Reduced myocardial antioxidant activity and increased oxidant damage have been described in animal models of coronary artery disease and heart failure (HF), and markers of oxidative stress are elevated in the same patients [1]. Among potential stimuli that contribute to myocardial failure progression, the over-activation of xanthine oxidase (XO) plays a central role. XO is an enzyme involved in the last two steps of purine catabolism that ultimately results in uric acid production [2]. The degree of activation of XO is directly related to the the circulating levels of uric acid that can be considered as an indirect marker of oxidative stress in several clinical conditions [2]. Serum uric acid is a potent antioxidant, but it can also promote oxidative stress at high concentrations and in surroundings with low pH or low levels of other antioxidants [3]. In vitro, uric acid may affect vascular smooth muscle and mononuclear cells that are important in pathophysiology of cardiovascular (CV) diseases [4]. Indeed, significantly increased serum uric acid levels have been identified in 25% of patients with heart failure and reduced ejection fraction and have been associated with worsening symptoms and increased mortality [5, 6]. Vascular activity of XO may also increase the oxidative stress that underlies endothelial dysfunction. XO inhibition with allopurinol 300 mg once daily for one month was compared with placebo to investigate its potential in endothelial function improvement after HF. Allopurinol significantly ameliorated endothelium-dependent vasodilation and reduced markers of oxidative stress [7]. In patients with HF allopurinol administration improved left ventricular ejection fraction [8], diastolic function, and coronary flow reserve [9] on the short-term, but showed no definitive benefits on the long-term [10]. In a placebo controlled cross-over study involving patients with chronic stable angina, treatment with allopurinol (600 mg per day) for 6 weeks showed anti-ischemic properties [11]. Thus, current evidence suggest that XOIs may have important cardioprotective activities [12].

Zofenopril is a potent sulfhydryl angiotensin-converting enzyme (ACE) inhibitor, characterized by high lipophilicity, selective cardiac ACE-inhibition, and antioxidant and tissue protective activities [13]. The effects of zofenopril treatment in terms of circulating adhesion molecules, oxidative stress parameters and endothelium-dependent vasodilation were analyzed in essential mildly hypertensive patients. Zofenopril has demonstrated a sustained antioxidant activity and reduced endothelial activation compared to carboxylic ACE-inhibitor ramipril and the beta-adrenoceptor blocker atenolol [14]. The analysis of the overall population of more than 3600 patients with coronary heart disease (CHD) and enrolled in the Survival of Myocardial Infarction Long-Term Evaluation (SMILE) project, showed that the treatment with zofenopril was associated with a lower risk of mortality and morbidity as compared to placebo or other ACE-inhibitors. The beneficial effect of zofenopril was associated with some degree of anti-ischemic effects and confirmed in patients with preserved or impaired left ventricular function [15‐18].

Considering the importance of the over-activation of XO in presence of hyperuricemia, we investigated the effects of the concomitant andministration of XO inhibitors and ACE-inhibitors in post-acute myocardial infarction (AMI) patients. We also postulated that the antioxidant activity of zofenopril may be more beneficial in comparison to other ACE-inhibitors in patients with AMI treated with XO inhibitor (XOI). Therefore, in this meta-analysis we assessed subgroups of patients involved in the SMILE studies who were concomitantly treated with XOIs to verify the potential enhanced antioxidant effect and survival improvement after zofenopril treatment.

This retrospective analysis on SMILE pooled patients demonstrated that the combination of an ACE-inhibitor and XOIs may be protective against MACE after AMI compared to treatment with an ACE-inhibitor without XOIs. A combination treatment of zofenopril and XOI seems to promote a statistically significant longer survival time without any events than treatment with other ACEIs not combined with XOIs, whereas a non-statistically significant trend to a large benefit of the XOI + zofenopril combination is observed vs. the combination of another ACE-inhibitor and XOI or vs. zofenopril alone. Possible explanations for our findings, as well as limitations of the present retrospective analysis, will be discussed in the next paragraphs.

In our population, the prevalence of patients who needed preparation inhibiting uric acid production was slightly higher than gout prevalence reported from 2000 to 2005 in European countries (4.5% vs 1.4%) [23]. This may be explained with the observation that there is an independent relationship between hyperuricemia and AMI [24] and patients with gout have an increased risk of AMI [25].

The analysis of pooled data confirmed the results of individual SMILE studies. In each SMILE study, treatment of AMI patients with zofenopril effectively reduced the risk of mortality and hospitalization for cardiovascular causes compared to placebo and other ACE-inhibitors, such as lisinopril and ramipril [26]. The results of pooled SMILE studies supported the notion that there are differences in terms of efficacy between different ACE-inhibitors, and that, in contrast to previous observations, a class effect is unlikely [27]. As a matter of fact, Hansen et al. investigated the risk of mortality and re-infarction after a first episode of AMI and demonstrated that there was none difference in terms of clinical efficacy between ACE inhibitors. However, in this study zofenopril was not included and the only sulfhydryl ACE inhibitor was captopril [27]. Zofenopril demonstrated higher anti-atherosclerotic and antioxidant effects than captopril in the arterial wall of hypercholesterolemic apoE(−/−) mice [28]. In rat models of experimental cardiac ischemia/reperfusion (IR) injury, zofenopril prevented the changes that usually occurred during IR injury and reduced lipid peroxidation, protein oxidation, and nitric oxide levels as well as XO and myeloperoxidase activities and increased the catalase and superoxide dismutase activities [29]. In untreated normocholesterolemic patients with moderate essential hypertension without clinically evident target organ damage, a 12-weeks treatment with zofenopril significantly reduced malondialdehyde levels, plasma NOx and asymmetrical dimethyl-L-arginine concentrations, compared to enalapril that was ineffective [30]. Consistently, the long term treatment with zofenopril in mildly hypertensive patients resulted in a favorable nitric oxide/oxidative stress profile and showed a slower progression of intima- media thickening of the carotid artery beyond lowering arterial pressure [31].

The present retrospective analysis is based on the observation that clinical benefits of early zofenopril treatment in post-AMI phase are related also to its anti-oxidant properties and, therefore, the combination with XOIs may have an enhanced effect. Previous studies have investigated the opportunity of a therapy with oxypurinol 600 mg daily in addition to an ACE-inhibitors or angiotensin receptor blocker and a beta-blocker in patients with moderate-to-severe HF. Oxypurinol significantly reduced serum uric acid by approximately 2 mg/dL, but did not improve the clinical status in unselected patients and when patients were stratified according to uric acid levels patients with elevated uric acid levels (≥ 9.5 mg/dL, n = 108) responded favorably to oxypurinol, whereas patients with UA < 9.5 mg/dL exhibited a trend towards worsening [32]. In our study, patients were clustered according to XOI treatment and not to uric acid levels. Therefore, we selected patients who already needed a preparation inhibiting uric acid production and had a therapy. These patients had major benefits in post-AMI phase when they were treated with zofenopril than lisinopril or ramipril. Further larger studies should be recommended to identify which population of hyperuremic patients may achieve major clinical benefits with zofenopril treatment post-AMI.

This study has some main limitations. First of all, it was a retrospective analysis and the number of patients concomitantly treated with ACE inhibitors and XOI in post-AMI phase was limited. This may explain why some comparisons (e.g. those between zofenopril and other ACE-inhibitors) did not achieve statistical significance. We tried to overcome this limitation in two ways: we down-sampled the control population of patients not treated with XOI, thus balancing the comparisons (yet, however, limited to a small sample), and we applied a propensity score analysis [19]. The latter confirmed the superiority of concomitant treatment with XO in terms of prevention of MACE and supported a better efficacy of a combination including zofenopril. However, we acknowledge that, given the retrospective nature of our study, the sample size could have been underpowered to demonstrate the study goal. Second, despite a very similar design of the four SMILE studies, there were some differences in the inclusion criteria, treatment duration, and follow-up. Our analysis included both thrombolyzed and non-thrombolyzed patients and those with and without left ventricular dysfunction; in addition, clinical effect was observed at different time-points. Thus, the patients considered in this study may be representative of those who could be encountered in post-AMI phase in clinical practice. However, in assessments of differences between treatments, variations in baseline characteristics may decrease the sensitivity of such analyses to show interaction.

Finally, we have no information about the exact type of XOI used by patients and the duration of treatment in the patients of these groups and thus we cannot know whether differences in outcomes may be related to differences in the type or duration of treatment. We also do not have information on serum levels of uric acid at baseline and during the follow-up period and thus we do not know the actual effectiveness of XOI treatment.

Our retrospective analysis on patients involved in the four SMILE studies suggests that concomitant use of zofenopril and urate lowering drugs with antioxidant activity (XOIs) may further improve survival free from MACE in post-AMI patients. Further prospective studies should be recommended to compare the activity of different ACE-inhibitors in hyperuremic patients and establish a treatment schedule to gain major clinical benefits.