Pharmacokinetic determinants of nitrate action

Urapidil, compared to nitroglycerin, has better clinical safety in the treatment of hypertensive patients with acute heart failure: a meta-analysis

Objectives

The application of urapidil for treating hypertensive patients with acute heart failure in the emergency department remains controversial. Our objective was to organize the relevant articles and assess the clinical indexes between urapidil and nitroglycerin.

Materials and methods

PubMed, EMBASE, the Cochrane Library and China National Knowledge Infrastructure were searched for randomized studies that compared urapidil treatment with nitroglycerin treatment for hypertensive patients with acute heart failure. The risk ratio, with 95% CI, was calculated by using a corresponding effects model, according to the value of I².

Results

Seven randomized controlled trials were identified, in order to compare the clinical indexes. On comparing the clinical indexes, the urapidil group was found to be better than the nitroglycerin group in regard to left ventricular ejection fraction, systolic blood pressure, N-terminal prohormone of brain natriuretic peptide, left ventricular end-diastolic volume, cardiac index, ALT, AST and health complications (P<0.05), but the indexes of creatinine were worse in the urapidil group. Furthermore, the two methods of treatment were comparable in diastolic blood pressure, left ventricular end-systolic volume, left ventricular end-systolic dimension, heart rate, fasting plasma glucose and total cholesterol levels (P>0.05).

Conclusion

Based on the current evidence, urapidil treatment had better clinical safety features than the traditional pharmaceutical treatment with nitroglycerin. For those indicators with a small amount of data, a greater number of randomized, high-quality controlled trials should be conducted in order to further verify the findings, which could give researchers a more comprehensive evaluation of urapidil treatment.

Efficacy of different forms of nitrates in angina pectoris

Nitroglycerin has maintained its position in the treatment of angina pectoris for more than a century. Efficacy of oral nitrates has been established and compares well with that of other anti-anginal drugs. New delivery systems are being developed for sustained systemic nitrate action. Beneficial action of nitrates in congestive heart failure and their crucial role in unstable angina and acute myocardial infarction has further widened their therapeutic use. A plausible hypothesis of the mechanism of nitrate-induced vasodilation has been presented, involving production of nitrosothiols and activation of guanylate cyclase in the vascular smooth muscle. Recent developments suggest that the rate degradation of nitrates and formation of nitrosothiols in the vascular smooth muscle are linked, offering an explanation to the relatively rapidly developing, but partial vascular tolerance during high-dose nitrate therapy.

Nitrate therapy today

A review of nitrate therapy including a short summary of their physiological effects is presented. Both cardiac and non-cardiac indications are reviewed including esophageal spasm, spasm of bile ducts and urinary tract, Raynaud's disease, pulmonary hypertensive disorders, portal hypertension, bronchial asthma, and effect on arrhythmias.

Response to changing plasma concentrations of isosorbide-bound NO2 during acute and sustained treatment with isosorbide dinitrate in patients with coronary artery disease

BACKGROUND

The mechanisms behind development of tolerance to nitrate effects during sustained, asymmetric isosorbide dinitrate (ISDN) therapy are not fully understood.

HYPOTHESIS

The study was undertaken to investigate the changes of the relationships between left ventricular (LV) function and plasma concentrations of ISDN and its vasoactive metabolites (2- and 5-ISMO) during acute and sustained, asymmetric ISDN therapy.

METHODS

Left ventricular function and plasma concentrations of ISDN, 2- and 5-isosorbide mononitrates (P-ISDN, P-2- and 5-ISMO) were measured at rest and at supine exercise before and for 4 h after peroral 30 mg ISDN in 15 patients with coronary artery disease, all with initial exercise pulmonary artery wedge pressure (PAWP) > 25 mmHg. Seven patients were untreated (acute group), while eight received 30 mg ISDN b.i.d. for 2 weeks before the invasive study. P-ISDN and the concentration of available isosorbide-bound nitrate (NO2) in plasma (P-ISDN.2 + P-2-ISMO + P-5-ISMO) (P-NO2) were used as measures of the nitric oxide (NO) offer to the tissues.

RESULTS

Throughout the study, after administration of medication, all plasma concentrations, in particular P-ISDN, were higher in the chronic than in the acute group. Peak P-ISDN was reached after 15 min in the chronic group and after 25 min in the acute group, while P-2- and 5-ISMO reached maximum only after 40 min in both groups. At rest, the full effect on PAWP was observed after 10 min in both groups, but at markedly higher levels of P-ISDN and P-NO2 in the chronic group. Afterward, no further changes in PAWP were observed. During exercise, 1 h after medication, PAWP and stroke index to PAWP ratio (SI/PAWP) were normal in both groups. Thereafter, at slowly declining P-NO2, PAWP rose and SI/PAWP declined toward the initial level in the chronic group, but remained unchanged in the acute group, in spite of higher P-NO2 and greater NO release in the former.

CONCLUSIONS

Patients receiving sustained, asymmetric 30 mg ISDN b.i.d. dosing had the same immediate beneficial effects on LV function during exercise after a morning dose as did untreated patients. However, in spite of higher P-NO2 and higher rate of NO release during sustained treatment, the effects deteriorated gradually 2 to 3 h after medication. The changes in metabolism and/or distribution of isosorbide-bound NO2 may possibly be part of the tolerance induced by long-term treatment, even with asymmetric dosing.

Vasodilators in the management of acute heart failure

Recent guidelines by the Heart Failure Society of America have recommended consideration for use of nitroprusside, nitroglycerin, or nesiritide in addition to diuretics to achieve hemodynamic and symptomatic improvement. This article reviews the results of previous studies evaluating the pharmacologic and clinical effects and safety profiles of these drugs in patients with heart failure.

The role of nitric oxide synthase/nitric oxide in vascular smooth muscle control

Vascular compliance is dependent on endogenous and exogenous sources of nitric oxide (NO). In a discussion of therapeutics and NO derived via nitric oxide synthase (NOS) enzymes, it is necessary to examine the pathways of each drug to provide the clinical perfusionist with a greater understanding of the role of NOS/NO in vascular function. Endothelial-derived NO is a contributor in the vasoregulation of vascular smooth muscle. Therapeutics seek to mimic the vasodilatory effects of the endogenous NO. The therapeutics included in this review are nitroglycerin, nitroprusside, amyl nitrite, and inhalation of NO. L-Arginine supplementation provides additional substrate for the endogenous pathway that can augment NO production. NO is a small bioactive molecule involved in various biochemical pathways. Dysregulation of NO production can impair normal physiologic control of vascular compliance. Therefore, the purpose of this review is to provide the perfusionist with an understanding of the biochemical and pharmacological aspects of NOS/NO associated with vascular function.

The influence of drug input rate on the development of tolerance to frusemide

AIMS

Understanding the impact of drug input rate on its pharmacokinetic-pharmacodynamic relationship may lead to a more optimal drug therapy. The aim of the present study was to investigate the influence of the rate of administration on tolerance development to frusemide, by giving the drug at four different infusion rates.

METHODS

Eight healthy volunteers were given 10 mg of frusemide on four different occasions, as a constant-rate intravenous infusion during 10, 30, 100 and 300 min, respectively. Urinary volume and contents of frusemide and sodium were measured in samples collected over 8 h.

RESULTS

The four different infusion rates systematically influenced the frusemide excretion rate versus diuretic and natriuretic response relationship. Counter-clockwise hysteresis occurred for the most rapid infusion rate, whereas a progressive clockwise hysteresis was observed for the slower infusions, indicating development of tolerance. For each subject, diuresis and natriuresis were modeled for all four treatments simultaneously, using a feedback tolerance model. It was not possible to describe the data using a model without tolerance. The time course of tolerance development showed remarkable differences between the infusion rates. The intensity of maximum tolerance development was significantly less for the slowest infusion rate compared with the more rapid infusions and it appeared significantly later. However, no differences in diuretic or natriuretic response were found between the treatments.

CONCLUSIONS

The direction of the hysteresis loop is dependent on the input rate of frusemide. After the administration of a single low dose of frusemide, the time course of tolerance, rather than the integrated time course of tolerance, is influenced by the drug input rate.

Application of pharmacodynamic modeling for designing time-variant dosing regimens to overcome nitroglycerin tolerance in experimental heart failure

PURPOSE

Prolonged continuous administration of nitroglycerin (NTG) leads to hemodynamic tolerance. We used a previously developed pharmacokinetic-pharmacodynamic (PK/PD) model of NTG tolerance in experimental heart failure to test whether dosage regimens, designed from this model, may allow avoidance of tolerance development upon continuous NTG infusion.

METHODS

Simulation experiments (using ADAPT II) were performed to evolve a time-variant infusion regimen that would theoretically provide sustained hemodynamic effect (30% reduction in left ventricular end-diastolic pressure, LVEDP) throughout 10 hours of drug dosing. A computer controlled infusion pump was utilized to deliver this time-variant input. Infusion experiments were then conducted in CHF rats to challenge the predictability of the applied PK/PD model.

RESULTS

Simulations showed that exponentially increasing input functions provided more sustained LVEDP effects when compared to linear or hyperbolic input functions delivering the same total NTG dose. A computer-selected infusion regimen of 6.56e0.00156 x minutes micrograms/min was anticipated to provide the desired hemodynamic profile in our animal model. Experiments conducted in rats with congestive heart failure (n = 4) confirmed the prediction of sustained hemodynamic effect without tolerance (28 +/- 4% reduction in LVEDP at 10 hrs).

CONCLUSIONS

These findings support the utility of our PK/PD model of NTG tolerance in predicting NTG action, and serve as an example of therapeutic optimization through PK/PD considerations.

Permeation behavior of salbutamol sulfate through hydrophilic and hydrophobic membranes embedded by thermo-responsive cholesteryl oleyl carbonate

PURPOSE

To investigate the suitability of hydrophilic or hydrophobic membranes for use as potential thermo-responsive drug delivery system.

METHODS

Liquid crystal was embedded in membranes using vacuum filtration method to control the penetration rate of salbutamol sulfate. Cholesteryl oleyl carbonate (COC) with smectic-cholesteric phase transition temperature near 18 degrees C was used as a model liquid crystal.

RESULTS

It indicates only hydrophilic salbutamol sulfate can penetrate through the hydrophilic membranes embedded with or without COC, in which the permeation is mainly governed by the adsorption of COC. However, the hydrophilic drug do not pass through the hydrophobic membranes even if not embedded with COC. The void volume of the membrane also influences the penetration of salbutamol sulfate. The higher thermo-response efficacy of the COC-embedded membranes can be explained not only by less permeability through matrix part of the membrane but also by higher thermal motion of the COC molecules due to above the phase transition temperature.

CONCLUSIONS

A COC-embedded membrane with rate-controlled and thermo-responsive function is easily prepared by vacuum filtration method. High reversibly thermo-responsive function can be achieved by choosing membrane and COC concentration properly.

Methionine restores the venodilative response to nitroglycerin after the development of tolerance

Depletion of sulfhydryl groups may contribute to nitroglycerin tolerance after long-term exposure. This study was performed to assess whether methionine, an amino acid capable of augmenting sulfhydryl availability, would restore the venodilative response to sublingual nitroglycerin once tolerance had developed. The venodilative response to organic nitrates was assessed with use of the equilibration technique of forearm plethysmography. Venous volume was measured before and after sublingual administration of 0.4 mg of nitroglycerin at baseline study and after 5 g of intravenous methionine. Retesting was performed 2 h after application of a 10 mg nitroglycerin patch and compared with the response after 74 h of nitroglycerin patch exposure before and after intravenous methionine. Methionine alone had no intrinsic venodilative action. Although the venous volume at rest was unchanged after methionine administration, the response to sublingual nitroglycerin was potentiated compared with baseline values (37 +/- 15% versus 32 +/- 13%, p less than 0.02). During nitroglycerin patch exposure, the response to sublingual nitroglycerin was significantly attenuated at 74 h compared with the response at 2 h of exposure (16 +/- 10% versus 31 +/- 13%, p less than 0.001). The venodilative response to sublingual nitroglycerin was restored at 74 h after methionine administration (35 +/- 14% versus 16 +/- 10%, p less than 0.001). Thus, methionine potentiates the venodilative effect of sublingual nitroglycerin both immediately and in the setting of nitrate tolerance.

Effect of nitroglycerin ointment placement on headache and flushing in healthy subjects

The purpose of this study was to determine the effect of site of nitroglycerin ointment placement on headache and flushing in healthy subjects. Twenty-six subjects were given two doses of nitroglycerin ointment 3 hr apart in a randomly assigned arm-chest sequence or chest-arm sequence. Subject reports of headache and flushing using the visual analogue scale were recorded. There were no significant differences in headache or flushing based on the site or sequence of placement. Thus, the practice by nurses of routinely teaching patients to vary the site of nitroglycerin ointment placement to minimize side-effects must be questioned.

Mechanisms of nitrate tolerance: a review

With the increased use of long-acting nitroglycerin preparations, there has been greater recognition of the problem of nitrate tolerance. In recent years extensive research has broadened our understanding of the mechanisms of nitroglycerin action and the mechanisms of drug attenuation. This paper reviews the current state of knowledge regarding nitroglycerin tolerance, with an emphasis on the concepts of cellular and neurohumoral mechanisms of drug attenuation. The discussion includes potential approaches to prevent nitrate tolerance, including the introduction of a nitrate-free interval, or concomitant administration of sulfhydryl donors or neurohumoral blocking agents.

Relationship of pharmacokinetic and pharmacodynamic properties of the organic nitrates

Glyceryl trinitrate (nitroglycerin), isosorbide dinitrate and isosorbide mononitrate are, in various formulations, available for clinical use. The pharmacokinetics of glyceryl trinitrate are complex and only 1% of the drug administered orally can be detected in the plasma due to a pronounced arteriovenous concentration gradient, hydrolysis in the blood, and rapid denitration in the liver. There is a poor and usually transient correlation between plasma concentrations and therapeutic effects, due in part to the complex pharmacokinetics of glyceryl trinitrate, but primarily due to development of tolerance during continuous administration, either via the intravenous or cutaneous route. Isosorbide dinitrate kinetics are complicated by its extensive metabolism into 2- and 5-mononitrates, which are pharmacologically active, and which also accumulate more than the parent drug during long term treatment. These facts, combined with development of tolerance during long term therapy, preclude the establishment of a concentration-response relationship. Isosorbide-5-mononitrate has ideal and dose-linear kinetics and is nearly 100% bioavailable after oral administration. However, tolerance develops during long term therapy, and therefore a relationship between plasma concentrations and clinical effects cannot be established. On the basis of available data, plasma concentrations of various nitrates do not reliably predict clinical effects.

Antianginal effects of intravenous nitroglycerin over 24 hours

To determine the constancy of hemodynamic and antianginal effects of the constant infusion of intravenous nitroglycerin (NTG) and their relationship to infusion rate and plasma NTG concentration, we administered maximal tolerated doses of intravenous NTG (range 10 to 120 micrograms/min, mean = 52 +/- 33 micrograms/min) and placebo to 10 patients with chronic stable angina for 25 hr each in a randomized, double-blind fashion. Sublingual NTG (0.4 mg) was given at 24.5 hr of infusion as a positive control. Bicycle exercise time (NIH protocol), blood pressure, heart rate, exercise ST response, and venous plasma NTG were determined before and at 1, 4, 8, 24, and 24.5 hr. Plasma NTG was linearly related to infusion rate, reached a steady state within 15 min and was unchanged over 24 hr (mean = 5.5 +/- 1.2 ng/ml). Mean plasma NTG clearance was 9.3 liters/min. However, during dose titration, patients demonstrated different relationships between plasma NTG and hemodynamic effects, with widely varying slopes and intercepts. Intravenous NTG produced a sustained reduction in blood pressure and a rise in heart rate at rest, and a reduction in blood pressure during submaximal exercise at as late as 24 hr, associated with reduced submaximal ST segment abnormality. In contrast, exercise tolerance to onset of angina showed a marked initial increase on intravenous NTG but fell progressively and did not differ from that with placebo at 24 hr. Increased exercise tolerance was associated with an increase in maximal heart rate and double product (heart rate X blood pressure), suggesting that direct coronary vasodilation and/or reduced left ventricular volume were the principal determinants of increased exercise tolerance.(ABSTRACT TRUNCATED AT 250 WORDS)

Sustained-release isosorbide mononitrate (50 mg): optimization of a once-daily dosage form for long-term treatment of angina pectoris

A capsule formulation containing 50 mg of isosorbide mononitrate has been developed, 30% of which is in a nonsustained-release form to ensure rapid attainment of initial plasma levels. The remaining 70% is in a sustained-release form that ensures a long-lasting effect and the convenience of once-daily dosing. This formulation of sustained-release isosorbide mononitrate (Elantan LA 50 mg) provides plasma levels greater than 100 ng/ml for 17 hours after oral administration. For the remaining 7 hours, the nitrate plasma level ranges between 50 and 100 ng/ml.

Nitrates

Nitroglycerin and the long-acting nitrates are playing an increasingly important role in cardiovascular medicine. These agents are recommended in all of the various anginal syndromes and are as effective as the beta-blockers and calcium channel antagonists. There is a definite place for nitrate therapy in treating the complications of acute myocardial infarction. These drugs are also highly effective as unloading therapy in congestive heart failure. The mechanisms of action of the nitrates are reviewed in this article. Information is provided regarding nitrate efficacy in all the major clinical syndromes in which these drugs are used. Finally, appropriate dosing strategies are suggested that should eliminate the potential problem of nitrate tolerance.

Pharmacokinetics and pharmacodynamics of organic nitrates

The pharmacokinetic and pharmacodynamic aspects of organic nitrates are discussed, with particular emphasis on the 3 major organic nitrates currently in use, nitroglycerin (NTG), isosorbide dinitrate and isosorbide-5-mononitrate. After intravenous administration, both NTG and isosorbide dinitrate exhibit large systemic clearances and both nitrates appear to be extensively distributed in vascular and other peripheral tissues. Two pharmacokinetic features appear particularly notable for NTG: there is a significant arteriovenous extraction of the drug, and its systemic clearance is related to cardiac output. Both of these features, plus other evidence, suggest that organic nitrates may be substantially removed from the systemic circulation by the vasculature itself. During nitrate tolerance, plasma drug concentrations remain elevated, but vascular activity is diminished. This apparent paradox might be explainable by a unifying hypothesis of reduced nitrate metabolism during vascular tolerance; thus, in the tolerant state, reduced systemic clearance of the intact drug brought about elevated plasma concentrations, whereas reduced cellular metabolism at the smooth muscle brought about a decrease in vascular activity. The complex relations among plasma kinetics, vascular metabolism and pharmacologic action of organic nitrates are still poorly understood.

Tolerance to organic nitrates in ischemic heart disease

The development of tolerance to organic nitrates in patients with ischemic heart disease is reviewed, with particular interest in alterations to both the hemodynamic and antiischemic effects over time. The article primarily focuses on how tolerance is defined, what biochemical mechanisms are involved when this condition occurs, which agents have been associated with the development of tolerance, and what can be done to prevent or reverse the condition in patients taking nitrates for ischemic heart disease. From a historical perspective, tolerance to organic nitrates has been a recognized phenomenon since the last century. The role that blood-level determinations and nitroglycerin pharmacokinetics have in the development of tolerance is discussed, and an extensive overview of currently marketed organic nitrate preparations and a few others available only through approved investigational protocols is presented. The role of cross-tolerance is discussed as is the role that nitrate-free intervals play in partially or completely reversing the effects of tolerance during chronic nitrate therapy. Additionally, a discussion of which specific nitrate formulation are least likely to have tolerance associated with their use is included, such as short-acting nitrate formulations with the exception of the intravenous dosage form. Finally, buccal nitroglycerin is presented as another new formulation that appears to be associated with minimal tolerance in studies already completed.

Glyceryl trinitrate (nitroglycerin) and the organic nitrates. Choosing the method of administration

Nitrate usage worldwide is on the increase as the indications for therapy expand. Present indications for nitrate therapy include chronic stable angina pectoris, unstable angina pectoris, complications of acute myocardial infarction, and 'unloading' therapy for acute and chronic congestive heart failure. Nitrates are also being used in the operating suite by anaesthesiologists to control systolic blood pressure during various surgical procedures. New nitrate delivery systems have recently become available which provide considerable dosing flexibility, further increasing the interest in this group of compounds. The dominant action of nitrates is a direct effect on vascular smooth muscle, producing vasodilation of the veins and arteries. These drugs decrease myocardial work by lowering systolic blood pressure, systemic vascular resistance, and reducing intracardiac dimensions. In addition, nitrates have a potent effect on cardiac preload as a result of systemic venodilatation. There is also some evidence that nitrates exert direct effects on the coronary circulation (vasodilatation of coronary arteries and coronary collateral vessels, and direct atherosclerotic stenosis dilatation). These actions may play a role in relieving myocardial ischaemia. Adverse sequelae of nitrate therapy are well known and serious adverse reactions are uncommon. Headache and dizziness are the most frequent side effects. Nitrate tolerance is a definite problem - present evidence indicates that long acting formulations, high doses, or frequent dosing regimens are particularly likely to induce vascular tolerance to nitrates. Consequently, provision of a nitrate-free interval has taken on increasing significance as a strategy to avoid tolerance. Nitrate delivery systems are numerous. Although availability varies from country to country, in most countries there are a wide variety of formulations of glyceryl trinitrate (nitroglycerin) available, including sublingual and oral tablets, oral spray, topical ointment as well as discs or patches for transdermal administration, a transmucosal tablet and an intravenous formulation. Similar formulations of isosorbide dinitrate, except buccal tablets, are available in some countries. Isosorbide 5-mononitrate, a potent metabolite of isosorbide dinitrate, is achieving increasing popularity as an antianginal drug. Optimum nitrate therapy requires a good understanding of the properties of the various formulations, particularly onset and duration of action and propensity to induce tolerance.(ABSTRACT TRUNCATED AT 400 WORDS)

Early tolerance to hemodynamic effects of high dose transdermal nitroglycerin in responders with severe chronic heart failure

Transdermal systems for delivery of nitroglycerin have been shown to provide sustained blood levels of the drug for at least 24 hours. Investigations of hemodynamic effects of transdermal nitroglycerin in patients with heart failure have demonstrated a transient reduction in pressure lasting less than the expected 24 hours. These findings could be due to the development of circulatory tolerance to the vasodilatory effects of nitroglycerin or to insufficient drug dosing. In the present study, we compared the hemodynamic effects of the first and the second doses of high dose (120 mg) transdermal nitroglycerin given 24 hours apart in 11 responders (greater than or equal to 20% reduction in mean pulmonary artery wedge pressure lasting greater than or equal to 2 hours). Initiation of nitroglycerin therapy resulted in a significant reduction in mean right atrial pressure lasting for 14 hours and in a reduction in mean pulmonary artery and mean pulmonary artery wedge pressures lasting 24 hours. After administration of the second dose, mean right atrial pressure at 2 hours (9 +/- 5 versus 7 +/- 4 mm Hg), 4 hours (8 +/- 5 versus 6 +/- 4 mm Hg) and 8 hours (8 +/- 5 versus 6 +/- 3 mm Hg) was higher than after the first dose (p less than 0.05). Both mean pulmonary artery and mean pulmonary artery wedge pressures were significantly higher after the second nitroglycerin dose.(ABSTRACT TRUNCATED AT 250 WORDS)

New therapeutic modalities for the treatment of elderly patients with ischemic heart disease

In developing a treatment plan for elderly patients with ischemic heart disease, it is important to appreciate that the pathophysiologic process and aging influence the type of response produced by various drugs. The aging process also alters the way drugs are absorbed, distributed, and eliminated. Each of these variables must be considered in deciding which drugs should be used and how they should be administered.

Clinical pharmacokinetics of glyceryl trinitrate following the use of systemic and topical preparations

Glyceryl trinitrate has been used for more than a century for the treatment of angina pectoris and, more recently, for the treatment of congestive heart failure. The introduction of transdermal delivery systems has renewed the controversy regarding the efficacy of the drug, mainly in the light of the development of tolerance. With concentrations of the order of 1 microgram/L or less, the measurement of glyceryl trinitrate in plasma is not easy: gas chromatography with electron capture detection has been used widely but recently gas chromatography-mass spectrometry has provided satisfactory results. Assay problems are most likely to be responsible for some of the unexpected results reported. Further factors which may confound the results of the study of plasma concentrations are the rapid metabolism of glyceryl trinitrate in blood in vitro, adsorption to containers and infusion sets, and the uptake and/or metabolism in vessel walls. From the intravenous infusion data, the large interindividual variability in plasma concentrations of glyceryl trinitrate is apparent. The plasma half-life is about 2 to 3 minutes; plasma clearance values reported vary from 216 to 3270 L/h, indicating extensive non-hepatic metabolism. With transdermal administration, mainly with the transdermal controlled delivery systems, plasma concentrations of glyceryl trinitrate appear to be maintained for up to 24 hours, with large interindividual variations. Despite the ability to maintain, for example with the transdermal delivery systems, relatively constant concentrations of glyceryl trinitrate, it has not been possible to find a relationship between plasma concentrations and pharmacological or clinical effects. This is in part due to the attenuation of the effects with time; from the available data it is clear that this attenuation occurs at a pharmacodynamic level (reflex adaptation and tolerance) and not at the pharmacokinetic level.

Nitrate tolerance

Nitrates are widely used in anginal prophylaxis. In spite of the fact that the concept of nitrate tolerance was first put forward many years ago, the question of tolerance has remained controversial. There is widespread agreement that tolerance does occur to the effects of nitrates on arterial pressure. In contrast, tolerance to the venous and pulmonary effects is disputed. Similarly, the possibility of tolerance to the antianginal effects remains at issue. In this review, I discuss the factors which may have contributed to conflicting results in different studies, in particular recent findings on the rapidity of onset and reversal of tolerance. Tolerance develops rapidly on initiation of treatment and disappears equally rapidly on its discontinuation. In addition, tolerance appears to be a function of plasma nitrate profile. It is most likely to occur when plasma nitrate levels are constant and least likely when nitrate levels fluctuate. Furthermore, the provision of a daily nitrate free interval may protect against the development of tolerance. I then discuss the implications of these findings for patient management. They suggest that nitrate prophylaxis should not be used continuously, if this can be avoided. Rather, prophylaxis should be tailored to the individual to provide protection at times of maximum susceptibility, while allowing nitrate levels to fall at other times. The adequacy of antianginal protection with drug regimens incorporating a nitrate free interval requires further assessment. Similarly, the possibility that a nitrate free interval might lead to withdrawal effects and exacerbation of angina needs to be excluded.

Influence of cardiopulmonary bypass on nitroglycerin clearance

The effect of cardiopulmonary bypass on the clearance of nitroglycerin (NTG) was studied in seven patients scheduled for coronary artery bypass graft surgery. Intravenous NTG was administered through nonadsorbing tubing at a starting dosage of 5-10 micrograms/min and was adjusted as needed. Blood samples were obtained from the radial artery and antecubital vein before bypass and from the arterial outlet of the oxygenator during bypass at least 30 minutes apart during a constant dosage or at least 30 minutes after a dosage change. Serum concentrations were analyzed for NTG by gas chromatography. Venous NTG concentrations were always lower than concurrent arterial concentrations, with an average arteriovenous extraction of 67.2%. Serum concentrations of NTG were generally within the range associated with a therapeutic response in congestive heart failure patients. Consistent with other reports, NTG concentrations varied widely among patients and considerable intrasubject fluctuations in drug concentrations were seen. The mean +/- SD apparent clearance of NTG before bypass of 0.044 +/- 0.02 L/kg/min increased 20% to 0.052 +/- 0.02 L/kg/min during bypass (P = .05). These results suggest that cardiopulmonary bypass increases the clearance of NTG; however, the magnitude appears to be small and only partially explains the reported increased dosage needed during cardiopulmonary bypass.

Pharmacology of the nitrates in angina pectoris

Nitrate therapy is the oldest treatment modality for angina pectoris. In addition to sublingual nitroglycerin, longer-acting preparations are available to provide angina prophylaxis. Nitrates work as venodilators and arterial dilators, and by these actions in patients with angina pectoris can reduce myocardial oxygen demands while maintaining or increasing coronary artery flow. On the cellular level, they may increase endothelial prostacyclin release to cause their vasodilating effects. Long-acting nitrates are available in sublingual, chewable, oral, buccal and topical forms. The topical forms include nitroglycerin paste and transcutaneous patches. Long-acting nitrates have a duration of action of 2 to 24 hours, depending on the preparation used. The transcutaneous patches, which provide the longest duration of activity, are being reexamined regarding long-term efficacy and drug tolerance. Nitrates are also available in an intravenous form and in a short-acting sublingual spray soon to be released. The safety of nitrates has been confirmed. Their hemodynamic and safety profiles allow them to be combined with calcium-channel blockers, beta-adrenergic blockers or both when trying to improve on the antianginal efficacy of single-drug therapy.

Kinetics of 1,2-dinitroglycerin following sustained release nitroglycerin: influence of propranolol and metoprolol

Ten healthy volunteers ingested a single 18-mg oral dose of sustained release nitroglycerin (TNG) (Giulini-Pharma) on three occasions: once in the control state, once during coadministration of propranolol (80-mg three times daily), and once during coadministration of metoprolol (100-mg twice daily). The degree of beta adrenergic blockade was evaluated by the metaproterenol infusion test. Plasma concentration of TNG and its major metabolite, 1,2-dinitroglycerin (DNG), during 12 h after each dose were measured by gas chromatography-mass spectrometry. Intact TNG was not detected in the plasma of any patient. The major metabolite, DNG, was easily measurable in blood, and had a biphasic plasma concentration profile. Coadministration of the beta-blockers had no influence on any of the kinetic variables for DNG. The mean values during control, propranolol, and metoprolol trials of DNG elimination half-life were: 1.35, 1.10, and 1.09 h; total area under the curve: 42, 38, and 42 ng/ml X h; oral clearance: 6.6, 7.2, and 6.4 liters/min. Thus TNG when administered as a sustained release oral preparation is rapidly and completely transformed to DNG. There was no pharmacokinetic interaction between sustained release TNG and two commonly used beta-blocking agents, suggesting that any clinical interaction that may-occur between sustained release nitroglycerin and beta-blocking agents is pharmacodynamic rather than pharmacokinetic in nature.

Pharmacokinetics and pharmacodynamics of isosorbide dinitrate

It has long been believed that organic nitrates, including isosorbide dinitrate (ISDN), are completely metabolized during their first passage through the liver and that oral therapy with this class of compounds is thus irrational. In the past few years, convincing data have been obtained in patients showing that intact ISDN is significantly bioavailable to the systemic circulation after oral administration; the oral bioavailability is about 20% relative to an intravenous dose and about 45% relative to a sublingual dose, with the balance metabolized to isosorbide mononitrates. These pharmacologically active metabolites have longer biologic half-lives than ISDN and are thus believed to contribute to the sustained duration of action of this drug. After acute dosing, changes in the pharmacologic effects of ISDN mirror those in plasma concentration. However, after long-term therapy, partial nitrate tolerance develops despite elevated plasma ISDN concentrations. Available evidence suggests that during sustained dosing, nitrate metabolism is generally reduced throughout the body; thus reduced hepatic and peripheral tissue metabolism raises plasma ISDN concentrations while reduced vascular tissue metabolism decreases the metabolic activation (perhaps to nitrosothiols?) necessary for vascular relaxation.

Dose requirements and hemodynamic effects of transdermal nitroglycerin compared with placebo in patients with congestive heart failure

The dose requirements and duration of effect of transdermal nitroglycerin in patients with heart failure are not clearly established. In a first series of eight patients with chronic heart failure we gave transdermal nitroglycerin in incremental doses until pulmonary capillary wedge pressure fell at least 30% within 4 hr in three consecutive patients. Thus we found that a single dose of 60 mg/24 hr (120 cm2) was the minimal effective dose. Transdermal nitroglycerin or placebo was then given as a single application of 60 mg/24 hr in random double-blind fashion to 15 additional patients with heart failure (eight received transdermal nitroglycerin and seven received placebo), and hemodynamics were monitored for up to 24 hr. After administration of transdermal nitroglycerin, the control pulmonary capillary wedge pressure of 22 +/- 7 mm Hg fell by 6 +/- 6 mm Hg at 2 hr (p less than .05) and reached maximal reduction of 8 +/- 6 mm Hg (p less than .01) at 4 hr. The reduction in wedge pressure remained significant through 12 hr but was no longer statistically significant by 18 hr after administration of the drug. Transdermal nitroglycerin also significantly reduced pulmonary arterial and right atrial pressures as well as pulmonary vascular resistance from 4 through 12 hr but did not affect systemic hemodynamics. No significant hemodynamic changes occurred after administration of placebo. Thus transdermal nitroglycerin is an effective vasodilator in patients with heart failure, but a dose of at least 60 mg/24 hr is needed. Even with this dose, hemodynamic effects do not last beyond 18 hr, suggesting altered absorption or development of tolerance.

Long-acting nitrates: relative utility of nitroglycerin patches

Cutaneous nitroglycerin patches offer an attractive, convenient approach to antianginal prophylaxis with nitroglycerin. Blood level data have suggested that nitroglycerin patches could have antianginal effects for 24 hours or more. However, direct documentation of antianginal efficacy is fragmentary at present. Existing studies using provocative exercise testing, with one exception, do not demonstrate antianginal effects of nitroglycerin patches at 24 hours, using either low or maximal tolerated doses in each subject. Furthermore, in contrast to other long-acting nitrates evaluated in similar fashion, the peak effects of nitroglycerin patches may not be as potent as those of sublingual nitroglycerin. Thus, at present, clinicians should not assume that nitroglycerin patches can provide 24-hour prophylaxis. Further, in patients on a patch regimen with continuing symptoms, consideration should be given to a trial of alternative or additional long-acting nitrates of other types.