What are the medications for pulmonary arterial hypertension and their mechanisms of action?

Medications for Pulmonary Arterial Hypertension and Mechanisms of Action

Pulmonary arterial hypertension is treated with medications targeting four major pathways: the endothelin pathway (endothelin receptor antagonists), the nitric oxide pathway (phosphodiesterase-5 inhibitors and soluble guanylate cyclase stimulators), the prostacyclin pathway (prostacyclin analogs and receptor agonists), and the activin signaling pathway (sotatercept). 1

Major Drug Classes and Mechanisms

Endothelin Receptor Antagonists (ERAs)

Endothelin-1 causes vasoconstriction and smooth muscle cell proliferation by binding to endothelin receptors type A and B in pulmonary vascular smooth muscle. ERAs block these receptors to prevent these harmful effects 2:

• Bosentan (125 mg twice daily): Dual antagonist blocking both endothelin receptor types A and B. Requires monthly liver function monitoring due to 10% risk of dose-dependent, reversible hepatic aminotransferase elevations 2, 1

• Ambrisentan (5-10 mg once daily): Selective endothelin receptor type A antagonist. Lower hepatotoxicity risk (0.8-3%), so monthly liver monitoring not mandated in the US, though peripheral edema is more common 2, 1

• Macitentan (10 mg once daily): Dual antagonist with demonstrated reduction in morbidity/mortality composite endpoints over 100 weeks. Watch for hemoglobin drops ≤8 g/dL (occurs in 4.3% of patients) 2, 1

Phosphodiesterase Type-5 Inhibitors (PDE5Is)

These drugs inhibit the enzyme that degrades cyclic GMP, causing vasodilation through the nitric oxide/cGMP pathway 2:

• Sildenafil (20 mg three times daily): First PDE5I approved for PAH 1

• Tadalafil (40 mg once daily): Longer half-life allowing once-daily dosing 1

Soluble Guanylate Cyclase Stimulators

• Riociguat (0.5-1.0 mg three times daily, titrated up to maximum 2.5 mg): Directly stimulates soluble guanylate cyclase to increase cGMP production, causing vasodilation through the nitric oxide pathway. Titrate by 0.5 mg every 2 weeks as tolerated 1

Critical caveat: Never combine riociguat with PDE5 inhibitors—both target the same pathway and combination causes dangerous hypotension.

Prostacyclin Pathway Agents

Prostacyclin causes vasodilation and inhibits smooth muscle proliferation. Multiple formulations exist 1:

Prostacyclin Analogs:

• Epoprostenol (IV infusion): Start 2 ng/kg/min, titrate as tolerated. Requires continuous infusion via central line. Most potent agent with mortality benefit in severe PAH 1

• Treprostinil: Available in multiple formulations

  • Subcutaneous/IV: Start 1.25 ng/kg/min, increase weekly
  • Inhaled: 18-54 mg (3-9 inhalations) four times daily
  • Oral: 0.25 mg twice daily or 0.125 mg three times daily, increase every 3-4 days 1

• Iloprost (inhaled): 2.5-5.0 mg, 6-9 inhalations daily 1

Prostacyclin Receptor Agonist:

• Selexipag (oral): Start 200 mg twice daily, titrate weekly as tolerated to maximum 1,600 mg twice daily. Selectively activates prostacyclin receptors without affecting the prostacyclin pathway directly 1

Calcium Channel Blockers

Only for acute vasoreactivity responders (approximately 10% of idiopathic PAH patients). High-dose CCBs (nifedipine, diltiazem, or amlodipine) are indicated exclusively for patients demonstrating acute vasoreactivity during right heart catheterization 2:

• Requires complete reassessment with repeat right heart catheterization at 3-4 months
• Continue only if patient achieves WHO functional class I or II with near-normalization of hemodynamics
• If inadequate response, immediately switch to PAH-specific therapy
• Never use CCBs in non-responders—can cause dangerous hemodynamic deterioration 2

Treatment Strategy Algorithm

For Treatment-Naïve Patients with WHO Functional Class II-III:

Initial combination therapy with ambrisentan (10 mg daily) plus tadalafil (40 mg daily) is preferred over monotherapy to improve exercise capacity (6-minute walk distance improved by 49 m vs 24 m with monotherapy, P<0.001) 1. This represents a weak recommendation with moderate quality evidence based on the AMBITION trial 1.

For Patients Already on Monotherapy:

Add agents from different pathways based on inadequate response. For example, adding bosentan to sildenafil showed statistically significant but clinically modest improvement (21.8 m increase in 6-minute walk distance, below the 33 m minimally important difference) 1.

For High-Risk Patients:

Consider triple therapy including parenteral prostacyclin (epoprostenol) from diagnosis, as parenteral therapy demonstrates greater efficacy than oral agents 3.

Common Pitfalls to Avoid

• Never combine riociguat with PDE5 inhibitors—severe hypotension risk
• Do not use CCBs without documented acute vasoreactivity—causes clinical deterioration in non-responders
• Monitor liver function monthly with bosentan—dose-dependent hepatotoxicity
• Watch for peripheral edema with ambrisentan—more common than other ERAs
• Ensure proper central line care with IV epoprostenol—line infections are serious complications
• Do not underdose tadalafil—40 mg daily is the evidence-based dose, not the 20 mg used for erectile dysfunction

Emerging Therapies

Sotatercept, targeting the activin signaling pathway within the BMP/TGF-β pathway, represents a novel fourth mechanistic approach. It acts as an activin receptor ligand trap to rebalance growth-promoting and growth-inhibiting signaling, reducing pulmonary vascular resistance and improving exercise capacity 4, 5. This addresses the underlying vascular remodeling rather than just vasodilation.