Antiarrhythmic versus antifibrillatory actions: inference from experimental studies.

Pathophysiology of the coronary circulation is a major contributor to altering the myocardial substrate, rendering the heart susceptible to the onset of arrhythmias associated with sudden cardiac death. Antiarrhythmic drug therapy for the prevention of sudden cardiac death has been provided primarily on the basis of trial and error and in some instances based on ill-suited preclinical evaluations. The findings of the Cardiac Arrhythmia Suppression Trial (CAST) requires a reexamination of the manner in which antiarrhythmic drugs are developed before entering into clinical testing. The major deficiency in this area of experimental investigation has been the lack of animal models that would permit preclinical studies to identify potentially useful or deleterious therapeutic agents. Further, CAST has emphasized the need to distinguish between pharmacologic interventions that suppresses nonlethal disturbances of cardiac rhythm as opposed to those agents capable of preventing lethal ventricular tachycardia or ventricular fibrillation. Preclinical models for the testing of antifibrillatory agents must consider the fact that the superimposition of transient ischemic events on an underlying pathophysiologic substrate makes the heart susceptible to lethal arrhythmias. Proarrhythmic events, not observed in the normal heart, may become manifest only when the myocardial substrate has been altered. We describe a model of sudden cardiac death that may more closely simulate the clinical state in humans who are at risk. The experimental results show a good correlation with clinical data regarding agents known to reduce the incidence of lethal arrhythmias as well as those showing proarrhythmic actions.

Atenolol, but not mexiletine, protects against stimulus-induced ventricular tachycardia in a chronic canine model.

1. In a placebo-controlled study of the antiarrhythmic and electrophysiological properties of atenolol and mexiletine, programmed electrical stimulation (PES) was performed in three groups of six conscious greyhounds, 7-30 days after coronary artery ligation. 2. In the placebo group, repeated PES challenge resulted in the consistent induction of ventricular tachycardias (VT) in 4/6 dogs and ventricular fibrillation in 2/6. Atenolol prevented arrhythmia induction in 4/6 dogs, one continued to demonstrate a VT and one died (P less than 0.05 compared with placebo). In the mexiletine group 5/6 dogs continued to demonstrate a VT and one died. 3. Electrocardiographic parameters were not affected by any treatment. There was no change in blood pressure in any group but when compared with placebo, heart rate fell (P less than 0.05) after atenolol (256 micrograms kg-1) and increased (P less than 0.05) after mexiletine (16 mg kg-1). Effective (ERP) and functional (FRP) refractory periods did not change after mexiletine, but ERP was prolonged (P less than 0.05) after atenolol. 4. The results indicate that atenolol but not mexiletine is effective in preventing re-entrant arrhythmias in this conscious canine model. Antiarrhythmic efficacy may be related to a fall in heart rate and/or a prolongation of refractoriness.

Pharmacokinetic and pharmacodynamic comparisons of twice daily and four times daily formulations of procainamide in patients with frequent ventricular premature depolarization.

A study was conducted to evaluate the pharmacokinetics of procainamide and its active metabolite, N-acetylprocainamide (NAPA), as a function of dose and formulation and to characterize the relationship between ventricular premature depolarization (VPD) rate and plasma concentrations of procainamide and NAPA. A subset of patients (n = 43) with frequent VPD who were enrolled in a double-blind, multicenter, activity trial were assigned in randomized fashion to receive 1 of 4 dose levels (placebo or 1,000, 2,000, or 4,000 mg/day procainamide) and to receive Procanbid (Parke-Davis) tablets every 12 hours or Procan SR (Parke-Davis) tablets every 6 hours during the first week of a blinded crossover phase. Patients crossed over to the alternative formulation after one week. Maximum and steady-state average concentrations of procainamide and NAPA after administration of Procanbid tablets were equivalent to those after administration of an equivalent daily dose of Procan SR tablets. Corresponding trough concentrations of procainamide were lower after administration of Procanbid tablets than after administration of Procan SR tablets. Both formulations produced disproportionate increases in procainamide concentrations with increasing dose; concentrations of NAPA increased in proportion to dose. Assessment of the relationship between VPD rate and drug concentration in plasma indicated no substantive difference between the two formulations. It was concluded that administration of Procanbid tablets every 12 hours is essentially equivalent to administration of procainamide extended-release tablets (Procan SR) every 6 hours with respect to pharmacokinetics of procainamide and NAPA and to VPD suppression.

A novel model of venous thrombosis in the vena cava of rabbits.

The objective of this study was to develop and validate a new experimental model of venous thrombosis in the rabbit. A 3-cm length of siliconized PE tubing was used as a veno-venous shunt inserted into the abdominal vena cava of anesthetized rabbits. The PE tubing contained six cotton threads which helped to restrict blood flow through the tubing and served as a foreign, thrombogenic surface upon which a thrombus could develop. By continuously measuring blood flow through the vena cava, the rate of thrombus development can be monitored until zero flow is achieved indicating that a completely occlusive thrombus is present. The shunt can be removed making it possible to weigh the thrombus and/or determine its composition. A second shunt can be placed in the vena cava to make a second determination of time to occlusion and thrombus weight, using the data from the first shunt as an internal control standard for comparison. Reproducibility of the technique was demonstrated in a control group (n = 7) in which two successive shunts were used without an antithrombotic intervention. In studies with the first and second shunts, time to occlusion averaged 20.6+/-5.2 min and 20.2+/-5.7 min (pNS), respectively. The net thrombus weights (less the wet weight of the cotton threads) were 49.0+/-3.5 mg and 47.0+/-3.3 mg (pNS). Histologic examination of the thrombi indicated that they were largely composed of fibrin and red blood cells, consistent with the characteristics of venous thrombi. The low molecular weight heparin (LMWH) enoxaparin was used as an antithrombotic intervention to validate the model. Dose-dependent changes in time to occlusion and thrombus weight were achieved which paralleled alterations in coagulation parameters (thrombin time and activated partial thromboplastin time) and bleeding time determined with an ear bleeding technique. The veno-venous shunt model is easy to use, reproducible, and responds appropriately to an antithrombotic intervention, indicating that it should be useful for experimental evaluation of antithrombotic agents designed for venous thromboembolic disorders.

Resolution and electrophysiological effects of mexiletine enantiomers.

Resolution of mexiletine enantiomers from the racemic mixture has been achieved by fractional crystallization through the formation of diastereoisomeric p-toluoyl tartrate salts. Following three crystallization steps in methanol, R-(-)- and S-(+)-mexiletine were resolved with an optical purity greater than 98% (yield approximately 30%) and their hydrochloride salts formed. Incremental doses of mexiletine enantiomers were administered to dogs with experimentally-induced arrhythmias to investigate the stereoselective antiarrhythmic and electrophysiological effects of these compounds. Using up to three extrastimuli, programmed electrical stimulation was performed in conscious animals 7-30 days after coronary ligation. R-(-)-Mexiletine prevented ventricular tachycardia in 3/6 dogs (2 after 0.5 mg kg-1, 1 after 8 mg kg-1); two animals died after 1 and 8 mg kg-1, respectively; one remained unchanged even at the highest dosage (16 mg kg-1). S-(+)-Mexiletine prevented ventricular tachycardia in only one dog (after 1 mg kg-1); two died after 4 and 8 mg kg-1, respectively; 2/5 remained unchanged even after the administration of 16 mg kg-1. No significant changes in any electrocardiographic intervals (PR, QRS, QTc) or refractory periods were induced by mexiletine enantiomers at any doses used (0.5-16.0 mg kg-1). These results suggest that R-(-)-mexiletine possesses greater antiarrhythmic properties than the opposite enantiomer.

Time-dependence of inducible ventricular tachycardia in a chronic canine model.

The natural history of inducible ventricular tachycardia in post-infarction dogs was followed with serial programmed electrical stimulation (PES: 1-3 extrastimuli, 4 msec duration, 2 x diastolic threshold). Arrhythmia inducibility was defined as a minimum of four unstimulated ventricular ectopic beats. Of 119 dogs prepared for chronic electrophysiological study, 87 (73.1%) were ambulant 24 hours after surgery. Mean infarct size was 11.1 +/- 1.5% of left ventricular (LV) mass for animals dying in the first week, before stimulation. 92.4% of 66 animals were inducible when stimulated at one week, 66.7% at two weeks and 64.3% and 55.6% at the third and fourth weeks respectively (p less than 0.01, Chi-square analysis). Infarct sizes fell from 7.0 +/- 0.5% LV mass at first stimulation to 4.6 +/- 0.8% at third stimulation and could not be visualized thereafter (p less than 0.01,ANOVA). There was no statistical difference between infarct sizes for inducible and non-inducible animals, but in both cases infarcts were smaller (p less than 0.01) than for those animals which died suddenly during the first week. This time-dependent decrease in arrhythmia inducibility, which may be related to infarct size, should be considered when similar models are employed for chronic electrophysiological studies.

Restenosis: is there a pharmacologic fix in the pipeline?

One of the most frustrating aspects of restenosis is that it is the result of advances in medical care (there was no restenosis before the days of balloon angioplasty), yet it seems to be resistant to all that science has to offer. Still we believe there is reason to be optimistic. We are at last beginning to see some promise from clinical trials, and data being generated confirm some of the hypotheses previously generated from animal experiments. Thus the effects seen with the GP IIb/IIIa antibody 7E3 suggest that thrombosis may be as important in its long-term sequelae as it is for acute reocclusion. The jury is still out on whether antiproliferative approaches will be a therapeutic option, but local delivery paradigms using novel formulations delivered by catheter or impregnated in stents may allow the concept to be tested without the risk of systemic toxicity. Plans are also underway for gene therapy trials, although we may have to wait for better vector technology before taking these into the coronary bed. Perhaps we should move away from the "single pill" approach and accept that, like many infections, malignancies, or even heart failure, a multifaceted approach with combination therapy will provide the first glimmer of that brighter tomorrow.

H2-receptor antagonism is not pro-arrhythmic in a chronic canine model.

Seven to 28 days after coronary artery ligation, programmed electrical stimulation was performed in conscious dogs. Groups of 6 previously inducible dogs in which no arrhythmia could presently be achieved were randomly allocated to receive quinidine, cimetidine, ranitidine or placebo. Results were assessed for the drugs' ability to induce ventricular tachycardia or fibrillation, and compared with placebo using Fisher's Exact Test. In the placebo group 4/6 dogs remained unchanged, one developed an arrhythmia, and one died. With quinidine, 3/6 dogs developed an arrhythmia (0.5 mg/kg, 4.0 mg/kg, 4.0 mg/kg) and three died (4 mg/kg, 8 mg/kg, 16 mg/kg) (p less than 0.05 compared with placebo). With cimetidine, 4/6 dogs remained unchanged, one developed an arrhythmia after 4 mg/kg, and one died after 0.5 mg/kg. After ranitidine 3/6 dogs remained unchanged and three died (1.0 mg/kg, 4.0 mg/kg, 16.0 mg/kg). PR, QTc, QRS, refractory periods, and mean systolic pressure remained unchanged after placebo, cimetidine, and ranitidine, but QTc increased (p less than 0.05) and mean systolic pressure fell (p less than 0.01) after quinidine. Heart rate did not change following placebo, but increased (p less than 0.05) after each of the three drug treatments. These results fail to show a significant arrhythmogenic effect of cimetidine or ranitidine in a model validated by the significant pro-arrhythmic effects of quinidine. The cause of death in all cases was ventricular fibrillation.

Discovery and development of an endothelin A receptor-selective antagonist PD 156707.

PD 156707 is a highly potent, selective antagonist of the ETA receptor that has demonstrated efficacy in a number of different disease models. The next few years will be exciting in the field of ET research as several compounds progress through clinical development. It is our hope that the efficacy that data demonstrated to date with PD 156707 will some day be translated into real hope for the patients who are waiting beyond the confines of our research laboratories.

Effects of tocainide enantiomers on experimental arrhythmias produced by programmed electrical stimulation.

The enantiomers of tocainide, a Class Ib antiarrhythmic agent, have recently been shown to exhibit differences in antiarrhythmic activity and pharmacokinetic characteristics. The present study examined the antiarrhythmic and electrophysiological effects of SR tocainide, S tocainide, and R tocainide on arrhythmias in a conscious canine arrhythmia model and compared the results with placebo. Using up to three extrastimuli, programmed electrical stimulation was performed in conscious dogs, 7-30 days after coronary artery ligation. Each treatment was assessed for its ability to abolish sustained ventricular tachycardia, prevent nonsustained ventricular tachycardia, and protect against death (ventricular fibrillation), in groups of six dogs. In the placebo group, arrhythmias in four of six dogs remained unchanged, and two dogs died. SR tocainide prevented the arrhythmia in three of six dogs (mean effective dose, 21.3 mg/kg), one remained unchanged, and two died. S tocainide prevented the arrhythmia in four of six dogs (mean effective dose, 7.1 mg/kg), one remained unchanged, and one died (p less than 0.05 compared with placebo). R tocainide prevented the arrhythmia in five of six dogs (mean effective dose, 9.0 mg/kg), one remained unchanged, and none died (p less than 0.01 compared with placebo). PR intervals, QRS durations, and corrected QT intervals were unaffected by any treatment and there was no change in effective or functional refractory periods. These results indicate that the enantiomers of tocainide are more effective than the racemic mixture in abolishing ventricular arrhythmias and in preventing death in this model; no additive antiarrhythmic effect occurs with the racemic mixture, and adverse effects may be potentiated.

Profibrillatory actions of pinacidil in a conscious canine model of sudden coronary death.

Pinacidil is one of a number of new antihypertensive agents possessing an action that involves an enhanced potassium efflux in cardiac and vascular smooth muscle. An associated feature of pinacidil is a shortening of the cardiac action potential duration, which may constitute a potentially proarrhythmic effect. The present study evaluated pinacidil (0.3 mg/kg/h i.v. for 6 h) on the postinfarcted canine heart in a subset of dogs unresponsive to programmed electrical stimulation during the subacute phase of anterior myocardial infarction, and known to be at low risk of ventricular fibrillation in response to acute posterolateral ischemia. Results were compared with a comparable control group of vehicle-treated, noninducible animals. Nonsustained ventricular tachyarrhythmia developed in 2 of 15 pinacidil-treated animals as compared to the initiation of ventricular tachycardia in 1 of 16 postinfarcted hearts (p = 0.96) in the control group. Thus, pinacidil did not alter the responsiveness of the postinfarcted heart with respect to the electrical induction of tachyarrhythmias. The subsequent development of an acute ischemic event at a site remote from the previous myocardial infarction was associated with a greater incidence of ventricular fibrillation within 1 h from the onset of ischemia in the pinacidil-treated animals (9/15; 60%) as compared to the control group (1/15; 6.7%; p = 0.007). The 24-h cumulative mortality, likewise, was greater in the pinacidil-treated group [13/15 (87%)] as compared to the vehicle-treated control group 3/15; 20%; p = 0.001. Significant cardiovascular and electrophysiologic effects of pinacidil included an increase in heart rate (124 +/- 6-143 +/- 10 beats/min, p less than 0.05) and reductions in the refractory periods of normal (178 +/- 2-166 +/- 4 ms, p less than 0.05) and peri-infarcted (170 +/- 5-185 +/- 5 ms, p less than 0.01) myocardial regions. It is concluded that pinacidil does not alter the responsiveness of the postinfarcted heart to programmed electrical stimulation. However, in the presence of a superimposed acute ischemic event, pinacidil increases the potential for the development of ventricular fibrillation in a subset of postinfarcted animals that otherwise show a low risk with respect to the development of lethal arrhythmias. It is hypothesized that the increased tendency to develop ventricular fibrillation is associated with the pinacidil-induced reduction in the ventricular refractory period. This conclusion is consistent with the known ability of pinacidil to enhance potassium efflux during myocardial repolarization and to decrease the duration of the action potential.

Functional consequence of big endothelin conversion: demonstration with isolated neutrophils but not in a postinfarction model.

The role of neutrophils in the conversion of big endothelin (Big-ET) to endothelin-1 (ET-1) was assessed in two models. In rabbit isolated hearts, ET-1 demonstrated significant increases in coronary perfusion pressure (PP) at doses greater than 100 pmol. With Big-ET, significant increases were obtained only at doses greater than 3 nmol. Similarly, this dose of Big-ET did not increase PP in isolated, perfused hearts from rabbits previously subjected to 60 min of coronary artery occlusion and 4 h of reperfusion (n = 7). However, when isolated rabbit neutrophils were incubated with Big-ET (n = 5) or vehicle (n = 4) and 3 nmol aliquots assayed in isolated hearts, results demonstrated a time-dependent conversion to a vasoactive product. These findings suggest that isolated neutrophils are capable of converting Big-ET to an ET-1-like vasoactive substance. The model may be useful in assessing antagonists of endothelin-converting enzyme activity and/or the endothelin receptor.

The QUinapril Ischemic Event Trial (QUIET) design and methods: evaluation of chronic ACE inhibitor therapy after coronary artery intervention.

The rationale, trial design, and statistical aspects of QUIET, the QUinapril Ischemic Event Trial, are described. QUIET is a prospective, double-blind placebo-controlled study that will assess the ability of the angiotensin-converting enzyme (ACE) inhibitor quinapril to reduce the rate of cardiac ischemic events and to slow or prevent the development of coronary artery atherosclerosis as assessed by serial angiography in a normolipidemic population without left ventricular dysfunction. The study began in September 1991 and has completed recruitment with 1740 patients across 38 centers (28 U.S., 4 Canada, 6 Europe) by the end of 1992. Patients are randomized to 20 mg of quinapril or placebo once daily and continue in the study for 3 years. Study completion is projected for 1995.

The therapeutic potential of novel anticoagulants.

Conventional anticoagulant therapy has been based on indirect inhibition of coagulation factors with heparin and warfarin. These agents display liabilities prompting the development of new anticoagulants over the last two decades. The first to be developed was a series of low molecular weight heparins(LMWHs). Their favourable pharmacokinetic profiles and risk/benefit ratios led to widespread use in Europe and, more recently, approval for their use in the USA. Paralleling the development of LMWHs has been the pursuit of a different strategy focused on direct rather than indirect inhibition of enzymes in the coagulation cascade. In contrast to heparin, LMWHs, or other glycosaminoglycans, direct inhibitors exert their effects independent of either antithrombin III (ATIII) or heparin cofactor II (HCII) and more effectively inhibit clot-bound thrombin or FXa. Highly potent, selective (versus other serine proteases)direct thrombin and FXa inhibitors have been identified and isolated from natural sources, such as leeches, ticks and hookworms. The recombinant forms and analogues of the senatural proteins have been produced using molecular biology techniques, i.e., rHirudin, Hirulogs, recombinant tick anticoagulant peptide (rTAP), recombinant antistasin (rATS) and recombinant nematode anticoagulant peptide-5 (rNAP-5). The design of novel structures or the modification of existing chemicals has led to the synthesis of many non-peptide, low molecular weight inhibitors of thrombin and FXa. Some of them are orally active and may be suitable for long-term clinical use. In addition, considerable progress has been made in developing specific TF/VIIa complex inhibitors. The anticoagulation properties of the new agents are being characterised in experimental studies. Some of them have been advanced to large scale clinical trials and their effectiveness, and sometimes relative ineffectiveness,in arterial and venous thromboembolic disorders has been demonstrated. They are being tested for their potential as new antithrombotic agents that act via direct enzyme inhibition. Thus,the clinician should in future be able to target different thrombotic conditions with proven, specific anticoagulant interventions.

Effects of combined thromboxane synthetase inhibition/thromboxane receptor antagonism in two models of sudden cardiac death in the canine: limited role for thromboxane.

The effects of combined thromboxane synthetase inhibition and thromboxane receptor antagonism (TSI/TRA) were studied in conscious and in anesthetized canine models of sudden cardiac death. Administration of the TSI/TRA, R68070 10 mg/kg intravenously (i.v.), decreased thrombin-stimulated thromboxane synthesis and significantly antagonized platelet aggregation in response to the thromboxane-mimetic U46,619. In the conscious canine model, R68070 did not change ventricular refractoriness, did not prevent induction of ventricular arrhythmias by programmed electrical stimulation, and failed to prevent development of spontaneous ventricular fibrillation (VF) in response to ischemia produced at a site remote from the area of previous myocardial infarction (R68070 mortality = 70%, vehicle = 100%, p = NS). In the anesthetized canine model, R68070 prevented development of ischemia in 7 of 11 animals and reduced mortality significantly (R68070 27% and vehicle 73%; p = 0.038). R68070 inhibited thrombus formation in both models (R68070 conscious 7.0 +/- 2.6 mg and vehicle conscious 15 +/- 7.6 mg, p = NS; R68070 anesthetized 5.9 +/- 1.9 mg and vehicle anesthetized 17.7 +/- 4.3 mg; p less than 0.05). The results suggest that inhibition of thromboxane-dependent activity during acute recovery from infarction was able to protect the myocardium from developing ischemia in response to current-mediated intimal damage in a noninfarct-related artery. In the subacute phase of recovery from infarction, when the underlying myocardial substrate is susceptible to electrical derangement induced by transient ischemia, thromboxane inhibition in itself was unable to prevent ischemia-induced sudden cardiac death. Although R68070 may delay onset of ischemia due to thrombotic occlusion of the coronary artery, there does not appear to be an antiarrhythmic/antifibrillatory action to be derived from interfering with the synthesis or receptor-mediated action of thromboxane. Furthermore, R68070 does not alter the electrophysiologic properties of the heart which would result in an antiarrhythmic or antifibrillatory action.