You may be surprised to learn that the drug Sildenafil, better known by the brand name Viagra, has other medical uses besides treating erectile dysfunction in men. It can also be used to treat lung disease, which often has a poor prognosis.
Sildenafil works by inhibiting an enzyme called phosphodiesterase.
Sildenafil works through a complex pathway involving other molecules, ultimately relaxing smooth muscle and dilating blood vessels. The latter effect is known as vasodilation.
Vasodilation results in increased blood flow to organs, both the penis and the lungs.
Vasodilation by sildenafil may be beneficial in pulmonary diseases such as pulmonary arterial hypertension (PAH) and idiopathic pulmonary fibrosis (IPF).
People with PAH and IPF have progressive shortness of breath and chronic cough. Besides the lungs, PAHs and IPFs can affect many other organ systems.
PAH is a disease in which high pressure is applied to the arteries of the lungs, which are normally designed for lower pressure, causing breathing difficulties and heart strain. Fortunately, it is a rare disease that affects 1–2 people per million people each year.
IPF is the more common lung disease, with an estimated incidence of 2 to 29 cases per 100,000 people per year. It is a chronic, repeated thickening, hardening, and scarring (fibrosis) of the lungs.
The etiology of both diseases is often unclear, and doctors and researchers do not fully understand why these diseases develop and progress.
Both diseases are incurable and often get worse over time despite the best treatments. Currently, there are few effective treatments and their discovery is of constant interest.
The use of sildenafil in PAH is now well established, effective, and approved in Canada. There are several high-quality randomized controlled clinical trials that have shown efficacy in improving exercise capacity and symptom burden.
Sildenafil is often called Revatio (rather than Viagra for erectile dysfunction) in PAH, but there is a difference between Viagra and Revatio, except that patients often take Revatio three times a day in small doses. , makes little difference.
Our recently published paper integrates the evidence for multiple treatments for PAH. The results examined the combination of drugs in the same class, such as sildenafil, tadalafil (trade name Cialis), and vardenafil (trade name Levitra), with other commonly used PAH drugs.
Results showed a 12.7% reduction in clinical adverse events such as disease progression and hospitalization compared to placebo. In addition, the exercise performance index measured by the 6‑minute walk test improved by nearly 50m.
The use of sildenafil in IPF is less certain as few randomized controlled trials, the gold standard of evidence, have been conducted.
Only four trials have been considered for use in IPF. This small number of meta-analyses approached statistical significance, suggesting that effects will become apparent as more trials are completed.
The latest European Respiratory Society/American Thoracic Society guidelines addressing this issue recommend against the use of sildenafil in IPF due to lack of data.
Recently, however, a drug with a similar effect to sildenafil (treprostinil) has shown promise for patients with interstitial lung disease (a generic term for lung diseases including IPF) and pulmonary hypertension. The greatest benefit was seen in patients diagnosed with interstitial lung disease.
This further demonstrates the promise of agents like sildenafil and similar vasodilatory mechanisms in the treatment of IPF.
For intractable diseases such as PAH and IPF, there are benefits to reusing drugs like sildenafil.
On the other hand, developing new drugs is very expensive. On the other hand, when it comes to the safety of new drugs, like Viagra, which is widely used, its side effect profile is well known in the medical community.
For example, sildenafil is known to lower blood pressure and should be avoided by people who are prone to hypotension or who are taking certain antihypertensive drugs. Other common side effects include hot flashes, headaches, and changes in vision.
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