Pharmacokinetic and pharmacodynamic interaction between allopurinol and probenecid in healthy subjects

Optimizing gout treatment: A comprehensive review of current and emerging uricosurics

Gout is the most common inflammatory arthritis, affecting approximately 5.1% of adults in the United States (US) population. Gout is a metabolic and autoinflammatory disease. Elevated uric acid pools lead to the precipitation of monosodium urate (MSU) crystals in and around joints, as well as other tissues, and the subsequent autoinflammatory response. Since elevated serum urate (SU) levels (hyperuricemia) correspond with gout severity, urate-lowering therapies (ULTs) are the cornerstone of gout treatment. ULTs include xanthine oxidoreductase inhibitors, uricosurics, less commonly used in the US but widely used in Europe and Asia, including benzbromarone, dotinurad, and probenecid (the only US Food and Drug Administration (FDA) approved uricosuric in the US), and uricases, including rasburicase and pegloticase (available only in the US). Over 90% of the daily load of uric acid filtered by the kidneys is reabsorbed through renal transporters. These urate transporters include uric acid transporter 1 (URAT1), glucose transporter 9, and organic anion transporters 1, 3, and 4 (OAT1, OAT3, OAT4). They are the target of approved and in-the-pipeline uricosurics. Any drug that increases renal excretion of uric acid, independently of the mechanism through which it exerts its effect, may be considered a uricosuric drug. This review discusses drugs that increase renal excretion of uric acid, either approved or in development, as well as off-label drugs with uricosuric properties.

Management of hypertension addressing hyperuricaemia: introduction of nano-based approaches

Uric acid (UA) levels in blood serum have been associated with hypertension, indicating a potential causal relationship between high serum UA levels and the progression of hypertension. Therefore, the reduction of serum UA level is considered a potential strategy for lowering and mitigating blood pressure. If an individual is at risk of developing or already manifesting elevated blood pressure, this intervention could be an integral part of a comprehensive treatment plan. By addressing hyperuricaemia, practitioners may subsidize the optimization of blood pressure regulation, which illustrates the importance of addressing UA levels as a valuable strategy within the broader context of hypertension management. In this analysis, we outlined the operational principles of effective xanthine oxidase inhibitors for the treatment of hyperuricaemia and hypertension, along with an exploration of the contribution of nanotechnology to this field.

Interactions between meropenem and renal drug transporters

BACKGROUND

Meropenem is a carbapenem antibiotic and commonly used with other antibiotics for the treatment of bacterial infections. It is primarily eliminated renally by glomerular filtration and renal tubular secretion.

OBJECTIVE

To evaluate the roles of renal uptake and efflux transporters in the excretion of meropenem and potential drug interactions mediated by renal drug transporters.

METHOD

Uptake and inhibition studies were conducted in human embryonic kidney 293 cells stably transfected with organic anion transporter (OAT) 1, OAT3, multidrug and toxin extrusion protein (MATE) 1 and MATE2K, as well as membrane vesicles containing breast cancer resistance-related protein (BCRP), multidrug resistance protein 1 (MDR1) and multidrug resistance-associated protein 2 (MRP2). Probenecid and piperacillin were used to assess potential drug interactions with meropenem in rats.

RESULTS

We observed that meropenem was a low-affinity substrate of OAT1/3 and had a weak inhibitory effect on OAT1/3 and MATE2K. BCRP, MDR1, MRP2, MATE1 and MATE2K could not mediate renal excretion of meropenem. Moreover, meropenem was not an inhibitor of BCRP, MDR1, MRP2 or MATE1. Among five tested antibiotics, moderate inhibition on OAT3-mediated meropenem uptake was observed for linezolid (IC50 value was 69.2 μM), weak inhibition was observed for piperacillin, benzylpenicillin and tazobactam (IC50 values were 282.2, 308.0 and 668.1 μM, respectively), and no inhibition was observed for sulbactam. Although piperacillin had a relatively high drug-drug interaction index (ratio of maximal unbound plasma concentration to IC50 was 1.42) in vitro, it had no meaningful impact on the pharmacokinetics of meropenem in rats.

CONCLUSION

Our results indicate that clinically significant interactions between meropenem and these five antibiotics are low.

Honorary Professor Garry Graham

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

Role of organic anion transporter 3 in the renal excretion of biapenem and potential drug-drug interactions

Biapenem is a carbapenem antibiotic. It is excreted predominantly through the kidney as unchanged forms. However, the molecular mechanism of renal excretion of biapenem and potential drug-drug interactions (DDIs) were still unknown. In the present study, the role of organic anion transporters (OAT) 1/3 and organic cation transporters (OCT) 2 in the renal excretion of biapenem, and the potential DDIs between biapenem and six clinical commonly prescribed antibiotics and antiviral drugs that acted as substrates or inhibitors of OAT3 were evaluated in vitro. Further, the effect of probenecid on the pharmacokinetics of biapenem was explored in the rats. We observed that biapenem could not inhibit the transport activities of OAT1 or OCT2, while mildly inhibited OAT3 (IC₅₀ >500 μM). Among the tested antibiotics and antiviral drugs, the relatively high DDI index values (maximal unbound plasma concentration over IC₅₀, I_max,u/IC₅₀) were found for piperacillin, linezolid and benzylpenicillin, which were 2.84, 1.7 and 0.62, respectively. Although probenecid had the highest DDI index (27.1) in vitro, no significant impact of it on the pharmacokinetics of biapenem was observed in the rats. Our results indicated that biapenem was primarily eliminated by the glomerular filtration, while OAT3-mediated renal tubular secretion was a minor route. Biapenem is not a clinically relevant substrate or inhibitor because of its low affinity to OAT3. According to current results, it would be safe to use biapenem with other antibiotics and antiviral drugs that acted as substrates or inhibitors of OAT3.

Individualising the dose of allopurinol in patients with gout

AIMS

The aims of the study were to: 1) determine if a plasma oxypurinol concentration-response relationship or an allopurinol dose-response relationship best predicts the dose requirements of allopurinol in the treatment of gout; and 2) to construct a nomogram for calculating the optimum maintenance dose of allopurinol to achieve target serum urate (SU) concentrations.

METHODS

A nonlinear regression analysis was used to examine the plasma oxypurinol concentration- and allopurinol dose-response relationships with serum urate. In 81 patients (205 samples), creatinine clearance (CL_CR ), concomitant diuretic use and SU concentrations before (U_P ) and during (U_T ) treatment were monitored across a range of allopurinol doses (D, 50-700 mg daily). Plasma concentrations of oxypurinol (C) were measured in 47 patients (98 samples). Models (n = 47 patients) and predictions from each relationship were compared using F-tests, r² values and paired t-tests. The best model was used to construct a nomogram.

RESULTS

The final plasma oxypurinol concentration-response relationship (U_T  = U_P  - C*(U_P  - U_R )/(ID₅₀  + C), r²  = 0.64) and allopurinol dose-response relationship (U_T  = U_P  - D^* (U_P  - U_R )/(ID₅₀  + D), r²  = 0.60) did not include CL_CR or diuretic use as covariates. There was no difference (P = 0.87) between the predicted SU concentrations derived from the oxypurinol concentration- and allopurinol dose-response relationships. The nomogram constructed using the allopurinol dose-response relationship for all recruited patients (n = 81 patients) required pretreatment SU as the predictor of allopurinol maintenance dose.

CONCLUSIONS

Plasma oxypurinol concentrations, CL_CR and diuretic status are not required to predict the maintenance dose of allopurinol. Using the nomogram, the maintenance dose of allopurinol estimated to reach target concentrations can be predicted from U_P .

Assessment of genetic polymorphisms associated with hyperuricemia or gout in the Hmong

AIM

Hyperuricemia commonly causes gout. Minnesota Hmong exhibit a two- to fivefold higher prevalence of gout versus non-Hmong. To elucidate a possible genomic contribution to this disparity, prevalence of risk alleles for hyperuricemia in Hmong was compared with European (CEU) and Han-Chinese (CHB).

METHODS

In total, 235 Hmong were genotyped for eight SNPs representing five candidate genes (SLC22A12, SLC2A9, ABCG2, SLC17A1 and PDZK1).

RESULTS

The frequency of seven out of eight risk alleles in the Hmong was significantly different than CEU; six higher and one with lower prevalence. The frequency of three out of eight risk alleles in the Hmong was significantly different than CHB; two higher and one with lower prevalence.

CONCLUSION

Hyperuricemia risk alleles are more prevalent in the Hmong than CEU and HB.

Predicting allopurinol response in patients with gout

AIMS

The primary aim of this research was to predict the allopurinol maintenance doses required to achieve the target plasma urate of ≤0.36 mmol l(-1) .

METHODS

A population analysis was conducted in nonmem using oxypurinol and urate plasma concentrations from 133 gout patients. Maintenance dose predictions to achieve the recommended plasma urate target were generated.

RESULTS

The urate response was best described by a direct effects model. Renal function, diuretic use and body size were found to be significant covariates. Dose requirements increased approximately 2-fold over a 3-fold range of total body weight and were 1.25-2 fold higher in those taking diuretics. Renal function had only a modest impact on dose requirements.

CONCLUSIONS

Contrary to current guidelines, the model predicted that allopurinol dose requirements were determined primarily by differences in body size and diuretic use. A revised guide to the likely allopurinol doses to achieve the target plasma urate concentration is proposed.

Insights into the poor prognosis of allopurinol-induced severe cutaneous adverse reactions: the impact of renal insufficiency, high plasma levels of oxypurinol and granulysin

OBJECTIVE

Allopurinol, an antihyperuricaemic agent, is one of the common causes of life-threatening severe cutaneous adverse reactions (SCAR), including drug rash with eosinophilia and systemic symptoms (DRESS), Stevens-Johnson syndrome (SJS) and toxic epidermal necrosis (TEN). The prognostic factors for allopurinol-related SCAR remain unclear. This study aimed to investigate the relationship of dosing, renal function, plasma levels of oxypurinol and granulysin (a cytotoxic protein of SJS/TEN), the disease severity and mortality in allopurinol-SCAR.

METHODS

We prospectively enrolled 48 patients with allopurinol-SCAR (26 SJS/TEN and 22 DRESS) and 138 allopurinol-tolerant controls from 2007 to 2012. The human leucocyte antigen (HLA)-B*58:01 status, plasma concentrations of oxypurinol and granulysin were determined.

RESULTS

In this cohort, HLA-B*58:01 was strongly associated with allopurinol-SCAR (p<0.001, OR (95% CI) 109 (25 to 481)); however, the initial/maintenance dosages showed no relationship with the disease. Poor renal function was significantly associated with the delayed clearance of plasma oxypurinol, and increased the risk of allopurinol-SCAR (p<0.001, OR (95% CI) 8.0 (3.9 to 17)). Sustained high levels of oxypurinol after allopurinol withdrawal correlated with the poor prognosis of allopurinol-SCAR. In particular, the increased plasma levels of oxypurinol and granulysin linked to the high mortality of allopurinol-SJS/TEN (p<0.01), and strongly associated with prolonged cutaneous reactions in allopurinol-DRESS (p<0.05).

CONCLUSIONS

Impaired renal function and increased plasma levels of oxypurinol and granulysin correlated with the poor prognosis of allopurinol-SCAR. Allopurinol prescription is suggested to be avoided in subjects with renal insufficiency and HLA-B*58:01 carriers. An early intervention to increase the clearance of plasma oxypurinol may improve the prognosis of allopurinol-SCAR.

Urate-lowering therapy: current options and future prospects for elderly patients with gout

Gout is increasingly seen in the elderly population, in large part due to physiological decline in renal function with age, and as a result of therapy for comorbidities, in particular the use of diuretic therapies for hypertension and congestive heart failure. Urate-lowering therapy (ULT) is the cornerstone of successful long-term gout management with the aim of achieving a sustained reduction in urate (<0.36 mmol/L, or lower [<0.30 mmol/L] in those with tophi). After decades during which there has been relatively little interest in developing new agents to treat gout, the last 5-10 years has seen a plethora of new agents with several now used in routine clinical practice. There has also been a renewed focus on the optimal use of established ULT, specifically allopurinol, which remains the first-line therapy for most patients. There is emerging data on its use in patients with renal impairment and better recognition of risk factors of the rare but potentially lethal allopurinol hypersensitivity syndrome (AHS). Febuxostat, a new xanthine oxidase inhibitor, is now established in everyday practice. Uricosuric agents may be indicated in certain patient groups, whilst a new class of recombinant uricases (pegloticase) given by intravenous infusion may achieve dramatic and rapid urate-lowering effects. Cost and other factors have thus far limited its use to the very severe cases. Furthermore, increased understanding of urate metabolism has led to the development of a number of drugs currently under clinical evaluation. Common therapeutic targets are the urate transporters in the kidney and alternative xanthine oxidase inhibition pathways. These advances bode well for the better management of gout and hyperuricaemia in our elderly patients.

Uricosuric medications for chronic gout

BACKGROUND

Uricosuric agents have long been used in the treatment of gout but there is little evidence regarding their benefit and safety in this condition.

OBJECTIVES

To assess the benefits and harms of uricosuric medications in the treatment of chronic gout.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL, Issue 4, 2013), Ovid MEDLINE and Ovid EMBASE for studies to the 13 May 2013. We also searched the World Health Organization Clinical Trials Registry, ClinicalTrials.gov and the 2011 to 2012 American College of Rheumatology and European League against Rheumatism abstracts. WE considered black box warnings and searched drug safety databases to identify and describe rare adverse events.

SELECTION CRITERIA

We considered all randomised controlled trials (RCTs) or quasi-randomised controlled trials (controlled clinical trials (CCTs)) that compared uricosuric medications (benzbromarone, probenecid or sulphinpyrazone) alone or in combination with another therapy (placebo or other active uric acid-lowering medication, or non-pharmacological treatment) in adults with chronic gout for inclusion.

DATA COLLECTION AND ANALYSIS

Two review authors independently selected the studies for inclusion, extracted data and performed a risk of bias assessment. Main outcomes were frequency of acute gout attacks, serum urate normalisation, study participant withdrawal due to adverse events, total adverse events, pain reduction, function and tophus regression.

MAIN RESULTS

The search identified four RCTs and one CCT that evaluated the benefit and safety of uricosurics for gout. One study (65 participants) compared benzbromarone with allopurinol for a duration of four months; one compared benzbromarone with allopurinol (36 participants) for a duration of nine to 24 months; one study (62 participants) compared benzbromarone with probenecid for two months and one study (74 participants) compared benzbromarone with probenecid. One study (37 participants) compared allopurinol with probenecid. No study was completely free from bias.Low-quality evidence from one study (55 participants) comparing benzbromarone with allopurinol indicated uncertain effects in terms of frequency of acute gout attacks (4% with benzbromarone versus 0% with allopurinol; risk ratio (RR) 3.58, 95% confidence interval (CI) 0.15 to 84.13), while moderate-quality evidence from two studies (101 participants; treated for four to nine months) indicated similar proportions of participants achieving serum urate normalisation (73.9% with benzbromarone versus 60% with allopurinol; pooled RR 1.27, 95% CI 0.90 to 1.79). Low-quality evidence indicated uncertain differences in withdrawals due to adverse events (7.1% with benzbromarone versus 6.1% with allopurinol; pooled RR 1.25, 95% CI 0.28 to 5.62), and total adverse events (20% with benzbromarone versus 6.7% with allopurinol; RR 3.00, 95% CI 0.64 to 14.16). The study did not measure pain reduction, function and tophus regression.When comparing benzbromarone with probenecid, there was moderate-quality evidence based on one study (62 participants) that participants taking benzbromarone were more likely to achieve serum urate normalisation after two months (81.5% with benzbromarone versus 57.1% with probenecid; RR 1.43, 95% CI 1.02 to 2.00). This indicated that when compared with probenecid, five participants needed to be treated with benzbromarone in order to have one additional person achieve serum urate normalisation (number needed to treat for an additional beneficial outcome (NNTB) 5). However, the second study reported no difference in the absolute decrease in serum urate between these groups after 12 weeks. Low-quality evidence from two studies (129 participants) indicated uncertain differences between treatments in the frequency of acute gout attacks (6.3% with benzbromarone versus 10.6% with probenecid; pooled RR 0.73, 95% CI 0.09 to 5.83); fewer withdrawals due to adverse events with benzbromarone (2% with benzbromarone versus 17% with probenecid; pooled RR 0.15, 95% CI 0.03 to 0.79, NNTB 7) and fewer total adverse events (21% with benzbromarone versus 47% with probenecid; pooled RR 0.43, 95% CI 0.25 to 0.74; NNTB 4). The studies did not measure pain reduction, function and tophus regression.Low-quality evidence based on one small CCT (37 participants) indicated uncertainty around the difference in the incidence of acute gout attacks between probenecid and allopurinol after 18 to 20 months' treatment (53% with probenecid versus 55% with allopurinol; RR 0.96, 95% CI 0.53 to 1.75). The study did not measure or report the proportion achieving serum urate normalisation, pain reduction, function, tophus regression, withdrawal due to adverse events and total adverse events.

AUTHORS' CONCLUSIONS

There was moderate-quality evidence that there is probably no important difference between benzbromarone and allopurinol at achieving serum urate normalisation, but that benzbromarone is probably more successful than probenecid at achieving serum urate normalisation in people with gout. There is some uncertainty around the effect estimates, based on low-quality evidence from only one or two trials, on the number of acute gout attacks, the number of withdrawals due to adverse events or the total number of participants experiencing adverse events when comparing benzbromarone with allopurinol. However, when compared with probenecid, benzbromarone resulted in fewer withdrawals due to adverse events and fewer participants experiencing adverse events. Low-quality evidence from one small study indicated uncertain effects in the incidence of acute gout attacks when comparing probenecid with allopurinol therapy. We downgraded the evidence because of a possible risk of performance and other biases and imprecision.

Allopurinol for chronic gout

BACKGROUND

Allopurinol, a xanthine oxidase inhibitor, is considered one of the most effective urate-lowering drugs and is frequently used in the treatment of chronic gout.

OBJECTIVES

To assess the efficacy and safety of allopurinol compared with placebo and other urate-lowering therapies for treating chronic gout.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE and EMBASE on 14 January 2014. We also handsearched the 2011 to 2012 American College of Rheumatology (ACR) and European League against Rheumatism (EULAR) abstracts, trial registers and regulatory agency drug safety databases.

SELECTION CRITERIA

All randomised controlled trials (RCTs) or quasi-randomised controlled clinical trials (CCTs) that compared allopurinol with a placebo or an active therapy in adults with chronic gout.

DATA COLLECTION AND ANALYSIS

We extracted and analysed data using standard methods for Cochrane reviews. The major outcomes of interest were frequency of acute gout attacks, serum urate normalisation, pain, function, tophus regression, study participant withdrawal due to adverse events (AE) and serious adverse events (SAE). We assessed the quality of the body of evidence for these outcomes using the GRADE approach.

MAIN RESULTS

We included 11 trials (4531 participants) that compared allopurinol (various doses) with placebo (two trials); febuxostat (four trials); benzbromarone (two trials); colchicine (one trial); probenecid (one trial); continuous versus intermittent allopurinol (one trial) and different doses of allopurinol (one trial). Only one trial was at low risk of bias in all domains. We deemed allopurinol versus placebo the main comparison, and allopurinol versus febuxostat and versus benzbromarone as the most clinically relevant active comparisons and restricted reporting to these comparisons here.Moderate-quality evidence from one trial (57 participants) indicated allopurinol 300 mg daily probably does not reduce the rate of gout attacks (2/26 with allopurinol versus 3/25 with placebo; risk ratio (RR) 0.64, 95% confidence interval (CI) 0.12 to 3.52) but increases the proportion of participants achieving a target serum urate over 30 days (25/26 with allopurinol versus 0/25 with placebo, RR 49.11, 95% CI 3.15 to 765.58; number needed to treat for an additional beneficial outcome (NNTB) 1). In two studies (453 participants), there was no significant increase in withdrawals due to AE (6% with allopurinol versus 4% with placebo, RR 1.36, 95% CI 0.61 to 3.08) or SAE (2% with allopurinol versus 1% with placebo, RR 1.93, 95% CI 0.48 to 7.80). One trial reported no difference in pain reduction or tophus regression, but did not report outcome data or measures of variance sufficiently and we could not calculate the differences between groups. Neither trial reported function.Low-quality evidence from three trials (1136 participants) indicated there may be no difference in the incidence of acute gout attacks with allopurinol up to 300 mg daily versus febuxostat 80 mg daily over eight to 24 weeks (21% with allopurinol versus 23% with febuxostat, RR 0.89, 95% CI 0.71 to 1.1); however more participants may achieve target serum urate level (four trials; 2618 participants) with febuxostat 80 mg daily versus allopurinol 300 mg daily (38% with allopurinol versus 70% with febuxostat, RR 0.56, 95% CI 0.48 to 0.65, NNTB with febuxostat 4). Two trials reported no difference in tophus regression between allopurinol and febuxostat over a 28- to 52-week period; but as the trialists did not provide variance, we could not calculate the mean difference between groups. The trials did not report pain reduction or function. Moderate-quality evidence from pooled data from three trials (2555 participants) comparing allopurinol up to 300 mg daily versus febuxostat 80 mg daily indicated no difference in the number of withdrawals due to AE (7% with allopurinol versus 8% with febuxostat, RR 0.89, 95% CI 0.62 to 1.26) or SAE (4% with allopurinol versus 4% with febuxostat, RR 1.13, 95% CI 0.71 to 1.82) over a 24- to 52-week period.Low-quality evidence from one trial (65 participants) indicated there may be no difference in the incidence of acute gout attacks with allopurinol up to 600 mg daily compared with benzbromarone up to 200 mg daily over a four-month period (0/30 with allopurinol versus 1/25 with benzbromarone, RR 0.28, 95% CI 0.01 to 6.58). Based on the pooled results of two trials (102 participants), there was moderate-quality evidence of no probable difference in the proportion of participants achieving a target serum urate level with allopurinol versus benzbromarone (58% with allopurinol versus 74% with benzbromarone, RR 0.79, 95% CI 0.56 to 1.11). Low-quality evidence from two studies indicated there may be no difference in the number of participants who withdrew due to AE with allopurinol versus benzbromarone over a four- to nine-month period (6% with allopurinol versus 7% with benzbromarone, pooled RR 0.80, 95% CI 0.18 to 3.58). There were no SAEs. They did not report tophi regression, pain and function.All other comparisons were supported by small, single studies only, limiting conclusions.

AUTHORS' CONCLUSIONS

Our review found low- to moderate-quality evidence indicating similar effects on withdrawals due to AEs and SAEs and incidence of acute gout attacks when allopurinol (100 to 600 mg daily) was compared with placebo, benzbromarone (100 to 200 mg daily) or febuxostat (80 mg daily). There was moderate-quality evidence of little or no difference in the proportion of participants achieving target serum urate when allopurinol was compared with benzbromarone. However, allopurinol seemed more successful than placebo and may be less successful than febuxostat (80 mg daily) in achieving a target serum urate level (6 mg/dL or less; 0.36 mmol/L or less) based on moderate- to low-quality evidence. Single studies reported no difference in pain reduction when allopurinol (300 mg daily) was compared with placebo over 10 days, and no difference in tophus regression when allopurinol (200 to 300 mg daily) was compared with febuxostat (80 mg daily). None of the trials reported on function, health-related quality of life or participant global assessment of treatment success, where further research would be useful.

Uricosuric drugs: the once and future therapy for hyperuricemia?

PURPOSE OF REVIEW

Although uricosuric agents provide the most time-honoured approach to the control of hyperuricemia, their place in the armamentarium has been eclipsed by that of xanthine oxidase inhibitors. This review considers the potential for uricosuric agents from the perspective of recent progress in the understanding of urate transport systems.

RECENT FINDINGS

No new agents have yet become available, but promising new drugs are under development. Better understanding of the transporters URAT1 and ABCG2 in particular would appear to provide opportunities for more selective, better tolerated agents to increase the renal clearance of uric acid and thereby control hyperuricemia.

SUMMARY

Conceptually, modest inhibition of renal tubular reabsorption should provide effective relief for the millions of individuals who are now hyperuricemic and who suffer from its principal consequence, gout.

Pharmacokinetics considerations for gout treatments

INTRODUCTION

Patients with gout often have comorbid conditions such as renal failure, cardiovascular disease and metabolic syndrome. The presence and required treatment of these conditions can make the treatment of gout challenging. Knowledge of the pharmacokinetics of the available drugs for the management of gout is mandatory.

AREAS COVERED

A MEDLINE PubMed search for articles published in English from January 1990 to January 2014 was completed using the terms: pharmacokinetics, colchicine, canakinumab, allopurinol, febuxostat, pegloticase, gout, toxicity, drug interaction.

EXPERT OPINION

Colchicine is a drug with a narrow therapeutic-toxicity window. Co-prescription with strong CYP3A4 or P-glycoprotein inhibitors can greatly modify its pharmacokinetics and is to be avoided. Elimination of canakinumab mainly occurs via intracellular catabolism, following receptor mediator endocytosis. Canakinumab appears to be a good alternative for patients with contraindications to colchicine, NSAIDs and corticosteroids. For patients with renal impairment, some authors recommend that the allopurinol maximum dosage should be adjusted to creatinine clearance. If the urate target cannot be achieved, the therapy should be switched to febuxostat, which is appropriate with mild-to-moderate renal failure. Anti-pegloticase antibodies affect the pharmacokinetics of the drug because they increase its clearance, with loss of pegloticase activity.

An audit of a therapeutic drug monitoring service for allopurinol therapy

BACKGROUND

Oxypurinol, the active metabolite of allopurinol, is the major determinant of the hypouricemic effect of allopurinol. Monitoring oxypurinol concentrations is undertaken to determine adherence to therapy, to investigate reasons for continuing attacks of acute gout and/or insufficiently low plasma urate concentrations despite allopurinol treatment, and to assess the risk of allopurinol hypersensitivity, an adverse effect that has been putatively associated with elevated plasma oxypurinol concentrations.

METHODS

An audit of request forms requesting plasma oxypurinol concentration measurements received by the pathology service (SydPath) at St Vincent's Hospital, Darlinghurst, Sydney was undertaken for the 7-year period January 2005-December 2011. Patient demographics, biochemical data, including plasma creatinine and uric acid concentrations, comorbidities, and concomitant medications were recorded.

RESULTS

There were 412 requests for determination of an oxypurinol concentration. On 48% of occasions, the time of allopurinol dosing was recorded, while just 79 (19%) blood samples were collected 6-9 hours postdosing, the time window used to establish the therapeutic range for oxypurinol. For these optimally interpretable concentrations, 32 (8%) were within the putative therapeutic range (5-15 mg/L), while 5 (1%) were below and 41 (10%) above this range. The daily dose of allopurinol was documented on only one-third of the request forms. Individually, plasma urate and creatinine concentrations were requested concomitantly with plasma oxypurinol concentrations in 66% and 58% of the cases, respectively; while plasma oxypurinol, urate, and creatinine concentrations were requested concomitantly in 49% of the cases.

CONCLUSIONS

Requesting clinicians and blood specimen collectors often fail to provide relevant information (dose, times of last dose, and blood sample collection) to allow the most useful interpretation of oxypurinol concentrations. Concomitant plasma urate and creatinine concentrations should be requested to allow more complete interpretation of the data.

How do taste cells lacking synapses mediate neurotransmission? CALHM1, a voltage-gated ATP channel

CALHM1 was recently demonstrated to be a voltage-gated ATP-permeable ion channel and to serve as a bona fide conduit for ATP release from sweet-, umami-, and bitter-sensing type II taste cells. Calhm1 is expressed in taste buds exclusively in type II cells and its product has structural and functional similarities with connexins and pannexins, two families of channel protein candidates for ATP release by type II cells. Calhm1 knockout in mice leads to loss of perception of sweet, umami, and bitter compounds and to impaired gustatory nerve responses to these tastants. These new studies validate the concept of ATP as the primary neurotransmitter from type II cells to gustatory neurons. Furthermore, they identify voltage-gated ATP release through CALHM1 as an essential molecular mechanism of ATP release in taste buds. We discuss these new findings, as well as unresolved issues in peripheral taste signaling that we hope will stimulate future research.

The pharmacokinetics of oxypurinol in people with gout

AIMS

Our aim was to identify and quantify the sources of variability in oxypurinol pharmacokinetics and explore relationships with plasma urate concentrations.

METHODS

Non-linear mixed effects modelling was applied to concentration-time data from 155 gouty patients with demographic, medical history and renal transporter genotype information.

RESULTS

A one compartment pharmacokinetic model with first order absorption best described the oxypurinol concentration-time data. Renal function and concomitant medicines (diuretics and probenecid), but not transporter genotype, significantly influenced oxypurinol pharmacokinetics and reduced the between subject variability in the apparent clearance of oxypurinol (CL/F(m)) from 65% to 29%. CL/F(m) for patients with normal, mild, moderate and severe renal impairment was 1.8, 0.6, 0.3 and 0.18 l h(-1), respectively. Model predictions showed a relationship between plasma oxypurinol and urate concentrations and failure to reach target oxypurinol concentrations using suggested allopurinol dosing guidelines.

CONCLUSIONS

In conclusion, this first established pharmacokinetic model provides a tool to achieve target oxypurinol plasma concentrations, thereby optimizing the effectiveness and safety of allopurinol therapy in gouty patients with various degrees of renal impairment.

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.

Fractional clearance of urate: validation of measurement in spot-urine samples in healthy subjects and gouty patients

INTRODUCTION

Hyperuricemia is the greatest risk factor for gout and is caused by an overproduction and/or inefficient renal clearance of urate. The fractional renal clearance of urate (FCU, renal clearance of urate/renal clearance of creatinine) has been proposed as a tool to identify subjects who manifest inefficient clearance of urate. The aim of the present studies was to validate the measurement of FCU by using spot-urine samples as a reliable indicator of the efficiency of the kidney to remove urate and to explore its distribution in healthy subjects and gouty patients.

METHODS

Timed (spot, 2-hour, 4-hour, 6-hour, 12-hour, and 24-hour) urine collections were used to derive FCU in 12 healthy subjects. FCUs from spot-urine samples were then determined in 13 healthy subjects twice a day, repeated on 3 nonconsecutive days. The effect of allopurinol, probenecid, and the combination on FCU was explored in 11 healthy subjects. FCU was determined in 36 patients with gout being treated with allopurinol. The distribution of FCU was examined in 118 healthy subjects and compared with that from the 36 patients with gout.

RESULTS

No substantive or statistically significant differences were observed between the FCUs derived from spot and 24-hour urine collections. Coefficients of variation (CVs) were both 28%. No significant variation in the spot FCU was obtained either within or between days, with mean intrasubject CV of 16.4%. FCU increased with probenecid (P < 0.05), whereas allopurinol did not change the FCU in healthy or gouty subjects. FCUs of patients with gout were lower than the FCUs of healthy subjects (4.8% versus 6.9%; P < 0.0001).

CONCLUSIONS

The present studies indicate that the spot-FCU is a convenient, valid, and reliable indicator of the efficiency of the kidney in removing urate from the blood and thus from tissues. Spot-FCU determinations may provide useful correlates in studies investigating molecular mechanisms underpinning the observed range of efficiencies of the kidneys in clearing urate from the blood.

TRIAL REGISTRATION

ACTRN12611000743965.

2011 recommendations for the diagnosis and management of gout and hyperuricemia

Gout is a major health problem in the United States; it affects 8.3 million people, which is approximately 4% of the adult population. Gout is most often diagnosed and managed in primary care practices; thus, primary care physicians have a significant opportunity to improve patient outcomes. Following publication of the 2006 European League Against Rheumatism (EULAR) gout guidelines, significant new evidence has accumulated, and new treatments for patients with gout have become available. It is the objective of these 2011 recommendations to update the 2006 EULAR guidelines, paying special attention to the needs of primary care physicians. The revised 2011 recommendations are based on the Grading of Recommendations Assessment, Development, and Evaluation approach as an evidence-based strategy for rating quality of evidence and grading the strength of recommendation formulated for use in clinical practice. A total of 26 key recommendations, 10 for diagnosis and 16 for management, of patients with gout were evaluated, resulting in important updates for patient care. The presence of monosodium urate crystals and/or tophus and response to colchicine have the highest clinical diagnostic value. The key aspect of effective management of an acute gout attack is initiation of treatment within hours of symptom onset. Low-dose colchicine is better tolerated and is as effective as a high dose. When urate-lowering therapy (ULT) is indicated, the xanthine oxidase inhibitors allopurinol and febuxostat are the options of choice. Febuxostat can be prescribed at unchanged doses for patients with mild-to-moderate renal or hepatic impairment. The target of ULT should be a serum uric acid level that is ≤ 6 mg/dL. For patients with refractory and tophaceous gout, intravenous pegloticase is a new treatment option. This article is a summary of the 2011 clinical guidelines published in Postgraduate Medicine. This article provides a streamlined, accessible overview intended for quick review by primary care physicians, with the full guidelines being a resource for those seeking additional background information and expanded discussion.

Probenecid inhibits the human bitter taste receptor TAS2R16 and suppresses bitter perception of salicin

Bitter taste stimuli are detected by a diverse family of G protein-coupled receptors (GPCRs) expressed in gustatory cells. Each bitter taste receptor (TAS2R) responds to an array of compounds, many of which are toxic and can be found in nature. For example, human TAS2R16 (hTAS2R16) responds to β-glucosides such as salicin, and hTAS2R38 responds to thiourea-containing molecules such as glucosinolates and phenylthiocarbamide (PTC). While many substances are known to activate TAS2Rs, only one inhibitor that specifically blocks bitter receptor activation has been described. Here, we describe a new inhibitor of bitter taste receptors, p-(dipropylsulfamoyl)benzoic acid (probenecid), that acts on a subset of TAS2Rs and inhibits through a novel, allosteric mechanism of action. Probenecid is an FDA-approved inhibitor of the Multidrug Resistance Protein 1 (MRP1) transporter and is clinically used to treat gout in humans. Probenecid is also commonly used to enhance cellular signals in GPCR calcium mobilization assays. We show that probenecid specifically inhibits the cellular response mediated by the bitter taste receptor hTAS2R16 and provide molecular and pharmacological evidence for direct interaction with this GPCR using a non-competitive (allosteric) mechanism. Through a comprehensive analysis of hTAS2R16 point mutants, we define amino acid residues involved in the probenecid interaction that result in decreased sensitivity to probenecid while maintaining normal responses to salicin. Probenecid inhibits hTAS2R16, hTAS2R38, and hTAS2R43, but does not inhibit the bitter receptor hTAS2R31 or non-TAS2R GPCRs. Additionally, structurally unrelated MRP1 inhibitors, such as indomethacin, fail to inhibit hTAS2R16 function. Finally, we demonstrate that the inhibitory activity of probenecid in cellular experiments translates to inhibition of bitter taste perception of salicin in humans. This work identifies probenecid as a pharmacological tool for understanding the cell biology of bitter taste and as a lead for the development of broad specificity bitter blockers to improve nutrition and medical compliance.

Pharmacokinetic and Pharmacodynamic Interaction Between Allopurinol and Probenecid in Patients with Gout

Objective.

To investigate the pharmacokinetic and pharmacodynamic interaction between probenecid and oxypurinol (the active metabolite of allopurinol) in patients with gout.

Methods.

This was an open-label observational clinical study. Blood and urine samples were collected to measure oxypurinol and urate concentrations. We examined the effects of adding probenecid to allopurinol therapy upon plasma concentrations and renal clearances of urate and oxypurinol.

Results.

Twenty patients taking allopurinol 100–400 mg daily completed the study. Maximum coadministered doses of probenecid were 250 mg/day (n = 1), 500 mg/day (n = 19), 1000 mg/day (n = 7), 1500 mg/day (n = 3), and 2000 mg/day (n = 1). All doses except the 250 mg daily dose were divided and dosing was twice daily. Estimated creatinine clearances ranged from 28 to 113 ml/min. Addition of probenecid 500 mg/day to allopurinol therapy decreased plasma urate concentrations by 25%, from mean 0.37 mmol/l (95% CI 0.33–0.41) to mean 0.28 mmol/l (95% CI 0.24–0.32) (p < 0.001); and increased renal urate clearance by 62%, from mean 6.0 ml/min (95% CI 4.5–7.5) to mean 9.6 ml/min (95% CI 6.9–12.3) (p < 0.001). Average steady-state plasma oxypurinol concentrations decreased by 26%, from mean 11.1 mg/l (95% CI 5.0–17.3) to mean 8.2 mg/l (95% CI 4.0–12.4) (p < 0.001); and renal oxypurinol clearance increased by 24%, from mean 12.7 ml/min (95% CI 9.6–15.8) to mean 16.1 ml/min (95% CI 12.0–20.2) (p < 0.05). The additional hypouricemic effect of probenecid 500 mg/day appeared to be lower in patients with renal impairment.

Conclusion.

Coadministration of allopurinol with probenecid had a significantly greater hypouricemic effect than allopurinol alone despite an associated reduction of plasma oxypurinol concentrations. Australian Clinical Trials Registry ACTRN012606000276550.

Lack of effect of hydrochlorothiazide and low-dose aspirin on the renal clearance of urate and oxypurinol after a single dose of allopurinol in normal volunteers

AIMS

To determine whether low-dose aspirin and hydrochlorothiazide (HCTZ) affect the renal clearance of oxypurinol and/or urate.

METHODS

Healthy volunteers (n = 8) were treated with allopurinol (600 mg, control), and allopurinol (600 mg) co-administered with single doses of aspirin (100 mg) or HCTZ (25 mg) or a combination of the two.

RESULTS

Hydrochlorothiazide, low-dose aspirin or a combination of the two, when co-administered with allopurinol, did not significantly alter (P > 0.05) the renal clearance of oxypurinol or urate. In particular, aspirin and HCTZ, when taken together and with allopurinol, did not change (P > 0.05) oxypurinol fractional renal clearance (allopurinol alone: 0.217, 0.173-0.262; combined: 0.202, 0.155-0.250) or urate fractional renal clearance (allopurinol alone: 0.066, 0.032-0.099; combined: 0.058, 0.038-0.078).

CONCLUSIONS

A single, low-dose of aspirin or an anti-hypertensive dose of hydrochlorothiazide, when administered alone or together with allopurinol, are unlikely to alter the hypouricaemic effect of allopurinol. The effect of chronic aspirin and HCTZ dosing taken together upon the efficacy of chronic allopurinol therapy in patients with hyperuricaemia needs to be investigated.

Progress in the pharmacotherapy of gout

PURPOSE OF REVIEW

To summarize new knowledge on approved and emerging drugs used to treat hyperuricemia or the clinical manifestations of gout.

RECENT FINDINGS

Results of several clinical trials provide new data on the efficacy and safety of the approved urate-lowering drugs, allopurinol and febuxostat. New recommendations have been presented on appropriate dosing of colchicine for acute gout flares and potential toxicities of combining colchicine with medications such as clarithromycin. Emerging therapies, including pegloticase, the uricosuric agent RDEA596, and the interleukin-1 inhibitors, rilonacept and canakinumab, have shown promise in early and late phase clinical trials.

SUMMARY

Recent publications demonstrate an opportunity to use existing gout therapies more effectively in order to improve both efficacy and safety. Emerging therapies for gout show promise for unmet needs in selected gout populations.

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.