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Clinical Comparison of Antiischemic Efficacy of Isosorbide Dinitrate and Molsidomine

Lehmann, Günter; Reiniger, Günther; Beyerle, Andrea; Schömig, Albert

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Journal of Cardiovascular Pharmacology: January 1998 - Volume 31 - Issue 1 - p 25-30
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Abstract

Nitric oxide (NO) donors (nitrates such as nitroglycerin, isosorbide dinitrate, and isosorbide-5-mononitrate, and sydnonimines such as molsidomine) have been shown to be of value in short-term as well as long-term treatment of heart failure (1,2) and ischemic heart disease (3-7). With nitrates, metabolization to NO by a cascade of events occurring in the vascular smooth-muscle cells is required before stimulation of cytosolic soluble guanylate cyclase (6). However, continuously high nitrate plasma concentrations lead to rapid loss of antiischemic and hemodynamic actions (4,6,8). Despite various attempts to overcome or circumvent tolerance development such as modification of nitrate molecules (9) or addition of sulfhydryl groups-donating agents (10), interval therapy has remained the only therapeutic strategy to preserve initial effects during long-term treatment (3,4,8). Moreover, although equally effective at identical times of day, nitrates' effects also vanish during one 24-h treatment cycle such that, consequently, therapeutic coverage encompassing 24 h is precluded with this substance group (5,6,8).

Sydnonimines, of which linsidomine (SIN-1) and its prodrug molsidomine are the best known, differ from nitrates mostly by their mode of activation and duration of effects: in contrast to the latter, no thiol-dependent intermediate steps are required, and except for a few instances, no clinically relevant tolerance development has been reported (11-13). On the other hand, effect duration of today's sustained-release formulations is confined to a few hours (2,14), thus requiring multiple dosing within one 24-h treatment cycle. Because there is no evidence of cross-tolerance, sydnonimines appear useful as an adjunct to nitrates or simply to bridge the inevitable therapeutic gap required during long-term treatment with nitrates (13,15).

Both substance groups, each with its peculiarities, are of proven efficacy during long-term treatment of ischemic heart disease (3-8). However, because no data exist comparing both treatments on clinical grounds, we investigated the extent and duration of antiischemic and hemodynamic actions of isosorbide dinitrate and molsidomine in patients with exercise-induced ischemia resulting from coronary artery disease.

PATIENTS AND METHODS

Patients

The study was carried out in 16 men with angiographically documented coronary artery disease with ≥75% stenosis of at least one major coronary artery. All had reproducible exercise-induced ST-segment depression ≥0.2 mV at 0.08 s after the J point in at least one precordial lead in the electrocardiogram. The presence of symptoms was not required, but more reliable parameters not amenable to subjective influences were chosen, reflecting myocardial ischemia and hemodynamic parameters (4,6,16).

Patients with concomitant cardiovascular disease were excluded from the study, as were those with unstable symptoms or a myocardial infarction ≤3 months before the beginning of the study. Other than the substances tested, no drugs capable of affecting the measured parameters were permitted. Nitrates, calcium channel blockers, and β-adrenergic blockers were discontinued at least five half-lives before the studies. If required, 0.8 mg nitroglycerin was given sublingually. All patients had given their informed written consent before participation in the study.

Study protocol

The study was performed with a randomized, double-blind, placebo-controlled, cross-over protocol. The extent and duration of action were assessed on 3 days, each separated from the subsequent day by a 24-h washout period. In the morning, either two tablets of placebo, identical in appearance to the respective substances, or placebo plus either one 120-mg tablet of sustained-release isosorbide dinitrate or one 8-mg tablet of sustained-release molsidomine were administered. On the day when molsidomine was given, patients received another 8-mg tablet of sustained-release molsidomine at 8 h after the first one and still another one at 16 h after the first dose in the morning; on the other 2 treatment days, placebo was administered at 8 and at 16 h after the morning dose. At 2, 8, 12, and 24 h after morning ingestion of active drug/placebo on the respective days of treatment, exercise testing was performed, and blood samples were taken for plasma concentration determinations.

Exercise testing was conducted in the semisupine position on a rpm-independent bicycle ergometer starting with an individual load chosen such that, for a given patient at 3-4 min after beginning, an ST-segment depression of ≥0.1 mV could be anticipated. At intervals of 3 min, the workload was increased by 30 W. Termination of exercise was either symptom limited (angina pectoris, exhaustion, shortness of breath), on blood pressure decreases, or on observation of 4 mm of ST-segment depression. Electrocardiographic tracings of the precordial leads V1-V6 were recorded each minute during and for 6 min after termination of exercise. Blood pressure and heart rate were measured at rest, at the end of each workload, at the termination of exercise, and at 3 and 6 min after exercise. ST-segment depression at 0.08 s after the J point in the lead with the most marked changes in the pretreatment phase was evaluated by two independent observers (5,6,15).

Blood samples, which had been placed in heparinized containers, were centrifuged at 4,000 rev/min at 4°C and then frozen at −20°C. Plasma concentrations of isosorbide dinitrate and of its metabolites, isosorbide-2-mononitrate and the antiischemically more active isosorbide-5-mononitrate, were analyzed by means of gas chromatography (17). For molsidomine, high-pressure liquid column chromatography was performed (18).

End-point parameters assessed were reduction in ST-segment depression at comparable workload (in millimeters); ischemia-free (i.e., 1 min before an ischemic reaction of 1-mm ST-segment depression) exercise capacity (in watts × minutes); time for occurrence of 1-mm ST-segment depression (in minutes; because a considerable number of patients often do not show any ischemic reaction, especially at the time of maximal effects, this parameter may lack natural distribution and therefore requires a different statistical analysis; see the following); heart rate, systolic blood pressure, and systolic blood pressure-heart rate product, each at rest and at highest comparable workload; heart rate at 1-mm ST-segment depression (percentage of maximal age-predicted heart rate); as well as the plasma concentrations of isosorbide dinitrate, isosorbide-2-mononitrate, isosorbide-5-mononitrate, and molsidomine.

For statistical evaluation of influence of treatment, analysis of variance was used, combined with Fisher's protected least-significant difference procedure; parameters not obeying a normal distribution were subjected to multiple comparison according to the Friedman test, followed by the Wilcoxon test. To exclude an impact of the day of the study, the cross-over experiment for clinical trials (19) was performed. A p value <0.05 was considered statistically significant.

Nonresponders were defined as patients with a reduction in ST-segment depression <33% or <1 mm at all times compared with responses observed in patients taking placebo, despite isosorbide-5-mononitrate plasma concentrations >100 ng/ml (6,8,20) or >8 ng/ml with molsidomine (21).

RESULTS

ST-segment depression at comparable workload

Sustained-release isosorbide dinitrate (120 mg). Compared with placebo, exercise-induced ST-segment depression at a comparable workload with isosorbide dinitrate was reduced, statistically significant by 82 and 64% at 2 and 8 h (Table 1 and Fig. 1). In contrast, at 12 h, only a residual reduction of 13% (NS) remained visible, vanishing completely at 24 h.

TABLE 1
TABLE 1:
Antiischemic effects and plasma concentrations
FIG. 1
FIG. 1:
ST-segment depression at comparable workload at 2, 8, 12, and 24 h after morning ingestion of placebo (open columns), 120 mg of sustained-release isosorbide dinitrate (hatched columns), or the first of three tablets of 8-mg sustained-release molsidomine (checked columns) administered every 8 h (columns represent mean ± SEM). Percentage changes compared with placebo are indicated. **p < 0.01; ***p < 0.001.

Sustained-release molsidomine (8 mg) t.i.d. Comparably fewer reductions by 68% at 2 h and by a residual, nonsignificant 9% at 8 h were found. At 12 h, equivalent to 4 h after renewed dosing, this parameter was significantly reduced by 38%. At 24 h, however, equivalent to 8 h after the last tablet, no meaningful effect could be measured, as was the case 16 h earlier.

Ischemia-free workload

Sustained-release isosorbide dinitrate (120 mg). Increases in workload of 116, 93, and 58% without signs of ischemia compared with placebo at 2, 8, and 12 h, respectively, were all statistically significant versus placebo (Table 1 and Fig. 2).

FIG. 2
FIG. 2:
Ischemia-free workload at 2, 8, 12, and 24 h after morning ingestion of placebo (open columns), 120 mg of sustained-release isosorbide dinitrate (hatched columns), or the first of three 8-mg tablets of sustained-release molsidomine (checked columns) administered every 8 h (columns represent mean ± SEM). Percentage changes compared with placebo are indicated. *p < 0.05; ***p < 0.001.

Sustained-release molsidomine (8 mg) t.i.d. At 2 h, obviously at the time of maximal effects, this parameter increased comparably versus placebo by 118%. At the two later times, at 8 and 12 h after the first tablet, increases by 47 and 100% versus placebo were found (i.e., opposite those of nitrate once daily). Up to 12 h after the first tablet in the morning, all changes were statistically significant for both treatments.

Time for occurrence of 1-mm ST-segment depression

Sustained-release isosorbide dinitrate (120 mg). With nitrate, time for occurrence of 1-mm ST-segment depression was increased by 74% at 2 h, by 42% at 8 h, and by 18% at 12 h, each of which was statistically significant. At 24 h, however, no more clinically relevant effects were detectable.

Sustained-release molsidomine (8 mg) t.i.d. At the respective times, increases by 44, 0, and 11% were found with sydnonimine, of which only the first and the third reached the level of significance. At 24 h, the effects of this regimen did not differ from those of placebo.

Responsiveness

Sustained-release isosorbide dinitrate (120 mg). With individual responses, all patients except one had a reduction in exercise-induced ST-segment depression ranging from 60 to 100% in association with isosorbide-5-mononitrate plasma concentrations of ≥100 ng/ml. The remaining patient showed no effect at any time in spite of adequate plasma concentrations and must be regarded as a nonresponder.

Sustained-release molsidomine (8 mg) t.i.d. At 2 h, the time of maximal effects, two patients did not have their ischemic reactions reduced by ≥33% despite sufficiently high molsidomine plasma concentrations (i.e., ≥8 ng/ml), as they did not at the remaining times.

Nevertheless, nonresponders to one drug always consistently responded to the other.

Heart rate and blood pressure

As opposed to the treatment day with molsidomine, tendencies to more marked increases of heart rate at rest were observed with sustained-release isosorbide dinitrate in a range of ≤8% for ≤8 h (Table 1). At comparable workload, however, both treatments led to only marginal influences of ≤2% during the entire 24-h treatment cycle. Blood pressure was reduced by ≤9% on both treatments for ≤12 h at rest only, but no meaningful effects could be observed during exercise. The systolic blood pressure-heart rate product, an indicator of myocardial oxygen consumption (22), was affected to the same degree only at rest and more with the nitrate. Nevertheless, none of these changes was statistically significant.

At 1-mm ST-segment depression, relevant increases in heart rate, a parameter of myocardial oxygen consumption, could be found only with the nitrate despite its meaningful effects on myocardial ischemia. Increases with placebo amounted to 14, 9, and 5% of predicted age-related maximal heart rate at 2 h, 8 h, and 12 h after morning ingestion of 120 mg sustained-release isosorbide dinitrate.

Plasma concentrations

With administration of 120 mg of sustained-release isosorbide dinitrate, mean isosorbide dinitrate plasma concentrations, after peaking at 2 h, continuously receded to 8, 3, and <1 ng/ml at 8, 12, and 24 h, respectively. For both major metabolites, the maximal values were found at 4 h. A residual plasma concentration of ≈100 ng/ml was found for isosorbide-5-mononitrate at 24 h, which is a prerequisite for unattenuated effects during long-term treatment with this formulation (3-6,8,20).

At 2 and 12 h (equivalent to 4 h after renewed dosing), molsidomine plasma concentrations of almost 20 ng/ml were found, above a limit considered necessary to elicit therapeutic effects (21). In contrast, borderline values of ≈8 ng/ml were measured at 8 h and 24 h (equivalent to 8 h after the third tablet) after initial dosing.

Adverse effects

None of the patients reported untoward side effects, such as headaches, dizziness, or symptomatic hypotension, at any time in this short-term study.

DISCUSSION

Although nitrates and sydnonimines release NO for stimulation of cytosolic soluble guanylate cyclase, forming the basis of their use in clinical heart failure and ischemic heart disease (1-7), both substance groups differ in potential for tolerance and extent and duration of effects, which impinge on therapeutic coverage in a 24-h treatment cycle. The purpose of our study was to characterize and to compare two established treatments of either substance group to optimize the therapeutic application of NO donors.

With 120 mg sustained-release isosorbide dinitrate, meaningful reduction of exercise-induced myocardial ischemia lasted for ≥8 h. Although approximately as effective as other high-dose sustained-release nitrates in other studies of the time of maximum effects (3-5), there were only residual effects at 12 h after dosing in our study. We attribute this to patient characteristics such as a greater individual variability over time or a tendency to more marked response at the time of maximum effects, because in one study, this formulation elicited a maximal reduction in exercise-induced ST-segment depression of >90% (3), whereas in another study, testing two tablets of nonsustained-release isosorbide dinitrate, a reduction by 15% was still significant at 12 h (6). Nevertheless, there is always a vanishing antiischemic activity over time despite further increase in isosorbide-5-mononitrate plasma concentrations even within one 24-h treatment cycle, irrespective of maintained effectiveness during long-term application (3,4). Antiischemic activity of a nitrate ≤12 h must be assumed, regardless of the formulation chosen (3-6,8). In spite of attempts to circumvent tolerance development by either altering the molecular structure (9) or coadministration of sulfhydryl groups-donating compounds (10), the former of which still lacks clinical testing and the latter of which appears simply impracticable (23), interval therapy continues to be the optimal strategy for maintained effectiveness during long-term use of nitrates. Because therapeutic coverage of 24 h is not feasible with nitrates, they appear best suited to cover the period of the day encompassing most physical activities.

With regard to sydnonimines, tolerance appears to be the exception rather than the rule (13), but the short duration of effects of even sustained-release formulations requires multiple dosing in a day (2,14,24). There exist only few recommendations with respect to this high-dose, sustained-release formulation of molsidomine to cover as much of the 24-h treatment cycle as possible (2,7,24), all of which consider its administration twice daily to provide sufficient antiischemic protection.

According to our data, there is less reduction of exercise-induced ST-segment depression, even at the time of maximal effects, and a shorter duration of antiischemic effects compared with the high-dose, sustained-release dinitrate. In parallel, there were molsidomine plasma concentrations in a range considered to lack antiischemic effects at 8 h after dosing (21), at 8 and 24 h after the first tablet. We must conclude that the duration of actions of 8-mg sustained-release molsidomine is confined to between 4 and 8 h, which would necessitate either dosing ≥4 times daily or administration of even higher doses when intake frequency is to be kept to a minimum (2). Disparities between the data of Messin et al. (7) and those of our study regarding the duration of effects rather than their extent must be ascribed to differing methods. In that study, parameters were assessed in subgroups of patients, which may detract from the statistical power of the analyses that were performed on pooled data by means of Student's t test. In addition, no plasma concentrations were measured, and the subgroup of patients with an effect duration >10 h underwent exercise testing after a second tablet of molsidomine, whereas all the other subgroups did so after one tablet (7). Apart from a putative accumulation of the substance at 10 h after two tablets in that subgroup and an altered hepatic transformation to the antiischemically active metabolite SIN-1A, an additional vasodilating role of another molsidomine metabolite, SIN-1C, lacking NO−1, cannot be ruled out (25).

With both treatments tested, only a minority of nonresponders responded to the alternative treatment. This is in keeping with literature reports in which nonresponsiveness is the exception rather than the rule for both substance groups and irrespective of the clinical setting (16,21,26). In addition, because nitrates and sydnonimines are well absorbed, a lack of effects is often the result of noncompliance rather than nonresponsiveness, which can be overcome with use of the alternative substance.

Hemodynamic parameters were affected only at rest and at single times by both treatments, which appears to be dependent on both the dose and the formulation used (2,4-6). Systolic blood pressure-heart rate product, an indicator of myocardial oxygen consumption (22), is often found to increase with nitrates during exercise (27,28) but was not affected at rest and at highest comparable workload. Obviously, this effect, reflecting improved blood supply, can only be detected at peak exercise (i.e., beyond highest comparable workload; 5,6,27,28). On the other hand, there was an increase in heart rate with an enhanced exercise capacity to elicit an ischemic reaction of 1-mm ST-segment depression. This points to preload as the major target of nitrates and sydnonimines, with which the same but lesser effects could be observed, because NO donors barely affect contractility and afterload as the remaining fundamental determinants of myocardial oxygen consumption in therapeutic dosages.

In summary, both treatments reduced myocardial ischemia to a comparable extent, although according to different patterns, and both have similar therapeutic indications (12). With a high-dose, sustained-release nitrate, maximal reductions in ischemia between 60 and 90% and a maximal duration of antiischemic effects between 8 and 12 h can be expected, allowing coverage of most physical activities (4-6,8,29). Because of tolerance development, this lapse of nitrate effects cannot be extended (6,30). Maximal antiischemic effects of sydnonimines appear comparably less, as does duration of actions on the doses and formulations tested. Possibly even higher-dosed preparations of molsidomine will overcome these shortcomings in the future (2); otherwise, repeated administration ≥4 times within the 24-h treatment cycle is necessary for complete therapeutic coverage, which is not feasible with a nitrate alone (4-6,8). The disadvantage of multiple dosing is accompanied by the advantage of lack of tolerance and even lack of cross-tolerance in the state of established nitrate tolerance (15). Consequently, molsidomine represents an alternative to nitrates with identical indications (12) and can bridge the therapeutic gap required for preservation of effects during long-term administration (8,30). In addition, it is suitable as an adjunct to nitrates, especially during the later course after nitrate administration, when their actions begin to fade away despite otherwise therapeutically effective plasma concentrations (5,6,8,15).

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Keywords:

Coronary artery disease; Hemodynamics; Ischemia; Isosorbide dinitrate; Molsidomine; Plasma concentrations

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