Isosorbide Mononitrate Versus Dinitrate

Isosorbide mononitrate (Imdur, ISMN) and isosorbide dinitrate (Isordil, ISDN) are two nitrates used to induce venodilation and alleviate angina pectoris, but they differ in their pharmacokinetics and clinical applications.

Both ISMN and ISDN are prodrugs that undergo biotransformation to release nitric oxide (NO). This occurs more directly in ISMN, whereas ISDN requires conversion to isosorbide-2-mononitrate and isosorbide-5-mononitrate. NO activates the enzyme guanylate cyclase, leading to an increase in cyclic guanosine monophosphate (cGMP). cGMP, in turn, relaxes vascular smooth muscle, resulting in vascular dilation. Due to their vasodilatory properties, typical side effects include headaches, hypotension, and dizziness. Remember that patients who are already on phosphodiesterase inhibitors should not be started on these meds due to the risk of severe hypotension.

As far as differences, ISDN has a faster onset of action (within 30 minutes), shorter duration (around 2-4 hours), requires more frequent dosing (2-3x/day), is more potent, and is also more likely to result in tolerance. In comparison, ISMN has a slower onset (30-60 minutes), longer duration (4-6 hours), requires less frequent dosing (1-2x/day), and does not undergo significant first-pass metabolism. Because of the longer duration, ISMN is a nice choice for chronic symptoms like stable angina (not to mention the more convenient dosing is great for patient compliance!) The faster onset of ISDN benefits both acute symptom relief and maintenance therapy.

When trying to recall dosing intervals/brand names, I remember IMdur (‘M’ for “mononitrate” → mono means one, so once-daily dosing) and IsorDil (‘D’ for “dinitrate” → “di” means twice, so twice-daily dosing).

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