Effect of allopurinol and benzbromarone on the concentration of uridine in plasma

Uric Acid Is a Biomarker of Oxidative Stress in the Failing Heart: Lessons Learned from Trials With Allopurinol and SGLT2 Inhibitors

Hyperuricemia increases the risk of heart failure, and higher levels of serum uric acid are seen in patients who have worse ventricular function, functional capacity, and prognosis. Heart failure is also accompanied by an upregulation of xanthine oxidase, the enzyme that catalyzes the formation of uric acid and a purported source of reactive oxygen species. However, the available evidence does not support the premise that either uric acid or the activation of xanthine oxidase has direct injurious effects on the heart in the clinical setting. Xanthine oxidase inhibitors (allopurinol and oxypurinol) have had little benefit and may exert detrimental effects in patients with chronic heart failure in randomized controlled trials, and the more selective and potent inhibitor febuxostat increases the risk of cardiovascular death more than allopurinol. Instead, the available evidence indicates that changes in xanthine oxidase and uric acid are biomarkers of oxidative stress (particularly in heart failure) and that xanthine oxidase may provide an important source of nitric oxide that quenches the injurious effects of reactive oxygen species. A primary determinant of the cellular redox state is nicotinamide adenine dinucleotide, whose levels drive an inverse relationship between xanthine oxidase and sirtuin-1, a nutrient deprivation sensor that exerts important antioxidant and cardioprotective effects. Interestingly, sodium-glucose cotransporter 2 inhibitors induce a state of nutrient deprivation that includes activation of sirtuin-1, suppression of xanthine oxidase, and lowering of serum uric acid. The intermediary role of sirtuin-1 in both uric acid-lowering and cardioprotection may explain why, in mediation analyses of large-scale cardiovascular trials, the effect of sodium-glucose cotransporter 2 inhibitors to decrease serum uric acid is a major predictor of the ability of these drugs to decrease serious heart failure events.

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.

Biochemistry of uridine in plasma

Uridine is a pyrimidine nucleoside that plays a crucial role in synthesis of RNA, glycogen, and biomembrane. In humans, uridine is present in plasma in considerably higher quantities than other purine and pyrimidine nucleosides, thus it may be utilized for endogenous pyrimidine synthesis. Uridine has a number of biological effects on a variety of organs with or without disease, such as the reproductive organs, central and peripheral nervous systems, and liver. In addition, it is used in clinical situations as a rescue agent to protect against the adverse effects of 5-fluorouracil. Since the biological actions of uridine may be related to its plasma concentration, it is important to examine factors that have effects on that concentration. Factors associated with an increase in plasma concentration of uridine include enhanced ATP consumption, enhanced uridine diphosphate (UDP)-glucose consumption via glycogenesis, inhibited uridine uptake by cells via the nucleoside transport pathway, increased intestinal absorption, and increased 5-phosphribosyl-1-pyrophosphate and urea synthesis. In contrast, factors that decrease the plasma concentration of uridine are associated with accelerated uridine uptake by cells via the nucleoside transport pathway and decreased pyrimidine synthesis.

Relationship between plasma uridine and urinary urea excretion

To investigate whether the concentration of uridine in plasma is related to the urinary excretion of urea, 45 healthy male subjects with normouricemia and normal blood pressure were studied after providing informed consent. Immediately after collection of 24-hour urine, blood samples were drawn after an overnight fast except for water. The contents of ingested foods during the 24-hour urine collection period were described by the subjects and analyzed by a dietician. Simple regression analysis showed that plasma uridine was correlated with the urinary excretions of urea (R = 0.41, P < .01), uric acid (R = 0.36, P < .05), and uridine (R = 0.30, P < .05), as well as uric acid clearance (R = 0.35, P < .05) and purine intake (R = 0.30, P < .05). In contrast, multiple regression analysis showed a positive relationship only between plasma uridine and urinary excretion of urea. These results suggest that an increase in de novo pyrimidine synthesis leads to an increased concentration of uridine in plasma via nitrogen catabolism in healthy subjects with normouricemia and normal blood pressure.

Selectivity of febuxostat, a novel non-purine inhibitor of xanthine oxidase/xanthine dehydrogenase

The purine analogue, allopurinol, has been in clinical use for more than 30 years as an inhibitor of xanthine oxidase (XO) in the treatment of hyperuricemia and gout. As consequences of structural similarities to purine compounds, however, allopurinol, its major active product, oxypurinol, and their respective metabolites inhibit other enzymes involved in purine and pyrimidine metabolism. Febuxostat (TEI-6720, TMX-67) is a potent, non-purine inhibitor of XO, currently under clinical evaluation for the treatment of hyperuricemia and gout. In this study, we investigated the effects of febuxostat on several enzymes in purine and pyrimidine metabolism and characterized the mechanism of febuxostat inhibition of XO activity. Febuxostat displayed potent mixed-type inhibition of the activity of purified bovine milk XO, with Ki and Ki' values of 0.6 and 3.1 nM respectively, indicating inhibition of both the oxidized and reduced forms of XO. In contrast, at concentrations up to 100 muM, febuxostat had no significant effects on the activities of the following enzymes of purine and pyrimidine metabolism: guanine deaminase, hypoxanthine-guanine phosphoribosyltransferase, purine nucleoside phosphorylase, orotate phosphoribosyltransferase and orotidine-5'-monophosphate decarboxylase. These results demonstrate that febuxostat is a potent non-purine, selective inhibitor of XO, and could be useful for the treatment of hyperuricemia and gout.

Allopurinol induces renal toxicity by impairing pyrimidine metabolism in mice

We investigated the relationship between the toxic effect of allopurinol and pyrimidine metabolism in mice. Allopurinol-induced increases in plasma transaminase levels in dinitrofluorobenzene (DNFB)-sensitized mice were not affected by uridine. In contrast, plasma creatinine and BUN tended to decrease 18 hr after the last injection of uridine. Both plasma and urinary orotidine (OD) were detected in DNFB-sensitized mice after administration of a single dose of allopurinol. In contrast, TEI-6720, a newly synthesized xanthine oxidase/xanthine dehydrogenase inhibitor, caused neither pyrimidine metabolism abnormality nor renal impairment in DNFB-sensitized mice. Also, normal mice administered high doses of allopurinol showed abnormal pyrimidine metabolism together with renal toxicity which could be ameliorated by uridine, indicating that allopurinol essentially causes pyrimidine metabolism abnormality leading to renal impairment. In DNFB-sensitized mice, allopurinol increased urinary OD excretion to an extent similar to that in normal mice administered the same dose of allopurinol. However, renal impairment by allopurinol was more striking in DNFB-sensitized mice than in normal mice. Histopathological observations showed that allopurinol induced calculus formation in the collecting tubules and papillary duct. Calculus formation was increased by DNFB and decreased by uridine. These observations indicate that the enhancement of the renal toxicity of allopurinol by DNFB-sensitization may be due to some biological interactions between DNFB and allopurinol. In humans, it is possible that there are some biological interactions which serve to enhance the toxicity of allopurinol, resulting in the development of allopurinol hypersensitivity syndrome (AHS). In contrast, TEI-6720, had no effect on pyrimidine metabolism and showed no toxic effect.