Recent developments in our understanding of the renal basis of hyperuricemia and the development of novel antihyperuricemic therapeutics

Drug discovery for hyperuricemia

Hyperuricemia is associated with an increased risk of developing gout. This increases with the degree and duration of hyperuricemia. Gout can be managed by dietary modification and pharmacologic urate-lowering therapies. The recent identification of the renal apical urate/anion exchanger URAT1 (SLC22A12) and several membrane proteins relevant to the transport of urate play an important role in gaining a better understanding of the mode of action of many drugs used to treat gout. As described in this review, therapeutics designed to modify URAT1 transport activities might be useful in treating pathologies associated with hyperuricemia such as gout and urolithiasis. Continuing studies into the urate transportsome hold promise for the development of new, more effective therapeutics for hyperuricemia.

Hyperuricemia and gout: new concepts in diagnosis and management

Gout is a chronic, progressive condition for which hyperuricemia is the primary risk factor. The initial episodes of gout may be brief, only lasting for 3 to 5 days, and patients may experience pain-free intercritical periods that last from months to years. However, as the disease progresses, acute gout flares become more frequent and prolonged (typically lasting ≥ 5-10 days). Chronic gouty arthritis develops, with shorter pain-free intervals; tophi become visible and interarticular joint damage occurs. Patients with advanced gout experience chronic pain and a decreased quality of life. Gout prevalence has increased significantly over time. Despite the increase in the number of gout cases, the disease is often mismanaged, especially in primary care. Hyperuricemia is inadequately controlled as a result of suboptimal dosing with urate-lowering drugs, intolerance to therapy, or poor patient compliance. This review article provides a comprehensive discussion of gout pathophysiology, risk factors, and approaches to treatment that encourage the clinician to appreciate hyperuricemia as a multifaceted disorder and manage the condition optimally.

Common genetic variants of the human uromodulin gene regulate transcription and predict plasma uric acid levels

Uromodulin (UMOD) genetic variants cause familial juvenile hyperuricemic nephropathy, characterized by hyperuricemia with decreased renal excretion of UMOD and uric acid, suggesting a role for UMOD in the regulation of plasma uric acid. To determine this, we screened common variants across the UMOD locus in one community-based Chinese population of 1000 individuals and the other population from 642 American twins and siblings of European and Hispanic ancestry. Transcriptional activity of promoter variants was estimated in luciferase reporter plasmids transfected into HEK-293 cells and mIMCD3 cells. In the primary Chinese population, we found that carriers of the GCC haplotype had higher plasma uric acid, and three promoter variants were associated with plasma uric acid. UMOD promoter variants displayed reciprocal effects on urine uric acid excretion and plasma uric acid concentration, suggesting a primary effect on renal tubular handling of urate. These UMOD genetic marker-on-trait associations for uric acid were replicated in the independent American cohort. Site-directed mutagenesis at trait-associated UMOD promoter variants altered promoter activity in transfected luciferase reporter plasmids. Thus, UMOD promoter variants seem to initiate a cascade of transcriptional and biochemical changes influencing UMOD secretion, leading to altered plasma uric acid levels.

Efficacy and tolerability of febuxostat in hyperuricemic patients with or without gout: a systematic review and meta-analysis

BACKGROUND

Febuxostat has been approved for the treatment of hyperuricemia in patients with/without gout.

OBJECTIVES

This meta-analysis and systematic review assessed the efficacy and tolerability of febuxostat in hyperuricemic patients with/without gout.

METHODS

Major electronic databases were searched for articles of all publication years (up to February 2012), as were the Web sites of the American College of Rheumatology, the European League Against Rheumatism, and the Chinese State Food and Drug Administration, and clinicaltrials.gov for unpublished studies. Only randomized, controlled trials (RCTs) were included.

RESULTS

Ten trials were included. A significantly greater proportion of patients achieved the target serum urate level (sUA ≤6.0 mg/dL) at the final visit in the febuxostat group compared with the placebo (OR = 235.73; P < 0.01) and allopurinol groups (OR = 3.14; P < 0.01). In subgroup analysis, the proportion of patients who achieved target sUA at the final visit was significantly greater in the febuxostat-treated group (40 mg/d) compared with the allopurinol-treated group (100-300 mg/d) (50.9% vs 45.6%; OR = 1.25; 95% CI, 1.05-1.49; P = 0.01). As the dosage was increased (40, 80, 120 mg/d), the proportion of patients who achieved target sUA in the febuxostat-treated group increased gradually (50.9%, 71.4%, 82%, respectively). There was no significant difference in the occurrence of adverse events (AEs) between the febuxostat- and allopurinol-treated groups.

CONCLUSION

Febuxostat was effective in reducing serum urate in hyperuricemic patients with/without gout, and febuxostat (40-120 mg/d) was more efficacious compared with allopurinol (100-300 mg/d). The doses of allopurinol to which febuxostat has been compared, although commonly prescribed, are low in the range of approved doses of allopurinol. The tolerability of febuxostat for the treatment of hyperuricemia with/without gout is similar to that of allopurinol.

High plasma uric acid concentration: causes and consequences

High plasma uric acid (UA) is a precipitating factor for gout and renal calculi as well as a strong risk factor for Metabolic Syndrome and cardiovascular disease. The main causes for higher plasma UA are either lower excretion, higher synthesis or both. Higher waist circumference and the BMI are associated with higher insulin resistance and leptin production, and both reduce uric acid excretion. The synthesis of fatty acids (tryglicerides) in the liver is associated with the de novo synthesis of purine, accelerating UA production. The role played by diet on hyperuricemia has not yet been fully clarified, but high intake of fructose-rich industrialized food and high alcohol intake (particularly beer) seem to influence uricemia. It is not known whether UA would be a causal factor or an antioxidant protective response. Most authors do not consider the UA as a risk factor, but presenting antioxidant function. UA contributes to > 50% of the antioxidant capacity of the blood. There is still no consensus if UA is a protective or a risk factor, however, it seems that acute elevation is a protective factor, whereas chronic elevation a risk for disease.

Tocilizumab treatment decreases circulating myeloid dendritic cells and monocytes, 2 components of the myeloid lineage

OBJECTIVE

Interleukin 6 (IL-6) and tumor necrosis factor-α (TNF-α) are proinflammatory cytokines involved in inflammatory response. Effective TNF-α blocker treatment is associated with an increase in circulating myeloid dendritic cells (mDC), suggesting their release from inflamed synovium. Currently, in vivo effects of IL-6 inhibition on DC are unknown. We monitored the changes in circulating mDC and plasmacytoid DC (pDC) during tocilizumab (TCZ) therapy in patients with rheumatoid arthritis (RA).

METHODS

DC subset levels were evaluated by flow cytometry in patients with RA (n = 43) and in healthy volunteers (n = 20). In patients with RA, these levels were measured before and during TCZ therapy (8 mg/kg every 4 weeks). Response to TCZ therapy was evaluated at 12 weeks. Statistical analysis was based on Mann-Whitney U tests or Wilcoxon signed-rank tests.

RESULTS

At baseline, patients with active RA were characterized by a significantly lower level of circulating mDC and pDC compared to healthy donors. However, this difference did not correlate with any disease activity score. TCZ-treated patients who met the European League Against Rheumatism (EULAR) improvement criteria at Week 12 had significant reductions in mDC and monocyte levels as compared with EULAR nonresponders. Levels of pDC, CD4+ T cells, and CD8+ T cells remained stable during the TCZ courses, regardless of treatment response.

CONCLUSION

Our study reveals an unexpected reduction of circulating mDC and monocytes in patients with RA in response to TCZ therapy. In accord with reports on neutrophils and platelets decreasing during TCZ therapy, our data suggest an effect of IL-6 inhibition on cells from myeloid lineage.

Uric acid as one of the important factors in multifactorial disorders--facts and controversies

With considering serum concentration of the uric acid in humans we are observing hyperuricemia and possible gout development. Many epidemiological studies have shown the relationship between the uric acid and different disorders such are obesity, metabolic syndrome, hypertension and coronary artery disease. Clinicians and investigators recognized serum uric acid concentration as very important diagnostic and prognostic factor of many multifactorial disorders. This review presented few clinical conditions which are not directly related to uric acid, but the concentrations of uric acid might have a great impact in observing, monitoring, prognosis and therapy of such disorders. Uric acid is recognized as a marker of oxidative stress. Production of the uric acid includes enzyme xanthine oxidase which is involved in producing of radical-oxigen species (ROS). As by-products ROS have a significant role in the increased vascular oxidative stress and might be involved in atherogenesis. Uric acid may inhibit endothelial function by inhibition of nitric oxide-function under conditions of oxidative stress. Down regulation of nitric oxide and induction of endothelial dysfunction might also be involved in pathogenesis of hypertension. The most important and well evidenced is possible predictive role of uric acid in predicting short-term outcome (mortality) in acute myocardial infarction (AMI) patients and stroke. Nephrolithiasis of uric acid origin is significantly more common among patients with the metabolic syndrome and obesity. On contrary to this, uric acid also acts is an “antioxidant”, a free radical scavenger and a chelator of transitional metal ions which are converted to poorly reactive forms.

Polymorphisms in the presumptive promoter region of the SLC2A9 gene are associated with gout in a Chinese male population

Background

Glucose transporter 9 (GLUT9) is a high-capacity/low-affinity urate transporter. To date, several recent genome-wide association studies (GWAS) and follow-up studies have identified genetic variants of SLC2A9 associated with urate concentrations and susceptibility to gout. We therefore investigated associations between gout and polymorphisms and haplotypes in the presumptive promoter region of GLUT9 in Chinese males.

Methodology/Principal Findings

The approximately 2000 bp presumptive promoter region upstream of the start site of exon 1 of GLUT9 was sequenced and subjected to genetic analysis. A genotype-phenotype correlation was performed and polymorphisms-induced changes in transcription factor binding sites were predicted. Of 21 SNPs identified in GLUT9, five had not been previously reported. Two of the SNPs (rs13124007 and rs6850166) were associated with susceptibility to gout (p = 0.009 and p = 0.042, respectively). The C allele of rs13124007 appeared to be the risk allele for predisposition to gout (p = 0.006, OR 1.709 [95% CI 1.162–2.514]). For rs6850166, an increased risk of gout was associated with the A allele (p = 0.029, OR 1.645 [95% CI 1.050–2.577]). After Bonferroni correction, there was statistically difference in rs13124007 allele frequencies between gout cases and controls (P = 0.042). Haplotype analyses showed that haplotype GG was a protective haplotype (p = 0.0053) and haplotype CA was associated with increased risk of gout (p = 0.0326). Genotype-phenotype analysis among gout patients revealed an association of rs13124007 with serum triglycerides levels (P = 0.001). The C to G substitution in polymorphism rs13124007 resulted in a loss of a binding site for transcription factor interferon regulatory factor 1 (IRF-1).

Conclusions/Significance

Polymorphisms rs13124007 and rs6850166 are associated with susceptibility to gout in Chinese males.

G109T polymorphism of SLC22A12 gene is associated with serum uric acid level, but not with metabolic syndrome

SLC22A12 gene, encoding urate transport 1, has been known to be responsible to urate metabolism. This study sought to determine the association between the novel G109T polymorphism in SLC22A12 with serum uric acid and the development of metabolic syndrome in Korean male subjects. A total of 132 healthy male subjects were enrolled in this study. Metabolic syndrome was determined using the modified guidelines for metabolic syndrome proposed by the National Cholesterol Education Program's Third Adult Treatment Panel. Genotyping for the SLC22A12 gene was assessed using denaturing high-performance liquid chromatography analysis. Serum uric acid and fractional excretion of uric acid (FEUA) from blood and urine samples were measured. Frequencies of the 109GG, 109GT, and 109TT genotypes were 57.6, 38.6, and 3.8%, respectively. Serum uric acid levels and FEUAs were significantly different among the three genotypes of the G109T polymorphism (P = 0.035 and P = 0.033, respectively). In addition, subjects of genotypes with the T allele had lower uric acid levels and higher FEUAs compared to those with the 109GG genotype (P = 0.007 and P = 0.031, respectively). The G109T polymorphism of the SLC22A12 gene has no association with metabolic syndrome. However, a number of metabolic syndrome components were related to serum uric acid level (r = 0.285, P = 0.001) and also significantly different between genotype with and without T allele (P = 0.008). The novel G109T polymorphism of the SLC22A12 gene is related to serum uric acid level, but not to the development of metabolic syndrome.

Decreased leukocyte accumulation and delayed Bordetella pertussis clearance in IL-6-/- mice

IL-6, a pleiotropic cytokine primarily produced by the innate immune system, has been implicated in the development of acquired immune responses, though its roles are largely undefined and may vary in the context of different diseases. Using a murine model of infection, we established that IL-6 influences the adaptive immune responses against the endemic human respiratory pathogen Bordetella pertussis. IL-6 was induced in the lungs of C57BL/6 mice by B. pertussis. IL-6(-/-) mice showed a protracted infectious course and were less efficiently protected by B. pertussis vaccination than wild-type mice. Abs from IL-6(-/-) mice, though lower in titer, efficiently reduced B. pertussis numbers in IL-6-sufficient mice. Pulmonary leukocyte recruitment and splenic or pulmonary T cell cytokine responses to B. pertussis, including Th1 and Th17 cytokine production, were lower in IL-6(-/-) mice than in wild-type mice. Adoptive transfer of immune wild-type CD4(+) cells ameliorated the defect of IL-6(-/-) mice in the control of B. pertussis numbers. Together, these results reveal the dysregulation of multiple aspects of adaptive immune responses in B. pertussis-infected IL-6(-/-) mice and suggest that IL-6 is involved in regulating Ab generation, pulmonary leukocyte accumulation, and T cell cytokine production in response to B. pertussis as well as the generation of effective vaccine-induced immunity against this pathogen.

Tetrahydroamentoflavone (THA) from Semecarpus anacardium as a potent inhibitor of xanthine oxidase

ETHNOPHARMACOLOGICAL RELEVANCE

Seed of Semecarpus anacardium L. is widely used in Indian traditional medicine; Ayurveda and Sidha, for treatment of inflammatory disorders and gout.

AIM OF THE STUDY

The present study was aimed at isolation of a compound for its potential to inhibit xanthine oxidase (XO), over expression of which lead to inflammation and gout.

MATERIALS AND METHODS

Activity guided fractionation of S. anacardium seed was conducted using liquid-liquid partition and preparative HPLC. The fractions were evaluated for their XO inhibition and antioxidant activity. The ethyl acetate fraction with the highest XO activity yielded a biflavonoid compound tetrahydroamentoflavone (THA). Lineweaver-Burk (LB) plot for the XO inhibition of THA and allopurinol was constructed from the kinetic data.

RESULTS

IC₅₀ values of THA and allopurinol for XO inhibition were 92 and 100 nM respectively and their corresponding values for K(i) were 0.982 and 0.612 μM respectively.

CONCLUSION

THA was a potent XO inhibitor which could be considered as a drug candidate or chemopreventive agent, after establishing its pharmacological and clinical evaluation. The study results appear to support the claim of the traditional medicine with respect to the efficacy of S. anacardium seed against inflammation and gout.

Chronic gout: epidemiology, disease progression, treatment and disease burden

BACKGROUND

Gout is a painful and disabling inflammatory arthritis of increasing prevalence associated with hyperuricemia and the deposition of monosodium urate crystals in soft tissues and joints. Diagnosed gout cases have been estimated at 2.13% of the 2009 US population. The highest incidence occurs in the 65+ year age group, with males more than twice as likely to be afflicted as females.

OBJECTIVE

To present the epidemiology of chronic gout and to discuss its disease burden.

METHODS

This commentary is based on expert opinion and supplemented with published/presented information identified through PubMed and rheumatology associations.

RESULTS

The steady rise of diagnosed gout cases can generally be linked to an aging population with multiple comorbidities, the use of certain prescription medications, and changes in diet and lifestyle. Progression to chronic gout has numerous causes such as poor compliance with, ineffectiveness of, or inability to tolerate prescribed regimens. Despite the availability of urate-lowering therapies (ULT), patients may either have contraindications to them or may not adequately respond. Patients with high flare frequency, tophi, and the inability to maintain serum urate levels below 6 mg/dL with ULT can be categorized as having chronic gout that is refractory, with a substantial disease burden. Based on lack of therapeutic options for urate-lowering for patients with chronic gout refractory to conventional therapy, the economic burden of this small but substantial population contributes disproportionately to the overall economic burden of chronic gout. Recent availability of gout-specific ICD-9-CM codes capturing the cost intense and impactful aspects of the disease - flares and tophi - is likely contribute to understanding the full health economic burden in gout.

CONCLUSION

The impact of chronic gout, especially if refractory to treatment, on functionality, productivity, quality of life and health care costs can be substantial and is deserving of future research.

New and improved strategies for the treatment of gout

The Western world appears to be in the midst of the third great gout epidemic of all time. In this century, gout is increasing in prevalence despite an increased understanding of its risk factors and pathophysiology, and the availability of reasonably effective treatment. The main cultural factors responsible for this appear to be diet, obesity, ethanol use and medications. Excess fructose consumption is a newly recognized modifiable risk factor. The debate has been renewed concerning hyperuricemia as an independent risk factor for renal insufficiency and cardiovascular disease. Prevention is still rooted in lifestyle choices. Existing treatments have proven to be unsatisfactory in many patients with comorbidities. New treatments are available today and on the horizon for tomorrow, which offer a better quality of life for gout sufferers. These include febuxostat, a nonpurine inhibitor of xanthine oxidase with a potentially better combination of efficacy and safety than allopurinol, and investigational inhibitors of URAT-1, an anion exchanger in the proximal tubule that is critical for uric acid homeostasis. New abortive treatments include interleukin-1 antagonists that can cut short the acute attack in 1 to 2 days in persons who cannot take nonsteroidal anti-inflammatory drugs, colchicine or corticosteroids. Lastly, newer formulations of uricase have the ability to dissolve destructive tophi over weeks or months in patients who cannot use currently available hypouricemic agents. Diagnostically, ultrasound and magnetic resonance imaging offer advanced ways to diagnose gout noninvasively, and just as importantly, a way to follow the progress of tophus dissolution. The close association of hyperuricemia with metabolic syndrome, hypertension and renal insufficiency ensures that nephrologists will see increasing numbers of gout-afflicted patients.

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.

Gout

Gout is a common arthritis caused by deposition of monosodium urate crystals within joints after chronic hyperuricaemia. It affects 1-2% of adults in developed countries, where it is the most common inflammatory arthritis in men. Epidemiological data are consistent with a rise in prevalence of gout. Diet and genetic polymorphisms of renal transporters of urate seem to be the main causal factors of primary gout. Gout and hyperuricaemia are associated with hypertension, diabetes mellitus, metabolic syndrome, and renal and cardiovascular diseases. Non-steroidal anti-inflammatory drugs and colchicine remain the most widely recommended drugs to treat acute attacks. Oral corticosteroids could be an alternative to these drugs. Interleukin 1beta is a pivotal mediator of acute gout and could become a therapeutic target. When serum uric acid concentrations are lowered below monosodium urate saturation point, the crystals dissolve and gout can be cured. Patient education, appropriate lifestyle advice, and treatment of comorbidities are an important part of management of patients with gout.

Gout. Novel therapies for treatment of gout and hyperuricemia

In the past few decades, gout has increased not only in prevalence, but also in clinical complexity, the latter accentuated in part by a dearth of novel advances in treatments for hyperuricemia and gouty arthritis. Fortunately, recent research reviewed here, much of it founded on elegant translational studies of the past decade, highlights how gout can be better managed with cost-effective, well-established therapies. In addition, the advent of both new urate-lowering and anti-inflammatory drugs, also reviewed here, promises for improved management of refractory gout, including in subjects with co-morbidities such as chronic kidney disease. Effectively delivering improved management of hyperuricemia and gout will require a frame shift in practice patterns, including increased recognition of the implications of refractory disease and frequent noncompliance of patients with gout, and understanding the evidence basis for therapeutic targets in serum urate-lowering and gouty inflammation.

Effects of febuxostat versus allopurinol and placebo in reducing serum urate in subjects with hyperuricemia and gout: a 28-week, phase III, randomized, double-blind, parallel-group trial

OBJECTIVE

To compare the urate-lowering efficacy and safety of febuxostat, allopurinol, and placebo in a large group of subjects with hyperuricemia and gout, including persons with impaired renal function.

METHODS

Subjects (n = 1,072) with hyperuricemia (serum urate level > or = 8.0 mg/dl) and gout with normal or impaired (serum creatinine level >1.5 to < or = 2.0 mg/dl) renal function were randomized to receive once-daily febuxostat (80 mg, 120 mg, or 240 mg), allopurinol (300 or 100 mg, based on renal function), or placebo for 28 weeks.

RESULTS

Significantly (P < or = 0.05) higher percentages of subjects treated with febuxostat 80 mg (48%), 120 mg (65%), and 240 mg (69%) attained the primary end point of last 3 monthly serum urate levels <6.0 mg/dl compared with allopurinol (22%) and placebo (0%). A significantly (P < 0.05) higher percentage of subjects with impaired renal function treated with febuxostat 80 mg (4 [44%] of 9), 120 mg (5 [45%] of 11), and 240 mg (3 [60%] of 5) achieved the primary end point compared with those treated with 100 mg of allopurinol (0 [0%] of 10). Proportions of subjects experiencing any adverse event or serious adverse event were similar across groups, although diarrhea and dizziness were more frequent in the febuxostat 240 mg group. The primary reasons for withdrawal were similar across groups except for gout flares, which were more frequent with febuxostat than with allopurinol.

CONCLUSION

At all doses studied, febuxostat more effectively lowered and maintained serum urate levels <6.0 mg/dl than did allopurinol (300 or 100 mg) or placebo in subjects with hyperuricemia and gout, including those with mild to moderately impaired renal function.

Association of three genetic loci with uric acid concentration and risk of gout: a genome-wide association study

BACKGROUND

Hyperuricaemia, a highly heritable trait, is a key risk factor for gout. We aimed to identify novel genes associated with serum uric acid concentration and gout.

METHODS

Genome-wide association studies were done for serum uric acid in 7699 participants in the Framingham cohort and in 4148 participants in the Rotterdam cohort. Genome-wide significant single nucleotide polymorphisms (SNPs) were replicated in white (n=11 024) and black (n=3843) individuals who took part in the study of Atherosclerosis Risk in Communities (ARIC). The SNPs that reached genome-wide significant association with uric acid in either the Framingham cohort (p<5.0 x 10(-8)) or the Rotterdam cohort (p<1.0 x 10(-7)) were evaluated with gout. The results obtained in white participants were combined using meta-analysis.

FINDINGS

Three loci in the Framingham cohort and two in the Rotterdam cohort showed genome-wide association with uric acid. Top SNPs in each locus were: missense rs16890979 in SLC2A9 (p=7.0 x 10(-168) and 2.9 x 10(-18) for white and black participants, respectively); missense rs2231142 in ABCG2 (p=2.5 x 10(-60) and 9.8 x 10(-4)), and rs1165205 in SLC17A3 (p=3.3 x 10(-26) and 0.33). All SNPs were direction-consistent with gout in white participants: rs16890979 (OR 0.59 per T allele, 95% CI 0.52-0.68, p=7.0 x 10(-14)), rs2231142 (1.74, 1.51-1.99, p=3.3 x 10(-15)), and rs1165205 (0.85, 0.77-0.94, p=0.002). In black participants of the ARIC study, rs2231142 was direction-consistent with gout (1.71, 1.06-2.77, p=0.028). An additive genetic risk score of high-risk alleles at the three loci showed graded associations with uric acid (272-351 mumol/L in the Framingham cohort, 269-386 mumol/L in the Rotterdam cohort, and 303-426 mumol/L in white participants of the ARIC study) and gout (frequency 2-13% in the Framingham cohort, 2-8% in the Rotterdam cohort, and 1-18% in white participants in the ARIC study).

INTERPRETATION

We identified three genetic loci associated with uric acid concentration and gout. A score based on genes with a putative role in renal urate handling showed a substantial risk for gout.

Effect of hypouricaemic and hyperuricaemic drugs on the renal urate efflux transporter, multidrug resistance protein 4

BACKGROUND AND PURPOSE

The xanthine oxidase inhibitors allopurinol and oxypurinol are used to treat hyperuricaemia, whereas loop and thiazide diuretics can cause iatrogenic hyperuricaemia. Some uricosuric drugs and salicylate have a bimodal action on urate renal excretion. The mechanisms of action of these hypo- and hyperuricaemic drugs on the handling of urate in renal tubules have not been fully elucidated. Recently, we identified the multidrug resistance protein (MRP) 4 as a luminal efflux transporter for urate in the proximal tubule.

EXPERIMENTAL APPROACH

Here, we studied the effect of these drugs on [(14)C]urate transport using human embryonic kidney 293 cells overexpressing human MRP4 and in membrane vesicles isolated from these cells.

KEY RESULTS

Allopurinol stimulated MRP4-mediated cellular urate efflux and allopurinol and oxypurinol both markedly stimulated urate transport by MRP4 in membrane vesicles. Bumetanide and torasemide had no effect, whereas furosemide, chlorothiazide, hydrochlorothiazide, salicylate, benzbromarone and sulfinpyrazone inhibited urate transport, at concentrations ranging from nanomolar up to millimolar. Probenecid stimulated urate transport at 0.1 microM and inhibited transport at higher concentrations.

CONCLUSIONS AND IMPLICATIONS

These data suggest that inhibition of MRP4-mediated urate efflux by furosemide and thiazide diuretics could have an important function in their hyperuricaemic mechanisms. Furthermore, stimulation of MRP4-mediated renal urate efflux could be a new mechanism in the hypouricaemic action of allopurinol and oxypurinol. In conclusion, MRP4 may provide a potential target for drugs affecting urate homoeostasis, which needs to be further evaluated in vivo.

Control of renal uric acid excretion and gout

PURPOSE OF REVIEW

Impaired renal uric acid excretion is the major mechanism of hyperuricemia in patients with primary gout. This review highlights recent advances in the knowledge of normal mechanisms of renal uric acid handling and derangement of these mechanisms in uric acid underexcretion.

RECENT FINDINGS

The discovery of URAT1 has facilitated identification of other molecules potentially involved in uric acid transport in the renal tubules. Some of these molecules show gender differential expression in animal experiments. Sodium-dependent monocarboxylate cotransporters have been shown to transport lactate and butyrate, and may have roles in hyperuricemia associated with diabetic ketoacidosis and alcohol ingestion. Certain polymorphisms in SLC22A12 may be associated with the development of hyperuricemia or gout, although confirmation is needed. Mechanisms of hyperuricemia associated with uric acid underexcretion in patients with familial juvenile hyperuricemic nephropathy also remain to be clarified. Distal tubular salt wasting and compensatory upregulation of the resorption of sodium and uric acid in the proximal tubule may explain the hyperuricemia associated with this disorder.

SUMMARY

Much progress has been made in understanding the mechanisms of renal uric acid handling. Elucidation of the mechanisms of hyperuricemia in patients with familial juvenile hyperuricemic nephropathy will shed light on the function of uromodulin, functional impairment of which eventually results in diminished uric acid excretion.

High-normal serum uric acid is associated with impaired glomerular filtration rate in nonproteinuric patients with type 1 diabetes

BACKGROUND AND OBJECTIVES

Early renal function decline begins before the onset of proteinuria in patients with type 1 diabetes. The association of elevated serum uric acid with advanced impaired renal function prompts an examination of its role in early renal function decline in patients before proteinuria develops.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Patients with type 1 diabetes and normoalbuminuria or microalbuminuria were recruited to the Second Joslin Kidney Study. A medical history and measurements of BP, hemoglobin A1c, albumin excretion rate, and serum concentrations of uric acid and cystatin C were obtained. Estimated glomerular filtration rate was measured by a cystatin C-based formula.

RESULTS

We studied 364 patients with normoalbuminuria and 311 patients with microalbuminuria. Mean glomerular filtration rate in these groups was 119 and 99 ml/min, respectively. Mildly or moderately impaired renal function (<90 ml/min) was present in 10% of those with normoalbuminuria and 36% of those with microalbuminuria. In univariate and multivariate analyses, lower glomerular filtration rate was strongly and independently associated with higher serum uric acid and higher urinary albumin excretion rate, older age, and antihypertensive treatment.

CONCLUSIONS

Serum uric acid concentration in the high-normal range is associated with impaired renal function in patients with type 1 diabetes. Follow-up studies are needed to confirm that this level of serum uric acid is a risk factor for early renal function decline in type 1 diabetes and to determine whether its reduction would prevent the decline.

Multiple organic anion transporters contribute to net renal excretion of uric acid

Excretion of uric acid, a compound of considerable medical importance, is largely determined by the balance between renal secretion and reabsorption. The latter process has been suggested to be principally mediated by urate transporter 1 (URAT1; slc22a12), but the role of various putative urate transporters has been much debated. We have characterized urate handling in mice null for RST, the murine ortholog of URAT1, as well as in those null for the related organic anion transporters Oat1 and Oat3. Expression of mRNA of other putative urate transporters (UAT, MRP2, MRP4, Oatv1) was unaffected in the knockouts, as were general indexes of renal function (glomerular filtration rate, fractional excretion of fluid and electrolytes). While mass spectrometric analyses of urine and plasma revealed significantly diminished renal reabsorption of urate in RST-null mice, the bulk of reabsorption, surprisingly, was preserved. Oat1- and Oat3-null mice manifested decreased secretion rather than reabsorption, indicating that these related transporters transport urate in the "opposite" direction to RST. Moreover, metabolomic analyses revealed significant alteration in the concentration of several molecules in the plasma and urine of RST knockouts, some of which may represent additional substrates of RST. The results suggest that RST, Oat1, and Oat3 each contribute to urate handling, but, at least in mice, the bulk of reabsorption is mediated by a transporter(s) that remains to be identified. We discuss the data in the context of recent human genetic studies that suggest that the magnitude of the contribution of URAT1 to urate reabsorption might vary with ethnic background.

Kristallarthropathien – alt, aber wichtig

Gout is the most common inflammatory arthropathy for men. Asymptomatic hyperuricemia, which should lead to diet, but not to medication, is much more common still. Increased uric acid levels mostly result from diminished renal excretion, which is more commonly familiar than secondary (renal failure, diuretics). With the first episode of often typical (red, hot, exquisitely painful first MTP joint) acute arthritis or with urate nephrolithiasis, increased uric acid turns pathological. Attacks are treated with NSAIDs or corticosteroids. More common attacks, chronic gout, or urate nephropathy are definite indications for long-term (at least 5 years) therapy with allopurinol or febuxostat. Additional anti-inflammatory medication will be necessary during the first months. Calcium pyrophosphate deposition arthropathy, the second common crystal-induced arthritis, is diagnosed by synovial fluid analysis or for chondrocalcinosis. Treatment for attacks resembles therapy of acute gout; causal therapy is possible in case of secondary forms (e.g. hypothyroidism. hyperparathyroidism, hemochromatosis).

Uric acid nephrolithiasis

Uric acid nephrolithiasis is typically found in individuals with a low urine pH and a normal concentration of urinary uric acid. Patients with a history of gout are at greater risk of forming uric acid stones, as are patients with obesity, diabetes, or the complete metabolic syndrome. The unifying renal tubular abnormality of these disorders appears to be the excretion of abnormally acidic urine. This article focuses on the relationship of these disorders to the development of uric acid stones. The diagnosis of uric acid stones can be elusive, because pure uric acid stones are radiolucent on plain radiographs. Ultrasound, or preferably noncontrast helical CT scanning, is required for their detection. The treatment of uric acid stones should focus on alkalinization of the urine with citrate or bicarbonate salts. Additional interventions such as increase in fluid intake and decrease in animal protein ingestion are often beneficial. Patients with documented hyperuricemia often require specific therapy to lower serum uric acid concentration and subsequent excretion.

Uric acid changes in urine and plasma: an effective tool in screening for purine inborn errors of metabolism and other pathological conditions

Purine inborn errors of metabolism (IEM) are serious hereditary disorders, which should be suspected in any case of neonatal fitting, failure to thrive, recurrent infections, neurological deficit, renal disease, self-mutilation and other manifestations. Investigation usually starts with uric acid (UA) determination in urine and plasma. UA, the final product of purine metabolism in humans, may be altered not only in purine IEM, but also in other related pathologies and clinical conditions. However, data and information about abnormal UA levels are scattered in the literature, often being controversial and confusing. A comprehensive overview has been elaborated, according to abnormal UA levels in urine and plasma, which associates these alterations with purine IEM. Other possible diseases, clinical conditions, diet and drug intake, related to the metabolism of uric acid, are also presented. The article includes tables that classify the disorders according to different patterns of UA alterations, with pertinent enzymes, clinical symptoms, inheritance and comments. Additionally, summarized pathophysiological mechanisms of important disorders are described. The overview is intended to assist in the interpretation of the results of UA analyses. It demonstrates that variation of UA concentrations in urine and plasma may constitute an effective tool in screening for purine IEM and other related pathological conditions.

New insights into renal transport of urate

PURPOSE OF REVIEW

This review focuses on recent progress in the understanding of various aspects of renal transport of urate.

RECENT FINDINGS

Since the molecular cloning of the renal apical urate/anion exchanger URAT1 (SLC22A12), several membrane proteins relevant to the transport of urate have been identified. The molecular identification of two sodium-coupled monocarboxylate transporters, SMCT1(SLC5A8) and SMCT2(SLC5A12), and the emerging role of PDZ (PSD-95, DglA, and ZO-1) scaffold for renal apical transporters have led to a new concept of renal urate transport: urate-transporting multimolecular complex, or 'urate transportsome', that may form an ultimate functional unit including the sodium-coupled urate transport system by linking URAT1 and sodium-coupled monocarboxylate transporters or the coordinated apical urate uptake system by balancing reabsorptive (URAT1) and efflux (NPT1/OATv1 and MRP4) transporters. In addition, genetic variations of the URAT1 gene are associated not only with idiopathic renal hypouricemia but also with reduced renal urate excretion.

SUMMARY

Although our knowledge of renal urate handling has been increased by the molecular identification of urate transport proteins and by results of genetic studies on patients with serum urate disorders, current evidence is insufficient to fully understand the precise mechanism governing the bi-directional transport of urate. Further studies are still necessary.