Learning Objective
By the end of this section, students should be able to classify PDE inhibitors by isoform specificity, understand their mechanisms, recognize clinical uses, and identify common adverse effects, which are high-yield for USMLE Step 1 pharmacology.
Phosphodiesterase (PDE) Inhibitors
Phosphodiesterase inhibitors block PDE enzymes, which normally hydrolyze cAMP and/or cGMP. This increases intracellular second messengers, producing smooth muscle relaxation, enhanced cardiac contractility, vasodilation, or platelet inhibition, depending on the PDE isoform targeted.
Types of PDE Inhibitors
| Type / Drug | Mechanism of Action | Clinical Uses | Adverse Effects |
|---|---|---|---|
| Nonselective PDE inhibitor – Theophylline | ↓ cAMP hydrolysis → ↑ cAMP → bronchial smooth muscle relaxation → bronchodilation | COPD, asthma (rarely used) | Cardiotoxicity: tachycardia, arrhythmias; Neurotoxicity: seizures, headache; GI: abdominal pain |
| PDE-5 inhibitors – Sildenafil, Vardenafil, Tadalafil, Avanafil | ↓ cGMP hydrolysis → ↑ cGMP → smooth muscle relaxation via enhanced NO → vasodilation | Erectile dysfunction, Pulmonary hypertension, BPH (tadalafil only) | Facial flushing, headache, dyspepsia, hypotension (especially with nitrates); Sildenafil: cyanopia (blue-tinted vision) via PDE-6 inhibition |
| PDE-4 inhibitor – Roflumilast | ↑ cAMP in neutrophils, granulocytes, and bronchial epithelium → anti-inflammatory effects | Severe COPD | GI: abdominal pain, weight loss; CNS: depression, anxiety, insomnia |
| PDE-3 inhibitor – Milrinone | Cardiomyocytes: ↑ cAMP → ↑ Ca²⁺ influx → ↑ inotropy & chronotropy; Vascular smooth muscle: ↑ cAMP → MLCK inhibition → vasodilation → ↓ preload & afterload | Acute decompensated heart failure with cardiogenic shock (inotrope) | Tachycardia, ventricular arrhythmias, hypotension |
| “Platelet inhibitors” – Cilostazol, Dipyridamole | ↑ cAMP in platelets → inhibition of platelet aggregation | Intermittent claudication, Stroke/TIA prevention (with aspirin), Cardiac stress testing (dipyridamole), Prevention of coronary stent restenosis | Nausea, headache, facial flushing, hypotension, abdominal pain |
Notes:
- Cilostazol is a PDE-3 inhibitor, but is categorized with platelet inhibitors due to its clinical indications.
- Dipyridamole is a nonspecific PDE inhibitor and also prevents adenosine reuptake, increasing extracellular adenosine → vasodilation.
Activity
USMLE Step 1 Pearls
- Theophylline → bronchodilator; narrow therapeutic index; toxicity: tachyarrhythmias & seizures.
- PDE-5 inhibitors → ED & pulmonary hypertension; avoid with nitrates (severe hypotension).
- Milrinone → short-term inotrope in cardiogenic shock; risk of arrhythmias.
- Cilostazol / Dipyridamole → ↑ cAMP in platelets → antiplatelet effects.
- Roflumilast → anti-inflammatory PDE-4 inhibitor in severe COPD.
