SB 209670, a rationally designed potent nonpeptide endothelin receptor antagonist

Triflic Acid-Catalyzed Synthesis of Indole-Substituted Indane Derivatives via In Situ Formed Acetal-Facilitated Nucleophilic Addition and 4π-Electron-5-Carbon Electrocyclization Sequence

An efficient protocol for the synthesis of indole-substituted indanes from o-alkenylbenzaldehydes under acetalization conditions has been presented. The cyclization occurs via a nucleophilic addition of indole on the oxacarbenium ion generated from acetal formed under the reaction condition followed by a conrotatory 4π-electrocyclization reaction, which takes care of the exclusive diastereoselectivity observed during the cyclization step. Olefin geometry of o-alkenylbenzaldehyde and the amount of indole play a decisive role in the success of this cyclization process.

Potassium-Zincate-Catalyzed Benzylic C-H Bond Addition of Diarylmethanes to Styrenes

Direct functionalization of the benzylic C-H bond of diarylmethanes is an important strategy for the synthesis of diarylmethine-containing compounds. However, the methods developed to date for this purpose require a stoichiometric amount (usually more) of either a strong base or an oxidant. Reported here is the first catalytic benzylic C-H bond addition of diarylmethanes to styrenes and conjugated dienes. A potassium zincate complex, generated from potassium benzyl and zinc amide, acts as a catalyst and displays good activity and chemoselectivity. Considering the atom economy of the reaction and the ready availability of the catalyst, this reaction constitutes a practical, efficient method for diarylalkane synthesis.

Organocatalytic synthesis of chiral tetrasubstituted allenes from racemic propargylic alcohols

Although chiral allene preparation via formal S_N2’ nucleophilic substitutions of enantioenriched propargylic derivatives or metal-catalyzed reactions of racemic propargylic derivatives has attracted considerable attention and found applications in many areas of research, direct use of propargylic alcohols instead of propargylic derivatives for catalytic asymmetric allene synthesis is unknown. Here, we show that a highly enantioselective synthesis of tetrasubstituted allenes from racemic propargylic alcohols has been realized by organocatalysis with good efficiency (up to 96% yield and 97% ee). The intermolecular C–C and C–S bond formation was achieved efficiently with simultaneous stereocontrol over the axial chirality. Furthermore, an adjacent quaternary stereocenter could also be constructed. Mechanistically, the reaction may involve efficient stereocontrol on the propargylic cation by its chiral counter anion or 1,8-conjugate addition of para-quinone methides. In sharp contrast to previous central chirality construction, this process employs quinone methides for axial chirality construction.

Axially chiral allenes that are normally present in natural products, bioactive molecules, organocatalysts, and functional materials are usually produced from propargylic derivatives. Here, the authors show direct use of propargylic alcohols for catalytic asymmetric allene synthesis.

Development and Analysis of a Pd(0)-Catalyzed Enantioselective 1,1-Diarylation of Acrylates Enabled by Chiral Anion Phase Transfer

Enantioselective 1,1-diarylation of terminal alkenes enabled by the combination of Pd catalysis with a chiral anion phase transfer (CAPT) strategy is reported herein. The reaction of substituted benzyl acrylates with aryldiazonium salts and arylboronic acids gave the corresponding 3,3-diarylpropanoates in moderate to good yields with high enantioselectivies (up to 98:2 er). Substituents on the benzyl acrylate and CAPT catalyst significantly affect the enantioselectivity, and multidimensional parametrization identified correlations suggesting structural origins for the high stereocontrol.

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.

Dual blockade of endothelin action exacerbates up-regulated VEGF angiogenic signaling in the heart of lipopolysaccharide-induced endotoxemic rat model

AIMS

Sepsis is a cluster of heterogeneous syndromes associated with progressive endotoxemic developments, ultimately leading to damage of multiple organs, including the heart. However, the pathogenesis of sepsis-induced myocardial dysfunction is still not fully understood. The present study is the first to examine alterations in expression of key angiogenic signaling system mediated by vascular endothelial growth factor (VEGF) in septic heart and the effects of endothelin dual blocker (ETDB) on it.

MAIN METHODS

Normal Wistar rats were either administered with: a) vehicle only (control group), b) lipopolysaccharide only (LPS: 15 mg/kg) and then sacrificed at different time points (1 h, 3 h, 6 h and 10 h), and c) the last group was co-administered with LPS and ETDB (SB-209670, 1 mg/kg body weight) for 6 h and then sacrificed.

KEY FINDINGS

Administration of LPS resulted in increases in levels of: a) serum tumor necrosis factor (TNF)-α, b) serum VEGF and c) serum endothelin (ET)-1 levels accompanied by up-regulation of cardiac VEGF and its downstream angiogenic signaling molecules. While cardiac TNF-α level was unchanged among experimental groups, cardiac ET-1 level was significantly higher in LPS-administered group.

SIGNIFICANCE

We conclude that elevation in VEGF angiogenic signaling may be triggered by diminished oxygenation in the myocardium following LPS administration as a consequence of sepsis-induced microvascular dysfunction. Because of this cardiac dysfunction, oxygen supply may be inadequate at microregional level to support the normal heart metabolism and function. ETDB at 6 h further increased the elevated levels of VEGF angiogenic signaling in endotoxemic heart.

The role of endothelin in the cerebrovascular response following intracerebral haemorrhage: experimental studies using the endothelin antagonist SB209670

Primary intracerebral haemorrhage (ICH) is associated with considerable morbidity and mortality. Local endothelin release following ICH may contribute to the pathophysiology of perilesional ischaemia. In diabetics, endothelin release can be enhanced by hyperglycaemia and cerebrovascular dilation may be inhibited by vascular endothelial dysfunction. To examine the effects of endothelin-mediated vasoconstriction after spontaneous ICH in the normal and diabetic brain, regional cerebral blood flow (rCBF) was examined in insulin dependent BB-rats and non-diabetic BB control rats. These experiments were performed 24 h following experimental ICH in both groups of animals that were either given the endothelin antagonist SB209670 or saline. Perilesional oligaemia was similar in control and SB209670 treated diabetic rats, but SB209670 reduced perilesional oligaemia in normal rats. In brain contralateral to the experimental ICH, rCBF was increased by SB209670 in diabetic rats, but not in non-diabetic rats. These studies show that there are differences in the cerebrovascular effects of endothelin in perilesional and contralateral brain in non-diabetic and diabetic rats following ICH.

Cardiopulmonary indications for endothelin receptor antagonists: review of recent efficacy trials

Recent clinical and experimental animal trials indicate that endogenously produced endothelin-1 (ET-1) contributes to the abnormal systemic and pulmonary vascular resistance associated with congestive heart failure (CHF) and pulmonary hypertension (PH). In experimental CHF, the chronic blockade of ET-1 actions by ET receptor antagonists clearly improves haemodynamic status, and improves cardiac structure and survival. The latter is based on limited results. In experimental PH there are consistent reports of prevention and reversal of PH, pulmonary vascular remodelling and right ventricular hypertrophy, independent of the inciting mechanisms. These results in experimental animals illustrate the potential efficacy of the ET receptor antagonists in future clinical trials. With five ET receptor antagonists in clinical development, and more on the way, their potential will soon be realised.

Endothelin-mediated induction of heme oxygenase-1 in the spinal cord is attenuated in transgenic mice overexpressing superoxide dismutase

Spinal cord blood flow and the induction of heme oxygenase-1 (HO-1), an indicator of oxidative stress, were studied in the spinal cords of adult wild-type and transgenic mice overexpressing the antioxidant copper, zinc superoxide dismutase (CuZn SOD) after intrathecal administration of the potent vasoactive peptide endothelin-1 (ET-1). Gelfoam, saturated with ET-1 (40, 80, or 400 micromol/L), was positioned in the intrathecal space at the midthoracic level in anesthetized animals. Blood flow was continuously monitored by laser Doppler for 10 min after the intrathecal application of ET-1. There was a significant reduction in spinal cord blood flow to approximately 40% of control values by 10 min after the intrathecal application of the peptide in both wild-type and transgenic mice. Moreover, SB209670, a nonselective endothelin receptor antagonist, blocked this reduction in flow. Each animal was euthanized 24 h after the intrathecal administration of ET-1, and the spinal cord was prepared for quantitative immunocytochemistry. HO-1 was primarily induced in astrocytes near the dorsal surface of the spinal cord in wild-type mice. This induction was attenuated in both wild-type, treated with SB209670, and untreated transgenic mice. Together, these findings suggest that ET-1 mediates oxidative stress in the spinal cord through the modulation of spinal cord blood flow.

The distinct alterations produced in cardiovascular functions by prednisolone and nitro-prednisolone (NCX-1015) in the rat highlight a causal role for endothelin-1

Daily administration of prednisolone, but not the derivative NCX-1015 (or prednisolone 21-[4'-nitrooxymethyl]benzoate), to rats resulted in a time- and dose-dependent increase in mean arterial blood pressure (MABP), significant after 1 week for the dose of 6.9 micromol/kg i.p. (n = 10; P < 0.05), and 3 weeks for the lower dose of 1.38 micromol/kg. A similar dichotomy of behavior was observed with respect to myocardial contractility and renal vascular resistance, in either case augmented by 3-week treatment with prednisolone but not NCX-1015. In contrast, both NCX-1015 and prednisolone reduced plasma levels of corticosterone in a dose- (dose range of 0.69-6.9 micromol/kg i.p.) and time-dependent (1-3 weeks) manner. Similar profiles were obtained for plasma nitrate values, although they were increased selectively after NCX-1015 administration. In contrast, prednisolone, but not NCX-1015, augmented plasma endothelin 1 (ET-1) with a profile that mirrored the changes observed in MABP and renal blood flow. Supply in the drinking water of the ET-1 receptor type A (ETA) antagonist FR139317 [(R-2-[(R)-2-[(S)-2-[[1-(hexahydro-1H-azepinyl)]-carbonyl]amino-4-methylpentanoyl]-amino-3-(2-pyridil)propionic] or mixed ETA/B, but not of selective ETB, antagonists prevented the changes produced by a 21-day treatment with prednisolone. In conclusion, this study indicates 1) a lack of occurrence of cardiovascular alterations by nitro-releasing derivative of prednisolone (NCX-1015), and 2) a functional link between prednisolone effects and the endogenous endothelin-1 system.

Endothelins and myocardial fibrosis

BACKGROUND

Endothelins are associated with cardiac remodeling. These peptides are the most powerful vasoconstrictor described-whether this remodeling is a direct effect of this hormone or indirect response to a relative ischemia promoted by vasoconstrictor effect. We evaluated the role of endothelin upon myocardial fibrosis despite of its hemodynamic effects and the benefits of its antagonism.

METHODS AND RESULTS

We used 40 Wistar rats: control, sham operated, rats had undergone myocardial infarction (MI) and MI rats treated with SB209670 which is an ET(A)/ET(B) endothelin antagonist. We evaluated tail systolic blood pressure (BP) and left ventricular end diastolic pressure (LVED) before surgery, just after, and at the end of the study. Remodeling was studied based on interstitial collagen and MI size by an image system analysis. BP decreased in MI groups after surgery, but did not differ between treated and untreated animals. LVED had increased levels in MI groups after surgery and did not differ between them. However, ICVF had an increase in MI group but significantly less in MI+SB209670. MI size was similar in both groups.

CONCLUSIONS

Endothelin may have a pivotal role in the myocardial fibrosis by direct stimulation of collagen accumulation despite of its hemodynamic effects.

Cortical expression of endothelin receptor subtypes A and B following middle cerebral artery occlusion in rats

This work aimed to define the spatial expression of endothelin A (ET(A)) and B (ET(B)) receptors in the cerebral cortex after permanent middle cerebral artery occlusion (MCAO) and to identify the phenotype of cells expressing ET(A) and ET(B) receptors. Cortical expression of ET(A) and ET(B) receptors was determined at the mRNA level by semi-quantitative reverse transcription-polymerase chain reaction and at the protein level by immunofluorescence staining, 12, 24 and 72 h after MCAO. Cells expressing endothelin receptors were phenotyped by double labelling with antibodies, anti-protein gene product (PGP9.5) and anti-ED1, towards neurons and activated microglia/macrophages, respectively. Both ET(A) and ET(B) receptor mRNA expressions increased significantly in the ipsilateral cortex in a time-dependent manner after MCAO. Robust expression of ET(A) receptors was noted in most neurons of the ischemic core and in several neurons in laminae 3 and 4 of the peri-infarct region 24 and 72 h after MCAO. ET(B) receptor immunoreactivity was observed in activated microglia/macrophages, beginning 24 h after MCAO. These results provide the first evidence that the action of endothelin during ischemia may be mediated by neuronal ET(A) receptors and activated microglia/macrophage ET(B) receptors. This differential localization of ET(A) and ET(B) receptors suggests that endothelin is involved in some complex neuron-glial interactions in addition to its vascular modulatory activity during ischemia.

Attenuation of hemolysate-induced cerebrovascular endothelial cell injury and of production of endothelin-1 and big endothelin-1 by an endothelin-converting enzyme inhibitor

BACKGROUND

Endothelin-1 (ET-1) is a potent and long-acting vasoconstrictive peptide that has been implicated in the pathogenesis of cerebral vasospasm after subarachnoid hemorrhage (SAH). ET-1 has been shown to be present in the cerebrospinal fluid of patients after SAH, and substances produced during hemolysis of subarachnoid blood clots are believed to be responsible for stimulating the production of ET-1. The biosynthesis of ET-1 is a multi-step process, involving the conversion of the relatively inactive precursor big ET-1 to the mature peptide by endothelin converting enzyme (ECE), a metalloprotease. Consequently, ECE inhibitors are expected to suppress the biosynthesis of ET-1 and reduce the pathologic impact resulting from overproduction of this peptide. The purpose of the present study was to investigate the effects of an ECE inhibitor, CGS 26303, on hemolysate-induced injury of cerebral vessel endothelial cells as well as the production of ET-1 from these cells.

METHODS

Different doses of CGS 26303 and hemolysate were added to the culture medium for 48 hours. Cell injury was assessed by cell morphology and density, while the productions of ET-1 and big ET-1 were determined by radioimmunoassays.

RESULTS

Hemolysate alone increased the levels of ET-1 and big ET-1 in culture medium and caused substantial cell loss. Treatment with CGS 26303 inhibited the hemolysate-induced increases in the levels of ET-1 and big ET-1 and reduced endothelial cell injury. The protective effects of CGS 26303 were modest when this inhibitor was added simultaneously with hemolysate, but were prominent and dose-dependent when the inhibitor was given 30 minutes before the addition of hemolysate.

CONCLUSION

These results suggest that overproduction of ET-1 contributes significantly to hemolysate-induced damage to cerebrovascular endothelial cells.

Altered mitogen-activated protein kinase activation in vascular smooth muscle cells from spontaneously hypertensive rats

1. We previously reported that activation function of mitogen-activated protein kinases (MAPK) is enhanced in aorta strips from both prehypertensive and hypertensive spontaneously hypertensive rats (SHR) and that this enhancement of MAPK activation results from enhanced MAPK activation reactivity to angiotensin (Ang) II in SHR aorta strips. 2. The purpose of the present study was to examine whether the enhanced function of the vascular angiotensin system observed in SHR aorta strips results from genetic alterations of vascular smooth muscle cells from SHR. 3. Basal MAPK activity was within normal limits in cells from 4-week-old SHR, whereas enzyme activity was enhanced in 9-week-old SHR compared with age-matched Wistar-Kyoto (WKY) rats. 4. Mitogen-activated protein kinase activation reactivity to AngII and endothelin-1 was enhanced in 9-week-old SHR cells but not in 4-week-old SHR cells. The enhancement of basal MAPK activity in 9-week-old SHR cells was abolished by a combination of the angiotensin AT(1) receptor antagonist losartan and the endothelin receptor antagonist BQ123. 5. These findings suggest that MAPK activation function in 4-week-old SHR cells is not enhanced. Thus, it appears that factors outside vascular smooth muscle cells are needed for the enhanced MAPK activation observed in 4-week-old SHR aorta strips. In 9-week-old SHR, MAPK activation function is enhanced in cells themselves and this function may, at least in part, contribute to the enhanced MAPK activation observed in SHR aorta strips.

Structure-activity relationships of a novel class of endothelin-A receptor antagonists and discovery of potent and selective receptor antagonist, 2-(benzo[1,3]dioxol-5-yl)-6-isopropyloxy-4-(4-methoxyphenyl)-2H-chromene-3-carboxylic acid (S-1255). 1. Study on structure-activity relationships and basic structure crucial for ET(A) antagonism

A novel series of endothelin-A (ET(A)) selective receptor antagonists having a 2H-chromene skeleton are described. A lead compound, 2-(benzo[1,3]dioxol-5-yl)-2H-chromene-3-carboxylic acid (3), was found by modifications of our own angiotensin II antagonist. A structure-activity relationship (SAR) study of 3 reveals that the structural requirements essential for potent and selective ET(A) receptor binding affinity are the m,p-methylenedioxyphenyl, carboxyl, and isopropoxy groups at the 2-, 3-, and 6-positions, respectively, on the (R)-2H-chromene skeleton. The substituent at the 4-position is also important for improving the activity, and various hydrophobic functional groups of 6-9 A such as liner, branched, and cyclic aliphatic groups, unsubstituted and substituted aryl groups, and even halogen atoms were acceptable. These results suggest that (R)-2-(benzo[1,3]dioxol-5-yl)-6-isopropoxy-2H-chromene-3-carboxylic acid, formula 108, is the crucial basic structure to be recognized by the ET(A) receptor. The most potent compound is (R)-48 (S-1255), which binds to the ET(A) receptor with an IC(50) value of 0.19 nM and is 630-fold selective for the ET(A) receptor than for the ET(B) receptor. This compound has 55% oral bioavailability in rats. On the basis of the SAR, the roles of each substituent in the receptor binding are discussed.

Dipeptide sulfonamides as endothelin ETA/ETB receptor antagonists

Endothelin-1 (ET-1) is a potent mitogen and modulator of vascular tone. It is synthesized and released from endothelial cells and acts upon two receptor subtypes designated as ETA and ETB. In this study, a series of potent dipeptide sulfonamide dual-endothelin ETA/ETB receptor antagonists were prepared to investigate their potential benefit in vascular diseases. CGS 31398 inhibited [125I]ET-1 binding to human ETA and ETB receptors expressed in Chinese hamster ovary (CHO) cells (ETA/CHO, ETB/CHO) with respective IC50 values of 0.26 and 0.12 nM. However, in anesthetized rats, this compound markedly potentiated ET-1-induced renal vascular resistance, a response normally observed with selective ETB receptor antagonists. To determine whether species differences account for these results, a direct comparison was made between binding to rat and rabbit aortic membranes versus functional antagonism in isolated rat aortic rings. It was found that CGS 31398 had potent affinity for the ETA receptor in rat and rabbit aorta with IC50 values of 0.87 and 0.79 nM, respectively. Inhibition of ET-1-induced contractions of rat aorta by the compound was considerably weaker than expected (pKB = 6.4), while that of sarafotoxin S6c induced contraction of dog saphenous vein (100% inhibition at 100 nM) was consistent with corresponding binding data. These results suggest that although CGS 31398 is a potent dual inhibitor of ETA/ETB receptor binding, it surprisingly displays potent ETB and weak ETA receptor antagonism in functional assays.

Endothelin and heart failure

The availability of potent and orally active nonpeptide endothelin (ET) receptor antagonists has generated a host of information on the pathophysiological role of ET-1 in a number of preclinical models including hypertension, renal failure, heart failure and pulmonary hypertension. Convincing data are available to show that ET-1 receptor antagonists are beneficial in humans as far as reversal of deranged systemic and regional hemodynamics associated with CHF and pulmonary hypertension. As in other disease areas, the issue of whether ET(A)-selective or ET(A/B) antagonists are more suited for CHF treatment remains unresolved. ET(B) receptors may mediate some critical processes in the kidney such as sodium and water excretion in addition to releasing vasodilator substances such as NO and prostacyclin from endothelial cells. In heart failure and chronic renal diseases, preservation of ET(B)-mediated responses in the kidney and pulmonary endothelium might be beneficial. On the other hand, blockade of ET(B)-mediated vasoconstriction, smooth muscle cell proliferation and fibrosis by ET(B) antagonists might be beneficial. In clinical trials so far, the hemodynamic effects of mixed antagonists of ET receptors and ET(A) selective antagonists seem equivalent.

Basic and therapeutic relevance of endothelin-mediated regulation

Three endothelin family peptides (endothelin-1, -2 and -3) exert an extremely potent and long-lasting vasoconstrictor action as well as other various actions through stimulating two subtypes of receptor (ETA and ETB). Vascular endothelial cells produce only endothelin-1. Although the pharmacological actions of exogenous endothelin-1 have been extensively analyzed, the physiological roles of endogenous endothelin-1 have long been obscure. Using potent and selective receptor antagonists, endothelin-1 has been demonstrated to contribute slightly to the maintenance of regional vascular tone. In gene-targeted mice, endothelin family peptides and their receptors have been shown to play an important role in the embryonic development of neural crest-derived tissues. In addition to its potent vasoconstrictor action, endothelin-1 has direct mitogenic actions on cardiovascular tissues, as well as co-mitogenic actions with a wide variety of growth factors and vasoactive substances. Endothelin-1 also promotes the synthesis and secretion of various substances including extracellular constituents. These effects of endogenous endothelin-1 would appear to be naturally concerned with the development and/or aggravation of chronic cardiovascular diseases, e.g. hypertension, pulmonary hypertension, vascular remodeling (restenosis, atherosclerosis), renal failure, and heart failure. A great many non-peptide and orally active endothelin receptor antagonists have been developed, and shown to exert excellent therapeutic effects in animal models as well as human patients with these diseases.

Pharmacologic characterization of the novel, orally available endothelin-A--selective antagonist SB 247083

Competition radioligand binding with [125I]ET-1 at human cloned ETA and ETB receptors demonstrated ET-A selective affinity by SB 247083 (Ki 0.41 and 467 nM, respectively). Accordingly, similar competitive, functional ETA receptor antagonism was observed. In vitro, SB 247083 exhibited a Kb of 3.5 +/- 0.3 nM (ET-1--induced rat aortic contraction). SB 247083 was significantly less potent as a functional ETB antagonist (Kb 0.34 +/- 0.01 microM; S6c-induced rabbit pulmonary artery contraction). In contrast to ETB-selective and mixed ETA/B antagonists, and consistent with its ETA-selective profile, in vivo administration of SB 247083 was not associated with an elevation in plasma ET-1 levels. Pharmacodynamic and pharmacokinetic studies revealed that SB 247083 was effectively absorbed from the gastrointestinal tract. A single bolus dose inhibited the hemodynamic actions of ET-1 for up to 8 h, consistent with a molecule shown to be 46% bioavailable. Therefore, the present study demonstrates that SB 247083, a unique chemical entity, represents a potent class of nonpeptide, orally active ETA-selective antagonists.

Design, syntheses, and structure-activity relationships of indan derivatives as endothelin antagonists; new lead generation of non-peptidic antagonist from peptidic leads

A new lead generation of non-peptidic ET(A) antagonists from two peptidic ET(A)-selective ones, BQ-123 and FR139317, was performed. Using computer assisted molecular modeling, a putative pharmacophore was constructed from the superposition of the reported three-dimensional structure of the cyclic peptide BQ-123 and a presumable beta-turn active conformation of the linear peptide FR139317 formed by an intramolecular hydrogen bond. According to this model, a new series of indan derivatives were designed and synthesized. Among these, 5-isobutyrylamino-6-(1-naphthylmethyloxy)-3-(2-thienyl)-1-indancarboxylic acid (1b) showed a moderate ET(A) antagonistic activity (IC50 = 28 microM).

Cardiovascular effects of endothelin-1 and endothelin antagonists in conscious, hypertensive ((mRen-2)27) rats

SB 209670 is a potent antagonist of the vasoconstrictor (ET(A)- and ET(B)-receptor-mediated) and vasodilator (ET(B)-receptor-mediated) effects of endothelin, whereas SB 234551 is relatively selective for the constrictor (ET(A)-receptor-mediated) effects. Since we had previously found SB 209670 exerted antihypertensive, vasodilator effects in conscious, heterozygous, transgenic ((mRen-2)27) (abbreviated to TG) rats, here we compared the two antagonists in that model, and assessed their chronic effects on responses to exogenous endothelin-1. We did this to test our global hypothesis, namely, that SB 209670, but not SB 234551, would cause inhibition of the depressor effects of exogenous endothelin-1 in vivo, and that this differential effect would be associated with a more marked antihypertensive action of SB 234551 in TG rats. SB 209670 and SB 234551 (infused for 50 h) exerted similar, sustained, antihypertensive effects in TG rats. The antihypertensive effects of the antagonists occurred at times when the pressor effects of exogenous endothelin-1 were not significantly inhibited. Furthermore, SB 234551 did not exert a greater antihypertensive effect than SB 209670 at a time (i.e., 2 - 4 h) when the depressor effects of endothelin-1 were abolished by the latter, but not by the former (although this differential action was lost after 24 h infusion). The results caused us to reject the hypothesis that selective antagonism of the vasoconstrictor effects of endothelin-1 would result in SB 234551 exerting a greater antihypertensive effect than SB 209670 in TG rats.

Endothelin: what does the radiologist need to know?

Endothelin (ET) is a potent endogenous vasoconstrictor peptide. It has been implicated in various pathological states since its discovery in 1988. The cardiovascular system and the kidneys are important sites for the action of this peptide. Two types of ET receptor, ETA and ETB, govern the biological effects of ET. Drugs that can prevent the endogenous synthesis of ET or block its binding to receptors may offer important therapeutic impact to patients with congestive heart failure, pulmonary hypertension and acute renal failure. Areas of particular interest to the radiologist include the role of ET in mediating some of the side effects of contrast media, particularly contrast medium nephropathy, and the involvement of ET in the pathogenesis of restenosis following angioplasty. This review outlines the basic biology of this important mediator and its role in health and disease.

Heterozygous knock-Out of ET(B) receptors induces BQ-123-sensitive hypertension in the mouse

Homozygous knock-out of ET(A) or ET(B) receptor genes results in lethal developmental phenotypes in the mouse. Such deleterious phenotypes do not occur in heterozygous littermates. However, it remains to be determined whether mice partially defective in ET(A) or ET(B) receptors display significant alterations in their responses to exogenous or endogenous endothelin-1 (ET-1). Furthermore, the anesthetized ET(B) (+/-) knock-out mice showed a significantly higher mean arterial blood pressure than the ET(A) (+/-) knock-out or their wild-type littermates. The pressor response to ET-1 but not to a selective ET(B) agonist, IRL-1620, was significantly reduced in the ET(A) (+/-) knock-out mice. In ET(B) (+/-) knock-out mice, the pressor effect of IRL-1620 was more markedly altered than those induced by ET-1. In wild-type mice, both ET(A) and ET(B) receptors were found to be involved in the pressor effect of ET-1, as confirmed by the significant and specific antagonism induced by either BQ-123 (ET(A) antagonist) or BQ-788 (ET(B) antagonist). Also, ET(A)-selective or mixed ET(A)/ET(B)- but not ET(B)-selective antagonists reversed the hypertensive state of the ET(B) (+/-) knock-out mice to the level of wild-type littermates. Finally, radiolabeled ET-1 plasmatic clearance was altered in ET(B) (+/-) but not ET(A) (+/-) knock-out mice when compared with wild-type animals. Thus, heterozygous knock-out of ET(B) receptors results in a hypertensive state, suggesting an important physiological role for that particular receptorial entity in opposing the endogenous ET-1-dependent pressor effects in the mouse.

Hypoxic modulation of striatal lesions induced by administration of endothelin-1

Levels of endothelin-1 (ET-1), a potent endogenous vasoconstrictor, are elevated in plasma and cerebrospinal fluid (CSF) following cerebral ischemia and reperfusion injury. The present study sought insight into the potential differential vasoactive effects on the cerebral vasculature and resultant neural damage of ET-1 during normoxic vs. ischemic conditions and upon reperfusion. Under normoxic conditions, intrastriatal stereotaxic injection of exogenous ET-1 (40 pmol) induced a significant (P<0.05) reduction (</=29+/-12%) in the regional (striatal) cerebral blood flow measured by Laser Doppler flowmetry (CBF(LDF)) for up to 40 min in halothane-anesthetized male Long-Evans rats. Intrastriatal injection of ET-1 10 min after the onset of hypoxia (12% O(2), balance N(2)) tended to blunt, but not significantly, the striatal CBF(LDF) responses to the 35 min period of hypoxia. ET-1 given during reoxygenation significantly (P<0.05) reduced striatal CBF(LDF), which was similar to the effect of ET-1 during normoxia. ET-1-induced infarction when administered prior to hypoxia, but not during or post-hypoxia, was significantly (P<0.05) exacerbated compared to infarction of ET-1 without hypoxia. These results suggest that exogenous ET-1 administered into the brain parenchyma can induce an infarction associated with modulation of CBF(LDF) during the normoxic or reoxygenation period, but not during the hypoxic period and that the increased release of ET-1 in any pathological phase of cerebral ischemia contributes to irreversible neural damage with associated hemodynamic disturbances.

Drug discovery in the next millennium

Selection and validation of novel molecular targets have become of paramount importance in light of the plethora of new potential therapeutic drug targets that have emerged from human gene sequencing. In response to this revolution within the pharmaceutical industry, the development of high-throughput methods in both biology and chemistry has been necessitated. This review addresses these technological advances as well as several new areas that have been created by necessity to deal with this new paradigm, such as bioinformatics, cheminformatics, and functional genomics. With many of these key components of future drug discovery now in place, it is possible to map out a critical path for this process that will be used into the new millennium.

Endothelin receptor subtype distribution predisposes coronary arteries to damage

Several vasoactive drugs that lower blood pressure and increase heart rate induce regional cardiotoxicity in the dog, most frequently of right coronary arteries and right atrium. The basis for this selective damage is thought to result from local changes in vascular tone and blood flow. Administration of an endothelin receptor antagonist (ETRA, SB 209670) to dogs induced damage most frequent and severe in the right coronary artery and right atrium. Because site predisposition may correlate with distribution of vasoactive receptors, the objectives of this study were to map endothelin (ET) receptor distribution and density within regions of dog heart using both gene (mRNA) and protein expression endpoints for dog ET(A) and ET(B) receptors, and, additionally, correlate ET receptor subtype density with regional cardiac blood flow. A 10- to 15-mmHg reduction in mean arterial pressure with a concomitant increase in heart rate (10-20%), a six- and twofold increase in regional blood flow to the right and left atrium, respectively, and acute hemorrhage, medial necrosis, and inflammation were observed in the right coronary arteries and arteries of the right atrium after ETRA infusion for 5 days. Radioligand protein binding to quantify both ET receptors in normal dog heart indicated a twofold greater density of ET receptors in atrial regions versus ventricular regions. Importantly, ET receptor density in coronary arteries was markedly (about five- to sixfold) increased above that in atrial or ventricular tissues. ET receptor subtype characterization indicated ET(B) receptors were three times more prevalent in right coronary arteries compared to left coronary arteries and in situ hybridization confirmed localization of ET(B) in vascular smooth muscle. ET(A) receptor density was comparable in right and left coronary arteries. Quantitative real-time polymerase chain reaction for ET(A) and ET(B) receptor mRNA transcripts supported the site prevalence for message distribution. Consequently, the composite of protein and message expression profiles for ET(A) and ET(B) receptors indicated a disproportionate distribution of ET(B) receptors within right coronary artery of dog and this, along with functional measures of blood flow after ETRA infusion indicated a predisposition for exaggerated pharmacological responses and subsequent damage to right coronary arteries by ET and/or ETRAs.

Conditions permitting suppression of stretch-induced and vasoconstrictor tone by basal nitric oxide activity in porcine cerebral artery

This study examined the ability of basal nitric oxide activity to suppress intrinsic and vasoconstrictor tone in isolated rings of porcine cerebral artery. Following stretch of approximately 1 g, N(G)-nitro-L-arginine methyl ester (L-NAME, 100 microM) produced a rise in tone in endothelium-containing but not endothelium-denuded rings. Thus, intrinsic tone was present and was powerfully suppressed by basal nitric oxide activity. Nevertheless, when concentration-response curves were constructed to U46619 and 5-hydroxytryptamine (5-HT), no endothelium-dependent depression of vasoconstriction was observed. It therefore appeared that basal nitric oxide activity was able to suppress intrinsic but not vasoconstrictor tone in these vessels. Stretch-tension curves generated following the application of stretch over the range 0 - 5. 5 g on endothelium-denuded rings showed that tension was stretch-induced. Experiments conducted in the presence of L-NAME (100 microM) revealed that the level of tone present in endothelium-containing rings was substantially higher than in endothelium-denuded rings across the entire range of stretch. When endothelium-containing and endothelium-denuded rings were set at similar levels of stretch-induced tone, rather than similar levels of stretch, the presence of the endothelium now depressed significantly vasoconstrictor responses to U46619 and 5-HT. Thus, when endothelium-containing and endothelium-denuded rings of porcine cerebral artery are set at similar points along their respective stretch-tension curves, rather than at similar levels of stretch, basal nitric oxide activity can be seen to inhibit both stretch-induced and vasoconstrictor tone.

Administration of selective endothelin receptor type A antagonist Ro 61-1790 does not improve outcome in focal cerebral ischemia in cat

The authors examined the effect of selective endothelin (ET) receptor type A (ET(A)) antagonism on histological and functional recovery in cat at 24 hours after reversible middle cerebral artery occlusion (MCAO). A novel and specific ET(A) antagonist, Ro 61-1790 [5-methylpyridine-2-sulfonic acid-6-(2-hydroxyethoxy)-5-(2-methoxyphenoxy)-2-(2-1H-tetrazol-5-y l-pyridin-4-yl)-pyrimidin-4-ylamide sodium salt (1:2)] (Roche, Basel, Switzerland), was used at doses that produced steady-state plasma concentrations and abolished ET-induced pial arteriolar vasoconstriction. In a cranial window preparation, 8 nmol/L ET constricted pial arterioles by 33 +/- 18% (mean +/- SD), but this response was ablated by intravenous Ro 61-1790 treatment (10-mg/kg bolus, 4-mg/kg/h infusion). In additional animal cohorts, halothane-anesthetized cats were treated with 90 minutes of MCAO and 24 hours of reperfusion. Animals received Ro 61-1790 infusion beginning at the onset of reperfusion and continuing for 6 or 24 hours (n = 41). Control cats were treated with 0.9% saline by intravenous infusion throughout reperfusion. There was no difference in injury volume or neurologic evaluation score in saline-treated cats (n = 11; caudate 24 +/- 28%, cortical injury 7.5 +/- 5% of ipsilateral structure; score 52 +/- 8) versus the results in cats treated with Ro 61-1790 for either 24 hours (n = 6; caudate 22 +/- 23%, cortex 6 +/- 5%, injury volume of ipsilateral structure; score 55 +/- 3) or 6 hours (n = 11; caudate 33 +/- 30%, cortex 12 +/- 14%, injury volume of ipsilateral structure; score 50 +/- 10). Mortality was greatest in the 24-hour drug treatment group. These data suggest that blockade of ET(A) receptor activity is not beneficial to tissue or functional outcomes from experimental stroke in cat.

Mixed endothelin receptor antagonist, SB209670, decreases portal pressure in biliary cirrhotic rats in vivo by reducing portal venous system resistance

BACKGROUND/AIMS

This study aimed to evaluate the hemodynamic effects of endothelin-1 or mixed endothelin receptor antagonist, SB209670 in cirrhotic rats, and to elucidate the role of endothelin in cirrhotic portal hypertension.

METHODS

Secondary biliary cirrhosis was induced by bile duct ligation. Hemodynamics were studied using the radioactive microsphere technique.

RESULTS

Plasma and hepatic endothelin levels in cirrhotic rats were significantly higher than those in normal rats (plasma, 9.0+/-1.3 vs. 2.6+/-0.5 pg/ml, p<0.001; liver, 74.8+/-13.3 vs. 12.6+/-2.5 pg/g wet tissue, p<0.001). Intraportal administration of endothelin-1 (3 nmol/kg) progressively raised portal pressure without an initial transient reduction, which was observed in systemic arterial pressure, in both cirrhotic and normal rats. SB209670 (5.4 micromol/kg) reduced portal pressure in cirrhotic rats (-19+/-5%, p<0.01) without modifying systemic arterial pressure and renal blood flow, but not in normal rats. This reduction was associated with reduced portal venous system resistance (vehicle, 2.5+/-0.2 vs. SB209670, 1.7+/-0.1 mmHg x min x 100 g bw/ml, p<0.01), but not with change in portal venous inflow and collateral blood flow.

CONCLUSIONS

Mixed endothelin antagonist, SB209670, decreased portal pressure by reducing portal venous system resistance without modifying systemic arterial pressure and renal blood flow in cirrhotic rats. This result, together with the findings that plasma and hepatic endothelin levels were elevated in cirrhotic rats and that exogenous endothelin-1 increased portal pressure, provides further support for a role of endothelin in portal hypertension and suggests a potential use of mixed endothelin antagonist in the pharmacological treatment of portal hypertension.

Differential haemodynamic effects of endothelin receptor antagonist, SB 209670, in conscious hypertensive and normotensive rats

Endothelin has been implicated in the pathogenesis and/or maintenance of hypertension. Endothelin receptor antagonists lower blood pressure in the spontaneously hypertensive rat (SHR), but the regional haemodynamic effects of such drugs in the SHR remain unknown. The aim of this study was to examine the regional haemodynamic effects of the endothelin receptor antagonist, (+/-)-(1S,2R,3S)-3-(2-carboxymethoxy-4-methoxyphenyl)-1-(3, 4-methylenedioxyphenyl)-5-(prop-1-yloxy)-indane-2-carboxylic acid (SB 209670), in SHR and Wistar-Kyoto (WKY) rats. Rats underwent a two-stage operation for implantation of Doppler flow probes and intravascular catheters. Recordings were made of mean arterial pressure (MAP), heart rate (HR) and renal (Ren), mesenteric (Mes) and hindquarters (HQ) blood flows and conductances (Cond). SHR and WKY received 4.5 h infusions of saline or SB 209670 (5 mg/kg priming dose+1 or 5 mg/kg/h, i.v.). SB 209670 lowered blood pressure in both SHR (-23+/-2 mm Hg) and WKY rats (-13+/-1 mm Hg). In addition, a lower dose infusion of SB 209670 also had an antihypertensive effect in SHR (-15+/-5 mm Hg). In SHRs which received the higher dose of antagonist, Ren, Mes and HQ Cond were significantly increased as was the HQ Cond in a low-dose group. In WKY rats, SB 209670 decreased Ren blood flow whilst increasing Mes and HQ blood flows and Cond. SB 209670 also attenuated the regional vasoconstrictor effects of endothelin-1, except in the Mes circulation in SHR. This study illustrates that SB 209670 causes differential haemodynamic effects in SHR and WKY rats. In SHR, the antihypertensive effect of SB 209670 was accompanied by a generalised vasodilatation in the Ren, Mes and HQ vascular beds. In WKY rats, the hypotensive effect of SB 209670 was accompanied by Mes and HQ vasodilatation, but with Ren vasoconstriction. Thus, endothelin is involved in the maintenance of blood pressure and vascular tone in both SHR and WKY rats, but the haemodynamic profiles of these effects differ between the two strains.

Endothelin mediates renal vasodilation and hyperfiltration during pregnancy in chronically instrumented conscious rats

Profound vasodilation of the kidneys and other nonreproductive organs transpires during early pregnancy. Because nitric oxide (NO) was found to mediate renal vasodilation and hyperfiltration in conscious pregnant rats, and endogenous endothelin (ET) was suggested to be vasodilatory in the renal circulation of nonpregnant rats, we tested whether endothelin mediates the NO-dependent changes in the renal circulation during pregnancy. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured in conscious pregnant and virgin rats before and during infusion of 30 micrograms/min RES-701-1 (a selective ETB receptor subtype antagonist). Baseline GFR and ERPF were significantly increased by 35% in gravid rats relative to virgin controls. During infusion of RES-701-1, the pregnant rats responded more robustly, showing a greater decline in both GFR and ERPF such that renal function converged in the two groups of rats. ERPF also converged in pregnant and virgin rats during infusion of SB-209760, a nonselective ETA/B receptor subtype antagonist. Combined infusion of Nomega-nitro-L-arginine methyl ester [L-NAME, an NO synthase (NOS) inhibitor] and RES-701-1 reduced GFR and ERPF to levels comparable to those reached with either agent given alone, suggesting inhibition of a common vasodilatory pathway. RES-701-1 and SB-209670 significantly lowered the cGMP content of small renal arteries from gravid and virgin rats in vitro, strengthening the link between the renal endothelial ETB receptor subtype and NO. Importantly, we showed that RES-701-1 is not a direct inhibitor of NOS. We conclude that endothelin mediates the NO-dependent changes in the renal circulation of conscious rats during pregnancy.

Heart failure and endothelin receptor antagonists

Cardiac myocytes and vascular endothelial cells produce endothelin-1, which increases the contractility of cardiac muscles and of vascular smooth muscles. Endothelin-1 also exerts long-term effects, such as myocardial hypertrophy, and causes cellular injury in cardiac myocytes. In heart failure, the production of endothelin-1 is markedly increased in the failing heart. Here, evidence that an endothelin receptor antagonist is a useful new drug for the treatment of heart failure is discussed. Long-term treatment with an endothelin receptor antagonist greatly improves the survival rate of animals (rat, hamster, etc.) with chronic heart failure. This beneficial effect is accompanied by amelioration of left ventricular dysfunction. The myocardial endothelin system appears to be a novel and important target for therapeutic intervention in heart failure.

Pathophysiology of endothelin in the cardiovascular system

In this article, we review the basic pharmacological and biochemical features of endothelin and the pathophysiological roles of endothelin in cardiovascular diseases. Development of receptor antagonists has accelerated the pace of investigations into the pathophysiological roles of endogenous endothelin-1 in various diseases, e.g. chronic heart failure, renal diseases, hypertension, cerebral vasospasm, and pulmonary hypertension. In chronic heart failure, the expression of endothelin-1 and its receptors in cardiomyocytes is increased, and treatment with an endothelin receptor antagonist improves survival and cardiac function. Endothelin receptor antagonists also improve other cardiovascular diseases. These results suggest that the interference with endothelin pathway either by receptor blockade or by inhibition of endothelin converting enzyme may provide novel therapeutic drugs strategies for multiple disease states.

Effects of hypertension, diabetes mellitus, and hypercholesterolemia on endothelin type B receptor-mediated nitric oxide release from rat kidney

BACKGROUND

Although endothelin-1 is a potent vasoconstrictor peptide, stimulation of endothelin type B receptor (ETBR) causes bidirectional changes in vascular tone, ie, vasodilation and vasoconstriction. Roles of ETBR in pathological conditions are largely unknown.

METHODS AND RESULTS

We studied the effect of BQ-3020, a highly selective ETBR agonist, on renal vascular resistance and nitric oxide (NO) release in the isolated, perfused kidney of rats with hypertension, diabetes mellitus, and hypercholesterolemia. Immunohistochemistry of endothelial NO synthase and ETBR was also examined. Infusion of BQ-3020 at concentrations of </=10(-10) mol/L reduced renal perfusion pressure in Dahl salt-resistant (R) rats but increased renal perfusion pressure in Dahl salt-sensitive (S) rats (10(-10) mol/L: -10.3+/-0. 6% versus 11.2+/-1.5%, R versus S; P<0.01). BQ-3020 caused a dose-dependent release of NO in both R and S rats, although the level of NO release in S rats was lower, as detected by chemiluminescence (10(-10) mol/L: 10.7+/-0.7 versus 3.1+/-0.4 fmol/min per gram of kidney, R versus S; P<0.01). Similar effects of BQ-3020 were observed in streptozotocin-induced diabetic rats and diet-induced hypercholesterolemic rats. Expression of endothelial NO synthase decreased in S rats but not in diabetic or hypercholesterolemic rats. In contrast, expression of ETBR in the endothelium was decreased in all 3 disease models compared with that in the vascular smooth muscle cell.

CONCLUSIONS

These results suggest that impaired NO release in response to stimulation of ETBR is due, at least in part, to a decrease in endothelial ETBR and may play a role in vascular dysfunction usually associated with arteriosclerosis-related diseases.

Pharmacology of A-216546: a highly selective antagonist for endothelin ET(A) receptor

Endothelins, 21-amino acid peptides involved in the pathogenesis of various diseases, bind to endothelin ET(A) and ET(B) receptors to initiate their effects. Here, we characterize the pharmacology of A-216546 ([2S-(2,2-dimethylpentyl)-4S-(7-methoxy-1,3-benzodioxol-5-yl )-1-(N,N-di(n-butyl) aminocarbonylmethyl)-pyrrolidine-3R-carboxylic acid), a potent antagonist with > 25,000-fold selectivity for the endothelin ET(A) receptor. A-216546 inhibited [125I]endothelin-1 binding to cloned human endothelin ET(A) and ET(B) receptors competitively with Ki of 0.46 and 13,000 nM, and blocked endothelin-1-induced arachidonic acid release and phosphatidylinositol hydrolysis with IC50 of 0.59 and 3 nM, respectively. In isolated vessels, A-216546 inhibited endothelin ET(A) receptor-mediated endothelin-1-induced vasoconstriction, and endothelin ET(B) receptor-mediated sarafotoxin 6c-induced vasoconstriction with pA2 of 8.29 and 4.57, respectively. A-216546 was orally available in rat, dog and monkey. In vivo, A-216546 dose-dependently blocked endothelin-1-induced pressor response in conscious rats. Maximal inhibition remained constant for at least 8 h after dosing. In conclusion, A-216546 is a potent, highly endothelin ET(A) receptor-selective and orally available antagonist, and will be useful for treating endothelin-1-mediated diseases.

Endothelin and its antagonists in hypertension: can we foresee the future?

Endothelin-receptor antagonists may soon become a new therapeutic class of agents used to treat cardiovascular diseases. Although the first clinical trials are anxiously awaited to position this new family of compounds in the treatment of essential or secondary forms of hypertension, we dispose of an impressive amount of studies in which plasma endothelin levels have been measured, in addition to chronic preclinical studies that provide a general picture of what we can expect from these drugs. The experimental models that do respond to endothelin- receptor antagonists share vascular overexpression of endothelin, which seems directly linked with vascular hypertrophy of resistance arteries. In addition, salt sensitivity may represent an unbalanced condition between relaxing and constrictive properties of the renal endothelium that can respond favorably to endothelin blockade. Thus, African-American hypertensives may well be a responsive target population for the new drugs. In addition to blood pressure control, endothelin may also be involved in the evolution of end-organ damage by its potent vasoactive and vasoproliferative properties. The kidney, heart, large arteries and brain may therefore benefit from these drugs, but it is still unclear if this benefit goes beyond what can be attributed to the reduction of arterial pressure. Moreover, most studies performed so far have looked at prevention of end-organ damage, while very few have addressed the clinically relevant question of regression of lesions already installed by the disease process.

Trigeminal nerve ganglion stimulation-induced neurovascular reflexes in the anaesthetized cat: role of endothelin(B) receptors in carotid vasodilatation

1. The effects of intravenous administration of endothelin (ET) receptor antagonists SB-209670 (0.001-10.0 mg kg(-1)), SB-217242, SB-234551 (0.01-10.0 mg kg(-1)) and BQ-788 (0.001-1.0 mg kg(-1)) were investigated on trigeminal nerve ganglion stimulation-induced neurovascular reflexes in the carotid vasculature of the anaesthetized cat. Comparisons were made with sumatriptan (0.003-3.0 mg kg(-1)) and alpha-CGRP8-37 (0.001-0.1 mg kg(-1)). 2. Trigeminal nerve ganglion stimulation produced frequency related increases in carotid blood flow, reductions in carotid vascular resistance and non-frequency related increases in blood pressure. Guanethidine (3 mg kg(-1), i.v.) blocked trigeminal nerve ganglion-induced increases in blood pressure but had no effect on changes in carotid flow or resistance. Maximal reductions in carotid vascular resistance was observed at 10 Hz, and this frequency was selected to investigate the effects of drugs on trigeminal nerve ganglion stimulation-induced responses in guanethidine treated cats. 3. Saline, alpha-CGRP8-37 SB-209670 and BQ-788 had little or no effect on resting haemodynamic parameters. SB-217242 (10 mg kg(-1), n=3) produced a 56% reduction in arterial blood pressure whereas SB-233451 (10 mg kg(-1), n=3) produced a 30% reduction in carotid vascular resistance. Sumatriptan produced dose-related reductions in resting carotid flow and increases (max. 104% at 0.3 mg kg(-1), n = 5) in vascular resistance. 4. SB-209670 (n=6-7), SB-217242 (n=3) and BQ-788 (n=3) produced inhibition of trigeminal nerve ganglion stimulation-induced reductions in carotid vascular resistance. Saline, SB-234551, alpha-CGRP8-37 and sumatriptan had no effect. 5. These data demonstrate ET(B) receptor blockade attenuates the vasodilator effects of trigeminal nerve ganglion stimulation in the carotid vascular bed of guanethidine pretreated anaesthetized cats.

PD 142893, SB 209670, and BQ 788 selectively antagonize vascular endothelial versus vascular smooth muscle ET(B)-receptor activity in the rat

The purpose of this study was to determine whether vascular endothelial and vascular smooth-muscle endothelin ET(B) receptors could be quantitatively differentiated by PD 142893 (PD), SB 209670 (SB), and BQ 788 (BQ) in the same species by using closely matched experimental conditions. The isolated perfused rat kidney (vascular smooth muscle) and isolated perfused rat mesentery (vascular endothelium) were challenged with increasing bolus doses of sarafotoxin S6c in the absence and presence of antagonist. PD, SB, and BQ produced parallel concentration-dependent rightward shifts in the S6c dose-response curve in the kidney. PD and SB also produced parallel concentration-dependent rightward shifts in the S6c dose-response curve in the mesentery. In contrast, BQ produced an insurmountable antagonism. Schild-derived pA2 values for PD and SB were significantly greater for inhibiting endothelial versus smooth-muscle ET(B) receptors. Furthermore, PD and SB differed in their relative potency between the two assays. Because BQ produced an insurmountable antagonism in the mesentery, it was not possible quantitatively to compare the antagonist activity in the two assays. These results indicate that PD, SB, and BQ selectively antagonize endothelial ET(B)-receptor activity over smooth-muscle ET(B)-receptor activity.

Pyrrolidine-3-carboxylic acids as endothelin antagonists. 3. Discovery of a potent, 2-nonaryl, highly selective ETA antagonist (A-216546)

Previously we have reported the discovery of ABT-627 (1, A-147627, active enantiomer of A-127722), a 2,4-diaryl substituted pyrrolidine-3-carboxylic acid based endothelin receptor-A antagonist. This compound binds to the ETA receptor with an affinity (Ki) of 0. 034 nM and with a 2000-fold selectivity for the ETA receptor versus the ETB receptor. We have expanded our structure-activity studies in this series, in an attempt to further increase the ETA selectivity. When the p-anisyl group of 1 was replaced by an n-pentyl group, the resultant antagonist 3 exhibited substantially increased ETB/ETA activity ratio, but a decreased ETA affinity. Structure-activity studies revealed that substitution and geometry of this alkyl group, and substitution on the benzodioxolyl ring, are important in optimizing this series of highly ETA selective antagonists. In particular, the combination of a (E)-2,2-dimethyl-3-pentenyl group and a 7-methoxy-1,3-benzodioxol-5-yl group provided hydrophobic compound 10b with subnanomolar affinity for human ETA receptor subtype and with an ETB/ETA activity ratio of over 130000. Meanwhile, synthetic efforts en route to olefinic compounds led to the discovery that 2-pyridylethyl (9o) and 2-(2-oxopyrrolidinyl)ethyl (9u) replacement of the p-anisyl group of 1yielded very hydrophilic ETA antagonists with potency and selectivity equal to those of 10b. On the basis of overall superior affinity, high selectivity for the ETA receptor (Ki, 0.46 nM for ETA and 13000 nM for ETB), and good oral bioavailability (48% in rats), A-216546 (10a) was selected as a potential clinical backup for 1.

Endothelin-1 expression in hearts of transgenic hypertensive mice overexpressing angiotensin II

Cardiac myocytes and vascular endothelial cells produce endothelin (ET)-1, which has potent hypertrophic effects on cardiac myocytes. Although in cultured cardiomyocytes, angiotensin II (Ang II) was reported to enhance ET-1 production in vitro, it is not known whether ET-1 production is enhanced by Ang II in vivo. We investigated the production and pathophysiologic roles of ET-1 in 20-week-old male transgenic hypertensive mice (THM), in which the renin-angiotensin system (RAS) was markedly activated because of the presence of both human renin and angiotensinogen genes. Systolic blood pressure and the ratio of left ventricular weight to body weight were significantly higher in the THM than in control mice, indicating that THM developed cardiac hypertrophy. ET-1 production was significantly increased in the heart of THM because both ET-1 mRNA expression and peptide levels were significantly higher than in controls. However, circulating plasma ET-1 levels did not differ between the groups, and blood pressure did not change after i.v. injection with a high dose (3 mg/kg) of the ETA/B-nonselective receptor antagonist SB209670. These findings suggest that increased cardiac ET-1 production may contribute to the progression of cardiac hypertrophy and that endogenous ET-1 may not be involved in the short-term modulation of blood pressure in THM of this age.

SB 234551, a novel endothelin--A receptor antagonist, unmasks endothelin-induced renal vasodilatation in the dog

Infusion of endothelin-1 (ET-1) into conscious, chronically instrumented dogs (10 ng/kg.min i.v.) resulted in a significant increase in mean arterial pressure and significant reductions in renal plasma flow, glomerular filtration rate, and sodium excretion. Intravenous infusion of SB 209670 (30 micrograms/kg.min, i.v.) abolished these responses, whereas infusion of SB 234551 (30 micrograms/kg.min, i.v.) resulted in significant increases in renal plasma flow and urinary sodium excretion. These data indicate that SB 234551 can unmask ETB receptor-induced renal vasodilatation and inhibition of sodium reabsorption.

SB 209670 inhibits the arrhythmogenic actions of endothelin-1 in the anesthetized dog

SB 209670 reduced basal mean arterial pressure (16%) without affecting left-circumflex coronary artery (LCX) flow, cardiac output, heart rate, or global/regional myocardial contractility. In vehicle-treated animals, i.e. endothelin (ET)-1 produced an initial hyperemic response in the LCX, followed by a secondary reduction in flow. This response was accomplished by decreases in LCX regional wall fractional shortening, +dP/dt and -dP/dt, but an increase in left anterior wall fractional shortening. ET-1 also produced dose-related, fatal ventricular fibrillation. Whereas SB 209670 administration did not inhibit the initial increase in coronary flow produced by ET-1, the secondary constrictor responses were markedly antagonized. SB 209670 also attenuated the reduction in LCX regional wall fractional shortening and converted the increase in left anterior wall contractility to a reduction in contractility. Although SB 209670 produced only a modest inhibition of the ET-1-mediated reductions in dP/dt, the induction of fatal ventricular arrhythmias was completely abolished. Therefore, the data are consistent with the hypothesis that the coronary ischemic and proarrythmic actions of ET-1 are distinct. Therefore, ET receptor antagonists may be useful in treatment of disturbances in cardiac rhythm.