Comparison of the effect of enalapril and losartan in conjunction with surgical coronary revascularisation versus revascularisation alone on systemic endothelial function

Pharmacogenetics of angiotensin-converting enzyme inhibitors (ACEI) and angiotensin II receptor blockers (ARB) in cardiovascular diseases

Cardiovascular diseases (CVDs) have a high mortality rate, and despite the several available therapeutic targets, non-response to antihypertensives remains a common problem. Angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers (ARBs) are important classes of drugs recommended as first-line therapy for several CVDs. However, response to ACEIs and ARBs varies among treated patients. Pharmacogenomics assesses how an individual's genetic characteristics affect their likely response to drug therapy. Currently, numerous studies suggest that genetic polymorphisms may contribute to variability in drug response. Moreover, further studies evaluating gene-gene interactions within signaling pathways in response to antihypertensives might help to unravel potential genetic predictors for antihypertensive response. This review summarizes the pharmacogenetic data for ACEIs and ARBs in patients with CVD, and discusses the potential pharmacogenetics of these classes of antihypertensives in clinical practice. However, replication studies in different populations are needed. In addition, studies that evaluate gene-gene interactions that share signaling pathways in the response to antihypertensive drugs might facilitate the discovery of genetic predictors for antihypertensive response.

The Role of Endothelial Cells in the Onset, Development and Modulation of Vein Graft Disease

Endothelial cells comprise the intimal layer of the vasculature, playing a crucial role in facilitating and regulating aspects such nutrient transport, vascular homeostasis, and inflammatory response. Given the importance of these cells in maintaining a healthy haemodynamic environment, dysfunction of the endothelium is central to a host of vascular diseases and is a key predictor of cardiovascular risk. Of note, endothelial dysfunction is believed to be a key driver for vein graft disease-a pathology in which vein grafts utilised in coronary artery bypass graft surgery develop intimal hyperplasia and accelerated atherosclerosis, resulting in poor long-term patency rates. Activation and denudation of the endothelium following surgical trauma and implantation of the graft encourage a host of immune, inflammatory, and cellular differentiation responses that risk driving the graft to failure. This review aims to provide an overview of the current working knowledge regarding the role of endothelial cells in the onset, development, and modulation of vein graft disease, as well as addressing current surgical and medical management approaches which aim to beneficially modulate endothelial function and improve patient outcomes.

Variants in ADRB1 and CYP2C9: Association with Response to Atenolol and Losartan in Marfan Syndrome

OBJECTIVE

To test whether variants in ADRB1 and CYP2C9 genes identify subgroups of individuals with differential response to treatment for Marfan syndrome through analysis of data from a large, randomized trial.

STUDY DESIGN

In a subset of 250 white, non-Hispanic participants with Marfan syndrome in a prior randomized trial of atenolol vs losartan, the common variants rs1801252 and rs1801253 in ADRB1 and rs1799853 and rs1057910 in CYP2C9 were analyzed. The primary outcome was baseline-adjusted annual rate of change in the maximum aortic root diameter z-score over 3 years, assessed using mixed effects models.

RESULTS

Among 122 atenolol-assigned participants, the 70 with rs1801253 CC genotype had greater rate of improvement in aortic root z-score compared with 52 participants with CG or GG genotypes (Time × Genotype interaction P = .005, mean annual z-score change ± SE -0.20 ± 0.03 vs -0.09 ± 0.03). Among participants with the CC genotype in both treatment arms, those assigned to atenolol had greater rate of improvement compared with the 71 of the 121 assigned to losartan (interaction P = .002; -0.20 ± 0.02 vs -0.07 ± 0.02; P < .001). There were no differences in atenolol response by rs1801252 genotype or in losartan response by CYP2C9 metabolizer status.

CONCLUSIONS

In this exploratory study, ADRB1-rs1801253 was associated with atenolol response in children and young adults with Marfan syndrome. If these findings are confirmed in future studies, ADRB1 genotyping has the potential to guide therapy by identifying those who are likely to have greater therapeutic response to atenolol than losartan.

Endothelium as a Therapeutic Target in Diabetes Mellitus: From Basic Mechanisms to Clinical Practice

Endothelium plays an essential role in human homeostasis by regulating arterial blood pressure, distributing nutrients and hormones as well as providing a smooth surface that modulates coagulation, fibrinolysis and inflammation. Endothelial dysfunction is present in Diabetes Mellitus (DM) and contributes to the development and progression of macrovascular disease, while it is also associated with most of the microvascular complications such as diabetic retinopathy, nephropathy and neuropathy. Hyperglycemia, insulin resistance, hyperinsulinemia and dyslipidemia are the main factors involved in the pathogenesis of endothelial dysfunction. Regarding antidiabetic medication, metformin, gliclazide, pioglitazone, exenatide and dapagliflozin exert a beneficial effect on Endothelial Function (EF); glimepiride and glibenclamide, dipeptidyl peptidase-4 inhibitors and liraglutide have a neutral effect, while studies examining the effect of insulin analogues, empagliflozin and canagliflozin on EF are limited. In terms of lipid-lowering medication, statins improve EF in subjects with DM, while data from short-term trials suggest that fenofibrate improves EF; ezetimibe also improves EF but further studies are required in people with DM. The effect of acetylsalicylic acid on EF is dose-dependent and lower doses improve EF while higher ones do not. Clopidogrel improves EF, but more studies in subjects with DM are required. Furthermore, angiotensin- converting-enzyme inhibitors /angiotensin II receptor blockers improve EF. Phosphodiesterase type 5 inhibitors improve EF locally in the corpus cavernosum. Finally, cilostazol exerts favorable effect on EF, nevertheless, more data in people with DM are required.

Engineering human ventricular heart tissue based on macroporous iron oxide scaffolds

Myocardial infarction (MI) is a primary cardiovascular disease threatening human health and quality of life worldwide. The development of engineered heart tissues (EHTs) as a transplantable artificial myocardium provides a promising therapy for MI. Since most MIs occur at the ventricle, engineering ventricular-specific myocardium is therefore more desirable for future applications. Here, by combining a new macroporous 3D iron oxide scaffold (IOS) with a fixed ratio of human pluripotent stem cell (hPSC)-derived ventricular-specific cardiomyocytes and human umbilical cord-derived mesenchymal stem cells, we constructed a new type of engineered human ventricular-specific heart tissue (EhVHT). The EhVHT promoted expression of cardiac-specific genes, ion exchange, and exhibited a better Ca²⁺ handling behaviors and normal electrophysiological activity in vitro. Furthermore, when patched on the infarcted area, the EhVHT effectively promoted repair of heart tissues in vivo and facilitated the restoration of damaged heart function of rats with acute MI. Our results show that it is feasible to generate functional human ventricular heart tissue based on hPSC-derived ventricular myocytes for the treatment of ventricular-specific myocardium damage. STATEMENT OF SIGNIFICANCE: We successfully generated highly purified homogenous human ventricular myocytes and developed a method to generate human ventricular-specific heart tissue (EhVHT) based on three-dimensional iron oxide scaffolds. The EhVHT promoted expression of cardiac-specific genes, ion exchange, and exhibited a better Ca²⁺ handling behaviors and normal electrophysiological activity in vitro. Patching the EhVHT on the infarct area significantly improved cardiac function in rat acute MI models. This EhVHT has a great potential to meet the specific requirements for ventricular damages in most MI cases and for screening drugs specifically targeting ventricular myocardium.

Stem/Progenitor Cells and Their Therapeutic Application in Cardiovascular Disease

Cardiovascular disease is the leading cause of death in the world. The stem/progenitor cell-based therapy has emerged as a promising approach for the treatment of a variety of cardiovascular diseases including myocardial infarction, stroke, peripheral arterial disease, and diabetes. An increasing number of evidence has shown that stem/progenitor cell transplantation could replenish damaged cells, improve cardiac and vascular functions, and repair injured tissues in many pre-clinical studies and clinical trials. In this review, we have outlined the major types of stem/progenitor cells, and summarized the studies in applying these cells, especially endothelial stem/progenitor cells and their derivatives, in the treatment of cardiovascular disease. Here the strategies used to improve the stem/progenitor cell-based therapies in cardiovascular disease and the challenges with these therapies in clinical applications are also reviewed.

Improving Arterial Wall Characteristics in Patients After Myocardial Infarction with a Very Low Dose of Fluvastatin and Valsartan: A Proof-of-Concept Study

Background

We tested the concept of improving arterial wall characteristics by treatment with a very low-dose combination of fluvastatin and valsartan (low-flu/val) in stable, post-myocardial infarction (MI) patients.

Material/Methods

We enrolled 36 post-MI middle-aged males in the treatment (n=20) or control (n=16) group receiving low-flu/val (10 mg/20 mg) or placebo, respectively. The parameters of endothelial function (flow-mediated dilatation (FMD), reactive hyperemia index), and arterial stiffness (carotid-femoral pulse wave velocity (cf-PWV), local carotid PWV, and beta stiffness coefficient) were measured before and after 30 days of therapy, and 10 weeks later.

Results

Treatment with low-flu/val improved FMD from 3.1±1.3% to 4.8±1.5% (p<0.001; by 54.8%) and cf-PWV from 7.8±1.1 to 6.7±1.5 m/s (p<0.01; by 14.1%) without affecting either lipids or blood pressure. In the treatment group, FMD and/or cf-PWV significantly improved in 17 patients, but the improvements did not correlate. The benefits obtained were still detectable 10 weeks after complete treatment cessation. No changes were obtained in the control group. No other vascular parameters changed.

Conclusions

Low-flu/val added “on top of” optimal therapy substantially improves endothelial function and arterial stiffness in post-MI patients. Since these improved parameters are well-known predictors of future coronary events, such treatment could decrease cardiovascular risk. Further studies are therefore warranted.

A Physiologic Approach to the Pharmacogenomics of Hypertension

Hypertension is a multifactorial condition with diverse physiological systems contributing to its pathogenesis. Individuals exhibit significant variation in their response to antihypertensive agents. Traditional markers, such as age, gender, diet, plasma renin level, and ethnicity, aid in drug selection. However, this review explores the contribution of genetics to facilitate antihypertensive agent selection and predict treatment efficacy. The findings, reproducibility, and limitations of published studies are examined, with emphasis placed on candidate genetic variants affecting drug metabolism, the renin-angiotensin system, adrenergic signalling, and renal sodium reabsorption. Single-nucleotide polymorphisms identified and replicated in unbiased genome-wide association studies of hypertension treatment are reviewed to illustrate the evolving understanding of the disease's complex and polygenic pathophysiology. Implementation efforts at academic centers seek to overcome barriers to the broad adoption of pharmacogenomics in the treatment of hypertension. The level of evidence required to support the implementation of pharmacogenomics in clinical practice is considered.

Effect of Angiotensin Receptor Blockers on Flow-Mediated Vasodilation: A Meta-Analysis of Randomized Controlled Trials

Background: In a meta-analysis, we investigated the effects of angiotensin receptor blockers (ARBs) in comparison to placebo or other classes of antihypertensive drugs on endothelial function, which was measured by brachial flow-mediated vasodilation (FMD). Methods: We searched for randomized controlled trials that compared ARBs with placebo or other classes of antihypertensive drugs in improving FMD. A random-effect model was used to compute pooled estimates. Results: In 13 trials (n = 529), ARBs were more efficacious in improving brachial FMD than placebo [pooled weighted mean change difference (WMD) 1.34%, 95% CI, 0.93-1.75%, p < 0.001]. In 15 trials (n = 918), treatment with ARBs had a significant effect on brachial FMD when compared with other antihypertensive drugs (pooled WMD 0.59%, 95% CI, 0.20-0.98%, p = 0.003 with significant heterogeneity). ARBs were also more efficacious in improving brachial FMD than calcium channel blockers (CCBs; pooled WMD 1.61%, 95% CI, 0.72-2.49%, p < 0.001) but not the other classes of drugs (p ≥ 0.072). Conclusions: This meta-analysis shows that ARBs improve brachial FMD, a marker of endothelial function, and that they are superior to placebo and CCBs. There was no significant difference in the effect on brachial FMD between ARBs and the other antihypertensive drugs.

Comparison of benazepril and losartan on endothelial function and vascular stiffness in patients with Type 2 diabetes mellitus and hypertension: A randomized controlled trial

INTRODUCTION

The purpose of this study was to compare the effects of benazepril and losartan on endothelial function and vascular stiffness, in patients with diabetes mellitus and hypertension.

MATERIALS AND METHODS

We included hypertensive diabetic patients with an office systolic blood pressure (BP) ⩾ 130 mmHg and/or diastolic BP ⩾ 80 mmHg. Patients were rolled over to amlodipine for 6 weeks, then we performed C-reactive protein assays, BP measurement and vascular tests; next, patients were randomized to benazepril or losartan. The tests were repeated after 12 weeks.

RESULTS

We randomized 14 patients to benazepril and 16 to losartan. There were no differences in systolic (139 versus 134 mmHg, p = 0.618) and diastolic (82 versus 80 mmHg, p = 0.950) BP at the end of the study. C-reactive protein values were lower in the benazepril group (0.38 versus 0.42 mg/dl, p = 0.020). There was a slightly higher flow-mediated vasodilation (FMD) response in the benazepril group (45% increase, p = 0.057) than in the losartan group (19% increase, p = 0.132). Both central systolic BP (129 versus 123 mmHg, p = 0.934) and carotid-femoral pulse wave velocity (cfPWV) (8.5 versus 8.5 m/s, p = 0.280) were the same between groups.

CONCLUSIONS

Hypertensive diabetic patients using benazepril had a greater reduction in C-reactive protein, and a slight improvement in FMD, than those taking losartan.

Endothelial dysfunction in conduit arteries and in microcirculation. Novel therapeutic approaches

The vascular endothelium not only is a single monolayer of cells between the vessel lumen and the intimal wall, but also plays an important role by controlling vascular function and structure mainly via the production of nitric oxide (NO). The so called "cardiovascular risk factors" are associated with endothelial dysfunction, that reduces NO bioavailability, increases oxidative stress, and promotes inflammation contributing therefore to the development of atherosclerosis. The significant role of endothelial dysfunction in the development of atherosclerosis emphasizes the need for efficient therapeutic interventions. During the last years statins, angiotensin-converting enzyme inhibitors, angiotensin-receptor antagonists, antioxidants, beta-blockers and insulin sensitizers have been evaluated for their ability to restore endothelial function (Briasoulis et al., 2012). As there is not a straightforward relationship between therapeutic interventions and improvement of endothelial function but rather a complicated interrelationship between multiple cellular and sub-cellular targets, research has been focused on the understanding of the underlying mechanisms. Moreover, the development of novel diagnostic invasive and non-invasive methods has allowed the early detection of endothelial dysfunction expanding the role of therapeutic interventions and our knowledge. In the current review we present the available data concerning the contribution of endothelial dysfunction to atherogenesis and review the methods that assess endothelial function with a view to understand the multiple targets of therapeutic interventions. Finally we focus on the classic and novel therapeutic approaches aiming to improve endothelial dysfunction and the underlying mechanisms.

Angiotensin II receptor blockers improve peripheral endothelial function: a meta-analysis of randomized controlled trials

OBJECTIVE(S)

Several studies have assessed the effect of angiotensin II receptor blockers (ARBs) on peripheral endothelial dysfunction as measured by flow-mediated vasodilatation (FMD), a widely-used indicator for endothelial function. We conducted a meta-analysis to investigate the effect in comparison to placebo or no treatment and other antihypertensives.

METHODS

MEDLINE, Cochrane library and EMBASE were searched to September 2013 for randomized controlled trials (RCTs) that assessed the effect of ARBs versus placebo or no treatment and other antihypertensives (angiotensin-converting enzyme inhibitors (ACEIs), calcium channel blockers (CCBs), β-blockers, diuretics) by forearm FMD. Furthermore, we also use meta-regression to analyze the relationship between the endothelial function and the duration of ARBs treatments.

RESULTS

In 11 trials including 590 patients, ARBs (n = 315) significantly improved FMD (1.36%, 95% confidence internal [CI]:1.28 to 1.44) versus placebo or no treatment (n = 275). In 16 trials that included 1028 patients, ARBs (n = 486) had a significant effect (0.59%, 95% CI: 0.25 to 0.94) on FMD when compared with other antihypertensives (n = 542). In 8 trials, ARBs (n = 174) had no significant effect (-0.14%, 95% CI: -0.32 to 0.03) compared with ACEI (n = 173). Compared with others, the benefits of ARBs, respectively, were 1.67% (95% CI: 0.65 to 0.93) in 7 trials with CCBs, 0.79% (95% CI: 0.42 to 1.01) with β-blockers in 3 trials and 0.9% (95% CI: 0.77 to 1.03) with diuretics in 3 trials. Importantly, we found ARBs were less effective in a long time span (95% CI: -1.990 to -0.622) than the first 6 months (95% CI: -0.484 to 0.360).

CONCLUSIONS

This study shows that ARBs improve peripheral endothelial function and are superior to CCBs, β-blockers and diuretics. However, the effect couldn't be maintained for a long time. In addition, there was no significant difference between ARBs and ACEI.

Persistence of arterial functional abnormalities after successful coronary revascularization

BACKGROUND

In patients with myocardial ischemia and coronary atherosclerosis, arterial stiffness and endothelial function are impaired. Whether these alterations can be favorably affected by successful coronary revascularization is debated.

METHODS

We studied 39 hospitalized patients 59.3 ± 3.2 years old (mean ± SEM). In 21 patients with angiographic evidence of significant coronary artery stenosis, revascularization procedures were performed (stenting n = 11 and bypass surgery n = 10). The remaining patients had no significant stenosis and thus served as controls. Prerevascularization measurements included carotido-femoral pulse wave velocity (PWV), radial artery flow-mediated vasodilatation and a complete echocardiographic examination. The same measurements were performed 6 months later. Pharmacological treatment consisted of different cardiovascular drugs and remained substantially unchanged over the 6-month follow-up period.

RESULTS

With the exception of an increased left ventricular mass index (LVMI; 130 ± 5.3 versus 105.8 ± 7.2 g/m(2), P < 0.05) and a lower Em/Am (0.6 ± 0.01 and 0.8 ± 0.01 respectively, P < 0.05) observed in patients with coronary stenosis, all other hemodynamic, cardiac and vascular variables were similar in the two groups. Following the 6-month follow-up period, all variables remained substantially unchanged, with the exception, in revascularized patients, of a significant reduction in LVMI (-12%, P < 0.05) and an improvement in Tissue Doppler Imaging-measured diastolic function (Em/Am + 30%, P < 0.05). This was not associated, however, with any significant change in PWV and in flow-mediated vasodilatation.

CONCLUSION

Cardiac revascularization has no effect on arterial function, assessed either as arterial stiffness or as flow-mediated vasodilatation. On the contrary, the improvement in coronary blood flow triggers local cardiac changes, namely, a reduction in LVMI and an improvement in diastolic function.

ACE (I/D) polymorphism and response to treatment in coronary artery disease: a comprehensive database and meta-analysis involving study quality evaluation

BACKGROUND

The role of angiotensin-converting enzyme (ACE) gene insertion/deletion (I/D) polymorphism in modifying the response to treatment modalities in coronary artery disease is controversial.

METHODS

PubMed was searched and a database of 58 studies with detailed information regarding ACE I/D polymorphism and response to treatment in coronary artery disease was created. Eligible studies were synthesized using meta-analysis methods, including cumulative meta-analysis. Heterogeneity and study quality issues were explored.

RESULTS

Forty studies involved invasive treatments (coronary angioplasty or coronary artery by-pass grafting) and 18 used conservative treatment options (including anti-hypertensive drugs, lipid lowering therapy and cardiac rehabilitation procedures). Clinical outcomes were investigated by 11 studies, while 47 studies focused on surrogate endpoints. The most studied outcome was the restenosis following coronary angioplasty (34 studies). Heterogeneity among studies (p < 0.01) was revealed and the risk of restenosis following balloon angioplasty was significant under an additive model: the random effects odds ratio was 1.42 (95% confidence interval:1.07-1.91). Cumulative meta-analysis showed a trend of association as information accumulates. The results were affected by population origin and study quality criteria. The meta-analyses for the risk of restenosis following stent angioplasty or after angioplasty and treatment with angiotensin-converting enzyme inhibitors produced non-significant results. The allele contrast random effects odds ratios with the 95% confidence intervals were 1.04(0.92-1.16) and 1.10(0.81-1.48), respectively. Regarding the effect of ACE I/D polymorphism on the response to treatment for the rest outcomes (coronary events, endothelial dysfunction, left ventricular remodeling, progression/regression of atherosclerosis), individual studies showed significance; however, results were discrepant and inconsistent.

CONCLUSION

In view of available evidence, genetic testing of ACE I/D polymorphism prior to clinical decision making is not currently justified. The relation between ACE genetic variation and response to treatment in CAD remains an unresolved issue. The results of long-term and properly designed prospective studies hold the promise for pharmacogenetically tailored therapy in CAD.

Angiotensin-converting enzyme gene polymorphism and coronary reactivity in young men

The purpose of the study was to examine whether the insertion/deletion (I/D) polymorphism of the angiotensin-converting enzyme (ACE) gene affects the vasodilatory properties of coronary arteries in healthy men. The ACE genotypes of 128 men (mean age 35 +/- 4 years) were determined and related to myocardial blood flow. The blood flow was measured by positron emission tomography at rest and during vasodilation caused by adenosine or dipyridamole infusion. The coronary flows and resistances at rest and during stimulation with adenosine or dipyridamole did not differ between the ACE genotypes. Furthermore, this polymorphism had no effect on coronary flow reserve corrected by a rate-pressure product. In conclusion, the ACE I/D polymorphism does not seem to affect myocardial reactivity--an early indicator of atherosclerosis--in healthy subjects.

Pharmacogenomics of Renin Angiotensin System Inhibitors in Coronary Artery Disease

Renin Angiotensin System (RAS) inhibitors comprise some of the most commonly used medications in coronary artery disease (CAD) and its related syndromes. Unfortunately, significant inter-patient variability seems likely in response to these agents; of which, the influence of genetic determinants is of interest. This review summarizes the available RAS inhibitor pharmacogenomic studies which have evaluated RAS polymorphisms that either elucidate mechanism via surrogate endpoint measurements, or predict efficacy via clinical outcomes in CAD related syndromes.Regardless of the endpoint, none of the RAS genotypes conclusively predicts efficacy of RAS inhibitors. In fact, the results of the pharmacogenomic studies were often in direct conflict with one another. Varied results appear due to methodological limitations (e.g., inadequate study power, genotyping error, methods of endpoint measurement), study conceptualization (e.g., overestimating the contribution of polymorphism to disease, lack of haplotype approach), and differences between studies (e.g., genotype frequency, study subject characteristics, the specific medication and dose used). Thus investigators should consider the various methodological limitations to improve upon the current approach to RAS inhibitor pharmacogenomic research in the vast CAD population.

Angiotensin II induces NF-kappa B activation in HUVEC via the p38MAPK pathway

Angiotensin II (Ang II) is the main active peptide of the renin-angiotensin system (RAS), producing a number of inflammatory mediators that lead to endothelial dysfunction and the progression of atherosclerosis. Ang II-induced NF-kappaB nuclear translocation plays a pivotal role in this response. This study examines the NF-kappaB activation mechanism elicited by Ang II in human umbilical vein endothelial cells (HUVEC). Electrophoretic mobility shift assays and Western blotting revealed that Ang II, signaling via AT(1), produces a time-dependent increase in NF-kappaB DNA binding and IkappaBalpha degradation. These results also demonstrate that Ang II leads to MAPK phosphorylation and p38MAPK pathway-induced NF-kappaB activation. Furthermore, AT(1) is required for p38MAPK phosphorylation induced by Ang II. This study provides evidence that Ang II elicits NF-kappaB activation via the p38MAPK pathway in HUVEC.

Novel therapies targeting vascular endothelium

Endothelial dysfunction has been identified as a major mechanism involved in all the stages of atherogenesis. Evaluation of endothelial function seems to have a predictive role in humans, and therapeutic interventions improving nitric oxide bioavailability in the vasculature may improve the long-term outcome in healthy individuals, high-risk subjects, or patients with advanced atherosclerosis. Several therapeutic strategies are now available, targeting both the synthesis and oxidative inactivation of nitric oxide (NO) in human vasculature. Statins seem to be currently the most powerful category of these agents, improving endothelial function and decreasing cardiovascular risk after long-term administration. Other cardiovascular agents improving endothelial function in humans are angiotensin-converting enzyme inhibitors/angiotensin receptors blockers, which increase NO bioavailability by modifying the rennin-angiotensin-aldosterone system. Newer therapeutic approaches targeting endothelial dysfunction in specific disease states include insulin sensitizers, L-arginine (the substrate for endothelial NO synthase [eNOS]) as well as substances that target eNOS "coupling," such as folates or tetrahydrobiopterin. Although there are a variety of strategies to improve NO bioavailability in human endothelium, it is still unclear whether they have any direct benefit at a clinical level.