihydroxy metabolite with benzbromarone-induced hepatotoxicity.

Treatment with benzbromarone can be associated with liver injury, but the detailed mechanism remains unknown. Our recent studies demonstrated that benzbromarone was metabolized to 1',6-dihydroxybenzbromarone and followed by formation of reactive intermediates that were trapped by glutathione, suggesting that the reactive intermediates may be responsible for the liver injury. The aim of this study was to clarify whether the reactive intermediates derived from 1',6-dihydroxybenzbromarone is a risk factor of liver injury in mice. An incubation study using mouse liver microsomes showed that the rates of formation of 1',6-dihydroxybenzbromarone from benzbromarone were increased by pretreatment with dexamethasone. Levels of a hepatic glutathione adduct derived from 1',6-dihydroxybenzbromarone were increased by pretreatment with dexamethasone. Furthermore, plasma alanine amino transferase activities were increased in mice treated with benzbromarone after pretreatment with dexamethasone. The results suggest that the reactive intermediate derived from 1',6-dihydroxybenzbromarone may be associated with liver injury.

Biotransformation and bioactivation reactions - 2015 literature highlights.

Since 1972, Drug Metabolism Reviews has been recognized as one of the principal resources for researchers in pharmacological, pharmaceutical and toxicological fields to keep abreast of advances in drug metabolism science in academia and the pharmaceutical industry. With a distinguished list of authors and editors, the journal covers topics ranging from relatively mature fields, such as cytochrome P450 enzymes, to a variety of emerging fields. We hope to continue this tradition with the current compendium of mini-reviews that highlight novel biotransformation processes that were published during the past year. Each review begins with a summary of the article followed by our comments on novel aspects of the research and their biological implications. This collection of highlights is not intended to be exhaustive, but rather to be illustrative of recent research that provides new insights or approaches that advance the field of drug metabolism. Abbreviations NAPQI N-acetyl-p-benzoquinoneimine ALDH aldehyde dehydrogenase AO aldehyde oxidase AKR aldo-keto reductase CES carboxylesterase CSB cystathionine ß-synthase CSE cystathionine γ-lyase P450 cytochrome P450 DHPO 2,3-dihydropyridin-4-one ESI electrospray FMO flavin monooxygenase GSH glutathione GSSG glutathione disulfide ICPMS inductively coupled plasma mass spectrometry i.p. intraperitoneal MDR multidrug-resistant NNAL 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol NNK 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone oaTOF orthogonal acceleration time-of-flight PBK physiologically based kinetic PCP pentachlorophenol SDR short-chain dehydrogenase/reductase SULT sulfotransferase TB tuberculosis.

Cysteine-Based Protein Adduction by Epoxide-Derived Metabolite(s) of Benzbromarone.

Benzbromarone (BBR) is a therapeutically useful uricosuric agent but can also cause acute liver injury. The hepatotoxicity of BBR is suggested to be associated with its metabolic activation. Our recent metabolic study demonstrated that BBR was metabolized to epoxide intermediate(s) by cytochrome P450 3A, and the intermediate(s) was reactive to N-acetylcysteine. The objectives of the present study were to determine the chemical identity of the interaction of protein with the epoxide intermediate(s) of BBR and to define the association of the protein modification with hepatotoxicity induced by BBR. Microsomal incubation study showed that the reactive intermediate(s) covalently modified microsomal protein at cysteine residues. Such adduction was also observed in hepatic protein obtained from liver of mice given BBR. The protein covalent binding occurred in time- and dose-dependent manners. Pretreatment with ketoconazole attenuated BBR-induced protein modification and hepatotoxicity, while pretreatment with dexamethasone or buthionine sulfoximine potentiated the protein adduction and hepatotoxicity induced by BBR. A good correlation was observed between BBR-induced hepatotoxicity and the epoxide-derived hepatic protein modification in mice. The present study provided in-depth mechanistic insight into BBR-induced hepatotoxicity.

Novel Bioactivation Pathway of Benzbromarone Mediated by Cytochrome P450.

Benzbromarone (BBR) is a hepatotoxic drug, but the detailed mechanism of its toxicity remains unknown. We identified 2,6-dibromohydroquinone (DBH) and mono-debrominated catechol (2-ethyl-3-(3-bromo-4,5-dihydroxybenzoyl)benzofuran; CAT) as novel metabolites of BBR in rat and human liver microsomal systems by comparison with chemically synthesized authentic compounds, and we also elucidated that DBH is formed by cytochrome P450 2C9 and that CAT is formed mainly by CYP1A1, 2D6, 2E1, and 3A4. Furthermore, CAT, DBH, and the oxidized form of DBH are highly cytotoxic in HepG2 compared with BBR. Taken together, our data demonstrate that DBH, a novel reactive metabolite, may be relevant to BBR-induced hepatotoxicity.

New approach to assess bioequivalence parameters using generalized gamma mixed-effect model (model-based asymptotic bioequivalence test).

In the pharmacokinetic (PK) study under a 2x2 crossover design that involves both the test and reference drugs, we propose a mixed-effects model for the drug concentration-time profiles obtained from subjects who receive different drugs at different periods. In the proposed model, the drug concentrations repeatedly measured from the same subject at different time points are distributed according to a multivariate generalized gamma distribution, and the drug concentration-time profiles are described by a compartmental PK model with between-subject and within-subject variations. We then suggest a bioequivalence test based on the estimated bioavailability parameters in the proposed mixed-effects model. The results of a Monte Carlo study further show that the proposed model-based bioequivalence test is not only better on maintaining its level but also more powerful for detecting the bioequivalence of the two drugs than the conventional bioequivalence test based on a non-compartmental analysis or the one based on a mixed-effects model with a normal error variable. The application of the proposed model and test is finally illustrated by using data sets in two PK studies.

A benefit-risk assessment of benzbromarone in the treatment of gout. Was its withdrawal from the market in the best interest of patients?

Benzbromarone, a potent uricosuric drug, was introduced in the 1970s and was viewed as having few associated serious adverse reactions. It was registered in about 20 countries throughout Asia, South America and Europe. In 2003, the drug was withdrawn by Sanofi-Synthélabo, after reports of serious hepatotoxicity, although it is still marketed in several countries by other drug companies. The withdrawal has greatly limited its availability around the world, and increased difficulty in accessing it in other countries where it has never been available.The overall aim of this paper is to determine if the withdrawal of benzbromarone was in the best interests of gouty patients and to present a benefit-risk assessment of benzbromarone. To determine this, we examined (i) the clinical benefits associated with benzbromarone treatment and compared them with the success of alternative therapies such as allopurinol and probenecid, particularly in patients with renal impairment; (ii) the attribution of the reported cases of hepatotoxicity to treatment with benzbromarone; (iii) the incidence of hepatotoxicity possibly due to benzbromarone; (iv) adverse reactions to allopurinol and probenecid. From these analyses, we present recommendations on the use of benzbromarone.Large reductions in plasma urate concentrations in patients with hyperuricaemia are achieved with benzbromarone and most patients normalize their plasma urate. The half-life of benzbromarone is generally short (about 3 hours); however, a uricosuric metabolite, 6-hydroxybenzbromarone, has a much longer half-life (up to 30 hours) and is the major species responsible for the uricosuric activity of benzbromarone, although its metabolism by cytochrome P450 (CYP) 2C9 in the liver may vary between patients as a result of polymorphisms in this enzyme. It is effective in patients with moderate renal impairment. Standard dosages of benzbromarone (100 mg/day) tend to produce greater hypouricaemic effects than standard doses of allopourinol (300 mg/day) or probenecid (1000 mg/day).Adverse effects associated with benzbromarone are relatively infrequent, but potentially severe. Four cases of benzbromarone-induced hepatotoxicity were identified from the literature. Eleven cases have been reported by Sanofi-Synthélabo, but details are not available in the public domain. Only one of the four published cases demonstrated a clear relationship between the drug and liver injury as demonstrated by rechallenge. The other three cases lacked incontrovertible evidence to support a diagnosis of benzbromarone-induced hepatotoxicity. If all the reported cases are assumed to be due to benzbromarone, the estimated risk of hepatotoxicity in Europe was approximately 1 in 17 000 patients but may be higher in Japan.Benzbromarone is also an inhibitor of CYP2C9 and so may be involved in drug interactions with drugs dependent on this enzyme for clearance, such as warfarin. Alternative drugs to benzbromarone have significant adverse reactions. Allopurinol is associated with rare life-threatening hypersensitivity syndromes; the risk of these reactions is approximately 1 in 56 000. Rash occurs in approximately 2% of patients taking allopurinol and usually leads to cessation of prescription of the drug. Probenecid has also been associated with life-threatening reactions in a very small number of case reports, but it frequently interacts with many renally excreted drugs. Febuxostat is a new xanthine oxidoreductase inhibitor, which is still in clinical trials, but abnormal liver function is the most commonly reported adverse reaction.Even assuming a causal relationship between benzbromarone and hepatotoxicity in the identified cases, benefit-risk assessment based on total exposure to the drug does not support the decision by the drug company to withdraw benzbromarone from the market given the paucity of alternative options. It is likely that the risks of hepatotoxicity could be ameliorated by employing a graded dosage increase, together with regular monitoring of liver function. Determination of CYP2C9 status and consideration of potential interactions through inhibition of this enzyme should be considered. The case for wider and easier availability of benzbromarone for treating selected cases of gout is compelling, particularly for patients in whom allopurinol produces insufficient response or toxicity.We conclude that the withdrawal of benzbromarone was not in the best interest of patients with gout.

Potentiation of anticoagulant effect of warfarin caused by enantioselective metabolic inhibition by the uricosuric agent benzbromarone.

OBJECTIVE: To clarify the mechanism(s) for the interaction between warfarin and benzbromarone, a uricosuric agent, and to predict changes in the in vivo pharmacokinetics of (S)-warfarin from in vitro data. METHODS: Warfarin enantiomers and benzbromarone in serum, 7-hydroxywarfarin in urine, and serum unbound fractions of warfarin enantiomers were measured in patients with heart disease given warfarin with (n = 13) or without (n = 18) oral benzbromarone (50 mg/d). In vitro inhibition constants (K(i)) of benzbromarone for (S)-warfarin 7-hydroxylation were determined with use of human CYP2C9 and liver microsomes. The magnitude of changes in the formation clearance for 7-hydroxylation (CLf), the unbound oral clearance (CL(oral,u)), and the oral clearance (CL(oral)) for (S)-warfarin were predicted by equations incorporating the in vitro Ki, the theoretical maximum unbound hepatic benzbromarone concentration, and the fractions of warfarin eliminated through metabolism and of CYP2C9-mediated metabolic reaction susceptible to inhibition by benzbromarone. RESULTS: The patients given warfarin with benzbromarone required a 36% less (P < .01) warfarin dose than those given warfarin alone (2.5 versus 3.9 mg/d) to attain similar international normalized ratios (2.1 and 2.2, respectively), and the former had 65%, 53%, and 54% lower (P < .05 or P < .01) CLf, CL(oral),u, and CL(oral) for (S)-warfarin than the latter, respectively. In contrast, no significant differences were observed for (R)-warfarin kinetics between the groups. Benzbromarone was found to be a potent competitive inhibitor (Ki < 0.01 micromol/L) for (S)-warfarin 7-hydroxylation mediated by CYP2C9. The average changes in the in vivo CLf, CL(oral),u, and CL(oral)values for (S)-warfarin induced by benzbromarone were largely predictable by the proposed equations. CONCLUSION: Benzbromarone would intensify anticoagulant response of warfarin through an enantioselective inhibition of CYP2C9-mediated metabolism of pharmacologically more potent (S)-warfarin. The magnitude of changes in the in vivo warfarin kinetics may be predicted by in vitro data.

Kinetics of allopurinol and its metabolite oxypurinol after oral administration of allopurinol alone or associated with benzbromarone in man. Simultaneous assay of hypoxanthine and xanthine by gas chromatography-mass spectrometry.

Allopurinol, oxypurinol, hypoxanthine and xanthine were assayed simultaneously using a highly specific method combining gas chromatography and mass spectrometry. Two hypo-uricaemic prescriptions were compared: i) 300 mg of allopurinol (AL); and ii) 100 mg of allopurinol plus 20 mg of benzbromarone (AL + BZB). When administered acutely, their effects on blood uric acid levels were similar. Analysis of the pharmacokinetic parameters of allopurinol and its metabolite after each treatment showed dose-linearity for the metabolite but not for the drug itself. The area under the concentration time curve for allopurinol was 40.3 +/- 9.3 mumol l-1 h after AL, against 8.4 +/- 3.9 mumol-1 h after AL + BZB, while for oxypurinol it was 948.0 +/- 125.4 mumol l-1 h after AL and 285.2 +/- 77.9 mumol l-1 h after AL + BZB. The difference in dosage form may partly account for this difference, but the benzbromarone also seems to be involved. Its role on the blood uric acid lowering action of the drug association is complex. Although benzbromarone appreciably favors the elimination of oxypurinol, which should result in a weakening of its hypo-uricaemic action, this is offset by enhanced elimination of hypoxanthine and xanthine. Renal clearance of xanthine was significantly increased under AL + BZB (173.1 +/- 65.6 ml/min against 112.2 +/- 32.9 ml/min after AL). Similarly, blood xanthine levels were proportionately higher in the presence of benzbromarone. The action of the two agents may thus be synergistic and not antagonistic, a pharmacological justification for the therapeutic use of this drug association.

Biotransformation and uric acid lowering effect of benzbromarone in patients with liver cirrhosis - evidence for active benzbromarone metabolites?

The disposition of benzbromarone and its uric acid lowering effect were investigated in 8 patients with compensated liver cirrhosis in order to obtain evidence whether dose requirements differ from subjects with normal liver function. Following a single oral dose of 100 mg benzbromarone, the plasma concentrations of the parent drug and the two hydroxylated main metabolites M1 and M2 as well as uric acid were determined up to at least 72 h. All patients were found to be rapid benzbromarone eliminators. In patients 2-8 the extent of systemic availability of benzbromarone, as estimated by the average AUC(0-infinite), was similar to previous observations in healthy individuals, whereas the values of both metabolites M1 and M2 tended to be lower in patients with liver cirrhosis. Cmax of benzbromarone and M1 also were lower in patients, M2 was equivalent to the data in subjects with normal liver function. tmax and the plasma elimination half-life t(1/2) varied within the same range as previously observed in healthy individuals. One patient exhibited much higher values in AUC(0-infinite); and Cmax of benzbromarone and both metabolites, and in addition of the elimination half-life of M1 and M2, whereas the plasma elimination of benzbromarone itself was not delayed. An effect of altered liver function cannot be excluded in this patient. Ten hours after benzbromarone administration the mean plasma uric acid in patients 2-8 was reduced by 31.5% and in patient 1 by 44.2% as compared to pretreatment values. Baseline levels were not regained until 72 h. These data are compatible with a prolonged uric acid lowering effect of an active benzbromarone metabolite. Altogether, the present observations do not suggest dose adjustment to be necessary in patients with compensated liver cirrhosis Child A and B.

Benzbromarone pharmacokinetics and pharmacodynamics in different cytochrome P450 2C9 genotypes.

Benzbromarone is a uricosuric drug and has been shown to be metabolized predominantly by cytochrome P450(CYP)2C9 in vitro findings. This study aims to investigate the influence of the CYP2C9 genotype on plasma levels of benzbromarone and 6-hydroxybenzbromarone, as well as uric acid lowering effects. A single oral dose pharmacokinetic and pharmacodynamic trial of benzbromarone (100 mg) was performed in 20 healthy volunteers, which included 15 with CYP2C9*1/*1, 4 with CYP2C9*1/*3, and 1 with CYP2C9*3/*3. The oral clearance of benzbromarone in the CYP2C9*1/*1 genotype and CYP2C9*1/*3 genotype was 58.8±25.2 L/hr/kg (mean±SD) and 51.3±7.9 L/hr/kg, respectively, whereas 8.58 L/hr/kg in the CYP2C9*3/*3 genotype. The metabolic ratio (6-hydroxybenzbromarone/benzbromarone) in urine was 38.6±10.7 in the CYP2C9*1/*1 genotype, 35.4±12.4 in the CYP2C9*1/*3 genotype and 12.9 in the CYP2C9*3/*3 genotype. Although benzbromarone significantly increased the urinary excretion and reduced the plasma concentration of uric acid, there were no significant differences in its effects for different CYP2C9 genotypes. These results suggest a critical role for CYP2C9 in the metabolism of benzbromarone in humans and a possible risk of toxicity in the CYP2C9*3 homozygote by lowering clearance of the drug. Further studies are required to assess the clinical impact of CYP2C9 on the metabolism of benzbromarone.

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.

Variation of benzbromarone elimination in man--a population study.

The plasma benzbromarone concentration-time profile in a healthy subject who retained the compound much longer than other individuals is described. The data suggested that determination of the 24 h plasma concentration of the parent drug after a single oral dose of 100 mg benzbromarone would be an appropriate procedure to determine the elimination phenotype. Based on this procedure, 148 of 153 healthy individuals (97%) in a population study were found to eliminate benzbromarone rapidly. In one subject the 24 h benzbromarone plasma concentration was very similar to that observed in the individual who had been more fully characterized. Four participants gave intermediate results. The data are compatible with a bimodal or trimodal distribution of different benzbromarone elimination phenotypes.

Liquid chromatography-mass spectrometry in metabolic research. I. Metabolites of benzbromarone in human plasma and urine.

Seven benzbromarone metabolites were identified in human plasma and urine by electron-impact mass spectrometry after semipreparative high-performance liquid chromatographic fractionation and/or by liquid chromatography-mass spectrometry using a thermospray interface. The major metabolite in plasma and urine was a hydroxybenzofuranoyl species; the 1-hydroxyethyl entity was identified as a minor metabolite. Five urinary metabolites occurred in trace amounts, all of them carrying OH and/or C = O groups in different positions. The hydroxybenzofuranoyl metabolite has often been mistaken for benzarone in previous studies.

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.

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.

Benzbromarone hydroxylation in man: defective formation of the 6-hydroxybenzbromarone metabolite.

To determine the elimination phenotype of the uricosuric agent benzbromarone 100 mg of the drug was administered as a single oral dose to 11 volunteers on a formula diet; plasma concentration-time profiles of the parent drug and the main metabolites M1 (1'-hydroxybenzbromarone) and M2 (6-hydroxy-benzbromarone) were measured by high-performance liquid chromatography for 168 h. Of the 11 subjects 2 showed higher plasma concentrations and delayed elimination of benzbromarone and metabolite M1 but reduced formation of metabolite M2 compared to the other 9 subjects. However, the plasma concentration-time profiles of the metabolites in these two slow eliminators, termed type 2, differed from those of a poor eliminator characterized during a previous study; the latter, termed type 1, eliminated benzbromarone as well as both metabolites M1 and M2 slowly. The differences in the elimination of benzbromarone and its metabolites are probably caused by differences in the activities of the cytochrome P450 mono-oxygenase isozymes. The results show that determination of the phenotype solely by measurement of the 24-h benzbromarone plasma concentration does not unequivocally characterize slow benzbromarone eliminators; additional plasma concentration-time profiles of the parent drug and metabolites are necessary. Metabolite M2 is characterized as 6-hydroxybenzbromarone; the formation and elimination of the chiral metabolite M1 is enantioselective.