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Mariana Rădoi1, Alina Bisoc2

1 Professor of cardiology, Faculty of Medicine, Transilvania University Braşov, Braşov Emergency Clinical Hospital, Cardiology II Department
2 Assistant Professor, cardiologist, Faculty of Medicine, Transilvania University Braşov, Braşov Emergency Clinical Hospital, Cardiology II Department

Contact address:
Mariana Rădoi, MD
Professor of cardiology, Faculty of Medicine, Transilvania University

Summary: In recent years, a lot of clinical and epidemiological studies have highlighted the association between increased serum levels of uric acid and metabolic syndromes, diabetes mellitus, hypertension, coronary artery disease1, cerebrovascular disease2, evolution with cardiovascular events3 and death2. Some of the clinical studies found that hyperuricemia is an independent cardiovascular risk factor or a risk factor for the development of cardiovascular events and other similar studies have considered hyperuricemia as a “confounding factor” due to it is frequent association with various cardiovascular risk factors4. Increased levels of seric uric acid have been linked to heart failure and have been found as an independent predictor of severe prognosis in patients with chronic heart failure2. In this context hyperuricemia may become a therapeutic target in the strategy for preventing chronic heart failure. Impact of hyperuricemia treatment with xanthine-oxidase inhibitors on the morbidity and mortality of patients with chronic heart failure is now evaluated in ongoing clinical trials.
Keywords: hyperuricemia, cardiovascular risk factors, heart failure, xanthine oxidase inhibitors.

Rezumat: În ultimii ani numerose studii clinice și epidemiologice pun în discuție asocierea nivelelor crescute ale acidului uric cu sindromul metabolic, diabetul zaharat, hipertensiunea arterială, boala coronariană1, boala cerevbrovasculară2, evoluția cu evenimente cardiovasculare3 și deces2. Rezultatele studiilor clinice și epidemiologice care apreciaza hiperuricemia ca factor independent de risc cardiovascular sau ca factor de risc al evoluției cu evenimente cardiovasculare sunt contrabalansate de rezultatele unor studii similare care au apreciat hiperuricemia drept „confounding factor”, datorită asocierii frecvente a acesteia cu diferti factori de risc cardiovascular4. În insuficiența cardiacă nivelele crescute ale acidului uric au fost implicate în patogeneza insuficienței cardiace și au fost apreciate ca predictor independent al prognosticului grav2. În acest context hiperuricemia devine o țintă terapeutică la pacienții cu incuficiență cardiacă. Date recente sugerează introducerea tratamentului hiperuricemiei în strategia de prevenție a apariției și progresiei insuficienței cardiace. Impactului tratamentului hiperuricemiei cu inhibitori ai xantinoxidazei asupra evoluției pacienților cu insuficiență cardiacă este în evaluare în trialuri clinice ce au ca obiectiv aprecierea influenței acestui tratament asupra morbidității și mortalității pacienților cu insuficiență cardiacă.
Cuvinte cheie: hiperuricemia, factor de risc cardiovascular, insuficienta cardiaca, inhibitori ai xantinoxidazei

Hyperuricemia

Hyperuricemia is defined as the increase of seric uric acid (UA) levels >7 mg/dl and it is reported in 2-20% of the population, depending on age, sex and other factors6. The increasing prevalence of hyperuricemia is associated both with the increasing prevalence of obesity and metabolic syndrome, as well as with the increased usage of drugs which can lead to the increase of seric levels of uric acid (UA) (thiazide diuretics and low doses of aspirin)7.
The seric level of UA is determined by the balance between the endogenous production of uric acid (UA) and the exogenous intake on one hand and renal excretion and UA catabolization on the other. In humans UA catabolism is absent due to the lack of uricase (urate oxidase) – the loss of this enzyme is the result of ancestral genetic mutations. The endogenous production of uric acid determines 70% of the UA “pool”, resulting from the purine catabolism under the action of xanthine oxidase, whereas the exogenous intake represents 30%.
The increase in the endogenous production can be determined by an excess in purine levels (severe cytolytic syndrome – rhabdomyolysis, tumor cell destruction) or by an increased activity of xhantine oxidase (XO). An increase in the activity of xanthine oxidase (found in important quantities in the liver and intestine) was incriminated in hyperuricemias from gout and the up-regulation of XO activity also described in the myocardium and vascular endothelium was associated with endogenous hyperuricemias found in dilatative cardiomyopathies and heart failure8. Endogenous hyperuricemia was also incriminated in the development of hyperuricemia in patients diagnosed with obesity, diabetes, or associated with increased levels of triglycerids, low levels of HDL-c, increased seric levels of C reactive protein. Leptin9 and insulin regulate UA levels10.
A diet rich in purines was assessed in clinical trials and revealed an increase in UA levels – meat and seafood, alcohol (beer) or industrial diet food, rich in fructose11. A decrease in UA levels was identified in connection with diets rich in dairy products, due to their uricosuric effect12. Recent data emphasizes the difficulties in appreciating the intervention of certain foods in the increase of UA levels and the low impact of the quality of the diet, without pointing out a specific element from the diet which can have a predictive value for the increase of UA levels13. The kidney excretes 10% of the UA “pool” on a daily basis. A decrease of UA renal excretion can be the consequence of renal failure/dysfunction, but also a result of affections concerning the different stages of renal UA excretion. In healthy subjects the glomerular filtration of UA is followed by an almost complete reabsorption in the proximal convoluted tubule, then by its secretion in the proximal convoluted tubule (50% of the quantity of UA reabsorbed) and in the end the postsecretion reabsorption of 40% of the UA secreted in the tubules14. The increase of UA renal excretion under the influence of estrogens explains lower levels of UA in women15. A decrease in the renal plasma flow, without variations of the glomerular filtration was associated with increased levels of UA. It is possible that the genetic polymorphism of UA “transtubule” transporters can be responsible for hyperuricemia in young people16 and that drug interferences with UA transtubule transporter proteins (aspirin)17 can lead to lower, or higher values of UA renal excretion (losartan)18.

The uric acid paradox and cardiovascular disease

Uric acid is the main hydrophilic antioxidant in the human body. Acute and moderate increases in UA levels have antioxidant properties directly linked to UA, which intervenes through the oxygen free radicals clearance (peroxynitrite) and by favoring the activity of superoxide dismutase19. However, chronic increases in UA levels are associated with the increase of intracellular oxidative stress, a phenomenon known as “the UA oxidant-antioxidant paradox”20. The oxidative stress associated with a chronic increase in UA values by an enhanced XO activity, directly involves the activity of XO in the increased production of reactive oxygen types (ROT)21. Thus, endogenous chronic hyperuricemia is associated with a larger production of oxygen free radicals. This phenomenon is important in an “atherosclerotic” environment22. Studies based on clinical observation and animal disease models have shown that chronic hyperuricemia determines endothelial dysfunction by increasing the oxidative stress23, as well as low grade systemic inflammation by directly stimulating the production of IL-6 and IL-124,25. Both conditions are important in the onset and progression of atherosclerotic cardiovascular disease, from risk factors to heart failure.
Data referring to the relationship between increased levels of UA and cardiovascular disease has shown that it is not linear, being influenced by age and associated diseases, mainly diabetes and chronic renal disease. Meta analyses conducted on studies which enrolled over 400000 patients have presented the increase of UA serum level as a predictive factor for coronary diseases26. Recently published, the results of the NHANES III study have excluded UA as an independent predictive risk factor for multiethnic populations without cardiovascular disease or diabetes27. However, these results do not exclude the possibility that increased levels of UA may be useful for predicting increased cardiovascular risk for population subgroups such as the elderly, patients diagnosed with diabetes or those with heart failure, for which previous studies have already proven that increased levels of UA are independent predictive factors. The SHEP study has shown that increased levels of UA in elderly people diagnosed with arterial hypertension represent an independent predictor for cardiovascular events28. Hyperuricemia was also appreciated to be an independent predictor for cardiovascular risk29. The clinical studies which have reported the UA levels as an independent predictor for cardiovascular events support the need to evaluate the efficiency of XO inhibitor treatment for hyperuricemia concerning cardiovascular risk, mainly for the population subgroups for which increased levels of UA were independent cardiovascular risk factors29.

Hyperuricemia in heart failure

Hyperuricemia was reported in 50-60% of the patients diagnosed with heart failure30.

Pathogenetic mechanisms
The rise of seric UA levels in patients diagnosed with heart failure was mainly correlated with the up-regulation of XO activity, including the myocardium31,32 and with a reduction in renal excretion. The decrease in renal excretion of UA is the consequence of a reduction of the renal plasmatic flow and its tubular secretion. The reduction of the renal plasmatic flow is involved in the rise of UA serum levels found in type-1 cardio-renal syndrome, encountered in the evolution of chronic heart failure. The decrease of tubular secretion is secondary to the renal ischemia provoked by heart failure, combined with the direct effect of the lactate33. However, the rise of UA seric levels in heart failure has components independent of renal dysfunction or diuretic treatment.
Hyperuricemia was involved in the onset and progression of heart failure4,35. The Framingham Offspring Study has shown that hyperuricemia is a risk factor for heart failure in the community starting from sUA levels of >6.3 mg/dl4. Studies conducted on animal disease models have shown that XO inhibition reduces cardiac remodeling36 and suggests that the reduction of serum UA levels might influence in a positive manner cardiac remodeling, thus preventing the onset of heart failure. The potential mechanisms through which hyperuricemia is involved in the onset of heart failure include the alteration of the energetic metabolism37 and the release of calcium from myocytic endoplasmic reticulum38, as well as the reduction in the myofibrile’s sensitivity to Calcium39 and apoptosis. The vascular mechanisms involving hyperuricemia in the progression of heart failure include endothelial dysfunction, provoked by a high oxidative stress (modulated through the activity of extracellular superoxide dismutase40) and vascular remodeling with the intervention of the rennin-angiotensin-aldosterone system41.
The rise of sUA levels is progressive and simultaneous with the functional class of the heart failure. In patients suffering from heart failure and cachexia sUA levels are significantly higher than in patients diagnosed with heart failure, without cachexia42.
Data published over a decade ago has shown that in heart failure high seric uric acid levels are a prognostic factor independent from the renal function, Sodium levels or age43. Later, solid data was published proving that in acute and chronic heart failure the rise of sUA levels can predict the rise of mortality and morbidity44. Values of sUA levels >9.50 mg/dl appear to be independent predictors for cardiovascular events in patients with heart failure43. UA levels predict a severe prognostic in association with systolic dysfunction (the decrease of the left ventricle’s EF) and the functional status of the heart (effort oxygen consumption)45 The HFSS score, useful for prognostic stratification, can be optimized for the prognostic of patients with heart failure by introducing UA levels next to the other 7 prognostic factors for heart failure45.
Some studies have even presented the rise of UA levels as a predictive factor for mortality overall46 and for the need for heart transplantation45,47. The predictive value of sUA levels >9.50 mg/dl is also supported by studies which have shown that oxypurinol therapy reduced the rate of cardiovascular events in patients with heart failure only in the patient population with sUA levels >9.50 mg/dl43.
Anker’s study described a gradual relationship between the level of sUA and the survival rate of patients with heart failure. Patients suffering from heart failure and UA levels <400 µmol/L had a better survival rate than those with levels between 400 and 600 µmol/L, 600 and 800 µmol/L and >800 µmol/L43.

XO inhibitor treatment

The prognostic implications of the rise of seric UA levels in heart failure lead to the legitimate question whether or not lowering UA levels should become a therapeutic goal for these patients and what are the therapeutic means to achieve this goal.
Clinical studies have revealed the fact that allopurinol treatment administered to patients with heart failure brings benefits regarding the correction of endothelial dysfunction48,49, the reduction of myocardial oxygen consumption, improving contraction50, reverse cardiac remodeling and the reduction of systolic dysfunction (the increase of the left ventricle’s ejection fraction)51,52.
The results of the studies evaluating allopurinol treatment in patients with heart failure and hyperuricemia were conflicting in regard to the effect of this treatment on the heart’s functional capacity (appreciated by the 6 minute walking test53), morbidity and mortality54. There is some data suggesting a reduction of cardiovascular events in patients with heart failure and hyperuricemia undergoing treatment with allopurinol55. The EXACT trial (Xanthine Oxidase Inhibition for Hyperuricemic HF Patients) sponsored by The National Heart, Blood and Lung Institute is ongoing and will assess the impact of allopurinol therapy on the heart failure patient’s hospitalization56.
Oxypurinol therapy was, however, able to induce a reduction of cardiovascular events when the evaluation was done on patients with sUA levels >9.50 mg/dl. For these patients, the rise of sUA levels was an independent predictor for the evolution with cardiovascular events54,57. The OPT-CHF (Efficacy and Safety Study of Oxypurinol Added to Standard Therapy in Patients with NYHA Class III-IV Congestive Heart Failure) is an ongoing study and will appreciate the safety and efficiency of adding oxypurinol to the standard therapy for patients with heart failure and its effects on morbidity, effort capacity and mortality58.

Conflict of interest: none declared.

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