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ISOSORBIDE DINITRATE
Extended-release Tablets, USP

DESCRIPTION

Isosorbide dinitrate (ISDN), an organic nitrate, is a vasodilator with effects on both arteries and veins. Isosorbide dinitrate is available as 40 mg extended-release tablets. The chemical name for isosorbide dinitrate 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.

Isosorbide dinitrate is a white, crystalline, odorless compound which is stable in air and in solution, has a melting point of 70°C and has an optical rotation of +134° (c=1.0, alcohol, 20°C). Isosorbide dinitrate is freely soluble in organic solvents, such as acetone, alcohol, and ether, but is only sparingly soluble in water.

Each Isosorbide Dinitrate Extended-release tablet, for oral administration, contains 40 mg of isosorbide dinitrate, in a matrix that causes the active drug to be released over a sustained period. In addition, each tablet also contains the following inactive ingredients: anhydrous lactose, colloidal silicon dioxide, FD&C Yellow #6 Aluminum Lake, hydroxypropyl methylcellulose, magnesium stearate, and stearic acid.

Conformance of Isosorbide Dinitrate Extended-release Tablets with the USP release test is pending.

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 anti-anginal 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 anti-anginal efficacy been restored.

Pharmacokinetics

The kinetics of absorption of isosorbide dinitrate has 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 Isosorbide Dinitrate Extended-release Tablets.

Once absorbed, the volume of distribution of isosorbide dinitrate is 2 to 4 L/kg, and this volume is cleared at the rate of 2 to 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 effected 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 daily dose-free 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 daily dose-free intervals of 10 to 12 hours are usually sufficient to minimize tolerance. Daily dose-free 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. A dose-free interval sufficient to avoid tolerance with Isosorbide Dinitrate Extended-release Tablets has not been demonstrated. Clinical trials using Isosorbide Dinitrate Extended-release Tablets in a regimen designed to avoid tolerance have not been conducted, but in a multiple-dose study of another controlled-release isosorbide dinitrate product, 40 mg capsules were administered at 0800 and 1400 hours. After two weeks of this regimen, the controlled-release isosorbide dinitrate product was statistically indistinguishable from placebo. For the formulation of controlled-release isosorbide dinitrate that was tested, the necessary dose-free interval must therefore be greater than 18 hours; the necessary interval for Isosorbide Dinitrate Extended-release Tablets remains unknown.

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 dose-free 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, immediate-release oral isosorbide dinitrate has been administered in a variety of regimens, with total daily doses ranging from 30 mg to 480 mg. Controlled trials of single doses of controlled-release isosorbide dinitrate have demonstrated effective reductions in exercise-related angina for up to 8 hours. Anti-anginal activity is present about 1 hour after dosing.

Adequate multiple-dose trials of Isosorbide Dinitrate Extended-release Tablets 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 anti-anginal efficacy for only 2 hours after dosing. Once-daily regimens, and regimens with one daily dose-free 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 anti-anginal effect from isosorbide dinitrate is about 12 hours. No dosing regimen for Isosorbide Dinitrate Extended-release Tablets has, however, ever actually been shown to achieve this duration of effect.

INDICATIONS AND USAGE

Isosorbide Dinitrate Extended-release Tablets 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 Extended-release Tablets are contraindicated in patients who are allergic to isosorbide dinitrate or to any of its other ingredients.

WARNINGS

Amplification of the vasodilatory effects of isosorbide dinitrate 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 nitrate overdose, with elevation of the extremities and with central volume expansion.

The benefits of controlled-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 controlled-release oral isosorbide dinitrate are so difficult to terminate rapidly, this formulation is not recommended in these settings.

PRECAUTIONS

ADVERSE REACTIONS

Adverse reactions to isosorbide dinitrate are generally dose-related, and almost all of these reactions are the result of isosorbide dinitrate's activity as a vasodilator. Headache, which may be severe, is the most commonly reported side effect. Headache may be recurrent with each daily dose, especially at higher doses. Transient episodes of lightheadedness, occasionally related to blood pressure changes, may also occur. Hypotension occurs infrequently, but in some patients it may be severe enough to warrant discontinuation of therapy. Syncope, crescendo angina, and rebound hypertension have been reported but are uncommon.

Extremely rarely, ordinary doses of organic nitrates have caused methemoglobinemia in normal-seeming patients. Methemoglobinemia is so infrequent at these doses that further discussion of its diagnosis and treatment is deferred (see “OVERDOSAGE”).

Data are not available to allow estimation of the frequency of adverse reactions during treatment with Isosorbide Dinitrate Extended-release Tablets.

OVERDOSAGE

DOSAGE AND ADMINISTRATION

As noted under “CLINICAL PHARMACOLOGY” multiple-dose studies with ISDN and other nitrates have shown that maintenance of continuous 24-hour plasma levels results in refractory tolerance. Every dosing regimen for Isosorbide Dinitrate Extended-release Tablets must provide a daily dose-free interval to minimize the development of this tolerance. With immediate-release ISDN, it appears that one daily dose-free interval must be at least 14 hours long. The necessary dose-free interval for Isosorbide Dinitrate Extended-release Tablets has not been clearly identified, but is presumably somewhat longer.

As also noted under “CLINICAL PHARMACOLOGY” only one trial has ever studied the use of controlled-release isosorbide dinitrate for more than one dose. In that trial, 40 mg of a different formulation of controlled-release ISDN was administered twice daily in doses given 6 hours apart. After 4 weeks, active treatment could not be distinguished from placebo.

Large controlled studies with other nitrates suggest that no dosing regimen with Isosorbide Dinitrate Extended-release Tablets should be expected to provide more than about 12 hours of continuous anti-anginal efficacy per day.

In clinical trials, immediate-release oral isosorbide dinitrate has been administered in a variety of regimens, with total daily doses ranging from 30 mg to 480 mg.

HOW SUPPLIED

Isosorbide Dinitrate Extended–release Tablets, USP 40 mg are available as round, scored, peach colored tablets, debossed IL/3613, in bottles of 100 (NDC 0258–3613–01) and 1000 (NDC 0258–3613–10).

STORAGE

Store at controlled room temperature 15°–30°C (59°–86°F). Dispense in well closed container.

Inwood Laboratories, Inc.
Inwood, NY 11096
Subsidiary of Forest Laboratories, Inc.
Revised February 2002
©2002 Forest Laboratories, Inc.

MG #7498
(08)

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