36.7 Phosphodiesterase 5 Inhibitors

Phosphodiesterase 5 Inhibitors

PDE5 inhibitors treat erectile dysfunction (ED) by preventing the breakdown of a molecule called cGMP, which is responsible for relaxing smooth muscle and increasing blood flow in the penis. For nursing practice, understanding their mechanism, drug interactions, and contraindications is critical because the most dangerous interaction (with nitrates) can be fatal.

PDE5 Inhibitor Mechanism of Action in Erectile Dysfunction

During sexual arousal, nitric oxide (NO) is released in the corpus cavernosum of the penis. NO activates an enzyme that produces cyclic guanosine monophosphate (cGMP), which relaxes smooth muscle cells and allows blood to flow in, producing an erection.

Normally, the enzyme phosphodiesterase type 5 (PDE5) breaks down cGMP, ending the erection. PDE5 inhibitors block this enzyme, so cGMP accumulates and smooth muscle relaxation is prolonged. The result is improved blood flow into the corpus cavernosum and better erection quality and duration.

One point that comes up repeatedly on exams: sexual stimulation is still required. These drugs don't initiate an erection on their own. They only enhance the NO/cGMP pathway that sexual arousal triggers.

Common PDE5 Inhibitors and Indications

• Sildenafil (Viagra) — Used for ED. Also marketed as Revatio for pulmonary arterial hypertension (PAH) at a lower dose. This dual indication is high-yield for pharmacology exams.
• Tadalafil (Cialis) — Used for ED and also approved for benign prostatic hyperplasia (BPH) symptoms. Has the longest duration of action (up to 36 hours), which is why it's sometimes called the "weekend pill." Can also be dosed daily at a lower strength.
• Vardenafil (Levitra) — Used for ED. Similar profile to sildenafil with a slightly longer duration of action.
• Avanafil (Stendra) — Used for ED. Has the fastest onset (as quick as 15 minutes), which distinguishes it from the others.

PDE5 Inhibitor Side Effects and Drug Interactions

Common side effects stem from vasodilation occurring beyond just the penile vasculature:

• Headache
• Facial flushing
• Nasal congestion
• Dyspepsia (indigestion)
• Back pain (especially with tadalafil)
• Visual disturbances (blurred vision, blue-tinted vision — rare, more associated with sildenafil)

Critical drug interactions:

• Nitrates (e.g., nitroglycerin, isosorbide mononitrate) — This is the most dangerous interaction and an absolute contraindication. Both nitrates and PDE5 inhibitors increase cGMP levels through different mechanisms. Together, they can cause severe, potentially fatal hypotension. Patients must not take nitrates within 24 hours of sildenafil/vardenafil or within 48 hours of tadalafil (due to its longer half-life).
• Alpha-blockers (e.g., tamsulosin, doxazosin) — Use with caution. Both drug classes lower blood pressure, so combining them raises the risk of orthostatic hypotension. If used together, the PDE5 inhibitor should be started at the lowest dose.
• CYP3A4 inhibitors (e.g., ketoconazole, ritonavir, erythromycin) — These slow the metabolism of PDE5 inhibitors, raising drug levels in the blood and increasing the risk of side effects. Dose adjustments may be needed.

Nursing Considerations and Patient Education for PDE5 Inhibitor Therapy

Before starting therapy, assess for:

• Recent myocardial infarction (within the past 90 days)
• Recent stroke (within the past 6 months)
• Unstable angina or uncontrolled hypertension/hypotension
• Current use of nitrates or nitrate-containing products (including recreational amyl nitrite/"poppers")
• Complete medication history, with specific attention to alpha-blockers, antifungals, HIV antiretrovirals, and macrolide antibiotics

Patient education points:

1. Take the medication as needed before anticipated sexual activity. Do not exceed the prescribed dose or frequency.
2. Onset of action varies by drug: avanafil can work in about 15 minutes, sildenafil and vardenafil typically take 30–60 minutes, and tadalafil may take up to 2 hours for peak effect. Taking the drug with a high-fat meal can delay absorption (except tadalafil, which is less affected by food).
3. Sexual stimulation is necessary for the medication to work.
4. Common side effects like headache and flushing are expected and usually mild.
5. Seek emergency care immediately if an erection lasts longer than 4 hours. This is called priapism, and it's a medical emergency. Prolonged priapism can cause permanent damage to penile tissue due to ischemia.
6. Never combine with nitrates. If chest pain occurs during or after sexual activity, do not take nitroglycerin. Call 911 and inform emergency personnel that a PDE5 inhibitor was taken.
7. Report any sudden vision loss or sudden hearing loss, as these are rare but serious adverse effects that require immediate evaluation.