Profile of Past and Current Clinical Trials Involving Endothelin Receptor Antagonists: The Novel “-Sentan” Class of Drug

Introduction

The first widely used endothelin (ET) receptor antagonists (ERAs) were BQ-123, a selective ET_A receptor antagonist (ERA-A; Ref. 1), and BQ-788, a selective ET_B receptor antagonist (ERA-B; Ref. 2), both from Banyu (Merck). We (3–6) and others (7–18) have reviewed the chemistry and pharmacology of these ERAs as well as the characteristics of other classes of blockers of the ET system, such as single ET-converting enzyme inhibitors or dual and triple vasopeptidase inhibitors incorporating ET-converting enzyme inhibitors (19, 20).

Currently, both BQ-123 and BQ-788 remain useful tools for defining the pathophysiology of the ET system; numerous studies have employed these agents. Unfortunately, their high cost and parenteral method of administration (peptidic nature) has precluded their use in large clinical trials (Table 1). Ro 47–0203 (bosentan) became the first ERA to undergo Phase I clinical trials, with subsequent studies demonstrating clear therapeutic benefit (efficacy) and safety, leading to Phase II–III clinical trials in patients with idiopathic pulmonary arterial hypertension (iPAH) and pulmonary arterial hypertension (PAH) secondary to connective tissue diseases. While the therapeutic applications for bosentan are expanding to address other related pathological conditions (such as secondary PAH), additional compounds have been chosen to also target PAH (ambrisentan and sitaxsentan). Further agents are aiming at resistant hypertension (darusentan and TBC3711), prostate cancer (atrasentan, ZD4054, and YM598), and cerebral vasospasm (clazosentan), as well as other disorders (bosentan, ambrisentan, and sitaxsentan).

Dual (Mixed) ERA (In Alphabetical Order).

Selective ERA-A (In Alphabetical Order).

Early work with the related compound, BMS-193884 (N-(3,4-dimethyl-5-isoxazolyl)-4′-(2-oxazolyl)[1,1′-biphenyl]-2-sulfonamide), revealed its high selectivity for the ERA-A (>13,000-fold with the constant of inhibition (Ki) = 1.4 nM for ET_A and 18.7 μM for ET_B; Ref. 28). An optimum pharmacologic profile contributed to its selection as a clinical candidate for studies in congestive heart failure (CHF; Tables 1, 2, and 5) following promising results from preclinical studies in rodent and porcine models (108–110). This compound was also tested in models of monocrotaline-induced PAH. It was replaced by edonentan (BMS-207940; (N-[[2′-[[(4,5-dimethyl-3-isoxazolyl)amino]sulfonyl]-4-(2-oxazolyl)[1,1′-biphenyl]-2-yl]methyl]-N,3,3 trimethylbutanamide), the backup compound, an even more potent and selective ERA-A (80,000-fold with Ki = 0.010 nM for ET_A-R) (33).

Sitaxsentan.

TBC11251 (N-(4-chloro-3-methyl-5-isoxazolyl)-2-[(6-methyl-1,3-benzodioxol-5-yl) acetyl]-3-thiophenesulphonamide) is a compound under clinical development by Encysive Pharmaceuticals (formerly Texas Biotechnology Corporation). Sitaxsentan has been reported to be an orally available (50%–60% and 90%–100% bioavailability in rat and dog, respectively) and highly selective ERA-A (6500-fold with Ki of 0.35 nM for ET_A; 34). In addition, sitaxsentan exhibits approximately 90% oral bioavailability in human subjects and has a t½ of 10 hrs in patients with PAH. Thus far, 31 clinical studies have been conducted with sitaxsentan, including three major randomized, placebo-controlled trials (Sitaxsentan to Relieve Impaired Exercise [STRIDE] trials; see below) in patients with PAH.

S-0139 (SB-737004).

S-0139 (27-O-3-[2-(3-carboxy-acryloylamino)-5-hydroxyphenyl]-acryloyloxy myricerone, sodium salt; also known as SB-737004) is a highly selective nonpeptidic ET_A-R antagonist (Ki: 1 nM on ET_A-R vs. 1000 nM on ET_B-R) developed by Shionogi (Japan). The compound has a 1000-fold more potency for ET_A as compared to ET_B receptors (Table 1; Ref. 188). A joint venture was finalized with GSK in 2001-Q4 to codevelop and commercialize four compounds contributed by Shionogi and one by GSK, including S-0139 for the potential treatment of hemorrhagic and ischemic stroke (187, 189).

Preclinical experimental data with S-0139 suggested its potential beneficial effects on brain edema and injury in rats, CHF in mice, and renal cortical blood flow reduction in rats (190–192).

TBC 3711.

TBC 3711 (N-(2-acetyl-4,6-dimethylphenyl)-3-(3,4-dimethylisoxazol-5-ylsulfamoyl)thiophene-2-carboxamide) is another of Encysive’s orally available, nonpeptidic, selective ERA-A (35). It has an oral bioavailability of approximately 100% in rats, high potency (mean inhibitory concentration [IC₅₀] on ET_A-R = 0.08 nM), and an ET_A selectivity of >100,000-fold (Table 1). It has completed a Phase I clinical trial and was well tolerated with reported pharmacokinetics in humans (t½ = 6–7 hrs, oral availability > 80%). In a preclinical experimental study, TBC3711 (22 mg/kg/day) has been reported to reduce neonatal hypoxia-induced pulmonary hypertension in 1-day-old piglets (112).

YM598.

Yamanouchi (now called Astellas Pharma after its merger with Fujisawa) developed ethenesulfonamide derivatives as a potent, orally active and selective type of ERA-A (36, 37) through the modification of bosentan. YM598 ((E)-N-[6-methoxy-5-(2-methoxyphenoxy)[2,2′-bi-pyrimidin]-4-yl]-2-phenylethenesulfonamide monopotassium salt; Fig. 1) is the lead compound and is 816-fold more selective for the ET_A receptor (Ki = 0.697 and 569 nM for ET_A and ET_B respectively; Table 1; Refs. 38–40). The first indication selected for YM598 was prostate cancer (as with Abbott and AstraZeneca), as suggested by preclinical data in mice (113, 114). YM598 was also studied and proven effective in animal models of chronic hypoxia-induced PAH, monocrotaline-induced PAH, hypertension (normal and low-renin), myocardial infarction-induced CHF, postischemic left-side CHF, and type 2 diabetic nephropathy.

ZD4054.

AstraZeneca has been working in the field since 1994 and developed ZD1611 and ZD2574 (115). More recently, they have developed an orally active, more potent and selective ERA-A, ZD4054 (5-(dimethylamino)-N-(5chloro-3-methoxy-2-pyrazinyl)-1-naphthalenesulfonamide; Refs. 41, 108). This novel antagonist emerged from structure activity relationship (SAR) studies conducted with BMS-182874 (41). Because ET_A-R activation by ET-1 mediates events that regulate mitogenesis, apoptosis, angiogenesis, and metastasis in tumors, AstraZeneca has entered this area as well as Abbott. AstraZeneca completed a Phase II-a trial (see below; Tables 2 and 5) and initiated a Phase II-b trial in 250 patients with prostate cancer (see below; Tables 2 and 7).

Approved New Drug Application (NDA): The Homologation of a New Class of Drug Through Clinical Trials

Actelion, with its bosentan, became the first to successfully emerge from the U.S. Food and Drug Administration (FDA)’s NDA homologation in 2001. Endothelin receptor antagonists became a new class of drug bearing the “-sentan” suffix.

Formally Completed and Ongoing Clinical Academic Studies and Trials in Control Subjects and Patients

Below, we briefly discuss clinical academic studies, as well as small-scale clinical trials, in healthy subjects and patients, that were conducted using the first generations of ERAs (peptidic, like BQ-123, and some others that are nonpeptidic). Such studies, many of which are ongoing and recruiting patients, are supported by public hospitals and/or government agencies in order to increase understanding of the pathophysiology and mechanisms of action of the ET system (Table 4).

Full-length clinical trials are mostly supported by biopharmaceutical industries. We have summarized them by separating between completed (Tables 4 and 5) and ongoing (Tables 6 and 7) clinical trials, while reviewing them one by one, elaborating on each trial’s name, phase of development, sponsor, target, study design, duration, formulation, route of administration, and, if completed, results, supported by key references when available.

Initial studies, performed to elucidate the role of the then-recently discovered ET-1, involved the infusion of the exogenous peptide (agonist at both ET_A and ET_B receptors) and/or sarafotoxin-6c (S6c; an ET_B receptor agonist) into the forearm circulation of healthy volunteers. These studies confirmed that, as in earlier animal experiments (141), ET-1 acts as a long-lasting, potent vasoconstrictor in man (116). Later, it was shown that ET-1 exerts a biphasic effect: low-dose ET-1 caused vasodilatation, whereas higher doses resulted in vasoconstriction (117). This biphasic response was also seen when bolus doses of S6c were infused into healthy subjects (118), suggesting that activation of ET_B receptors might cause vasodilatation as well as vasoconstriction. When infused systemically, ET-1 exerted a marked pressor effect associated with a reduction in renal blood flow and decreased natriuresis (119). Such changes were not seen following systemic infusion of ET-3 (an ET_B receptor agonist), indicating that these systemic and renal vasoconstrictor effects of ET-1 in healthy volunteers are mediated predominantly by ET_A receptors.

Infusion of ERAs.

Since their development by Banyu (Merck), appropriate doses of the ERA-A BQ-123 (1) and the ERA-B BQ-788 (2) have been used as highly selective probes to determine the physiological role of ET-1 both in health and in a range of cardiovascular, high-risk disease states. Nevertheless, BQ-123 and BQ-788 have been shown to exert nonselective ET blocking effects at higher doses (142, 123). Recently, such studies have been extended to investigate the potential therapeutic role of these ERAs in reversing the endothelial dysfunction seen in cardiovascular disease.

Physiological Role of ET-1.

When first used in humans, BQ-123 infusion into the forearm circulation of healthy volunteers caused marked vasodilatation, strongly suggesting that endogenous ET-1 maintains vascular tone through basal activation of ET_A receptors (120). In contrast, local infusion of BQ-788 resulted in vasoconstriction, indicating that ET_B receptors act to reduce basal vascular tone (121). The vasodilatory action of BQ-123 was significantly reduced by ET_B blockade and almost abolished when nitric oxide (NO) synthesis was inhibited (121). This indicated that it is attributable, to not only direct blockade of ET-1 binding to vasoconstrictive ET_A receptors on vascular smooth muscle, but also enhanced endogenous NO generation and the preservation of ET_B receptor-mediated vasodilator tone. In contrast, the response to BQ-123 is not altered following cyclooxygenase inhibition with aspirin, providing strong evidence that this response is not dependent upon endothelium-derived prostaglandins.

Systemic infusion of BQ-788 into healthy volunteers resulted in a significant rise in peripheral vascular resistance, although, because of a compensatory fall in heart rate, no change in BP was seen (122). Within the vasculature, ET_B receptors are expressed on both vascular smooth muscle cells, where they cause vasoconstriction, and on the endothelium, where their activation results in vasodilatation. Along with the forearm studies detailed above (121), this suggests that the net effect of ET_B activation in healthy subjects favors vasodilatation via the endothelial ET_B receptors. This study also investigated the effect of systemic ET_B antagonism on plasma ET-1 concentrations. As expected, and as observed in preclinical studies with BQ-788, ET_B selective antagonism leads to a 5- to 7-fold increase in ET-1 plasma concentrations in the rat, the administration of an ERA-B, which impairs clearance of ET-1 (143), resulted in an increase in the plasma concentration of ET-1. In contrast, when BQ-123 was infused systemically over a wide range of doses, no difference in plasma ET-1 was found (123). Here, BQ-123 dose-dependently decreased systemic vascular resistance and mean BP (123), providing further evidence that endogenous ET_A activation contributes to the maintenance of basal systemic vascular tone.

Role of ET-1 in Cardiovascular Disease.

Having used BQ-123 and BQ-788 to help reveal the physiological roles of ET-1, investigators were swift to perform similar experiments in patients with either risk factors for, or established, atherosclerotic disease, in order to determine whether the ET system is active in such pathological states.

BQ-123 infused into the forearms of CHF patients resulted in an increase in forearm blood flow (124). Two independent groups then showed that local infusions of BQ-123 and BQ-788 (125) or of the mixed ERA, TAK-004 (126), causes a greater vasodilatation in patients with essential hypertension than in healthy subjects. Moreover, vasoconstriction to N^G-monomethyl-l-arginine was significantly decreased in hypertensive patients compared with controls, suggesting that the increased ET-1–mediated vascular tone in these patients is associated with decreased tonic NO release (126). Recent work has demonstrated not only that this increased vasoconstrictor activity is largely ET_A-dependent, but also that it is more pronounced in hypertensive patients with an increased body mass index (129). This suggests that increased vascular ET_A-R activity could be a feature of the metabolic syndrome.

In hypercholesterolemic patients, BQ-123 infusion also increased forearm blood flow compared to healthy controls. However, this was reversed by coinfusion with the ERA-B, BQ-788 (127). A similar result was seen in an equivalent study in Type 2 diabetic patients (128).

Studies have also been performed to investigate the effect of ERAs in patients with established atherosclerosis (defined in this study as the combination of intermittent claudication with coronary artery disease; Ref. 131). As expected, BQ-123 caused an increase in forearm blood flow in both groups. However, BQ-123 and BQ-788 infused together had a negligible effect on vascular tone in control subjects. In contrast, combined ERA blockade caused a greater dilatation in atherosclerotics than that seen with BQ-123 alone. This suggests a greater functional significance of vasoconstrictive vascular smooth muscle ET_B than dilatory endothelial ET_B in these patients.

The important role of ET_A-dependent vascular tone in the coronary circulation was demonstrated by the vasodilatory effect of intracoronary infusions of BQ-123 (132). It was found to be exaggerated in subjects with known coronary artery disease (133, 137). Intracoronary BQ-788 caused microvascular constriction, partly as a result of reduced ET-1 clearance and reduction in NO production (134).

As well as being used in forearm and coronary circulation studies, ERAs have also been used following systemic administration. In patients suffering from hypertensive chronic renal failure, BQ-123 substantially increased renal blood flow and reduced renal vascular resistance. These effects were not seen with mixed ET blockade (135). In contrast, infusion of BQ-788 resulted in both renal and systemic vasoconstriction. Similarly, infusion of BQ-123 reduced both systemic and pulmonary vascular resistance in CHF patients to a greater degree than following coinfusion of both BQ-123 and BQ-788 (136). BQ-788 alone worsened hemodynamic variables and had deleterious effects on a number of patients involved in the study.

ERA Reverse Endothelial Dysfunction.

The demonstration that ERA-As cause vasodilatation, not only through competitive inhibition of ET-1 binding to ET_A receptors, but also through increasing the generation of endogenous NO (121), encouraged many groups to study the effect of ERAs on endothelial function more directly.

The dilatation of small arteries in response to acetylcholine is dependent upon endothelial-derived NO, and has been taken as a measure of endothelial function. Intra-coronary infusion of BQ-123 enhanced acetylcholine-induced vasodilatation of the coronary arteries of patients with atherosclerosis (137) to a greater extent to that seen following coinfusion of BQ-788 and BQ-123 (134). Neither compound altered the endothelium-independent vasodilatation to sodium nitroprusside. In the forearms of both hypertensive patients (138) and patients with atherosclerosis (139), mixed blockade with BQ-123 and BQ-788 similarly potentiated the response to acetylcholine without affecting the response seen to sodium nitroprusside. Mental stress–induced impairment of endothelial function, brought on in healthy subjects by a 3-min mental stress task, could also be partially reversed by infusion with BQ-123 (140).

Clinical studies with the Banyu peptide ERAs, as probes for ET_A and ET_B receptor blockade, have allowed a much fuller understanding of the ET system in health and disease as well as of its role in modifying endothelial function. Such knowledge has informed the choice of targets for subsequent clinical trials.

Completed Clinical Trials in Control Subjects and Patients

This was a small (n = 16), prospective, noncomparative cohort study investigating the effects of bosentan in human immunodeficiency virus (HIV)-associated PAH patients (49). Treatment resulted in improvements in the endpoints of exercise capacity, cardiopulmonary hemodynamics, functional class, and quality of life (Tables 2 and 4; Ref. 49). During the study, there were no deaths, and no patients required epoprostenol treatment. Hepatic tolerability was similar to that reported in patients with iPAH. Bosentan treatment did not worsen either CD4 count or HIV viral load. During 12 months of follow-up in 12 patients, during which bosentan treatment was continued, no deterioration in functional class was seen. Despite the co-administration of potentially hepatotoxic antiretroviral therapies, the safety profile of bosentan was good and no clinically relevant drug interactions were observed (49).

Comparing Bosentan with Prostacyclin Treatment.

This study compared the survival of patients with class III iPAH treated with first-line oral bosentan versus the historical cohort of patients started on iv epoprostenol (50). The use of bosentan delayed the initiation of epoprostenol. Kaplan-Meier survival estimates were 97% and 91%, after 1 and 2 years, respectively, in the bosentan cohort, and 91% and 84% in the epoprostenol cohort. When matched cohorts of 83 patients each were selected, survival estimates were similar. In the bosentan cohort, 87% and 75% of patients followed for 1 and 2 years remained on monotherapy. Therefore, no evidence was found to suggest that initial therapy with oral bosentan, followed by other treatment if needed, adversely affected long-term outcome compared with initial intravenous epoprostenol in class III iPAH patients (50).

ENABLE.

In 1998, oral bosentan was used in a double-blind and randomized short-term trial as an add-on therapy in patients (n =24 +12 placebo) with symptomatic severe CHF conventionally treated with diuretics, digoxin, and angiotensin-converting enzyme inhibitor (ACEi) over 2 weeks (143). On Day 1, bosentan significantly decreased mean arterial BP, PAP, PCWP and RAP. Cardiac output (CO) increased, whereas heart rate (HR) remained unchanged. After 2 weeks of bosentan therapy, CO further increased, whereas systemic vascular resistance (SVR) and PVR fell (144). Following this encouraging short-term hemodynamic effect of oral therapy with bosentan in heart failure patients who were symptomatic with standard triple-drug therapy, Endothelin Antagonist Bosentan for Lowering Cardiac Events in Heart Failure (ENABLE) was swiftly organized.

In this large trial, 1613 patients with severe heart failure (left ventricular ejection fraction <35%, NYHA class III-b–IV) were randomized to receive either bosentan (125 mg bid after a 4-week titration phase at 62.5 mg bid) or placebo. The preliminary results were presented at the 51st Annual Scientific Session of the American College of Cardiology (March 2002, Atlanta, Georgia) and did not demonstrate a benefit with bosentan treatment. The primary endpoint of all-cause mortality or hospitalization for heart failure was reached in 321 of 808 patients on placebo, and in 312 of 805 patients receiving bosentan. Treatment with bosentan appeared to confer an early risk of worsening heart failure necessitating hospitalization, as a consequence of fluid retention. It has been suggested that further studies using even lower doses of bosentan or more aggressive concomitant diuretic therapy may avoid this adverse effect. Such results from the ENABLE study created doubt about the potential benefits of ERA in the treatment of CHF (51).

REACH-1 with Bosentan.

In the Research on Endothelin Antagonism in Chronic Heart Failure (REACH) study (52), which preceded the ENABLE study, the long-term effects of bosentan (target dose 500 mg bid; n = 244) versus placebo (n =126) in patients with CHF (NYHA class IIIB/IV) were assessed. This trial was halted prematurely because of increased incidence of elevated liver transaminase levels. When the trial was stopped, however, patients who had been maintained on therapy over a 6-month period demonstrated a trend toward reduced risk of heart failure-related mortality and morbidity (52).

ENCOR.

In the Enrasentan Cooperative Randomized Evaluation (ENCOR), treatment of 419 patients with CHF (Class II–III) randomized to either the dual ERA, enrasentan, or placebo failed to show benefit in a composite end point including NYHA class, hospitalization rate, and global assessment. In fact, the study showed a trend in favor of placebo: there was a significantly greater likelihood of being hospitalized for heart failure—almost 3-fold—for patients randomized to enrasentan (53).

RITZ-1.

There were four Randomized Intravenous Tezosentan (RITZ) studies in patients with acute decompensated heart failure (ADHF). Phase I investigated the tolerability, pharmacokinetics, and pharmacodynamics during chronic infusions of tezosentan in healthy male subjects (145, 54). Both doses tested (Table 4) were well tolerated with headaches being the most frequently reported adverse event (incidence of 75%–100% for tezosentan and 50% for placebo). Plasma concentrations of tezosentan rapidly approached steady state and did not change upon prolonged infusion. A two-compartment model described its pharmacokinetic profile. The half-lives of the two disposition phases were approximately 0.10 and 3.2 hrs. Circulating plasma concentrations of ET-1 increased rapidly during infusion, compared with predose values, and did not change during the 72-hr infusion. The volume of distribution at steady state (approximately 16 liters) and the clearance (approximately 30 liters/hr) were considered independent of dose, in view of the wide dose range explored. Additional doses (5, 20, 50, 100, 200, 400, and 600 mg/hr) were also infused for 1 hr (55). No additional clinically relevant changes in vital signs or in electrocardiographic or clinical laboratory parameters occurred.

A second safety trial was conducted with the co-administration of cyclosporine and tezosentan. Cyclosporine caused a 4-fold increase in the exposure to tezosentan (56). All subjects on the combined regimen reported headache, hot flushes, and nausea/vomiting. Some of these symptoms were of severe intensity. The symptoms did not correlate with the circulating plasma concentrations of tezosentan.

In a third safety study, no notable differences in safety and tolerability variables were detected between tezosentan-treated and placebo-treated patients when infused over 4 to 6 hrs.

A fourth pilot safety trial was conducted in patients with advanced heart failure infused over 48 hrs (57). The safety and tolerability, as well as the hemodynamic stability, following tezosentan treatment revealed no episodes of hypotension requiring withdrawal of therapy occurred. Hemodynamic rebound was not observed after abrupt cessation of the infusion. In addition, there were no reports of worsening heart failure in tezosentan-treated patients up to 28 days following the infusion. The most common side effect observed during the infusion was headache. Echocardiographic Doppler measurements suggested improvements in CI, PCWP, and relaxation properties as well as in diastolic and systolic function. Thus, tezosentan was well tolerated with no new safety concerns emerging (57).

RITZ-2.

This study was designed to determine the effective dosage range, hemodynamic effects, and tolerability of tezosentan in patients with advanced NYHA class III heart failure (58, 59). It acutely improved hemodynamic parameters (significant increase in CI and decreases in PVR and SVR without changes in heart rate). However, a consistent greater decrease in right-sided pressures and mean systemic BP did not reach statistical significance. Hemodynamic changes were dose-dependent with maximal effects at 20 and 50 mg per hour. Tezosentan was well tolerated. Despite an increase in circulating plasma concentrations of ET-1, hemodynamic rebound was not observed. Thus, the favorable effects on CI and vascular resistance, without changes in heart rate, suggested that tezosentan could be beneficial in the treatment of ADHF (58).

RITZ-3.

This was a further Phase II-b trial intended to enroll more patients (NYHA class III–IV) and study them over a longer duration (from 1 [Ref. 58] to 6 hrs follow-up) in combination with a diuretic (60). However, it was deemed unnecessary from a regulatory point of view by Actelion and was never conducted.

RITZ-4.

Thus far, tezosentan, as a dual ERA, was shown to improve CI/CO and reduce PVR and SVR in initial human acute decompensated heart failure studies (see above). The next step included a dose-optimization trial (design in Refs. 146, 147) as part of Phase III studies that would provide valuable data regarding the efficacy and tolerability benefits, as well as the morbidity and mortality. However, following disappointing trial results, a new 2-year Phase III study in ADHF was planned using lower doses of the compound (75). This additional Phase III trial was designed as a multicenter, randomized, double-blind, placebo-controlled trial with 193 enrolled patients with ADHF and acute coronary syndromes. No significant differences were observed between placebo and 50 mg/hr tezosentan infusion in the composite primary end point (e.g., death, worsening of heart failure, recurrent ischemia, and recurrent or new myocardial infarction within 72 hrs after the initiation of the treatment with tezosentan). Symptomatic hypotension was more frequent in the treatment group. Thus, at the doses studied, tezosentan did not result in a significant improvement (61).

Study 205 with Tezosentan from Actelion.

This study was a Phase II trial designed to optimize tezosentan dosing in patients with ADHF who required right heart pressure monitoring. The study compared the effects of iv tezosentan (0.2, 1, 5, and 25 mg/hr) versus placebo on CI, PCWP, urine output, and serum creatinine. The three highest doses showed significant increases in CI and decreases in PCWP. However, the maximum effect was evident only at 5 mg/hr and the highest dose decreased urine output.

VERITAS-1 and -2.

RITZ studies left us uncertain whether ERA could be effective in patients with ADHF. Morbidity and mortality had not been evaluated. The Value of Endothelin Receptor Inhibition with Tezosentan in Acute Heart Failure Study (VERITAS) consisted of two identical double-blind, randomized, placebo-controlled, multicenter trials, designed to enroll at least 1760 patients (Table 4) and study the effect of tezosentan on mortality and rate of clinical worsening, The program was discontinued in November 2005 because of the low probability of achieving a significant treatment effect over the first 24 hrs of treatment and at 7 days (62).

Avosentan by Speedel.

Based on a pilot study, Phase I through II-b trials have been completed. 286 patients were enrolled in a randomized, placebo-controlled, double-blind, parallel-design dose-range study (see Table 4; Ref. 44). The study compared the effects of a 12-week therapy with SPP301 (5, 10, 25, 50 mg) or placebo, in addition to standard treatment on urinary albumin excretion rate. SPP301 decreased urinary albumin excretion rate significantly, whereas total cholesterol also decreased significantly. Creatinine clearance and BP were unaffected. The main adverse effects were peripheral edema and headache. Because it was administered concomitantly with ACEi, SPP301 presents additional benefits toward treating this disease.

Completed Clinical Trials with Selective ERA-A (See Tables 2 and 5).

The purpose of this Phase II study was to examine the efficacy and safety of four doses of ambrisentan in patients with iPAH of various etiologies (associated with collagen vascular disease, anorexigen use, or HIV). This double-blind, dose-ranging (four doses) study revealed an improvement for all primary efficacy end points: 6-min walk distance (6MWD; +36.1 m, P < 0.0001), Borg dyspnea index, WHO functional class, and cardiopulmonary hemodynamics, such as mean PAP (−5.2 mm Hg, P < 0.0001) and CI (+0.33 liters min⁻¹ m⁻² P < 0.0008). Adverse events, such as elevated liver function tests, were mild and unrelated to the dose, but peripheral edema reached 25% (see section below; Table 5; Ref. 93).

A long-term study of subjects who participated in AMB-220 has demonstrated durable efficacy and safety for more than two years.

ARIES-2 with Ambrisentan.

This was a Phase III, randomized, double-blind, placebo-controlled, multicenter, efficacy and safety studies in subjects with PAH (Table 7). This study evaluated two doses of ambrisentan 2.5 and 5 mg per day in 186 PAH subjects (62 subjects/arm). The primary efficacy endpoint was the change from baseline in the 6MWD evaluated after 12 weeks of therapy compared to placebo. Secondary endpoints include Borg dyspnea index, WHO functional class, the SF-36 health survey, and time to clinical worsening. Results of the trial demonstrated that both doses of ambrisentan significantly improved the placebo-corrected mean 6MWD (32.3 m and 59.4 m for the 2.5 mg and 5 mg dose groups, respectively). Significant improvements in time to clinical worsening compared to placebo were also observed for both doses (Table 5). The most frequent adverse event was headache, which occurred in 12.7% of patients in the 5 mg dose group and 7.8% in the 2.5 mg dose group, compared to 6.2% in the placebo group. No patients treated with ambrisentan developed serum aminotransferase concentrations greater than three-times the upper limit of the normal range and ambrisentan had no apparent effect on the activity or dosage of warfarin-type anticoagulants.

Edonentan.

BMS-207940, a second-generation analog of BMS-193884, entered Phase I clinical trials by April 2002. Filing for NDA was expected to take place in 2004, aimed at CHF (148). No news has emerged from these trials and the program was seemingly dropped (the Bristol-Myers Squibb website does not reveal any information).

EARTH with Darusentan.

The Endothelin-A Receptor Antagonist Trial in Heart Failure (EARTH) study aimed at measuring the effects of long-term ET blockade on left-ventricular (LV) remodeling and clinical outcomes in patients with CHF (Table 5). In this study, 642 patients with NYHA Class II–IV CHF (receiving either an ACEi, beta blocker or an aldosterone antagonist) were randomized to treatment with darusentan or placebo over 24 weeks (99). The primary endpoint was the change in left ventricular end systolic volume (LVESV) measured by magnetic resonance imaging. No significant difference was seen in left ventricular end systolic volume (assessed in 485 (76%) paired magnetic resonance imaging data at baseline and 6 months) from that following placebo treatment for any dose of darusentan. Furthermore, there was no difference seen in terms of mortality or the progression of CHF, even though heart failure worsened in 71 (11.1%) patients, and 30 (4.7%) died during the study. Circulating plasma concentrations of ET-1 increased dose-dependently in all groups receiving darusentan. Darusentan did not improve cardiac remodeling (Table 5).

HEAT-HTN with Darusentan.

The Hypertension Endothelin Antagonist Treatment (HEAT) study was a randomized, double-blind, placebo-controlled, multicenter, parallel-group study that investigated the antihypertensive efficacy and safety of darusentan in subjects with moderate essential hypertension (Tables 2 and 5). A total of 392 subjects were randomized to placebo or one of three doses of darusentan (10, 30, or 100 mg) for 6 weeks. The study results demonstrated a statistically significant and dose-dependent decrease in systolic and diastolic BP as compared to placebo. The mean, placebo-corrected change from baseline in systolic BP was −6.0 mm Hg on 10 mg, −7.3 mm Hg on 30 mg, and −11.3 mm Hg on 100 mg darusentan, following the 6-week treatment period. Significant placebo-corrected reductions in diastolic BP were also observed (−3.7, −4.9, and −8.3 mm Hg, respectively). Darusentan was well tolerated in this subject population.

A pre-“CONSCIOUS-1” Trial with Clazosentan.

The pre-“Clazosentan to Overcome Neurological Ischemia and Infarct Occurring after Subarachnoid Hemorrhage (CONSCIOUS)” study is a Phase II-a study that has just been completed investigating whether clazosentan prevents cerebral vasospasm in patients following aneurysmal subarachnoid hemorrhage (Ref. 97; Tables 2 and 5), and its safety and tolerability profile. Treatment reduced the incidence and severity of angiographically-evident cerebral vasospasm as well as the incidence of new infarctions.

STRIDE-1 to STRIDE-6.

Early nonpivotal clinical studies with sitaxsentan included: (i) a double-blind, placebo-control, acute hemodynamic study in 48 CHF patients (103), (ii) a 31-patient trial in essential hypertension (104), and (iii) an open-label trial in 20 patients with PAH (including six children; Tables 2, 5 and 7; Refs. 105, 111).

The first large-scale randomized clinical trial (STRIDE-1) by Encysive was aimed at patients with pulmonary arterial hypertension (PAH; Tables 2, 5 and 7). Overall, six STRIDE studies have targeted primary and secondary PAH patients, with doses of sitaxsentan ranging from 50 to 300 mg (150). In addition, four long-term extension studies, using sitaxsentan in patients for up to 58 weeks, have been completed (150). A small 1-year follow-up study to STRIDE-1 showed persistent improvement (151). STRIDE-6 (an open-label pilot) was designed to evaluate sitaxsentan treatment for patients with PAH who were discontinuing bosentan treatment because of safety or efficacy failure. The company completed the submission of an NDA with the FDA in May 2005 for Thelin (sitaxsentan) 100 mg as once-daily oral treatment for patients with PAH. The NDA contains the largest database ever assembled in a regulatory filing for PAH, with approximately 900 PAH patients receiving Thelin in clinical evaluations. In August 2005, the European Agency for the Evaluation of Medicinal Products accepted for review the company’s Marketing Authorization Application for Thelin. It is unclear to date whether selectivity for the ET_A-R confers superior effects against PAH compared to dual ERA. The safety profile (e.g., incidence of liver enzymes abnormalities) might prove to be better. Only a direct head-to-head comparison (dose-related efficacy with a surrogate marker, patient selection, and primary and secondary end points) would answer this issue.

YM598 by Astellas Pharma (Formerly Yamanouchi).

In a Phase II multicenter clinical trial, YM598, added to mitoxantrone and prednisone, failed to control pain in hormone-refractory prostate cancer and prostatic neoplasms. The first record was received in November 2002 and terminated in June 2005, with no significant signs of improvement in the pain associated with prostate cancer metastases in the bone (Tables 2 and 5; www.clinical-trials.gov).

ZD4054 by AstraZeneca.

In a randomized placebo-controlled trial in eight healthy subjects, a single oral dose of ZD4054 reduced forearm vasoconstriction in response to brachial artery infusion of ET-1, thus providing clinical relevance of ET_A blockade. ET_B blockade was also assessed in an ascending, single-dose, placebo-controlled trial in 28 volunteers. For all doses of ZD4054, mean plasma ET-1 concentrations measured at 4 and 24 hrs were within the placebo reference range (indicating no significant ET_B receptor blockade). There was no evidence of dose-related changes (Tables 2 and 5; Ref. 108). ZD4054 was then developed for prostate cancer with bone metastases. A Phase II-a trial has also been completed (see Tables 2 and 5).

BENEFIT with Bosentan.

Bosentan Effects in Inoperable Forms of Chronic Thromboembolic Pulmonary Hypertension (BENEFIT) is a randomized, placebo-controlled, Phase III trial aimed at determining the efficacy and safety of bosentan in patients with chronic thromboembolic pulmonary hypertension (CTEPH). Various forms of pulmonary embolism are associated with PAH for which the role of the ET system has recently been reviewed (154).

Already, two open-label multicenter pilot trials have been conducted in 19 and 16 inoperable cases of CTEPH (155, 156). The first study found that following 3 months of treatment with bosentan, PVR decreased, whereas functional class and peak maximum rate of oxygen consumption remained unchanged. However, the 6MWD increased. Treatment was well tolerated by all patients. In the second study, patients were treated over 6 months. NYHA functional class was improved by one class in 11 patients. Mean 6MWDs increased. Regardless of uncontrolled design and small sample size, bosentan may offer a therapeutic option for patients with inoperable CTEPH.

BREATH-2 with Bosentan and Flolan.

This Phase II randomized, double-blind, placebo controlled trial will be conducted by CoTherix in 60 patients (Table 6), with a treatment arm combining bosentan with existing prostanoid therapy (iv epoprostenol (Flolan)). The combination of ERAs (at a lower dose) with established therapeutic agents has always been on the agenda toward increasing efficacy and reducing the extent of side effects. Other future options to be considered and tested include investigating the effects of combined treatment of bosentan with other prostacyclin preparations such as Treprostinil (sc) or Beraprost (inhaled Iloprost; see below).

Toward that end, a prospective, nonrandomized, open-label study in a university hospital setting has recently been reported. The addition of bosentan led to an increased exercise capacity (6MWD) and right ventricular function (Tei index) in 16 patients with PAH already receiving either Beraprost (inhaled Iloprost) or Iloprost iv at 6 months after initiation of combination therapy, and every 3 months thereafter (157). Another limited study revealed that bosentan was effective at replacing inhaled Iloprost, as reflected by early and sustained positive hemodynamic changes, precluding the return to previous therapy (158). This case report (n =2) was the first to document a switch of that sort.

BREATH-3 with Bosentan.

The safety and efficacy of bosentan therapy in children suffering from severe PAH has been demonstrated in two retrospective observational studies (159–161). The group of Maiya et al. (159) reported that in 40 children (mean age of 9.3 years, range 0.6–16 years) with various forms of PAH (class III and IV), bosentan was well tolerated and helped stabilize the children. However, parenteral epoprostenol was also needed in 60% of cases. In another retrospective study recently published (160), 86 children with PAH (idiopathic, associated with congenital heart or connective tissue disease) started bosentan with or without concomitant intravenous epoprostenol or subcutaneous Treprostinil therapy. Bosentan improved WHO functional class in 46% of cases and also decreased mean PAP and PVR. In addition, overall it was safe and efficacious for the treatment of PAH in children after a median exposure of 14 months.

Actelion is now planning a large Phase III trial of bosentan treatment in children with PAH.

BREATH-5 with Bosentan.

A Phase III clinical trial has been conducted with bosentan looking at PAH in patients associated with Eisenmenger’s syndrome (e.g., with congenital heart disease [CHD]), a target identified in 1993 by Cacoub et al. (Tables 2 and 6; Ref. 162). Initial experience with bosentan in such patients (n = 9, started on 62.5 mg/day for 4 weeks, than 125 mg bid; Ref. 163), revealed that six of nine patients (67%) had an improvement in NYHA classification and that oxygen saturation levels increased. In another open-label pilot study in 10 patients, similar results were obtained (164). The first open-label, prospective, multicenter trial (Phase II-a; n =33 patients/n = 23 with the syndrome) revealed that, after 2.1 ± 0.5 years of treatment, bosentan increased 6MWD and improved NYA class, in association with slight trends in improvement of transcutaneous oxygen saturation and maximum oxygen uptake, as well as decreased right ventricular systolic pressure (165).

BUILD-1 and -2.

Bosentan Use in Interstitial Lung Disease (BUILD) is a Phase III clinical trial that will investigate the safety and efficacy of bosentan in patients suffering from idiopathic pulmonary fibrosis (IPF), a chronic fibrosing lung disease limited to the lungs, with an estimated prevalence of 35,000–55,000 annual cases in the United States. Conventional therapy (corticosteroids, azathioprine, cyclophosphamide) provides only marginal benefit. Emerging strategies to treat patients with IPF include agents that inhibit epithelial injury or enhance repair, anticytokine approaches, agents that inhibit fibroblast proliferation or induce fibroblast apoptosis, and other novel approaches such as ERAs. The BUILD program with bosentan in patients suffering from either IPF (BUILD-1) or pulmonary fibrosis related to systemic sclerosis (BUILD-2), showed no effect on the primary endpoint of exercise improvement as measured by the 6-min-walk test (MWT). In the IPF study, BUILD-1, although not statistically significant, positive trends were observed for predefined secondary endpoints, such as the combined incidence of death or treatment failure at 12 months (36.1% in the placebo group vs. 22.5% in the bosentan group; P = 0.076; 95% confidence limits (CL) 0.37, 1.05), representing a relative risk reduction of 38%. Treatment failure (per protocol) was defined as worsening in pulmonary function tests (PFTs) or acute decompensation of IPF. Actelion intends to pursue a mortality-morbidity Phase III study.

COMBI.

Combination of Bosentan and Iloprost (COMBI) is a Phase IV trial, reminiscent of the BREATH-2 study, is to be conducted at the Hannover Medical School with 72 patients. It will assess the combined efficacy of bosentan with existing Iloprost in patients with iPAH (Table 6).

GRH01 with TBC 3711 by Encysive.

This is a dose-ranging Phase II-a clinical trial initiated in 2005 in patients with resistant hypertension (Table 7). Darusentan (another ERA-A), and a Phase II-b trial named DAR-201 was previously conducted in patients with resistant hypertension (see previous section).

Safety and Pharmacotoxicology of ERAs

Summary and Conclusions

As put forward by Lee and Rubin (178), the lack of absolute significant improvement in clinical parameters in studies involving ERAs does not equate to: FAILURE (i.e., the latest Phase III trial conducted with atrasentan in prostate cancer). Rather than causing an overt improvement, ERA treatment might result in stabilization or slowing of the rate of disease progression. Studies over longer periods, albeit more costly, may reveal such effects.

Bosentan, the most studied, both preclinically and clinically, has a good safety profile (as shown by the TRAX database, made up of over 5000 treated patients in Europe) with an incidence of abnormal LFT of 7.4%. In long-term follow-up studies (after 1, 2, and 3 years), bosentan treatment improved survival (estimates of 96%, 89%, and 86%, respectively (179) when compared to the NIH-PPH registry equation predictor (180). Pediatric experience over 2 years has also revealed a 91% survival rate in this sensitive population (181). In the context of polypharmacy related to cases of PAH, Actelion is also pioneering trials to compare bosentan with various prostacyclin analogues and Sildenafil.

By detailing all completed, stopped, and ongoing clinical trials involving ERAs, in even further detail than what we are presenting here, lessons can be learned in terms of dosing, efficacy, toxicity, and receptor selectivity. In this way the design of new trials can be improved.

Perspectives

Preclinical studies conducted in several experimental animal models of disease suggest that a number of therapeutic targets involving the dysregulation of the ET system remain to be studied in clinical trials. Such targets include other forms of PAH associated with connective tissue diseases such as lupus, rheumatoid arthritis, derma-topolymyositis, and Sjogren’s syndrome (94). ERA could also be developed as a novel pharmacotherapy for chronic obstructive pulmonary disease, which is associated with pulmonary artery intimal thickening and vessel narrowing; chronic obstructive pulmonary disease has excess ET receptor expression and elevated circulation plasma concentrations of ET-1. Additional targets include cancer (bone, lung, ovarian); renal diseases (hepatorenal syndrome [in which indication, however, tezosentan was evaluated and stopped in the second quarter of 2003] and chronic renal insufficiency); and cardiovascular-remodeling conditions (various other forms of hypertension, including nonidiopathic related forms of PAH, CHF, and acute myocardial infarction). They also include obesity-related Type 2 diabetes and oxidative stress, respiratory-fibrotic-hypoxic diseases (allergic asthma, and also sarcoidosis-related PAH, for which bosentan may constitute an effective treatment; Ref. 182), pain control, various diseases of the eye, and subarachnoid hemorrhage (as detailed in articles in these Proceedings of the Ninth International Conference on Endothelins). ERAs might also offer benefit in preventing complications of certain treatments, such as nephrotoxicity following the injection of radio contrast, rejection after graft transplantation, restenosis postangioplasty, and cardiorenopathy and edema following administration of blood substitutes.

For instance, bosentan could be used as a bridging therapy before (heart)-lung transplantation in patients with severe left ventricular dysfunction secondary to iPAH; such a treatment might improve ventricular function, making the patient more amenable to transplantation (to the point that the heart component of the whole approach is no longer necessary; Ref. 183). ERAs could be added to transplant solution for organ stabilization and preservation, and also to reduce inflammation, prevent post-transplant ischemia-reperfusion injury and some of cyclosporine-mediated (or other antirejection therapy) side effects, nephrovascular-related side effects, post-transplant complications, and transplant failure.

For many of these potential applications, the presence of existing therapeutic options means that the financial incentives for pharmaceutical companies to launch expensive and decisive clinical trials are currently lacking. Furthermore, benefit from ERAs must be shown in addition to treatment with established therapy. For example, there is a reluctance to develop ERAs as an antihypertensive treatment, despite many patients having high BP that is resistant to multidrug treatment, though two “non-big pharmaceutical” companies are doing so. By learning lessons from previous clinical trials, summarized in this review, we hope that the design of future studies of ERAs in these new potential therapeutic areas will be improved.

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Table 1.

List of Endothelin Receptor Antagonists (ET-Ra = ERA) that Underwent Extensive Preclinical Development Leading to Clinical Studies and/or Further Drug Development.

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Table 2.

Summary of Clinical Trials Conducted Using an Endothelin Receptor Antagonist (ERA).

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Table 3.

A Selection of Completed and Ongoing Clinical Academic Studies and Formal Trials Conducted with ET-1 and Endothelin Receptor Antagonists (ERAs) in Healthy Control Subjects and Patients.

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Table 4.

Completed Clinical Trials Conducted with Dual Endothelin Receptor Antagonists (dual (mixed) ERAs).

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Table 5.

Completed Clinical Trials Conducted with Selective Endothelin Receptor Antagonists to the ET_A Subtype (ERA-A).

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Table 6.

Ongoing and/or Planned Clinical Trial Studies Undergoing Recruitment of Dual Endothelin Receptor Antagonists (dual (mixed) ERAs).

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Table 7.

Ongoing and Planned Clinical Trial Studies Undergoing Recruitment with Selective Endothelin Receptor Antagonists (ERAs) of the A Subtype.

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Figure 1.

(a) Structures of the first generation of peptidic ERA.

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Figure 1.

(b) Chemical structures of selective ERA-A.

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Figure 1.

(c) Chemical structures of dual (mixed) ERA.