In vitro characterization of Ro 46-2005, a novel synthetic non-peptide endothelin antagonist of ETA and ETB receptors

The heart as an extravascular target of endothelin-1 in particulate matter-induced cardiac dysfunction

Exposure to particulate matter air pollution has been causally linked to cardiovascular disease in humans. Several broad and overlapping hypotheses describing the biological mechanisms by which particulate matter exposure leads to cardiovascular disease have been explored, although linkage with specific factors or genes remains limited. These hypotheses may or may not also lead to particulate matter-induced cardiac dysfunction. Evidence pointing to autocrine/paracrine signaling systems as modulators of cardiac dysfunction has increased interest in the emerging role of endothelins as mediators of cardiac function following particulate matter exposure. Endothelin-1, a well-described small peptide expressed in the pulmonary and cardiovascular systems, is best known for its ability to constrict blood vessels, although it can also induce extravascular effects. Research on the role of endothelins in the context of air pollution has largely focused on vascular effects, with limited investigation of responses resulting from the direct effects of endothelins on cardiac tissue. This represents a significant knowledge gap in air pollution health effects research, given the abundance of endothelin receptors found on cardiac tissue and the ability of endothelin-1 to modulate cardiac contractility, heart rate, and rhythm. The plausibility of endothelin-1 as a mediator of particulate matter-induced cardiac dysfunction is further supported by the therapeutic utility of certain endothelin receptor antagonists. The present review examines the possibility that endothelin-1 release caused by exposure to PM directly modulates extravascular effects on the heart, deleteriously altering cardiac function.

Endothelin-Receptor Antagonists beyond Pulmonary Arterial Hypertension: Cancer and Fibrosis

The endothelin axis and in particular the two endothelin receptors, ETA and ETB, are targets for therapeutic intervention in human diseases. Endothelin-receptor antagonists are in clinical use to treat pulmonary arterial hypertension and have been under clinical investigation for the treatment of several other diseases, such as systemic hypertension, cancer, vasospasm, and fibrogenic diseases. In this Perspective, we review the molecules that have been evaluated in human clinical trials for the treatment of pulmonary arterial hypertension, as well as other cardiovascular diseases, cancer, and fibrosis. We will also discuss the therapeutic consequences of receptor selectivity with regard to ETA-selective, ETB-selective, or dual ETA/ETB antagonists. We will also consider which chemical characteristics are relevant to clinical use and the properties of molecules necessary for efficacy in treating diseases against which known molecules displayed suboptimal efficacy.

Defining the affinity and receptor sub-type selectivity of four classes of endothelin antagonists in clinically relevant human cardiovascular tissues

AIMS

We have compared the endothelin receptor subtype affinity (K(D)) and selectivity of four structural classes of antagonists (peptide, sulphonamide-based, carboxylic acid-based, myceric acid-based) in human cardiovascular tissues to determine whether these are predicted by values reported for human cloned receptors. Additionally, affinities (K(B)) for these antagonists, determined in ET-1-mediated vasoconstriction assays in human blood vessels, were used to identify discrepancies between K(B) and K(D) determined in the same tissues.

MAIN METHODS

Competition binding experiments were carried out in sections of human left ventricle, coronary artery and homogenates of saphenous vein to determine K(D) values for structurally different ET(A)-selective (FR139317, BMS 182874, S97-139, sitaxentan, ambrisentan) and mixed (PD142893, Ro462005, bosentan, L-749329, SB209670) antagonists. Schild-derived values of antagonist affinity were obtained in vascular functional studies.

KEY FINDINGS

When compared with previously reported data in human cloned endothelin receptors, those antagonists reported to be ET(A)-selective exhibited even greater ET(A) selectivity in human ventricle (BMS 182874, sitaxentan, ambrisentan) that expressed both receptor subtypes. Those antagonists reported to have <100 fold selectivity in cloned receptors (PD142893, Ro-462005, bosentan, SB209670, L-749329) did not distinguish between receptor subtypes in human left ventricle. For antagonists where we determined affinity in vascular functional and binding assays (Ro462005, bosentan, BMS 182874, L-749329, SB209670) there was no correlation between the degree of discrepancy in K(B) and K(D) and structural class.

SIGNIFICANCE

For an antagonist to retain ET(A)-selectivity in vivo it may be necessary to identify those compounds that have at least 1000 fold ET(A):ET(B) selectivity in in vitro assays.

Clinical trials with endothelin receptor antagonists: what went wrong and where can we improve?

In the early 1990s, within three years of cloning of endothelin receptors, orally active endothelin receptor antagonists (ERAs) were tested in humans and the first clinical trial of ERA therapy in humans was published in 1995. ERAs were subsequently tested in clinical trials involving heart failure, pulmonary arterial hypertension, resistant arterial hypertension, stroke/subarachnoid hemorrhage and various forms of cancer. The results of most of these trials - except those for pulmonary arterial hypertension and scleroderma-related digital ulcers - were either negative or neutral. Problems with study design, patient selection, drug toxicity, and drug dosing have been used to explain or excuse failures. Currently, a number of pharmaceutical companies who had developed ERAs as drug candidates have discontinued clinical trials or further drug development. Given the problems with using ERAs in clinical medicine, at the Twelfth International Conference on Endothelin in Cambridge, UK, a panel discussion was held by clinicians actively involved in clinical development of ERA therapy in renal disease, systemic and pulmonary arterial hypertension, heart failure, and cancer. This article provides summaries from the panel discussion as well as personal perspectives of the panelists on how to proceed with further clinical testing of ERAs and guidance for researchers and decision makers in clinical drug development on where future research efforts might best be focused.

Novel Benzo[1,4]diazepin-2-one Derivatives as Endothelin Receptor Antagonists

Since its discovery in 1988 by Yanagisawa et al., endothelin (ET), a potent vasoconstrictor, has been widely implicated in the pathophysiology of cardiovascular, cerebrovascular, and renal diseases. Many research groups have embarked on the discovery and development of ET receptor antagonists for the treatment of such diseases. While several compounds, e.g., ambrisentan 2, are in late clinical trials for various indications, one compound (bosentan, Tracleer) is being marketed to treat pulmonary arterial hypertension. Inspired by the structure of ambrisentan 2, we designed a novel class of ET receptor antagonists based on a 1,3,4,5-tetrahydro-1H-benzo[e][1,4]diazepin-2-one scaffold. Here, we report on the preparation as well as the in vitro and in vivo structure-activity relationships of these derivatives. Potent dual ET(A)/ET(B) receptor antagonists with affinities in the low nanomolar range have been identified. In addition, several compounds efficiently reduced arterial blood pressure after oral administration to Dahl salt sensitive rats. In this animal model, the efficacy of the benzo[e][1,4]diazepin-2-one derivative rac-39au was superior to that of racemic ambrisentan, rac-2.

The use of sulfonylamido pyrimidines incorporating an unsaturated side chain as endothelin receptor antagonists

A series of compounds structurally related to bosentan 1 featuring an unsaturated side chain at position 6 of the core pyrimidine have been studied for their potential to block the ET(A) and ET(B) receptor. Incorporation of a 2-butyne-1,4-diol linker bearing a pyridyl carbamoyl moiety led to in vitro highly potent endothelin receptor antagonists (e.g., 70 and 75). The propargyl derivative 26 significantly reduced blood pressure in in vivo model studies with hypertensive salt-sensitive Dahl rats.

Function of the endothelinB receptor in cardiovascular physiology and pathophysiology

One of the two receptors by which the potent vasoactive effects of endothelin (ET)-1 are mediated is the ET(B) receptor (ET(BR)), which is found in several tissues, but, more importantly from a cardiovascular point of view, on the endothelial cell. The endothelial cell also has the unique capability of releasing ET-1, as well as other factors, such as the endothelial-derived relaxing factors and prostacyclin, which counteract the myotropic effects of the peptide. The secretory and contractile responses to ET-1 rely on G-protein-coupled ET(BR)s, as well as ET(A)-G-protein-coupled receptor-like proteins. The mitogenic properties of ET-1 via ET(A) receptors (ET(AR)s) coupled to mitogen-activated protein kinases and tyrosine kinases on the vascular smooth muscle may occur in conjunction with the anti-apoptotic characteristics of the endothelial ET(BR)s. Interestingly, most of the relevant antagonists and agonists for both ET(AR)s and ET(BR)s have been developed by the pharmaceutical industry. This highlights the therapeutical potential of compounds that act on ET receptors. In normal as well as in physiopathological conditions, the ET(BR) plays an important role in the control of vascular tone, and must be taken into account when using ET receptor antagonists for the treatment of cardiovascular diseases. For the management of congestive heart failure, renal failure and primary pulmonary hypertension, the most recent literature supports the use of selective ET(AR) antagonists rather than mixed antagonists of ET(AR)s and ET(BR)s. Nonetheless, validation of this view will have to await the first clinical trials comparing the actions of ET(A) to mixed ET(A)/ET(B) receptor antagonists.

Inhibition of endothelin-converting enzyme activity in the rabbit basilar artery

OBJECTIVE

Endothelin (ET)-1 may be involved in the regulation of cerebrovascular resistance under pathological conditions, most notably during the development of vasospasm after subarachnoid hemorrhage. Blocking ET-converting enzyme activity may be a promising approach to the prevention of cerebral vasospasm after subarachnoid hemorrhage.

METHODS

In this study, the effects of several putative ET-converting enzyme inhibitors were investigated after intracisternal application in rabbits, to inhibit basilar artery contractions induced by big ET-1 (2 x 10(-6) mol/L).

RESULTS

In the group pretreated with [D-Val22]big ET-1[16-38] (2 x 10(-5) mol/L) (n = 8), the angiographically measured diameter of the basilar artery changed from 0.63 +/- 0.12 mm to 0.66 +/- 0.12 mm. In the control group (n = 8), the diameter of the basilar artery decreased from 0.71 +/- 0.13 mm to 0.57 +/- 0.15 mm. These results corresponded to an increase in vessel diameter of 5 +/- 10% in the treatment group and a decrease in vessel diameter of 20 +/- 16% in the control group (P = 0.014). In the group pretreated with captopril (2 x 10(-4) mol/L) (n = 8), the angiographically measured diameter of the basilar artery changed from 0.64 +/- 0.11 mm to 0.71 +/- 0.10 mm. These results corresponded to an increase in vessel diameter of 14 +/- 19% in the treatment group, compared with a decrease in vessel diameter of 20 +/- 16% in the control group (P = 0.014).

CONCLUSION

These results demonstrate that [D-Val22]big ET-1[16-38] and captopril act as highly potent ET-converting enzyme inhibitors, affecting big ET-1-induced contraction of the rabbit basilar artery.

Contribution of upregulated airway endothelin-1 expression to airway smooth muscle and epithelial cell DNA synthesis after repeated allergen exposure of sensitized Brown-Norway rats

Endothelin-1 is a potent bronchoconstrictor peptide with pro-inflammatory and growth-promoting properties. After exposure of sensitized Brown-Norway rats to six repeated ovalbumin exposures, there was an increase in pro-endothelin (ET)-1 messenger RNA compared with saline-exposed control rats 24 h after the final exposure (P < 0.01). ET-1 immunoreactivity was increased sixfold in the bronchial epithelium of the larger conducting airways in the repeated allergen-exposed rats (P < 0.001). After repeated allergen exposure, there were increased rates of DNA synthesis in the airway smooth muscle (ASM) cells (P < 0.001) and epithelial cells (P < 0. 001) compared with saline-exposed controls, as measured by bromodeoxyuridine incorporation. Treatment with a dual endothelin A and B (ET(A+B)) receptor antagonist caused a significant attenuation in both ASM (P < 0.001) and epithelial cell (P < 0.001) bromodeoxyuridine incorporation compared with the allergen-challenged and vehicle-treated group. The dual ET(A+B) antagonist attenuated eosinophil recruitment into the airways (P < 0. 05) but had no significant effect on increased bronchial reactivity to acetylcholine in allergen-exposed rats. Increased levels of ET-1 in the airways may contribute to inflammation and ASM and epithelial cell DNA synthesis after repeated allergen exposure. Such processes may underlie increased proliferation of resident cells leading to airway wall remodeling in asthmatics.

Endothelin and subarachnoid hemorrhage: an overview

INTRODUCTION

Delayed cerebral vasospasm occurring after subarachnoid hemorrhage (SAH) is still responsible for a considerable percentage of the morbidity and mortality in patients with aneurysms. It has been suggested that the pathogenesis of delayed cerebral vasospasm is related to a number of pathological processes, including endothelial damage and smooth muscle cell contraction resulting from spasmogenic substances generated during lysis of subarachnoid blood clots, changes in vascular responsiveness, and inflammatory or immunological reactions of the vascular wall. It has been recognized that the endothelium plays an important role in the regulation of the cerebral vascular tone. In 1988, endothelin (ET)-1, a potent vasoconstrictor, was isolated from cultured porcine aortic endothelial cells.

RESULTS

ET-1, which is one of three distinct isoforms of ETs (ET-1, ET-2, and ET-3), has a more marked effect on cerebral arteries than do the other two isoforms. Elevated levels of ETs have been demonstrated in the cerebrospinal fluid and plasma of patients after SAH and cerebral infarction. ETs act by at least three different receptor subtypes, the ET(A) receptor, which is localized in vascular smooth muscle cells and mediates vasoconstriction, and two different ET(B) receptor subtypes. The ET(B1) receptor subtype is present in vascular endothelial cells and mediates the endothelium-dependent vasodilation. The ET(B2) receptor subtype is present in smooth muscle cells causing vasoconstriction. ET-1 acts from the adventitial but not from the luminal side of cerebral arteries. In vivo and in vitro ET-1 causes a dose-dependent and long-lasting vasoconstriction, similar to cerebral vasospasm after SAH. The vasoconstriction caused by ET-1 can be reversed by selective ET(A) receptor antagonists or combined ET(A) and ET(B) receptor antagonists.

CONCLUSION

The results of current clinical and experimental investigations support the hypothesis that ET-1 is a major cause of cerebral vasospasm after SAH. Other studies indicate that SAH causes complex changes in the ET system and increased ET-1 levels after SAH, which are not solely responsible for the development of vasospasm but may occur after cerebral ischemia. Further investigations are therefore needed to clarify these different hypotheses.

A new family of orphan G protein-coupled receptors predominantly expressed in the brain

The cloning of a cDNA encoding a G protein-coupled receptor homologous to the endothelin type B receptor, but unable to bind endothelin, was recently reported and termed ET(B)R-LP. We report here the isolation of a human cDNA encoding a receptor that is highly related to ET(B)R-LP and which was therefore termed ET(B)R-LP-2. Comparison of the two amino acid sequences revealed 68% overall homology and 48% identity. As is the case for ET(B)R-LP, the new receptor is strongly expressed in the human central nervous system (e.g. in cerebellar Bergmann glia, cerebral cortex, internal capsule fibers). Membranes of HEK-293 cells stably expressing ET(B)R-LP-2 did not bind endothelin-1, endothelin-2, endothelin-3, bombesin, cholecystokinin-8 or gastrin-releasing peptide.

Additional hypotensive effect of endothelin-1 receptor antagonism in hypertensive dogs under angiotensin-converting enzyme inhibition

BACKGROUND

Endothelin-1 (ET-1) may play a role in hypertension. ET-1 receptor antagonism by bosentan lowers blood pressure in hypertension. We evaluated whether the effect of bosentan is still observed under ACE inhibitors (ACEI).

METHODS AND RESULTS

Thirty anesthetized and 18 conscious hypertensive dogs were studied randomly. Anesthetized dogs were divided into 4 groups: group 1 received cumulative doses of bosentan (bolus+30-minute infusion: 0.1 mg/kg+/-0.23 mg/kg per hour to 3 mg/kg+/-7 mg/kg per hour); group 2, the same dose-responses after 1 mg/kg enalaprilat; group 3, the vehicle after enalaprilat; and group 4, the dose responses to bosentan followed by enalaprilat. The conscious dogs were divided into 3 groups: group 5 received 2 cumulative doses of bosentan; group 6, the vehicle; and group 7, enalaprilat alone. In groups 1 and 2, bosentan produced dose-related decreases (P=.0001) in left ventricular systolic pressure and mean aortic pressure (AOP). In group 1, bosentan decreased mean AOP by 22%. In group 2, enalaprilat decreased mean AOP by 25% (from 173+/-26 to 130+/-25 mm Hg; P<.005); an additional 18% decrease was obtained with bosentan, the mean AOP reaching 98+/-21 mm Hg (P<.01). In group 3, the effect of enalaprilat alone was a 22% decrease in mean AOP (P<.005). The additive effect of the bosentan-ACEI association was also observed in group 4. In group 5, bosentan reduced mean AOP by 20% (P<.005), whereas mean AOP remained unchanged in group 6. The effect of ACEI alone (group 7) was similar to that of bosentan.

CONCLUSIONS

Bosentan produces an additional hypotensive effect to that of ACEI, which opens new therapeutic perspectives.

A new functional isoform of the human CRF2 receptor for corticotropin-releasing factor

We have identified the human counterpart of the corticotropin-releasing factor receptor subtype 2beta. Its functional response to human urocortin was demonstrated after stable expression in HEK-293 cells. The receptor was also shown to bind sauvagine, corticotropin-releasing factor and urocortin. In contrast to rodents, the human CRF(2beta) receptor is only weakly expressed in heart and skeletal tissues, where the CRF(2alpha) isoform is predominant. Moreover, we have identified additional mRNAs of the CRF(2beta) type which are probably a consequence of aberrant splicing events.

Absence of ET(B)-mediated contraction in Piebald-lethal mice

Activation of the endothelin (ET) ET(B) receptor can mediate opposite effects, endothelium-dependent vasodilation but also direct vasoconstriction. So far one gene encoding an ET(B) receptor has been identified and associated with endothelium-dependent relaxation. It has been suspected that the presence of another ET(B) gene could explain ET(B)-mediated contraction. The goal of the present study was to evaluate in Piebald-lethal (s[1]) mice, a naturally occurring mutant with deletion of the known ET(B) receptor gene, whether ET(B) receptor-mediated constriction is lost. Piebald-lethal (s[1]) mice, in contrast to control mice, completely lacked ET(B) specific ligand binding. The pressor effect of the ET(B) receptor selective agonist sarafotoxin S6c was completely absent. In vitro, contraction of stomach strips induced by sarafotoxin S6c was also abolished in Piebald-lethal (s[1]) mice. These results demonstrate the responsibility of the known ET(B) receptor gene in ET(B)-mediated constriction.

Endothelin and calcium antagonists in the skin microcirculation of patients with coronary artery disease

BACKGROUND

Endothelin, a potent endothelium-derived vasoconstrictor peptide, is elevated in coronary artery disease (CAD); however, its pathophysiological role is uncertain. Calcium antagonists are widely used in patients with CAD. Using laser Doppler flowmetry, we investigated the influence of two endothelin antagonists and the calcium antagonist diltiazem on endogenous and exogenous endothelin in the skin microcirculation of CAD patients and healthy control subjects.

METHODS AND RESULTS

Both endothelin antagonists and diltiazem applied intradermally induced vasodilation in CAD patients, which was more pronounced with the ETA/ETB antagonist than with the ETA antagonist or diltiazem. Exogenous endothelin led to profound vasoconstriction in CAD patients and healthy volunteers. Both endothelin antagonists and diltiazem blunted the vasoconstriction to exogenous endothelin in CAD patients and young healthy volunteers and less so in old healthy volunteers. However, compared with both endothelin antagonists, a 10-times-higher dose of diltiazem was required. Systemic diltiazem (240 mg, slow release) attenuated endothelin-induced vasoconstriction in CAD patients. Neurogenic vasodilation to exogenous endothelin was inhibited by both endothelin antagonists.

CONCLUSIONS

This study demonstrates that endogenous endothelin of CAD patients contributes to the regulation of vascular tone in the skin microcirculation not only through ETA receptors but also possibly through ETB receptors. Diltiazem inhibited endothelin-induced vasoconstriction, but endothelin antagonists were slightly more effective. Thus, endothelin antagonists represent potent new tools to interfere with the vascular effects of endothelin in CAD patients. Future studies must confirm these findings in other areas of the circulation.

Characterization of [125I]-endothelin-1 and [125I]-BQ3020 binding to rat cerebellar endothelin receptors

1. We performed radioligand binding experiments on rat cerebellar homogenates using [125I]-endothelin-1 ¿[1251]-ET-1¿ and [125I]-BQ3020 to examine the pharmacology of endothelin receptors in rat brain. Saturation experiments demonstrated a single population of binding sites with high affinity for both radioligands ([125I]-ET-1, pKd = 8.94 +/- 0.17; [125I]-BQ3020, pKd = 9.18 +/- 0.14 nM; mean +/- s.e.mean). However, [125I]-BQ3020 only recognised approximately one third the number of endothelin receptors measured with [125I]-ET-1. 2. Saturation binding experiments with [125I]-PD151242 revealed high affinity binding to a single population of ETA receptors in the cerebellar homogenates (pKd = 9.95 +/- 0.14; Bmax = 30 +/- 15 fmol mg-1 protein). 3. Competition experiments were performed with ligands that are either non-selective for endothelin receptor subtypes. The rat cerebellar endothelin receptor displayed a high affinity for endothelin-1 (ET-1), endothelin-3 (ET-3) and sarafotoxin-S6c (STX-6c) although the affinity for ET-3 was slightly higher than the affinity for ET-1 using both radioligands. The selective ETA antagonists, BQ123, BMS-182,874 and JKC-301 all displayed low affinities at the endothelin receptors. In contrast the selective ETB agonists, IRL1620 and [Ala1,3,11,15]ET-1 and the selective ETB antagonist, BQ-788 had moderate affinities at the endothelin receptor, in the low nanomolar range. The ETB agonist, BQ3020, had approximately 10 fold higher affinity than IRL1620 and [Ala1,3,11,15]ET-1 at the rat cerebellar endothelin receptors. The non-selective antagonists, Ro-46,2005, Ro-47,0203 and PD-142,893 displayed moderate affinities at the cerebellar receptor. 4. Since [125I]-BQ3020 recognises only a fraction of the [125I]-ET-1 binding sites, the majority of the endothelin receptors in the cerebellum cannot be classed as ETB. Although [125I]-PD151242 was able to detect ETA receptors in the rat cerebellar homogenates, the small population of ETA receptors (2% of the total endothelin population as measured with [125I]-ET-1) could not account for the non-ETB receptor population. We conclude that the rat brain cerebellar receptor has a profile similar to the ETB1 receptor as it has a high affinity for ET-1, ET-3, STX-6c and was moderately sensitive to PD-142,893. However, as the ETB ligands BQ-788, IRL1620 and [Ala1,3,11,15]ET-1 have only a moderate affinity for the rat cerebellar endothelin receptor and since ET-3 has a higher affinity as compared to ET-1, our findings suggest that the rat cerebellum contains predominately ETc receptors.

In vitro characterisation of Ro 46-8443, the first non-peptide antagonist selective for the endothelin ETB receptor

We describe here Ro 46-8443, the first non-peptide endothelin ETB receptor selective antagonist. It displays up to 2000-fold selectivity for ETB receptors both in terms of binding inhibitory potency and functional inhibition. The observed parallel rightward shift of concentration-response curves with different antagonist concentrations is consistent with a competitive binding mode. Since R0 46-8443 selectively inhibits ETB receptor mediated responses, it is a valuable tool for clarifying the role of ETB receptors in pathology.

Prevention of subarachnoid hemorrhage-induced cerebral vasospasm by oral administration of endothelin receptor antagonists

The purpose of this study was to investigate the effectiveness of oral treatment with the endothelin (ET) A/B receptor antagonist Ro 47-0203, 4-tert-butyl-N-[6-(hydroxy- ethoxy)-5-(2-methoxy-phenoxy)-2'-bipyrimidin-4-yl]-benzenesulfonam ide (bosentan), and the ET A receptor antagonist 2-benzo[1,3]dioxol-5-yl-3-benzyl-4-(4-methoxy- phenyl)-4-oxo-but-2-enoic acid monosodium salt (PD155080), in the prevention of subarachnoid hemorrhage (SAH)-induced delayed cerebral vasospasm. Double hemorrhage in the rabbit constricted the basilar artery to 34% if control as determined by angiography. Oral bosentan and PD155080 administration after the initial SAH decreased the magnitude of constriction to 9% and 16% of control, respectively. Plasma and cerebrospinal fluid bosentan levels and plasma PD155808 levels were consistent with concentrations reported to inhibit ET-1 constriction of blood vessels in vitro. These results support the use of oral administration of ET A/B and ET A receptor antagonists as potential specific treatment for vasospasm resulting from SAH in humans.

Role of endothelin in mediating neurogenic plasma extravasation in rat dura mater

In addition to their potent vasoconstrictor properties, the endothelins (endothelin-1 and -3) may possess neurotransmitter/neuromediator and neuroendocrine actions. The aim of the present study was to evaluate the role of endothelins (ET) in mediating neurogenic inflammation of cephalic tissues in the rat. For this purpose, bosentan, a specific non-peptide mixed antagonist of ET receptors, was tested in rat models of neurogenic and non-neurogenic plasma extravasation in the dura mater and extracranial tissues (eyelid, conjunctiva, lip, tongue). Bosentan was effective for preventing neurogenic inflammation in the dura mater induced by unilateral electrical stimulation of the trigeminal ganglion or intravenous injection of capsaicin, whereas it was ineffective in extracranial tissues or after injection of substance P (non-neurogenic inflammation). The effect of nerve fiber stimulation on ET plasma concentrations in superior sagittal sinus was measured using selective radioimmunoassays for ET-1 and -3. Endothelin-3 concentration significantly increased after intravenous injection of capsaicin, whereas ET-1 levels remained unchanged. Competition binding assays on microsomal membranes from the trigeminal ganglion revealed a single class of binding sites with equal affinity for ET-1 and ET-3, suggesting a homogenous population of ETB receptors. The role of ETB receptors in mediating inflammation was evidenced by the lack of efficacy of a selective ETA receptor antagonist, in contrast to the full efficacy of a selective ETB receptor antagonist, for preventing neurogenic inflammation induced by unilateral stimulation of the trigeminal ganglion. The role of ETB receptors was finally confirmed by the observation that exogenous administration of the ETB receptor agonist sarafotoxin S6c also induced plasma protein extravasation in the dura mater. This extravasation was not a direct effect of ETB receptor stimulation, because it was inhibited by spantide, a selective tachykinin receptor antagonist. These data strongly suggest that ET, acting through ETB receptors, may play an important role in mediating neurogenic inflammation in the meninges of rats. Since the profile of activity of bosentan is similar to that of the 5-HT1D/B agonists, sumatriptan and ergot alkaloids, one may speculate that ET receptor antagonists might be potentially effective in the treatment of acute migraine attacks.

Up-regulation of endothelin-B receptors in atherosclerotic human coronary arteries

Both endothelin-A (ETA) and endothelin-B (ETB) receptors are known to be present in human coronary arteries. However, their absolute and relative amounts, functional roles, and the influence of pathology are uncertain. The goal of the present study was to characterize endothelin receptors mediating constriction in human coronary arteries and to assess the influence of cardiomyopathy (CMP) and coronary artery disease (CAD) on ET receptors in human tissue. For comparison, porcine coronary arteries were evaluated in parallel. Competition binding experiments using [125I]ET-1 and different selective and nonselective ETA- and ETB-receptor agonists or antagonists revealed similar relative densities (relative Bmax) of ETA and ETB receptors in coronary arteries from human cardiomyopathic hearts (83% ETA and 17% ETB; n = 5) and porcine hearts (78% ETA and 22% ETB; n = 5). In marked contrast, the relative Bmax of ETB receptors were significantly higher in coronary arteries from human atherosclerotic hearts (51% ETA and 49% ETB; n = 3). Total receptor density (Bmax; fmol/mg protein) was highest in porcine (385 +/- 29) arteries, followed by human CAD (253 +/- 41) and CMP (174 +/- 20) coronary arteries. The relative and absolute Bmax values for ETA and ETB receptors in coronary arteries from a donor heart were similar to those obtained in CMP hearts. There were no significant differences in affinity constants (KD) values for ET-1, ET-3, Sarafotoxin S6c (SRTX S6c), BQ-123, and bosentan (Ro 47-0203) between tissues. In human coronary arteries from CMP hearts, ET-induced constriction seemed to be solely mediated via ETA receptors. In contrast, in porcine coronary arteries 20% of the maximal effect mediated by ET-1 could be attributed to ETB receptors, in agreement with the binding data. The functional role of ETB receptors in CAD tissue could not be evaluated because of the occurrence of spontaneous phasic contractions. We conclude that ETB receptors are up-regulated in human atherosclerotic coronary arteries. Further studies are needed to determine the pathophysiological importance of these receptors.

Prevention of cerebral vasospasm after experimental subarachnoid hemorrhage by RO 47-0203, a newly developed orally active endothelin receptor antagonist

Since their discovery in 1988, endothelins have attracted scientific interest because of their extremely potent and long-lasting vasoconstrictive effects, similar to cerebral vasospasm in humans. In this study, the efficacy of the orally active endothelin receptor antagonist RO 47-0203 for prevention of cerebral vasospasm after experimental subarachnoid hemorrhage, using the two-hemorrhage dog model, was investigated. Twenty-eight beagle dogs were used in this laboratory experiment. Fourteen animals each were assigned to the treatment and control groups. In the treatment group, each dog received two single doses of 30 mg/kg RO 47-0203 orally per day. The diameter of the basilar artery decreased from 1.36 +/- 0.17 mm on Day 1 to 1.19 +/- 0.23 mm on Day 8 in the treatment group, whereas in the control group, the vessel diameter decreased from 1.48 +/- 0.19 mm on Day 1 to 1.02 +/- 0.22 mm on Day 8. These results corresponded to a decrease of vessel diameter of 13.1% +/- 11.2% in the treatment group and a decrease of vessel diameter of 30.7% +/- 12.4% in the control group (P < 0.001). Concentrations of endothelin-1 in cerebrospinal fluid significantly increased with time after subarachnoid hemorrhage. These results emphasize the important role of endothelin in the development of cerebral vasospasm and present first-time evidence that prevention of cerebral vasospasm can be achieved by the endothelin receptor antagonist RO 47-0203.

Bosentan antagonizes the effects of endothelin-1 on rat gastric blood flow and mucosal integrity

Bosentan, a new type of orally effective, mixed (ETA+ETB) endothelin receptor antagonist has been recently introduced and tested in a variety of experimental models. We studied the effect of bosentan on the changes in gastric mucosal hemodynamics and mucosal integrity, induced by the exogenous application of endothelin-1, in rats. Bosentan (10 mg/kg iv) pretreated rats were injected with endothelin-1 (500-1000-2000 pmol/kg, iv) and gastric mucosal hemodynamics were monitored. After combined oral (30 mg/kg) and systemic pretreatment with bosentan we studied the effects of submucosal injection of endothelin-1 (50 pmol) on blood flow and gastric mucosa. Bosentan antagonized the vasodilator, vasoconstrictor and ulcerogenic effects of endothelin-1 in the rat gastric mucosa. These results show that bosentan can be a useful probe in the study of endogenous endothelin in the gastrointestinal tract.

Endothelin receptor antagonism

Following the original report by Yanagisawa et al. (1988) more than 7 years ago, compelling evidence that ET plays an important role in the local regulation of smooth muscle tone and cell growth has been reported. In addition, many studies point to a significant role for endothelin in nonvascular function. The investigation of the endothelin system has been greatly advanced in the last 2 to 3 years through significant advances in the development of potent and selective ET receptor antagonists. These agents have proven to be essential tools for elucidating the biological significance of the ET system, leading to the realization that antagonism of the ET system may have significant therapeutic potential. As emphasized in this review, the importance of chronic blockade of the ET system may be a critical aspect of future research in this exciting area. Confounding issues remain the lack of information about the role of the ETB receptor, the apparent pharmacological evidence for additional ET receptor subtypes, and species variation in the tissue distribution of ET isoforms and receptor subtypes. Along with the greater ability to understand the endothelin system provided by potent and selective pharmacological agents, is the important contribution of modern molecular biology techniques, highlighted by the insights gained from recent reports of results from ET gene disruption studies. Kurihara et al. (1994) found that ET-1-deficient homozygous mice die at birth of apparent respiratory failure secondary to severe craniofacial abnormalities. Subsequently, Yanagisawa's laboratory has presented and published a series of complementary gene disruption studies. First, Hosoda et al. (1994) demonstrated remarkably, that ETA receptor knockout mice bear morphological abnormalities nearly identical to ET-1 knockout mice. Second, they found that disruption of the ET-3 peptide and ETB receptor genes result in homozygous mice that share identical phenotypic traits (i.e., coloration changes and aganglionic megacolon) which are similar to a previously known natural mutation, the Piebald-Lethal mouse (Hosoda et al., 1994; Baynash et al., 1994). This phenotype has a human corollary known as Hirschsprung's Disease and it is now known that the disease, though multigenic, results from a missense mutation of the ETB receptor gene in some individuals (Puffenberger et al., 1994). Taken together these data indicate that the endothelin system is essential to correct embryonic neural crest development, a completely novel finding within the superfamily of guanine-protein-linked receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Endothelin and endothelin antagonists: pharmacology and clinical implications

Endothelins (ET) are a family of peptides with potent biological properties. Endothelial cells produce exclusively ET-1 while other tissues produce ET-2 and ET-3. The production of ET requires an increase in intracellular Ca2+. This increase can be induced by physical chemicals (i.e. hypoxia) or receptor-operated stimuli (i.e. thrombin, angiotensin II, arginine vasopressin, transforming growth factor beta 1, interleukin-1). Most of ET is released abluminally towards vascular smooth muscle and less luminally. The main vascular effect of ET are vasodilation (transient), profound and sustained vasoconstriction as well as proliferation of vascular smooth muscle. These biological effects are mediated by distinct receptors. Three ET receptors have been cloned, i.e. ETA-, ETB- and ETC-receptors. In vascular tissue ETA-receptors are expressed on vascular smooth muscle and responsible for vasoconstriction. ETB-receptors are expressed on endothelium and linked to nitric oxide and/or prostacyclin release. Activation of these receptors explains the transient vasodilation with intraluminal application of ET. Vascular smooth muscle cells can express ETB-receptors which contribute to ET-induced vasoconstriction particularly at lower concentrations. The role of the recently cloned ETC-receptor in the vasculature is still uncertain. ET production is increased (as judged from circulating plasma levels) in vascular disease and atherosclerosis in particular, in myocardial infarction and heart failure, pulmonary hypertension and renal disease. ET production is increased in arterial hypertension remains controversial. Non-peptidic ET antagonists have been developed which either block ETA- receptors or ETA- and ETB-receptors simultaneously. The advantage of ETA-receptors is that they leave the endothelium-dependent vasodilation to ET (via ETB-receptor) intact. However, ETB-mediated contraction remains unaffected by these antagonists. In contrast ETA-/ETB-antagonists fully prevent ET-induced vasoconstriction, however, they also inhibit the endothelial effects of the peptide. ET antagonists interfere with the effects of ET in isolated vascular tissue (including that obtained from humans) as well as in vivo. In humans, ETA as well as ETA-/ETB-antagonists inhibit endothelin-induced vasoconstriction. Hence in summary ET are a family of potent peptides with profound effects in the vasculature. Several studies suggest a role of ET in cardiovascular disease. The newly developed ET-antagonists are potent and selective tools to delineate the (patho-)physiological roles of ET and may become a new class of cardiovascular drugs.

The current endothelin receptor classification: time for reconsideration?

The possible involvement of endothelins in a variety of diseases has attracted the attention of many pharmacologists in search of a novel therapeutic approach. The rapid development of endothelin research has resulted in the molecular characterization and pharmacological recognition of ETA and ETB receptors, and in the development of compounds selective for these receptors. However, the characterization of receptors in various assays has shown that a number of effects are mediated by receptors that do not fit the present criteria for ETA or ETB receptors. In this article, Willem Bax and Pramod Saxena address endothelin receptors in general, and atypical receptors in particular.

Effect of a nonselective endothelin antagonist on vascular remodeling in deoxycorticosterone acetate-salt hypertensive rats. Evidence for a role of endothelin in vascular hypertrophy

We have previously shown that the endothelin content in arteries of deoxycorticosterone acetate (DOCA)-salt hypertensive rats is increased. We designed this study to examine, using the new orally active nonselective endothelin receptor antagonist bosentan, whether this increase in vascular endothelin may contribute to elevated blood pressure and vascular hypertrophy in DOCA-salt hypertensive rats. Rats received bosentan (100 mg/kg body wt per day) for 3 weeks mixed with their food. Systolic blood pressure of DOCA-salt hypertensive rats rose to 197 +/- 5 mm Hg, and that of bosentan-treated DOCA-salt hypertensive rats was 177 +/- 4 mm Hg (P < .01). Mesenteric resistance arteries were studied on a wire myograph. The media width, ratio of media width to lumen diameter, and cross-sectional area of the media of resistance arteries of bosentan-treated DOCA-salt hypertensive rats were significantly smaller than those of untreated DOCA-salt hypertensive rats. The lumen diameter and cross-sectional area of the media of vessels of bosentan-treated rats were not different from those of uninephrectomized control rats. Vasoconstrictor responses, which were altered in DOCA-salt hypertensive rats, approached control in the bosentan-treated rats. We conclude that these results with a nonselective endothelin receptor antagonist may suggest a role for endothelin in the elevation of blood pressure and vascular hypertrophy and remodeling in DOCA-salt hypertensive rats.

Characterisation of the endothelin receptor mediating contraction of human pulmonary artery using BQ123 and Ro 46-2005

We have characterised the endothelin receptor mediating contraction of human isolated pulmonary artery. Endothelin-1 induced a concentration-dependent contraction of human endothelium-denuded pulmonary artery (EC50 5.6 nM). In contrast, endothelin-3 produced only a small contraction (approximately 12% of maximum endothelin-1 response) at the highest concentration tested (1 microM). The ETB receptor-selective agonist, sarafotoxin S6c (0.1 nM to 1 microM) did not cause contraction of human pulmonary artery. Pretreatment of human pulmonary artery with BQ123 (1-10 microM), an ETA receptor-selective blocking drug, resulted in a concentration-dependent, surmountable antagonism of endothelin-1-induced contractions (apparent pKB 6.6-7.0). Schild analyses yielded a shallow slope (0.58), which was significantly less than unity and, consequently, the calculated pA2 (8.1) was greater than the individual pKB values. Pretreatment of human pulmonary artery with Ro 46-2005 (30 microM), a non-peptide. non-selective endothelin receptor-blocking drug, resulted in a surmountable antagonism of endothelin-1-induced contractions (apparent pKB 5.5). In conclusion, endothelin-1-induced contraction of human pulmonary artery appears to be mediated predominantly via ETA receptors, although the shallow Schild slope observed with BQ123 indicates possible receptor heterogeneity.

Characterization of three non-peptide endothelin receptor ligands using human cloned ETA and ETB receptors

1. A number of putative endothelin (ET) receptor ligands were synthesized with a view to assessing their relative affinity for human recombinant ET receptors. 2. Human (h) and endothelin ETA and ETB receptor open reading frames were cloned by reverse transcription-polymerase chain reaction into the mammalian expression vector pcDNA1 and stable cell lines were created by transfection of Chinese hamster ovary cells. 3. Scatchard analyses of saturation isotherms for the specific binding of [125I]-endothelin-1 ([125I]-ET-1) to membranes, prepared from Chinese hamster ovary cells transfected with hETA or hETB receptors, yielded values for equilibrium dissociation constants (Kd) of 20.5 +/- 1.8 pM and 25.5 +/- 5.5 pM, respectively. Hill coefficients did not differ significantly from unity, suggesting binding to homogeneous, non-interacting receptor populations. 4. Pharmacological characterization of the transfected hETA and hETB receptors was undertaken by measuring the relative abilities of ETA and ETB receptor-selective peptide ligands to inhibit binding of [125I]ET-1. For interaction with hETA receptors, the relative order of potency was ET-1 > ET-3 = FR139317 = BQ123 >[Ala1,3,11,15]-ET-1 = sarafotoxin S6c (S6c). In contrast, the relative order of potency, at hETB receptors, was ET-1 = ET-3 = [Ala1,3,11,15]-ET-1 = S6c >> FR139317 = BQ123. 5. The novel non-peptide ligands, Ro 46-2005, SB 209670 and BMS 182874, were found to inhibit [125I]-ET-1 binding to human recombinant ETA and ETB receptors. At hETA receptors, the calculated pIC50 values were 6.7 (Ro 46-2005), 8.7 (SB 209670) and 5.8 (BMS 182874), while at hETB receptors, the corresponding pIC50 values were 6.8, 7.5 and <5, respectively.6. In conclusion, we have characterized the pharmacology of human cloned ETA and ETB receptors and used these in membrane binding assays to determine the affinity and selectivity of three structurally diverse non-peptide ET receptor ligands. SB 209670 is, to date, the highest affinity non-peptide ligand to be described for ET receptors. As such, it may prove to be a valuable tool in further examination of the physiological and pathophysiological roles of endothelins.

Magnifying endoscopic observation of the gastric mucosa, particularly in patients with atrophic gastritis

The gastric mucosal surface was observed using the magnifying fibergastroscope (FGS-ML), and the fine gastric mucosal patterns, which were even smaller than one unit of gastric area, were examined at a magnification of about 30. For simplicification, we classified these patterns by magnifying endoscopy in the following ways; FP, FIP, FSP, SP and MP, modifying Yoshii's classification under the dissecting microscope. The FIP, which was found to have round and long elliptical gastric pits, is a new addition to our endoscopic classification. The relationship between the FIP and the intermediate zone was evaluated by superficial and histological studies of surgical and biopsy specimens. The width of the band of FIP seems to be related to the severity of atrophic gastritis. Also, the transformation of FP to FIP was assessed by comparing specimens taken from the resected and residual parts of the stomach, respectively. Moreover, it appears that severe gastritis occurs in the gastric mucosa which shows a FIP. Therefore, we consider that the FIP indicates the position of the atrophic border.