The effects of prostacyclin (PGI2) on ventricular arrhythmias following 20 min of coronary occlusion and release were studied in 34 conscious dogs. We administered PGI2 at 100 ng/kg/min and did not observe significant changes in heart rate, blood pressure, or systemic vascular resistance. During the control period, heart rate was 97 +/- 30 (mean +/- SEM) vs. 99 +/- 28 in the PGI2-treated group. Mean arterial pressure was 115 +/- 26 mm Hg and 109 +/- 10 mm Hg in the control and PGI2 groups, respectively. Systemic vascular resistance declined minimally from 2,985 +/- 221 dyn . s . cm-5 to 2,484 +/- 135 dyn . s . cm-5 during the PGI2 infusion (p = NS). Following coronary occlusion, the frequency of ventricular fibrillation was reduced from 53% (9/17) in the control group to 6% (1/17) in the PGI2 group (p less than 0.01). Overall 80-min postinfarction survival was 64% in the group receiving PGI2 infusion compared to 24% in the control group (p less than 0.05). The effects of PGI2 in preventing ventricular fibrillation following acute coronary occlusion can be ascribed to a direct action of this prostaglandin on the myocardium, rather than to an indirect effect due to a reduction in systemic vascular resistance.

Animals↗

PubMedDate not supplied

Electrophysiology of O-demethyl encainide in a canine model of sustained ventricular tachycardia.

The antiarrhythmic agent encainide produces marked suppression of ventricular arrhythmias in most patients. However, in some with sustained ventricular tachycardia, worsening of clinical arrhythmias can occur. Since the effects of this agent are mediated by its O-demethyl metabolite in most patients, we have evaluated the effects of O-demethyl encainide in dogs susceptible to the induction of ventricular tachycardia. Nonsedated animals were studied 3-5 days after 90-min left anterior descending coronary artery occlusions. Electrophysiologic evaluations were carried out at baseline, and then during a series of infusions of O-demethyl encainide that achieved low (58 +/- 5 ng/ml) (mean +/- SE), moderate (190 +/- 16 ng/ml), and high (758 +/- 98 ng/ml) plasma concentrations compared with the range seen in patients (50-300 ng/ml). Ventricular tachycardia induction was unaffected by the drug. Effective refractory period was prolonged in a dose-related fashion at both normal and infarcted epicardial sites. However, local electrogram duration was prolonged only in the infarcted zone. We conclude that O-demethyl encainide exerted no consistent effect on susceptibility to induction of ventricular tachycardia in this study. This agent appears to alter infarcted zone conduction disproportionately.

Anilides↗

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The antiarrhythmic activity of meobentine sulfate in man.

Meobentine (sulfate) has antifibrillatory and antiarrhythmic activity in canine models. The antiarrhythmic, pharmacokinetic, and adrenergic neuronal blocking effects of meobentine were assessed in 15 patients with chronic, high-frequency ventricular ectopic depolarizations (VEDs). Eleven of the 15 patients had recurrent nonsustained ventricular tachycardia. The patients were given a series of gradually increasing single doses of meobentine; six received oral meobentine and nine had infusions. The antiarrhythmic efficacy of meobentine was assessed by a comparison of arrhythmia frequency during placebo given on days just prior to meobentine. Oral therapy with meobentine at dosages above 20 mg/kg caused diarrhea, and well-tolerated dosages achieved peak concentrations of 0.69 micrograms/ml (range 0.5-1.0 micrograms/ml). Antiarrhythmic activity was seen in only one patient with oral meobentine. In contrast, intravenous infusions (6.75-34.2 mg/kg) achieved concentrations ranging from 1.3-9.8 micrograms/ml. There was a linear relationship between pseudo-steady-state plasma concentrations and dosage, r = 0.82, p less than 0.01. Antiarrhythmic activity was seen in four of nine patients who received intravenous meobentine over a range of concentrations from 2.5-4.5 micrograms/ml. Four patients developed evidence of adrenergic neuronal blockage (loss of the venous reflex response); two at dosages of 16.2 mg/kg, one at 24.3 mg/kg, and one at 34.2 mg/kg. In one individual (24.3 mg/kg), the adrenergic neuronal blockade was associated with an acute episode of shortness of breath, orthopnea, and cough. With intravenous meobentine, there was a linear relationship between dosage and AUC, and the elimination half-life ranged from 11-27 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Administration, Oral↗

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Electropharmacologic synergism with mexiletine and quinidine.

We have previously shown that combination therapy with mexiletine and quinidine is more effective and causes fewer side effects than either agent alone in the treatment of patients with serious ventricular arrhythmias. To further assess this enhanced antiarrhythmic effect, the electrophysiologic actions of mexiletine and quinidine alone and in combination were evaluated in isolated perfused rabbit hearts. Dose-response curves were constructed for right ventricular effective refractory period, epicardial monophasic action potential duration, and conduction time (the time from pacing stimulus to the upstroke of the monophasic action potential signals) during constant rate pacing. No significant changes in these parameters were seen in 15 preparations treated with saline over a duration of 1 h (a time equal to the longest experiments). With gradually increasing concentrations of mexiletine (2.3-36 microM) prolongation of conduction time (12 +/- 5 msec, mean +/- SE, p less than 0.05) paralleled change in ventricular refractoriness (20 +/- 5 msec, mean +/- SE, p less than 0.05) but occurred in the absence of any significant change in monophasic action potential duration. With gradually increasing concentrations of quinidine (0.55-34 microM/L) prolongation of ventricular refractoriness (65 +/- 7 msec, mean +/- SE, p less than 0.01) and monophasic action potential duration (65 +/- 7 msec, mean +/- SE, p less than 0.01) occurred in parallel and at concentrations less than those required to prolong conduction time.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

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Reducing inappropriate prescribing of sublingual nifedipine.

OBJECTIVE: To report a program to reduce the practice of prescribing sublingual nifedipine. MONITORING AND EDUCATIONAL PROGRAM: Pharmacy records were used to identify orders for sublingual nifedipine at Georgetown University Medical Center. Initial review showed 30-40 orders/month, or approximately 11% of all nifedipine orders. A newsletter was published outlining Pharmacy and Therapeutics Committee guidelines for the use of nifedipine when rapid onset of action is desired. Further educational efforts involved correspondence with each attending physician responsible for the sublingual nifedipine orders. A reduction in orders for sublingual nifedipine to approximately 10 orders/month (3.9% of total nifedipine orders) was observed after using this educational approach. The reduction in orders has been maintained by frequent contact with the attending physicians. CONCLUSIONS: Repeated educational measures have resulted in a reduction in the inappropriate prescribing of sublingual nifedipine.

Administration, Sublingual↗

PubMedDate not supplied

Acetylator phenotype and lupus erythematosus.

There are several known therapeutic implications of acetylator phenotype; among them, the association of a higher incidence of procainamide- and hydralazine-induced lupus in slow acetylators. Presumably, this is because acetylation of the aromatic amine or hydrazine functional group leads to a non-toxic product. Several other drugs which have been implicated in drug-induced lupus also contain an aromatic amine or hydrazine group. The clinical and laboratory characteristics of drug-induced and idiopathic lupus are similar but the degree to which the pathophysiological mechanisms are related, if at all, is unknown. There is also evidence reported for an association between the slow acetylator phenotype and idiopathic lupus. If true, this relationship should provoke some new experimental approaches to investigation into the mechanism of idiopathic lupus.

Acetylation↗

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Clinical pharmacokinetics of disopyramide.

Disopyramide is an antiarrhythmic agent with proven efficacy in the management of atrial and ventricular arrhythmias. The drug is well absorbed and undergoes virtually no first-pass metabolism. Peak concentrations are achieved approximately 0.5 to 3.0 hours after a dose. Absorption is reduced and slightly slowed in patients with acute myocardial infarction. Disopyramide is excreted as unchanged drug (two-thirds) or as the metabolite mono-N-desisopropyldisopyramide, with elimination via both renal and biliary routes. Elimination half-life is approximately 7 hours in normal subjects and patients, but is prolonged in patients with renal insufficiency (creatinine clearance less than 60 ml/min). Disopyramide exhibits complex protein binding. It is bound to alpha 1-acid glycoprotein (AAG), an acute phase reactant, and binds in a concentration-dependent (saturable) manner. The unbound fraction is reduced in the presence of elevated concentrations of AAG, as are found in acute myocardial infarction and in some chronic haemodialysis patients and renal transplant recipients. Free disopyramide concentrations are low relative to total concentration in these patients. Because the pharmacological effects of disopyramide are determined by unbound drug, changes in the unbound fraction could make total disopyramide concentrations misleading as a guide to therapy. Changes in protein binding do not, however, alter free disopyramide or metabolite concentrations, both of which are dependent only on dosage and intrinsic clearance. Free drug concentration measurement could potentially improve therapeutic monitoring, but is as yet of unproven clinical value. Disopyramide is cleared more rapidly in children than in adults, and therefore children require higher dosages to attain therapeutic concentrations.

Aged↗

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N-Desethyacecainide is a metabolite of procainamide in man; convenient method for the preparation of an N-dealkylated drug metabolite.

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Biotransformation↗

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Metabolism of procainamide to a hydroxylamine by rat and human hepatic microsomes.

We have previously demonstrated that procainamide is oxidized to a reactive metabolite. We speculated that this reactive metabolite might be a hydroxylamine and further that it might be responsible for the syndrome of procainamide-induced lupus. We now report that procainamide is metabolized to a hydroxylamine by rat and human hepatic microsomes. The extent of this metabolic oxidation was quantitated by HPLC after conversion of the hydroxylamine to the more stable nitro derivative of procainamide. Formation of the hydroxylamine required NADPH, active microsomes, and oxygen and was inhibited by carbon monoxide, SKF 525-A, and cimetidine. Formation of the hydroxylamine was also studied as a function of time, microsomal protein concentration, and procainamide concentration.

Adult↗

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Metabolism of procainamide in the perfused rat liver.

We have previously produced evidence that procainamide forms a reactive metabolite. This study of the metabolism of procainamide in a perfused rat liver was aimed at providing clues to the identity of this reactive metabolite. Several metabolites were found that had not been previously described and three of these were identified. Probably the most significant of these metabolites is the phenol, N-acetyl-3-hydroxyprocainamide. This phenol could have been formed from either a hydroxylamine or an arene oxide intermediate and either of these could represent a reactive metabolite. In contrast to the metabolism of procainamide, the phenol metabolite, N-acetyl-3-hydroxyprocainamide, is not formed in significant quantities from N-acetylprocainamide. This implies that oxidation of procainamide to 3-hydroxyprocainamide precedes acetylation to give N-acetyl-3-hydroxyprocainamide. As N-acetylation of procainamide decreases both its toxicity and the formation of the oxidative phenol metabolite, among the hypotheses to be explored is relationship between the formation of the phenol metabolite and the toxicity of procainamide. The other metabolites that were identified were N-acetylprocainamide N-oxide and N-acetyl-4-aminohippuric acid.

Acecainide↗

PubMedDate not supplied

Covalent binding of procainamide in vitro and in vivo to hepatic protein in mice.

Procainamide (PA) formed reactive metabolites capable of covalently binding to protein both in vivo and in vitro. The in vitro covalent binding of PA to washed hepatic microsomal protein prepared from control male mice was dependent upon mixed-function oxidase activity. The binding was proportional with time and protein concentration. Glutathione and SKF 525-A inhibited the in vitro covalent binding by 88 and 51%, respectively. The addition of NaF to the incubation medium produced a concentration-dependent decrease in covalent binding. Covalent binding of N-acetylprocainamide in vitro was 90% less than that of procainamide and was not increased by NaF. The in vivo covalent binding of PA to hepatic protein in male mice was increased with phenobarbital and 3-methylcholanthrene pretreatment, resulting in increase in binding of 29 and 56%, respectively, compared to control mice. Pretreatment of mice with SKF 525-A inhibited binding by 39%. Depletion of hepatic glutathione with diethyl maleate pretreatment increased the amount of covalent binding in vivo. Bioactivation of PA by hepatic microsomal enzymes in the mouse produces a metabolite capable of covalent interactions with cellular macromolecules.

Acecainide↗

PubMedDate not supplied

Mechanism of the cardiotoxic actions of terfenadine.

OBJECTIVES AND METHODS: To gain insight into possible mechanisms of and predisposing factors for torsades de pointes during terfenadine therapy, spontaneous reports in the US Food and Drug Administration's Spontaneous Reporting System database were examined. Based on the characteristics of the cases, in vitro cardiac electrophysiologic studies were conducted to test the hypothesis that terfenadine, and not its major metabolite, has actions similar to those of quinidine and is responsible for this form of cardiac toxicity. DESIGN: Spontaneous reports from the general medical community. RESULTS: As of April 1, 1992, 25 cases of torsades de pointes had been reported to the Food and Drug Administration's Spontaneous Reporting System. Predisposing factors in these cases indicated that the parent drug, but not its metabolite, may have actions similar those of quinidine that are responsible for inducing arrhythmia. In vitro studies found that terfenadine is equipotent to quinidine as a blocker of the delayed rectifier potassium current in isolated feline myocytes. The metabolite, terfenadine carboxylate, did not inhibit this potassium current even at concentrations 30 times higher than the concentration of terfenadine producing a half-maximal effect. CONCLUSIONS: Since blockade of the potassium channel did not occur with the major metabolite of terfenadine, episodes of torsades de pointes are most likely the result of a quinidinelike action of the parent drug and of factors that impair the normally rapid metabolism of terfenadine. Dosage restriction and awareness of the clinical conditions and drug interactions capable of inhibiting the metabolism of terfenadine are essential for prevention of this serious reaction.

Adolescent↗