PTCB Guide to PDE5 Inhibitors!

What are PDE5 Inhibitors?

PDE5 inhibitors inhibit the enzyme – phosphodiesterase type 5 – a class of medicines used in the treatment of erectile dysfunction.

Originally, PDE5 inhibitors were developed to reduce high blood pressure but they were then found to promote erections to a significant degree – and so the therapeutic direction moved accordingly. Today, PDE5 inhibitors are still used in the treatment of primary pulmonary hypertension.

Examples of PDE5 inhibitors include:

• Sildenafil (Revatio; Viagra)
• Tadalafil (Cialis; Adcirca)
• Vardenafil (Levitra; Staxyn)
• Avanafil (Stendra)

On the PTCB test, candidates can expect questions of the style that ask about identifying brand and drug; what they are used to treat; and why the drugs work in the way that they do.

Below, you can find more information about how PDE5 inhibitors work and the common side effects and drug interactions that students need to know.

Mechanism of Action.

The phosphodiesterase type 5 enzyme can be found in the smooth muscle of the corpus cavernosum of the penis and the arteries of the lung – hence why inhibitors of this enzyme are used to treat impotence and pulmonary hypertension.

Erections require stimulation – which ordinarily releases a substance called nitric oxide. Nitric oxide is necessary to promote dilation of blood vessels, what is known as vasodilation. PDE5 interferes with this process to prevent the release of nitric oxide. As a result, blood vessels do not dilate, and blood does not flow into the penis to produce an erection.

These drugs block the PDE5 enzyme – meaning that nitric oxide can be produced as it is expected to be produced. In this way, PDE5 inhibitors produce erections – on average, 27 minutes after the medicine has been administered.

Side Effects.

Common side effects of PDE5 inhibitors include:

• Flushing
• Headache
• Nasal congestion
• Hypotension
• Dizziness
• Elevated heart rate
• Palpitations

A small number of patients experience visual disorders – including color distortion.

A small number of patients may also experience a dangerously prolonged erection known as priapism. This requires urgent medical attention to prevent damage to the penis.

Clinical Pharmacology.

Because inhibitors of the PDE5 enzyme cause hypotension, the risk of severe or dangerous hypotension can result if these medicines are taken with other medicines that also cause hypotension. For example, nitrates and alpha blockers, to name a few.

It’s also worth noting that inhibitors of PDE5 are associated with an elevated risk of vascular events such as heart attack and stroke. These drugs should be avoided in patients who have recently experienced a cardiovascular event.

Similarly, PDE5 inhibitors should be avoided (or receive a much lower dose) in patients with severe hepatic or renal impairment.

If taken with food, absorption of PDE5 inhibitors will be delayed.

Final Thoughts.

Phosphodiesterase type 5 inhibitors are a widely prescribed drug class.

Pharmacy technicians are expected to understand this drug class – its members and dominant brand medicines; what these drugs are used to treat; and how they work in the way that they do. Furthermore, technicians should be familiar with common side effects and drug interactions.

Whilst in-depth detail is not required, technicians should have a broad understanding of these commonly prescribed medicines.

PTCB practice test questions on this topic and every other major drug class in pharmacology can be found in our registered member’s online learning portal.