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. Author manuscript; available in PMC: 2011 Nov 28.
Published in final edited form as: Br J Haematol. 2009 Sep 22;147(5):737–743. doi: 10.1111/j.1365-2141.2009.07906.x

Endothelin receptor antagonists for pulmonary hypertension in adult patients with sickle cell disease

Caterina P Minniti 1, Roberto F Machado 2, Wynona A Coles 3, Vandana Sachdev 4, Mark T Gladwin 5, Gregory J Kato 1
PMCID: PMC3225273  NIHMSID: NIHMS333033  PMID: 19775299

Abstract

Pulmonary Hypertension is a serious complication of sickle cell disease (SCD), with a high risk of morbidity and mortality. Endothelin-1, a potent vasoconstrictor peptide known to be elevated in SCD, acts through two receptors: ETR-A and ETR-B. Bosentan and ambrisentan are ETR blockers approved for use in primary pulmonary hypertension. We report on the use of ETR blocking agents in a cohort of 14 high-risk adult patients with SCD and pulmonary hypertension. Patients underwent right heart catheterization, 6-min walk test, echocardiogram, physical examination and laboratory studies prior to starting ETR blocking agents. Six patients received bosentan and two ambrisentan as initial therapy. Six additional patients were already on sildenafil when ETR blocking agents were added. Over at least six months of therapy, sequential measurements of 6-min walk distance increased significantly (mean ± standard error baseline 357 ± 22 m, 1–2 months 367 ± 17 m, 3–4 months 387 ± 16 m, 5–6 months 398 ± 18 m, n=12, p<0.05, ANOVA with repeated measures). Nonsignificant downward trends were also observed for amino-terminal brain natriuretic peptide and tricuspid regurgitant velocity. Pulmonary artery mean pressures decreased in the three patients that had a repeat right heart catheterization from a mean of 44 to 38 mmHg, although this was not statistically significant. Adverse events while on ETR blocking therapy were similar to the ones reported on patients with primary PH: increase in serum alanine aminotransferase (2), increased peripheral edema (3), rash (4), and headache. Therapy was stopped in two patients, who were switched to the other ETR blocking agent, with resolution of symptoms. These data suggest preliminary evidence for benefit from the use of bosentan and ambrisentan and supports their use in pulmonary hypertension in SCD.

INTRODUCTION

Pulmonary hypertension (PH) is a recently recognized complication of sickle cell disease (SCD)(Gladwin, et al 2004). Modestly elevated pulmonary artery pressures are seen in 24% of patients, with overt PH in approximately 9%. This is a severe condition with a two-year mortality rate as high as 50% (Castro, et al 2003). The pathogenesis of PH in SCD is likely to be multi-factorial, with systemic scavenging of nitric oxide by free plasma hemoglobin; endothelial dysfunction resulting in pulmonary vascular intimal hyperplasia; chronic hypoxemia exacerbated by sleep hypoventilation; excessive platelet activation (Villagra, et al 2007, Wun, et al 1997) and asplenia, all contributing to its development. Current treatments for SCD such as Hydroxycarbamide and/or chronic red cell transfusions have shown no consistent benefit in reducing pulmonary artery pressure, but two recent case series report improvement in tricuspid regurgitant velocity (TRV) in patients with mildly elevated pulmonary artery systolic pressures (Olnes M 2009, In Press, Pashankar, et al 2009). Endothelin-1 (ET-1) is a potent vasoconstrictor (Yanagisawa, et al 1988), which can also promote fibrosis, cell proliferation, remodeling and inflammation. ET-1 concentrations in tissues and plasma are increased in PH suggesting a pathogenic role of ET-1 in this disease (Giaid, et al 1993). In primary PH and heart failure elevated ET-1 concentrations are strongly correlated with severity of disease and prognosis (Galie, et al 2004).

Bosentan is a dual endothelin receptor antagonist with affinity for both endothelin ETR-A and ETR-B receptors. Bosentan competes with the binding of ET-1 to both ETR-A and ETR-B receptors with a slightly higher affinity for ETR-A receptors than for ETR-B receptors. Ambrisentan is a selective ETR-A receptor antagonist that has demonstrated clinical efficacy with low incidence of side effects in patients with primary PH (Barst 2007). Both ETR blockers have been approved in the U.S. and other countries on the basis of data from randomized clinical trials in patients without sickle cell disease (Galie, et al 2008, Rubin, et al 2002).

ET-1 levels are elevated in SCD patients and increase during episodes of acute chest syndrome and vaso-occlusive crises (Graido-Gonzalez, et al 1998, Rybicki and Benjamin 1998) and elevations of ET-1 levels precede the manifestation of clinical symptoms (Hammerman, et al 1997, Rybicki and Benjamin 1998), thus supporting a potential pathogenic role of ET-1 in the development of SCD complications as well as potential value of bosentan and ambrisentan therapy in this patient population. ET-1 modulates the Gardos channel activity in sickle cell mouse and human erythrocytes via the ETR-B receptor (Rivera 2007, Rivera, et al 2002), which might potentially contribute to sickling of the red blood cells. As a result, ETR blockade has been hypothesized to reduce acute vaso-occlusive events and ameliorate the disease process (Graido-Gonzalez, et al 1998). In a mouse model of SCD, bosentan has been found effective in preventing hypoxia-induced mortality and morbidity (Sabaa, et al 2008) and a recent case report has suggested clinical improvement in leg ulceration in a sickle cell patient being treated with bosentan for pulmonary hypertension (Lionnet, et al 2008). We investigated whether treatment with the ETR blocking agents bosentan and ambrisentan, either as single agents or in combination with sildenafil, was tolerated in a high-risk group of adults with SCD and clinical significant PH and whether it improved PH parameters.

METHODS

In order to assess the safety and efficacy of ETR blocking therapy in patients with SCD and pulmonary hypertension, a retrospective chart review was performed to identify patients with sickle cell disease treated at the Clinical Center of the National Institutes of Health from 2005 to 2008 who had received clinically indicated bosentan or ambrisentan therapy. Before initiating ETR blocking therapy, data collected included echocardiographic imaging, hemodynamics obtained at right heart catheterization, 6-min walk test, laboratory analysis, physical examinations, pulse oximeter measurements, NYHA categorization, and Borg dyspnea scores. Echocardiography and right heart catheterization were performed as previously described (Gladwin, et al 2004). For the purpose of this analysis, baseline ETR antagonist therapy measurements were compared to those obtained after 6 to 14 months of therapy (mean of 8.5 months).

Subjects were screened with echocardiographic determination of tricuspid regurgitant jet velocity (TRV), with confirmatory right heart catheterization (Machado, et al 2005). After initiation of ETR therapy, patients' laboratory values were evaluated bi-weekly then every one to three months as required for optimal clinical care. Baseline laboratory analysis included complete blood count, chemistries, LDH, pro-brain natriuretic peptide (NT-proBNP), human immunodeficiency virus, hepatitis A, B and C, and serum pregnancy test in females. Additionally, TRV measurements and 6-min walks were performed every one to three months to observe any efficacy of ETR antagonist therapy on cardiopulmonary function. The 6 minute walk test was performed after measuring TRV to avoid potentially confounding exercise-induced increases in TRV. Pulse oximeter measurements were obtained at each visit. The first three patients on combination therapy with sildenafil and bosentan had a repeat right heart catheterization after six months of ETR blocking therapy.

PATIENT CHARACTERISTICS AND SELECTION

All patients were enrolled in a National Heart Lung and Blood Institute approved natural history human subject protocol (ClinicalTrials.gov # NCT00081523); all subjects provided written, informed consent a subgroup of 6 patients (patients #1 to #6 in table 1) were enrolled in the open label continuation phase of the ASSET trial (ClinicalTrials.gov # NCT00360087) and the results from the randomized, double blind placebo controlled portion of the study will be reported separately. The diagnosis of pulmonary hypertension was definitively established by right heart catheterization, with a pulmonary artery mean pressure greater than 25 mmHg, using baseline hemodynamics information obtained on NIH protocols (ClinicalTrials.gov # NCT00081523 and NCT00352430), followed by 6–14 months follow up on NIH open label bosentan. There were 10 females and 4 males, 36 to 62 years of age. Patients were selected based on severity of pulmonary hypertension. In general individuals in this study had clinically severe symptomatic pulmonary hypertension characterized by significant functional impairment reflected by a NYHA functional class and a low six minute walk distance.

Table 1.

Patients Characteristics and Drug Therapy

Pt. # Hb Age (years) PH Therapy Renal Insufficiency/ Failure HIV O2 Atrial Fibrillation Iron Overload DVT/PE HC or Tx Concomitant Therapy
l SS 36 Bos Yes No No No Yes No Both erythropoietin, insulin
2 SS 50 Bos Yes No No No Yes Yes HC deferasirox, warfarin
3 SC 62 Bos Yes No No No No Yes HC warfarin
4 SS 45 Bos No No No No No No HC warfarin
5 SS 49 Bos No No Yes No Yes No HC erythropoietin, CPAP
6 SS 38 Bos Yes No No No Yes Yes TX warfarin, deferasirox, low mol weight heparin
7 SC 49 Amb No No No No No Yes HC warfarin, CPAP
8 SS 51 Amb No No Yes No Yes Yes HC warfarin erythropoietin
9 SS 58 Sil/Bos Yes No Yes No Yes no HC darbopoietin
10 SS 55 Sil/Bos Yes No Yes No No Yes HC low mol. weight heparin, erythropoietin
11 SS 51 Sil/Bos No Yes No Yes Yes No TX warfarin, deferoxamine HAART
12 SS 46 Sil/Bos No No Yes No No yes HC warfarin
13 SS 42 Sil/Bos No Yes No No No yes TX low mol. weight heparin
14 SS 53 Sil/Amb Yes No No Yes Yes No TX deferasirox warfarin, tacrolimus
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Hb, Haemoglobin phenotype; HbSS, homozygous sickle cell; HbSC, double heterozygous haemoglobin S and C; PH, pulmonary hypertension; Bos, bosentan; Amb, ambrisentan; Sil, sildenafil; HIV, human immunodeficiency virus; O2, oxygen supplementation; DVT, deep vein thrombosis; PE, pulmonary embolus; HC, hydroxycarbamide; TX, transfusion therapy; LMWH, low molecular weight heparin; HAART, highly active anti-retroviral treatment; HC, hydroxycarbamide, TX, transfusion therapy; insulin, patient with diabetes mellitus on insulin therapy; CPAP, patient with obstructive sleep apnea on continuous positive airway pressure at night-time; Tacrolimus for immunosuppressive therapy after liver transplant.

Table 1 shows the patient's characteristics, co-morbidities and concomitant therapies. Twelve patients had homozygous sickle cell disease (HbSS) and two had HbSC disease, determined by high performance liquid chromatography of their hemoglobin.

Co-morbidities included: Renal insufficiency in 7 patients, deep vein thrombosis and/or pulmonary embolism in 8 patients, five patients had severe chronic hypoxia requiring oxygen supplementation, two were HIV positive (one on anti-retroviral medications), 8 patients had severe iron overload and were on chelating therapy, 3 had chronic multifocal osteonecrosis, and two had atrial fibrillation on anticoagulation therapy. Therapies directed at co-morbidities, such as anticoagulation, and iron chelation had been initiated prior to the start of ETR blocking agents and maintained on a stable dose.

PHARMACOLOGICAL THERAPY

The patients' SCD was stabilized as previously described (Machado, et al 2005) as follows: Five patients were placed on chronic transfusion therapy; nine patients received hydroxycarbamide and one patient both (Table 1). Transfused patients received packed red blood cells every three to four weeks, targeted to maintain hemoglobin S level <40%. Hydroxycarbamide was generally titrated to the maximum tolerated dose. Patients' transfusion regimen or hydroxycarbamide therapy was initiated at least three months before starting ETR blocking therapy and was not modified during the ETR blocking therapy. One patient did not receive hydroxycarbamide or transfusion prior to starting bosentan, however, hydroxycarbamide therapy was initiated one week later (patient #4). Six patients were already on oral sildenafil as initial therapy for PH, with ETR blocking agents added for worsening PH (5 with bosentan and 1 with ambrisentan) after a period of 6 to 36 months (mean 20.5 months). These patients remained on a stable dose of sildenafil for the time period of this report. An additional six patients received bosentan and two ambrisentan as initial PH therapy. The starting dose of bosentan was 62.5 mg twice a day for 4 weeks, increased to 125 mg twice a day if no side effects were noted. The starting dose of ambrisentan was 5 mg once daily for 4 weeks, increased to 10 mg once daily if no side effects were noted.

STATISTICAL METHODS

Data represented are mean and standard error of the mean. Data were analyzed as appropriate by student's t-test or one way ANOVA with repeated measures. Statistical significance was assumed at p<0.05.

RESULTS

The clinical characteristics and the PH therapy for all patients are shown in Table I. Right heart catheterization demonstrated the following group characteristics (mean): pulmonary artery mean pressure 38 mmHg; right atrial pressure 8.4 mmHg, cardiac output 7.6 L/min, pulmonary vascular resistance 282 dynes/s per cm3 and pulmonary capillary wedge pressure 14.6 mmHg. PH parameters at baseline and on ETR blocking therapy are shown in Table II. Outcome observations were grouped for analysis in four groups: baseline, 1–2, 2–3 and 5–6 months after initiation of ETR blocking therapy. The 6-min walk distance increased significantly over 6 months, suggesting improved cardiopulmonary functional performance (mean ± standard error 357 ± 22, 367 ± 17, 387 ± 16, 398 ± 18 m, n = 12, P <0.05, ANOVA with repeated measures)(Figure 1). Consistent with this, decreases were observed for NT-proBNP (407 ± 172, 286 ± 63, 224 ± 46, 209 ± 50 pg/mL, n = 11) and TRV (3.4 ± 0.1, 3.4 ± 0.1, 3.3 ± 0.1, 3.3 ± 0.2 m/s, n = 13), but these were not statistically significant (Figure 1). The NT-proBNP level of patient #14 was an outlier due to end stage renal disease on hemodialysis, and was excluded from analysis. Three patients had a repeat right heart catheterization, which showed a decrease in mean pulmonary artery pressure from a group mean of 44 to 38 mmHg, although this was not statistically significant in this small number of patients (Table 3). All three catheterized patients showed some decrease in their pulmonary artery mean pressure, associated with a decrease in pulmonary vascular resistance and improvement in cardiac output (Table 3).

Table 2.

Tricuspid regurgitant velocity, 6 minute walk distance and NT-proBNP at baseline and on ETR blocking therapy.

Patient Number PH Therapy TRV (m/sec) 6 MW (m) NT-proBNP (pg/l)
Baseline On Therapy Baseline On Therapy Baseline On Therapy
1 Bos 2.9 3.1 360 414 1412 353
2 Bos 3.2 2.8 408 510 825 672
3 Bos 3.1 2.8 270 294 58 114
4 Bos 3 2.8 465 464 90 189
5 Bos 3.1 3.2 379 429 107 106
6 Bos 4.1 4.5 330 379 106 786
7 Amb 3.7 3.4 462 452 196 63
8 Amb 3.0 2.9 306 409 90 67
9 Sil/Bos 4.3 3.8 204 388 234 423
10 Sil/Bos 3.2 3.0 376 340 87 268
11 Sil/Bos 4.3 3.9 379 302 548 31
12 Sil/Bos 3.1 4.0 389 396 570 540
13 Sil/Bos 4.8 4.2 453 420 473 149
14 Sil/Amb 3.2 3.1 362 NA *7984 *14786
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PH, pulmonary hypertension; TRV, tricuspid regurgitant velocity; 6MW, 6-min walk distance; NT-proBNP, pro-brain natriuretic peptide; Bos, bosentan; Amb, ambrisentan; Sil, sildenafil.

*

Indicates patients with end stage renal disease, where NT-proBNP levels are elevated and not reliable as markers of cardiac dysfunction.

Figure 1.

Figure 1

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Changes in 6-min walk distance (A), NT-proBNP a serum levels (B) and tricuspid regurgitant velocity as measured by echocardiogram (C) during the time of observation on ETR blocking therapy (6–14 months, mean 8.5 months). Sequential measurements of 6-min walk distance increased significantly (mean ± standard error baseline, 357 ± 22 m; 1–2 months, 367 ± 17 m; 3–4 months, 387 ± 16 m; 5–6 months, 398 ± 18 m; n = 12, P < 0.05, ANOVA with repeated measures).

Table 3.

Effect of ETR blocking therapy on hemodynamic parameters, as assessed by right heart catheterization.

Patient Number PAM RA CO PVR PCWP
Baseline On Therapy Baseline On Therapy Baseline On Therapy Baseline On Therapy Baseline On Therapy
9 35 34 0 7 5.3 6.2 468 284 4 12
10 40 34 10 16 7.2 7.4 178 195 24 16
11 57 47 16 11 5.9 8.3 444 270 24 19
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PAM, mean pulmonary artery pressure (mmHg); RA, right atrial pressure (mmHg); CO, cardiac output (L/min); PVR, pulmonary vascular resistance (dynes/s per cm3); PCWP, pulmonary capillary wedge pressure (mmHg).

Seven patients improved their NYHA classification by at least one classification and 3 worsened by one classification. Three out of 13 patients had no change in NYHA classification (one patient did not have follow up six minute walk data). Patient #6 had worsening six minute walk distance associated with increased fluid retention, which improved upon switching from bosentan to ambrisentan.

Symptomatically, 8 out of 13 had a decrease in their Borg dyspnea score, 4 out of 13 had no change, and only 1 experienced an increase (data not shown). Serum LDH levels decreased in ten patients, with the average values for the entire group falling modestly from a mean of 386 U/L prior to ETR blocking therapy compared to 341 U/L after. Five patients were on continuous oxygen therapy prior to starting ETR blocking agents and continued to require the same level of oxygen support. Pulse oximeter mean values were 95.8% before ETR blocking therapy and 97% after six months.

Adverse events while on therapy occurred in 7 out of 14 patients during and extended follow up of 14 months. They were: increased serum alanine aminotransferase (3 patients), increased peripheral edema (5 patients), rash (2 patients), headache (3 patients), facial edema (2 patients). Few patients experienced multiple side effects. ETR blocking therapy was switched in two patients (#14 from bosentan to ambrisentan and #10 from ambrisentan to bosentan), with resolution of symptoms (Table 4). Patient, #14, developed hypotension attributed primarily to progressive cirrhosis and hypoadrenalism, and his ambrisentan dose was decreased from 10 to 5 mg/day, with stabilization of his blood pressure. Another patient, #2 experienced a subarachnoid hemorrhage six months after initiating bosentan therapy, while also on concurrent sildenafil and warfarin, which was attributed to a cerebral aneurysm. Bosentan was continued at the same dose without recurrence. All other side effects were treated medically without the need for suspension of ETR blocking therapy.

Table 4.

Adverse Events on ETR blocking therapy

Adverse Events on ETR blocking therapy Number of patients
Lower extremities edema 4
Headache 3
Skin rash 1
Increased LFT's 2
Decreased Hb 2
Subarachnoid hemorrhage 1
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Adverse events occurring in patients while on ETR blocking therapy. Six of the 14 patients were also receiving sildenafil. Seven patients had more than one adverse events, the other seven did not have any recorded. LFTs, liver function tests; Hb, haemoglobin concentration.

DISCUSSION

This is a retrospective case series of ETR blocking agents in SCD patients with clinically significant PH, with the necessary limitations due to its non prospective and non controlled design and relatively small size. However, this is the largest series of patients with SCD reported to date treated with ETR antagonists. In this extended case series, we found that ambrisentan and bosentan doses were well tolerated, with most adverse events mild in severity, despite their baseline anemia and end organ damage. Remarkably, only two patients, one with an allergic reaction to ambrisentan, and another with liver abnormalities, required discontinuation of therapy with that specific ETR blocking agent and switching to the other one. Other side effects reported, such as increase in peripheral edema and fluid retention, or headache, uniformly resolved with addition of diuretics. Our results mirror the experience with ETR antagonists in large clinical trials with patients with primary PH, where the dose of 125 mg bid of bosentan was not associated with a significant increase in adverse events when compared to placebo. The most frequent side effect was headache (19 out of 74 patients), while elevated AST occurred only in 3 out of 74 patients (Rubin, et al 2002). Similarly, patients treated with ambrisentan in ARIES-1 and 2, had mild side effects (Galie, et al 2008).

While the efficacy of ETR blocking agents is well documented in large trials in idiopathic PH (Galie, et al 2008, Rubin, et al 2002), little is known about their efficacy in SCD-related PH. We present preliminary data in support of the efficacy of this class of drugs in patients with sickle cell disease and catheterization documented PH. In this small study, none of the changes in PH parameters were statistically significant, but every one of the average values improved in a mutually consistent direction. The treatment associated improvement in 6 MW distances, which is an independent predictor of mortality in PH (Miyamoto, et al 2000) was 11%. This is a magnitude considered clinically significant in primary PH patients treated with similar therapy (Rubin, et al 2002). Plasma NT-proBNP levels have been shown to correlate with PH severity in primary PH (Leuchte, et al 2003), and an independent predictor of mortality in SCD patients with PH (Machado, et al 2006). In our patients NT-proBNP decreased, although non-significantly, suggesting amelioration of hemodynamic and right ventricle dysfunction.

Interesting data was gathered from the three paired right heart catheterizations, which demonstrate a decrease in pulmonary artery pressure in all three patients, associated with a decrease in pulmonary vascular resistance and increase in cardiac output. A mild increase in right atrial pressure and in wedge pressure, in patient #10 may be attributed to fluid retention, as previously reported in several clinical trials in patients with primary PH treated with bosentan or ambrisentan. Two of the patients had high baseline PCWP that decreased with therapy. This might be due to improvement in left diastolic function, induced by bosentan, which has been seen in primary PH patients.

Excessive red cell Gardos channel activity is known to promote an increase in the number of dense cells in patients with SCD (Lew, et al 2005) and Gardos channel blockade reduces hemolysis in SCD (Ataga, et al 2008). Since ETR-b blockade is believed also to inhibit Gardos channel activity (Rivera, et al 2002), it is conceivable that dual ETR blocking agents, such as bosentan, might decrease intravascular hemolytic rates. In our study 10 out of 14 patients had a decrease in their serum LDH. Even though the decrease did not reach statistical significance, this trend is intriguing and points to the need for a larger, prospective trial.

Finally, eleven out of the 14 patients reported in this analysis are alive to date with a follow up ranging from 4 to 10 years since the initial diagnosis of right heart catheterization documented moderate to severe pulmonary hypertension. In the absence of a control group, it is impossible to comment on what the natural progression of the disease would have been in a similar group of patients without the addition of ETR blocking agents. On the other hand, given the published 50% mortality at 2 years in an SCD PH cohort of comparable baseline clinical severity (Castro, et al 2003), it is plausible to speculate that the addition of ETR blocking agents contributed to our patients' survival. These are limited observations from which it is premature to draw any firm conclusions regarding efficacy.

The demonstration that ETR antagonists are safe and potentially effective in this case series of patients with SCD may generate further investigation of their role in the therapy of PH and other complications of SCD patients.

ACKNOWLEDGEMENTS

We acknowledge the careful work of Heather Kennedy in editing the manuscript and Mary Hall for exceptional protocol support. This work was supported by the NIH Division of Intramural Research.

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