Pulmonary Hypertension & Peripheral Vascular Drugs
One of the world's most famous drugs was invented to treat the heart, failed, became a bedroom blockbuster by accident — and then circled back to save people with a rare, deadly lung disease. Sildenafil's strange journey is the perfect way into the drugs for pulmonary hypertension and the peripheral vessels, because they all work by relaxing arteries through a handful of precise molecular switches.
Sildenafil began life as a heart drug — designed to relax blood vessels for angina. In trials it disappointed for the heart, but volunteers reported a striking side effect, and it was reborn as Viagra. Yet the same vessel-relaxing mechanism turned out to be exactly what's needed in pulmonary arterial hypertension — a rare disease where the lung arteries clamp down and strangle the right heart. So the drug came full circle, back to serious cardiovascular medicine. Its story reveals the molecular switches that all these specialist vasodilators flip.
Three pathways to relax the lung arteries
Pulmonary hypertension is treated by dilating the lung vessels. In pulmonary arterial hypertension the small lung arteries are abnormally constricted and remodelled, forcing the right side of the heart to pump against a crushing resistance until it fails. The drugs target three vessel-tone pathways. The nitric oxide pathway: PDE5 inhibitors (sildenafil, tadalafil) block the enzyme that breaks down cGMP, so cGMP builds up and relaxes the artery — the same mechanism as nitrates, and the same reason these drugs must NEVER be combined with nitrates (both raise cGMP, causing dangerous hypotension). The endothelin pathway: endothelin receptor antagonists (bosentan, ambrisentan) block endothelin, a powerful natural vasoconstrictor — but they can harm the liver and are teratogenic. The prostacyclin pathway: prostacyclin analogues (epoprostenol, iloprost, treprostinil) are potent dilators given by infusion or inhalation for severe disease. Three pathways, three drug classes, one goal — open the lung arteries.
The peripheral vessels
Out in the limbs, peripheral arterial disease narrows the arteries to the legs, causing pain on walking (claudication). The mainstays are treating the underlying atherosclerosis — an antiplatelet and a statin, plus stopping smoking and exercising — but one specific symptom drug is cilostazol, which inhibits a phosphodiesterase (PDE3) to both dilate vessels and inhibit platelets, improving walking distance. For Raynaud's phenomenon, where the fingers' arteries spasm and blanch in the cold, calcium channel blockers (the dihydropyridines) are first-line. And, closing the circle, the PDE5 inhibitors that treat pulmonary hypertension are the same drugs used for erectile dysfunction — with the same absolute rule that they must never be taken with nitrates.
Notice the cGMP thread running through the whole cardiovascular section. Nitrates make nitric oxide, which raises cGMP to relax vessels (angina). PDE5 inhibitors block the breakdown of cGMP, raising it too (pulmonary hypertension, erectile dysfunction). Because BOTH raise cGMP, combining a nitrate with a PDE5 inhibitor stacks the effect and can cause fatal hypotension — the single most important interaction in this field. One molecule, cGMP, ties nitrates, sildenafil, and the lung-artery drugs together.
- Pulmonary arterial hypertension is treated by dilating the lung arteries via three pathways.
- PDE5 inhibitors (sildenafil) ↑cGMP; endothelin antagonists (bosentan) block a vasoconstrictor; prostacyclins dilate.
- NEVER combine a PDE5 inhibitor with a nitrate — both raise cGMP → dangerous hypotension.
- Peripheral arterial disease: antiplatelet + statin + risk-factor control; cilostazol for symptoms.
- Raynaud's phenomenon: calcium channel blockers are first-line.
- Combining a PDE5 inhibitor with a nitrate. It can cause fatal hypotension.
- Using systemic blood-pressure drugs for pulmonary hypertension without targeting the lung pathways.
- Forgetting endothelin antagonists (bosentan) are hepatotoxic and teratogenic — monitor & avoid in pregnancy.
- Treating claudication symptoms while ignoring the atherosclerosis (antiplatelet, statin, smoking).
Why must PDE5 inhibitors (like sildenafil) never be combined with nitrates?
- Pulmonary hypertension: dilate lung arteries via PDE5 inhibitors, endothelin antagonists, prostacyclins.
- PDE5 inhibitors and nitrates both raise cGMP — never combine them (fatal hypotension).
- Peripheral arterial disease: antiplatelet + statin + risk control; cilostazol for claudication.
- Raynaud's is treated with calcium channel blockers.
- Katzung BG. Basic & Clinical Pharmacology — Drugs for pulmonary hypertension & vasodilators.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Pulmonary vasodilators; PDE inhibitors.
- ESC/ERS Pulmonary Hypertension Guidelines — Endothelin antagonists, PDE5 inhibitors & prostacyclins.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Vascular smooth muscle & vasodilators.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Drugs for pulmonary & peripheral vascular disease.
