Leif Carlsson

Relationships between preclinical cardiac electrophysiology, clinical QT interval prolongation and torsade de pointes for a broad range of drugs: evidence for a provisional safety margin in drug development

OBJECTIVE To attempt to determine the relative value of preclinical cardiac electrophysiology data (in vitro and in vivo) for predicting risk of torsade de pointes (TdP) in clinical use. METHODS Published data on hERG (or I(Kr)) activity, cardiac action potential duration (at 90% repolarisation; APD(90)), and QT prolongation in dogs were compared against QT effects and reports of TdP in humans for 100 drugs. These data were set against the free plasma concentrations attained during clinical use (effective therapeutic plasma concentrations; ETPC(unbound)). The drugs were divided into five categories: (1) Class Ia and III antiarrhythmics; (2) Withdrawn from market due to TdP; (3) Measurable incidence/numerous reports of TdP in humans; (4) Isolated reports of TdP in humans; (5) No reports of TdP in humans. RESULTS Data from hERG (or I(Kr)) assays in addition to ETPC(unbound) data were available for 52 drugs. For Category 1 drugs, data for hERG/I(Kr) IC(50), APD(90), QTc in animals and QTc in humans were generally close to or superimposed on the ETPC(unbound) values. This relationship was uncoupled in the other categories, with more complex relationships between the data. In Category 1 (except amiodarone), the ratios between hERG/I(Kr) IC(50) and ETPC(unbound) (max) ranged from 0.1- to 31-fold. Similar ranges were obtained for drugs in Category 2 (0.31- to 13-fold) and Category 3 (0.03- to 35-fold). A large spread was found for Category 4 drugs (0.13- to 35700-fold); this category embraced an assortment of mechanisms ranging from drugs which may well be affecting I(Kr) currents in clinical use (e.g. sparfloxacin) to others such as nifedipine (35700-fold) where channel block is not involved. Finally, for the majority of Category 5 drugs there was a >30-fold separation between hERG/I(Kr) activity and ETPC(unbound) values, with the notable exception of verapamil (1.7-fold), which is free from QT prolongation in man; this is probably explained by its multiple interactions with cardiac ion channels. CONCLUSIONS The dataset confirms the widely-held belief that most drugs associated with TdP in humans are also associated with hERG K(+) channel block at concentrations close to or superimposed upon the free plasma concentrations found in clinical use. A 30-fold margin between C(max) and hERG IC(50) may suffice for drugs currently undergoing clinical evaluation, but for future drug discovery programmes, pharmaceutical companies should consider increasing this margin, particularly for drugs aimed at non-debilitating diseases. However, interactions with multiple cardiac ion channels can either mitigate or exacerbate the prolongation of APD and QT that would ensue from block of I(Kr) currents alone, and delay of repolarisation per se is not necessarily torsadogenic. Clearly, an integrated assessment of in vitro and in vivo data is required in order to predict the torsadogenic risk of a new candidate drug in humans.

Instability and Triangulation of the Action Potential Predict Serious Proarrhythmia, but Action Potential Duration Prolongation Is Antiarrhythmic

BackgroundProlongation of action potential duration (APD) is considered a major antiarrhythmic mechanism (class III), but paradoxically, it frequently is also proarrhythmic (torsade de pointes). Methods and ResultsThe cardiac electrophysiological effects of 702 chemicals (class III or HERG channel block) were studied in 1071 rabbit Langendorff-perfused hearts. Temporal instability of APD, triangulation (duration of phase 3 repolarization), reverse use-dependence, and induction of ectopic beats were measured. Instability, triangulation, and reverse use-dependence were found to be important determinants of proarrhythmia. Agents that lengthened the APD by >50 ms, with induction of instability, triangulation, and reverse use-dependence (n=59), induced proarrhythmia (primarily polymorphic ventricular tachycardia); in their absence (n=19), the same prolongation of APD induced no proarrhythmia but significant antiarrhythmia (P <0.001). Shortening of APD, when accompanied by instability and triangulation, was also markedly proarrhythmic (primarily monomorphic ventricular tachycardia). In experiments in which instability and triangulation were present, proarrhythmia declined with prolongation of APD, but this effect was not large enough to become antiarrhythmic. Only with agents without instability did prolongation of APD become antiarrhythmic. For 20 selected compounds, it was shown that instability of APD and triangulation observed in vitro were strong predictors of in vivo proarrhythmia (torsade de pointes). ConclusionsLengthening of APD without instability or triangulation is not proarrhythmic but rather antiarrhythmic.

QTU-prolongation and torsades de pointes induced by putative class III antiarrhythmic agents in the rabbit: etiology and interventions.

When low doses of clofilium were administered to conscious rabbits, ventricular tachyarrhythmia (VT) with features typical of torsades de pointes developed. This arrhythmia was further studied and characterized in cholralose-anesthetized rabbits. In 20 of 20 rabbits, VT developed after a mean cumulative dose of 0.53 ± 0.04 μmol/kg clofilium, provided an infusion of the α1-agonist methoxamine (15 (μg/kg/min) was given concomitantly. The arrhythmia was preceded by a marked prolongation of the QTU interval and the monophasic action potential duration, as well as signs of early afterdepolarizations (EADs). In seven of 10 rabbits receiving only clofilium (cumulative dose, 20.8 μmol/kg), no VT occurred (p < 0.001 compared to the incidence in animals given both methoxamine and clofilium). In animals given both methoxamine and clofilium, pretreatment with prazosin (1 mg/kg i.v., n = 4) attenuated the arrhythmia, whereas diltiazem (0.5 mg/kg i.v., n = 4) or propranolol (0.5 mg/kg i.v., n = 8) was ineffective. Acute intervention with prazosin, isoproterenol, pinacidil, or magnesium sulfate promptly regularized the rhythm in animals with VT. Prazosin and pinacidil were equally effective in β-blocked rabbits. When other agents known to retard the repolarization currents (sematilide, UK-68,798, LY97119, amperozide, and cesium chloride) were examined, a strong correlation (r = 0.99, p < 0.001) between the potency of the drug to prolong the QTU interval and the proarrhythmic potential was obtained. This experimental model may represent an appropriate alternative for studying the acquired (“pause-dependent”) long QT syndrome.

Atrial-selective effects of chronic amiodarone in the management of atrial fibrillation

BACKGROUND Although amiodarone is one of the most effective pharmacologic agents used in clinical management of atrial fibrillation (AF), little is known about its differential effects in atrial and ventricular myocardium. OBJECTIVES This study sought to compare the electrophysiological effects of chronic amiodarone in atria and ventricles. METHODS We compared the electrophysiological characteristics of coronary-perfused atrial and ventricular wedge preparations isolated from untreated and chronic amiodarone-treated dogs (amiodarone, 40 mg/kg/day for 6 weeks, n = 12). RESULTS Chronic amiodarone prolonged action potential duration (APD(90)) predominantly in atria compared to ventricles and prolonged the effective refractory period (ERP) more than APD(90) in both ventricular and atrial preparations (particularly in the latter) due to the development of postrepolarization refractoriness. Amiodarone reduced dispersion of APD(90) in both atria and ventricles. Although the maximum rate of increase of the action potential upstroke (V(max)) was significantly lower in both atria and ventricles of amiodarone-treated hearts versus untreated controls, the reduction of V(max) was much more pronounced in atria. Amiodarone prolonged P-wave duration more significantly than QRS duration, reflecting greater slowing of conduction in atria versus ventricles. These atrioventricular distinctions were significantly accentuated at faster activation rates. Persistent acetylcholine-mediated AF could be induced in only 1 of 6 atria from amiodarone-treated versus 10 of 10 untreated dogs. CONCLUSION Our results indicate that under the conditions studied, chronic amiodarone has potent atrial-predominant effects to depress sodium channel-mediated parameters and that this action of the drug is greatly potentiated by its ability to prolong APD predominantly in the atria, thus contributing to its effectiveness to suppress AF.

Blocking Characteristics of hERG, hNav1.5, and hKvLQT1/hminK after Administration of the Novel Anti-Arrhythmic Compound AZD7009

Introduction: AZD7009 is a novel anti‐arrhythmic compound under development for short‐ and long‐term management of atrial fibrillation and flutter. Electrophysiological studies in animals have shown high anti‐arrhythmic efficacy, predominant action on atrial electrophysiology, and low proarrhythmic activity. The main aim of this study was to characterize the blocking effects of AZD7009 on the human ether‐a‐go‐go‐related gene (hERG), the hNav1.5, and the hKvLQT1/hminK currents.

Characterization of the in vivo and in vitro electrophysiological effects of the novel antiarrhythmic agent AZD7009 in atrial and ventricular tissue of the dog.

This study evaluated the effects of the novel antiarrhythmic agent AZD7009 on atrial and ventricular repolarization and on the Na+-current system, using Vmax as an index. Anesthetized dogs were infused with AZD7009 or azimilide to produce three pseudo steady-state plasma concentrations in vivo. Microelectrode techniques were used to record action potentials and effective refractory period (ERP) in vitro. Whereas AZD7009 concentration-dependently increased atrial ERP (AERP, by 48 ± 7 milliseconds maximum, P < 0.001 versus vehicle), the increases in ventricular ERP (VERP, 8 ± 4 milliseconds) and QT interval (2 ± 5.5 milliseconds) were small and not concentration-dependent. For azimilide, the AERP increase was less, whereas VERP and QT increases were substantially larger than with AZD7009. In vitro, AZD7009 concentration-dependently reduced Vmax and increased action potential duration (APD). ERP was increased through APD lengthening and post-repolarization refractoriness. The suppression of Vmax, but not APD prolongation, showed frequency-dependence. APD and ERP increases were more pronounced in atrial than ventricular tissue: in atria, 2 μM AZD7009 increased APD90 and ERP from 224 ± 7 to 318 ± 7 milliseconds and 241 ± 7 milliseconds to 378 ± 17 milliseconds; versus 257 ± 5 to 283 ± 7 milliseconds and 253 ± 12 to 300 ± 11 milliseconds respectively in ventricles. Thus, AZD7009 potently and predominantly increases atrial refractoriness in the dog, with actions mediated by combined effects on repolarization and the Na+-current system.

Assessment of the proarrhythmic potential of the novel antiarrhythmic agent AZD7009 and dofetilide in experimental models of torsades de pointes.

Background: This study examined the proarrhythmic potential of the novel antiarrhythmic agent AZD7009 and dofetilide.

Local release of myocardial norepinephrine during acute ischemia: an experimental study in the isolated perfused rat heart.

The appearance of an early ischemia-induced local release of myocardial norepinephrine (NE) was examined in the isolated Langendorff-perfused rat heart prelabeled with [3H]NE. Either glucose or lactate was used as the perfusion substrate. Ischemia for a 60-min period was produced by global flow reduction (by 90%) or left coronary artery ligation followed by a 15-min reperfusion period. During the first 20 min of ischemia both the concentration of [3H]NE and the fraction of total 3H representing nonmetabolized [3H]NE were increased in the coronary venous effluent. This early increase in [3H]NE concentration was most pronounced in hearts with global ischemia perfused with lactate as substrate (from 19 ± 2 to 524 ± 76 fmol/ml/g after 20 min of ischemia). The quantity of [3H]NE released was then further increased during the 60-min period of ischemia. Reperfusion of the ischemic myocardium was associated with a marked outflow of [3H]NE and [3H]NE metabolites, primarily representing a washout from the ischemic tissue. Under the present experimental conditions the ischemia-induced release of NE was attenuated by glucose, probably owing to an ongoing glycolytic ATP formation. This effect was most pronounced during global low-flow ischemia. It is concluded that ischemia is associated with a local release of myocardial NE. In the nonworking Langendorff heart preparation, clear evidence of such a local release was already obtained after 10–20 min of ischemia.

Beat-by-beat QT interval variability, but not QT prolongation per se, predicts drug-induced torsades de pointes in the anaesthetised methoxamine-sensitized rabbit

INTRODUCTION Accumulating evidence suggest that drug-induced QT prolongation per se poorly predicts repolarisation-related proarrhythmia liability. We examined whether beat-by-beat variability of the QT interval may be a complementary proarrhythmia marker to QT prolongation. METHODS Anaesthetised rabbits sensitized towards developing torsades de pointes (TdP) were infused for 30 min maximum with explorative antiarrhythmic compounds characterised as mixed ion channel blockers. Based on the outcome in this model the compounds were classified as having a low (TdPlow; n=5), intermediate (TdPintermediate; n=7) or high (TdPhigh; n=10) proarrhythmic potential. Dofetilide (n=4) was included as a representative of a selective IKr-blocking antiarrhythmic with known high proarrhythmic potential. QT interval prolongation and beat-by-beat QT variability (quantified as the short-term variability, STV) were continuously assessed during the infusion or up to the point where ventricular proarrhythmias were induced. RESULTS All compounds significantly prolonged the QT interval. For TdPlow and TdPhigh compounds the QT interval maximally increased from 169 ± 14 to 225 ± 28 ms (p<0.05) and from 186 ± 21 to 268 ± 42 ms (p<0.01), respectively. Likewise, in the dofetilide-infused rabbits the QT interval maximally increased from 177 ± 11 to 243 ± 25 ms (p<0.01). In contrast, whereas the STV in rabbits administered the TdPhigh compounds or dofetilide significantly increased prior to proarrhythmia induction (from 1.6 ± 0.4 to 10.5 ± 5.6 ms and from 1.6 ± 0.5 to 5.9 ± 1.8 ms, p<0.01) it remained unaltered in the TdPlow group (1.3 ± 0.6 to 2.2 ± 0.9 ms). In the TdPintermediate group, rabbits experiencing TdP had a similar maximal QT prolongation as the non-susceptible rabbits whereas the change in the STV was significantly different (from 0.9 ± 0.5 to 8.7 ± 7.3 ms vs 0.8 ± 0.3 to 2.5 ± 1.1 ms). DISCUSSION It is concluded from the present series of experiments in a sensitive rabbit model of TdP that increased beat-by-beat QT interval variability precedes drug-induced TdP. In addition, assessment of this potential proarrhythmia marker may be useful in discriminating highly proarrhythmic compounds from compounds with a low proarrhythmic potential.

Assessment of the Ion Channel-blocking Profile of the Novel Combined Ion Channel Blocker Azd1305 and Its Proarrhythmic Potential Versus Dofetilide in the Methoxamine-sensitized Rabbit In Vivo

AZD1305 is a novel antiarrhythmic agent under clinical evaluation for management of atrial fibrillation. This study assessed its ion channel-blocking potency by the whole cell patch-clamp technique in vitro and its proarrhythmic liability in anesthetized methoxamine-sensitized rabbits in comparison with dofetilide. AZD1305 predominantly blocked the hERG, the L-type calcium and the hNav1.5 currents in a concentration-dependent manner. In vivo AZD1305 increased the QT interval (from 145 ± 8 to 196 ± 18 ms, P < 0.01) without inducing ventricular extrasystoles or torsades de pointes (TdP). In contrast, dofetilide prolonged the QT interval from 161 ± 3 to 256 ± 15 ms (P < 0.001) and caused TdP in 12/17 rabbits (P < 0.01 vs. AZD1305). During AZD1305 and dofetilide infusion, the QTend-peak interval maximally increased by 14 ± 4 and 30 ± 6 ms (P < 0.05 vs. AZD1305) and the beat-by-beat QT interval variability (quantified as the short-term variability, STV) changed from 2 ± 0.8 to 2 ± 0.3 ms (NS) and from 2 ± 0.2 to 12 ± 1.1 ms (P < 0.001), respectively. Following dofetilide-induced TdP, 6 rabbits each were injected with saline or AZD1305. In contrast to saline, AZD1305 abbreviated the QT interval (from 275 ± 25 to 216 ± 9 ms, P < 0.05), reduced the STV to 1 ± 0.1 ms (P < 0.001) and suppressed TdP in all 6 rabbits (P < 0.01 vs. saline). In conclusion, AZD1305 can be characterised as a combined ion channel blocker that delays repolarization without increasing beat-by-beat variability of repolarization (BVR) or inducing TdP whereas selective IKr blockade by dofetilide prolongs the QT interval and eventually increases BVR resulting in TdP.