Drugs for Pulmonary Hypertension

History and the Modern Era of PAH Therapy

“I would comment now upon the clinical picture of pure right ventricular failure. Usually one sees it secondary to failure of the left side, or the case is one of mitral disease with embarrassment of the pulmonary circulation... It is rare in my experience to witness such a pure and rapidly progressive failure of the right ventricle.” Dr. Terence East, London, 1940, describing a series of three young women with rapidly fatal PAH.

PAH was traditionally an invariably progressive and often fatal condition. Although it remains incurable (except by lung transplantation), PAH has become viewed in more recent years a more manageable chronic condition that can be marked by long periods of clinical stability. This is in large part due to the number of agents now available for patients with PAH and the expansion of dedicated PAH comprehensive care centers with expert multidisciplinary teams dedicated to this disease.

The modern era of PAH therapy began in the mid-1990s with the remarkable success of intravenous prostacyclin. In a landmark 1996 study including 81 patients with PAH followed for only 12 weeks, intravenous epoprostenol was associated with a reduction in pulmonary vascular resistance, 6-minute walk test (6MWT) distance, and mortality—100% of epoprostenol patients were alive at study end versus 80% in the control group. Prior to the epoprostenol era, registry data of patients with idiopathic PAH showed a 1-year and 5-year survival of 68% and 34%, respectively. These figures have slowly improved over the years. Over a decade later, results from the Registry to Evaluate Early and Long-term Pulmonary Arterial Hypertension Disease Management (REVEAL Registry) showed that the 1- and 5-year survival for idiopathic PAH patients had improved to 91% and 65%, respectively.

Prostanoids

The first truly efficacious therapy in PAH, the prostanoid class of medications has now grown to include medications available in four routes of administration: intravenous, subcutaneous, inhaled, and oral.

Approved in 1996 on the basis of the randomized trial described previously, epoprostenol is the prototypical example of a vasodilatory agent used in PAH. Available only in intravenous formulation, epoprostenol has two available preparations: Flolan and Veletri, the latter designed to have enhanced room temperature stability.

Treprostinil is a synthetic prostanoid that is now available for use intravenously or subcutaneously (Remodulin), via inhalation (Tyvaso), or orally (Orenitram). Like epoprostenol, treprostinil is a potent pulmonary vasodilator. It is distinguished from epoprostenol by its much longer half-life (4 hours versus 2.7 minutes for epoprostenol) and greater flexibility with respect to route of administration.

Approved in 2004, and only available in inhaled formation, Iloprost is the third currently available prostanoid for the treatment of PAH. It is marketed under the trade name Ventavis.

While not a true prostanoid by molecular structure, Selexipag (Uptravi) is an IP receptor antagonist, administered orally, and approved for use since 2015.

Endothelin Receptor Antagonists (ERAs)

With action against the ET-1 receptor, there are three ERAs currently available for use: bosentan (Tracleer), ambrisentan (Letaris), and macitentan (Opsumit). All are oral agents and have been proven efficacious in improving hemodynamics and symptoms in PAH.

Phosphodiesterase Type 5 Inhibitors (PDE-5is)

Orally taken, the PDE-5is vasodilate the pulmonary circulation via augmentation of the cGMP pathway. Two agents are available for use in the US, sildenafil (Revatio) and tadalafil (Adcirca). Vardenafil has been studied for use in a PAH population but is not approved for use in PAH by the FDA.

Closely related to PDE-5is are the sGC stimulators, of which the recently approved oral agent riociguat (Adempas) is the only approved agent.

Calcium Channel Blockers (CCBs)

Off-label use of CCBs, particularly diltiazem and nifedipine , and occasionally amlodipine , has long had a role in PAH, specifically in those with a positive vasoreactivity study on right heart catheterization. To perform such a study, an operator with experience in PAH would place a pulmonary artery catheter in position, then administer a vasodilator, typically inhaled NO, intravenous epoprostenol, adenosine, or sodium nitroprusside. A drop in mPAP of ≥ 10 mmHg to a mPAP of ≤ 40 mmHg with stable cardiac output is defined as a positive vasoreactivity test. Approximately 5%–10% of treatment naïve patients will have a positive vasoreactivity test—a figure that may be higher in idiopathic PAH patients. A 2010 analysis of approximately 2400 registry patients showed that 8.7% took CCBs specifically for PAH, a number that is consistent with the expected prevalence of positive vasoreactivity studies.

Rationale and General Principles

In general, more aggressive therapy, including parental medications, is reserved for patients with more advanced and higher-risk disease. Correctly matching PAH therapy to a patient requires a careful and complete physiologic and historical assessment. Diligent evaluation of PAH patients, both at index presentation and longitudinally, is essential. The most important historical element is the patient’s level of functioning. Often referred to as the WHO functional class (FC), it is modeled on the frequently used New York Heart Association functional status. Briefly, FC I patients have no functional limitation. FC II patients have a slight impairment in physical activity, where ordinary causes symptoms such as dyspnea, chest pain, fatigue, or presyncope. FC III patients recall symptoms with less than ordinary daily activity, and FC IV patients are either symptomatic at rest, often with signs of overt right heart failure.

Patients with PAH often suffer from delayed diagnosis, meaning it is not unusual for PAH patients to first present with severe disease and advanced symptoms. In one study, a series of retrospective interviews with PAH patients revealed that patients averaged 5.3 general practitioner visits and 3.0 specialist visits prior to being seen in a PAH referral center—a process that took 47 months on average. In REVEAL, 61% of patients were FC III at the time of diagnostic right heart catheterization and 12% suffered from FC IV.

Initial workup of PAH will include detailed history and physical examination to determine the WHO functional class and assess for signs of right ventricular failure, including syncope, ascites, and edema. An electrocardiogram and echocardiogram are included in the initial evaluation of all patients. At times, cardiac magnetic resonance imaging (MRI) can be helpful in quantifying the level of right ventricular dysfunction. In patients in whom initial evaluation raises concern for PAH, and in patients with established disease but a change in clinical status, a right heart catheterization is required.

Risk Stratification

The evaluation of the PAH patient should allow for risk stratification. Patients with more significant functional limitation are higher risk. In addition, patients with so-called “sentinel events” such as hospital admission, intensification of PAH therapy, decrease in 6MWT distance by 15%, or worsening WHO FC should be given heightened scrutiny, as these morbidity events have been shown to be predictive of incident mortality.

The 2015 European Society of Cardiology/European Respiratory Society (ESC/ERS) guidelines proposed a simple low/moderate/high risk framework based on history, laboratory, and hemodynamic assessment ( Table 11.1 ). Patients in the highest risk category should be considered for aggressive escalation of therapy.

Patients With Positive Vasoreactivity Studies

Guidelines support vasoreactivity testing during right heart catheterization in select patients with an initial diagnosis of PAH. Contraindications include low blood pressure, reduced cardiac output, a diagnosis of pulmonary vasoocclusive disease, or pulmonary capillary hemangiomatosis (PVOD or PCH), as these patients may not tolerate an acute vasodilator challenge. Patients with FC IV would not be candidates for a trial of CCB even if vasoreactivity test were positive; therefore, it is of questionable utility to proceed with such testing in these patients.

For the 5%–10% of patients who do respond to vasodilator challenge and have FC II or III symptoms, current guidelines suggest a trial of CCB as initial therapy, with early repeat catheterization in 3 months to assess for response. Required doses are typically high; up to 240 mg for nifedipine, 20 mg for amlodipine, and 720 mg for diltiazem. Verapamil is not typically used or recommended for use in this indication, mostly over concern (perhaps based more on experience than data ) that verapamil is more potent negative inotrope than the other CCBs, including diltiazem.

Multiple concerns have been voiced for the use of long-term CCB in the PAH population. Most acutely, there is concern for the lack of direct evidence of benefit in the modern vasodilator therapy era, along with the theoretical risk of worsening inotropy on high-dose CCBs.

Asymptomatic Patients

Patients without symptoms (WHO FC I) at diagnosis represent a rare clinical scenario. Current guidelines suggest close monitoring for the development of symptoms while withholding active therapy. In such cases, careful examination, history, and functional testing may be required to determine if patients are truly asymptomatic. Occasionally, cardiopulmonary exercise testing, either invasive or noninvasive, may be considered to provide an objective assessment of severity of functional limitation and may be of use in borderline cases.

Initial Combination Therapy

From the early 1990s until 2004, all PAH clinical drug trials tested single drug regimens against placebo or alternatives. It was not until the BREATHE-2 trial, which compared epoprostenol plus bosentan to epoprostenol plus placebo in patients with WHO FC III or IV, that initial combination therapy was rigorously tested. Although it did not show significant difference in hemodynamics or clinical status, BREATHE-2 was an important first step toward initial combination therapy in treatment naïve individuals, and led the way for the AMBITION trial, published in 2015. Comprised of patients with WHO FC II and III on no current PAH therapy, AMBITION was a three-armed protocol that randomized patients to ambrisentan plus tadalafil combination therapy or to each drug (plus placebo) alone. In contrast to BREATHE-2, AMBITION showed a marked reduction in a combined clinical endpoint of all-cause mortality, PAH hospitalization, and disease progression.

The 2015 ESC/ERS guidelines make no value judgment on starting combination therapy versus monotherapy in treatment naïve patients presenting with FC II or III, citing the possible benefit of combination therapy seen in AMBITION but also the potential for increased drug interactions and side effects with upfront combination therapy. In contrast, the 2019 update to the American College of Chest Physicians (ACCP) guideline now recommends initial combination ambrisentan plus tadalafil for patients with FC II and III, reflecting the advancing acceptance of the overall benefit profile of upfront combination therapy. This represents a change from the 2014 guidelines and is given a grade of weak recommendation with moderate quality of evidence. Tables 11.2 and 11.3 outline selected regimens for initial combination therapy as supported by current guidelines and expert consensus.

Initial Monotherapy

Starting monotherapy in treatment naïve PAH patients, then adding additional agents sequentially based on patient response and tolerability is the more traditional approach to the management of PAH—albeit a decreasingly popular choice in light of the AMBITION trial data. In general, patients presenting with WHO FC II or III symptoms are recommended to begin with oral therapy; parenteral therapy is reserved for WHO functional class IV patients and those with high-risk features ( Table 11.4 ). In the 2019 ACCP guidelines, the same therapy might have several different recommendation grades for a particular WHO FC, as the guidelines’ authors may provide a separate recommendation grade for multiple outcomes. For example, there may be two recommendations provided for one particular drug: one for the endpoint of improving 6MWT and one for improving functional class. The summary information in Table 11.4 records the strongest recommendation given for each drug.

Several key differences exist among the presently offered expert guidelines. The ACCP currently recommends starting an intravenous or subcutaneous prostanoid (epoprostenol or treprostinil) in patients with FC IV and issue no recommendation for oral agents in this scenario. The ESC/ERS guidelines do issue recommendations for specific oral agents as monotherapy in FC IV; however, these guidelines also make a statement recommending upfront combination therapy that includes an intravenous or subcutaneous prostanoid in such patients.

Sequential Therapy

For patients already started on initial monotherapy, a common strategy for treatment of PAH consists of short interval reassessment of symptoms and risk ( Table 11.1 ) with consideration of sequential addition of therapies. This has been the approach often taken in the trial literature as well, where novel agents are often tested when added to established medications.

In patients who continue to be symptomatic on stable doses of an ERA or PDE-5i, the addition of inhaled iloprost or inhaled treprostinil is guideline-supported. Riociguat is recommended as an addition to bosentan, ambrisentan, or inhaled prostanoid. Macitentan receives a recommendation as an addition to symptomatic patients on stable doses of a PDE-5i or inhaled prostanoid, and tadalafil as add-on therapy to ambrisentan.

In practice, drugs within a class are often substituted for each other, based on side effect profiles, availability or formulary access, and cost. This is done with the presumption that class effects may be interchangeable to varying degrees, but evidence does exist regarding the specific nature of activity of single drugs independent of drug class as well.