Selective Endothelin Research Articles

Effectiveness of Spironolactone Plus Ambrisentan for Treatment of Pulmonary Arterial Hypertension (from the [ARIES] Study 1 and 2 Trials)

In translational models of pulmonary arterial hypertension (PAH), spironolactone improves cardiopulmonary hemodynamics by attenuating the adverse effects of hyperaldosteronism on endothelin type-B receptor function in pulmonary endothelial cells. This observation suggests that coupling spironolactone with inhibition of endothelin type-A receptor-mediated pulmonary vasoconstriction may be a useful treatment strategy for patients with PAH. We examined clinical data from patients randomized to placebo or the selective endothelin type-A receptor antagonist ambrisentan (10 mg/day) and in whom spironolactone use was reported during ARIES-1 and -2, which were randomized, double-blind, placebo-controlled trials assessing the effect of ambrisentan for 12 weeks on clinical outcome in PAH. From patients randomized to placebo (n = 132) or ambrisentan (n = 67), we identified concurrent spironolactone use in 21 (15.9%) and 10 (14.9%) patients, respectively. Compared with patients treated with ambrisentan alone (n = 57), therapy with ambrisentan + spironolactone improved change in 6-minute walk distance by 94% at week 12 (mean ± SE, +38.2 ± 8.1 vs +74.2 ± 27.4 m, p = 0.11), improved plasma B-type natriuretic peptide concentration by 1.7-fold (p = 0.08), and resulted in a 90% relative increase in the number of patients improving ≥1 World Health Organization functional class (p = 0.08). Progressive illness, PAH-associated hospitalizations, or death occurred as an end point for 5.3% of ambrisentan-treated patients; however, no patient treated with ambrisentan + spironolactone reached any of these end points. In conclusion, these pilot data suggest that coupling spironolactone and endothelin type-A receptor antagonism may be clinically beneficial in PAH. Prospective clinical trials are required to further characterize our findings.

Bioactivation of Sitaxentan in Liver Microsomes, Hepatocytes, and Expressed Human P450s with Characterization of the Glutathione Conjugate by Liquid Chromatography Tandem Mass Spectrometry

Sitaxentan is a selective endothelin-A receptor antagonist that was marketed as Thelin in several European countries and Canada for pulmonary arterial hypertension. Sitaxentan was undergoing further clinical trials in the United States but due to four deaths and one case of liver transplantation from severe liver toxicity that appeared to be idiosyncratic in nature, it was withdrawn worldwide in December, 2010. Sitaxentan contains a 1,3-benzodioxole ring that undergoes enzymatic demethyleneation to an ortho-catechol metabolite that can further oxidize to a reactive ortho-quinone metabolite. Here, we report the detection and mass spectral characterization of a glutathione conjugate of this sitaxentan quinone reactive metabolite that was trapped in vitro using mouse, rat, dog, and human liver microsomes supplemented with NADPH and glutathione and that was also observed in rat and human hepatocytes. Using human liver microsomes, we also demonstrated that P450 3A4 undergoes time-dependent inhibition. Density functional calculations on the catechol metabolite of sitaxentan indicated that the reaction leading to the quinone was thermodynamically favorable with an enthalpy change of -6.3 kcal/mol. Using density functional methodology, we modeled the attack of glutathione on the quinone with an S-methyl thiolate anion which allowed us to predict, based on the difference in transition state energies, that the 2-position on the phenyl ring was more likely than the 5-position as the site of glutathione conjugation. Overall, our results demonstrated that sitaxentan is capable of facile formation of a reactive ortho-quinone metabolite capable of reacting with glutathione and may rationalize the idiosyncratic nature of the hepatotoxicity that led to its withdrawal.

Ambrisentan for the treatment of pulmonary arterial hypertension: improving outcomes

Pulmonary arterial hypertension (PAH) is a progressive disease of the pulmonary vasculature that is associated with severe functional impairment and a poor prognosis. Ambrisentan is a selective endothelin type A receptor antagonist approved for the treatment of patients with PAH World Health Organization group 1. The efficacy and safety of ambrisentan has been evaluated in the ARIES series (Ambrisentan for the Treatment of Pulmonary Arterial Hypertension, Randomized, Double-Blind, Placebo-Controlled, Multicenter, Efficacy Studies), which has established its use as both monotherapy or in conjunction with other PAH therapies. Specifically, ambrisentan is effective at increasing exercise tolerance, decreasing the risk of functional class deterioration, and prolonging time to clinical worsening. Further, ambrisentan has a favorable effect on mortality, with an 88% patient survival rate after two years of therapy compared with a 61% survival rate as estimated by the National Institute of Health Registry. Ambrisentan is generally well tolerated in all patient groups, with the main side effects of peripheral edema, sinusitis, flushing, and nasal congestion considered to be mild to moderate in nature. Ambrisentan has several favorable qualities that potentially make it more acceptable to patients, including once-daily administration, limited adverse drug reactions and drug-drug interactions, and minimal risk of liver enzyme elevation. Because of the potential risk of teratogenicity associated with ambrisentan, it is only available through a limited distribution program, ie, LEAP (the Letairis Education and Access Program). Ongoing clinical trials will help to clarify the role of ambrisentan in the treatment of PAH.

Clinical Pharmacokinetics and Drug‐Drug Interactions of Endothelin Receptor Antagonists in Pulmonary Arterial Hypertension

The authors review the basic pharmacology and potential for adverse drug-drug interactions (DDIs) of bosentan and ambrisentan, the 2 endothelin receptor antagonists currently approved for pulmonary arterial hypertension (PAH) treatment. Bosentan, an endothelin (ET) receptor-type ET(A) and ET(B) antagonist, is metabolized to active metabolites by and an inducer of cytochrome P450 (CYP)2C9 and CYP3A. Ambrisentan, a selective ET(A) receptor antagonist, is metabolized primarily by uridine 5'diphosphate glucuronosyltransferases (UGTs) 1A9S, 2B7S, and 1A3S and, to a lesser extent, by CYP3A and CYP2C19. Drug interactions observed with bosentan DDI studies have demonstrated a potential for significant clinical implications during PAH management: bosentan is contraindicated with cyclosporine A and glyburide, and additional monitoring/dose adjustments are required when coadministered with hormonal contraceptives, simvastatin, lopinavir/ritonavir, and rifampicin. As bosentan carries a boxed warning regarding risks of liver injury and showed dose-dependant increases in serum aminotransferase abnormalities, drug interactions that increase bosentan exposure are of particular clinical concern. Ambrisentan DDI studies performed to date have shown only one clinically relevant DDI, an interaction with cyclosporine A that requires ambrisentan dose reduction. As the treatment of PAH moves toward multimodal combination therapy, scrutiny should be placed on ensuring that drug combinations achieve maximal clinical benefit while minimizing side effects.

EndothelinA–EndothelinB Receptor Cross Talk in Endothelin-1–Induced Contraction of Smooth Muscle

The efficacy of selective endothelin (ET) receptor antagonists may be limited by a functional interaction between the ET(A) and ET(B) receptors. This interaction, also termed "cross talk", is characterized by the dependency of the inhibition of an ET-1 response due to antagonism of one ET receptor subtype upon concomitant antagonism of the other ET receptor subtype. Although a reduction in ET(A)-ET(B) receptor cross talk would presumably increase the efficacy of selective ET receptor antagonists, an approach that accomplishes this aim is largely absent due to a lack of mechanistic understanding. Toward this goal, we evaluated the characteristics and potential dependencies of cross talk in smooth muscle. Smooth muscle was adopted as an exemplar not only because cross talk is widely reported in this tissue type, thereby allowing numerous comparisons, but also significant controversy surrounds the use of selective versus nonselective ET receptor antagonists in ET-1-related pathophysiologies involving smooth muscle. Based on this evaluation, we suggest that ET(A)-ET(B) receptor cross talk is a dynamic process directed by either or both ET receptor subtypes and expressed to varying magnitudes depending on the ET-1 and selective ET receptor antagonist concentrations, tone due to intraluminal pressure/stretch, agonists acting at receptors other than the ET(A)/ET(B) receptors, and endothelial/epithelial function. It is speculated that ET(A)-ET(B) receptor cross talk occurs through signal transduction pathways along with changes at the receptor level. Pharmacologic intervention of the signaling pathways could increase the therapeutic efficacy of ET receptor antagonists.

Influence of specific endothelin-1 receptor blockers on hemodynamic parameters and antioxidant status of plasma in LPS-induced endotoxemia

BackgroundThe potent vasoconstrictor endothelin-1 has been implicated in the pathogenesis of plasma oxidative stress seen in sepsis. The selective endothelin receptor blockers BQ123 andBQ788 were used to investigate the importance of selective endothelin receptor blockage in modulating oxidative stress during endotoxemia. MethodsThe study was performed on male Wistar rats (n=6 per group) divided into groups: (1) saline, (2) lipopolysaccharide (LPS) (15mg\\kg)-saline, (3) BQ123 (0.5mg\\kg)-LPS, (4) BQ123 (1mg\\kg)-LPS, (5) BQ788 (3mg\\kg)-LPS. The endothelin receptor type A (ETA-R) or type B (ETB-R) antagonist was injected intravenously 30min before LPS administration. Blood pressure was monitored and blood was taken before, 90min and 300min after saline or LPS administration. ResultsInjection of LPS alone resulted in a decrease in mean arterial pressure (MAP) (p<0.05), a decrease in ferric reducing ability of plasma (FRAP) value (p<0.01) and a marked increase in plasma tumor necrosis factor α (TNF-α) and thiobarbituric acid reactive substances (TBARS) (p<0.001, p<0.001, respectively). Administration of BQ123 before LPS administration deteriorated MAP in a dose dependent way. Moreover, BQ123 (1mg\\kg) decreased plasma level of TBARS and TNF-α (p<0.01 and p<0.05, respectively) and increased FRAP value (p<0.001). On the contrary, BQ788 prevented LPS-induced decrease in MAP (p<0.001) and led to a significant reduction in plasma TBARS concentration (p<0.01). ConclusionOur study showed that blockage of ETB-R during endotoxemia improved blood hemodynamics and decreased plasma lipid peroxidation. Blockage of ETA-R improved plasma antioxidant status and decreased lipid peroxidation and TNF-a production, but it deteriorated hemodynamic conditions.

An Evaluation of Reproductive and Developmental Toxicity of Sitaxentan (Thelin) in Rats

Sitaxentan sodium (Thelin) is a once daily, orally bioavailable, highly selective endothelin A receptor antagonist. Initially approved for the treatment of pulmonary arterial hypertension, sitaxentan was withdrawn in 2010 following the recognition of a pattern of idiosyncratic liver injury. During development of this drug, a series of nonclinical studies investigated the effects of orally administered sitaxentan on fertility, embryofetal development, and pre- and postnatal development in the rat; results of these studies are reported here. In the fertility study, sitaxentan did not affect mating behavior, fertility, sperm morphology, or estrous cycle. Sitaxentan was teratogenic in the embyrofetal development study, which was expected based on its pharmacologic mechanism of action. Teratogenic effects included malformations of the head, mouth, face, and large blood vessels. In the pre- and postnatal study, sitaxentan administration was associated with reduced pup survival, large or abnormally shaped livers, and delays in markers of auditory and sexual development. Sitaxentan was detected in plasma of suckling pups receiving milk from females dosed with sitaxentan. These nonclinical study findings were reflected in the sitaxentan product label warnings.

Changes in Exercise Capacity and Cardiac Performance in a Series of Patients with Eisenmenger's Syndrome Transitioned from Selective to Dual Endothelin Receptor Antagonist

Differences in clinical effects between selective and dual endothelin (ET) receptor antagonists (ERA) in patients with pulmonary arterial hypertension (PAH) are currently unknown. We aimed to assess prospectively how transition from selective (sitaxsentan) to dual (bosentan) ERA affected exercise capacity and cardiocirculatory performance in patients with Eisenmenger's syndrome. A series of seven stable patients with Eisenmenger's syndrome aged 40.0 (30.0-56.0) years old treated with sitaxsentan were assessed before and three months after transition to bosentan. Six minute walk test and magnetic resonance to assess LV and RV mass, volume and ejection fraction, and pulmonary flow, and laboratory tests were performed. We observed an increase in LV mass [96.5 (66.0-116.0) vs. 123.0 (93.0-146.0)g; p=0.03], LV ejection fraction [55.0 (44.0-63.0) vs. 65.0 (58.0-70.0)%; p=0.02)], and pulmonary flow [64 (53.0-71.0) vs. 69.0 (55.0-84.0)ml/beat; p=0.046]. This was accompanied by an increase of oxygen saturation, elongation of 6MWD [435.0 (378.0-482.3) vs. 474 (405.0-534.7); p=0.02], decrease of NTproBNP level and increase of ET-1 level. Three month follow-up of stable patients with Eisenmenger's syndrome transitioned from sitaxsentan to bosentan revealed improvement of exercise capacity despite significant elevation of ET-1 level. Concurrent increase of LV ejection fraction and pulmonary flow might have contributed to these favourable effects.

Endothelin B receptor agonist, IRL-1620, provides long-term neuroprotection in cerebral ischemia in rats

We have earlier shown that stimulation of endothelin B receptors by IRL-1620 provides significant neuroprotection at 24h following cerebral ischemia. However, the effect of IRL-1620 is not known in the subacute phase of cerebral ischemia, where development of cerebral edema further contributes towards brain damage. This study was designed to determine the effect of IRL-1620 on neurological functions, infarct volume, oxidative stress, and endothelin receptors following permanent middle cerebral artery occlusion for 7days. Rats received three intravenous injections of either vehicle or IRL-1620 [Suc-[Glu9,Ala11,15]-Endothelin-1(8–12)] at 2, 4, and 6h post occlusion. Treatment with IRL-1620 reduced infarct volume (54.06±14.12mm3 vs. 177.06±13.21mm3), prevented cerebral edema and significantly improved all neurological and motor function parameters when compared to the vehicle-treated group. Vehicle-treated middle cerebral artery occluded rats demonstrated high levels of malondialdehyde and low levels of reduced glutathione and superoxide dismutase; these effects were reversed in IRL-1620 treated rats. No change in expression of endothelin A receptor was observed 7days after induction of cerebral ischemia in vehicle or IRL-1620 treated rats. Rats receiving IRL-1620 demonstrated an upregulation of endothelin B receptor only in the infarcted hemisphere 7days following occlusion. All effects of IRL-1620 were blocked by endothelin B receptor antagonist, BQ788. Results of the present study demonstrate that IRL-1620, administered on day 1, provides significant neuroprotection till 7days after the induction of cerebral ischemia in rats. Selective endothelin B receptor activation may prove to be a novel therapeutic target in the treatment of cerebral ischemia.

Non-endothelial cell endothelin-B receptors limit neointima formation following vascular injury

The potent vasoconstrictor endothelin-1 (ET-1), acting on the endothelin-A (ETA) receptor, promotes intimal lesion formation following vascular injury. The endothelin-B (ETB) receptor, which mediates nitric oxide release and ET-1 clearance in endothelial cells, may moderate lesion formation, but this is less clear. We used selective ET receptor antagonists and cell-specific deletion to address the hypothesis that ETB receptors in the endothelium inhibit lesion formation following arterial injury. Neointimal proliferation was induced by wire or ligation injury to the femoral artery in mice treated with selective ETA (ABT-627) and/or ETB antagonists (A192621). Measurement of lesion formation by optical projection tomography and histology indicated that ETA blockade reduced lesion burden in both models. Although ETB blockade had little effect on ligation injury-induced lesion formation, after wire injury, blockade of the ETB receptor increased lesion burden (184% of vehicle; P < 0.05) and reversed the protective effects of an ETA antagonist. Selective deletion of ETB receptors from the endothelium, however, had no effect on neointimal lesion size. These results are consistent with ETB receptor activation playing an important role in limiting neointimal lesion formation following acute vascular injury, but indicate that this protective effect is not mediated by those ETB receptors expressed by endothelial cells. These data support the proposal that selective ETA antagonists may be preferable to mixed ETA/ETB antagonists for targeting the arterial response to injury.

Ambrisentan and Tadalafil Synergistically Relax Endothelin-Induced Contraction of Rat Pulmonary Arteries

Endothelin receptor antagonists and phosphodiesterase type 5 inhibitors are used to treat pulmonary arterial hypertension. We tested the hypothesis that a selective endothelin type A receptor antagonist (ambrisentan) and a phosphodiesterase type 5 inhibitor (tadalafil) may act synergistically to relax endothelin-constricted pulmonary arteries. Rat isolated intrapulmonary arterial rings contracted with 8 nmol/L endothelin-1 were relaxed by 10 nmol/L ambrisentan and 30 nmol/L tadalafil alone by 26±3% and 21±1%, respectively, whereas both drugs in combination acted synergistically to relax arterial rings by 83±6%. The nonselective endothelin type A and B receptor antagonists bosentan (100 nmol/L) and macitentan (30 nmol/L) alone relaxed endothelin-contracted rings by 30±5% and 24±3%, respectively. Combinations of 30 nmol/L tadalafil with 100 nmol/L bosentan or 30 nmol/L macitentan relaxed endothelin-contracted rings by 53±5% or 46±7%, respectively; these values are similar to the calculated sums of the individual effects of these compounds. Denudation of endothelium from pulmonary arterial rings abolished the vasodilator response to 30 nmol/L tadalafil and the synergistic vasorelaxant effect of tadalafil with ambrisentan. In the presence of 1 μmol/L BQ-788, a selective endothelin type B receptor antagonist, the vasorelaxant effects of 10 nmol/L ambrisentan and 30 nmol/L tadalafil were additive but not synergistic. These data can be interpreted to suggest that ambrisentan and tadalafil synergistically inhibit endothelin-1-induced constriction of rat intrapulmonary arteries and that endothelin type B receptors in endothelium are necessary to enable a synergistic vasorelaxant effect of the drug combination.

Clinical efficacy of the selective endothelin A receptor antagonist, atrasentan, in patients with diabetes and chronic kidney disease (CKD)

AimsProgression of chronic kidney disease (CKD) in patients with diabetes is a growing problem. Diabetes is associated with elevated endothelin-1 (ET-1) and enhanced renal expression of the endothelin A receptor (ETAR). Atrasentan, a highly selective ETAR antagonist, reduces albuminuria in patients with DN. Key methodsThis was a randomized, double-blind trial of subjects with type 2 diabetes on renin–angiotensin system (RAS) inhibitors having eGFR >20ml/min, and urine albumin-to-creatinine ratio (UACR) of 100–3000mg/g, who were allocated to placebo, 0.25, 0.75 or 1.75mg atrasentan. Key findingsUACR was reduced in the 0.75mg and 1.75mg groups (42% and 35% vs placebo, P<0.011) over the 8week treatment period. Edema was reported in 21 subjects: 62% of edema events emerged during the first 4weeks. There were no significant changes in serum hsCRP, IL-6, NT-pro-BNP, ET-1, urine TGFb or MCP-1. Urine NGAL was reduced 24% in the 1.75mg group (P=0.044). Hispanic subjects (58% of total) tended to have greater UACR reductions than non-Hispanics (0.75mg dose: Hispanic: 41–60%; non-Hispanic: 18–37%; P=0.012 and 0.048 vs placebo, respectively) without different rates of edema. Mean UACR reduction in subjects receiving maximum doses of RAS inhibitors (38%) was 32% and 35% in the 0.75 and 1.75mg groups, respectively, and similar to overall UACR changes. SignificanceEdema formation was dose-dependent and occurred early. The decrease in urine NGAL warrants further study in renal tubular disease attenuation. UACR responses based on ethnicity need further characterization. Results suggest atrasentan may have additive effects to RAS inhibition in treatment of DN.

EndothelinA–endothelinB receptor cross-talk in rat basilar artery in situ

The rationale for the therapeutic use of dual as opposed to selective endothelin (ET) receptor antagonists stems in part from cross-talk between the ET(A) and ET(B) receptors. However, whether ET(A)-ET(B) receptor cross-talk is present in the cerebral vasculature is difficult to discern since findings of cross-talk contrast even among the few reports available. Thus, this study tested whether ET(A)-ET(B) receptor cross-talk is present in the rat basilar artery. In an in situ cranial window, 0.1μM sarafotoxin S6c, an ET(B) receptor agonist, relaxed basilar artery basal tone by 54%. ET-1 (3nM) in the absence and presence of 10μM BQ123, an ET(A) receptor agonist, induced 13% contraction and 15% relaxation, respectively. In contrast, the 3-nM ET-1 plateau contraction was relaxed by only ∼50% by 3-10μM BQ123 and 10μM BQ610, ET(A) receptor antagonists. N(ω)-nitro-L: -arginine, an NO synthase inhibitor, did not enhance contraction to 3nM ET-1, suggesting that the partial relaxation of the ET-1 plateau contraction did not involve unmasked endothelial ET(B) receptor-mediated relaxation. The ∼50% ET-1 contraction that remained following ET(A) receptor antagonist was relaxed by 3-10μM BQ788, consistent with an ET(B) receptor-mediated component of contraction. However, 10μM BQ788 in the absence of prior ET(A) receptor antagonist did not cause relaxation. Subsequent BQ123 addition in the presence of BQ788 completely relaxed the ET-1 contraction. PD145065 (1μM), an ET(A/B) receptor antagonist, completely relaxed 3-nM ET-1 contracted vessels in both the absence and presence of BQ123. These findings suggest that the inability of ET(A) receptor antagonist to completely relax the ET-1 plateau contraction in rat basilar artery is due to ET(A)-ET(B) receptor cross-talk.

Pathological Importance of the Endothelin-1/ETBReceptor System on Vascular Diseases

Activation of the endothelin (ET)-1/ET receptor system is involved in the development of vascular diseases such as atherosclerosis, vascular hypertrophy, and restenosis. Some issues still remain unresolved including whether ET receptor antagonists are expected to become the new therapeutic tools for the treatment of vascular diseases. One of the unresolved critical points is the functional role of ET receptor subtypes on each vascular disease, in particular the pathophysiological roles of the ETB receptor. We recently demonstrated that selective inhibition of the ETB receptor system showed harmful effects in the development of neointimal formation after vascular injury. However, there was no apparent difference in the therapeutic effects between a nonselective ETA/ETB receptor antagonist and selective ETA receptor antagonist. These findings indicate that antagonism of the ETA receptor system is essential for suppressing vascular remodeling, irrespective of the presence of ETB-receptor-mediated actions, although the selective ETB receptor antagonist worsens vascular remodeling. In addition, we found that ET receptor systems contribute to sex differences in the severity of vascular disease, thereby suggesting that the efficacy of ET receptor antagonists for vascular diseases may differ between sexes. In this paper, we outline the roles of the ET-1/ETB receptor system on vascular diseases and its sex differences.

Sitaxsentan-Induced Acute Severe Hepatitis Treated with Glucocorticoid Therapy

Endothelin receptor antagonists are commonly used in the treatment of pulmonary hypertension. Sitaxsentan, a selective endothelin A receptor blocker, induces a mild transaminitis in approximately 3% to 5% of patients, but rarely an acute severe hepatitis. A case involving a 61-year-old female with sitaxsentan-induced acute severe liver failure is presented. Depite withdrawal of therapy, her liver tests failed to improve. After six weeks of monitoring, the patient was administered high-dose corticosteroids, with a good clinical and biochemical response. While endothelin receptor antagonists are postulated to cause hepatitis by inhibition of a bile salt transporter pump, an immune-mediated or idiosyncratic mechanism should be considered.

The Effect of Selective Endothelin Receptor a Antagonism by Bq-123 on Myocardial Ischemia-Reperfusion Induced Apoptotic Cell Death

The objective of the present study was to investigate the possible impact of specific endothelin-A (ETA) receptor blockage by BQ-123 on myocardial ischemia-reperfusion (MI/R) induced apoptosis in rats. To produce MI/R, a branch of the descending left coronary artery was occluded for 30 min followed by 2 h reperfusion. Thirty-two rats were randomly assigned to four groups equally: (1) sham-operated group, (2) MI/R group, (3) MI/R+BQ-123-treated group, and (4) MI/R+ET-1+BQ-123-treated group. TUNEL staining, caspase-3 and caspase-9 activities were determined immunohistologically. MI/R group revealed extensive TUNEL positive cardiomyocytes especially in the free wall of the left ventricule, interventicular septum, and apex zone. Intensity of TUNEL-positive cardiomyocytes reduced as a result of BQ-123 treatment compared to the sham group in the same sections. Result of the caspase activity was found to correlate with TUNEL evaluation. BQ-123 administrations to MI/R group with or without ET-1 caused significant decrease both in lipid peroxidation and nitric oxide (NO) generation. Also, BQ-123 leads to augmentation of superoxide dismutase, catalase and glutathione contents. We propose that selective ETA antagonism by BQ-123 has a worthwhile effect on apoptotic cell death following MI/R, and that scavenging of free radicals by selective ETA antagonist is part of this beneficial effect.

Effects of Endothelin Receptor Antagonists on Renal Hemodynamics in Angiotensin II-Infused Rats on High NaCl Intake

Aim: The aim was to investigate effects of selective endothelin (ET) receptor antagonists on renal hemodynamics and dynamic renal blood flow autoregulation (RBFA) in angiotensin II (Ang II)-infused rats on a high NaCl intake. Methods: Sprague-Dawley rats received Ang II (250 ng/kg/min, s.c.) and an 8 % NaCl diet for 14 days after which renal clearance experiments were performed. After baseline measurements animals were administered either: (a) saline vehicle; (b) ET_A receptor antagonist BQ-123 (30 nmol/kg/min); (c) ET_B receptor antagonist BQ-788 (30 nmol/kg/min); or (d) BQ-123 + BQ-788, for six consecutive 20-minute clearance periods. Results: BQ-123 reduced arterial pressure (AP) and selectively increased outer medullary perfusion versus vehicle (p<0.05). These effects were attenuated or abolished by combined BQ-123 and BQ-788. BQ-788 reduced renal blood flow and increased renovascular resistance (p<0.05). Ang II-infused rats on high NaCl intake showed abnormalities in dynamic RBFA characterized by an impaired myogenic response that were not significantly affected by ET receptor antagonists. Conclusion: In hypertensive Ang II-infused rats on a high-NaCl intake selective ET_A antagonism with BQ-123 reduced AP and specifically increased OM perfusion and these effects were dependent on intact ET_B receptor stimulation. Furthermore, ET receptor antagonists did not attenuate abnormalities in dynamic RBFA.

Fluorescence Correlation Spectroscopy in Drug Discovery: Study of Alexa532-Endothelin 1 Binding to the Endothelin ETAReceptor to Describe the Pharmacological Profile of Natural Products

Fluorescence correlation spectroscopy and the newly synthesized Alexa532-ET1 were used to study the dynamics of the endothelin ETA receptor-ligand complex alone and under the influence of a semisynthetic selective antagonist and a fungal extract on living A10 cells. Dose-dependent increase of inositol phosphate production was seen for Alexa532-ET1, and its binding was reduced to 8% by the selective endothelin ETA antagonist BQ-123, confirming the specific binding of Alexa532-ET1 to the endothelin ETA receptor. Two different lateral mobilities of the receptor-ligand complexes within the cell membrane were found allowing the discrimination of different states for this complex. BQ-123 showed a strong binding affinity to the “inactive” receptor state characterized by the slow diffusion time constant. A similar effect was observed for the fungal extract, which completely displaced Alexa532-ET1 from its binding to the “inactive” receptor state. These findings suggest that both BQ-123 and the fungal extract act as inverse agonists.

Therapeutic efficacy of TBC3711 in monocrotaline-induced pulmonary hypertension.

BackgroundEndothelin-1 signalling plays an important role in pathogenesis of pulmonary hypertension. Although different endothelin-A receptor antagonists are developed, a novel therapeutic option to cure the disease is still needed. This study aims to investigate the therapeutic efficacy of the selective endothelin-A receptor antagonist TBC3711 in monocrotaline-induced pulmonary hypertension in rats.MethodsMonocrotaline-injected male Sprague-Dawley rats were randomized and treated orally from day 21 to 35 either with TBC3711 (Dose: 30 mg/kg body weight/day) or placebo. Echocardiographic measurements of different hemodynamic and right-heart hypertrophy parameters were performed. After day 35, rats were sacrificed for invasive hemodynamic and right-heart hypertrophy measurements. Additionally, histologic assessment of pulmonary vascular and right-heart remodelling was performed.ResultsThe novel endothelin-A receptor antagonist TBC3711 significantly attenuated monocrotaline-induced pulmonary hypertension, as evident from improved hemodynamics and right-heart hypertrophy in comparison with placebo group. In addition, muscularization and medial wall thickness of distal pulmonary vessels were ameliorated. The histologic evaluation of the right ventricle showed a significant reduction in fibrosis and cardiomyocyte size, suggesting an improvement in right-heart remodelling.ConclusionThe results of this study suggest that the selective endothelin-A receptor antagonist TBC3711 demonstrates therapeutic benefit in rats with established pulmonary hypertension, thus representing a useful therapeutic approach for treatment of pulmonary hypertension.

Safety and efficacy of sitaxsentan 50 and 100 mg in patients with pulmonary arterial hypertension

ObjectiveTo assess safety and efficacy of sitaxsentan 50 and 100 mg in patients with pulmonary arterial hypertension (PAH). BackgroundSitaxsentan is a highly selective endothelin-A receptor antagonist that was recently withdrawn by the manufacturer because of a pattern of idiosyncratic liver injury. MethodsBefore sitaxsentan withdrawal, this 18-week double-blind, placebo-controlled study randomized patients with PAH to receive placebo or sitaxsentan 50 or 100 mg once daily. The primary efficacy endpoint was change from baseline in 6-min walk distance (6MWD) at week 18. Changes in World Health Organization (WHO) functional class and time to clinical worsening (TTCW) were secondary endpoints. The primary efficacy analysis was powered for sitaxsentan 100 mg versus placebo. ResultsOf 98 randomized patients, 61% were WHO functional class II at baseline. Improvement from baseline to week 18 in 6MWD occurred with sitaxsentan 100 but not 50 mg; a strong placebo effect was observed. At week 18, WHO functional class was improved or maintained in more patients receiving sitaxsentan 100 mg than placebo (P = 0.038); 0% versus 12% of patients deteriorated, respectively. TTCW was not significantly different for 100-mg sitaxsentan patients than placebo (P = 0.090). Adverse events (AEs) occurring more frequently with sitaxsentan (50 or 100 mg) included headache, peripheral edema, dizziness, nausea, extremity pain, and fatigue; most AEs were of mild or moderate severity. ConclusionSitaxsentan 100 mg improved functional class but not 6MWD in PAH patients who were mostly WHO functional class II at baseline. No patient receiving sitaxsentan 100 mg experienced clinical worsening; sitaxsentan was well tolerated.