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* From the University of Colorado Health Sciences Center (Dr. Badesch), Denver, CO; The Children’s Hospital (Dr. Abman), Denver, CO; Babies Hospital (Dr. Barst), Columbia-Presbyterian Medical Center, Pediatric-Cardiology Center, New York, NY; Duke University Medical Center (Drs. Ahearn and McCrory), Durham, NC; Hôpital Antoine Béclère (Dr. Simonneau), Clamart, France; and University of Michigan (Dr. McLaughlin), Ann Arbor, MI.
Correspondence to: David B. Badesch, MD, FCCP, University of Colorado Health Sciences Center, Box C-272, 4200 E Ninth Ave, Denver, CO 80262; e-mail: David.Badesch{at}UCHSC.edu
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Abstract |
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 TOP Abstract IntroductionOverview of the Approach...General CareProstanoidsEndothelin AntagonistsPhosphodiesterase InhibitorsNO and ArginineCombination TherapySpecial SituationsSummarySummary of RecommendationsReferences |
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Key Words: anticoagulation • arginine • beraprost • bosentan • calcium-channel blockers • endothelin • endothelin receptor antagonist • epoprostenol • idiopathic pulmonary arterial hypertension • iloprost • medical therapy • oxygen • primary pulmonary hypertension • prostacyclin • pulmonary arterial hypertension • pulmonary hypertension • secondary pulmonary hypertension • sildenafil • therapy • treatment • treprostinil • vasoreactivity • warfarin
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Introduction |
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TOPAbstractIntroductionOverview of the Approach...General CareProstanoidsEndothelin AntagonistsPhosphodiesterase InhibitorsNO and ArginineCombination TherapySpecial SituationsSummarySummary of RecommendationsReferences |
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Methodologically, a computerized search of the MEDLINE bibliographic database from 1992 to October 2002 (see Methods chapter, page 11S) was conducted using the term hypertension, pulmonary. While the background provided in some sections includes selected information from basic and animal studies, the formal search was limited to articles concerning human subjects that were published in the English language and accompanied by an abstract. In addition, we searched the reference lists of included studies, practice guidelines, systematic reviews, and meta-analyses, and clinical experts identified relevant studies missed by the search strategy or published before 1992. We selected studies of oxygen, diuretics, inotropic agents (digoxin), anticoagulants, calcium antagonists, angiotensin-converting enzyme inhibitors, prostanoids (eg, epoprostenol, treprostinil, inhaled iloprost), L-arginine, endothelin-receptor antagonists (eg, bosentan, sitaxsentan, ambrisentan), phosphodiesterase-5 inhibitors (sildenafil), nitric oxide (NO), and thromboxane inhibitors (eg, terbogrel). We considered studies conducted among patients with known or suspected IPAH or PAH occurring in association with underlying collagen vascular disease, congenital heart disease, or chronic thromboembolic disease. We excluded studies of pulmonary hypertension (PH) associated with COPD or other parenchymal lung disease, high-altitude PH, or cardiac disease (eg, left-heart failure, valvular heart disease) except congenital heart disease. The summary evidence tables can be viewed on-line at http://www.chestjournal.org/content/vol126/1_suppl/.
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Overview of the Approach to the Patient With PAH |
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TOPAbstractIntroductionOverview of the Approach...General CareProstanoidsEndothelin AntagonistsPhosphodiesterase InhibitorsNO and ArginineCombination TherapySpecial SituationsSummarySummary of RecommendationsReferences |
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Following the identification of underlying associated disorders and contributing factors, specific therapy for PAH should be considered. Recognition that there is some similarity across subgroups of patients, in terms of hemodynamic and functional response to therapy (though not necessarily survival), has lead to the development of similar treatment strategies. Exceptions do, of course, exist for special situations, some of which will be addressed separately near the end of this chapter.
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General Care |
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TOPAbstractIntroductionOverview of the Approach...General CareProstanoidsEndothelin AntagonistsPhosphodiesterase InhibitorsNO and ArginineCombination TherapySpecial SituationsSummarySummary of RecommendationsReferences |
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40 mm Hg, with an increase or unchanged cardiac output (CO). It should be noted, however, that the precise definition of a favorable acute response to vasodilator is still somewhat controversial. Although initially IV vasodilators including isoproterenol and hydralazine were used to assess vasodilator responsiveness, the use of oral CCBs came into favor in the early 1990s. In 1992, Rich et al1 studied 64 patients with IPAH with oral nifedipine (20 mg), or diltiazem (60 mg) if they had a resting tachycardia, and the doses were repeated every hour until either a favorable response was achieved or intolerable side effects developed. In this series,1 a favorable response was defined as a > 20% reduction in mPAP and PVR. Seventeen of the 64 patients (26.5%) were considered responders, and were subsequently treated with high-dose, oral CCBs. This group had a 94% 5-year survival. However, acute testing with oral CCBs can result in deterioration in patients who are not responsive, and this can be exacerbated by the long half-life of these agents. After several case reports of fatal outcomes associated with testing patients with IPAH and CCBs, interest developed in the use of shorter-acting vasodilators to assess vasoreactivity prior to institution of CCBs.
In 1993, Groves et al2 studied the acute response to IV epoprostenol in 44 patients with IPAH. An initial dose of 1 ng/kg/min was administered and increased by 1 to 2 ng/kg/min every 5 to 15 min to a maximum dose of 12 ng/kg/min. The mean tolerated dose was 8.0 ng/kg/min. The mean hemodynamic effects of epoprostenol included a 14% increase in heart rate, 5% decrease in mPAP, 47% increase in CO, and 32% decrease in PVR. Using the definition of a favorable response as > 30% decrease in PVR and > 10% decrease in mPAP, 13 of 44 patients with IPAH (30%) were considered responders. The acute response to IV epoprostenol was predictive of subsequent response to oral CCB therapy. Similarly, in a study of 35 consecutive patients with IPAH, Sitbon et al3 evaluated the acute vasodilator response to IV epoprostenol at a starting dose of 2.5 ng/kg/min increased stepwise by increments of 2.5 ng/kg/min every 10 min to a maximal dose of 10 ng/kg/min. Using the definition of a response as a reduction of > 30% in total pulmonary resistance, 13 of 35 patients (37%) were considered responders.
IV adenosine has been shown to be a potent vasodilator through its actions on specific vascular receptors. Adenosine produces coronary vasodilatation, decreases systemic vascular resistance, and causes relaxation of smooth muscles including pulmonary arteries. Because of its short serum half-life, adenosine is a desirable agent to use as a vasodilator in the assessment of PH. Shrader et al4 studied 15 patients (11 patients with IPAH) with IV adenosine administered at a dose of 50 µg/kg/min and increased by 50 µg/kg/min every 2 min to a maximum dose of 500 µg/kg/min to acutely assess vasodilator response. Subsequently, patients were administered hourly doses of nifedipine to assess vasodilator response. Importantly, the correlation of PVR reduction with both agents was high (R = 0.714; p = 0.01). The three patients who did not respond to adenosine also did not respond to nifedipine.
Because both IV epoprostenol and IV adenosine have the potential to cause a reduction in systemic vascular resistance and systemic hypotension, more pulmonary selective vasodilators have been sought. In 1998, Sitbon et al5 reported the results of inhaled NO testing (10 ppm) via face mask in 33 patients with IPAH. A significant acute vasodilator response was defined by a fall in both mPAP and total pulmonary resistance of > 20%. Ten of the 33 patients responded acutely to NO, 9 of whom responded acutely to CCBs without any complications. Of the other 23 patients who failed to respond to NO, none had a response to CCBs. Notably, there were nine serious adverse events associated with the administration of CCBs in the patients not responsive to NO. This led to the conclusion that acute vasodilator testing with NO is safer than testing with CCBs. Similarly, in 1998, Ricciardi et al5a reported results of acute testing with NO in 17 patients with IPAH prior to undergoing a trial with nifedipine. Patients were considered responders if they had a > 20% decrease in mPAP or a > 20% decrease in PVR. Seven of the 17 patients responded to NO, while 8 of the 17 patients responded to nifedipine. There were three adverse events, including one death during challenge with nifedipine. All responders to NO responded to nifedipine, while 9 of the 10 NO nonresponders were also nifedipine nonresponders. There was a highly significant correlation between the effects of NO and nifedipine on PVR (R = 0.67, p = 0.003). More limited data exist for the use of inhaled iloprost as an acute vasoreactivity testing agent. In a recent study, Opitz et al6 compared the response of oxygen inhalation, iloprost inhalation, IV epoprostenol, and IV iloprost. IV iloprost and epoprostenol had very similar hemodynamic profiles in terms of reduction in PVR and PAP. Inhaled iloprost exerted a selective pulmonary dilatation, resulting in a reduction of PVR and PAP without systemic vasodilatation. This study suggested that inhaled iloprost should be equivalent to IV epoprostenol and inhaled NO in terms of predicting response to CCBs.
The primary objective of acute vasodilator testing in patients with IPAH is to delineate the subset of patients who might effectively be treated with oral CCBs. As such, unstable patients or those with severe right-heart failure, who should not be treated with CCBs, need not undergo vasodilator testing. The weight of the evidence favors either IV epoprostenol or inhaled NO as preferred agents for vasodilator testing. IV adenosine may be used if neither of the other agents are available. Testing with a short-acting agent should always take place before testing with oral CCBs, given the potential complications of vasoreactivity testing with CCBs. Only those patients who have had a substantial reduction in both PAP and PVR with an acute vasodilator should undergo further testing with CCBs. Vasodilator testing is best described in the setting of IPAH. The literature does, however, suggest that the pediatric population has a higher response rate to acute vasodilators. Rates of responsiveness in patients with collagen vascular diseases have been low when tested with inhaled NO.
Calcium-Channel Antagonists/Blockers
Smooth-muscle cell hypertrophy and vasoconstriction have long been known to contribute to the pathogenesis of IPAH. More recently, abnormalities of the voltage-gated 1.5 potassium channels have been demonstrated in IPAH. Many vasodilators have been studied in the setting of PAH. The most notable and successful of these have been the CCBs. These agents have been studied for IPAH since the mid-1980s.
As discussed above in the section on "Vasodilator Testing," in 1992 Rich et al1 reported the results of a prospective but not randomized trial of high-dose CCBs in patients with IPAH. Patients were included in this prospective, single-center study if they had IPAH and were not too ill for a cardiac catheterization and vasoreactivity testing. The patients who had a favorable acute response were treated with high-dose CCBs for up to 5 years. The 1-, 3-, and 5-year survival was 94%, 94%, and 94% in the patients treated with CCBs compared to 68%, 47%, and 38% in those who were classified as nonresponders, a statistically significant improvement. When compared to patients enrolled in the National Institutes of Health Registry, their survival was also significantly better.
In 1993, Ogata et al7 reported the results of an uncontrolled, retrospective, open-label, single-center study in which a combination of anticoagulant and vasodilator therapy was evaluated in patients with IPAH. Seven patients were treated with the anticoagulant warfarin combined with a vasodilator: three patients with isoproterenol, and four patients with nifedipine. The remaining 13 patients were not treated and constituted the control group. The 5-year survival was significantly higher in the group treated with anticoagulants and vasodilators: 57% vs 15%.
There has not been an RCT of oral CCBs for IPAH, nor have there been any substantial reports of success with CCBs for other forms of PAH. Based on the available data, it would be prudent to test patients with PAH with an acute vasodilator such as prostacyclin, NO, or adenosine. Patients who demonstrate a significant response to the acute administration of a short-acting vasodilator (see above) should be treated cautiously with oral CCBs, and monitored closely to determine both the efficacy and safety of such therapy. CCBs with a significant negative inotropic effect, such as verapamil, should be avoided. Nifedipine, diltiazem, or amlodipine are used most frequently, with the choice often based on the heart rate at baseline (relative bradycardia favoring nifedipine, and relative tachycardia favoring diltiazem). While early recommendations seemed to favor relatively high doses of CCBs, most experts now seem to introduce these agents more cautiously, and gradually titrate the dose as tolerated.
Warfarin, Supplemental Oxygen, Diuretics, Digoxin
In situ microscopic thrombosis has been documented in some patients with IPAH. In addition, patients with right ventricular failure and resultant venous stasis are likely at increased risk for pulmonary thromboembolism. Improved survival has been reported with oral anticoagulation in patients with IPAH.18 The target international normalized ratio (INR) in patients with IPAH treated with warfarin is approximately 1.5 to 2.5, but this varies somewhat from center to center. Anticoagulation of patients with PAH occurring in association with other underlying processes, such as scleroderma or congenital heart disease, is controversial. Some experts extrapolate the evidence supporting anticoagulation in patients with IPAH to other patients with PAH, while others may not. When deciding whether or not to anticoagulate patients with PAH occurring in association with underlying processes, the risk/benefit ratio should be carefully considered. It is generally thought that the risk of GI bleeding may be higher in patients with PAH occurring in association with scleroderma. Patients with PAH occurring in association with congenital heart disease may be at some increased risk of hemoptysis. However, patients with significant right-to-left intracardiac shunting may be at increased for paradoxical embolism to the CNS. Patients with portopulmonary hypertension may be at increased risk for GI bleeding due to the presence of varices. Generally, patients with PAH receiving therapy with long-term IV epoprostenol are anticoagulated in the absence of contraindications, due in part to the additional risk of catheter-associated thrombosis.
Hypoxemia is a potent pulmonary vasoconstrictor, and can contribute to the development and/or progression of PAH. It is generally considered important to maintain oxygen saturations at > 90% at all times. This may be difficult in patients with concomitant intrinsic lung disease, or right-to-left intracardiac shunting. The use of supplemental oxygen may be somewhat more controversial in patients with large right-to-left shunts due to congenital heart disease with Eisenmenger physiology, but may help to decrease the need for phlebotomy, and potentially reduce the incidence of neurologic dysfunction and complications.
Diuretics are indicated in patients with evidence of right ventricular failure (ie, peripheral edema and/or ascites). Maintaining near-normal intravascular volume with diuretics, and careful dietary restriction of sodium and fluid intake is generally considered to be important in the long-term management of patients with IPAH. However, rapid and excessive diuresis may lead to systemic hypotension, renal insufficiency, and syncope. Serum electrolytes and indices of renal function should be followed closely in patients receiving diuretic therapy.
Although not extensively studied in PAH, digitalis is sometimes utilized in patients with refractory right ventricular failure and/or atrial dysrrhythmias. Drug levels must be followed closely, especially in patients with impaired renal function.
Prevention and Treatment of Respiratory Tract Infections
Due to the potentially devastating effects of respiratory tract infections in patients with PAH, they should be immunized against influenza and pneumococcal pneumonia per the usual standards for patients with serious cardiopulmonary disease. If respiratory tract infections do develop, they should be treated aggressively.
Recommendations
40 mm Hg, with an increased or unchanged CO), should be considered candidates for a trial of therapy with an oral calcium-channel antagonist. Level of evidence: low; benefit: substantial; grade of recommendation: B. 40 mm Hg, with an increased or unchanged CO), should be considered candidates for a trial of therapy with an oral calcium-channel antagonist. Level of evidence: expert opinion; benefit: intermediate; grade of recommendation: E/B. |
Prostanoids |
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Epoprostenol
In a 12-week, prospective, multicenter, randomized, controlled, open-label trial,11 continuously IV infused epoprostenol plus conventional therapy (oral vasodilators, anticoagulation, etc) was compared to conventional therapy alone in 81 patients with severe IPAH (New York Heart Association [NYHA] functional class III or IV). Exercise capacity improved in the 41 patients treated with epoprostenol (median distance walked in 6 min of 362 m at 12 weeks vs 315 m at baseline), and decreased in the 40 patients treated with conventional therapy alone (204 m at 12 weeks vs 270 m at baseline; p < 0.002 for the comparison of the treatment groups). Indices of the quality of life were improved in the epoprostenol group (p < 0.01). Hemodynamics improved at 12 weeks in the epoprostenol-treated group; the changes in mPAP for the epoprostenol group and control group were – 8% and + 3%, respectively (difference in mean change, – 6.7 mm Hg; 95% confidence interval [CI], – 10.7 to – 2.6 mm Hg; p < 0.002), and the mean changes in PVR for the epoprostenol and control groups were – 21% and + 9%, respectively (difference in mean change, – 4.9 mm Hg/L/min; 95% CI, – 7.6 to – 2.3 mm Hg/L/min; p < 0.001). Eight patients died during the study, all of whom had received conventional therapy (p = 0.003). Serious complications included four episodes of catheter-related sepsis and one thrombotic event. It was concluded that, as compared with conventional therapy, continuous IV infusion of epoprostenol produced symptomatic and hemodynamic improvement, as well as improved survival in patients with severe IPAH.
A multicenter, randomized, controlled, open-label study of long-term IV epoprostenol showed improvement in exercise capacity and hemodynamics in patients with PAH occurring in association with the scleroderma spectrum of disease.12 Epoprostenol plus conventional therapy was compared to conventional therapy alone. The primary outcome measure was exercise capacity. Other measures were cardiopulmonary hemodynamics, signs and symptoms of PH and scleroderma, and survival. Exercise capacity improved with epoprostenol (median distance walked in 6 min, 316 m at 12 weeks compared with 270 m at baseline) but decreased with conventional therapy (192 m at 12 weeks compared with 240 m at baseline). The difference between treatment groups in the median distance walked at week 12 was 108 m (95% CI, 55.2 to 180.0 m; p < 0.001). Hemodynamics improved at 12 weeks with epoprostenol therapy. The changes in mPAP for the epoprostenol and conventional therapy groups were – 5.0 and 0.9 mm Hg, respectively (difference, – 6.0 mm Hg; 95% CI, – 9.0 to – 3.0 mm Hg), and the mean changes in PVR were – 4.6 mm Hg/L/min and 0.9 mm Hg/L/min, respectively (difference, – 5.5 mm Hg/L/min; 95% CI, – 7.3 to – 3.7 mm Hg/L/min). Twenty-one patients treated with epoprostenol, and no patients receiving conventional therapy, showed improved NYHA functional class. Borg dyspnea scores and dyspnea-fatigue ratings improved in the epoprostenol group. Trends toward greater improvement in severity of the Raynaud phenomenon and fewer new digital ulcers were seen in the epoprostenol group. Four patients in the epoprostenol group and five patients in the conventional therapy group died (p value not significant). Side effects of epoprostenol therapy included jaw pain, nausea, and anorexia. Adverse events related to the epoprostenol delivery system included sepsis, cellulitis, hemorrhage, and pneumothorax (4% incidence for each condition). It was concluded that continuous epoprostenol therapy improves exercise capacity and cardiopulmonary hemodynamics in patients with PAH due to the scleroderma spectrum of disease.
Epoprostenol therapy is complicated by the need for continuous IV infusion. The drug is unstable at room temperature, and is generally best kept cold prior to and during infusion (necessitating the use of ice packs). It has a very short half-life in the blood stream (< 6 min), is unstable at acidic pH, and cannot be taken orally. Due to the short half-life, the risk of rebound worsening with abrupt/inadvertent interruption of the infusion (see below), and its effects on peripheral veins, it should be administered through an indwelling central venous catheter. Due to the long duration of therapy, and the ongoing risk of catheter-associated infection, tunneled central venous catheters are generally preferred. Patients are typically begun on a very low dosage of epoprostenol (1 to 2 ng/kg/min), and the dose is gradually titrated upward in increments of 1 to 2 ng/kg/min, based on side effects and tolerance. Many patients seem to eventually reach a "plateau" dose, and may not require continued uptitration from that point. While this dose may be between 20 ng/kg/min and 40 ng/kg/min for many patients, the dose range tends to be quite wide, with considerable interindividual variability.
Common side effects of epoprostenol therapy include headache, flushing, jaw pain with initial mastication, diarrhea, nausea, a blotchy erythematous rash, and musculoskeletal aches and pain (predominantly involving the legs and feet). These tend to be dose dependent, and often respond to a cautious reduction in dose. Severe side effects can occur with overdosage of the drug. Acutely, overdosage can lead to systemic hypotension. Long-term overdosage can lead to the development of a hyperdynamic state and high-output cardiac failure.13
Abrupt or inadvertent interruption of the epoprostenol infusion should be avoided, as this may, in some patients, lead to a rebound worsening of their PH with symptomatic deterioration and perhaps even death. Patients with severe disease, who are highly dependent on the hemodynamic effects of the drug, can experience clinical deterioration in a relatively short period of time (20 to 30 min) with abrupt interruption of the infusion. Patients are advised to always carry with them a spare cassette of premixed epoprostenol, as well as a spare infusion pump. If central venous access is lost for whatever reason (clogging or dislodgement of the catheter), patients are generally advised to access the emergency medical system immediately. The infusion is then re-established with placement of peripheral IV access, until central venous access can be restored. Patients should generally remain within the medical system until stable central venous access has been secured.
Other complications of long-term IV therapy with epoprostenol include line-related infections (which can range from small exit site reactions, to tunnel infections and cellulitis, to bacteremic infections with sepsis), catheter-associated venous thrombosis, thrombocytopenia, and ascites. Central venous catheter placement can occasionally be associated with the development of pneumothorax or hemothorax.
Due to the complexity of administration of epoprostenol (long-term indwelling catheters, reconstitution of the drug, operation of the infusion pump, etc), strong consideration should be given to referring patients to centers of excellence in PH. Management of patients receiving long-term epoprostenol therapy requires a considerable infrastructure, including experienced nurses and physicians.
It has become more challenging to predict prognosis in the era of epoprostenol therapy. Some patients who might previously have been considered to have a poor prognosis can now enjoy relatively long-term survival receiving long-term IV epoprostenol therapy. The beneficial effects of epoprostenol therapy appear to be sustained for years in many patients with IPAH. Barst et al14 reported long-term benefit in a small group of patients from several centers involved in the earliest clinical usage of epoprostenol. More recently, Shapiro et al15 and McLaughlin et al16 described sustained benefit in larger numbers of patients with continuously infused epoprostenol. It appears as though decreases in mPAP and PVR, and improvement in CO can be sustained over a period of years in many patients.
Most recently, McLaughlin et al17 reported long-term epoprostenol therapy in 162 consecutive patients with IPAH followed up for a mean of 36.3 months (median, 31 months). Data obtained included functional class, exercise tolerance, and hemodynamics. Observed survival with epoprostenol therapy at 1 year, 2 years, and 3 years was 87.8%, 76.3%, and 62.8%, and was significantly greater than the expected survival of 58.9%, 46.3%, and 35.4% based on historical data. Baseline predictors of survival included exercise tolerance, functional class, right atrial pressure, and vasodilator response to adenosine. Predictors of survival after the first year of therapy included functional class and improvement in exercise tolerance, cardiac index, and mPAP. Similarly, Sitbon et al18 sought to determine the factors associated with long-term survival in patients with IPAH treated with continuous epoprostenol infusion. They began with the concept that epoprostenol generally improves survival in patients with IPAH in NYHA functional class III or IV, but some patients do not benefit and must be considered for lung transplantation. The best timing for listing these patients for lung transplantation is unknown. Between December 1992 and January 2001, 178 patients with IPAH in NYHA functional class III or IV were treated with epoprostenol. The 6-min walk test and right-sided heart catheterization were performed at baseline, after 3 months receiving epoprostenol, and thereafter once a year. Overall survival rates at 1 year, 2 years, 3 years, and 5 years were 85%, 70%, 63%, and 55%, respectively. On univariate analysis, the baseline variables associated with a poor outcome were a history of right-sided heart failure, NYHA functional class IV, 6-min walk test 250 m (median value), right atrial pressure 
12 mm Hg, and mPAP 65 mm Hg. On multivariate analysis, including both baseline variables and those measured after 3 months receiving epoprostenol, a history of right-sided heart failure, persistence of NYHA functional class III or IV at 3 months, and the absence of a fall in total pulmonary resistance of 30%, relative to baseline, were associated with poor survival. They concluded that survival of patients with IPAH treated with epoprostenol depends on the severity at baseline, as well as the 3-month response to therapy. They inferred that the findings suggest lung transplantation should be considered in a subset of patients who remain in NYHA functional class III or IV, or in those who cannot achieve significant hemodynamic improvement after 3 months of epoprostenol therapy, or both.
In summary, long-term IV epoprostenol therapy has had a dramatic effect on the treatment of patients with moderately severe to severe PAH. It has been studied most thoroughly in patients with IPAH and PAH occurring in association with the scleroderma spectrum of disease. Due to the requirement for constant IV infusion, it is complicated therapy, and it is strongly recommended that patients be referred to clinical centers of excellence.
Treprostinil
The success of epoprostenol therapy, coupled with the limitations of its delivery system, has led to the development of prostacyclin analogues with alternative routes of delivery. Treprostinil is a prostacyclin analog with a half-life of 3 h. The drug is stable at room temperature. To test the hypothesis that the hemodynamic effects of treprostinil are similar to those of epoprostenol, 14 patients with IPAH were tested acutely with IV epoprostenol and then IV treprostinil.19 The two drugs had similar effects on hemodynamics. There was a 22% reduction in PVR with epoprostenol vs a 20% reduction in PVR with treprostinil. To test the subcutaneous delivery method, the effects of IV and subcutaneous treprostinil were compared in 25 patients with IPAH. In the IV treprostinil and subcutaneous treprostinil groups, there were 6% and 13% declines in mPAP, and 23% and 28% declines in PVR, respectively. Having demonstrated that the drug favorably affects cardiopulmonary hemodynamics when administered subcutaneously acutely, an 8-week, placebo-controlled, 2:1, randomized trial of subcutaneous treprostinil was performed. Twenty-six patients with IPAH were enrolled. Two patients in the treprostinil group did not complete the study due to intolerable side effects. The remaining 15 patients were receiving a mean dose of 13.0 ± 3.1 ng/kg/min of treprostinil, while the 9 patients receiving placebo were receiving 38.9 ± 6.7 ng/kg/min at the end of the 8-week period (± SE). There was an improvement of 37 ± 17 m in the 6-min walk distance in patients receiving the active therapy (from 373 to 411 m), compared to a 6 ± 28 m reduction in those receiving placebo (379 m vs 384 m), a nonstatistically significant trend. There was also a favorable, but nonstatistically significant trend in hemodynamic improvement, with a 20% reduction in PVR index over the 8-week period in the group receiving active treprostinil. Adverse events including headache, diarrhea, flushing, jaw pain, and foot pain were common in the treprostinil group, as they are with epoprostenol. An unexpected adverse effect was pain, which was occasionally severe and often associated with erythema and induration, at the site of the subcutaneous infusion. This occurred in nearly all the patients receiving active therapy. This proof-of-concept trial19 demonstrated that this novel subcutaneous agent could be administered safely and effectively on an outpatient basis, and paved the way for the larger pivotal trial.
The largest placebo-controlled randomized study for PAH was an international trial20 assessing the efficacy of subcutaneously delivered treprostinil in patients with PAH, either IPAH or PAH associated with connective tissue disease or congenital systemic to pulmonary shunts. Patients were enrolled between November 1998 and October 1999 in 24 centers in North America and 16 centers in Europe, Australia, and Israel. Four hundred seventy patients were randomly assigned to receive either continuous subcutaneous infusion of treprostinil plus conventional therapy or continuous infusion of placebo (vehicle solution without treprostinil) plus conventional therapy. Because of the infusion site pain and reaction that occurred in the proof-of-concept trial,19 the dosing strategy called for lower doses at initiation with a maximal allowable dose at the end of 12 weeks of 22.5 ng/kg/min. The primary end point of this trial was exercise capacity as measured by the 6-min walk distance, which improved in the treprostinil group and was unchanged with placebo. The median between treatment group difference was 16 m (p = 0.006). This effect on exercise tolerance appeared to be dose related. The patients in the lowest two quartiles of dosing experienced little improvement in 6-min walk distance, while patients in the highest quartile of dosing (> 13.8 ng/kg/min) demonstrated an improvement of 36 m in 6-min walk distance. Other indices of well-being, including the dyspnea fatigue rating and the Borg dyspnea scale, confirmed an improvement with treprostinil therapy. Treprostinil also demonstrated a significant improvement in the hemodynamic parameters of mean right atrial pressure, mPAP, cardiac index, PVR, and mixed venous oxygen saturation. Common side effects included headache, diarrhea, nausea, rash, and jaw pain. Side effects related to the infusion site were common. Eighty-five percent of patients complained of infusion site pain, and 83% had erythema or induration at the infusion site.
Although statistically significant, the 16-m improvement in 6-min walk distance was relatively modest, and less than the improvements demonstrated in the trials,1112 with IV epoprostenol for both IPAH and PAH related to the scleroderma spectrum of disease, which demonstrated treatment effects of 47 m and 99 m, respectively. The reasons for this are multifactorial. The entry criteria for the treprostinil trial were much broader than for either of the epoprostenol trials. The epoprostenol trials included only patients who were functional class III and IV. Fifty-three functional class II patients were enrolled into the treprostinil trial. Their treatment effect in the 6-min walk distance was only 2 m, compared to 17 m for the 382 patients who were functional class III and 54 m for the 34 patients who were functional class IV. The baseline 6-min walk distance in the treprostinil study was 326 ± 5 m (± SE) in the active treprostinil group and 327 ± 6 m in the placebo group; in comparison, the baseline 6-min walk distance in the IPAH epoprostenol trial was 315 m in the epoprostenol-plus-conventional therapy group vs 270 m in the conventional therapy alone group.11 In the scleroderma epoprostenol trial,12 the baseline 6-min walk distance was 272 m in the epoprostenol-plus-conventional therapy group and 240 m in the conventional therapy alone group. This suggests that the patient population was less ill in the treprostinil trial,20 and this may have contributed to the less impressive treatment effect. The treatment effect was also related to the baseline walk in the treprostinil trial. Patients who were able to walk between 351 m and 450 m did not demonstrate a treatment effect at all, whereas those patients who were able to walk in the lowest category of 50 to 150 m demonstrated a treatment effect of 51 m. The treprostinil trial included a broader range of patients with PAH. In addition to the inclusion of patients with IPAH and PAH associated with the scleroderma spectrum of diseases, PAH associated with congenital heart disease was included. This group had been untested in the past, and in the treprostinil study did not demonstrate any treatment effect at all. This may, in part, be related to their long-standing disease and the difficulty of making an impact on such a process over a short 12-week period.
The nemesis of subcutaneous treprostinil has been pain and erythema at the infusion site. A variety of therapies have been attempted to control this adverse effect, although none have emerged as uniformly successful. Local remedies such as topical hot and cold packs and topical analgesics and anti-inflammatory agents have been variably effective. Some patients respond to oral analgesics such as nonsteroidal anti-inflammatory drugs. Site pain and erythema sometimes improve after several months of therapy. Some patients find that moving the infusion site every 3 days as opposed to every day is useful. The infusion site most commonly used is the subcutaneous abdominal fat, although some patients were able to use the outer hips and thighs and underside of the upper arm. Because of the longer half-life of treprostinil, interruptions of the drug due to dislodgment of the catheter or pump malfunction tend to be less serious. In such instances, the catheter can be replaced or the pump switched with the individual’s backup pump without any serious consequences. The Mini-Med pump (Medtronic Mini-Med, Northridge, CA) used to administer treprostinil is smaller than the CADD pump (Smith’s Medical MD, St. Paul, MN) used to administer epoprostenol, and is approximately the size of a pager. The drug comes in a premixed and prefilled syringe, and therefore the patient needs only to place the syringe in the pump and does not have to mix the medication in a sterile fashion on a daily basis.
Inhaled Iloprost
Iloprost is a chemically stable prostacyclin analog available for IV, oral, and aerosol administration. It has a serum half-life of 20 to 25 min.21 Inhaled therapy for PAH is an attractive concept that has been applied to clinical practice, > 10 years ago, with the use of inhaled NO.22 Since intra-acinar pulmonary arteries are closely surrounded by alveolar units, it is possible to vasodilate these vessels by an alveolar deposition of vasodilators. It is critical that aerosolized particles be small enough (diameter, 3 to 5 pm) to ensure alveolar deposition.23
In IPAH, acute inhalation of iloprost resulted in a more potent pulmonary vasodilator effect than acute NO inhalation.24 For long-term use, the relatively short duration of action of inhaled iloprost requires six to nine inhalations a day to obtain a sustained clinical benefit. With jet nebulizers, the duration of each inhalation takes approximately 15 min23; with alternative devices such as ultrasound nebulizers, the inhalation time can be reduced to approximately 5 min.23
In a 3-month, open, uncontrolled study25 of 19 patients with various forms of PAH, inhaled iloprost at a daily dose of 50 to 200 µg in 6 to 12 inhalations a day improved functional class, exercise capacity (mean increase of the 6-min walking distance of 148 m), and pulmonary hemodynamics. Four patients died during the 3-month study period.
In a 1-year, open, uncontrolled study26 of 24 patients with IPAH, aerosolized iloprost at a daily dose of 100 to 150 µg in six to eight inhalations per day improved exercise capacity (mean increase of the 6-min walking distance of 75 m) and pulmonary hemodynamics. The treatment was generally well tolerated except for mild coughing, minor headache, and jaw pain in some patients.
A 3-month, randomized, double-blind, placebo-controlled European multicenter trial27 with inhaled iloprost was performed. A total of 203 NYHA functional class III and IV patients with IPAH, and PAH occurring in association with collagen vascular disease or inoperable chronic thromboembolic PAH, were enrolled. The daily dose of iloprost was 2.5 µg or 5 µg six times or nine times a day (maximum dose, 45 µg/d; median dose, 30 µg/d). The primary combined end point of a 10% improvement in the 6-min walking distance and NYHA functional class improvement in the absence of clinical deterioration or death was achieved in 17% of treated patients, compared to 5% in patients receiving placebo (p = 0.007). The treatment effect on the 6-min walking distance was a mean increase of 36 m in the overall population in favor of iloprost (p = 0.004), and 59 m in the subgroup of patients with IPAH. There was also a statistically significant beneficial effect of iloprost on NYHA functional class (p < 0.05), quality of life (p < 0.05), and the Mahler dyspnea index (p < 0.05). As compared with baseline values, hemodynamic variables were significantly improved at 3 months when measured after iloprost inhalation. Importantly, these hemodynamic variables were largely unchanged when measured before inhalation, and were significantly worse in the placebo group. One patient died during the study period in the iloprost group vs four patients in the placebo group (not significant). Overall, inhaled iloprost was well tolerated; cough, flushing, and headache occurred more frequently in the iloprost group. These adverse events were mild and mostly transient. Syncope occurred with similar frequency in the two groups, but was more frequently considered to be serious in the iloprost group, although this adverse effect was not associated with clinical deterioration.
In conclusion, based on available data, it appears as though inhaled iloprost is a safe, effective, and well-tolerated treatment for severe PAH. It is currently approved in Europe for IPAH in patients in NYHA functional class III. The most important drawback of inhaled iloprost is related to the relatively short duration of action, requiring the use of six to nine inhalations a day, which is not convenient for patients. In addition, the hemodynamic effects of inhaled iloprost disappear within 30 to 90 min after inhalation.
Beraprost
Beraprost sodium is the first chemically stable and orally active prostacyclin analog.28 It is absorbed rapidly in fasting conditions; peak concentration is reached after 30 min and elimination half-life is 35 to 40 min after oral administration.29 In a monocrotaline-induced PH model, beraprost sodium has been shown to have a protective effect on the development of PH lesions.30 In addition, high doses of beraprost appear to have inotropic and chronotropic effects in the isolated guinea pig myocardium.31 Beraprost has also been evaluated in peripheral vascular disorders such as intermittent claudication,32 Raynaud phenomenon, and digital necrosis in systemic sclerosis,33 with variable results.
Since 1995, beraprost has been used to treat PAH in Japan. Several small, open, uncontrolled studies2834 have reported beneficial hemodynamic effects with beraprost in patients with IPAH. In a retrospective, open uncontrolled study, Nagaya et al35 reported improved survival in 24 patients with IPAH treated with beraprost, compared to a similar group of 34 patients receiving conventional therapy. In this study,35 the 3-year survival rate was 76% in the beraprost group, compared with 44% in the conventional therapy group.
To date, two randomized, double-blind, placebo-controlled trials3637 studied beraprost in PAH. The first study36 was a 12-week, double blind, randomized, placebo-controlled trial performed in 130 patients in NYHA functional class II and III with PAH of various etiologies (IPAH, PAH associated with connective tissue diseases, congenital systemic-to-pulmonary shunts, portal hypertension, or HIV infection). Beraprost (median dose, 80 µg po qd) increased exercise capacity as assessed by the 6-min walking distance. The treatment effect was 25 m in the overall population; however, the treatment effect was 45 m in the patients with IPAH, whereas there were no significant changes in the exercise capacity of subjects with PAH and associated conditions. There were no significant changes in cardiopulmonary hemodynamics, and no difference in survival was detected between the two treatment groups. Side effects linked to systemic vasodilatation were frequent, mainly in the initial titration period, suggesting that tolerance may affect the long-term results with beraprost in the treatment of PAH. A second trial37 evaluated the effects of beraprost therapy for PAH in 116 patients in NYHA functional class II and III: a 12-month, double-blind, randomized, placebo-controlled study. This study37 showed that the beraprost-treated patients had less disease progression at 6 months and confirmed the results of the previous trial36: improved 6-min walk distance at 3 months (+ 22 m from baseline) and 6 months (+ 31 m from baseline), as compared to placebo; however, this improvement was no longer present at 9 months or 12 months. There were no significant changes in hemodynamics at month 12 vs baseline. The survival rate was similar for both treatment groups. These data raise the possibility that the beneficial effects of beraprost may attenuate with time.
Beraprost is an approved therapy for PAH in Japan, and is currently under evaluation by the European Agency for the Evaluation of Medicinal Product. Whether beraprost will prove efficacious as a concomitant medical therapy in combination/multimodal treatment regimens requires further study. In addition, the development of an extended-release form should improve the overall risk-benefit profile for treating PAH patients with beraprost.
Specific recommendations regarding the use of prostanoids in the treatment of PAH follow the section immediately below on "Endothelin Antagonists." This facilitates the use of an approach based on disease and functional severity, which is more applicable to clinical practice and is summarized in the treatment algorithm (Fig 1 ).
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Endothelin Antagonists |
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TOPAbstractIntroductionOverview of the Approach...General CareProstanoidsEndothelin AntagonistsPhosphodiesterase InhibitorsNO and ArginineCombination TherapySpecial SituationsSummarySummary of RecommendationsReferences |
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Two distinct endothelin-receptor isoforms have been identified, ETA and ETB.44 Activation of ETA receptors facilitates vasoconstriction and proliferation of vascular smooth-muscle cells.44 In contrast, ETB receptors are thought to be principally involved in the clearance of endothelin, particularly in the vascular beds of the lung and kidney.44 Activation of ETB receptors may also cause vasodilation and NO release. There is considerable debate as to whether it is preferable to block both the ETA and ETB receptors, or to target the ETA receptor alone. It is argued by some that selective antagonism of ETA receptors may be beneficial for the treatment of PAH, due to maintenance of the vasodilator and clearance functions of ETB receptors. In PH models,39454647 bosentan, an orally active nonpeptide antagonist of both endothelin-receptor subtypes (ETA and ETB), prevents and even reverses the development of PH, pulmonary vascular remodeling, and right ventricular hypertrophy, independent of the triggering mechanisms. Sitaxsentan sodium, hereafter referred to as sitaxsentan, is a potent endothelin-receptor antagonist that has oral bioavailability and a long duration of action.48 Sitaxsentan is approximately 6,000-fold more selective as an antagonist for ETA compared with ETB receptors. Both bosentan and sitaxsentan have undergone randomized and controlled clinical trials in patients with PAH. The results of these studies will be summarized below.
Bosentan
The first randomized, double-blind, placebo-controlled, multicenter study49 of bosentan was designed to assess the effects of bosentan on exercise capacity and cardiopulmonary hemodynamics, as well as to assess its safety and tolerability in patients with severe PAH. Patients eligible for this study had symptomatic, severe IPAH or PAH occurring in association with scleroderma (in functional classes III or IV, according to the 1998 modified NYHA classification), despite prior treatment, which included vasodilators, anticoagulants, diuretics, cardiac glycosides, or supplemental oxygen. No class IV patients were actually enrolled in the study. Patients were excluded if they had started or stopped any of the above treatments within 1 month of screening, or if they were receiving long-term treatment with epoprostenol. To avoid potential drug interactions, patients were excluded if they had received glibenclamide (glyburide) or cyclosporine within 1 month of enrollment. A baseline 6-min walking distance between 150 m and 500 m, an mPAP > 25 mm Hg, a pulmonary capillary wedge pressure < 15 mm Hg, and a PVR > 240 dyne·s·cm–5 were required for inclusion. The study was conducted in five centers in the United States and one center in France. Thirty-two patients were randomized to receive bosentan or placebo (2:1 ratio). Patients received either bosentan 62.5 mg bid for the first 4 weeks, followed by the target dose (125 mg bid), unless drug-related adverse events were observed (eg, hypotension), or matching doses of placebo. Treatment groups were well matched with respect to baseline characteristics. After 12 weeks of treatment with bosentan, the distance walked in 6 min improved by 70 m (from 360 ± 19 m at baseline to 430 ± 14 m at week 12; p < 0.05) [± SEM], whereas no improvement was seen with placebo (355 ± 25 m at baseline and 349 ± 44 m at week 12). The median change from baseline was 51 m with bosentan and – 6 m with placebo. The difference between treatment groups in the mean change in the 6-min walking distance was 76 ± 31 m in favor of bosentan (95% CI, 12 to 139 m; p = 0.021). Treatment with bosentan significantly improved cardiopulmonary hemodynamics from baseline to week 12 compared with placebo. Bosentan improved cardiac index; the difference between treatment groups in the mean change at week 12 was 1.0 ± 0.2 L/min/m2 (mean ± SEM) in favor of bosentan (95% CI, 0.6 to 1.4 L/min/m2; p < 0.001). PVR was significantly decreased with bosentan, whereas it was increased with placebo (95% CI, – 608 to – 221 dyne·s·cm–5; p < 0.001). Treatment with bosentan decreased the mPAP, the pulmonary capillary wedge pressure, and the mean right atrial pressure. In contrast, all three variables increased in the placebo group. Functional class also improved in patients treated with bosentan. No patient received a lung transplant or died during the study. During the first 12 weeks of treatment, adverse events were transient and similar in frequency and nature in the two groups (7 of 11 patients [63.6%] in the placebo group, and 9 of 21 patients [42.9%] in the bosentan group). Asymptomatic increases in hepatic aminotransferases were observed in two bosentan-treated patients, but these normalized without discontinuation or change of dose.
In a second double-blind, placebo-controlled study, bosentan was evaluated in 213 patients with PAH (either primary or associated with connective tissue disease) who were equally randomized to placebo, bosentan 125 bid, or bosentan 250 mg bid for a minimum of 16 weeks (62.5 mg bid for 4 weeks, then target dose).50 The primary end point was the change in exercise capacity as assessed by the 6-min walk. Secondary end points included changes in Borg dyspnea index, World Health Organization functional class, and time from randomization to clinical worsening. Enrolled patients had symptomatic, severe PAH (World Health Organization functional classes III to IV)51 despite treatment with anticoagulants, and/or vasodilators, diuretics, cardiac glycosides, or supplemental oxygen. Inclusion and exclusion criteria were similar to those in the first study of bosentan, described above. The study was conducted in 27 centers in Europe, North America, Israel, and Australia. Two hundred thirteen patients were equally randomized to receive either bosentan (62.5 mg bid for 4 weeks), followed by the target dose (125 mg bid or 250 mg bid) or matching doses of placebo (144 patients received bosentan and 69 patients received placebo). The placebo and bosentan groups were well matched with respect to demographics and baseline characteristics. After 16 weeks of treatment, bosentan improved the distance walked in 6 min by 36 m, whereas deterioration (– 8 m) was seen with placebo. The difference between treatment groups in the mean change in the 6-min walking distance was 44 m in favor of bosentan (95% CI, 21 to 67 m; p = 0.0002). Although both bosentan dosages induced a significant treatment effect, the improvement was more pronounced for the 250 mg bid dosage than for the 125 mg bid dosage (+ 54 m and + 35 m, respectively). However, no dose response for efficacy could be ascertained; the observed difference in the walking distance at week 16 for the two dose groups (
= 20 m) was not statistically significant and was already present at week 4 ( = 12 m) when all patients were treated with 62.5 mg bid. The risk of clinical worsening was significantly reduced by bosentan compared to placebo (p = 0.0015, with log-rank test). The most frequent adverse event in both treatment groups was headache (21% in the bosentan group and 19% in the placebo group). Adverse events that were more frequent in the placebo group than in the bosentan group were disease related, and included dizziness, aggravated PAH, cough, and dyspnea. Conversely, abnormal hepatic function (as indicated by elevated levels of alanine aminotransferase and/or aspartate aminotransferase), syncope, and flushing occurred more frequently in the bosentan group. Study medication had to be prematurely discontinued in nine patients (6%) in the bosentan group and five patients (7%) in the placebo group. The most frequent cause for withdrawal was abnormal hepatic function in the bosentan group (2% for bosentan vs 0% for placebo), and aggravated PAH and syncope in the placebo group (respectively, 6% and 3% for placebo vs 1.4% and 0% for bosentan). Abnormal hepatic function was found to be dose dependent. It was more frequently reported as an adverse event in the high-dosage bosentan group (250 mg bid) than in the low-dosage group (125 mg bid) [14% vs 5%, respectively]. Increases in hepatic enzymes more than three times the upper limit of normal occurred in 10 patients (14%) in each bosentan-dosage group; two patients (2.7%) in the 125 mg bid group and five patients (7.1%) in the 250 mg bid group experienced elevations more than eight times the upper limit of normal. Hepatic function abnormalities were transient except for three patients (all in the high-dosage bosentan group); these patients had to be withdrawn prematurely from the study. Three patients died during the course of the study: two placebo patients died of aggravated PAH, and one bosentan patient (125 mg bid) died of cardiac failure.
There are several notable potential toxicities associated with the use of bosentan. Due to the risk of potential hepatic toxicity discussed above, the US FDA requires that liver function tests be performed at least monthly in patients receiving this drug. Bosentan use may also be associated with the development of anemia, which seems typically to be mild. The hemoglobin/hematocrit should be checked regularly. Due to the potential teratogenic effects of bosentan, careful attention must be paid to the use of adequate contraception in women of childbearing age. It is important to note that bosentan may decrease the efficacy of hormonal contraceptive techniques, and for this reason they should not be used alone. Rather, it is suggested that some other form of contraception be included, such as the use of double-barrier techniques (condom and diaphragm) with a spermicide. Regular pregnancy testing is recommended in women of childbearing age. There is concern that the endothelin antagonists as a class may be capable of causing testicular atrophy and male infertility. Younger men who may consider conceiving should be counseled regarding this possibility prior to taking these drugs.
Sitaxsentan
In the first randomized, double-blind, placebo-controlled trial with sitaxsentan in PAH,51a 178 NYHA class II, III and IV patients with either IPAH, PAH related to connective tissue disease, or PAH related to congenital systemic to pulmonary shunts were equally randomized to receive placebo, sitaxsentan 100 mg po qd, or sitaxsentan 300 mg po qd. Sitaxsentan improved exercise capacity (6-min walk distance) and functional class after 12 weeks of treatment. These functional benefits occurred with both the 100-mg and the 300-mg doses. The treatment effects in the sitaxsentan groups were 35 m (p < 0.01) for the 100-mg dose, and 33 m (p < 0.01) for the 300-mg dose. NYHA functional class improved in 16 of 55 patients (29%) in the 100-mg group, and in 19 of 63 patients (30%) in the 300-mg group. In contrast, only 9 of 60 patients (15%) in the placebo group had improvement in NYHA functional class. PVR significantly decreased with sitaxsentan treatment from baseline to week 12 (mean ± SD for 100-mg group, 1,025 ± 694 to 805 ± 553 dyne·s·cm–5 [p < 0.001]; and 300-mg group, 946 ± 484 to 753 ± 524 dyne·s·cm–5 [p < 0.001], and increased with placebo (911 ± 484 to 960 ± 535 dyne·s·cm–5). Cardiac index did not change in the placebo group after 12 weeks of treatment (2.4 ± 0.8 to 2.4 ± 0.9 L/min/m2), but increased significantly with sitaxsentan treatment (100 mg, 2.4 ± 0.8 to 2.7 ± 0.8 L/min/m2 [p < 0.02]; 300 mg, 2.3 ± 0.7 to 2.7 ± 0.9 L/min/m2 [p < 0.001]. Similar improvements in 6-min walk, functional class, and hemodynamics with both doses suggest that significant saturation of ETA receptors occurred with the 100-mg and 300-mg doses, which were at or near the top of the dose-response curve for efficacy. In contrast, the incidence of liver function abnormalities was more favorable for the 100-mg dose, ie, the incidence of elevated aminotransferase values (more than three times normal), which reversed in all cases, was 3% for the placebo group, 0% for the 100-mg group, and 10% for the 300-mg group. It should be noted that in an earlier pilot study,52 sitaxsentan was associated with fatal hepatitis when used at higher doses. In the larger randomized trial,51a the most frequently reported clinical adverse events with sitaxsentan treatment (and more frequent than in placebo) were headache, peripheral edema, nausea, nasal congestion, and dizziness, reactions previously noted with endothelin-receptor antagonists. The most frequently reported laboratory adverse event was increased INR or prothrombin time, related to the effect of sitaxsentan on inhibition of CYP2C9 P450 enzyme, the principal hepatic enzyme involved in the metabolism of warfarin. Observations during the trial showed that this interaction can be managed by reducing the warfarin dose to achieve the desired INR. A second randomized, double-blind, placebo-controlled multicenter study is in progress.
Ambrisentan
A third endothelin antagonist, ambrisentan, is currently in phase III clinical trials in patients with PAH. This ETA-selective antagonist is slightly different biochemically. Information on relative safety and efficacy will hopefully be forthcoming in the near future.
Recommendations
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Phosphodiesterase Inhibitors |
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Dipyridamole
Early clinical studies6064 demonstrated that dipyridamole can lower PVR, attenuate hypoxic pulmonary vasoconstriction, decrease PH and, at least in some cases, augment or prolong the effects of inhaled NO in children with PH. Some patients who failed to respond to inhaled NO responded to the combination of inhaled NO plus dipyridamole.60 These findings suggest that phosphodiesterase type 5 inhibition may be an effective clinical strategy for the treatment of PAH, but have been limited by the lack of potency and selectivity, and potential systemic effects of dipyridamole.
Sildenafil
Sildenafil is a potent and highly specific phosphodiesterase type 5 inhibitor, which has been proven as a safe and effective therapy for erectile dysfunction. Based on growing understanding of phosphodiesterase type 5 activity in the pulmonary circulation, uncontrolled clinical studies have examined the acute hemodynamic effects of sildenafil and its potential role in the long-term treatment of patients with PAH. Reports have shown that sildenafil blocks acute hypoxic pulmonary vasoconstriction in healthy adult volunteers,65 and acutely reduces mPAP in patients with PAH.66 Michelakis et al66 studied the effects of sildenafil in 13 patients with PAH, and reported a reduction in mPAP and PVR, with an increase in cardiac index. In comparison with inhaled NO, sildenafil had similar effects on the reduction in mPAP; unlike NO, sildenafil also had apparent systemic hemodynamic effects.6667
