Endothelin as a therapeutic target in the treatment of cardiovascular disease

Application of structure–metabolism relationships in the identification of a selective endothelin A antagonist, BMS-193884, with favourable pharmacokinetic properties

1. Based on binding affinity, 2'-amino-N-(3,4-dimethyl-5-isoxazolyl)-4'-(2-methylpropyl)[1,1'-biphenyl]-2-sulfonamide (2) was identified as an initial lead in a programme to identify selective endothelin (ET) receptor antagonists. However, the compound was extensively metabolized in preclinical animal species and human in vitro systems due to oxidative biotransformation. 2. To optimize this structural class, the site of metabolism of 2 was determined. This allowed for focussed structure-activity and structure-metabolism studies aimed at finding more metabolically stable analogues that maintained potency. New analogues were screened for their ET binding characteristics and their stability in rat and human liver microsomes. 3. The use of the microsomal stability screen was tested by the determination of the pharmacokinetic parameters of select analogues. A good correlation was found between reduced rates of rat microsomal metabolism and reduced clearance in the rat. 4. N-(3,4-dimethyl-5-isoxazolyl)-4'-(2-oxazolyl)[1,1'-biphenyl]-2-sulfonamide (3) was identified as an analogue with improved in vitro properties and further studies revealed that the compound had improved pharmacokinetic properties. 5. N-[[2'-[[(3,4-dimethyl-5-isoxazolyl)amino]sulfonyl]-4-(2-oxazolyl)[1,1'-biphenyl]-2-yl]methyl]acetamide (4) was subsequently identified as a compound with superior in vitro properties compared with compound 3, but when tested in vivo it had a substantially increased rate of clearance. Further studies demonstrated that the clearance of this closely related structural analogue was not dictated by metabolic processes, but was mediated by transport-mediated direct biliary excretion. 6. The utility of screening for in vitro liver microsomal stability as part of the lead optimization process for compounds with metabolic liabilities was shown. It was also shown that relatively small molecular changes can dramatically change the disposition of closely related analogues and care must be used when screening for a single property.

Role of endothelins as mediators of injury-induced alterations of glial glutamate turnover

Astroglia terminate glutamatergic neurotransmission and prevent excitotoxic extracellular glutamate concentration by clearing synaptically released glutamate through the high-affinity, sodium-dependent glutamate transporters GLT-1 and GLAST. Many brain injures are associated with the disturbed expression of glial glutamate transporters and a subsequent increase of extracellular glutamate to neurotoxic levels. We have now followed up initial hints pointing to endothelins, a family of injury-regulated peptides, as mediators of this injury-induced loss of glial glutamate transporter expression. We observed that, in line with such a role, endothelins not only act as potent inhibitors of basal and exogenously (dbcAMP)-induced expression of GLT-1 in cortical astrocytes as shown before, but likewise inhibit expression of GLT-1 or GLAST in astrocytes cultured from the diencephalon, mesencephalon, cerebellum, and spinal cord. We further demonstrate that endothelins equally inhibit GLT-1 expression in cortical slice cultures, a culture system closely resembling the in vivo situation. Although brain injuries are usually associated with an increase in the expression of the glutamate-converting enzyme glutamine synthetase, cultured cortical astrocytes maintained with endothelins showed an almost complete loss of glutamine synthetase. Interestingly, the inhibitory effects of endothelins on the expression of glutamine synthetase, but not of glutamate transporters, was overridden by high extracellular glutamate, indicating that the primarily inhibitory action of endothelins on the various components of glial glutamate turnover dissociates in the injured brain.

Neuroprotective effect of topically applied brimonidine tartrate 0.2% in endothelin-1-induced optic nerve ischaemia model

BACKGROUND

To investigate the neuroprotective effects of topically applied brimonidine tartrate 0.2% (BMD), an alpha(2)-receptor agonist, on the retinal ganglion cell (RGC) layer and inner nuclear layer (INL) of rabbit retina in endothelin-1 (ET-1)-induced optic nerve (ON) ischaemia model.

METHODS

Osmotic minipumps were surgically implanted into one eye of 16 New Zealand Albino rabbits to deliver ET-1 at the constant rate of 0.5 microL/h for 2 weeks. Eyes were divided into four groups. ET-1 was given with (Group 3) and without topical BMD therapy (Group 1). Groups 2 and 4 were taken as controls. Rabbits were sacrificed at day 14. Morphological alterations, total cell number and proportion of cells undergoing apoptosis in INL and RGC layer were assessed by histopathological analysis to determine the survival of the cells of the INL and RGC layer.

RESULTS

Endothelin-1 led to severe reduction of cells in both the RGC layer and INL in Group 1 (P < 0.05). In Group 3, the total cell number and the proportion of cells undergoing apoptosis in the RGC layer were comparable with the control group (Group 4), whereas the former was found to be higher and the latter was found to be lower than those recorded for Group 1. However, the total cell number in the INL was found to be lower in Group 3 compared with that of Group 4, despite topical BMD therapy (P < 0.05).

CONCLUSIONS

Topically applied BMD seems to be neuroprotective and antiapoptotic in the ET-1-induced ON ischaemia model, especially for RGCs. BMD might be used as an adjuvant agent for its neuroprotective effects in hypoxic-ischaemic conditions such as diabetic retinopathy, normotensive glaucoma and other retinal vascular occlusive conditions which require further investigations.

Eyeing endothelins: a cellular perspective

Endothelin is an endogenous vasoactive peptide that is considered among the most potent vasoconstrictor substances known. In addition to its vascular effects, endothelins and their receptors have been shown to be present in the eye and to have a number of ocular actions that may be important for ocular homeostasis, but, in excess can be a potential contributor to ocular neuropathy in glaucoma. The current review focuses on the cellular and molecular aspects of endothelins and its receptors in the eye with an emphasis on its relationship to ocular function and its potential role in the etiology of glaucoma pathophysiology.

Tezosentan in the Treatment of Acute Heart Failure

OBJECTIVE

To evaluate the pharmacology, pharmacokinetics, clinical efficacy, and tolerability of tezosentan, a new intravenous endothelin (ET)-1 receptor antagonist.

DATA SOURCES

Literature was identified through a MEDLINE search (1990-June 2003) using the search terms endothelin-1, heart failure, RITZ, and tezosentan. References listed in articles and abstracts from scientific meetings were also used.

STUDY SELECTION AND DATA EXTRACTION

English-language literature reporting controlled animal and human clinical studies was reviewed to evaluate the pharmacology, pharmacokinetics, therapeutic use, and adverse effects of tezosentan. Clinical trials selected for inclusion were limited to those with human subjects and included data from animal studies if human data were not available.

DATA SYNTHESIS

Tezosentan is a dual ET-1 receptor antagonist that has demonstrated efficacy in improving cardiac index and reducing pulmonary capillary wedge pressure in patients with acute, decompensated heart failure. Following infusion, tezosentan's plasma concentration approaches steady-state within the first 6 hours, with a relatively small volume of distribution (17 L) and clearance (39 L/h) that are dose independent. Tezosentan is excreted almost entirely unchanged via the bile (>95%), with the rest (<5%) excreted in the urine. Elimination can be explained by a biphasic profile that has a rapid elimination phase (half-life 6 min) followed by a slow phase (half-life 3 h) that accounts for distribution from tissues. The adverse event profile is significant for a higher incidence of headaches, nausea, and hypotension compared with placebo.

CONCLUSIONS

Phase II and III clinical trials have rendered mixed results for the efficacy and tolerability of tezosentan. A dose optimization trial yet to be published and an ongoing Phase III registration study will provide valuable data regarding the efficacy and tolerability benefits, as well as the morbidity and mortality, of tezosentan. Until then, tezosentan's role in the treatment of patients with acute heart failure will remain unclear.

Endothelin-1 activity in pregnancy

Plasma endothelin-1 activity was measured by radioimmunoassay in 24 normotensive non-pregnant women and in 24 normotensive pregnant, 24-aproteinuric hypertensive and 24 pre-eclamptic women. Endothelin-1 activity was increased in the pre-eclamptic group (2.7 +/- 06 pg/ml) compared to the normotensive non-pregnant (1.0 +/- 0.8 pg/ml; P < 0.0001), normotensive pregnant (1.2 +/- 0.4 pg/ml; P < 0.0001) group and the aproteinuric hypertensive group (1.4 +/- 0.7 pg/ml; P < 0.0001). There was no difference in endothelin-1 activity between the normotensive non-pregnant and normotensive pregnant group (1.0 +/- 0.8 vs. 1.2 +/- 0.4 pg/ml; P = 0.3). However, there was a difference between the aproteinuric hypertensive group (1.4 +/- 0.7 pg/ml) and both the normotensive nonpregnant (1.0 +/- 0.8 pg/ml; P < 0.01) and the normotensive pregnant group (1.2 +/- 0.4 pg/ml; P < 0.06). The birth weight in the pre-eclamptic group (2.48 +/- 0.61 kg) was significantly lower than that of the normotensive pregnant group (2.85 +/- 0.33 kg; P < 0.001) and the aproteinuric hypertensive group (2.99 +/- 0.46 kg; P < 0.001). In addition, there was no difference in birth weight between the normotensive pregnant group and aproteinuric hypertensive group (2.85 +/- 0.33 vs. 2.99 +/- 0.46 kg; P = 0.3). A significant Pearson's correlation of plasma endothelin-l versus birth weight in the pre-eclamptic group was obtained (r = 0.64; P < 0.0001). Endothelin-1 activity is increased with pre-eclampsia in black African women with pre-eclampsia. The results of our study also suggests an ethnic difference in plasma endothelin-1 activity when compared to result of studies carried out in Caucasian women with hypertensive disorders of pregnancy.

Primary pulmonary hypertension and cor pulmonale

Primary pulmonary hypertension and cor pulmonale represent forms of precapillary pulmonary hypertension due to intrinsic lung disease. In the case of primary pulmonary hypertension, this is due to disease of the pulmonary vasculature while cor pulmonale is related to diseases of the pulmonary vasculature, airways, or interstitium. Patients present with signs and symptoms of right ventricular dysfunction and low cardiac output including dyspnea, chest pain and peripheral edema. Therapy is directed at the underlying disease and may include supplemental oxygen for diseases causing chronic hypoxemia and anticoagulation for thrombotic disease. Vasodilator therapy has variable efficacy for pulmonary vascular disorders. Postacyclin by continuous infusion has been a major advance in the therapy of primary pulmonary hypertension and has prolonged survival and delayed the need for lung transplantation. Bosentan, an endothelin receptor blocking agent is the first oral medication approved for the therapy of pulmonary hypertension.

Bosentan

Bosentan is a nonpeptide, specific, competitive, dual antagonist at both endothelin receptor subtypes (ET(A) and ET(B)). Orally administered bosentan effectively prevents endothelin 1-induced vasoconstriction in pulmonary vessels in patients with pulmonary arterial hypertension. Improvement in exercise capacity from baseline was significantly greater with bosentan than with placebo in two phase III trials in patients with WHO functional class III or IV pulmonary arterial hypertension (primary or associated with connective tissue disease) despite treatment with vasodilators, diuretics, anticoagulants, cardiac glycosides, or supplemental oxygen. The beneficial effects of bosentan on exercise capacity were maintained for at least 20 weeks. Compared with placebo, bosentan led to a significantly greater improvement from pretreatment values in secondary efficacy endpoints such as the Borg dyspnea index, WHO functional class, and cardiopulmonary hemodynamic parameters (cardiac index, pulmonary vascular resistance, pulmonary artery pressure, pulmonary capillary wedge pressure, mean right atrial pressure). Bosentan significantly reduced the incidence, and delayed the onset, of clinical worsening of pulmonary arterial hypertension compared with placebo. In published clinical trials, adverse events that occurred with similar or greater frequency with bosentan 125 mg twice daily than with placebo included headache, syncope, flushing and abnormal hepatic function. Those that occurred less frequently with bosentan 125 mg twice daily than with placebo included dizziness, worsening of symptoms of pulmonary arterial hypertension, cough and dyspnea.