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Dilatrate-SR (Isosorbide Dinitrate) - Description and Clinical Pharmacology



Isosorbide dinitrate (ISDN) is 1,4:3,6-dianhydro-D-glucitol 2,5 dinitrate, an organic nitrate whose structural formula is

and whose molecular weight is 236.14. The organic nitrates are vasodilators, active on both arteries and veins. Each dilatrate®-SR sustained release capsule contains 40 mg of isosorbide dinitrate, in a microdialysis delivery system that causes the active drug to be released over an extended period. Each capsule also contains ethylcellulose, lactose, pharmaceutical glaze, starch, sucrose and talc. The capsule shells contain D&C Red 33, D&C Yellow 10, gelatin and titanium dioxide.


The principal pharmacological action of isosorbide dinitrate is relaxation of vascular smooth muscle and consequent dilatation of peripheral arteries and veins, especially the latter. Dilatation of the veins promotes peripheral pooling of blood and decreases venous return to the heart, thereby reducing left ventricular end-diastolic pressure and pulmonary capillary wedge pressure (preload). Arteriolar relaxation reduces systemic vascular resistance, systolic arterial pressure, and mean arterial pressure (afterload). Dilatation of the coronary arteries also occurs. The relative importance of preload reduction, afterload reduction, and coronary dilatation remains undefined.

Dosing regimens for most chronically used drugs are designed to provide plasma concentrations that are continuously greater than a minimally effective concentration. This strategy is inappropriate for organic nitrates. Several well-controlled clinical trials have used exercise testing to assess the antianginal efficacy of continuously-delivered nitrates. In the large majority of these trials, active agents were no more effective than placebo after 24 hours (or less) of continuous therapy. Attempts to overcome nitrate tolerance by dose escalation, even to doses far in excess of those used acutely, have consistently failed. Only after nitrates have been absent from the body for several hours has their antianginal efficacy been restored.


The kinetics of absorption of isosorbide dinitrate from dilatrate®-SR sustained release capsules have not been well studied. Studies of immediate-release formulations of ISDN have found highly variable bioavailability (10 to 90%), with extensive first-pass metabolism in the liver. Most such studies have observed progressive increases in bioavailability during chronic therapy; it is not known whether similar increases in bioavailability appear during the course of chronic therapy with dilatrate®-SR sustained release capsules.

Once absorbed, the distribution volume of isosorbide dinitrate is 2-4 L/kg and this volume is cleared at the rate of 2-4 L/min, so ISDN's half-life in serum is about an hour. Since the clearance exceeds hepatic blood flow, considerable extrahepatic metabolism must also occur. Clearance is affected primarily by denitration to the 2-mononitrate (15 to 25%) and the 5-mononitrate (75 to 85%).

Both metabolites have biological activity, especially the 5-mononitrate. With an overall half-life of about 5 hours, the 5-mononitrate is cleared from the serum by denitration to isosorbide; glucuronidation to the 5-mononitrate glucuronide; and denitration/hydration to sorbitol. The 2-mononitrate has been less well studied, but it appears to participate in the same metabolic pathways with a half-life of about 2 hours.

The interdosing interval sufficient to avoid tolerance to ISDN has not been well defined. Studies of nitroglycerin (an organic nitrate with a very short half-life) have shown that dosing intervals of 10-12 hours are usually sufficient to prevent or attenuate tolerance. Dosing intervals that have succeeded in avoiding tolerance during trials of moderate doses (e.g., 30 mg) of immediate release ISDN have generally been somewhat longer (at least 14 hours), but this is consistent with the longer half-lives of ISDN and its active metabolites.

An interdosing interval sufficient to avoid tolerance with dilatrate®-SR has not been demonstrated. In an eccentric dosing study, 40 mg capsules of dilatrate®-SR were administered daily at 0800 and 1400 hours. After two weeks of this regimen, dilatrate®-SR was statistically indistinguishable from placebo. Thus, the necessary interdosing interval sufficient to avoid tolerance remains unknown, but it must be greater than 18 hours.

Few well-controlled clinical trials of organic nitrates have been designed to detect rebound or withdrawal effects. In one such trial, however, subjects receiving nitroglycerin had less exercise tolerance at the end of the daily interdosing interval than the parallel group receiving placebo. The incidence, magnitude, and clinical significance of similar phenomena in patients receiving ISDN have not been studied.

Clinical Trials

In clinical trials, extended-release oral isosorbide dinitrate has been administered in a variety of regimens, with total daily doses ranging from 40 to 160 mg. A controlled trial using a single 40 mg sustained-release oral dose of isosorbide dinitrate (dilatrate®-SR) has demonstrated effective reductions in exercise-related angina for up to 8 hours. Antianginal activity is present about 1 hour after dosing.

Adequate multiple-dose trials of dilatrate®-SR sustained release capsules have not been reported.

Most controlled trials of multiple-dose immediate-release oral ISDN taken every 12 hours (or more frequently) for several weeks have shown statistically significant antianginal efficacy for only 2 hours after dosing. Once-daily regimens, and regimens with one daily interdosing interval of at least 14 hours (e.g., a regimen providing doses at 0800, 1400 and 1800 hours), have shown efficacy after the first dose of each day that was similar to that shown in the single dose studies cited above. The efficacy of subsequent doses has not been demonstrated. From large, well-controlled studies of other nitrates, it is reasonable to believe that the maximal achievable daily duration of antianginal effect from isosorbide dinitrate is about 12 hours. No dosing regimen for dilatrate®-SR sustained release capsules has actually been shown to achieve this duration of effect.

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