Tolerance to the circulatory effects of oral isosorbide dinitrate. Rate of development and cross-tolerance to glyceryl trinitrate

Organic mononitrites of 1,2-propanediol act as an effective NO-releasing vasodilator in pulmonary hypertension and exhibit no cross-tolerance with nitroglycerin in anesthetized pigs

Purpose

Clinically available intravenous (IV) nitric oxide (NO) donor drugs such as nitroglycerin (GTN) cause systemic hypotension and/or tolerance development. In a porcine model, novel NO donor compounds – the organic mononitrites of 1,2-propanediol (PDNO) – were compared to GTN with regard to pulmonary selectivity and tolerance development. The vasodilatory effects of inorganic nitrite were investigated.

Materials and methods

In anesthetized piglets, central hemodynamics were monitored. At normal pulmonary vascular resistance (PVR), IV infusions of PDNO (15–60 nmol kg⁻¹ min⁻¹), GTN (13–132 nmol kg⁻¹ min⁻¹), and inorganic nitrite (dosed as PDNO) were administered. At increased PVR (by U46619 IV), IV infusions of PDNO (60–240 nmol kg⁻¹ min⁻¹) and GTN (75–300 nmol kg⁻¹ min⁻¹) before and after a 5 h infusion of GTN (45 nmol kg⁻¹ min⁻¹) were given.

Results

At normal PVR, PDNO (n=12) and GTN (n=7) caused significant dose-dependent decreases in mean systemic and pulmonary arterial pressures, whereas inorganic nitrite (n=13) had no significant effect. At increased PVR, PDNO (n=6) and GTN (n=6) significantly decreased mean systemic and pulmonary pressures and resistances, but only PDNO reduced the ratio between pulmonary and systemic vascular resistances significantly. After the 5 h GTN infusion, the hemodynamic response to GTN infusions (n=6) was significantly suppressed, whereas PDNO (n=6) produced similar hemodynamic effects to those observed before the GTN infusion.

Conclusion

PDNO is a vasodilator with selectivity for pulmonary circulation exhibiting no cross-tolerance to GTN, but GTN causes non selective vasodilatation with substantial tolerance development in the pulmonary and systemic circulations. Inorganic nitrite has no vasodilatory properties at relevant doses.

The Role of Nitroglycerin and Other Nitrogen Oxides in Cardiovascular Therapeutics

The use of nitroglycerin in the treatment of angina pectoris began not long after its original synthesis in 1847. Since then, the discovery of nitric oxide as a biological effector and better understanding of its roles in vasodilation, cell permeability, platelet function, inflammation, and other vascular processes have advanced our knowledge of the hemodynamic (mostly mediated through vasodilation of capacitance and conductance arteries) and nonhemodynamic effects of organic nitrate therapy, via both nitric oxide-dependent and -independent mechanisms. Nitrates are rapidly absorbed from mucous membranes, the gastrointestinal tract, and the skin; thus, nitroglycerin is available in a number of preparations for delivery via several routes: oral tablets, sublingual tablets, buccal tablets, sublingual spray, transdermal ointment, and transdermal patch, as well as intravenous formulations. Organic nitrates are commonly used in the treatment of cardiovascular disease, but clinical data limit their use mostly to the treatment of angina. They are also used in the treatment of subsets of patients with heart failure and pulmonary hypertension. One major limitation of the use of nitrates is the development of tolerance. Although several agents have been studied for use in the prevention of nitrate tolerance, none are currently recommended owing to a paucity of supportive clinical data. Only 1 method of preventing nitrate tolerance remains widely accepted: the use of a dosing strategy that provides an interval of no or low nitrate exposure during each 24-h period. Nitric oxide's important role in several cardiovascular disease mechanisms continues to drive research toward finding novel ways to affect both endogenous and exogenous sources of this key molecular mediator.

Dietary Nitrate Lowers Blood Pressure: Epidemiological, Pre-clinical Experimental and Clinical Trial Evidence

Nitric oxide (NO), a potent vasodilator critical in maintaining vascular homeostasis, can reduce blood pressure in vivo. Loss of constitutive NO generation, for example as a result of endothelial dysfunction, occurs in many pathological conditions, including hypertension, and contributes to disease pathology. Attempts to therapeutically deliver NO via organic nitrates (e.g. glyceryl trinitrate, GTN) to reduce blood pressure in hypertensives have been largely unsuccessful. However, in recent years inorganic (or 'dietary') nitrate has been identified as a potential solution for NO delivery through its sequential chemical reduction via the enterosalivary circuit. With dietary nitrate found in abundance in vegetables this review discusses epidemiological, pre-clinical and clinical data supporting the idea that dietary nitrate could represent a cheap and effective dietary intervention capable of reducing blood pressure and thereby improving cardiovascular health.

Organic nitrates: update on mechanisms underlying vasodilation, tolerance and endothelial dysfunction

Given acutely, organic nitrates, such as nitroglycerin (GTN), isosorbide mono- and dinitrates (ISMN, ISDN), and pentaerythrityl tetranitrate (PETN), have potent vasodilator and anti-ischemic effects in patients with acute coronary syndromes, acute and chronic congestive heart failure and arterial hypertension. During long-term treatment, however, side effects such as nitrate tolerance and endothelial dysfunction occur, and therapeutic efficacy of these drugs rapidly vanishes. Recent experimental and clinical studies have revealed that organic nitrates per se are not just nitric oxide (NO) donors, but rather a quite heterogeneous group of drugs considerably differing for mechanisms underlying vasodilation and the development of endothelial dysfunction and tolerance. Based on this, we propose that the term nitrate tolerance should be avoided and more specifically the terms of GTN, ISMN and ISDN tolerance should be used. The present review summarizes preclinical and clinical data concerning organic nitrates. Here we also emphasize the consequences of chronic nitrate therapy on the supersensitivity of the vasculature to vasoconstriction and on the increased autocrine expression of endothelin. We believe that these so far rather neglected and underestimated side effects of chronic therapy with at least GTN and ISMN are clinically important.

Challenges with nitrate therapy and nitrate tolerance: prevalence, prevention, and clinical relevance

Nitrate therapy has been an effective treatment for ischemic heart disease for over 100 years. The anti-ischemic and exercise-promoting benefits of sublingually administered nitrates are well established. Nitroglycerin is indicated for the relief of an established attack of angina and for prophylactic use, but its effects are short lived. In an effort to increase the duration of beneficial effects, long-acting orally administered and topical applications of nitrates have been developed; however, following their continued or frequent daily use, patients soon develop tolerance to these long-acting nitrate preparations. Once tolerance develops, patients begin losing the protective effects of the long-acting nitrate therapy. By providing a nitrate-free interval, or declining nitrate levels at night, one can overcome or reduce the development of tolerance, but cannot provide 24-h anti-anginal and anti-ischemic protection. In addition, patients may be vulnerable to occurrence of rebound angina and myocardial ischemia during periods of absent nitrate levels at night and early hours of the morning, and worsening of exercise capacity prior to the morning dose of the medication. This has been a concern with nitroglycerin patches but not with oral formulations of isosorbide-5 mononitrates, and has not been adequately studied with isosorbide dinitrate. This paper describes problems associated with nitrate tolerance, reviews mechanisms by which nitrate tolerance and loss of efficacy develop, and presents strategies to avoid nitrate tolerance and maintain efficacy when using long-acting nitrate formulations.

Differential metabolism of organic nitrates by aldehyde dehydrogenase 1a1 and 2: substrate selectivity, enzyme inactivation, and active cysteine sites

Organic nitrate vasodilators (ORN) exert their pharmacologic effects through the metabolic release of nitric oxide (NO). Mitochondrial aldehyde dehydrogenase (ALDH2) is the principal enzyme responsible for NO liberation from nitroglycerin (NTG), but lacks activity towards other ORN. Cytosolic aldehyde dehydrogenase (ALDH1a1) can produce NO from NTG, but its activity towards other ORN is unknown. Using purified enzymes, we showed that both isoforms could liberate NO from NTG, isosorbide dinitrate (ISDN), and nicrorandil, while only ALDH1a1 metabolized isosorbide-2-mononitrate and isosorbide-5-mononitrate (IS-5-MN). Following a 10-min incubation with purified enzyme, 0.1 mM NTG and 1 mM ISDN potently inactivated ALDH1a1 (to 21.9% ± 11.1% and 0.44% ± 1.04% of control activity, respectively) and ALDH2 (no activity remaining and 4.57% ± 7.92% of control activity, respectively), while 1 mM IS-5-MN exerted only modest inactivation of ALDH1a1 (reduced to 89% ± 4.3% of control). Cytosolic ALDH in hepatic homogenates incubated at the vascular EC(50) concentrations of ORN was inactivated by NTG (to 45.1% ± 8.1% of control activity) while mitochondrial ALDH was inactivated by NTG and nicorandil (to 68.2% ± 10.0% and 78.7% ± 19.8% of control, respectively). Via site-directed mutagenesis, the active sites of ORN metabolism of ALDH2 (Cys-319) and ALDH1a1 (Cys-303) were found to be identical to those responsible for their dehydrogenase activity. Cysteine-302 of ALDH1a1 and glutamate-504 of ALDH2 were found to modulate the rate of ORN metabolism. These studies provide further characterization of the substrate selectivity, inactivation, and active sites of ALDH2 and ALDH1a1 toward ORN.

Organic nitrates and nitrate tolerance--state of the art and future developments

The hemodynamic and antiischemic effects of nitroglycerin (GTN) are lost upon chronic administration due to the rapid development of nitrate tolerance. The mechanism of this phenomenon has puzzled several generations of scientists, but recent findings have led to novel hypotheses. The formation of reactive oxygen and nitrogen species in the mitochondria and the subsequent inhibition of the nitrate-bioactivating enzyme mitochondrial aldehyde dehydrogenase (ALDH-2) appear to play a central role, at least for GTN, that is, bioactivated by ALDH-2. Importantly, these findings provide the opportunity to reconcile the two "traditional" hypotheses of nitrate tolerance, that is, the one postulating a decreased bioactivation and the concurrent one suggesting a role of oxidative stress. Furthermore, recent animal and human experimental studies suggest that the organic nitrates are not a homogeneous group but demonstrate a broad diversity with regard to induction of vascular dysfunction, oxidative stress, and other side effects. In the past, attempts to avoid nitrate-induced side effects have focused on administration schedules that would allow a "nitrate-free interval"; in the future, the role of co-therapies with antioxidant compounds and of activation of endogeneous protective pathways such as the heme oxygenase 1 (HO-1) will need to be explored. However, the development of new nitrates, for example, tolerance-free aminoalkyl nitrates or combination of nitrate groups with established cardiovascular drugs like ACE inhibitors or AT(1)-receptor blockers (hybrid molecules) may be of great clinical interest.

Nitrate tolerance. State of the art--chairman's summary

The development of tolerance to the effects of nitrates is well documented in the literature. It can be induced and rapidly reversed with all types of nitrates and it is more pronounced in the arterial as compared to the venous circulation. Cross-tolerance between different nitrates exists. Furthermore, tolerance seems more related to high and continuous plasma concentrations and may possibly be avoided by intermittent dosing. The importance of such factors as patient selection, type of nitrate preparation, duration of therapy, the significance of target organs, methods of evaluating the development of nitrate tolerance as well as a summary of the discussions during the workshop is presented.

Treatment of congestive heart failure

The past decade has improved our understanding of the pathophysiological mechanisms underlying the congestive heart failure syndrome. The same decade has seen a considerable expansion in modes of therapy for this syndrome. A review of the present forms of treatment is given.

Angiotensin II-Receptor Antagonist Losartan Does not Prevent Nitroglycerin Tolerance in Patients with Coronary Artery Disease

The study evaluated the effect of Losartan in preventing nitrate tolerance during continuous transdermal nitroglycerin (TD-GTN) therapy in patients with coronary disease. Fifteen subjects with chronic stable ischemia evaluated by exercise test, were randomized to 28 days of TD-GTN 20 mg once a day without free interval plus Losartan 100 mg or Losartan-placebo with a double blind crossover design. Myocardial ischemic parameters during stress test were evaluated after each test period and results of Losartan therapy were compared to those with placebo. Time to onset 1 mm ST-depression was significantly higher after acute TD-GTN 20 mg with respect to placebo run-in, sustained TD-GTN 20 mg plus Losartan 100 mg or Losartan-placebo (p < 0.001). ST-depression at peak exercise and time to recovery of ST segment were markedly lower after acute TD-GTN 20 mg compared to placebo run-in (p < 0.05), sustained TD-GTN 20 mg plus Losartan 100 mg (p < 0.001) or Losartan-placebo (p < 0.05). At 1 mm-ST depression and at peak exercise, systolic blood pressure and rate-pressure product significantly decreased after sustained TD-GTN 20 mg plus Losartan 100 mg (p < 0.001, p < 0.05 respectively) with respect to placebo run-in, acute and sustained TD-GTN 20 mg plus Losartan-placebo. Moreover at peak exercise, these data were also observed after acute TD-GTN 20 mg compared to placebo run-in and sustained TD-GTN 20 mg plus Losartan-placebo (p < 0.001). The AT(1) antagonist Losartan administration does not prevent the development of nitrate tolerance during continuous TD-GTN therapy.

Nitrate tolerance and the links with endothelial dysfunction and oxidative stress

Identification of nitric oxide as the molecule responsible for endothelial dependant vasodilatation has led to an explosion of interest in endothelial function. Oxidative stress has been identified as an important factor in the development of tolerance to organic nitrates. This review examines the evidence supporting this recently developed theory and how mechanisms of nitrate tolerance may link with the wider picture of primary nitric oxide resistance.

Echo-dipyridamole stress test evaluation of isosorbide-5-mononitrate efficacy and tolerance in patients with coronary heart disease: interplay with sympathetic activity

In 22 patients with stable myocardial ischemia, we prospectively studied the short- and long-term effects of isosorbide-5-mononitrate (5-ISMN) on dipyridamole-induced myocardial ischemia, the ability of dipyridamole-stress echocardiography to evaluate nitrate tolerance, and the role of activation of the neurohumoral system in nitrate tolerance development, assessed by modifications of catecholamines plasma levels and heart rate variability. After brief treatment with 5-ISMN, dipyridamole-stress echocardiography was negative in 19 of 22 patients (p < 0.001 vs. placebo). During the sustained phase, dipyridamole-stress echocardiography was positive after both placebo and active drug (p = NS vs. placebo). Heart rate variability showed significantly higher values in power of the low frequency (LF) band and low- to high-frequency ratio (L/H), as well as significantly lower values of the power of the high-frequency (HF) band (all p < 0.001) during brief but not during sustained administration of 5-ISMN. Norepinephrine plasma levels were significantly higher (p < 0.001) during short-term 5-ISMN administration but not during the sustained phase. Our results indicate that short-term administration of 5-ISMN antagonizes dipyridamole-induced myocardial ischemia and show the loss of antiischemic efficacy in 95% of patients during sustained treatment, demonstrating that dipyridamole-stress echocardiography is a useful tool to assess the presence of nitrate tolerance. Spectral analysis of heart rate variability and norepinephrine values confirm that brief nitrate administration increases sympathetic activity, a possible crucial trigger event in the development of nitrate tolerance, whereas prolonged nitrate treatment is not associated with prolonged neurohumoral activation.

Efficacy and rebound phenomenon related to intermittent nitroglycerin therapy for the prevention of nitrate tolerance

Intermittent transdermal therapy of nitroglycerin (NTG) has been recommended for the prevention of nitrate tolerance, but a rebound phenomenon has been reported to occur following removal of the NTG tape. The present study investigated the effects of intermittent NTG therapy on vasodilatory response and the intracellular production of cyclic GMP (cGMP). The study group comprised 12 healthy adults and measurements were taken of the platelet cGMP level, the venous volume (VV) (by forearm plethysmography) and the plasma levels of neurohormonal factors before and 5 min after administration of 0.3 mg of sublingual nitroglycerin (NTG) during the following 4 phases: (i) the control phase (8.00 h); (ii) the continuous phase (8.00 h; 7 days after continuous application of a 10 mg/24 h NTG tape); (iii) the intermittent application phase (8.00 h; 7 days after intermittent application of NTG tape, applied at 21.00 h and removed at 9.00 h); and (iv) the intermittent removal phase (13.00 h; 4 h after removal of the NTG tape in the intermittent phase). The percentage increase in cGMP (%cGMP) and venous volume (%VV) were significantly lower in the continuous phase than the control phase, but there was no difference between the control and the intermittent application phases. However, in the intermittent removal phase, the cGMP level before sublingual NTG, the %cGMP and the %VV were unchanged, but the VV before sublingual NTG was significantly lower than in the control phase. Plasma renin activity and the plasma level of angiotensin II were significantly increased in the continuous phase, the intermittent application phase, and the intermittent removal phase. In conclusion, intermittent transdermal NTG therapy prevented nitrate tolerance in the production of cGMP and vasodilation, but induced a rebound phenomenon after removal of the NTG tape. The rebound phenomenon following the tape removal may be related to some other mechanism, such as activation of neurohormonal factors.

Preventive effects of angiotensin-converting enzyme inhibitors on nitrate tolerance during continuous transdermal application of nitroglycerin in patients with chronic heart failure

This study was designed to investigate the effect of angiotensin-converting enzyme (ACE) inhibitors with and without a sulfhydryl group on intracellular production of cGMP, forearm blood flow, and neurohormonal factors during continuous transdermal application of nitroglycerin in patients with chronic heart failure. Platelet cGMP level and forearm blood flow were measured before and 5 min after sublingual administration of nitroglycerin (NTG) in 20 patients with chronic heart failure during the following 4 phases: (1) baseline phase; (2) NTG phase (1 week after NTG tape 10 mg/day); (3) CPT phase (1 week after both captopril 37.5 mg/day and NTG tape 10 mg/day); and (4) ENL phase (1 week after both enalapril 5 mg/day and NTG tape 10 mg/day). The platelet GMP level before sublingual NTG and forearm blood flow were significantly higher during the 3 phases with NTG tape than during the control phase. The percent increases in platelet cGMP level and forearm blood flow after sublingual NTG were significantly lower during the NTG phase than during the baseline phase. In contrast, concomitant application of ACE inhibitors maintained the percent increase in platelet cGMP level and forearm blood flow. These results indicate that concomitant therapy with ACE inhibitors may be helpful in preventing the attenuation of intracellular cGMP production in patients with chronic heart failure during continuous transdermal application of NTG.

Effects of enalapril during continuous nitrate therapy: analysis of diameter of coronary arteries and platelet cyclic guanosine monophosphate

To investigate the effects of enalapril, an angiotensin-converting enzyme inhibitor, on nitrate tolerance during continuous nitrate therapy, coronary artery diameters and platelet cyclic guanosine monophosphate (cGMP) levels were measured before and 2 minutes after intracoronary injection of nitroglycerin 200 microg in 60 patients with coronary artery disease and were compared among 20 patients treated with nitrates (nitrate group), 20 patients treated with both nitrates and enalapril (enalapril group), and 20 untreated patients (control group). The percent increase in platelet cGMP and coronary dilatation in the nitrate group was significantly less than in the control group, but the percent increase in the enalapril group was significantly greater than that in the nitrate group. These results indicate that enalapril may be helpful as concomitant therapy to maintain the effect of nitrates during continuous nitrate therapy.

Nitrate tolerance, rebound, and their clinical relevance in stable angina pectoris, unstable angina, and heart failure

Vascular tolerance develops rapidly in isolated vascular strips exposed to millimolar concentrations of nitroglycerin. Several mechanisms, including depletion of sulfhydryl groups, reduced biotransformation of nitrates to NO or nitrosothiols, oxygen free radical injury, and downregulation of a membrane-bound enzyme or a nitrate receptor, have been proposed, but the exact mechanism responsible for in-vitro tolerance remains unknown. In-vivo tolerance of the beneficial effects of nitrates on hemodynamics, myocardial ischemia, and exercise performance develops rapidly. It has been suggested, but remains to be proven, that development of venous tolerance and not arterial tolerance is responsible for the attenuation of nitrate effects during long-term nitrate therapy. Several mechanisms, including neurohormonal activation, depletion of sulfhdryl groups, and the shift of fluid from the extravascular to intravascular compartment have been implicated. However, the use of agents to counteract these mechanisms (ACE inhibitors, sulfhydryl donors, diuretics) has produced conflicting results. Thus, at present the mechanism responsible for in vivo tolerance to nitrates remains unknown. Both in vitro and in vivo vascular tolerance to nitrates can be prevented or minimized by providing nitrate-free or low-nitrate intervals. However, during nitrate-free periods, rebound phenomena (rest angina in patients with ischemic heart disease or a deterioration in exercise performance prior to the renewal of the morning dose in patients with stable angina) remain a clinical concern. When treating patients with stable angina pectoris, it must be recognized that none of the nitrate preparations or formulations can provide round-the-clock antianginal or antiischemic prophylaxis. In these patients, beneficial antianginal and antiischemic effects of nitrates for 10-14 hours during the daytime can be maintained by using formulations and dosing regimens that avoid or minimize the development of tolerance (standard formulation of isosorbide-5-mononitrate, 20 mg in the morning and 7 hours later; slow-release formulation of isosorbide-5-mononitrate, 120-240 mg once a day; or nitroglycerin patch delivering 0.6 nitroglycerin per hour for 10-12 hours each day). Only the patch on and off treatment is associated with nitrate rebound. Although intermittent nitrate therapy is not associated with the development of tolerance, this strategy cannot be recommended for treating unstable angina because rebound angina during nitrate-free periods complicates clinical decision making. In the acute phase of unstable angina, continuous treatment with intravenous nitroglycerin is recommended because it permits rapid up- or down-titration. Tolerance towards antianginal and antiischemic effects does develop in a substantial number of patients with 24 hours, but this can be overridden by dose escalation and restoration of the therapeutic effectiveness of nitroglycerin. Tolerance towards the beneficial effects of nitrates on hemodynamics and on exercise performance also develops rapidly during continuous or long-term nitrate therapy, and for these reasons nitrates are not used as first-line therapy to treat chronic heart failure. In combination with hydralazine, high-dose isosorbide dinitrate (30-40 mg four times a day) improves survival, but this combination therapy is inferior to ACE inhibitors.

Hemodynamic time course of acute and chronic isosorbide dinitrate treatment at rest and during exercise in patients with stable ischemic heart disease

HYPOTHESIS

The study was undertaken to establish differences between venous and arterial isosorbide dinitrate (ISDN) effects during acute and chronic treatment, hemodynamics at rest, and during supine exercise.

METHODS

These effects were assessed invasively in 16 patients with stable ischemic heart disease before and at hourly intervals for 4 h after administration of peroral 30 mg ISDN. Eight patients were previously untreated (acute group), and eight were treated with 30 mg ISDN asymmetrically b.i.d. for two weeks (chronic group).

RESULTS

Prior to ISDN administration, right atrial, mean pulmonary artery, pulmonary artery wedge, and mean arterial pressure (RAP, MPAP, PAWP, and MAP) rose from normal resting to pathologic values during exercise. One h after ISDN administration, all exercise pressures were normalized (p < 0.001). During the following 3 h, exercise RAP rose similarly in both groups (p < 0.01), while MPAP rose particularly in the chronic group (p < 0.001). Exercise PAWP and MAP, however, remained low in the acute group, but increased markedly in the chronic group (p < 0.01), particularly from the third to the fourth hour after ISDN.

CONCLUSION

The daily, asymmetric administration of 30 mg ISDN b.i.d. maintained beneficial, anti-ischemic effects for 2 to 3 h after a morning dose of the drug, but thereafter attenuation of the effects occurred in the arteries but not in the veins.

KATP channel activation mediates nicorandil-induced relaxation of nitrate-tolerant coronary arteries

We compared the tolerance-inducing effects of nitroglycerin (NTG) and nicorandil (NIC) in porcine isolated coronary arteries and assessed the role of KATP channels in the response to NIC in nitrate-tolerant and nontolerant preparations. In coronary arteries contracted with U46619 (1-3 x 10(-9) M), NTG, NIC, sodium nitroprusside (SNP), and cromakalim produced concentration-dependent relaxations. The rank order of potency was NTG > or = SNP > cromakalim > nicorandil. Exposure of the rings to NTG (10(-4) M) for 90 min, followed by repeated rinsing for 1 h, produced a parallel, rightward shift of the subsequent concentration-response curves to NTG and SNP; a slight but significant reduction in the maximal response to NTG was also observed. Previous exposure to NTG had no effect on the NIC or cromakalim concentration-response curves. When the tissues were exposed to NIC (3 x 10(-4) M) for 90 min, followed by repeated rinsing for 1 h, there was no effect on the subsequent concentration-response curves to NTG, NIC, SNP, or cromakalim. In both nitrate-tolerant and nontolerant coronary arteries, glibenclamide (GLI 10(-6) M), a selective KATP channel blocker, caused a parallel rightward shift in the concentration-response curve to cromakalim, but had no effect on responses to NTG or SNP. In nontolerant coronary arteries, GLI had no effect on NIC-induced relaxation, but in nitrate-tolerant preparations, GLI produced a significant rightward shift in the NIC concentration-response curve. The results demonstrate that prolonged exposure to NTG, but not NIC, causes tolerance in isolated porcine coronary arteries and that the response to NIC is not affected by nitrate tolerance. The data also suggest that NIC-induced relaxation of nitratetolerant, but not nontolerant, coronary arteries is mediated by activation of KATP channels.

Effects of chronic oral administration of isosorbide dinitrate on in vitro contractility of rat arterial smooth muscle

In this study, we examined the effects of in vitro and in vivo treatment with isosorbide dinitrate (ISDN) on the in vitro response of isolated rat aorta. The in vitro treatment of isolated aorta with ISDN (100 microM) for 2 hr had no effect on the ISDN-induced relaxation of norepinephrine-induced contraction. In the aorta isolated from the rats treated with a high dose (90 mg/kg) of ISDN for 7-14 days, in contrast, the relaxant effect of ISDN was significantly reduced. However, the relaxant effect of sodium nitroprusside was only slightly attenuated by the treatment with a high dose of ISDN for 14 days; and the relaxant effects of 8-bromo-cGMP, levcromakalim and verapamil were unchanged. These results suggest that tolerance to ISDN was obtained only after the in vivo chronic treatment with a high dose of ISDN. ISDN may desensitize the nitric oxide-generating step rather than inactivate guanylate cyclase or the downstream pathways.

Short-term hemodynamic, anti-ischemic, and antianginal effects of pirsidomine, a new sydnonimine

Pirsidomine is a new sydnonimine compound in clinical development. As a prodrug, it is transformed into a nitric oxide-releasing metabolite in vivo. In animal tests there were no signs of tolerance with repeated administration. The short-term effects of 10, 20, and 40 mg of the drug on pulmonary hemodynamics and ischemic parameters were examined at rest and during exercise in a double-blind, randomized, placebo-controlled study. The study included 48 patients with documented coronary artery disease and exercise-induced ST-segment depression. Compared with the baseline test, there was a reduction of diastolic pulmonary artery pressure with pirsidomine at rest (placebo: -0.4 +/- 0.5 mm Hg; 10 mg: - 1.5 +/- 2.4 mm Hg; 20 mg: - 1.4 +/- 1.1 mm Hg; 40 mg: - 2.3 +/- 1.3 mm Hg [p < 0.05 ]) and at the highest comparable workload (placebo: -2.8 +/- 1.9 mm Hg; 10 mg: -7.3 +/- 6.8 mm Hg; 20 mg: -8.4 +/- 7.9 mm Hg [p <0.05]; 40 mg: -13.8 +/- 7.1 mm Hg [p <0.05]). ST-segment depression decreased at the highest comparable workload (placebo: -0.33 +/- 0.49 mm; 10 mg: -1.33 +/- 1.37 mm [p <0.05]; 20 mg: -1.33 +/- 0.83 mm [p <0.05]; 40 mg: -1.96 +/- 0.86 mm [p <0.05]) and total exercise time increased (placebo: 15 +/- 48 s; 10 mg: 98 +/- 126 s; 20 mg: 165 +/- 251 s [p <0.05]; 40 mg: 155 +/- 174 s [p <0.05]). Of 40 patients who complained of angina pectoris symptoms in the baseline test, 15 became free of angina pectoris with pirsidomine. Compared with placebo, blood pressure, heart rate during exercise, and cardiac output during exercise showed no significant change. Plasma concentration response relations of the metabolite revealed concentrations that caused a half-maximum effect of 6 ng/ml, 13 ng/ml, 20 ng/ml, and 28 ng/ml in reduction of ST-segment depression, reduction of diastolic pulmonary artery pressure, relief of angina pectoris symptoms, and an increase in exercise duration, respectively. Thus, pirsidomine is an effective anti-ischemic and antianginal agent. A significant preload reduction was obtained with plasma metabolite concentrations lower than those necessary to achieve a satisfactory antianginal effect.

Dissociation of coronary vascular tolerance and neurohormonal adjustments during long-term nitroglycerin therapy in patients with stable coronary artery disease

OBJECTIVES

We sought to examine whether long-term nitroglycerin treatment causes tolerance in large coronary arteries and whether the loss of vascular effects parallels neurohormonal adjustments.

BACKGROUND

Nitroglycerin therapy is associated with increased plasma renin activity and aldosterone levels and a decrease in hematocrit. It is assumed that nitroglycerin tolerance results in part from these neurohormonal adjustments and intravascular volume expansion.

METHODS

Three groups were studied: group I (n = 10), no prior nitroglycerin therapy; and group II (n = 10) and group III (n = 8), 24- and 72-h long-term nitroglycerin infusion (0.5 micrograms/kg body weight per min), respectively. Coronary artery dimensions were assessed using quantitative angiography. Plasma renin activity, plasma aldosterone and vasopressin levels and hematocrit were monitored before and during nitroglycerin infusions.

RESULTS

In group I, increasing intravenous concentrations of nitroglycerin caused a dose-dependent increase of the midportion of the left anterior descending coronary artery (baseline diameter 2.13 +/- 0.07 mm [mean +/- SEM], maximally by 22 +/- 2%) and left circumflex coronary artery (baseline diameter 2.08 +/- 0.07) mm, maximally by 22 +/- 3%). An intracoronary nitroglycerin bolus (0.2 mg) caused no further significant increase in diameter, indicating maximal dilation. In group II (n = 10), the baseline large coronary artery diameter under ongoing nitroglycerin was significantly larger than that in group I (left anterior descending artery 2.61 +/- 0.08 mm, left circumflex artery 2.57 +/- 0.08 mm). Additional intravenous and intracoronary nitroglycerin challenges did not cause further dilation, indicating maximally dilated vessels. At the same time, plasma renin activity, plasma aldosterone and vasopressin levels were significantly increased, and hematocrit significantly decreased. In group III patients, the baseline diameter of the left anterior descending artery and the left circumflex artery did not differ from that in patients without nitroglycerin pretreatment, indicating a complete loss of nitroglycerin coronary vasodilative effects. These patients showed no significant increase in circulating neurohormonal levels but a significant decrease in hematocrit.

CONCLUSIONS

Within 24 h of continuous nitroglycerin treatment, the coronary arteries were maximally dilated despite neurohormonal adjustments and signs of intravascular volume expansion. Within 3 days of nitroglycerin infusion, tolerance developed in the absence of neurohormonal activation. The dissociation of neurohormonal adjustments and tolerance in large coronary arteries indicates that after long-term nitroglycerin treatment, true vascular tolerance, perhaps from an intracellular tolerance step, may have developed.

Prevention of nitroglycerin tolerance with diuretics

Tolerance to prolonged nitroglycerin (NTG) may be due to drug-induced intravascular expansion. To test the hypothesis that concomitant diuretics may reduce volume expansions and thus prevent NTG tolerance, we studied 23 healthy volunteers randomized to hydrochlorthiazide or placebo before and during 74-hour continuous NTG exposure. Venodilator response to NTG was tested serially with venous forearm plethysmography. In the preNTG patch phase venodilator response was similar in both the placebo and diuretic groups. Venodilator capacity was compared after "acute" (2-hour) and "chronic" (74-hour) NTG exposure with NTG patches. Attenuation to NTG was seen in the placebo group, but NTG venodilator activity was fully maintained in the diuretic group. These data suggest that diuretics may be helpful in the prevention of NTG tolerance.

FR144420, a novel, slow, nitric oxide-releasing agent

We report that (+/-)-(E)-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexeneamide (FK409) decomposes and releases nitric oxide (NO) spontaneously in solution. (+/-)-N-[(E)-4-Ethyl-3-[(Z)-hydroxyimino]-5-nitro-3-hexen-1- yl]-3- pyridinecarboxamide (FR144420) was synthesized with the aim of discovering a compound with longer duration of effects in vivo, compared with FK409. FR144420, like FK409, released NO spontaneously in solution, but the amount of NO released from FR144420 during a 5-min incubation was half the amount from FK409. In addition, FR144420 spontaneously decomposed and generated nitrite, which is an oxidative metabolite of NO, at half the rate of FK409. In a vasorelaxant study with isolated rat aorta, FR144420 had a weaker potency than FK409 (EC50 = 54 and 8.1 nM, respectively). In in vivo studies, FR144420 decreased mean blood pressure immediately after intravenous and oral administration to conscious rats. The maximum hypotensive effects of FR144420 were less than those of FK409. However, the durations of FR144420-induced (i.v. and p.o.) hypotensive effects were longer than those of FK409-induced effects. In conclusion, FR144420 is more stable and releases NO more slowly in solution than does FK409. In in vivo experiments, FR144420 showed a longer duration of effects than FK409. FR144420 may be very useful for investigating the in vivo actions of NO.

Phenomenon of nitrate tolerance

The phenomenon of nitrate tolerance has now been appreciated for almost a century, and our understanding of this process has greatly improved during the past 20 years. Therapeutic nitrates are now recognized as exogenous sources of nitric oxide (or nitrosothiols), which appears to be a primary mediator of natural vasodilatation. Nitrates have been clearly shown to have vasodilatory and antiplatelet effects, both of which diminish during continuous exposure. Nitrate tolerance has been documented with most nitrate preparations when the patient is given continuous nitrate therapy. Tolerance to nitrates may occur in any patient, regardless of underlying illness, medication dose, or serum concentration of NTG. The cause of this phenomenon is multifactorial; there appear to be both cellular and systemic processes involved. To date, no adjuvant pharmacologic intervention has conclusively demonstrated benefit in preventing, abating, or reversing nitrate tolerance. Interruption of nitrate exposure for as little as 8 to 12 hours does appear to be the best means of preventing or reversing tolerance. Nevertheless, some patients with objective tolerance continue to experience relief of symptoms. In addition, despite laboratory-documented cross-tolerance, patients receiving continuous nitrate therapy at usual clinical doses may continue to benefit from the hemodynamic and antianginal effects of SL NTG. Hence, nitrate tolerance is a real entity, but the clinical importance of this phenomenon remains controversial. Finally, further investigation will need to address quality-of-life issues and perhaps assess relief of ischemia by other means.

Antihypertensive and vasorelaxant effects on KRN2391 in spontaneously hypertensive rats

1. In conscious spontaneously hypertensive rats (SHR), the oral administration of KRN2391 (0.1-3.0 mg/kg) produced a dose-dependent decrease in blood pressure. The antihypertensive effect of KRN2391 was about 2 and 20 times more potent than those of pinacidil and nifedipine, respectively, but about 2 times less potent than that of cromakalim. 2. During oral administration of KRN2391 (0.5 and 1.0 mg/kg) once daily for 5 weeks, its antihypertensive effect did not diminish in conscious SHR. 3. In anaesthetized SHR, KRN2391 (3-100 micrograms/kg, i.v.) produced a decrease in blood pressure in a dose-dependent manner. Its antihypertensive effect was antagonized by glibenclamide (20 mg/kg, i.v.). 4. In isolated aorta obtained from SHR, KRN2391 (0.01-100 microM) produced a concentration-dependent relaxation. Its concentration-relaxation curve was shifted to the right by glibenclamide (1 microM) and methylene blue (3 microM). 5. These results indicate that the antihypertensive effect of KRN2391 in SHR is due to its direct action on vascular smooth muscle based on a K+ channel opening action and a nitrate action. In addition, KRN2391 is absorbed from the gastrointestinal tract into blood and does not induce tolerance despite possessing some nitrate action.

Influence of glyceryl trinitrate on venous and arterial effects of chronic, asymmetric isosorbide dinitrate treatment in patients with ischemic heart disease

Asymmetric dosage regimes have been introduced to circumvent development of nitrate tolerance. This study assessed invasively the hemodynamics during supine rest and exercise before and after 4 weeks treatment with 30 mg isosorbide dinitrate (ISDN) or placebo asymmetrically b.i.d. in 14 randomized patients with stable ischemic heart disease in a double-blinded study. An intravenous infusion of glyceryl trinitrate (GTN) was used to assess possible nitrate tolerance. During the initial, medication-free exercise all patients had increased pulmonary arterial wedge pressure (PAWP) 31.4 +/- 5.56 mmHg (mean +/- SD), showing impaired left ventricular function, while mean arterial pressures (MAP) rose from 112 +/- 16.3 mmHg at rest to 141 +/- 15.9 mmHg during exercise. After 4 weeks ISDN treatment, mean exercise PAWP and MAP, 3 h after morning dose, were reduced to 22.4 +/- 7.09 mmHg and 127 +/- 18.2 mmHg, respectively. Before the ISDN treatment, GTN reduced exercise PAWP to 13.9 +/- 5.27 mmHg and MAP to 119 +/- 11.2 mmHg, whereas after 4 weeks ISDN treatment, the addition of GTN did not reduce exercise PAWP and MAP to the same low levels. Thus, the applied ISDN regimen improved the hemodynamics, but induced a definite, partial nitrate tolerance.

Eccentric dosing with isosorbide-5-mononitrate in angina pectoris

In a placebo-controlled, parallel, double-blind study, 214 patients with stable angina were studied. Doses of 5, 10 and 20 mg of immediate-release isosorbide-5-mononitrate (IS-5-MN) or placebo were administered at 0800 and 1500 hours for a period of 3 weeks. On days 1 and 14, patients underwent exercise testing before, and 2 and 7 hours after both doses; on days 7 and 21, testing was performed 2 hours after morning dosing. Study variables included total exercise duration and time to moderate angina. On day 1, the active treatment groups showed improvement over the placebo group 2 and 7 hours after dose 1, and 2 hours after dose 2. Throughout the subsequent study period, each active treatment group was superior to placebo 2 hours after the first and second doses. Only the 20 mg dose increased exercise duration 7 hours after dosing, and this was observed after both doses 1 and 2 on day 14. There were fewer episodes of angina in patients receiving the 20 mg dose. There was no increase in nocturnal angina during eccentric dosing with IS-5-MN. In comparing exercise times throughout the study, tolerance did not develop during eccentric dosing. The morning exercise tests before tablet administration showed a progressive increase during the study period, with slightly greater improvement in patients receiving active therapy. This absence of a zero hour effect suggests that rebound does not occur during eccentric dosing with immediate-release IS-5-MN.

Release of nitric oxide from glyceryl trinitrate by captopril but not enalaprilat: in vitro and in vivo studies

1. The hypotensive effects of glyceryl trinitrate (GTN, 0.5 mg kg-1) but not of 3-morpholino-sydnonimine (SIN-1, 0.125 mg kg-1) in anaesthetized rats were attenuated following a seven day (using a q.i.d. dosing schedule) oral treatment with isosorbide-5-mononitrate (IS-5-MN; 5 mg kg-1) indicative of the induction of tolerance to GTN but not to SIN-1. The hypotensive effects of GTN did not decline when the sulphydryl (SH) containing angiotensin converting enzyme inhibitor (ACE-1), captopril (CPT, 5 mg kg-1) or the structurally unrelated SH-containing, N-acetylcysteine (NAC, 10 mg kg-1) but not the non-SH-containing ACE-I, enalaprilat (ENA, 5 mg kg-1) were given together with IS-5-MN for the seven days treatment. 2. The attenuated hypotensive effects of GTN (0.5 mg kg-1) in rats treated with IS-5-MN were also restored when CPT (1 mg kg-1) or NAC (2.5 mg kg-1) but not ENA (1 mg kg-1) was administered intraperitoneally (i.p.) 30 min before GTN. Furthermore, in control rats, CPT or NAC but not ENA given i.p. 30 min before GTN, potentiated its haemodynamic effects. These effects were blocked by methylene blue (10 mg kg-1). At the same doses, CPT or NAC did not affect the hypotensive effects of SIN-1. 3. The reduced ability of cultured tolerant smooth muscle cells (SMC, 24 x 103 cells) or endothelial cells(EC, 40 x 103 cells) to potentiate the anti-platelet effects of GTN (44 microM) was restored by CPT or NAC but not by ENA or glutathione (all at 0.5 mM). Potentiation of the anti-platelet effects of tolerant SMC or EC by CPT or NAC was abolished by co-incubation with oxyhaemoglobin (Oxy-Hb, 10 microM)indicative of nitric oxide (NO) formation.4. When GTN (150-2400 microM) was incubated with CPT, NAC or glutathione but not ENA (all at 0.1 mM) for 30 min in Krebs buffer at 37 degrees C a concentration-dependent increase in nitrite (NO2-)formation was observed. 5. The antiplatelet effects of GTN (5.5-352 microM) were potentiated by co-incubation with CPT or NAC but not with ENA or glutathione (all at 0.5 mM). The concentration of GTN required to inhibit platelet aggregation by 50% (IC50) was 110 +/- 2 microM for GTN alone, 14 +/- 2 microM for GTN in the presence of NAC and 30 +/- 2 microM for GTN in the presence of CPT. The potentiation of the effects of GTN by CPT or NAC was inhibited by co-incubation with Oxy-Hb (10 microM). By themselves, CPT or NAC did not inhibit platelet aggregation.6. The ability of CPT to restore (a) the haemodynamic effects of GTN in tolerant rats and (b) the reduced capacity of tolerant SMC or EC to potentiate the anti-platelet effects of GTN is not related to its ACE inhibitory activity.7. CPT also potentiated the hypotensive effects of GTN in non-tolerant rats, and in vitro CPT released NO from GTN in the absence of a GTN to NO converting cell, so that it is unlikely that reversal of tolerance by CPT is due to the replenishment of intracellular thiols. Rather it can be explained by the ability of CPT to release NO from GTN in the extracellular space. This extracellular formation of NO from GTN by CPT would then compensate for the impaired enzymic biotransformation of GTN to NO that develops during tolerance as was originally proposed for NAC.

Differences of intravenous nitroglycerin responses in left ventricular systolic and end-diastolic pressures and coronary artery diameters during long-term treatment with cutaneous nitroglycerin patches

The differences of intravenous nitroglycerin responses in left ventricular (LV) systolic and end-diastolic pressures and in coronary artery diameters (cross-tolerance) were investigated in patients receiving nitroglycerin patches. During diagnostic cardiac catheterization, graded doses of 50, 100, and 150 mcg of intravenous nitroglycerin were given. Left ventricular systolic and end-diastolic pressures and left coronary arteriograms were obtained during each dose. Twenty patients with coronary artery disease were studied. Before cardiac catheterization, 10 received nitroglycerin patch (patch group), and 10 did not (control group). In the control group, graded intravenous nitroglycerin doses of 50, 100, and 150 mcg caused decrease in LV systolic pressure of 18 +/- 7%, 20 +/- 5%, and 23 +/- 6%, respectively. In the patch group, the same intravenous nitroglycerin doses decreased LV systolic pressure by 12 +/- 6% (p < 0.05), 19 +/- 7% (NS), and 18 +/- 6% (p < 0.05), respectively, (p value: vs. control group). At the same intravenous nitroglycerin doses, LV end-diastolic pressures were decreased by 48 +/- 14%, 52 +/- 17%, and 56 +/- 9%, respectively, in the control group. However, there were no significant differences in LV end-diastolic pressure between the two groups for any of the three intravenous nitroglycerin doses. The same intravenous nitroglycerin doses caused increase in diameter of the left anterior descending coronary artery and circumflex coronary artery in the control group, which was attenuated significantly in the patch group. Tolerance may develop in LV systolic pressure and coronary artery diameters, whereas it may not develop in LV end-diastolic pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of nitrates in angina pectoris

Nitrates are used extensively for the treatment of angina pectoris. However, continuous therapy with either oral nitrates or nitroglycerin patches leads to rapid development of tolerance, with loss or diminution of antianginal and anti-ischemic effects. The only practical way to avoid the development of tolerance is to use intermittent daily therapy with nitrates. Nitroglycerin patches applied for 10-12 hours during the day increase exercise duration for 8-12 hours, but a rebound increase in anginal attacks during the nitrate-free interval may occur. Oral isosorbide-5-mononitrate, 20 mg twice a day, with the first dose administered in the morning and the second dose 7 hours later, increases exercise duration for at least 12 hours without the development of tolerance to either the morning or afternoon dose. This dosing regimen has been shown not to produce a rebound phenomenon during the periods of low nitrate levels at night and early hours of the morning. Isosorbide dinitrate (30 mg) prescribed at 7 AM and 1 PM does not produce tolerance to the 7 AM dose, but effects of the afternoon dose have not been evaluated. Recent data suggest that isosorbide dinitrate given 3 or 4 times daily produces tolerance and this dosing schedule is inadequate for antianginal prophylaxis. It should be recognized that intermittent oral or patch therapy with nitrates during the day leaves the patient unprotected at night and early hours of the morning. If this is of concern, additional therapy with another class of antianginal agent, preferably a long-acting beta blocker or a long-acting calcium antagonist should be instituted.

Effects of 8:00 a.m. and 2:00 p.m. doses of isosorbide-5-mononitrate during twice-daily therapy in stable angina pectoris

"Trough" plasma concentrations of isosorbide-5-mononitrate (IS-5-MN), an active metabolite of isosorbide dinitrate, of less than 95 ng/ml are considered necessary to prevent development of tolerance to isosorbide dinitrate and IS-5-MN. In a double-blind, crossover, placebo-controlled study, the effects of IS-5-MN during twice daily eccentric therapy were evaluated in 18 patients with reproducible exercise-induced angina who were nitrate responders. In a random order, patients received either placebo or IS-5-MN (20 mg) at 8 a.m. and 2 p.m. for 1 week each. Average trough plasma IS-5-MN concentrations before the 8 a.m. and 2 p.m. doses were 67 and 226 ng/ml, respectively, and increased to 382 and 488 ng/ml 2 hours after the 8 a.m. and 2 p.m. doses, respectively. Despite a more than threefold higher trough plasma IS-5-MN concentration before the 2 p.m. dose than before the 8 a.m. dose, the increase in exercise duration 2 hours after the doses was similar (1.21 minutes [21%] after 8 a.m. dose, and 1.08 minutes [19%] after 2 p.m. dose). These increases in exercise duration after IS-5-MN were significantly (p less than 0.01) greater than those observed after placebo (0.17 minutes [3%] after 8 a.m. dose, and -0.05 minute [-0.5%] after 2 p.m. dose). Reduction in standing systolic blood pressure at 2 hours after the doses was also nearly identical after the 8 a.m. and 2 p.m. doses of IS-5-MN (21 [15%] and 19 [14%] mm Hg, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Acute hemodynamic effects of carvedilol in comparison with propranolol in patients with coronary heart disease

In a randomized, double-blind study oral doses of 50 mg carvedilol (Dilatrend) were compared with 40 mg propranolol in 16 male patients with coronary heart disease, CHD [12 without significant stenoses following percutaneous transluminal coronary angioplasty (PTCA), 4 with multivessel disease]. Bicycle ergometry in the supine position was performed before and 80 min after drug application; measurements were done at rest, during and after exercise. Clinically, the total exercise time and the onset of angina and exhaustion were noted, while the investigated hemodynamic parameters were heart rate, systemic and pulmonary pressures and resistances, cardiac index, and lower limb blood flow. Clinically, carvedilol improved the exercise tolerance more than propranolol as regards angina and exhaustion. Hemodynamically, carvedilol did not lead, as the classic betablocker propranolol does, to an increase in systemic or pulmonary resistance, to a decrease in cardiac output, or to an increase of the pulmonary capillary wedge pressure during exercise, but instead caused opposite changes. In contrast to propranolol, the post exercise lower limb blood flow had increased significantly. The differences in action between the two betablockers can be explained by the vasodilating properties of carvedilol. Due to these acute effects, carvedilol may be preferred to propranolol in the treatment of CHD patients with hypertension, peripheral occlusive artery disease, and/or coronary vasospasm.

A combined single and multiple dose pharmacokinetic study of oral isosorbide-5-mononitrate in healthy volunteers

Pharmacokinetics of 20 mg isosorbide-5-mononitrate (IS-5-MN) after single and multiple administration of two different tablet formulations were investigated in twelve healthy human subjects using an open, randomized, two-way crossover experimental design. Pentacard 20 mg tablets were compared with Ismo 20 mg tablets. After single-dose administration, both preparations caused a rapid increase in IS-5-MN plasma levels with the peak plasma concentration occurring between 0.5 and 1.5 h. For both formulations, the mean plasma half-life was found to be approximately 5 h after a single dose. In steady state during multiple dosing (t.i.d. at 8 h dosing intervals), a reduced elimination rate was observed. In line with this observation, the area under the plasma concentration-time curve (AUC) for one 8 h dosing interval during multiple dosing was higher than the extrapolated AUC after a single dose. Based on statistical evaluation of the various relevant pharmacokinetic parameters calculated from the plasma concentrations occurring after single and multiple dosing, the tablet formulations are judged to be bioequivalent.

Effects of sublingual nitroglycerin in patients receiving transdermal nitroglycerin for coronary artery disease: prevention of cross-tolerance

The systemic hemodynamic and coronary dilative responses to sublingual nitroglycerin were studied in patients receiving transdermal nitroglycerin. A total of 48 patients with coronary artery disease were divided into 4 groups: 12 patients receiving 1 tablet of sublingual nitroglycerin without transdermal nitroglycerin (Group 1), 12 patients receiving 1 tablet of sublingual nitroglycerin with 12-hour-daily intermittent therapy of transdermal nitroglycerin (Group 2), 12 patients receiving 1 tablet of sublingual nitroglycerin with continuous therapy of transdermal nitroglycerin with continuous therapy of transdermal nitroglycerin (Group 3), and 12 patients receiving 2 tablets of sublingual nitroglycerin with continuous therapy of transdermal nitroglycerin (Group 4). Before and during administration of sublingual nitroglycerin, aortic pressure, left ventricular pressure, and coronary artery diameter were examined at diagnostic cardiac catheterization in all patients. During sublingual nitroglycerin, the decreases of aortic systolic pressure and left ventricular end-diastolic pressure were greater in Group 1, 2, and 4 than in Group 3. Dilation of coronary arteries by sublingual nitroglycerin tended to be greater in Group 1, 2, and 4 than in Group 3. Thus, the effects of sublingual nitroglycerin for the relief of ischemia might be more prominent in patients with intermittent therapy of transdermal nitroglycerin than in those with continuous therapy. The increased dose of sublingual nitroglycerin for the relief of ischemia might be more effective in patients with continuous therapy of transdermal nitroglycerin.

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.

Long-term haemodynamic effects of isosorbide 5-mononitrate in patients with cirrhosis and portal hypertension

Since it is well known that pharmacological tolerance may rapidly occur on continuous administration of organic nitrates, in this study we attempted to investigate whether isosorbide 5-mononitrate (Is-5-Mn), a long-acting vasodilator that decreases portal pressure in acute haemodynamic studies, causes a significant reduction in portal pressure following long-term oral administration. Eleven patients with cirrhosis and portal hypertension were studied prior to and following 3 months of continuous administration of Is-5-Mn, 40 mg b.i.d. The hepatic venous pressure gradient decreased significantly following long-term Is-5-Mn treatment (from 18.6 +/- 3.4 to 17.2 +/- 3.1 mmHg; p less than 0.01). This was associated with a moderate increase in hepatic blood flow. Azygos blood flow and portal blood flow did not change. There were significant decreases in mean arterial pressure (from 89.4 +/- 13.7 to 82.6 +/- 10.8 mmHg; p less than 0.05) and heart rate (from 77 +/- 10 to 73 +/- 10 b.p.m.; p less than 0.05). In contrast, there were no changes in portal pressure or hepatic and systemic haemodynamics in a control group of 17 patients receiving placebo. Repeated nitroglycerin cross-tolerance studies in five patients receiving Is-5-Mn indicated the development of a partial pharmacological tolerance (as shown by blunted haemodynamic response to nitroglycerin after long-term Is-5-Mn administration). This study shows that Is-5-Mn continues to cause a significant decrease in portal pressure during long-term therapy, with only partial pharmacological tolerance to this compound.

Dissociation of exercise tolerance and total myocardial ischemic burden in chronic stable angina pectoris

Exercise treadmill tests and ambulatory monitoring were used in a double-blind, placebo-controlled, double-dummy crossover comparison of nifedipine (10 mg, 3 times daily) and transdermal nitroglycerin (15 mg). All patients (n = 20) had chronic stable angina with symptomatic and silent events. All patients had 3 episodes of angina/week and 3 episodes of ischemia/24 hr. The protocol was made up of 2 weeks of placebo followed by 2 weeks of active drug, then crossed over for 2 weeks of placebo followed by the other active drug. At the end of each 2-week period, patients had ambulatory monitoring and exercise treadmill testing. All ambulatory monitoring reports were read blind and entered into an independent data base. The results were the following: on transdermal nitroglycerin, the duration of ischemia decreased by 57% from 140 min/24 hr to 60 min/24 hr (p = 0.0054). The exercise time increased by 5.5% from 4.8 to 5.0 minutes (p = 0.16). With nifedipine, the duration of ischemia decreased by 22% from 175 min/24 hr to 137 min/24 hr (p = 0.16). The exercise tolerance time increased by 13% from 4.5 to 5.0 minutes (p = 0.0264). Nifedipine increased exercise time without altering total ischemic time, while transdermal nitroglycerin decreased total ischemic time without increasing exercise time. Thus, changes in exercise time do not necessarily predict changes in total ischemic time.

Relationship between the inhibition of receptor-induced increase in cytosolic free calcium concentration and the vasodilator effects of nitrates in patients with congestive heart failure

Nitroglycerin, isosorbide dinitrate and sodium nitroprusside, like nifedipine, were found to inhibit the receptor-provoked increase of cytosolic free calcium concentration in human platelets loaded with 2-[(2-amino-5-methylphenoxy)methyl]-6-methoxy-8-aminoquinoline-N,N,N',N' - tetraacetate. Sodium nitroprusside and nitroglycerin induced elevation of cyclic guanosine 3',5'-monophosphate content in platelets which correlated with their calcium-blocking activity. Methylene blue and epinephrine decreased the calcium-blocking effect and the influence of nitroglycerin on cyclic guanosine 3'-5'-monophosphate content, but failed to suppress the inhibitory effect of sodium nitroprusside. Ascorbic acid increased the calcium blocking effect of sodium nitroprusside and its influence on cyclic guanosine 3'-5'-monophosphate content, but did not alter the inhibitory effect of nitroglycerin. In order to evaluate the relationship between the mode of action of nitrates at cellular level and their vasodilatory effectiveness, we studied the circulatory response of the forearm to isosorbide dinitrate and the influence of nitroglycerin on free calcium concentration in the platelets in 10 patients with chronic heart failure. We established a significant positive correlation between the basal values for venous tone and its peak decrease after administration of the 10-mg dose of isosorbide dinitrate. A correlation was also found between the deviation of maximal decrease of venous tone by this dose of isosorbide dinitrate from the regression line (the relationship between the basal venous tone and its lowering by the drug) and mean inhibitory concentration values for nitroglycerin in blocking that proportion of the rise of calcium ion concentration in platelets due to blocking of the platelet-activating factor. Thus, nitrates, like calcium antagonists, inhibit the receptor-provoked calcium supply to the contractile system of the cells so neutralizing the effects of increased concentrations of vasoconstrictors. This suggests that the effectiveness of nitrates appears to be positively related to the contribution of receptor-induced increase of cytosolic free calcium concentration in vasoconstriction together with their capacity to raise cyclic guanosine 3',5'-monophosphate.

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.

Nitrates: why and how should they be used today? Current status of the clinical usefulness of nitroglycerin, isosorbide dinitrate and isosorbide-5-mononitrate

Nitrates are highly effective both in terminating acute attacks of angina pectoris and in the prophylaxis of symptomatic and asymptomatic myocardial ischemia. Preload reduction by venodilatation is the prevailing mechanism of nitrates in patients with chronic stable angina and is the unique feature distinguishing them from beta and calcium-channel blockers. Nitrates dilate coronary arteries not only in pre- and poststenotic vessels, but also in eccentric lesions. In patients with endothelial dysfunction, nitrates seem to be the physiological substitute for endothelium-derived relaxing factor. During the past decade, however, there has been substantial evidence of a clinically relevant loss of the anti-ischemic effects ("nitrate tolerance"). Many studies with oral dosing of isosorbide dinitrate or isosorbide-5-mononitrate at least three times daily have proven nitrate tolerance in patients with coronary artery disease and/or congestive heart failure. Complete loss of anti-ischemic effects after repetitive, continuous patch attachments has also been found. As we first showed in 1983, intermittent therapy with once-daily ingestion of high-dose sustained-release isosorbide dinitrate was successful in preventing the development of tolerance. Similarly, tolerance to isosorbide-5-mononitrate also does not develop when it is ingested once daily. It is now generally accepted that a daily low-nitrate interval is required to prevent tolerance development. Although the minimal patch-free interval required to prevent tolerance needs further investigation, a 12-h patch-free interval should prevent tolerance in most patients. The prolonged duration of action of once-daily high-dosage administration of sustained-release formulations, the improved patient compliance with a single daily administration, and the increased likelihood of maximal anti-ischemic effects are important reasons for recommending high single daily doses of isosorbide dinitrate or isosorbide-5-mononitrate.

Nitrate tolerance. A problem during continuous nitrate administration

The organic nitrates are effective agents in the management of patients with angina pectoris. They are the agents of choice in the treatment of acute episodes of angina pectoris and are useful in angina prophylaxis. While the organic nitrates are extremely effective in angina prophylaxis during acute therapy, there is increasing evidence that with many dosing regimens for oral and transdermal therapy, substantial attenuation of the antianginal effects develops. Thus, during acute therapy the organic nitrates improve exercise tolerance for many hours, but during sustained therapy designed to provide antianginal efficacy throughout the 24-h period there is significant attenuation of the beneficial effects. It has been documented that treatment regimens designed to provide a period of nitrate washout prevent or reverse nitrate tolerance, and such changes in dosing regimens have been shown to provide continued antianginal protection. It is clear that the objective of providing 24-h antianginal protection with the organic nitrates cannot be achieved. With appropriate dosing schedules, however, it is possible to improve exercise tolerance throughout the major portion of the 24-h dosing period.

Drug-related syncope

The records of 483 patients admitted to the emergency room because of syncope were reviewed. Forty-one patients were found to have drug-related syncope. Thirty-nine experienced syncope related to drugs administered for cardiovascular disease. The most frequently associated diseases were anginal syndrome (22 patients), hypertension (13 patients), and a history of myocardial infarction (6 patients). Thirty-eight patients experienced symptomatic orthostatic hypotension following drug taking (nitrates in 19 patients, beta blockers in 10 patients, nifedipine in 3 patients, prazosin and quinidine in 2 patients each, methyldopa and verapamil in 1 patient each). One patient developed complete heart block as a result of digoxin intoxication. Two patients developed the characteristic picture of anaphylactic reaction (1 with ampicillin, 1 with dipyrone). During one-year follow-up, without the offending medications, no further syncopal episodes were reported by these patients. We conclude that drug-related syncope was more common among our patients with syncope than had been reported previously. It is suggested that drug-related syncope should be taken into consideration in any patient with syncope who is treated by any of the above-mentioned drugs.

Effects of sublingual nitrate in patients receiving sustained therapy of isosorbide dinitrate for coronary artery disease

To examine the effects of sublingual isosorbide dinitrate (ISDN) in patients receiving sustained ISDN therapy, 24 patients with coronary artery disease were divided into 2 groups. Group C comprised 12 patients without sustained ISDN therapy and group N included 12 patients with sustained ISDN therapy. Before and during administration of sublingual ISDN in both groups, aortic systolic pressure, left ventricular end-diastolic pressure and coronary artery diameter were examined at cardiac catheterization. During sublingual ISDN, the aortic systolic pressure decreased by 20 +/- 6% (138 +/- 26 to 112 +/- 27 mm Hg, p less than 0.01) in group C and 10 +/- 6% (127 +/- 26 to 113 +/- 23 mm Hg, p less than 0.01) in group N (p less than 0.01, group C vs group N). The left ventricular end-diastolic pressure decreased by 65 +/- 16% (11 +/- 5 to 4 +/- 3 mm Hg, p less than 0.01) in group C and 43 +/- 14% (12 +/- 5 to 7 +/- 3 mm Hg, p less than 0.01) in group N (p less than 0.01, group C vs group N). During sublingual ISDN, the diameters of the proximal and distal segments of the left anterior descending and circumflex coronary arteries increased more significantly in group C than in group N (p less than 0.01, group C vs group N). Thus, sublingual ISDN produced less reduction of aortic systolic pressure and left ventricular end-diastolic pressure, and less dilation of coronary artery diameter in patients receiving sustained therapy with ISDN than in those without sustained therapy.(ABSTRACT TRUNCATED AT 250 WORDS)