Endothelin as a Clinical Target in the Treatment of Systemic Hypertension

New Potential Treatments for Resistant Hypertension

PURPOSE OF REVIEW

To provide an update and review approaches to the treatment of resistant hypertension (RH) with a focus on emerging potential therapies.

RECENT FINDINGS

Resistant hypertension is defined as a blood pressure that remains elevated above a patient's individualized target despite the concurrent use of 3 antihypertensive agents of different classes including a diuretic or use of 4 or more antihypertensive agents. Patients with RH have an increased risk of adverse cardiovascular and renal outcomes. Most RH is attributed to apparent RH and is not true RH. True RH is a diagnosis of exclusion after apparent RH has been excluded. Treatment of RH is challenging, and blood pressure goal is often difficult to achieve. Currently several new therapies have emerged with forthcoming data that provide promise for improved blood pressure control in those with resistant hypertension. Once RH has been diagnosed, patients should be on standardized therapy that includes agents from three different classes including a diuretic with addition in most cases of a mineralocorticoid as a fourth line agent. There are newer agents in development currently being studied in clinical trials including dual endothelin receptor antagonists and aldosterone synthase inhibitors that appear to be efficacious. Other approved medications including SGLT2 inhibitors and non-steroidal mineralocorticoids such as finerenone also need to be incorporated into treatment paradigms. Renal denervation with catheter based devices is another potential promising treatment option in this population.

Antiaging gene Klotho regulates endothelin-1 levels and endothelin receptor subtype B expression in kidneys of spontaneously hypertensive rats

OBJECTIVE

Klotho is an antiaging gene and is predominately expressed in kidneys. The endothelin system is critical in the regulation of kidney function. The objective of this study is to assess whether klotho affects the renal endothelin system in spontaneously hypertensive rats (SHRs).

METHOD

Four groups of male SHRs and one group of male Wistar-Kyoto (WKY) rats were used. In-vivo expression of klotho was achieved by AAV2 delivery of mouse klotho full-length cDNA (AAV.mKL). Four groups of SHRs were given (intravenously) AAV.mKL, AAV.LacZ, AAV.GFP, and phosphate-buffered saline, respectively. The WKY group was given phosphate-buffered saline and served as a control. At the end of week 12 after gene delivery, all animals were euthanized.

RESULTS

Plasma endothelin-1 (ET-1) and renal ET-1 levels were increased in SHRs vs. WKY rats. In-vivo expression of klotho reversed the elevated ET-1 levels in SHRs. ETB receptor protein expression was decreased in both kidney cortex and medulla of SHRs. Interestingly, in-vivo expression of klotho abolished the downregulation of ETB protein expression in SHRs, suggesting that klotho regulates ETB receptor expression. Klotho gene delivery also eliminated the increase in the ratio of ETA/ETB in SHRs. Mitochondrial superoxide dismutase (Mn-SOD) protein expression was decreased in kidneys of SHRs, which was rescued by in-vivo expression of klotho.

CONCLUSION

Klotho gene delivery abolished the upregulation of ET-1 levels and the downregulation of ETB and Mn-SOD expression in kidneys of SHRs. These findings revealed a previously unidentified role of klotho in the regulation of the renal ET system and Mn-SOD in SHRs.

Combined endothelin a blockade and chlorthalidone treatment in a rat model of metabolic syndrome

Experiments determined whether the combination of endothelin A (ETA) receptor antagonist [ABT-627, atrasentan; (2R,3R,4S)-4-(1,3-benzodioxol-5-yl)-1-[2-(dibutylamino)-2-oxoethyl]-2-(4-methoxyphenyl)pyrrolidine-3-carboxylic acid] and a thiazide diuretic (chlorthalidone) would be more effective at lowering blood pressure and reducing renal injury in a rodent model of metabolic syndrome compared with either treatment alone. Male Dahl salt-sensitive rats were fed a high-fat (36% fat), high-salt (4% NaCl) diet for 4 weeks. Separate groups of rats were then treated with vehicle (control), ABT-627 (ABT; 5 mg/kg per day, in drinking water), chlorthalidone (CLTD; 5 mg/kg per day, in drinking water), or both ABT plus CLTD. Mean arterial pressure (MAP) was recorded continuously by telemetry. After 4 weeks, both ABT and CLTD severely attenuated the development of hypertension, whereas the combination further reduced MAP compared with ABT alone. All treatments prevented proteinuria. CLTD and ABT plus CLTD significantly reduced nephrin (a podocyte injury marker) and kidney injury molecule-1 (a tubulointerstitial injury marker) excretion. ABT, with or without CLTD, significantly reduced plasma 8-oxo-2'-deoxyguanosine, a measure of DNA oxidation, whereas CLTD alone had no effect. All treatments suppressed the number of ED1(+) cells (macrophages) in the kidney. Plasma tumor necrosis factor receptors 1 and 2 were reduced only in the combined ABT and CLTD group. These results suggest that ABT and CLTD have antihypertensive and renal-protective effects in a model of metabolic syndrome that are maximally effective when both drugs are administered together. The findings support the hypothesis that combined ETA antagonist and diuretic treatment may provide therapeutic benefit for individuals with metabolic syndrome consuming a Western diet.

Hypertension: physiology and pathophysiology

Despite major advances in understanding the pathophysiology of hypertension and availability of effective and safe antihypertensive drugs, suboptimal blood pressure (BP) control is still the most important risk factor for cardiovascular mortality and is globally responsible for more than 7 million deaths annually. Short-term and long-term BP regulation involve the integrated actions of multiple cardiovascular, renal, neural, endocrine, and local tissue control systems. Clinical and experimental observations strongly support a central role for the kidneys in the long-term regulation of BP, and abnormal renal-pressure natriuresis is present in all forms of chronic hypertension. Impaired renal-pressure natriuresis and chronic hypertension can be caused by intrarenal or extrarenal factors that reduce glomerular filtration rate or increase renal tubular reabsorption of salt and water; these factors include excessive activation of the renin-angiotensin-aldosterone and sympathetic nervous systems, increased formation of reactive oxygen species, endothelin, and inflammatory cytokines, or decreased synthesis of nitric oxide and various natriuretic factors. In human primary (essential) hypertension, the precise causes of impaired renal function are not completely understood, although excessive weight gain and dietary factors appear to play a major role since hypertension is rare in nonobese hunter-gathers living in nonindustrialized societies. Recent advances in genetics offer opportunities to discover gene-environment interactions that may also contribute to hypertension, although success thus far has been limited mainly to identification of rare monogenic forms of hypertension.

Endothelin antagonists in hypertension and kidney disease

The endothelin (ET) system seems to play a pivotal role in hypertension and in proteinuric kidney disease, including the micro- and macro-vascular complications of diabetes. Endothelin-1 (ET-1) is a multifunctional peptide that primarily acts as a potent vasoconstrictor with direct effects on systemic vasculature and the kidney. ET-1 and ET receptors are expressed in the vascular smooth muscle cells, endothelial cells, fibroblasts and macrophages in systemic vasculature and arterioles of the kidney, and are associated with collagen accumulation, inflammation, extracellular matrix remodeling, and renal fibrosis. Experimental evidence and recent clinical studies suggest that endothelin receptor blockade, in particular selective ETAR blockade, holds promise in the treatment of hypertension, proteinuria, and diabetes. Concomitant blockade of the ETB receptor is not usually beneficial and may lead to vasoconstriction and salt and water retention. The side-effect profile of ET receptor antagonists and relatively poor antagonist selectivity for ETA receptor are limitations that need to be addressed. This review will discuss what is currently known about the endothelin system, the role of ET-1 in the pathogenesis of hypertension and kidney disease, and summarize literature on the therapeutic potential of endothelin system antagonism.

Status of end organs in newly detected rural essential hypertensives: a study from southern India

BACKGROUND

Hypertension is associated with end-organ damage (EOD). Since EOD is a risk factor for cardio- and cerebrovascular complications, it is a major requirement for these to be detected, prevented, and treated.

OBJECTIVE

The objectives of our study were to determine out the prevalence of EOD in newly detected rural essential hypertensives, identify the pattern of EOD involved, and also assess the interrelationship among them.

METHODS

A total of 147 consecutive patients with newly diagnosed essential hypertension and, attending the outpatient clinic, were included in this study based on a set of inclusion and exclusion criteria. Patients with comorbid illnesses were excluded from the investigation. The significance of a range of variables involved was evaluated using statistical software.

RESULTS

Amongst the cases, there were 79 males (M) and 68 females (F). Their mean and median ages were 55 (standard deviation (SD) = 9.90) and 54 years, respectively. Of these 86% (70 M and 56 F) had one or more EODs; an observation which was very close to statistically significant (P = .054). In our patients, microalbuminuria (35%), diastolic dysfunction (59%), increased left ventricular (LV) mass (68%) and retinopathy (69%) were particularly notable. Increased LV mass had a strong association with retinopathy, microalbuminuria, and diastolic dysfunction.

CONCLUSION

The presence of one or more EODs in newly detected hypertension indicates widespread vascular damage that carries a high risk for cardio- and cerebrovascular morbidity and mortality. Hence, there is an urgent requirement not only for complete evaluation but also to stimulate an increased awareness amongst physicians and the general public, regarding the control of blood pressure.

ETA activation mediates angiotensin II-induced infiltration of renal cortical T cells

T cells and endothelin (ET-1) both contribute to angiotensin II (AngII)-dependent hypertension. To determine whether ET-1, via the ET(A) receptor, facilitates T cell infiltration in the kidney during AngII-dependent hypertension, we measured T cell infiltration in response to four different treatments: saline, AngII infusion, AngII infusion with an ET(A) receptor antagonist, or AngII infusion with triple-antihypertensive therapy. After 14 days, AngII increased both BP and the numbers of CD3(+) and proliferating cells in the kidney. Mice treated concomitantly with the ET(A) receptor antagonist had lower BP and fewer CD3(+) and proliferating cells in the renal cortex. Mice treated with triple therapy had similar reductions in BP but no change in renal cortical CD3(+) cells compared with kidneys from AngII-infused hypertensive mice. In the outer medulla, both the ET(A) receptor antagonist and triple therapy reduced the number of CD3(+) cells and macrophages. Taken together, these data suggest that ET(A) receptor activation in AngII-mediated hypertension increases CD3(+) cells and proliferation in the renal cortex independent of changes in BP, but changes in the number of inflammatory cells in the renal medulla are BP dependent.

Insights into ET(A) subtype selectivity of benzodiazepine endothelin receptor antagonists by 3D-QSAR approaches

ET(A) subtype selective antagonists constitute a novel and potentially important class of agents for the treatment of pulmonary hypertension, heart failure, and other pathological conditions. In this paper, 60 benzodiazepine derivatives displaying potent activities against ET(A) and ET(B) subtypes of endothelin receptor were selected to establish the 3D-QSAR models using CoMFA and CoMSIA approaches. These models show excellent internal predictability and consistency, external validation using test-set 19 compounds yields a good predictive power for antagonistic potency. Statistical parameters of models were obtained with CoMFA-ET(A) (q (2) = 0.787, r (2) = 0.935, r (2) ( pred ) = 0.901), CoMFA-ET(B) (q (2) = 0.842, r (2) = 0.984, r (2) ( pred ) = 0.941), CoMSIA-ET(A) (q (2) = 0.762, r (2) = 0.971, r (2) ( pred ) = 0.958) and CoMSIA-ET(B) (q (2) = 0.771, r (2) = 0.974, r (2) ( pred ) = 0.953) respectively. Field contour maps (CoMFA and CoMSIA) corresponding to the ET(A) and ET(B) subtypes reflects the characteristic similarities and differences between these types. The results of this paper provide valuable information to facilitate structural modifications of the title compounds to increase the inhibitory potency and subtype selectivity of endothelin receptor.

O-GlcNAcylation: a novel pathway contributing to the effects of endothelin in the vasculature

Glycosylation with O-linked β-N-acetylglucosamine (O-GlcNAc) or O-GlcNAcylation on serine and threonine residues of nuclear and cytoplasmic proteins is a posttranslational modification that alters the function of numerous proteins important in vascular function, including kinases, phosphatases, transcription factors, and cytoskeletal proteins. O-GlcNAcylation is an innovative way to think about vascular signaling events both in physiological conditions and in disease states. This posttranslational modification interferes with vascular processes, mainly vascular reactivity, in conditions where endothelin-1 (ET-1) levels are augmented (e.g. salt-sensitive hypertension, ischemia/reperfusion, and stroke). ET-1 plays a crucial role in the vascular function of most organ systems, both in physiological and pathophysiological conditions. Recognition of ET-1 by the ET(A) and ET(B) receptors activates intracellular signaling pathways and cascades that result in rapid and long-term alterations in vascular activity and function. Components of these ET-1-activated signaling pathways (e.g., mitogen-activated protein kinases, protein kinase C, RhoA/Rho kinase) are also targets for O-GlcNAcylation. Recent experimental evidence suggests that ET-1 directly activates O-GlcNAcylation, and this posttranslational modification mediates important vascular effects of the peptide. This review focuses on ET-1-activated signaling pathways that can be modified by O-GlcNAcylation. A brief description of the O-GlcNAcylation biology is presented, and its role on vascular function is addressed. ET-1-induced O-GlcNAcylation and its implications for vascular function are then discussed. Finally, the interplay between O-GlcNAcylation and O-phosphorylation is addressed.

Lactoferricin B-derived peptides with inhibitory effects on ECE-dependent vasoconstriction

Endothelin-converting enzyme (ECE), a key peptidase in the endothelin (ET) system, cleaves inactive big ET-1 to produce active ET-1, which binds to ET(A) receptors to exert its vasoconstrictor and pressor effects. ECE inhibition could be beneficial in the treatment of hypertension. In this study, a set of eight lactoferricin B (LfcinB)-derived peptides, previously characterized in our laboratory as angiotensin-converting enzyme (ACE) inhibitory peptides, was examined for their inhibitory effects on ECE. In vitro inhibitory effects on ECE activity were assessed using both the synthetic fluorogenic peptide substrate V (FPS V) and the natural substrate big ET-1. To study vasoactive effects, an ex vivo functional assay was developed using isolated rabbit carotid artery segments. With FPS V, only four LfcinB-derived peptides induced inhibition of ECE activity, whereas the eight peptides showed ECE inhibitory effects with big ET-1 as substrate. Regarding the ex vivo assays, six LfcinB-derived peptides showed inhibition of big ET-1-induced, ECE-dependent vasoconstriction. A positive correlation between the inhibitory effects of LfcinB-derived peptides on ECE activity when using big ET-1 and the inhibitory effects on ECE-dependent vasoconstriction was shown. ECE-independent vasoconstriction induced by ET-1 was not affected, thus discarding effects of LfcinB-derived peptides on ET(A) receptors or intracellular signal transduction mechanisms. In conclusion, a combined in vitro and ex vivo method to assess the effects of potentially antihypertensive peptides on the ET system has been developed and applied to show the inhibitory effects on ECE-dependent vasoconstriction of six LfcinB-derived peptides, five of which were dual vasopeptidase (ACE/ECE) inhibitors.

Darusentan, a selective endothelin A receptor antagonist, for the oral treatment of resistant hypertension

Resistant hypertension is defined as failure to lower blood pressure to target when a patient adheres to the maximum tolerated doses of three antihypertensive drugs including a diuretic. Notwithstanding the wide availability of several antihypertensive agents and the continued recommendation of dietary and lifestyle modifications, the prevalence of resistant hypertension remains high and is expected to increase thus underscoring the need for potential new treatment modalities in resistant hypertension. Endothelin-1 is a long-lasting potent vasoconstrictor and plays a key role in cardiovascular haemostasis. Endothelin mediates its biological activity in humans through the endothelin A and B receptors. The clinical experience and the evidence for therapy with darusentan in resistant systemic hypertension are reviewed. The leading journals that publish basic science and clinical research in the area of cardiovascular diseases and PubMed were scanned. While results from early clinical studies suggested that darusentan might emerge as new treatment option in patients with resistant hypertension, results from recent studies suggests that darusentan appears unlikely to find its way in the armamentarium for treatment of resistant hypertension.

Review: Update on Newer Antihypertensive Medicines and Interventions

The incidence and prevalence of systemic hypertension are reaching global epidemic proportions. Despite a diverse pharmacologic armamentarium of agents to treat high blood pressure, suboptimal control remains a significant problem in as many as 43% of patients and this rate has not significantly improved over the past 2 decades. There are a variety of factors contributing to this including patient nonadherence due to complex drug regimens and medication side effects, undertreatment, and treatment resistance. There, thus, remains a need to develop novel agents and approaches to antihypertensive therapy that facilitate attainment of optimal blood pressure levels. This monograph will review a number of new pharmacologic targets and interventions as well as a novel method of drug delivery to patients.

Involvement of imidazoline and opioid receptors in the enhancement of clonidine-induced analgesia by sulfisoxazole

Clonidine, an α2-adrenergic agonist, has been demonstrated to produce significant analgesia and potentiate morphine analgesia. Endothelin (ETA) receptor antagonists have also been found to potentiate the antinociceptive response to morphine. Clonidine and ET have been reported to have cardiovascular interactions involving the sympathetic nervous system, but it is not known whether ETA receptor antagonist affects clonidine analgesia. This study examined the influence of sulfisoxazole (ETA receptor antagonist) on clonidine analgesia. Male Swiss Webster mice were used to determine antinociceptive response of drugs by measuring tail-flick latency. The effect of clonidine (0.3, 1.0, and 3.0 mg/kg, i.p.) alone or in combination with sulfisoxazole (25, 75, and 225 mg/kg, p.o.) on analgesia and body temperature was determined. Clonidine produced a dose-dependent analgesia and hypothermia. Sulfisoxazole (25, 75, and 225 mg/kg), when administered with clonidine (0.3 mg/kg), significantly potentiated (31% increase in area under the curve (AUC)) the analgesic effect of clonidine. Yohimbine (α2-adrenergic receptor antagonist) did not affect analgesic effect of clonidine plus sulfisoxazole. Idazoxan (I1-imidazoline and α2-adrenergic receptor antagonist) reduced (47% decrease in AUC) the analgesic effect of clonidine plus sulfisoxazole. Treatment with naloxone reduced (46% decrease in AUC) the analgesic effect of clonidine plus sulfisoxazole. The effect of another ETA receptor antagonist, BMS-182874 (2, 10, and 50 µg, i.c.v.) was studied, and it was found that the dose of 10 µg significantly potentiated (26% increase in AUC) the analgesic effect of clonidine. These results indicate that sulfisoxazole, an ETA receptor antagonist, potentiates the analgesic effect of clonidine, which could be mediated through I1-imidazoline receptors and opioid receptors.

Antihypertensive agents in hemodialysis patients: a current perspective

Hypertension affects most hemodialysis patients and is often poorly controlled. Adequate control of blood pressure is difficult with conventional hemodialysis alone but is important to improve cardiovascular outcomes. Nonpharmacologic interventions to improve blood pressure include educating patients about limiting sodium intake, ensuring adequate sodium solute removal during hemodialysis, and achieving target "dry weight." However, most patients require a number of antihypertensive medications to achieve an appropriate blood pressure. First-line antihypertensive agents include angiotensin converting enzyme inhibitors and angiotensin receptor blockers given their safety profile and demonstrated benefit on cardiovascular outcomes in clinical trials. beta-blockers and combined alpha- and beta-blockers should also be used in patients with cardiovascular disease or congestive heart failure and may improve outcomes in these populations. Calcium channel blockers and direct vasodilators are also effective for controlling blood pressure. Many blood pressure agents can be dosed once daily and should preferentially be administered at night to control nocturnal blood pressure and minimize intradialytic hypotension. In patients who are noncompliant with therapy, renally eliminated agents (such as lisinopril and atenolol) can be given thrice weekly following hemodialysis. Older antihypertensive agents which require thrice daily dosing ought to be avoided given the high pill burden with these regimens and the concern for noncompliance resulting in rebound hypertension. Newer antihypertensive agents, such as direct renin inhibitors, may provide alternative options to improve blood pressure but require testing for efficacy and safety in hemodialysis patients.

Endothelin, hypertension and chronic kidney disease: new insights

PURPOSE OF REVIEW

Endothelin is important in the development of cardiorenal disease. This review discusses recent developments in understanding endothelin's role in hypertension and chronic kidney disease (CKD).

RECENT FINDINGS

Endothelin-1 production is increased in hypertension and CKD. Endothelin-1 stimulates vasoconstriction, inflammation and fibrosis, thereby promoting hypertension, atherosclerosis and CKD. These effects are closely linked to angiotensin II and reactive oxygen species. In preclinical studies, endothelin receptor antagonists were effective in treating hypertension (particularly with endothelial dysfunction) and CKD. In preclinical studies, endothelin A-selective, as opposed to combined endothelin A and B, receptor blockers have generally been more efficacious. Few clinical trials have been conducted in hypertension and/or kidney disease, partly due to concerns over side effects of testicular toxicity and fluid retention. Endothelin blockade reduces blood pressure in patients with resistant hypertension, with additional beneficial metabolic effects. Endothelin antagonism improves proteinuria in CKD (diabetic or not), particularly in patients taking inhibitors of angiotensin II action.

SUMMARY

Endothelin is a promising target in the treatment of resistant hypertension and CKD, with additional potential benefits on atherosclerosis and the metabolic syndrome. The nature and mechanisms of drug side effects require elucidation before the potential of this new class of drugs can be fully realized.

Intradialytic hypertension: a less-recognized cardiovascular complication of hemodialysis

Intradialytic hypertension, defined as an increase in blood pressure during or immediately after hemodialysis that results in postdialysis hypertension, has long been recognized to complicate the hemodialysis procedure, yet often is largely ignored. In light of recent investigations suggesting that intradialytic hypertension is associated with adverse outcomes, this review broadly covers the epidemiologic characteristics, prognostic significance, potential pathogenic mechanisms, prevention, and possible treatment of intradialytic hypertension. Intradialytic hypertension affects up to 15% of hemodialysis patients and occurs more frequently in patients who are older, have lower dry weights, are prescribed more antihypertensive medications, and have lower serum creatinine levels. Recent studies associated intradialytic hypertension independently with higher hospitalization rates and decreased survival. Although the pathophysiologic mechanisms of intradialytic hypertension are uncertain, it likely is multifactorial and includes subclinical volume overload, sympathetic overactivity, activation of the renin-angiotensin system, endothelial cell dysfunction, and specific dialytic techniques. Prevention and treatment of intradialytic hypertension may include careful attention to dry weight, avoidance of dialyzable antihypertensive medications, limiting the use of high-calcium dialysate, achieving adequate sodium solute removal during hemodialysis, and using medications that inhibit the renin-angiotensin-aldosterone system or decrease endothelin 1 levels. In summary, although intradialytic hypertension often is underappreciated, recent studies suggest that it should not be ignored. However, further work is necessary to elucidate the pathophysiologic mechanisms of intradialytic hypertension and its appropriate management and determine whether treatment of intradialytic hypertension can improve clinical outcomes.