1. Clinical Overview
Isosorbide dinitrate (ISDN) is a potent organic nitrate vasodilator used primarily for the prophylaxis and treatment of angina pectoris. It is a prodrug that undergoes denitration to form active metabolites, primarily isosorbide-5-mononitrate (IS-5-MN) and isosorbide-2-mononitrate (IS-2-MN). It reduces myocardial oxygen demand by causing venous and arterial dilation, leading to decreased preload and afterload. In the Indian context, it is a widely available, cost-effective antianginal agent, though its use has been partially supplanted by the longer-acting IS-5-MN.
| Onset | Duration | Bioavailability |
|---|---|---|
| Oral: 45-60 minutes; Sublingual: 2-5 minutes. | Oral: 4-6 hours; Sublingual: 1-2 hours. | Oral: Approximately 10-25% due to extensive first-pass metabolism. |
2. Mechanism of Action
Isosorbide dinitrate is a prodrug that is converted to nitric oxide (NO) intracellularly, primarily in vascular smooth muscle cells. NO activates soluble guanylyl cyclase, leading to increased intracellular cyclic guanosine monophosphate (cGMP). Elevated cGMP causes dephosphorylation of myosin light chains, resulting in smooth muscle relaxation and vasodilation.
3. Indications & Uses
- Prophylaxis and treatment of angina pectoris (chronic stable angina)
- Acute relief of angina attacks (via sublingual formulation)
4. Dosage & Administration
Adult Dosage: For angina prophylaxis: 10-40 mg orally 2-3 times daily. A typical regimen is 10-20 mg every 6-8 hours, allowing for a 10-14 hour nitrate-free interval (e.g., last dose at 6 PM, first dose at 8 AM). For acute angina: 5-10 mg sublingually, repeated every 5-10 minutes for up to 3 doses. If pain persists, seek emergency care.
Administration: Oral tablets: Swallow whole with a glass of water, with or without food. Do not crush or chew sustained-release formulations. For sublingual: Place tablet under the tongue and allow to dissolve completely. Do not swallow immediately. Do not eat, drink, or smoke until tablet is dissolved. Patient must be sitting or lying down when taking the first dose to avoid syncope.
5. Side Effects
Common side effects may include:
- Headache (often dose-limiting, may diminish with continued use)
- Dizziness, lightheadedness
- Flushing
- Postural hypotension
- Reflex tachycardia
- Weakness
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Phosphodiesterase-5 Inhibitors (Sildenafil, Tadalafil, Vardenafil) | Profound, life-threatening hypotension | Contraindicated |
| Other Vasodilators (ACE inhibitors, ARBs, Alpha-blockers, Calcium channel blockers) | Additive hypotensive effect | Major |
| Alcohol | Enhanced vasodilation and hypotension | Major |
| Heparin | ISDN may antagonize anticoagulant effect of heparin (mechanism unclear) | Moderate |
| Dihydroergotamine | ISDN may increase the bioavailability and vasoconstrictive effects of dihydroergotamine | Moderate |
| Antihypertensives | Potentiated hypotensive effect | Moderate |
| Tricyclic Antidepressants (TCAs) | May potentiate hypotensive effects | Moderate |
| Aspirin | Increased nitrate bioavailability | Minor |
7. Patient Counselling
- DO take the medication exactly as prescribed, allowing for a nitrate-free interval as directed.
- DO sit or lie down when taking the first dose or if you feel dizzy.
- DO keep sublingual tablets handy at all times for acute angina. Check expiration date regularly.
- DO inform all healthcare providers (including dentists) that you are on nitrates.
- DONT take with medications for erectile dysfunction (e.g., Sildenafil, Tadalafil).
- DONT stop taking this medicine suddenly without consulting your doctor.
- DONT crush, chew, or split sustained-release tablets.
- DONT consume alcohol while on this medication.
8. Toxicology & Storage
Overdose: Symptoms are primarily related to vasodilation: Severe hypotension with throbbing headache, vertigo, palpitations, visual disturbances, flushing, sweating, nausea, vomiting, syncope, methemoglobinemia (cyanosis, dyspnea, chocolate-brown blood), air hunger, slow pulse, heart block, hyperpnea, paralysis, coma, and death due to circulatory collapse.
Storage: Store at a temperature not exceeding 30°C, in a cool, dry place, protected from light and moisture. Keep sublingual tablets in their original glass container with the cotton plug, tightly closed. Keep out of reach of children. Do not use if tablets are discolored or brittle.