Pharmacokinetics of nitroglycerin and long-acting nitrate esters

In vitro and in vivo-release of nitroglycerin from a new transdermal therapeutic system

A new transdermal therapeutic system (TTS) for nitroglycerin is presented that controls release of the active substance by means of desorption and diffusion. The drug release, in the dosage range examined under sink conditions, is independent of electrolytes and pH of the aqueous acceptor medium, but it does depend on its temperature as expected. Batches obtained on a production scale were highly reproducible. The validity of an "in vitro" dissolution model is demonstrated by the good correlation between the amount of nitroglycerin liberated "in vitro" and "in vivo". The amount of nitroglycerin released in vivo is approximately 10 µg/cm(2)/h from 4 hours after application, and, it is controlled by the system.

Nitrates and bone turnover (NABT) - trial to select the best nitrate preparation: study protocol for a randomized controlled trial

Background

Organic nitrates uncouple bone turnover, improve bone mineral density, and improve trabecular and cortical components of bone. These changes in turnover, strength and geometry may translate into an important reduction in fractures. However, before proceeding with a large fracture trial, there is a need to identify the nitrate formulation that has both the greatest efficacy (with regards to bone turnover markers) and gives the fewest headaches. Ascertaining which nitrate formulation this may be is the purpose of the current study.

Methods and design

This will be an open-label randomized, controlled trial conducted at Women’s College Hospital comparing five formulations of nitrates for their effects on bone turnover markers and headache. We will recruit postmenopausal women age 50 years or older with no contraindications to nitroglycerin. Our trial will consist of a run-in phase and a treatment phase. We will enroll 420 women in the run-in phase, each to receive all of the 5 potential treatments in random order for 2 days, each with a 2-day washout period between treatments. Those who tolerate all formulations will enter the 12-week treatment phase and be randomly assigned to one of five groups: 0.3 mg sublingual nitroglycerin tablet, 0.6 mg of the sublingual tablet, a 20 mg tablet of isosorbide mononitrate, a 160 mg nitroglycerin transdermal patch (used for 8 h), and 15 mg of nitroglycerin ointment as used in a previous trial by our group. We will continue enrolment until we have randomized 210 women or 35 women per group. Concentrations of bone formation (bone-specific alkaline phosphatase and procollagen type I N-terminal propeptide) and bone resorption (C-telopeptides of collagen crosslinks and N-terminal crosslinks of collagen) agents will be measured in samples taken at study entry (the start of the run in phase) and 12 weeks. Subjects will record the frequency and severity of headaches daily during the run-in phase and then monthly after that. We will use the ‘multiple comparisons with the best’ approach for data analyses, as this strategy allows practical considerations of ease of use and tolerability to guide selection of the preparation for future studies.

Discussion

Data from this protocol will be used to develop a randomized, controlled trial of nitrates to prevent osteoporotic fractures.

Trial registration

ClinicalTrials.gov Identifier: NCT01387672. Controlled-Trials.com: ISRCTN08860742.

Organic nitrates for osteoporosis: an update

The number of osteoporotic fractures is increasing worldwide as populations age. An inexpensive and widely available treatment is necessary to alleviate this increase in fractures. Current treatments decrease fractures at trabecular bone sites (spine) but have limited effects at cortical sites (hip, legs, forearm and upper arm)-the most common sites of osteoporotic fracture. Treatments are also limited by costs, side effects and lack of availability. Nitric oxide (NO) is a novel agent that has the potential to influence cortical bone, is inexpensive, widely available and has limited side effects. In this review, we will evaluate the in vitro and in vivo data that support the concept that NO is important in bone cell function, review the observational, case control and randomized trial data on organic nitrates and the effects of these agents on bone turnover, geometry and strength.

Nitric oxide donors for the treatment of osteoporosis

The number of osteoporotic fractures is increasing worldwide as populations age. An inexpensive and widely available treatment is necessary to alleviate this increase in fractures. Current treatments decrease fractures at trabecular bone sites (spine) but have limited effects at cortical sites (hip, legs, forearm, and upper arm)-the most common sites of osteoporotic fracture. Treatments are also limited by costs, side effects, and lack of availability. Nitric oxide is a novel agent that has the potential to influence cortical bone, is inexpensive, is widely available, and has limited side effects. In this review we evaluate the in vitro and in vivo data which support the concept that nitric oxide is important in bone cell function, review the observational and case-control studies reporting on subjects taking organic nitrates that act as nitric oxide donors, and review the effects of nitrates on bone mineral density measurements and fracture risk.

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.

The effects of organic nitrates on osteoporosis: a randomized controlled trial [ISRCTN94484747]

Background

Osteoporotic fractures are common and are associated with increased morbidity, mortality and health care costs. The most effective way to moderate increases in health care costs and the sickness and premature death associated with osteoporotic fractures, is to prevent osteoporosis. Several lines of evidence suggest that nitrates, drugs typically prescribed for the treatment of angina, may be effective in preventing postmenopausal osteoporosis.

Methods

We have designed a multicentre randomized controlled trial to determine the effects of nitrates on bone. The trial consists of two studies. The objective of the first study is to determine whether isosorbide mononitrate at 20 mg/day or nitroglycerin ointment at 15 mg/day leads to fewer headaches. The nitrate that is best tolerated will be used in a second study with one main objective: To determine if postmenopausal women with a T-score at the lumbar spine (L1 to L4) between 0 and -2.0 randomized to two years of treatment with intermittent nitrates have a greater increase in spine bone mineral density as compared to women randomized to placebo.

We hypothesize that: 1. Women will report fewer headaches when they are randomized to intermittent nitroglycerin ointment at 15 mg/day compared to intermittent oral isosorbide mononitrate at 20 mg/day, and, 2. After two years, women randomized to intermittent nitrates will have a greater percent increase in lumbar spine bone mineral density compared with women randomized to placebo.

Discussion

We have completed our pilot study and found that transdermal nitroglycerin was associated with fewer headaches than oral isosorbide mononitrate. We are currently recruiting patients for our second main study.

Stable angina and acute coronary syndromes are associated with nitric oxide resistance in platelets

OBJECTIVES

The study examined possible clinical determinants of platelet resistance to nitric oxide (NO) donors in patients with stable angina pectoris (SAP) and acute coronary syndromes (ACS), relative to nonischemic patients and normal subjects.

BACKGROUND

We have shown previously that platelets from patients with SAP are resistant to the antiaggregating effects of nitroglycerin (NTG) and sodium nitroprusside (SNP).

METHODS

Extent of adenosine diphosphate (1 micromol/liter)-induced platelet aggregation (impedance aggregometry in whole blood) and inhibition of aggregation by NTG (100 micromol/liter) and SNP (10 micromol/liter) were compared in normal subjects (n = 43), nonischemic patients (those with chest pain but no fixed coronary disease, (n = 35) and patients with SAP (n = 82) or ACS (n = 153). Association of NO resistance with coronary risk factors, coronary artery disease (CAD), intensity of angina and current medication was examined by univariate and multivariate analyses.

RESULTS

In patients with SAP and ACS as distinct from nonischemic patients and normal subjects, platelet aggregability was increased (both p < 0.01), and inhibition of aggregation by NTG and SNP was decreased (both p < 0.01). Multivariate analysis revealed that NO resistance occurred significantly more frequently with ACS than with SAP (odds ratio [OR] 2.3:1), and was less common among patients treated with perhexiline (OR 0.3:1) or statins (OR 0.45:1). Therapy with other antianginal drugs, extent of CAD, intensity of angina and coronary risk factors were not associated with variability in platelet responsiveness to NO donor.

CONCLUSIONS

Patients with symptomatic ischemic heart disease, especially ACS, exhibit increased platelet aggregability and decreased platelet responsiveness to the antiaggregatory effects of NO donors. The extent of NO resistance in platelets is not correlated with coronary risk factors. Pharmacotherapy with perhexiline and/or statins may improve platelet responsiveness to NO.

Use of intra-aortic nitroglycerin in the cardiac catheterization laboratory

The purpose of this study was to determine whether nitroglycerin (NTG) injected into the ascending aorta or left ventricle would safely and effectively lower blood pressure in hypertensive patients undergoing cardiac catheterization. Fifty bolus injections of 297 +/- 67 micrograms (mean +/- SD) NTG were given to patients with a systolic blood pressure (SBP) of > or = 140 mm Hg (mean SBP 188 +/- 20.1 mm Hg). An average drop in systolic blood pressure of 36 +/- 16 mmHg (P < 0.001), diastolic blood pressure of 19 +/- 7 mm Hg (P < 0.001), and left ventricular end-diastolic pressure of 4.7 +/- 4 mm Hg (P = 0.001) was well tolerated in each patient. The mean time to response was 11 +/- 3 sec. Intra-aortic injection of NTG is a safe and effective means to treat hypertensive patients in the cardiac catheterization laboratory.

Portal and systemic haemodynamic response to acute and chronic administration of low and high dose isosorbide-5-mononitrate in patients with cirrhosis

Oral isosorbide-5-mononitrate (Is-5-Mn) was given in doses of 10 and 40 mg acutely and chronically (twice daily for four weeks), allowing a nitrate free interval to 25 patients with cirrhosis. Both 10 mg and 40 mg Is-5-Mn reduced the hepatic venous pressure gradient acutely and chronically, without evidence of tolerance. This was achieved by a reduction in the wedged hepatic venous pressure. The effect on mean azygos blood flow was variable with no significant mean change seen acutely or after chronic use with either dose. The variability was dependent not on the dose used but on the initial azygos flow; the flow in patients with initially low values increased and those with high azygos flows decreased after nitrate challenge. The development of the porto-collateral flow seems an important parameter in predicting haemodynamic response to Is-5-Mn.

Correlation between respiratory symptoms and pulmonary function in asbestos-exposed workers

Studies of respiratory questionnaire efficacy have emphasized reliability of responses; few have validated symptom response with objective measures of pulmonary function. To determine whether respiratory symptoms are associated with diminished pulmonary function, symptoms reported on the American Thoracic Society (ATS-DLD-78A) questionnaire were correlated cross-sectionally with measured spirometric volumes in 816 asbestos-exposed workers. Cough, phlegm, wheeze, and dyspnea were inversely related to pulmonary function. Cough, phlegm, and chronic bronchitis were associated with a 2 to 8% reduction (p < 0.001) in predicted values for forced vital capacity (FVC) and forced expiratory volume (FEV1); wheeze and dyspnea were clinically more significant, with an 11 to 17% reduction (p < 0.001). Wheeze, dyspnea, and roentgenographic fibrosis were all significant independent predictors of risk for restrictive impairment. These results support the validity of the ATS questionnaire as an epidemiologic tool and emphasize the importance of clinical history in assessing respiratory status.

Blood pressure management during aortic surgery: urapidil compared to isosorbide dinitrate

The efficacy and hemodynamic effects of urapidil, an arteriolar vasodilator, and isosorbide dinitrate, a venodilator, were compared, when used for blood pressure control during abdominal aortic surgery. Urapidil is an alpha-adrenergic receptor antagonist with serotonin-1A receptor-agonist activity in the central nervous system. Hemodynamic profiles were recorded before and after the administration of the study drug (+/- 10 minutes before aortic clamping), 3 and 10 minutes following aortic clamping, and before and 3 and 10 minutes following the removal of the aortic clamp. Arterial and mixed venous oxygen contents were compared. Both groups of 18 patients were similar with respect to demographic profiles, anesthetic technique, and perioperative fluid therapy. Identical heart rate and blood pressure profiles were obtained. In contrast to isosorbide dinitrate, urapidil produced a 17% (P < 0.05) increase in cardiac index as a result of a 30% (P < 0.001) decrease in systemic vascular resistance before placement of the aortic clamp. In patients treated with urapidil, cardiac index was higher (P < 0.05) 10 minutes after aortic clamping, before removal of the clamp, and 10 minutes later. The arterio-venous oxygen content difference decreased from 3.2 +/- 0.8 mL O2/dL to 2.4 +/- 1.0 mL O2/dL (P < 0.01) following urapidil, but did not change during the administration of isosorbide dinitrate. It is concluded that urapidil is an effective and safe drug for the prevention of the hemodynamic consequences of aortic clamping. Compared to a venodilator (isosorbide dinitrate), urapidil offers the advantage of improving cardiac output and oxygen delivery.

Nitroglycerin metabolism by Phanerochaete chrysosporium: evidence for nitric oxide and nitrite formation

We have demonstrated that a filamentous fungus Phanerochaete chrysosporium converts glyceryl trinitrate (GTN) into its di- and mononitrate derivatives concurrently with the formation of nitric oxide detected by electron paramagnetic resonance (EPR), and the formation of nitrite. The metabolisms of nitrite and nitrate by the fungus are evaluated and taken into account when considering GTN degradation. Lack of evidence for nitrate formation from GTN suggests that an esterase-type activity is not involved. Furthermore, the kinetics of appearance of the hemoprotein-NO and non-heme protein-NO (FeS-NO) complexes indicate that an enzymatic process producing NO directly from GTN may be involved concurrently with a glutathione transferase-like system.

Pharmacokinetics of nicorandil

This report presents the findings of some studies on single intravenous and oral dosing performed in healthy volunteers to determine the pharmacokinetics and preliminary metabolism of nicorandil, a new vasodilator acting via increase of both membrane potassium conductance and intracellular cyclic guanosine monophosphate in vascular smooth muscle. Nicorandil (5 to 40 mg) is rapidly and completely absorbed after oral administration. Absolute bioavailability is 75 +/- 23% (mean +/- standard deviation) indicating that no significant hepatic first-pass effect exists; peak plasma levels occur within 0.30 to 1.0 hours after dosing. Maximal concentration and area under the plasma concentration time curve of the parent drug are linearly related to a dose range of 5 to 40 mg, which covers the therapeutic regimen proposed for the treatment of patients with angina pectoris. The apparent distribution volume is about 1.4 liters/kg and the plasma concentrations decline according to 2 different processes: (1) a rapid elimination phase (apparent t1/2 beta congruent to 1 hour) that involves about 96% of the dose found in plasma, and a slower phase between the eighth and twenty-fourth hour that could be the consequence of the vascular affinity of the compound. Nicorandil is weakly bound to human plasma proteins (free fraction greater than 75%) and its mean residence time is close to 1.25 hour. Both in animals and in humans, preliminary metabolic studies show that the main biotransformation pathways are denitration and then introduction into the nicotinamide metabolism. However, unchanged nicorandil and denitrated metabolite excreted into the urine represent only about 1 and 4% of the dose, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic mechanisms for the concentration dependency of in vitro degradation of nitroglycerin and glyceryl dinitrates in human blood: metabolite inhibition or cosubstrate depletion?

The in vitro degradation of nitroglycerin (NTG) and its dinitrate metabolites in human blood and red blood cells (RBC) has been shown to exhibit apparent first-order kinetics. The decay rates of NTG and its dinitrate metabolites, however, were dependent on the initial concentration. We showed that this unusual kinetic behavior can be described mathematically by models of Michaelis-Menten kinetics combined with either competitive product inhibition or cosubstrate depletion. Experimental studies were conducted to determine the relative contribution of these two mechanisms to the observed kinetics. The effect of added thiols (the likely cosubstrates) on [14C]NTG degradation was studied separately in whole blood, reconstituted RBC, lysed RBC, and plasma. N-Acetylcysteine, L-cysteine, and D-cysteine accelerated NTG degradation in whole blood, while a similar concentration of glutathione had no effect. However, all four thiols exerted no effect on NTG kinetics in reconstituted and lysed RBC. In contrast, these thiols, as well as dithiothreitol, produced a marked increase (3-14 fold) in NTG degradation rate in plasma compared with buffer controls. Since thiol replenishment in reconstituted and lysed RBC did not abolish the concentration dependency, cosubstrate depletion due to thiols appeared unimportant as a contributor to the kinetic phenomenon. In human blood, metabolite inhibition of NTG degradation occurred along with the existence of concentration dependency. Both phenomena, however, were absent when NTG degradation was examined in rat blood. Concentration-dependent degradation in human blood was not observed for glyceryl-1-mononitrate, a compound that does not produce a nitrated metabolite.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitrate tolerance in angina pectoris

Tolerance to the hemodynamic and antianginal effects of the organic nitrates develops rapidly during therapy. This has been documented with a variety of nitrate preparations and with different routes of administration. Dosing strategies designed to provide therapeutic plasma nitrate concentrations throughout the 24 hours of the day are regularly associated with the development of tolerance. Recent information indicates that dosing schedules providing a nitrate-free period will permit continued efficacy of the organic nitrates without tolerance development. This can be accomplished in several ways. With oral preparations of isosorbide dinitrate, medication can be given 3 times daily, omitting the evening dose. Buccal nitroglycerin is usually given 3 times daily after meals, and this has been shown not to be associated with tolerance. Likewise, preliminary studies suggest that removal of the nitroglycerin patches for a period of several hours each day will prevent the development of tolerance. Much needs to be learned about the nitrate-free period. With oral isosorbide dinitrate, it appears that 12 hours is required, but this could be substantially less with nitroglycerin preparations. It is also possible that several short periods with low nitrate levels each day would suffice. Thus, simply increasing the interval between dosing of oral preparations may provide an adequate, low, nitrate-free period.

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.

In vivo induction and reversal of nitroglycerin tolerance in human coronary arteries

The mechanism by which tolerance to the clinical effects of organic nitrates develops has not been elucidated. This study was done to determine whether an intravenous infusion of nitroglycerin induces tolerance in the coronary vascular bed and whether such tolerance is reversed by the sulfhydryl-group donor N-acetylcysteine. We studied 19 subjects--17 with coronary artery disease and 2 without it--who had a mean age (+/- SD) of 54 +/- 9 years. Coronary sinus blood flow, which approximates blood flow to the left ventricle, was measured before and during intracoronary injections of nitroglycerin (10, 25, 50, and 100 micrograms). The patients then received a 24-hour intravenous infusion of saline (n = 7) or of nitroglycerin, 45 +/- 13 micrograms per minute (n = 12), after which the responses of coronary sinus flow to the same doses of intracoronary nitroglycerin used earlier were measured. In the seven patients given saline, the four doses of intracoronary nitroglycerin caused similar percentage increases in coronary sinus flow before and after the saline infusion. In the 12 patients given intravenous nitroglycerin, the four intracoronary doses caused percentage increases in coronary flow before the infusion of 30 +/- 9, 35 +/- 14, 41 +/- 12, and 52 +/- 15, respectively. After the infusion, the same doses of nitroglycerin caused smaller (P less than 0.05) percentage increases (16 +/- 6, 21 +/- 11, 23 +/- 12, and 27 +/- 11, respectively), indicating the development of partial tolerance. Subsequently, 7 of the 12 patients received N-acetylcysteine, after which intracoronary nitroglycerin caused percentage increases in coronary sinus flow similar to the values measured before the intravenous nitroglycerin was given (34 +/- 13, 32 +/- 8, 38 +/- 11, and 44 +/- 16, respectively). We conclude that the coronary vasodilator effect of nitroglycerin is attenuated by an intravenous infusion of nitroglycerin (that is, partial tolerance develops) and that tolerance to the agent can be reversed by administration of the sulfhydryl-group donor N-acetylcysteine. The mechanism by which N-acetylcysteine reverses tolerance will require further investigation.

Antihypertensive effect of sustained-release isosorbide dinitrate for isolated systolic systemic hypertension in the elderly

A double-blind, randomized trial was performed in 40 patients, mean age (+/- standard deviation) 80 +/- 4 years, with isolated systolic systemic hypertension to evaluate the antihypertensive effect of oral sustained-release isosorbide dinitrate (ISDN), 20 to 40 mg twice daily, vs placebo. After 12 weeks of treatment, supine systolic blood pressure (BP) decreased from 192 +/- 10 to 162 +/- 12 mm Hg with ISDN (p less than 0.001) and from 189 +/- 10 to 175 +/- 15 mm Hg with placebo (p less than 0.001). On the basis of variance analysis, the decrease in systolic BP was significantly lower with ISDN (27 mm Hg) than with placebo (13 mm Hg). Similar results were observed for supine and erect systolic BP measured at 8 AM and 4 PM, 8 and 12 hours after drug intake. No significant differences in diastolic BP, heart rate or side effects occurred. After the ISDN tapering off-period (2 weeks), systolic BP increased significantly but did not change with placebo. The study provided evidence that in elderly patients with systolic hypertension, sustained-release ISDN induced a selective and sustained decrease in systolic BP, antihypertensive effect was observed 8 and 12 hours after drug administration, and no tolerance phenomenon was noted.

Potentiation of nitroglycerin-induced coronary dilatation by N-acetylcysteine

Previous studies have suggested that (1) nitroglycerin causes vasodilatation by interacting with sulfhydryl groups in vascular smooth muscle, thereby activating guanylate cyclase and increasing the intracellular concentration of cyclic GMP, and (2) N-acetylcysteine, a source of sulfhydryl groups, potentiates the peripheral vasodilatory effect of nitroglycerin. This study was performed to explore the influence of N-acetylcysteine on nitroglycerin-induced coronary dilatation. In 18 patients (13 men and five women, 30 to 76 years old), coronary sinus blood flow (by thermodilution) was measured before and during intracoronary administration of nitroglycerin, 25 micrograms, both before and 5 min after a 15 min intravenous infusion of (1) 5% dextrose in water (n = 8, control) or (2) 100 mg/kg N-acetylcysteine (n = 10). Nitroglycerin caused no change in heart rate or systemic arterial pressure. In the control patients, coronary sinus blood flow behaved similarly during the two injections: it was 134 +/- 36 ml/min (mean +/- SD) before and 183 +/- 50 ml/min during injection No. 1 (average increase, 49 +/- 25 ml/min; average percent increase, 38 +/- 21%); and it was 131 +/- 34 ml/min before and 178 +/- 45 ml/min during injection No. 2 (average increase, 47 +/- 23 ml/min; average percent increase, 37 +/- 20%) (NS compared with injection 1). In the patients who received N-acetylcysteine, coronary sinus blood flow was 149 +/- 48 ml/min before and 191 +/- 54 ml/min during injection 1 (average increase, 42 +/- 15 ml/min; average percent increase, 30 +/- 12%) (NS compared with eight control values).(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolite inhibition of nitroglycerin metabolism in sheep tissue homogenates

The metabolism of nitroglycerin in sheep tissue homogenates has been examined using tritiated nitroglycerin and a HPLC separation procedure. Nitroglycerin was metabolized by liver, lung, muscle, arterial and venous tissue to its dinitrometabolites and subsequently to mononitroglycerin. Addition of the dinitrometabolites substantially inhibited the degradation of nitroglycerin in all tissue homogenates.

Pharmacology of nitroglycerin and long-acting nitrates

Organic nitrates are available in a remarkably diverse variety of formulations, including sublingual, buccal and oral tablets, capsules, topical creams, ointments, patches, tapes, inhalable sprays and intravenous solutions. Although not all of these formulations are available in the United States, the array of drugs and dosages approved for use is extensive. It is only by weighing the pharmacologic properties of these agents against the patient's clinical status and needs that a concise and appropriate treatment regimen may be derived. Numerous recent studies have confirmed the protracted efficacy of the organic nitrates in the treatment of patients with angina pectoris and congestive heart failure (CHF) as evidenced by improvements in cardiac hemodynamics and desired clinical parameters. It is appropriate that the patient's dosage of nitrates be administered with a formulation most likely to be both clinically effective and well tolerated. The use of nitroglycerin and isosorbide dinitrate in the acute and chronic treatment of CHF will be discussed in the context of their unique pharmacologic and pharmacokinetic properties. A rationale for the most efficacious use of these agents will be presented. Tolerance phenomena and adverse effects (i.e., headache) will also be discussed from the perspective of their significance in chronic nitrate therapy.

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.

Transdermal nitroglycerin in angina pectoris

The hemodynamic and antianginal efficacy of transdermal nitroglycerin (TGTN) patches were assessed in patients with chronic, stable angina pectoris. In 11 patients, acute dosing with 10, 20 and 30 cm2 of TGTN (designed to deliver 5, 10 and 15 mg GTN over 24 hours) improved treadmill walking time 2 and 4 hours after application, but no clinical effects were seen at 24 hours. In a second study in 6 patients with doses of 30, 60 and 90 cm2 of TGTN treadmill walking time was improved at 2 and 4 hours, but no changes were seen at 24 hours except with the 90-cm2 preparation. After daily therapy with 30-cm2 patches of TGTN for 1 to 2 weeks, exercise tolerance was similar to that seen during daily placebo therapy. These results suggest that TGTN patches are of inadequate size to produce 24-hour antianginal protection. During sustained therapy, tolerance develops to the antianginal efficacy of this form of GTN administration.

Specialized delivery systems for intravenous nitroglycerin. Are they necessary?

Nitroglycerin is absorbed in vitro into polyvinyl chloride tubing, and it has been recommended that nitroglycerin be administered intravenously through specialized polyethylene infusion sets. To determine if tubing type is essential to achieve physiologic effectiveness, we studied dose responses to intravenous nitroglycerin in 15 patients with heart failure using standard polyvinyl chloride tubing in seven (group 2) and special polyethylene infusion sets in seven (group 1) (one patient was excluded from analysis because of technical difficulties). We monitored heart rate, blood pressure, right atrial pressure, pulmonary artery pressure, pulmonary capillary wedge pressure, and cardiac output. Cardiac index, systemic and pulmonary vascular resistance, triple index, rate pressure product, stroke volume, stroke volume index, and stroke work index were calculated. Baseline and treatment measurements were obtained from five to 15 minutes after the infusion of 10, 20, 40, and 80 micrograms of nitroglycerin per minute. Over-all, systolic blood pressure decreased (p less than 0.05) about 8 percent and mean blood pressure approximately 12 percent, mean pulmonary artery pressure and mean pulmonary capillary wedge pressure decreased 30 to 40 percent, and the decline in mean right atrial pressure was 35 percent of baseline (all p less than 0.05). Heart rate and cardiac index did not change (p greater than 0.05). Pulmonary vascular resistance decreased slightly (p = 0.07) and systemic vascular resistance significantly (p less than 0.05). When the two groups were compared physiologic changes were virtually identical (p less than 0.05). Two-way analysis of variance for baseline corrected data proved no differences between tubing sets (p less than 0.05), but the infusion concentration rate was highly related to response (p = 0.0001). A significant (p less than 0.05) decrease in mean blood pressure and mean right atrial pressure was noted at lower dose rates (20 micrograms per minute and 40 micrograms per minute, respectively) in group 1. Beneficial hemodynamic effects in heart failure patients can, then, be predicted to occur at 80 micrograms per minute infusion rates; these responses seem independent of the type of infusion tubing system employed. Additionally, when patients given intravenous nitroglycerin for various reasons were followed for 48 hours, the majority receiving infusions via polyvinyl chloride tubing (group 2) did not require dosage adjustments. Also, at lower flow rates, more solution than calculated may be delivered when polyethylene tubing infusion sets are employed with volumetric infusion pumps.

Pharmacokinetic determinants of nitrate action

An update on some of the recent studies relating to organic nitrate pharmacokinetics and pharmacodynamics is presented. The systemic clearance of nitroglycerin was found to be unaffected by portacaval shunting in animals. Thus, the liver only plays a minor role in the metabolism of systemic nitroglycerin. Organic nitrates are extensively taken up by blood vessels in which metabolic activation can occur to produce vascular activity. During sustained therapy, nitrate metabolites may decrease the systemic and hepatic clearance of the parent drug, thus increasing its plasma concentration. Metabolites could also decrease the extent of metabolism in vascular tissues, thus contributing to vascular nitrate tolerance. Therefore, during long-term angina therapy when metabolites are present, the same plasma nitrate concentration may produce less effects compared with that obtained after acute dosing. Nitrate action was shown to be possibly dependent on the rate of drug input. An alternate dosing mode is proposed that speculatively may provide an improvement in producing and maintaining nitrate action in long-term angina therapy.

Nitrate delivery systems in perspective. A decade of progress

In recent years the use of nitroglycerin and long-acting nitrate compounds in clinical practice has been increasing. Only 10 to 15 years ago these drugs, at least in oral formulation, were felt to have no clinical utility because of concern that potent hepatic enzyme activity would degrade sufficient amounts of the nitrate compounds from reaching the systemic circulation. However, it is now recognized that oral nitrate administration when given in sufficient amounts achieves therapeutic plasma concentrations and desired clinical effects. Nitrates are routinely used for the treatment of stable and unstable angina and also play a role in therapy for complications of myocardial infarction. Nitrates are very effective agents for preload reduction in vasodilator therapy of congestive heart failure. A wide variety of nitrate delivery systems, including the standard oral and sublingual formulations, and as well as the new transdermal nitroglycerin discs and buccal nitroglycerin, are now available. Sublingual nitroglycerin, isosorbide dinitrate, and buccal nitroglycerin are used for acute treatment of attacks of ischemic chest pain. For ambulatory patients, long-acting therapy can be administered by oral, topical ointment, transdermal disc, and buccal nitroglycerin formulations. Each compound has a slightly different onset and duration of action, which is in part dose-dependent. The relative merits and problems with each of the formulations are reviewed. Intravenous nitroglycerin is now commercially available and plays an increasing role in the intensive care units. It is an ideal drug for acute chest pain syndromes, including acute myocardial infarction. Specialized tubing does not need to be employed. The wide variety of nitrate delivery systems available to physicians makes use of this tried and true therapy practical and easy to carry out for clinicians.