Dilatrate-SR

DILATRATE-SR- isosorbide dinitrate capsule, extended release
Schwartz Pharma

Description

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

Image from Drug Label Content

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 micro-dialysis 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.

Clinical Pharmacology

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.

Pharmacokinetics

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 (75-85%).

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 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 trails, 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.

Indications and Usage

dilatrate®-SR sustained release capsules are indicated for the prevention of angina pectoris due to coronary artery disease. The onset of action of controlled-release oral isosorbide dinitrate is not sufficiently rapid for this product to be useful in aborting an acute anginal episode.

Contraindications

Allergic reactions to organic nitrates are extremely rare, but they do occur. Isosorbide dinitrate is contraindicated in patients who are allergic to it.

Warnings

Amplification of the vasodilatory effects of dilatrate® SR by sildenafil can result in severe hypotension. The time course and dose dependence of this interaction have not been studied. Appropriate supportive care has not been studied, but it seems reasonable to treat this as a nitrate overdose, with elevation of the extremities and with central volume expansion.

The benefits of extended-release oral isosorbide dinitrate in patients with acute myocardial infarction or congestive heart failure have not been established. If one elects to use isosorbide dinitrate in these conditions, careful clinical or hemodynamic monitoring must be used to avoid the hazards of hypotension and tachycardia. Because the effects of extended-release oral isosorbide dinitrate are so difficult to terminate rapidly, this formulation is not recommended in these settings.

Precautions

General

Severe hypotension, particularly with upright posture, may occur with even small doses of isosorbide dinitrate. This drug should therefore be used with caution in patients who may be volume depleted or who, for whatever reason, are already hypotensive. Hypotension induced by isosorbide dinitrate may be accompanied by paradoxical bradycardia and increased angina pectoris.

Nitrate therapy may aggravate the angina caused by hypertrophic cardiomyopathy.

As tolerance to isosorbide dinitrate develops, the effects of sublingual nitroglycerin on exercise tolerance, although still observable, is somewhat blunted.

Some clinical trials in angina patients have provided nitroglycerin for about 12 continuous hours of every 24-hour day. During the interdosing intervals in some of these trials, anginal attacks have been more easily provoked than before treatment and patients have demonstrated hemodynamic rebound and decreased exercise tolerance. The importance of these observations to the routine, clinical use of controlled-release oral isosorbide dinitrate is not known.

In industrial workers who have had long-term exposure to unknown (presumably high) doses of organic nitrates, tolerance clearly occurs. Chest pain, acute myocardial infarction, and even sudden death have occurred during temporary withdrawal of nitrates from these workers demonstrating the existence of true physical dependence.

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