Allopurinol dosage selection: relationships between dose and plasma oxipurinol and urate concentrations and urinary urate excretion

Xanthine oxidase and aldehyde oxidase contribute to allopurinol metabolism in rats

BACKGROUND

Allopurinol is used to treat hyperuricemia and gout. It is metabolized to oxypurinol by xanthine oxidase (XO), and aldehyde oxidase (AO). Allopurinol and oxypurinol are potent XO inhibitors that reduce the plasma uric acid levels. Although oxypurinol levels show large inter-individual variations, high concentrations of oxypurinol can cause various adverse effects. Therefore, it is important to understand allopurinol metabolism by XO and AO. In this study we aimed to estimate the role of AO and XO in allopurinol metabolism by pre-administering Crl:CD and Jcl:SD rats, which have known strain differences in AO activity, with XO inhibitor febuxostat.

METHODS

Allopurinol (30 or 100 mg/kg) was administered to Crl:CD and Jcl:SD rats with low and high AO activity, respectively, after pretreatment with or without febuxostat. The serum concentrations of allopurinol and oxypurinol were measured, and the area under the concentration-time curve (AUC) was calculated from the 48 h serum concentration-time profile. In vivo metabolic activity was measured as the ratio AUC_oxypurinol /AUC_allopurinol.

RESULTS

Although no strain-specific differences were observed in the AUC_oxypurinol/AUC_allopurinol ratio in the allopurinol (30 mg/kg)-treated group, the ratio in Jcl:SD rats was higher than that in Crl:CD rats after febuxostat pretreatment. Contrastingly, the AUC ratio of allopurinol (100 mg/kg) was approximately 2-fold higher in Jcl:SD rats than that in Crl:CD rats. These findings showed that Jcl:SD rats had higher intrinsic AO activity than Crl:CD rats did. However, febuxostat pretreatment substantially decreased the activity, as measured by the AUC ratio using allopurinol (100 mg/kg), to 46 and 63% in Crl:CD rats and Jcl:SD rats, respectively, compared to the control group without febuxostat pretreatment.

CONCLUSIONS

We elucidated the role of XO and AO in allopurinol metabolism in Crl:CD and Jcl:SD rats. Notably, AO can exert a proportionately greater impact on allopurinol metabolism at high allopurinol concentrations. AO's impact on allopurinol metabolism is meaningful enough that individual differences in AO may explain allopurinol toxicity events. Considering the inter-individual differences in AO activity, these findings can aid to dose adjustment of allopurinol to avoid potential adverse effects.

Honorary Professor Garry Graham

An appreciation of the contribution of Professor Gary Graham to anti-inflammatory and antirheumatic pharmacology and clinical pharmacology.

Xanthine oxidoreductase and its inhibitors: relevance for gout

Xanthine oxidoreductase (XOR) is the rate-limiting enzyme in purine catabolism and converts hypoxanthine to xanthine, and xanthine into uric acid. When concentrations of uric acid exceed its biochemical saturation point, crystals of uric acid, in the form of monosodium urate, emerge and can predispose an individual to gout, the commonest form of inflammatory arthritis in men aged over 40 years. XOR inhibitors are primarily used in the treatment of gout, reducing the formation of uric acid and thereby, preventing the formation of monosodium urate crystals. Allopurinol is established as first-line therapy for gout; a newer alternative, febuxostat, is used in patients unable to tolerate allopurinol. This review provides an overview of gout, a detailed analysis of the structure and function of XOR, discussion on the pharmacokinetics and pharmacodynamics of XOR inhibitors-allopurinol and febuxostat, and the relevance of XOR in common comorbidities of gout.

Modeling and Simulation for Estimating the Influence of Renal Dysfunction on the Hypouricemic Effect of Febuxostat in Hyperuricemic Patients Due to Overproduction or Underexcretion of Uric Acid

Whether renal dysfunction influences the hypouricemic effect of febuxostat, a xanthine oxidase (XO) inhibitor, in patients with hyperuricemia due to overproduction or underexcretion of uric acid (UA) remains unclear. We aimed to address this question with a modeling and simulation approach. The pharmacokinetics (PK) of febuxostat were analyzed using data from the literature. A kinetic model of UA was retrieved from a previous human study. Renal UA clearance was estimated as a function of creatinine clearance (CLcr) but non-renal UA clearance was assumed constant. A reversible inhibition model for bovine XO was adopted. Integrating these kinetic formulas, we developed a PK-pharmacodynamic (PK-PD) model for estimating the time course of the hypouricemic effect of febuxostat as a function of baseline UA level, febuxostat dose, treatment duration, body weight, and CLcr. Using the Monte Carlo simulation method, we examined the performance of the model by comparing predicted UA levels with those reported in the literature. We also modified the models for application to hyperuricemia due to UA overproduction or underexcretion. Thirty-nine data sets comprising 735 volunteers or patients were retrieved from the literature. A good correlation was observed between the hypouricemic effects of febuxostat estimated by our PK-PD model and those reported in the articles (observed) (r=0.89, p<0.001). The hypouricemic effect was estimated to be augmented in patients with renal dysfunction irrespective of the etiology of hyperuricemia. While validation in clinical studies is needed, the modeling and simulation approach may be useful for individualizing febuxostat doses in patients with various clinical characteristics.

Understanding and improving the use of allopurinol in a teaching hospital

BACKGROUND

An emphasis on renal function in deciding maintenance doses of allopurinol to prevent allopurinol hypersensitivity has resulted in ineffective prevention of attacks of gout. New therapeutic guidelines for gout have shifted the focus back to titrating maintenance doses to reach a serum uric acid (SUA) concentration target of ≤ 0.36 mmol/L.

AIMS

To examine trends in the prescribing of allopurinol in a teaching hospital and their concordance with the new guidelines for gout management, and to explore prescribers' approaches and attitudes to the use of allopurinol.

METHODS

An audit was conducted of all inpatients prescribed allopurinol at a teaching hospital between January 2008 and December 2012. Allopurinol dose, SUA, serum creatinine concentrations and estimated glomerular filtration rates were extracted from the hospital databases. Doctors from medical units who regularly prescribed allopurinol were interviewed.

RESULTS

The allopurinol dose prescribed in gout patients most commonly was a continuation of the pre-admission dosage. Dosage change during admission was rarely observed. Dosages reflected a consideration of renal function. SUA concentrations were measured in only 21% (n = 269) of gout patients. Prescriber interviews (n = 12) reflected adequate knowledge regarding allopurinol use, but most maintained that the primary care setting was more suitable for the management of dose titration in gout.

CONCLUSIONS

SUA concentrations were not routinely measured in the majority of admitted gout patients taking allopurinol. Without SUA measurements and allopurinol dose titration, patients with SUA > 0.36 mmol/L are at increased risk for acute attacks of gout in hospital.

Switching from allopurinol to febuxostat: efficacy and tolerability in hemodialysis patients

Background

Febuxostat is a novel xanthine oxidase inhibitor. However, few studies have examined the long-term efficacy and tolerability of febuxostat after switching from allopurinol in hemodialysis (HD) patients. Therefore, the present study evaluated the long-term efficacy and tolerability of febuxostat in HD patients after switching from allopurinol.

Findings

We monitored the levels of hemoglobin, hematocrit, platelet count, blood urea nitrogen, serum creatinine, serum sodium, serum potassium, serum chloride, serum calcium, serum inorganic phosphorus, aspartate transaminase, alanine aminotransferase, alkaline phosphatase, lactate dehydrogenase, and total protein that were considered overall as a tolerability index, while the serum uric acid (UA) level was considered an index of efficacy. All values were measured at baseline and at 1, 6, 12, and 16 months after the switch to febuxostat therapy. All subjects switched from allopurinol (100 mg/day) to febuxostat (10 mg/day) in August 2013. Clinical laboratory data were collected at baseline in July 2013 until December 2014. Nine patients were included in the study analysis. Results showed that clinical laboratory data at baseline versus those at 16 months were not significantly different. Serum UA levels, which represented the efficacy index, were significantly different between the baseline level (6.8 ± 1.4) and those at 1, 6, 12, and 16 months (5.2 ± 1.1, 5.1 ± 1.1, 4.6 ± 0.9, and 5.4 ± 1.8 mg/dL, respectively; all p < 0.05).

Conclusion

Switching from allopurinol to febuxostat in HD patients reduced serum UA levels, with no changes in other clinical laboratory data in the long term.

Allopurinol reduces antigen-specific and polyclonal activation of human T cells

Allopurinol is the most popular commercially available xanthine oxidase inhibitor and it is widely used for treatment of symptomatic hyperuricaemia, or gout. Although, several anti-inflammatory actions of allopurinol have been demonstrated in vivo and in vitro, there have been few studies on the action of allopurinol on T cells. In the current study, we have assessed the effect of allopurinol on antigen-specific and mitogen-driven activation and cytokine production in human T cells. Allopurinol markedly decreased the frequency of IFN-γ and IL-2-producing T cells, either after polyclonal or antigen-specific stimulation with Herpes Simplex virus 1, Influenza (Flu) virus, tetanus toxoid and Trypanosoma cruzi-derived antigens. Allopurinol attenuated CD69 upregulation after CD3 and CD28 engagement and significantly reduced the levels of spontaneous and mitogen-induced intracellular reactive oxygen species in T cells. The diminished T cell activation and cytokine production in the presence of allopurinol support a direct action of allopurinol on human T cells, offering a potential pharmacological tool for the management of cell-mediated inflammatory diseases.

The history and future of probenecid

Probenecid was initially developed with the goal of reducing the renal excretion of antibiotics, specifically penicillin. It is still used for its uricosuric properties in the treatment in gout, but its clinical relevance has sharply fallen and is rarely used today for either. Interestingly, throughout the last 60 years, there have been a host of apparently unrelated studies using probenecid in the clinical and basic research arena, including its potential use in the diagnosis and treatment of depression and its use to prevent fura-2 leakage in calcium transient studies. Recently, it has been shown that it is also an agonist of the Transient Receptor Potential Vanilloid 2 channel. Due to its unique action and new findings implicating TRPV channels in physiology and in disease, probenecid may have a new future as a research tool, and perhaps as a clinical agent in the neurology and cardiology fields. We review the history of probenecid in this paper and its potential future uses.

Allopurinol to Febuxostat: How far have we come?

For decades allopurinol has been used as a xanthine oxidase inhibitor for treatment of hyperuricemia and gout. Although effective in many patients, some experience sensitivity to the drug. In some cases, this sensitivity may lead to allopurinol hypersensitivity disorder, which if untreated can be fatal. Recently the Food and Drug Administration has approved the use of febuxostat as an alternative therapy for hyperuricemia and gout. Febuxostat is a new xanthine oxidase inhibitor, but is not purine based and therefore decreases adverse reactions due to patient sensitivity. This review is a comprehensive look at the background of hyperuricemia and gout treatment with allopurinol compared to recent clinical studies with febuxostat. Each clinical study is evaluated and summarized, identifying the advances in treatment that have been made as well as the concerns that still exist with either treatment.

Febuxostat: the evidence for its use in the treatment of hyperuricemia and gout

Introduction:

Gout is a common and disabling cause of arthritis in middle-aged and elderly populations, with its main predisposing factor being hyperuricemia (serum urate > 6.8 mg/dL). Options for treatment of chronic gout until 2008 were allopurinol, a xanthine oxidase inhibitor, and the group of drugs known as uricosurics that stimulate the renal excretion of uric acid. A proportion of patients, including some with chronic kidney disease and solid organ transplantations, could not be treated with the those therapies because of intolerance, drug interactions, or adverse events. Febuxostat is a nonpurine xanthine oxidase inhibitor, recently approved in Europe and the United States for the treatment of chronic gout.

Aim:

To review the clinical evidence (phase II and III studies) of the effectiveness and safety of febuxostat for treatment of hyperuricemia and gout.

Evidence review:

Febuxostat, at doses ranging from 40 to 240 mg/day, is efficacious in reducing serum urate in patients with hyperuricemia and gout, comparing favorably with fixed doses of allopurinol in that respect. Early safety signals with respect to liver test abnormalities and cardiovascular outcomes have not been confirmed in recent large prospective trials but need to be further monitored.

Clinical potential:

Given its low cost and extensive clinical experience, allopurinol will likely remain the first-line drug for management of hyperuricemia and gout. Febuxostat may provide an important option in patients unable to use allopurinol, those with very high serum urate levels, or in the presence of refractory tophi.

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.

Pharmacokinetic and pharmacodynamic interaction between allopurinol and probenecid in healthy subjects

BACKGROUND AND OBJECTIVE

Combination therapy with allopurinol and probenecid is used to treat tophaceous gout in patients who do not respond sufficiently to allopurinol alone. However, the potential interaction between these drugs has not been systematically investigated. The objective of this study was to investigate the pharmacokinetics and hypouricaemic effect of oxypurinol (the active metabolite of allopurinol) and probenecid when administered alone and in combination in healthy subjects.

METHODS

An open-label, randomized, three-way crossover clinical trial was conducted in 12 healthy adults. Subjects were randomized to receive treatment for 7 days with allopurinol (150 mg twice daily), probenecid (500 mg twice daily) or combination therapy with both drugs, with a 7-day washout period between treatments. Venous blood samples were collected predose (at 0 hours) and 1, 2, 3, 4, 6, 8, 10 and 12 hours after dosage for determination of oxypurinol and/or probenecid concentrations. Plasma and urinary urate concentrations were determined on each study day and at the end of each washout period. Pharmacokinetic and pharmacodynamic parameters were analysed using two-way ANOVA.

RESULTS

Coadministration of allopurinol and probenecid significantly reduced average steady-state plasma oxypurinol concentrations (mean+/-SD: allopurinol alone 9.7+/-2.1 mg/L vs combination 5.1+/-1.0 mg/L, p<0.001). Probenecid concentrations were unaffected. Plasma urate concentrations decreased (p<0.01) during allopurinol therapy (0.16+/-0.05 mmol/L), probenecid therapy (0.13+/-0.02 mmol/L) and combination therapy (0.09+/-0.02 mmol/L) compared with baseline (0.30+/-0.05 mmol/L).

CONCLUSION

Coadministration of allopurinol and probenecid to healthy subjects had a greater hypouricaemic effect than either allopurinol or probenecid alone, despite a reduction in plasma oxypurinol concentrations when the drugs were taken concomitantly.

Optimizing therapy with allopurinol: factors limiting hypouricemic efficacy

The aim of this review is to examine clinical aspects of the use of the hypouricemic drug allopurinol. Allopurinol is a moderately active hypouricemic drug. Its activity is largely the result of the inhibition of xanthine oxidoreductase by oxypurinol, the active metabolite of allopurinol. The activity of allopurinol may be limited by oxypurinol, reducing the renal clearance of urate. Optimal use of allopurinol involves individualization of dose to attain a sufficient decrease in plasma urate concentrations. This may require a dose greater than recommended based on creatinine clearance. The initial use of an anti-inflammatory drug or low-dose colchicine decreases but does not eliminate the development of acute attacks of gout during the initiation of therapy with allopurinol. Monitoring of oxypurinol concentrations has shed some light on the efficacy of allopurinol but more data are required particularly in patients with renal impairment. Probenecid increases the hypouricemic effect of allopurinol but the favorable interaction may be significant only in patients with glomerular filtration rates greater than about 50 mL/min.

Utilization of allopurinol in the Australian community

BACKGROUND

International data suggest that suboptimal use of allopurinol is common. Allopurinol dose should be lower in renal impairment, but higher when gout is not controlled. The aim of the study was to examine trends in the usage of allopurinol in the Australian community.

METHODS

Community dispensing data on the urate-lowering drugs allopurinol and probenecid were obtained from databases kept by Medicare Australia and the Drug Utilization Sub-Committee, for January 1992 to December 2005.

RESULTS

Allopurinol comprised 98.4% of all prescriptions for urate-lowering drugs dispensed during 2005. Most prescriptions were for allopurinol 300 mg, but there was a steady shift towards use of allopurinol 100 mg in all states and territories over the period of the study. There were marked variations in prescribing rates across the country. New South Wales had the highest rate of subsidized prescribing for allopurinol 300 mg (39.3 per 1000 population). Tasmania had the highest rate for allopurinol 100 mg (14.3 per 1000 population), which coincided with an educational programme to decrease allopurinol dose in patients with renal impairment. Prescribing rates in the Northern Territory were substantially lower than all other regions, at 10.8 and 3.3 prescriptions per 1000 population for allopurinol 300 and 100 mg, respectively.

CONCLUSION

The increased uptake of allopurinol 100 mg suggests greater adherence to dosing guidelines and that there is value in educational programmes to optimize drug usage. Variability in utilization rates across regions indicates the need for research on factors responsible. Precise understanding of dosing trends requires access to deidentified, individual dosing data.

Clinical pharmacokinetics and pharmacodynamics of allopurinol and oxypurinol

Allopurinol is the drug most widely used to lower the blood concentrations of urate and, therefore, to decrease the number of repeated attacks of gout. Allopurinol is rapidly and extensively metabolised to oxypurinol (oxipurinol), and the hypouricaemic efficacy of allopurinol is due very largely to this metabolite. The pharmacokinetic parameters of allopurinol after oral dosage include oral bioavailability of 79 +/- 20% (mean +/- SD), an elimination half-life (t((1/2))) of 1.2 +/- 0.3 hours, apparent oral clearance (CL/F) of 15.8 +/- 5.2 mL/min/kg and an apparent volume of distribution after oral administration (V(d)/F) of 1.31 +/- 0.41 L/kg. Assuming that 90 mg of oxypurinol is formed from every 100mg of allopurinol, the pharmacokinetic parameters of oxypurinol in subjects with normal renal function are a t((1/2)) of 23.3 +/- 6.0 hours, CL/F of 0.31 +/- 0.07 mL/min/kg, V(d)/F of 0.59 +/- 0.16 L/kg, and renal clearance (CL(R)) relative to creatinine clearance of 0.19 +/- 0.06. Oxypurinol is cleared almost entirely by urinary excretion and, for many years, it has been recommended that the dosage of allopurinol should be reduced in renal impairment. A reduced initial target dosage in renal impairment is still reasonable, but recent data on the toxicity of allopurinol indicate that the dosage may be increased above the present guidelines if the reduction in plasma urate concentrations is inadequate. Measurement of plasma concentrations of oxypurinol in selected patients, particularly those with renal impairment, may help to decrease the risk of toxicity and improve the hypouricaemic response. Monitoring of plasma concentrations of oxypurinol should also help to identify patients with poor adherence. Uricosuric drugs, such as probenecid, have potentially opposing effects on the hypouricaemic efficacy of allopurinol. Their uricosuric effect lowers the plasma concentrations of urate; however, they increase the CL(R) of oxypurinol, thus potentially decreasing the influence of allopurinol. The net effect is an increased degree of hypouricaemia, but the interaction is probably limited to patients with normal renal function or only moderate impairment.

Allopurinol dosing in renal impairment: walking the tightrope between adequate urate lowering and adverse events

Allopurinol is the mainstay of urate-lowering therapy for patients with gout and impaired renal function. Although rare, a life-threatening hypersensitivity syndrome may occur with this drug. The risk of this allopurinol hypersensitivity syndrome (AHS) is increased in renal impairment. The recognition that AHS may be because of delayed-type hypersensitivity to oxypurinol, the main metabolite of allopurinol, and that oxypurinol concentrations are frequently elevated in patients with renal impairment prescribed standard doses of allopurinol has led to the widespread adoption of allopurinol-dosing guidelines. These guidelines advocate allopurinol dose reduction according to creatinine clearance in patients with renal impairment. However, recent studies have challenged the role of these guidelines, suggesting that AHS may occur even at low doses of allopurinol, and that these guidelines lead to under-treatment of hyperuricemia, a key therapeutic target in gout. Based on current data, we advocate gradual introduction of allopurinol according to current treatment guidelines, with close monitoring of serum uric acid concentrations. In patients with severe disease and persistent hyperuricemia, allopurinol dose escalation above those recommended by the guidelines should be considered, with careful evaluation of the benefits and risks of therapy. Further work is needed to clarify the safety and efficacy of allopurinol dose escalation, particularly in patients with renal impairment.

A simple method for quantification of allopurinol and oxipurinol in human serum by high-performance liquid chromatography with UV-detection

OBJECTIVES

Allopurinol is a uric acid lowering drug used in the treatment of gout and the prevention of tumor lysis syndrome. Allopurinol and its active metabolite oxipurinol inhibit xanthine oxidase, which forms uric acid from xanthine and hypoxanthine. Therapeutic drug monitoring is an important option for evaluation and optimization of allopurinol treatment in case of renal impairment, interaction with uricosuric drugs or to verify patient adherence. In this study we developed and validated a simple quantitative assay using reverse phased high-performance liquid chromatography (HPLC) with UV-detection as a method for quantification of allopurinol and oxipurinol in human serum in the presence of different frequently used drugs.

METHODS

The HPLC-UV method uses a mobile phase consisting of sodium acetate (0.02 M; pH 4.5), at a flow rate of 1.0 mL/min. Allopurinol and oxipurinol are detected by UV-absorption at 254 nm with a retention time of 9.9 min for oxipurinol and 12.3 min for allopurinol. Aciclovir is used as internal standard.

RESULTS

Validation showed for allopurinol lower and upper limits of quantification of 0.5 and 10mg/L and for oxipurinol 1 and 40 mg/L, respectively. The assay was linear over the concentration range of 0.5-10mg/L (allopurinol) and 1-40 mg/L (oxipurinol). Intra- and inter-day precision showed coefficients of variation <15% over the complete concentration range; accuracy was within 5% for allopurinol and oxipurinol. Endogenous purine-like compounds were separated from allopurinol, oxipurinol and aciclovir with a resolution factor >1.5. Exogenous purine-like compounds and co-medication frequently used by gout patients did not hinder the analysis due to the dichloromethane washing step or to low UV-absorpion at 253 nm. Serum levels of 66 patients prescribed allopurinol 300 mg/day were determined using this HPLC-UV method. Measured serum allopurinol and oxipurinol concentrations in clinical practice showed large variability with a range of <0.5-4.3 mg/L for allopurinol and <1.0-39.2 mg/L for oxipurinol, respectively.

CONCLUSION

We developed an easy-to-operate and validated HPLC-UV method for the quantification of allopurinol and oxipurinol in human serum. This method was proven to be valid for samples of gout patients frequently using concomitant medications.

Concordance of the management of chronic gout in a UK primary-care population with the EULAR gout recommendations

OBJECTIVES

To assess concordance of the management of chronic gout in UK primary care with the European League Against Rheumatism (EULAR) gout recommendations.

METHODS

A postal questionnaire was sent to all adults aged >30 years registered with two general practices. Patients with possible gout attended for clinical assessment, at which the diagnosis was verified clinically. Aspects of chronic gout management, including provision of lifestyle modification advice, use of urate-lowering therapies (ULT) including dose titration to serum urate (SUA) level, prophylaxis against acute attacks, and diuretic cessation were assessed in accordance with the EULAR recommendations.

RESULTS

Of 4249 (32%) completed questionnaires returned, 488 reported gout or acute attacks and were invited for clinical assessment. Of 359 attendees, 164 clinically confirmed cases of gout were identified. Advice regarding alcohol consumption was recalled by 59 (41%), weight loss by 36 (25%) and diet by 42 (29%). Allopurinol was the only ULT used and was taken by 44 (30%); 31 (70%) were taking 300 mg daily. Mean SUA was lower in allopurinol users than non-users (318 vs 434 micromol/l) and was less often >360 micromol/l in allopurinol users (23% vs 75%). Eight patients had recently commenced allopurinol; two of these also were taking prophylactic colchicine or non-steroidal anti-inflammatory drugs. Of 25 patients with diuretic-induced gout, 16 (64%) were still taking a diuretic.

CONCLUSION

Treatment of chronic gout is often suboptimal and poorly concordant with EULAR recommendations. Lifestyle advice is infrequently offered, and allopurinol is restricted to a minority. Persistent hyperuricaemia was often seen in allopurinol non-users, but was also in allopurinol users, suggesting that doses >300 mg are often necessary.

Treatment of crystal arthropathy--history and advances

The history of gout and the many distinguished historical figures who have suffered the agonies of this crystal deposition disorder have claimed the attention of medical historians like no other disease. Its treatment with uric acid lowering drugs became a twentieth century paradigm for the successful management and prevention of a chronic rheumatic disease, but the colorful history of the treatment of gout and crystal deposition disorders stretches back over 4,000 years.

Pathophysiology, Clinical Presentation and Treatment of Gout

Gout is a common form of inflammatory arthritis that has been managed primarily in general medical practices for centuries. It appears that there has been an increasing prevalence of gout over the past decades, implying a growing public health burden. Accurate diagnosis and recognition of the various stages and manifestations of gout enable realistic goal setting for management. Recent evidence suggests new risk factors and potentially refutes others. Management of gout requires characterising and modifying risk factors and associated disorders, and commonly initiating drug therapy. Pharmacotherapy of gout includes the management of acute flares with anti-inflammatory agents such as NSAIDs and glucocorticoids and long-term treatment with urate-lowering drugs. Although pharmacotherapy is generally safe and effective, there are caveats and limitations to all gout therapies. Patient non-adherence and errors with the use of drugs for gout treatment are important factors leading to medical failures. With early intervention, careful monitoring and patient education, gout is a condition that can be managed very effectively. The advent of new drugs (such as febuxostat and urate oxidase [uricase]) and enhanced understanding of the pathogenesis of gout continue to improve our therapeutic options, particularly in a subset of patients with refractory disease and those who are intolerant to currently available medications.

Appropriate dosing regimen of allopurinol in Japanese patients1

OBJECTIVE

Approved dosage regimens for prescription drug products are developed with a view to obtaining a favourable therapeutic index in the overall exposed population. The purpose of this study was to examine differences between the approved dosage regimen and the clinically prescribed doses of allopurinol in major hospitals in Japan.

METHODS

The prescribing records for allopurinol were scrutinized at five national hospitals in Japan. Prescription information, including mean dose and the distribution of doses, was extracted for each hospital and the data compared with the dosage recommended in the approved labelling for the product. In addition, therapeutic drug monitoring (TDM) data were examined to evaluate relationships between dose administered, serum concentration of oxypurinol, and clinical efficacy.

RESULTS

The mean dose of allopurinol prescribed in the five institutions, 131.7 mg/day, was lower than the approved dosage of 200-300 mg/day. There were no differences in the mean dose between the hospitals, and similar dose distributions were seen among the hospitals. Approximately 60-70% of patients were treated with 100 mg/day and 20-30% with 200 mg/day of allopurinol. The most frequent dosage of allopurinol used in clinical practice was 100 mg/day. In the TDM study, the mean trough serum concentrations of oxypurinol were 9.5+/-3.6 microg/mL (50 mg/day), 13.0+/-6.8 microg/mL (100 mg/day), 19.8+/-12.9 microg/mL (200 mg/day) and 15.7+/-7.3 microg/mL (300 mg/day). The mean values of creatinine clearance were 17.0+/-16.4 mL/min (50 mg/day), 33.5+/-32.8 mL/min (100 mg/day), 57.8+/-33.8 mL/min (200 mg/day) and 94.3+/-35.8 mL/min (300 mg/day, in patients with normal renal function), and showed a downward trend together with a reduction of dosage of allopurinol. Allopurinol was given to 91% (91/100) of patients at a daily dose of 100-200 mg, and the oxypurinol trough serum concentration attained (>4.6 microg/mL) was sufficient to maintain a therapeutic effect in 92.3% (84/91) of these patients. A daily dose of 100-200 mg may be enough to obtain therapeutic serum oxypurinol concentrations in most Japanese patients.

CONCLUSIONS

Dose of 100-300 mg/day was an effective and commonly used dosing regimen for allopurinol in Japanese patients. The approved dosage range (200-300 mg/day) may be too high for patients with renal dysfunction, suggesting the recommended dosing regimen for allopurinol should be revised to include the lower doses.

Quality of care indicators for gout management

OBJECTIVE

Despite the significant health impact of gout, there is no consensus on management standards. To guide physician practice, we sought to develop quality of care indicators for gout management.

METHODS

A systematic literature review of gout therapy was performed using the Medline database. Two abstractors independently reviewed each of the articles for relevance and satisfaction of minimal inclusion criteria. Based on the review of the literature, 11 preliminary quality indicators were developed and then reviewed and refined by an initial feasibility panel of community and academic rheumatologists. A twelfth indicator was added at the request of the first panel. Using a modification of the RAND/University of California at Los Angeles appropriateness method (bridging teleconference and white-board Internet technology were added), a second expert panel rated each of the proposed indicators for validity using a 9-point scale, in which ratings of 1-3, 4-6, and 7-9 were considered "invalid," "indeterminate," and "highly valid," respectively. Indicators were considered valid if the median panel rating was > or =7 and there was no evidence of panel disagreement (defined to occur when 2 of 6 panelists provided a validity rating of 1-3 and 2 panelists provided a validity rating of 7-9).

RESULTS

Ten of the 12 draft indicators were rated to be valid by our second expert panel. Validated indicators pertained to 1) the use of urate-lowering medications in chronic gout, 2) the use of antiinflammatory drugs, and 3) counseling on lifestyle modifications.

CONCLUSION

Using a combination of evidence and expert opinion, 10 indicators for quality of gout care were developed. These indicators represent an important initial step in quality improvement initiatives for gout care.

The optimal use of allopurinol: an audit of allopurinol use in South Auckland

BACKGROUND

Gout is a common and challenging problem in South Auckland, New Zealand. Allopurinol is widely used but urate reduction remains unsatisfactory. Allopurinol dosing guidelines and a therapeutic range for plasma oxypurinol levels have been published.

AIMS

We aimed to determine the appropriateness of allopurinol dosing according to current guidelines and to assess the relationship between plasma creatinine, oxypurinol and urate. In addition, we assessed the clinical usefulness of the oxypurinol level.

METHODS

Thirty-one patients, on a stable dose of allopurinol for at least three weeks, had plasma creatinine, urate and oxypurinol measured as part of routine clinical assessment. Relationships between the various methods were examined using regression analysis. Fisher's exact test was used to test associations with categorical variables.

RESULTS

Fifty-five per cent of patients were on higher than recommended doses of allopurinol. There was a statistically significant relationship between calculated creatinine clearance and plasma oxypurinol level. Only 50% of patients with a plasma oxypurinol within the therapeutic range (30-100 micromol/L) had a plasma urate < 0.42 mmol/L and this did not increase significantly in the patients with an oxypurinol level > 100 micromol/L.

CONCLUSIONS

There is poor adherence to the current recommended dosing guidelines for allopurinol. Creatinine clearance rather than plasma creatinine needs to be used to predict the dose of allopurinol. The current role of the oxypurinol level is to identify non-compliers with allopurinol therapy. We need further research to clarify whether increasing the dose of allopurinol outside the recommended dose range to reach an oxypurinol level of close to 100 micromol/L may be of benefit in those who have not had sufficient urate reduction.

Gout in the elderly. Clinical presentation and treatment

Gout in the elderly differs from classical gout found in middle-aged men in several respects: it has a more equal gender distribution, frequent polyarticular presentation with involvement of the joints of the upper extremities, fewer acute gouty episodes, a more indolent chronic clinical course, and an increased incidence of tophi. Long term diuretic use in patients with hypertension or congestive cardiac failure, renal insufficiency, prophylactic low dose aspirin (acetylsalicylic acid), and alcohol (ethanol) abuse (particularly by men) are factors associated with the development of hyperuricaemia and gout in the elderly. Extreme caution is necessary when prescribing nonsteroidal anti-inflammatory drugs (NSAIDs) for the treatment of acute gouty arthritis in the elderly. NSAIDs with short plasma half-life (such as diclofenac and ketoprofen) are preferred, but these drugs are not recommended in patients with peptic ulcer disease, renal failure, uncontrolled hypertension or cardiac failure. Colchicine is poorly tolerated in the elderly and is best avoided. Intra-articular and systemic corticosteroids are increasingly being used for treating acute gouty flares in aged patients with medical disorders contraindicating NSAID therapy. Urate-lowering drugs are indicated for the treatment of hyperuricaemia and chronic gouty arthritis. Uricosuric drugs are poorly tolerated and the frequent presence of renal impairment in the elderly renders these drugs ineffective. Allopurinol is the urate-lowering drug of choice, but its use in the aged is associated with an increased incidence of both cutaneous and severe hypersensitivity reactions. To minimise this risk, allopurinol dose must be kept low. A starting dose of allopurinal 50 to 100mg on alternate days, to a maximum daily dose of about 100 to 300mg, based upon the patient's creatinine clearance and serum urate level, is recommended. Asymptomatic hyperuricaemia is not an indication for long term urate-lowering therapy; the risks of drug toxicity often outweigh any benefit.

The use of caffeine as a metabolic probe for human drug metabolizing enzymes

1. Caffeine (CA) is metabolized extensively and at least 17 metabolites arising from primary and secondary biotransformation pathways are found in urine following CA ingestion. The enzymes responsible for the formation of most of the metabolites derived from CA have been identified. 2. Given the near ubiquitous consumption of CA, this compound potentially constitutes a useful substrate probe for assessment of certain xenobiotic metabolizing enzyme activities in vivo. Indeed, various ratios of CA metabolites excreted in urine (urinary metabolic ratios; MRs) are now utilized widely for the population screening of enzyme activities. 3. Excretion of the acetylated secondary metabolite 5-actylamino-6-formylamino-3-methyluracil (AFMU) is dependent on the activity of the polymorphic N-acetyltransferase (NAT2), and certain MRs incorporating AFMU may be used for NAT2 phenotyping. 4. The conversion of 1-methylxanthine (1-MX), another secondary metabolite of CA, to 1-methyluric acid (1-MU) is catalyzed by xanthine oxidase (XO), and the urinary 1-MU to 1MX ratio reflects XO activity. 5. N3-demethylation to form paraxanthine (PX), a reaction mediated by cytochrome P4501A2 (CYP1A2), is the dominant primary metabolic pathway of CA. CA N3-demethylation activity may be used as a measure of human hepatic CYP1A2 in vitro. 6. Plasma CA clearance is considered to reflect CYP1A2 activity in vivo. Although a number of MRs are based on the excretion of PX metabolites (PX derived from CA is employed for the assessment of CYP1A2 activity in vivo), factors other than enzyme activity may affect these ratios.

The management of gout

We now have sufficient knowledge to be able to identify the factors contributing to hyperuricemia in most patients with gout. Some of these factors, such as obesity, a high-purine diet, regular alcohol consumption, and diuretic therapy, may be correctable. In patients with persistent hyperuricemia, regular medication should lower the serum urate concentration to an optimal level. The continuing challenge is to educate patients about correctable factors and the importance of regular medication and ensure their compliance so that attacks of gout do not recur.

Lack of a pharmacokinetic interaction between oral famciclovir and allopurinol in healthy volunteers

Famciclovir has been shown to have potent and selective activity against herpesviruses. The possibility of a pharmacokinetic interaction between the anti-viral agent, famciclovir and allopurinol has been investigated in twelve healthy male volunteers following a single oral dose of famciclovir (500 mg) in the presence and absence of steady-state levels of allopurinol (300 mg). Similarly, the pharmacokinetic profiles of allopurinol and oxypurinol prior to and following a single dose of famciclovir were compared. Mean values of Cmax, AUC and terminal-phase half-life for penciclovir following administration of famciclovir alone at 3.3 micrograms.ml-1, 8.8 micrograms.h.ml-1 and 2.1 h, respectively were unchanged by co-administration of allopurinol. Similarly, mean urinary recovery and renal clearance values of penciclovir following famciclovir alone were 56.8% and 27 l.h-1, and when given with allopurinol 59.7% and 27.5 l.h-1, respectively. No evidence of accumulation of the inactive precursor to penciclovir, BRL 42359, was noted as a result of co-administration of the two drugs. Mean steady-state Cmax, AUC and terminal-phase half-life values for allopurinol after co-administration of allopurinol with famciclovir also appeared unchanged from values obtained after dosing of allopurinol alone, at 2.12 micrograms.ml-1, 5.73 micrograms.h.ml-1 and 1.38 h, respectively. Mean Cmax and AUC values of the active metabolite of allopurinol, oxypurinol were 11.2 micrograms.ml-1 and 96.0 micrograms.h.ml-1, respectively, and these were also unaltered by co-administration of famciclovir with allopurinol, with values of 10.6 micrograms/ml and 89.8 micrograms.h/ml, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Allopurinol hypersensitivity syndrome: a review

OBJECTIVE

To review the pathophysiology, pathology, and clinical findings of allopurinol hypersensitivity syndrome (AHS), an infrequent but life-threatening adverse effect of allopurinol therapy.

DATA SOURCES

A MEDLINE search (key terms hepatitis, interstitial nephritis, severe hypersensitivity, severe toxicity, vasculitis, toxic epidermal necrolysis, Lyell's syndrome, erythema multiforme, and Stevens-Johnson syndrome) was used to identify cases reported in the literature through the end of 1990.

STUDY SELECTION

All cases evaluated met Singer and Wallace's diagnostic criteria for AHS.

DATA EXTRACTION

We extracted data from 101 cases of AHS reported in the literature. The following information, when available, was analyzed: (1) patient data (age, gender, medical history), (2) treatment data (daily dosage of allopurinol, duration of treatment, indications, concomitant medications, and (3) adverse-event data.

DATA SYNTHESIS

Patients were mostly middle-aged men with hypertension and/or renal failure receiving excessive doses of allopurinol primarily for asymptomatic hyperuricemia. Cutaneous rash and fever were the most common clinical findings.

CONCLUSIONS

Although the pathophysiologic pathway leading to the development of AHS is unknown, it probably involves an immunologic mechanism following allopurinol accumulation in patients with poor renal function. Our findings suggest that the accepted diagnostic criteria for AHS may be too broad, and we recommend the application of more restrictive criteria. There is no effective treatment for AHS. The use of allopurinol only for accepted indications and in dosages adjusted for a patient's renal function may be the only means of minimizing the incidence of AHS.

1-Methylxanthine derived from theophylline as an in vivo biochemical probe of allopurinol effect

The urinary 1-methyluric acid (1MU) to 1-methylxanthine (1MX) ratio has been assessed as a biochemical index of oxipurinol effect in vivo in man. Dosing with theophylline was used to produce 1MX as an intermediate metabolite in six healthy volunteers. A sigmoid Emax model was fitted to the data and gave a mean plasma oxipurinol IC50 of 3.0 +/- 1.1 mg l-1, a mean exponent n of 3.4 +/- 2.1 and a mean IC90 of 8.5 +/- 5.9 mg l-1. There was marked interindividual variability in the steepness of the plasma oxipurinol concentration response relationship, and in the plasma oxipurinol IC90 values. The study has confirmed the feasibility of using single doses of allopurinol to construct individual plasma oxipurinol concentration-response curves.

Dosage prescribing and plasma oxipurinol levels in patients receiving allopurinol therapy

This study examined dosage prescribing patterns and steady-state oxipurinol plasma concentrations in 66 patients receiving chronic allopurinol therapy. Most patients (65%) were taking 300 mg allopurinol daily, although renal impairment was common. Using published guidelines, it was estimated that 35% of patients were receiving excessive dosages of allopurinol. Consequently, the plasma oxipurinol concentrations were often very high (mean (SD) was 156 (109) mumol.l-1). Accumulation of oxipurinol was inversely related to renal function. Plasma concentrations of oxipurinol and urate were not significantly related. However, most patients with oxipurinol concentrations of up to 100 mumol.l-1 had urate concentrations within the normal reference range.

A survey of current prescribing practices of anti-inflammatory and urate-lowering drugs in gouty arthritis in New South Wales and Queensland

We recently have conducted a cross-sectional survey to determine the prescribing practices of rheumatologists and a random sample of general practitioners in New South Wales and Queensland. While in general there was agreement as to the preferred management of gout, several important differences were noted between the two groups of doctors. In particular, general practitioners were more liberal than were rheumatologists in their use of allopurinol. However, they were less likely to cover the introduction of allopurinol with anti-inflammatory agents, to titrate the dose against the serum uric acid level or to adjust the dose according to the serum creatinine level. A small number of doctors continued to use urate-lowering drugs as a routine in the treatment of entirely asymptomatic hyperuricaemia. The data indicate a continuing need to disseminate information regarding the preferred management of hyperuricaemic states.