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Tadalafil: Mechanism of Action & Structure

Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

In this lesson, you're going to learn about a drug known generically as tadalafil. You'll find out what it's used to treat and how it actually works to treat these conditions.

Tadalafil

Tadalafil is the generic name for two better heard-of medications known as Cialis and Adcirca. Cialis is used to manage erectile dysfunction and benign prostatic hyperplasia (BPH). Adcirca is used for pulmonary arterial hypertension (PAH), which is basically high blood pressure within the lungs.

You can spot the structure of this medication in the image herein.

The structure of tadalafil.
The structure of tadalafil.

This lesson is going to go over the ways by which tadalafil exerts its therapeutic effects, its mechanism of action.

Erectile Dysfunction

Overall, tadalafil is classified as a phosphodiesterase-5 enzyme inhibitor. More simply we'll just call it by its abbreviation of PDE-5 inhibitor. PDE-5 is a type of enzyme, which is something that helps speed up biochemical reactions.

PDE-5 can be found in something known as the corpus cavernosum, which is spongy erectile tissue that engorges with blood in order to produce, you guessed it, an erection.

When a man is sexually stimulated, a substance called nitric oxide (NO) is released in the corpus cavernosum. This eventually leads to the production of cyclic guanosine monophosphate, or cGMP. cGMP causes the smooth muscles of the corpus cavernosum to relax. This enables blood to rush into the corpus cavernosum. This is what leads to an erection. At least, that's the basic but important gist of it for this lesson.

So what in the world does PDE-5 have to do with all of this? Well, PDE-5 is the enzyme that lowers the amount of cGMP, which means the smooth muscles aren't as relaxed as before. This, in turn, causes less blood to flow into the penis. And, of course, this means the erection can't be sustained very well.

So tadalafil inhibits PDE-5 from working. This means there are higher levels of cGMP in the corpus cavernosum. And, as you just learned, this will result in more blood in the corpus cavernosum and thus an erection, or a stronger one at that.

So, at the recommended dose, tadalafil does not directly lead to an erection. It only affects the biochemical response to sexual stimulation by enhancing the effects of nitric oxide.

BPH & PAH

With respect to BPH, it's not entirely clear how tadalafil actually works in this scenario. It could be that it helps minimize or reduce the proliferation of cells that lead to the signs and symptoms of BPH, such as difficulty urinating. There is supposition that tadalafil relaxes the smooth muscles of the prostate and bladder as well. Or, perhaps, it somehow affects the nervous system in a way that minimizes the signs and symptoms of this condition. It's just not clear at the moment.

With respect to PAH, tadalafil works by inhibiting PDE-5 in the smooth muscles of the blood vessels of the lungs. As with erectile dysfunction and the mechanism of action therein, this increases the levels of cGMP, and that leads to the relaxation of the pulmonary vasculature (blood vessels in the lungs). By relaxing the blood of the lungs, this helps lower the blood pressure within the lungs.

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