單硝酸異山梨酯（Isosorbide Mononitrate）
Overview
Isosorbide mononitrate is a long-acting nitrate medication prescribed to prevent angina (chest pain) in patients with coronary artery disease. Unlike fast-acting nitrates used for acute attacks, it is taken regularly to keep blood vessels dilated and reduce the frequency of angina episodes.
Mechanism of Action
Similar to nitroglycerin, it acts as a nitric oxide (NO) donor. The released NO stimulates guanylate cyclase, increasing cGMP levels and causing relaxation of vascular smooth muscle. This leads to the dilation of both veins and arteries, which decreases the heart’s workload (preload and afterload) and myocardial oxygen demand, effectively preventing angina.
Dosage and Administration
It is typically administered as an immediate-release tablet taken twice daily, or an extended-release tablet taken once daily. For immediate-release formulations, the doses should be taken 7 hours apart (e.g., 8 AM and 3 PM) to allow for a nitrate-free interval of 10-14 hours, which is crucial to prevent the development of nitrate tolerance.
Side Effects
The most frequently reported side effect is a headache, which is often dose-related and may subside with continued use. Other side effects include dizziness, lightheadedness, hypotension (low blood pressure), tachycardia (fast heart rate), and flushing.
Contraindications
It is strictly contraindicated in patients concurrently using PDE5 inhibitors (e.g., sildenafil, tadalafil) due to the risk of severe and potentially fatal hypotension. It should not be used in patients with severe anemia, closed-angle glaucoma, or known hypersensitivity to nitrates.
Pregnancy and Lactation
Safety during pregnancy has not been established, so it should only be used if the potential benefits outweigh the risks. It is unknown if the drug is excreted in breast milk; therefore, caution is advised for nursing mothers.
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