Nitrates & Nitrites

NITRATES & NITRITES

Chemistry

These agents are simple nitric and nitrous acid esters of polyalcohols. Nitroglycerin may be considered the prototype of the group. Although nitroglycerin is used in the manufacture of dynamite, the systemic formulations used in medicine are not explosive. The conventional sublingual tablet form of nitroglycerin may lose potency when stored as a result of volatilization and adsorption to plastic surfaces. Therefore, it should be kept in tightly closed glass containers. Nitroglycerin is not sensitive to light.

All therapeutically active agents in the nitrate group appear to have identical mechanisms of action and similar toxicities, although susceptibility to tolerance may vary. Therefore, pharma-cokinetic factors govern the choice of agent and mode of therapy when using the nitrates.

Pharmacokinetics

The liver contains a high-capacity organic nitrate reductase that removes nitrate groups in a stepwise fashion from the parentmolecule and ultimately inactivates the drug. Therefore, oral bio-availability of the traditional organic nitrates (eg, nitroglycerin and isosorbide dinitrate) is low (typically<10–20%). For this rea-son, the sublingual route, which avoids the first-pass effect, is preferred for achieving a therapeutic blood level rapidly. Nitroglycerin and isosorbide dinitrate both are absorbed effi-ciently by this route and reach therapeutic blood levels within a few minutes. However, the total dose administered by this route must be limited to avoid excessive effect; therefore, the total dura-tion of effect is brief (15–30 minutes). When much longer dura-tion of action is needed, oral preparations can be given that contain an amount of drug sufficient to result in sustained sys-temic blood levels of the parent drug plus active metabolites. Other routes of administration available for nitroglycerin include transdermal and buccal absorption from slow-release preparations (described below). Amyl nitrite and related nitrites are highly volatile liquids.Amyl nitrite is available in fragile glass ampules packaged in a protective cloth covering. The ampule can be crushed with the fingers, resulting in rapid release of vapors inhalable through the cloth covering. The inhalation route provides very rapid absorp-tion and, like the sublingual route, avoids the hepatic first-pass effect. Because of its unpleasant odor and short duration of action, amyl nitrite is now obsolete for angina.

Once absorbed, the unchanged nitrate compounds have half-lives of only 2–8 minutes. The partially denitrated metabolites have much longer half-lives (up to 3 hours). Of the nitroglycerin metabolites (two dinitroglycerins and two mononitro forms), the 1,2-dinitro derivative has significant vasodilator efficacy and prob-ably provides most of the therapeutic effect of orally administered nitroglycerin. The 5-mononitrate metabolite of isosorbide dinitrate is an active metabolite of the latter drug and is available for oral use as isosorbide mononitrate. It has a bioavailability of 100%.Excretion, primarily in the form of glucuronide derivatives of the denitrated metabolites, is largely by way of the kidney.

Pharmacodynamics

A. Mechanism of Action in Smooth Muscle

After more than a century of study, the mechanism of action of nitroglycerin is still not fully understood. There is general agree-ment that the drug must be bioactivated with the release of nitric oxide. Unlike nitroprusside and some other direct nitric oxide donors, nitroglycerin activation requires enzymatic action. Nitroglycerin can be denitrated by glutathione S-transferase in smooth muscle and other cells. A mitochondrial enzyme, aldehyde dehydrogenase isoform 2 (ALDH2) and possibly isoform 3, ALDH3, is also capable of activating nitroglycerin and releasing nitric oxide. The differential selectivity of glutathione S-transferase and ALDH2 for different organic nitrates suggests that the ALDH2 may be the more important enzyme for nitroglycerin bioactivation. Free nitrite ion is released, which is then converted to nitric oxide . Nitric oxide (probably com-plexed with cysteine) combines with the heme group of soluble guanylyl cyclase, activating that enzyme and causing an increase in cGMP. As shown in Figure 12–2, formation of cGMP represents a first step toward smooth muscle relaxation. The production of prostaglandin E or prostacyclin (PGI₂) and membrane hyperpo-larization may also be involved. There is no evidence that auto-nomic receptors are involved in the primary nitrate response. However, autonomic reflex responses, evoked when hypotensive doses are given, are common.

As described in the following text, tolerance is an important consideration in the use of nitrates. Although tolerance may be caused in part by a decrease in tissue sulfhydryl groups, eg, on cysteine, it can be only partially prevented or reversed with a sulfhydryl-regenerating agent. Increased generation of oxygen free radicals during nitrate therapy may be another important mecha-nism of tolerance. Recent evidence suggests that diminished avail-ability of calcitonin gene-related peptide (CGRP, a potent vasodilator) is also associated with nitrate tolerance. Nicorandil and several other investigational antianginal agents appear to combine the activity of nitric oxide release with potas-sium channel-opening action, thus providing an additional mechanism for causing vasodilation. Nitroglycerin has not been reported to open potassium channels.

B. Organ System Effects

Nitroglycerin relaxes all types of smooth muscle regardless of the cause of the preexisting muscle tone (Figure 12–3). It has practi-cally no direct effect on cardiac or skeletal muscle.

Vascular smooth muscle— All segments of the vascularsystem from large arteries through large veins relax in response to nitroglycerin. Most evidence suggests a gradient of response, with veins responding at the lowest concentrations, arteries at slightly higher ones. The epicardial coronary arteries are sensitive, but concentric atheromas can prevent significant dilation. On the other hand, eccentric lesions permit an increase in flow when nitrates relax the smooth muscle on the side away from the lesion. Arterioles and precapillary sphincters are dilated least, partly because of reflex responses and partly because different vessels vary in their ability to release nitric oxide from the drug.

A primary direct result of an effective dose of nitroglycerin is marked relaxation of veins with increased venous capacitance and decreased ventricular preload. Pulmonary vascular pressures and heart size are significantly reduced. In the absence of heart failure, cardiac output is reduced. Because venous capacitance is increased, orthostatic hypotension may be marked and syncope can result. Dilation of large epicardial coronary arteries may improve oxygen delivery in the presence of eccentric atheromas. Temporal artery pulsations and a throbbing headache associated with meningeal artery pulsations are common effects of nitroglycerin and amyl nitrite. In heart failure, preload is often abnormally high; the nitrates and other vasodilators, by reducing preload, may have a beneficial effect on cardiac output in this condition .

The indirect effects of nitroglycerin consist of those compensa-tory responses evoked by baroreceptors and hormonal mecha-nisms responding to decreased arterial pressure (see Figure 6–7); this often results in tachycardia and increased cardiac contractility. Retention of salt and water may also be significant, especially with intermediate- and long-acting nitrates. These compensatory responses contribute to the development of tolerance.

In normal subjects without coronary disease, nitroglycerin can induce a significant, if transient, increase in total coronary blood flow. In contrast, there is no evidence that total coronary flow is increased in patients with angina due to atherosclerotic obstruc-tive coronary artery disease. However, some studies suggest that redistribution of coronary flow from normal to ischemic regionsmay play a role in nitroglycerin’s therapeutic effect. Nitroglycerin also exerts a weak negative inotropic effect on the heart via nitric oxide.

Other smooth muscle organs—Relaxation of smoothmuscle of the bronchi, gastrointestinal tract (including biliary system), and genitourinary tract has been demonstrated experi-mentally. Because of their brief duration, these actions of the nitrates are rarely of any clinical value. During recent decades, the use of amyl nitrite and isobutyl nitrite (not nitrates) by inhalation as recreational (sex-enhancing) drugs has become popular with some segments of the population. Nitrites readily release nitric oxide in erectile tissue as well as vascular smooth muscle and activate guanylyl cyclase. The resulting increase in cGMP causes dephospho-rylation of myosin light chains and relaxation (Figure 12–2), which enhances erection. The pharmacologic approach to erectile dys-function is are discussed in the Box: Drugs Used in the Treatment of Erectile Dysfunction.

Action on platelets—Nitric oxide released from nitroglycerinstimulates guanylyl cyclase in platelets as in smooth muscle. The increase in cGMP that results is responsible for a decrease in platelet aggregation. Unfortunately, recent prospective trials have estab-lished no survival benefit when nitroglycerin is used in acute myo-cardial infarction. In contrast, intravenous nitroglycerin may be of value in unstable angina, in part through its action on platelets.

Other effects—Nitriteion reacts with hemoglobin (whichcontains ferrous iron) to produce methemoglobin (which contains ferric iron). Because methemoglobin has a very low affinity for oxygen, large doses of nitrites can result in pseudocyanosis, tissue hypoxia, and death. Fortunately, the plasma level of nitrite result-ing from even large doses of organic and inorganic nitrates is too low to cause significant methemoglobinemia in adults. In nursing infants, the intestinal flora is capable of converting significant amounts of inorganic nitrate, eg, from well water, to nitrite ion. In addition, sodium nitrite is used as a curing agent for meats, eg,

corned beef. Thus, inadvertent exposure to large amounts of nitrite ion can occur and may produce serious toxicity.

One therapeutic application of this otherwise toxic effect of nitrite has been discovered. Cyanide poisoning results from com-plexing of cytochrome iron by the CN⁻ ion. Methemoglobin iron has a very high affinity for CN⁻; thus, administration of sodium nitrite (NaNO₂) soon after cyanide exposure regenerates active cytochrome. The cyanmethemoglobin produced can be further detoxified by the intravenous administration of sodium thiosulfate (Na₂S₂O₃); this results in formation of thiocyanate ion (SCN⁻), a less toxic ion that is readily excreted. Methemoglobinemia, if excessive, can be treated by giving methylene blue intravenously. This antidotal procedure is now being replaced by hydroxocobala-min, a form of vitamin B₁₂, which also has a very high affinity for cyanide and converts it to another form of vitamin B₁₂.

Toxicity & Tolerance

A. Acute Adverse Effects

The major acute toxicities of organic nitrates are direct extensions of therapeutic vasodilation: orthostatic hypotension, tachycardia, and throbbing headache. Glaucoma, once thought to be a con-traindication, does not worsen, and nitrates can be used safely in the presence of increased intraocular pressure. Nitrates are con-traindicated, however, if intracranial pressure is elevated. Rarely, transdermal nitroglycerin patches have ignited when external defi-brillator electroshock was applied to the chest of patients in ven-tricular fibrillation. Such patches should be removed before use of external defibrillators to prevent superficial burns.

B. Tolerance

With continuous exposure to nitrates, isolated smooth muscle may develop complete tolerance (tachyphylaxis), and the intact human becomes progressively more tolerant when long-acting preparations (oral, transdermal) or continuous intravenous infu-sions are used for more than a few hours without interruption. The mechanisms by which tolerance develops are not completely understood. As previously noted, diminished release of nitric oxide resulting from reduced bioactivation may be partly respon-sible for tolerance to nitroglycerin. Systemic compensation also plays a role in tolerance in the intact human. Initially, significant sympathetic discharge occurs, and after one or more days of therapy with long-acting nitrates, retention of salt and water may reverse the favorable hemodynamic changes normally caused by nitroglycerin. Tolerance does not occur equally with all nitric oxide donors. Nitroprusside, for example, retains activity over long periods. Other organic nitrates appear to be less susceptible than nitroglyc-erin to the development of tolerance. In cell-free systems, soluble guanylate cyclase is inhibited, possibly by nitrosylation of the enzyme, only after prolonged exposure to exceedingly high nitro-glycerin concentrations. In contrast, treatment with antioxidants that protect ALDH2 and similar enzymes appears to prevent or reduce tolerance. This suggests that tolerance is a function of diminished bioactivation of organic nitrates and to a lesser degree, a loss of soluble guanylate cyclase responsiveness to nitric oxide.

Continuous exposure to high levels of nitrates can occur in the chemical industry, especially where explosives are manufactured. When contamination of the workplace with volatile organic nitrate compounds is severe, workers find that upon starting their work week (Monday), they suffer headache and transient dizziness (“Monday disease”). After a day or so, these symptoms disappear owing to the development of tolerance. Over the weekend, when exposure to the chemicals is reduced, tolerance disappears, so symp-toms recur each Monday. Other hazards of industrial exposure, including dependence, have been reported. There is no evidence that physical dependence develops as a result of the therapeutic use of short-acting nitrates for angina, even in large doses.

C. Carcinogenicity of Nitrate and Nitrite Derivatives

Nitrosamines are small molecules with the structure R₂–N–NO formed from the combination of nitrates and nitrites with amines. Some nitrosamines are powerful carcinogens in animals, appar-ently through conversion to reactive derivatives. Although there is no direct proof that these agents cause cancer in humans, there is a strong epidemiologic correlation between the incidence of esophageal and gastric carcinoma and the nitrate content of food in certain cultures. Nitrosamines are also found in tobacco and in cigarette smoke. There is no evidence that the small doses of nitrates used in the treatment of angina result in significant body levels of nitrosamines.

Mechanisms of Clinical Effect

The beneficial and deleterious effects of nitrate-induced vasodila-tion are summarized in Table 12–2.

A. Nitrate Effects in Angina of Effort

Decreased venous return to the heart and the resulting reduction of intracardiac volume are important beneficial hemodynamic effects of nitrates. Arterial pressure also decreases. Decreased intraventricular pressure and left ventricular volume are associ-ated with decreased wall tension (Laplace relation) and decreased myocardial oxygen requirement. In rare instances, a paradoxical increase in myocardial oxygen demand may occur as a result ofexcessive reflex tachycardia and increased contractility.

Intracoronary, intravenous, or sublingual nitrate administra-tion consistently increases the caliber of the large epicardial coro-nary arteries except where blocked by concentric atheromas. Coronary arteriolar resistance tends to decrease, though to a lesser extent. However, nitrates administered by the usual systemic routes may decrease overall coronary blood flow (and myocardial oxygen consumption) if cardiac output is reduced due to decreased venous return. The reduction in oxygen consumption is the major mechanism for the relief of effort angina.

B. Nitrate Effects in Variant Angina

Nitrates benefit patients with variant angina by relaxing the smooth muscle of the epicardial coronary arteries and relieving coronary artery spasm.

C. Nitrate Effects in Unstable Angina

Nitrates are also useful in the treatment of the acute coronary syndrome of unstable angina, but the precise mechanism for their beneficial effects is not clear. Because both increased coronary vascular tone and increased myocardial oxygen demand can pre-cipitate rest angina in these patients, nitrates may exert their beneficial effects both by dilating the epicardial coronary arteries and by simultaneously reducing myocardial oxygen demand. As previously noted, nitroglycerin also decreases platelet aggregation, and this effect may be of importance in unstable angina.

Clinical Use of Nitrates

Some of the forms of nitroglycerin and its congeners are listed in Table 12–3. Because of its rapid onset of action (1–3 minutes), sublingual nitroglycerin is the most frequently used agent for the immediate treatment of angina. Because its duration of action is short (not exceeding 20–30 minutes), it is not suitable for main-tenance therapy. The onset of action of intravenous nitroglycerin is also rapid (minutes), but its hemodynamic effects are quickly reversed when the infusion is stopped. Clinical use of intravenous nitroglycerin is therefore restricted to the treatment of severe, recurrent rest angina. Slowly absorbed preparations of nitroglyc-erin include a buccal form, oral preparations, and several transder-mal forms. These formulations have been shown to provide blood concentrations for long periods but, as noted above, this leads to the development of tolerance.

The hemodynamic effects of sublingual or chewable isosorbide dinitrate and the oral organic nitrates are similar to those of nitro-glycerin given by the same route. The recommended dosage sched-ules for commonly used long-acting nitrate preparations, along with their durations of action, are listed in Table 12–3. Although transdermal administration may provide blood levels of nitroglyc-erin for 24 hours or longer, the full hemodynamic effects usually do not persist for more than 6–8 hours. The clinical efficacy of slow-release forms of nitroglycerin in maintenance therapy of angina is thus limited by the development of significant tolerance. Therefore, a nitrate-free period of at least 8 hours between doses should be observed to reduce or prevent tolerance.

OTHER NITRO-VASODILATORS

Nicorandil is a nicotinamide nitrate ester that has vasodilatingproperties in normal coronary arteries but more complex effects in patients with angina. Clinical studies suggest that it reduces both preload and afterload. It also provides some myocardial protection via preconditioning by activation of cardiac K_ATP channels. One large trial showed a significant reduction in relative risk of fatal and nonfatal coronary events in patients receiving the drug. Nicorandil is currently approved for use in the treatment of angina in Europe and Japan and has been submitted for approval in the USA.