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Nitroglycerin (Nitroglycerin) - Description and Clinical Pharmacology

 
 



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DESCRIPTION:

Nitroglycerin is 1,2,3-propanetriol trinitrate, an organic nitrate whose structural formula is:

Molecular Structure


Molecular Structure

CLINICAL PHARMACOLOGY

The principal pharmacological action of nitroglycerin is relaxation of vascular smooth muscle and consequent dilation 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 indistinguishable from 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 had been absent from the body for several hours was their anti-anginal efficacy restored.

Pharmacokinetics: The volume of distribution of nitroglycerin is about 3 L/kg, and nitroglycerin is cleared from this volume at extremely rapid rates, with a resulting serum half-life of about 3 minutes. The observed clearance rates (close to 1 L/kg/min) greatly exceed hepatic blood flow; known sites of extrahepatic metabolism include red blood cells and vascular walls.

The first products in the metabolism of nitroglycerin are inorganic nitrate, and the 1,2- and 1,3-dinitroglycerols. The dinitrates are less effective vasodilators than nitroglycerin, but they are longer-lived in the serum, and their net contribution to the overall effect of chronic nitroglycerin regimens is not known. The dinitrates are further metabolized to (non-vasoactive) mononitrates and, ultimately, to glycerol and carbon dioxide.

To avoid development of tolerance to nitroglycerin, drug-free intervals of 10-12 hours are known to be sufficient; shorter intervals have not been well studied. In one well-controlled clinical trial, subjects receiving nitroglycerin appeared to exhibit a rebound or withdrawal effect, so that their exercise tolerance at the end of the daily drug-free interval was less than that exhibited by the parallel group receiving placebo.

Reliable assay techniques for plasma nitroglycerin levels have only recently become available, and studies using these techniques to define the pharmacokinetics of oral nitroglycerin preparations have not been reported. Published studies using older techniques provide results that often differ, in similar experimental settings, by an order of magnitude.

Clinical Trials: Controlled trials of single oral doses of nitroglycerin have demonstrated that nitroglycerin capsules can effectively reduce exercise-related angina for up to 5 hours. Anti-anginal activity is present about 1 hour after ingestion of a capsule.

Controlled trials of multiple-dose oral nitroglycerin have shown statistically significant anti-anginal efficacy 2½ and 4 hours after a dose when oral nitroglycerin had been administered four times a day for 2 weeks or three times a day for 1 week. As noted above, careful studies with other formulations of nitroglycerin have shown that maintenance of continuous 24-hour plasma levels of nitroglycerin results in insurmountable tolerance. Presumably, the studied 1-week and 2-week regimens of oral nitroglycerin therapy achieved adequate nitrate-free intervals by non-uniformity of dosing interval, with longer intervals overnight. The investigators did not report how subjects interpreted their dosing instructions, and they similarly did not report which dose of the day was the one after which they obtained the end-of-trial exercise results.

Thus, these studies of oral nitroglycerin should not be interpreted as demonstrations that these regimens provide round-the-clock anti-anginal protection. From large, well-controlled studies of other nitroglycerin formulations, it is reasonable to believe that the maximal achievable daily duration of anti-anginal effect from Nitroglycerin Extended-Release Capsules is about 12 hours.

In some controlled trials of other organic nitrate formulations, efficacy has declined with time. Because the controlled, multiple-dose trials of oral nitroglycerin did not include exercise tests before the last day of treatment, it is not known how the efficacy of Nitroglycerin Extended-Release Capsules may vary during extended therapy.

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