Endothelin Receptor Antagonists in the Treatment of Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a life-threatening disease characterized by a progressive increase in pulmonary artery pressure and pulmonary vascular resistance, leading to right ventricular failure and death.¹ The recent World Health Organization (WHO) classification designates PAH as Group I and includes idiopathic PAH (IPAH)—formerly known as primary pulmonary hypertension (PPH)—familial PAH, and PAH associated with various systemic disorders such as collagen vascular diseases, congenital systemic-to-pulmonary shunts, portal hypertension, anorexigen use, and HIV infection.²

Whether idiopathic or related to a systemic disorder, the histological appearance of lung tissue in each of these conditions shares common features, including intimal fibrosis, increased medial thickness, pulmonary arteriolar occlusion, and formation of plexiform lesions.1 Left untreated, PAH is associated with poor outcome.3 Significant advances in understanding the molecular mechanisms in PAH have delivered insight into the key role played by endothelial dysfunction in initiating and propagating the pulmonary vascular remodeling process: specifically, impaired production of vasoactive mediators, such as prostacyclin and nitric oxide, along with chronic overproduction of vasoconstrictors such as endothelin-1.^4-6 The biology of endothelin-1 (ET-1) and the effect of endothelin receptor blockade in the treatment of PAH will be the focus of this review.

The Biology of ET-1 and Endothelin Receptors in PAH

Since its initial discovery by Yanagisawa and co-workers in 1988, ET-1 has been extensively studied and is recognized as a key mediator of pulmonary vascular biology and pathophysiology in PAH.⁷ It is a 21-amino peptide predominantly produced by endothelial cells, though it can be also produced by smooth-muscle cells, leukocytes, macrophages, cardiac myocytes, and mesangial cells.⁸ The effects of ET-1 include vasoconstriction, and it is recognized as the most potent vasoconstrictive agent discovered to date.⁷ It also acts as a strong mitogen by stimulating production of several growth factors, and it induces myocardial fibrosis and hypertrophy, as well as vascular fibrosis with extracellular matrix proliferation.⁹ Furthermore,ET-1 is also produced in leukocytes, where it increases the production of cytokines in the inflammatory response.¹⁰ In PAH, not only are the plasma levels of ET-1 increased, its level is also inversely proportional to the magnitude of the pulmonary blood flow and cardiac output, suggesting that these hemodynamic changes are influenced directly by this vascular effector.^5,11,12

ET-1 exerts its vascular effects through activation of the ET_A and ET_B receptors.¹³ ET_A receptors are located on smooth-muscle cells, whereas ET_B receptors are found predominantly on vascular endothelial cells and, to a lesser extent, on smooth muscle cells. Activation of the ET_A and ET_B receptors on smooth-muscle cells induces vasoconstriction and cellular proliferation and hypertrophy. In contrast, stimulation of ET_B receptors on endothelial cells results in production of the vasodilators nitric oxide and prostacyclin. In addition, ET_B receptors are involved in the clearance of ET-1 from the circulatory system, particulary in the lungs and kidneys.^13-15 In a non-diseased state, a regulated balance is maintained between the production and clearance of ET-1 to keep the circulating level low, a process that is mediated by ET_B.

Selective versus Dual Endothelin Receptor Antagonists in PAH

Dual ET_A and ET_B receptor blockers and selective ET_A receptor antagonist have been developed for treatment of PAH.^16-21 The proposed rationale for utilizing selective ET_A receptor blockades stems from the observation that there may be advantages for selectively blocking the vasoconstrictive and proliferative effects of ET_A receptor only while maintaining the vasodilator and ET-1 clearance function of the ET_B receptors.²² Conversely, blocking both ET_A and ET_B receptors has been stated as necessary to completely negate all the deleterious effects of ET-1 on pulmonary vasculature, since ET_B receptors have been reported to be upregulated in certain disease states (scleroderma lung disease,²³ congenital heart disease,²⁴ and thromboembolic disease²⁵).

Indeed, evidence exists to support the effectiveness of both strategies in animal and human studies. In a rat model of PAH, both selective ET_A receptor antagonists and dual ET_A/ET_B receptor blockers have been shown to modify pulmonary vascular remodeling, which occurs in response to chronic hypoxic exposure. Furthermore, both groups of animals demonstrated reversal of pulmonary vascular remodeling with their respective therapies.^26,27 Similarly, studies have been performed on healthy volunteers and on those with vascular diseased states that demonstrate superiority of one type of receptor antagonist over the other.With ET_A-selective antagonist sitaxsentan, the degree of vasodilation was shown to be greater with selective antagonists than with dual-receptor antagonists.²⁸

Conversely, the dual ET_A/ET_B receptor antagonist bosentan was demonstrated to cause more percent change in forearm blood flow in hypertensive patients than the selective ET_A blockade.²⁹ Further complicating the question is the fact that the spatial distribution of the upregulated ET_B receptors in certain disease states—whether the receptors are increased in endothelial cells or smooth-muscle cells—has not been well elucidated.A recent study has also reported finding that ET_B receptor-mediated extraction of ET-1 is highly preserved, and that high circulating ET-1 levels in PAH patients must be predominantly due to excess synthesis rather than reduced clearance.³⁰ Indeed, whether selective or dual endothelin receptor antagonism will provide the most benefit in patients with PAH remain to be seen.

The Clinical Efficacy and Safety of Endothelin Receptor Antagonists in PAH

Bosentan (Tracleer™), a dual ET_A/ET_B endothelin receptor antagonist, was the first oral therapy to be approved for treatment of PAH. The first multicenter study by Channick et al. reported the result of a 12-week randomized, placebo-controlled, double-blind trial in 32 patients with IPAH (n=27) and PAH associated with scleroderma (n=5).¹⁶ The participants were all WHO functional class III and there was 2:1 randomization to the bosentan group in relation to placebo. Patients in the bosentan group received the drug at a dose of 62.5mg twice a daily for four weeks followed by 125mg twice daily. A 70m improvement was seen in the six-minute walk distance (6MWD) at 12 weeks, whereas it worsened by 6m for the patients receiving placebo (p<0.05). Treatment with bosentan also improved hemodyanmics (increased cardiac index and decreased pulmonary vascular resistance and mean pulmonary arterial pressure) and functional class.

In a subsequent study—Bosentan Randomized Trial of Endothelin Antagonist Therapy (BREATHE-1)—213 patients with IPAH (n=150) or PAH related to scleroderma (n=47) or systemic lupus erythematosus (n=16) were enrolled.The majority of the patients were in WHO functional class III (94% in placebo and 90% in treatment group) and a few were in functional class IV.The mean 6MWD was about 330m and the mean pulmonary artery pressure was about 55mmHg. Patients were randomly assigned to receive placebo or bosentan titrated to a target dose of either 125mg twice daily or 250mg twice daily for 16 weeks.¹⁷ At the end of the treatment period, 6MWD improved by 36m in the bosentan group, whereas a deterioration of 8m was seen in the placebo group.

The differences were statistically significant for both the 125mg and 250mg groups; there was no significant difference between the two bosentan groups. Patients treated with bosentan also had improvement in the time to clinical worsening, defined as death, lung transplantation, hospitalization for pulmonary hypertension, lack of improvement or worsening leading to discontinuation, or need for epoprostenol therapy or atrial septostomy.¹⁷

The primary safety issue in prescribing bosentan relates to its effect on liver function tests. Bosentan is primarily metabolized by hepatic metabolism through the P450 enzyme systems CYP2C9 and CYP3A4. In the BREATHE-1 study, there was a dose-dependent increase in hepatic transaminases noted with a significant elevation in 14% of the patients randomized to 250mg bosentan and 4% among patients receiving 125mg. The liver abnormalities were often asymptomatic and all resolved with dose reduction or cessation. Animal studies have demonstrated that bosentan-induced liver injury is likely mediated by drug-induced inhibition of the hepatocanalicular bilesalt export pump.³¹ All patients receiving bosentan are required to have liver function tests before drug initiation, and then monthly. Patients with baseline hepatic dysfunction should not treated with bosentan.

Bosentan is teratogenic, so is contraindicated in pregnancy, and pregnancy must be excluded before therapy initiation.Women of childbearing age must be counseled that hormonal forms of contraception cannot be relied on as sole form of contraception while on bosentan. Several drugs have been shown to have significant interaction with bosentan through the P460 system, and glyburide and cyclosporine A are contraindicated with bosentan therapy. No clinically significant effect has been reported on warfarin level using the approved doses.

Bosentan received approval in November 2001 for treatment of PAH for WHO functional class III or IV symptoms to improve exercise capacity and decrease the rate of clinical worsening. Since then, data on the long-term efficacy of bosentan have emerged. Sitbon et al. reported the result of an open-label extension of the initial double-blind, placebo-controlled study of 32 patients.³² Twenty-nine patients received bosentan for an additional year (62.6mg twice a day for four weeks and then 125mg twice daily). Patients continuing bosentan treatment maintained the improvement in walk distance observed at the end of the initial study and patients starting bosentan improved their walk distance by 45±13m. Hemodynamic measurements performed on 11 patients after a mean of 15 months of therapy demonstrated significant improvement in cardiac index and pulmonary vascular resistance. Overall, bosentan treatment was well tolerated in this cohort.There were no deaths during the course of the study and one patient was initiated on epoprostenol therapy due to worsening PAH.

McLaughlin et al. presented survival data in 169 patients treated with bosentan as first-line therapy.³³ The Kaplam-Meier survival estimate at two years was 89%.The predicted two-year survival of these patients based on the NIH equation was 57%. In this study, 70% of the patients remained on bosentan monotherapy while 23% of patients received additional therapies or were transitioned to prostanoid-based therapies. The rate of liver function test abnormality reported was 14.9%, although no serious irreversible hepatic dysfunction occurred. This long-term observation emphasizes the importance of following patients every month for this potentially serious adverse event, and that for stable functional class III patients with PAH it is reasonable to consider bosentan as a first-line therapy.

Selective ETA Receptor Antagonist

Sitaxsentan is a selective ET_A antagonist currently under review by the FDA for treatment of PAH. Sitaxsentan sodium is approximately 6,500-fold more selective as an antagonist for ET_A compared with ET_B receptors. In the Sitaxsentan To Relieve Impaired Exercise Trial (STRIDE-1), 178 New York Heart Association (NYHA) functional class II (33%), III (66%), and IV (1%) patients with either IPAH (53%) or PAH related to connective tissue disease (24%) or congenital systemic-to-pulmonary shunts (24%) were randomized equally to receive placebo, sitxsentan 100mg or 300mg orally once daily.¹⁹ The primary efficacy end-point of maximum oxygen consumption was not improved by sitaxsentan compared with placebo; however, the 6MWD improved for both the 100mg (35m; p<0.01) and 300mg groups (33m; p<0.01). Improvements in the NYHA functional class and hemodyanmic parameters also resulted from treatment with sitaxsentan.

Liver function abnormalities were also noted with sitaxsentan therapy in a dose-dependent manner. In the 12-week trial, the incidence of hepatic enzyme elevations (aminotransferase values >3 times upper limit of normal) was 3% (2/59) for the placebo group, 0% for the 100mg group, and 10% (6/63) for the 300mg group. All abnormalities were reversed in this study group; however, in an earlier pilot study, sitaxentan was associated with fatal hepatitis when used at higher doses.³⁴ The investigators note that when the placebo-controlled trial results were combined with an extension trial that randomized all patients to receive sitaxsentan 100mg or 300mg, the incidence of liver function abnormalities increased to 5% (4/77) for the 100mg group and 21% (19/91) for the 300mg group for the duration of therapy (median 26 weeks).¹⁹

The other significant laboratory abnormality reported was an increase in international normalized ratio (INR) with both doses of sitaxsentan. This is because sitaxsentan is an inhibitor of CYP2C9 P450 enzyme, the principal hepatic enzyme involved in the metabolism of warfarin. The investigators reported the need to reduce the dose of warfarin when patients are receiving concurrent therapy with sitaxsentan.

Recently, STRIDE-2 was published, which addressed some unresolved questions from the STRIDE-1 trial.²⁰ First, since the safety profile was not acceptable with the 300mg dose, the optimal dose on the basis of risk and benefit considerations were studied. This was a double-blind, placebo-controlled, 18-week study of 245 PAH patients (IPAH 59%; connective tissue disease 30%; congenital heart disease 11%) randomized to receive placebo (n=62); sitaxsentan 50mg (n=62) or 100mg (n=61), or open-label bosentan (n=60). The placebo-subtracted treatment effects were 24.2m (p=0.07) for the sitaxsentan 50mg group, 31.4m (p=0.03) for the 100mg group, and 29.5m (p=0.05) for the bosentan group.

The incidence of abnormal transaminases (>3 times the upper limit of normal) was 6% for placebo, 5% for sitaxsentan 50mg, 3% for sitaxsentan 100mg, and 11% for bosentan.The warfarin dose was reduced by 80% for patients not randomized to bosentan at baseline, with adjustments as needed to maintain a therapeutic INR. Using the decreased dosing schedule, the proportion of patients with INR >3.5 was comparable for the sitaxsentan and placbo groups (33% versus 42%) and lower for the bosentan group (20%; p=0.17). From an efficacy standpoint, a direct comparison between bosentan and sitaxsentan cannot be made since bosentan therapy was given in open-label for observational purposes only; however, it is interesting to note that the 6MWD was similar between the 100mg sitaxsentan- and bosentan-treated patients. In terms of safety issues, the incidence of liver function abnormalities was lower for the 100mg sitaxsentan than bosentan patients; however, the episodes of increased INR were greater for the sitaxsentan-treated group despite lowered warfarin doses, although the bleeding events were reported to be rare.

Ambrisentan is a second selective ET_A receptor antagonist currently under evaluation. A study examining efficacy and safety demonstrated that at 12 weeks all four doses (1, 2.5, 5, or 10mg) of ambrisentan increased 6MWD (+36.1m; p<0.0001) which were statistically significant for each dose group.²¹ Improvements were also reported in WHO functional class and hemodynamic parameters. The incidence of liver function abnormalities >3 times the upper limit of normal were low (3.1%) and no clinically relevant changes were observed for INR or dose of anticoagulation therapy. Ambrisentan is currently in phase III placebo-controlled trial.

Endothelin Receptor Antagonists as Part of Combination Therapy

Endothelin receptor antagonists have been considered as part of combination therapy for treatment of PAH. One of the main rationales behind considering combining therapies is to target multiple pathophysiological pathways in patients with suboptimal response to monotherapy with the hope of improving efficacy and outcome. Theoretically, the potential to obtain synergistic action by combining certain classes of drugs and/or lessening side effects by being able to use lower doses with the same degree of efficacy are desirable approaches. Several studies have been conducted studying efficacy and safety of combination therapies in PAH patients, and two studies (BREATHE-2 and STEP) investigated the safety of treating with bosentan and a prostacyclin-based regimen.³⁵ Whether there will be a difference in efficacy or long-term outcome in utilizing selective versus dual endothelin receptor antagonists with other therapies needs to be investigated. All the studies to date, however, have included only small numbers of patients and thus it is difficult to draw any firm conclusions regarding use of the combination approach. There are several large studies currently underway to determine if the theoretical benefits of combining therapy hold true.

Conclusion

PAH is a deadly disease marked by adverse remodeling within small pulmonary arteries and progressive pulmonary vascular constriction leading to right heart failure and death. Approval of the dual endothelin receptor antagonist bosentan has provided a tremendous breakthrough in the treatment of PAH, not only as the first oral therapy to be available, but also by being a new class of drug, providing a way of targeting a different pathway in fighting the progression of disease and opening doors for combining therapies. With selective endothelin receptor antagonists likely to be available soon, the future of treatment for PAH holds great promise.