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Sildenafil: Mechanism of Action & Structure

Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

In this lesson, you're going to learn about sildenafil. Namely, what conditions it's used for, what it looks like structurally, as well as how it works in treating these conditions.

Sildenafil

Have you ever heard of sildenafil? If you said no, then you've definitely heard of Viagra, the brand name for the generic term of sildenafil. Viagra is used to treat erectile dysfunction (impotence). Another brand name version of sildenafil, Revatio, is used to manage pulmonary arterial hypertension (PAH), or high blood pressure (hypertension) in the arteries of the lungs ('pulmon-').

You can check out the structure of sildenafil in this lesson's image. And if you keep reading, you'll also learn exactly how it is that sildenafil works at treating both of these conditions.

The structure of sildenafil.
The structure of sildenafil.

Erectile Dysfunction

An erection occurs and disappears as a result of an interplay of forces. Not to be funny, just for simplicity's sake, we'll call the forces that lead to an erection as the good guys and the ones that cause one to disappear as the bad guys. As a side note, the bad guys can actually be good since an indefinite erection can be painful and dangerous so you'd be hoping for the 'bad' guys to win in that case!

But for men with erectile dysfunction, they need the good guys to win. What are these good guys? One of them is called nitric oxide, or NO. When a man is sexually stimulated, this chemical is released in the corpus cavernosum. The latter is spongy erectile tissue that fills with blood during an erection.

Anyways, NO eventually leads to the appearance of another good guy, called cGMP. cGMP does something important. cGMP causes the smooth muscles of the corpus cavernosum to relax. This allows the corpus cavernosum to fill up with blood, producing an erection.

Good. Now the bad part. The bad guys here is an enzyme known as phosphodiesterase-5, or PDE-5. PDE-5 is the party pooper. This enzyme degrades and reduces the amount of cGMP. And working backwards logically, this means the smooth muscles of the corpus cavernosum can't relax well. This means the penis can't engorge with blood and the erection disappears (or never appears in the first place).

Sildenafil (Viagra) to the rescue! Sildenafil is a phosphodiesterase-5 inhibitor. It rescues the party by stopping PDE-5 from reducing the amount of cGMP in the penis. This means the levels of cGMP stay high, the corpus cavernosum relaxes, fills with blood, and an erection occurs and is maintained so long as sexual stimulation is present.

That last bit is actually important. At the recommended dose, sildenafil does not produce an erection directly. Without sexual stimulation, an erection isn't going to appear out of nowhere. Instead, sildenafil enhances the effect started by the good guys, NO and cGMP, by inhibiting the effect of the bad guy, PDE-5.

Pulmonary Arterial Hypertension

And with that, you're in a solid spot to understand how sildenafil, as Revatio, is used to help manage pulmonary arterial hypertension, or high blood pressure in the arteries of the lungs.

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