Antifibrillatory efficacy of long-term tedisamil administration in a postinfarcted canine model of ischemic ventricular fibrillation

HBI-3000 prevents secondary sudden cardiac death

BACKGROUND

In postmyocardial infarction patients, transient episodes of ischemia are associated with a greater incidence of sudden cardiac death (SCD). Ventricular tachycardia and ventricular fibrillation (VF) are responsible for the majority of SCDs, but current pharmacological interventions for prevention of lethal ventricular arrhythmias are less than satisfactory. We investigated the efficacy of HBI-3000 (HBI), a novel antiarrhythmic agent, in preventing SCD in a conscious canine model.

METHODS

After 3 to 7 days of a surgically induced myocardial infarction (ie, 90-minute occlusion of the left anterior descending coronary artery followed by 30 minutes of reperfusion), conscious animals were administered vehicle (0.9% NaCl solution for injection) or HBI (15 mg/kg) intravenously. An occlusive thrombus at a site remote from the previous myocardial infarction was induced by electrolytic injury to the intimal surface of the left circumflex coronary artery.

RESULTS

Control animals developed premature ventricular complexes (PVCs) followed by ventricular tachycardia, which terminated in VF in 5 of the 8 dogs. HBI reduced the frequency of PVCs, and only 1 of the 9 HBI-treated animals developed VF (P < .05). In a separate group of postinfarcted animals, the electrical conversion threshold was assessed before and after the intravenous administration of HBI (5, 10, or 15 mg/kg) or flecainide (3 mg/kg), a class IC antiarrhythmic agent. The electrical conversion threshold was not altered by HBI, whereas the administration of flecainide increased the threshold (P < .01 vs baseline).

CONCLUSIONS

The data indicate that HBI is an effective antiarrhythmic and antifibrillatory agent for the prevention of VF or sudden cardiac death.

New model of ventricular fibrillation

The purpose of this study was to develop a more efficient and stable model of ventricular fibrillation (VF) in the isolated rabbit heart, because there is not a satisfactory model with this animal. We also observed the effects of increasing extracellular calcium in the stability and reversibility of the arrhythmia. After suspending the hearts in a classical Langendorff preparation, VF was induced by burst stimulation (current = 2.0 mA, pulse duration = 3 milliseconds, frequency = 50 Hz, voltage = 10 V, duration of stimulation = 5 minutes). The hearts were then divided into 2 groups, A and B. The hearts in group B were perfused with a modified Krebs-Henseleit solution, which contained twice as much calcium as the solution used in the other group. The rate of success with this model was 100% for both groups. The hearts fibrillated up to 30 minutes in group A and more than 40 minutes in group B, longer then all studies ever published in rabbit hearts. Ventricular fibrillation reverted to sinus rhythm in 100% of the hearts of group A when treated with an antifibrillatory drug, whereas no reversion at all was observed in the hearts of group B. We conclude that high extracellular calcium makes the reversion to sinus rhythm more difficult in this model. Our high rate of success and the exceptionally stable and long-lasting VF turn our model very effective for the study of antiarrhythmic interventions in the isolated rabbit heart.

Tedisamil: a new novel antiarrhythmic

Solvay Pharmaceuticals is currently developing tedisamil (KC-8857), a novel antiarrhythmic with additional anti-ischaemic properties, which acts via potassium channel blockade. This drug can be categorised as a class III antiarrhythmic agent due to its effects of action potential and QT interval prolongation in these patients. This agent was initially developed for its anti-ischaemic properties and Phase I trials have shown tedisamil to be an effective bradycardic agent, as well as causing a reverse rate-dependent QT interval prolongation. Subsequent Phase II results have confirmed that in patients with ischaemic heart disease, tedisamil had beneficial haemodynamic and anti-ischaemic effects. Phase III studies in patients with ischaemic heart disease indicated that tedisamil is an effective agent for the treatment of angina, resulting in a dose-dependent increase in anginal threshold (with a decrease in anginal attacks, increased exercise capacity during treadmill exercise and decreased electrocardiographic signs of exercise induced ischaemia) in comparison to placebo. Although tedisamil has been shown to be an effective anti-ischaemic agent, with Phase III trials for angina pectoris now completed, the company are now pursuing the use of tedisamil for the treatment of atrial fibrillation, for which tedisamil is still in Phase II/III clinical trials. Launch data are not yet known.

Antiarrhythmic drugs in atrial fibrillation: an overview of new agents, their mechanisms of action and potential clinical utility

Despite recent advances in our understanding of the mechanism of atrial fibrillation (AF), effective treatment remains difficult in many patients. Pharmacotherapy remains the mainstay of treatment and includes control of ventricular rate as well as restoration and maintenance of sinus rhythm. The currently available antiarrhythmic drugs are particularly effective in converting paroxysmal AF to sinus rhythm and in enhancing the positive effect of electrical cardioversion, but are limited in their efficacy in maintaining sinus rhythm. Moreover, there are limited options in the setting of co-existing ischaemic heart disease, left ventricular dysfunction and structural heart diseases. New drugs added to our clinical armamentarium have been, or are being, developed to combine better efficacy and lack of pro-arrhythmic effects. These developments have gained more interest particularly with the recent debate over rate control versus rhythm control for AF. Although some of these agents are promising, their uptake in clinical practice will not only depend on their efficacy as antiarrhythmic agents but also on their safety in acutely terminating AF and in long-term maintenance of sinus rhythm or rate control in the community.

Risk of Ventricular Proarrhythmia with Selective Opening of the Myocardial Sarcolemmal versus Mitochondrial ATP-Gated Potassium Channel

Myocardial ATP-gated potassium channels (K-ATPs) are critical in the intracellular signaling cascade resulting in ischemic preconditioning (IP). Mitochondrial K-ATP channels seem to be responsible for IP, whereas the functions of K-ATP channels in the sarcolemmal membrane are less well understood. The proarrhythmic potential of specific versus nonspecific opening of K-ATP channels has not been investigated. In this study, Langendorff-perfused rabbit hearts were exposed to either pinacidil (1.25 microM), a nonselective K-ATP channel agonist, or selective mitochondrial or sarcolemmal K-ATP channel agonists or antagonists. The hearts were then subjected to 12 min of hypoxic perfusion and 40 min of reoxygenation. Hearts were monitored for the induction of ventricular fibrillation (VF). No heart subjected to hypoxia-reoxygenation without drug treatment developed VF (0 of 5). Pinacidil pretreatment induced VF (12 of 14; p = 0.004 versus control). Pinacidil's effect was blocked by HMR-1098 (1-[5-[2-(5-chloro-o-anisamide)ethyl]-2-methoxyphenyl]sulfonyl]-3-methylthiourea) (1 microM), a selective sarcolemmal K-ATP channel antagonist (1 of 7; p = 0.007 versus pinacidil; N.S. versus control). Hearts pretreated with 5-hydroxydecanoate (5-HD) (100 microM), a putatively selective mitochondrial K-ATP channel blocker developed VF in one of eight trials (N.S. versus control). 5-HD did not alter the effects of pinacidil (6 of 8; p < 0.05 versus control; N.S. versus pinacidil alone). Selective mitochondrial K-ATP channel activation with [(3R)-trans-4-((4-chlorophenyl)-N-(1H-imidazol-2-ylmethyl)dimethyl-2H-1-benzopyran-6-carbonitril monohydrochloride] (BMS-191095) (6 microM) resulted in zero of five hearts developing VF (N.S. versus control). Our data suggest that selective opening of the sarcolemmal K-ATP channel during hypoxia-reoxygenation induced VF, whereas opening of the mitochondrial channel was not associated with VF. The findings suggest that caution should be exercised when developing compounds aimed at inducing IP, and nonspecific opening of the K-ATP channel should be avoided.

Antiarrhythmic drug therapy of atrial fibrillation: focus on new agents

The precise mechanisms of clinical effect of antiarrhythmic agents and the ideal "molecular targets" against arrhythmias, in particular atrial fibrillation, are poorly understood. Current antiarrhythmic drug development, particularly for drugs expected to be active against atrial fibrillation, has focused on drugs with multiple ionic mechanisms of action, in particular on those that block multiple potassium channels. Investigation of antiarrhythmic agents is complicated by the diversity of animal-disease models studied, by the potential multiple mechanisms of arrhythmias, and by the incompletely understood relationships between risks and benefits of antiarrhythmic drug therapy. Furthermore, rhythm control strategies in large groups of patients with atrial fibrillation have failed to show substantial clinical benefit. Nevertheless, drugs that block multiple potassium channels and appear to have relatively little organ toxicity, such as tedisamil, may represent an important new avenue in the therapeutic approach to highly symptomatic arrhythmias such as atrial fibrillation.

Effects of tedisamil on cardiovascular tissues isolated from normo- and hypertensive rats

BACKGROUND

This study was undertaken to characterize the effects of tedisamil on isolated rat cardiovascular tissues, and identify actions that could be beneficial or detrimental in the treatment of cardiac disease.

RESULTS

Tedisamil prolonged the Wistar Kyoto normotensive rat (WKY) left ventricular action potential and augmented the force of contraction of left ventricle strips. On the 12-month-old SHR model of cardiac hypertrophy, the augmenting effects of tedisamil at 10(-6) and 3 x 10(-6) M were reduced. On the 21-month-old SHR model of heart failure, the augmenting effects of tedisamil at 10(-6) and 3 x 10(-6) M were further reduced. The augmenting effect of tedisamil at 10(-5) M was reduced to 47%. The rate of the right atrium of 16- to 17-month-old WKY was reduced by tedisamil at 10(-5) and 10(-4) M, and tedisamil had a similar effect on the SHR right atrium. Tedisamil at 10(-6)--3 x 10(-5) M contracted the portal veins of WKY and aortae of 12-month-old WKY and SHR.

CONCLUSIONS

The positive inotropic and negative chronotropic effects of tedisamil in the rat, which are partially or fully maintained in hypertrophied or failing myocardium would be beneficial in the treatment of heart failure. In contrast, the vasoconstrictor action of tedisamil will be detrimental in heart failure.

Tedisamil and dofetilide-induced torsades de pointes, rate and potassium dependence

1. Tedisamil is a bradycardiac agent that prolongs the QT interval of the ECG and prevents cardiac arrhythmias. Given this profile, tedisamil might be expected to have proarrhythmic actions similar to Class III antiarrhythmic drugs. To address this question, the actions of dofetilide and tedisamil were examined in rabbit isolated hearts in which bradycardia was induced by AV ablation. 2. The QT interval was prolonged in a reverse rate-dependent fashion by dofetilide (3 and 30 nM) and tedisamil (0.3 and 3 microM). 3. Torsades de pointes was observed in 1/7 hearts treated with 3 nM dofetilide and 0/7 hearts treated with 0.3 microM tedisamil. The incidence of torsades de pointes was increased to 5/7 in hearts treated with 30 nM dofetilide and to 7/7 in hearts treated with 3 microM tedisamil (both P < 0.05 vs control). 4. The actions of 30 nM dofetilide and 3 microM tedisamil were also examined in hearts paced at 50, 100, 200 and 50 beats min(-1) successively. Both drugs caused torsades de pointes in 5/5 hearts paced at 50 beats min(-1); however, the incidence was reduced to 0/5 during pacing at 200 beats min(-1). Thus, drug-induced proarrhythmia was bradycardia-dependent. 5. Drug-induced prolongation of the interval between the peak and end of the T-wave (QTa-e) was reverse rate-dependent and was associated with the occurrence of torsades de pointes (r = 0.91, P < 0.01). 6. The results suggest that tedisamil, like dofetilide, presents a risk for development of torsades de pointes.

Tedisamil: master switch of nature?

Decreasing heart rate is potentially useful in ischaemic heart disease. Tedisamil is a bradycardic agent resulting from its ability to inhibit transient outward current (I(to)) in atria. Tedisamil inhibits I(to), potassium current (IK), K(ATP) and the protein kinase A-activated chloride channel in ventricles as well as vascular IK and Ca(2+)-activated IK (IK((Ca))). Tedisamil prolongs cardiac action potentials and the corrected QT (QTc) of the ECG and also increases cardiac refractoriness. Tedisamil is anti-arrhythmic in animal models of ventricular arrhythmias and atrial flutter. The bradycardic effect of tedisamil is associated with a reduction in myocardial oxygen demand. On isolated rat ventricle, tedisamil is a positive inotrope and on isolated rabbit atria, tedisamil reverses the negative inotropic effect of pinacidil. Tedisamil contracts the isolated rat portal vein and aorta, reduces cromakalim-induced relaxations of contracted rat aorta and increases blood pressure in animals and humans. Tedisamil is 96% bound to plasma proteins, has a plasma half-life of about 10 h and is cleared from the kidney unchanged. Clinical trials have shown that the electrophysiology of tedisamil is that of a class III anti-arrhythmic. In coronary artery disease, tedisamil has no effect on inotropism and increases the threshold for angina. Potassium channel blockade with tedisamil may have advantages over calcium channel blockers or K(ATP) channel openers as an anti-ischaemic mechanism in coronary artery disease. In exercise-induced myocardial ischaemia, beta-blockers are probably favourable to tedisamil, as they will limit the increase in heart rate, contractility and blood pressure caused by sympathetic stimulation, whereas tedisamil will not. In heart failure patients, tedisamil reduces heart rate, but increases blood pressure. The usefulness of tedisamil as a bradycardic agent is limited by the increase in blood pressure. A drug that is bradycardic without increasing blood pressure would be an improvement on tedisamil as the master switch of nature for ischaemic heart disease.

RSD1019 suppresses ischaemia-induced monophasic action potential shortening and arrhythmias in anaesthetized rabbits

The electrophysiological actions of lidocaine, tedisamil and RSD1019 were assessed on normal and ischaemic cardiac tissue using monophasic action potentials (MAPs) recorded from the epicardium of anaesthetized rabbits. Drug effects on ischaemia-induced arrhythmias were assessed simultaneously in the same rabbits. Lidocaine, infused at 2.5, 5 and 10 micromol kg(-1) min(-1) i.v., accelerated and worsened the electrophysiological derangement caused by ischaemia, had profibrillatory actions and reduced the time to the occurrence of ventricular fibrillation (VF) relative to controls. Tedisamil, infused at 0.063, 0.125 and 0.25 micromol kg(-1) min(-1) i.v., prolonged MAP duration at 90% repolarization (MAPD(90%)) before induction of ischaemia in a dose-related manner; however, this effect was not maintained 5 min after induction of ischaemia. Tedisamil had no significant antiarrhythmic actions over the dose-range tested. RSD1019, infused at 2, 4 and 8 micromol kg(-1) min(-1) i.v., produced a small increase in MAPD(90%) before induction of ischaemia and only at the highest dose tested. In contrast to tedisamil, RSD1019 suppressed ischaemia-induced MAP shortening assessed 5 min after induction of ischaemia. This effect was dose-related. RSD1019 completely prevented ischaemia-induced tachyarrhythmias at the mid and highest infusion levels tested. The results of this study illustrate a pathologically targeted approach for preventing ischaemia-induced arrhythmias. Suppression of ischaemia-induced MAP shortening, demonstrated herein for RSD1019, represents a novel antifibrillatory approach.

New advances in class III antiarrhythmic drug therapy

In the past 2 years, significant advances have been made in class III antiarrhythmic drug therapy. In patients with ventricular arrhythmias and implantable cardioverter defibrillators (ICDs), antiarrhythmic agents are increasingly being used as adjunct therapy to decrease the frequency of ICD discharges. Sotalol was recently shown to be effective in reducing tachyarrhythmias in patients with ICDs. Intravenous amiodarone is being used for the acute treatment of unstable ventricular arrhythmia and is being investigated for the treatment of acute out-of-hospital cardiac arrest. Class III agents are increasingly being used for prophylaxis in patients who have atrial fibrillation or atrial flutter, and data point to an important role for these agents in reducing supraventricular tachyarrhythmias after cardiac surgery. Future studies will need to directly compare these agents with pure anti-adrenergic maneuvers in postoperative patients. In addition to terminating atrial fibrillation and atrial flutter, ibutilide significantly reduces human atrial defibrillation thresholds and increases the percentage of patients who can be cardioverted from atrial fibrillation to sinus rhythm. The US Food and Drug Administration is expected to approve dofetilide for clinical use soon, and it is currently reviewing azimilide (which seems to be devoid of frequency-dependent effects on repolarization) for prophylaxis against atrial fibrillation and atrial flutter. Dronedarone, tedisamal, and trecetilide are now under active study intended to determine their usefulness in patients with cardiac arrhythmias. Experimental studies are ongoing to identify pharmacologic agents that will selectively prolong repolarization in the atria without exerting electrophysiologic effects in the ventricles.

Are the antiarrhythmic-defibrillating effects of D-sotalol due to or despite the prolongation of the action potential duration?

These results support our hypothesis that class III compounds, with a positive inotropic effect, increase intercellular coupling and synchronization, mainly by preventing intracellular Ca overload. They act as defibrillating compound, similar to cAMP and adrenaline, most probably due to their so called sympathomimetic effect. In our opinion, their cardioprotective effects, resembling cardioversion, are not related to their ability to prolong APD and ERP. Moreover, we suggest that any compound that possesses these sympathomimetic effects, but without inducing the arrhythmogenic prolongation of APD, may exhibit a potent, safety and more efficient antiarrhythmic - defibrillating ability.

Tedisamil in a chronic canine model of atrial flutter

Tedisamil inhibits several cardiac potassium channels including Ito, Ikr, and the adenosine triphosphate (ATP)-sensitive potassium channel (I(KATP)), which may be important in the initiation and maintenance of atrial arrhythmias. We herein report the efficacy of tedisamil in terminating and protecting against the reinduction of atrial flutter (AFL) in a conscious canine model. Sustained AFL (> 15 min) was induced in eight of 10 mongrel dogs by programmed atrial stimulation (PAS) 2-41 days after producing a surgical barrier to conduction in the right atrium. At the time of surgery, an epicardial electrode was attached to the right atrial appendage for pacing and recording. Normal saline, 1 ml/kg, was infused after 15 min of AFL as placebo. Tedisamil (1.0 mg/kg) was given intravenously after 30 min of sustained AFL while recording surface ECGs and atrial electrograms. Conversion to sinus rhythm was achieved in 10 of 10 trials (eight dogs) in a mean time of 20.5 s (SD, +/- 11.8 s). Tedisamil had a negative chronotropic effect lasting > or =2 h and was protective against the reinduction of AFL. In five dogs, PAS was able to induce AFL in only two of seven trials 2 h after drug infusion. The corrected QT interval (QTc) was lengthened for the first 15 min after tedisamil administration (mean, +/- 39.3 ms; p < 0.05), but thereafter returned to baseline. The QRS interval was not altered by tedisamil. Saline alone, given after 15 min of sustained AFL, converted AFL in one of 11 trials (eight dogs) but did not alter the RR interval, QTc, or QRS interval compared with values measured during AFL. No significant adverse effects of tedisamil were observed. The results indicate that tedisamil is effective in interrupting and/or preventing reinduction of canine AFL, possibly by prolonging atrial refractoriness through inhibition of one or more potassium ion repolarizing currents in atrial muscle. Further studies are required to address the exact mechanism by which tedisamil exerts its antiarrhythmic effect.