Use of a Failing Rabbit Heart as a Model to Predict Torsadogenicity

Evaluation of cardiac arrhythmic risks using a rabbit model of left ventricular systolic dysfunction

Patients with heart disease have a higher risk to develop cardiac arrhythmias, either spontaneously or drug-induced. In this study, we have used a rabbit model of myocardial infarction (MI) with severe left ventricular systolic dysfunction (LVSD) to study potential drug-induced cardiac risks with N-(piperidin-2-ylmethyl)-2,5-bis(2,2,2-trifluoroethoxy)benzamide (flecainide). Upon ligation of the left circumflex arteries, male New Zealand White rabbits developed a large MI and moderate or severe LVSD 7 weeks after surgery, in comparison to SHAM-operated animals. Subsequently, animals were exposed to escalating doses of flecainide (0.25-4 mg/kg) or solvent. Electrocardiograms (ECG) were recorded before surgery, 1 and 7 weeks after surgery and continuously during the drug protocol. The ECG biomarker iCEB (index of Cardio-Electrophysiological Balance = QT/QRS ratio) was calculated. During the ECG recording at week 1 and week 7 post MI, rabbits had no spontaneous cardiac arrhythmias. When rabbits were exposed to escalating doses of flecainide, 2 out of 5 rabbits with MI and moderate LVSD versus 0 out of 5 solvent-treated rabbits developed arrhythmias, such as ventricular tachycardia/ventricular fibrillation. These were preceded by a marked decrease of iCEB just before the onset (from 4.09 to 2.42 and from 5.56 to 2.25, respectively). Furthermore, 1 out of 5 MI rabbits with moderate LVSD and 1 out of 7 MI rabbits with severe LVSD developed total atrioventricular block after flecainide infusion and died. This rabbit model of MI and severe LVSD may be useful for preclinical evaluation of drug (similar mechanism as flecainide)-induced arrhythmic risks, which might be predicted by iCEB.

Dehydroevodiamine and hortiamine, alkaloids from the traditional Chinese herbal drug Evodia rutaecarpa, are IKr blockers with proarrhythmic effects in vitro and in vivo

Evodiae fructus is a widely used herbal drug in traditional Chinese medicine. Evodia extract was found to inhibit hERG channels. The aim of the current study was to identify hERG inhibitors in Evodia extract and to investigate their potential proarrhythmic effects. Dehydroevodiamine (DHE) and hortiamine were identified as I_Kr (rapid delayed rectifier current) inhibitors in Evodia extract by HPLC-microfractionation and subsequent patch clamp studies on human embryonic kidney cells. DHE and hortiamine inhibited I_Kr with IC₅₀s of 253.2±26.3nM and 144.8±35.1nM, respectively. In dog ventricular cardiomyocytes, DHE dose-dependently prolonged the action potential duration (APD). Early afterdepolarizations (EADs) were seen in 14, 67, 100, and 67% of cells after 0.01, 0.1, 1 and 10μM DHE, respectively. The proarrhythmic potential of DHE was evaluated in 8 anesthetized rabbits and in 8 chronic atrioventricular block (cAVB) dogs. In rabbits, DHE increased the QT interval significantly by 12±10% (0.05mg/kg/5min) and 60±26% (0.5mg/kg/5min), and induced Torsade de Pointes arrhythmias (TdP, 0.5mg/kg/5min) in 2 rabbits. In cAVB dogs, 0.33mg/kg/5min DHE increased QT duration by 48±10% (P<0.05*) and induced TdP in 2/4 dogs. A higher dose did not induce TdP. In human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), methanolic extracts of Evodia, DHE and hortiamine dose-dependently prolonged APD. At 3μM DHE and hortiamine induced EADs. hERG inhibition at submicromolar concentrations, APD prolongation and EADs in hiPSC-CMs and dose-dependent proarrhythmic effects of DHE at micromolar plasma concentrations in cAVB dogs should increase awareness regarding proarrhythmic effects of widely used Evodia extracts.

Characteristics of electromechanical window in anesthetized rabbit models of short QT and long QT syndromes

The current regulatory guidelines recommend the use of QT interval to assess the risk of arrhythmogenic potential of new chemical entities. Recently, the electromechanical window (EMW), the difference in duration between electrical and mechanical systole, has been proposed as markers for drug-induced torsades de pointes (TdP); however, data of EMW in short QT model are not available. This study aimed to characterize the EMW as a marker for drug-induced ventricular arrhythmias in anesthetized rabbit model of long QT syndrome type 2 (LQT2) and short QT syndrome (SQTS) infused with reference compounds known to lengthen or shorten QT intervals. After rabbits were anesthetized with isoflurane, body surface electrocardiograms and left ventricular pressure were recorded. The LQT2 was produced by intravenous infusion with dofetilide (n = 6), quinidine (n = 6) and sotalol (n = 6) whereas the SQTS was induced by intravenous escalating concentrations of nicorandil (n = 7), pinacidil (n = 5) and cromakalim (n = 5). The EMW in anesthetized rabbits ranged from 1.3 to 53.3 msec. All three drugs known to lengthen QT intervals prolonged QT and QTcF interval while the EMW was markedly decreased to negative values. Pinacidil significantly produced QT and QTcF shortening and significantly abbreviated the EMW (p < 0.05). This study demonstrated that the EMW is associated with QT intervals (p < 0.001). It is negative in the presence of QT-prolonging drugs while it is more positive in the presence of QT-shortening drugs. The results suggest that the EMW in anesthetized rabbits can be used in drug safety evaluation in addition to the QT interval.

An increasing electromechanical window is a predictive marker of ventricular fibrillation in anesthetized rabbit with ischemic heart

The QTc interval is widely used in Safety Pharmacological studies to predict arrhythmia risk, and the electromechanical window (EMW) and short-term variability of QT intervals (STV_QT) have been studied as new biomarkers for drug-induced Torsades de Pointes (TdP). However, the use of EMW and STV_QT to predict ventricular fibrillation (VF) has not been elucidated. This study aimed to evaluate EMW and STV_QT to predict VF in anesthetized rabbit model of VF. VF was induced by ligation of the left anterior descending and a descending branch of the left circumflex coronary arteries in a sample population of rabbits (n=18). VF was developed 55.6% (10/18). In rabbit with VF, the EMW was significantly higher than in rabbits without VF (96.3 ± 15.6 ms and 49.5 ± 5.6 ms, respectively, P<0.05). STV_QT had significantly increased before the onset of VF in rabbits that experienced VF, but not in rabbits that did not experience VF (11.7 ± 1.8 ms and 3.7 ± 0.4 ms, respectively, P<0.05). The EMW and STV_QT had better predictive power for VF with higher sensitivity and specificity than the QTc measure. The result suggested that the increasing of EMW, as well as the elevation of STV_QT, can potentially be used as biomarkers for predicting of VF.

Assessment of QT-prolonging drugs in the isolated normal and failing rabbit hearts

Lengthening of QTc is the usual signal to indicate torsadogenic potential of a therapeutic agent. The ICH S7B guideline recommends that new chemical entities should be assessed for potential of delayed ventricular repolarization in animal models. The aim of this study was to determine a feasibility of using isolated failing heart rabbit to assess the QT-lengthening drugs in comparison with their effects on isolated normal heart rabbits. Heart failure was induced by ligation of the left anterior descending and descending branch of left circumflex coronary arteries. One month after ligation, all rabbits were anesthetized and the hearts were removed quickly, and they were perfused with the oxygenated Krebs-Henseleit solution to which escalating concentrations of QT-lengthening compounds were added. RR, QT, and QTc(F) were not significantly different, at rest, between failing and normal hearts. During baseline, dP/dt<inf>max</inf> was lower and dP/dt<inf>min</inf> was higher for failing hearts than for normals. In responses to all three QT-lengthening compounds, RR, QT and QTc(F) lengthened similarly in a dose-response manner in both the failing and normal hearts. Neither the failing nor the normal hearts developed fatal arrhythmias, torsades de pointes. Langendorff preparations of failing hearts are as good as normal isolated hearts and can be use to assess the potential of delayed ventricular repolarization of test articles.

Chronically increased intra-abdominal pressure: validating a model

BACKGROUND

Pregnancy is the only physiologic condition in which we encounter chronically elevated intra-abdominal pressure (IAP), while pathologically several pathologies, such as ascites and morbid obesity, are affected by this phenomenon. This paper introduces and validates a new model that is able to create and maintain chronically increased IAP, facilitating the study of phenomena related to chronically elevated IAP, i.e., obesity.

METHODS

An experimental device was implanted in 15 rabbits, which consisted of an intra-abdominal balloon (IAB), an external control valve, and a connecting tube. A Foley catheter was inserted in their urinary bladders. IAPs were measured simultaneously transvesically and via the device. During the acute phase, IAB was gradually inflated to 16 cmH(2)O, and IAPs were consecutively measured. During the chronic phase, residual IAPs were measured in a weekly rate for 8 weeks. Statistical significances, mean bias, and precisions were calculated.

RESULTS

During the acute phase, the saline in the IAB efficiently increases IAP to 16 cmH(2)O. IAPs measured both through the urinary bladder and the device correlate well with small bias and high precision. Our model maintains sufficiently chronically increased IAP for at least 8 weeks. No mortality was observed.

CONCLUSIONS

A rabbit model establishing and maintaining chronically increased IAP was successfully created and proved to be simple, effective, and repeatable. This model established chronically increased IAP permitting this way the study of its effect on organs and systems.

CPU228, a derivative of dofetilide, relieves cardiac dysfunction by normalizing FKBP12.6, NADPH oxidase and protein kinase C ε in the myocardium

Objectives

The aim of this study was to determine if CPU228, a derivative of dofetilide, is more effective than dofetilide in attenuating isoproterenol-induced heart failure by recovering downregulated FK506 binding protein (FKBP12.6), and suppressing oxidative stress, upregulated NADPH oxidase and protein kinase C ε (PKCε) hyperphosphorylation in the myocardium.

Methods

Heart failure was induced by isoproterenol (1 mg/kg s.c. for 5 days) in male Sprague-Dawley rats. Intervention with either CPU228 or dofetilide (2 mg/kg on Days 3–5) was then conducted in vivo and in vitro.

Key findings

Isoproterenol produced compromised left ventricular systolic pressure, left ventricular pressure rise (dp/dtmax) and fall (dp/dtmin), and left ventricular end-diastolic pressure, associated with oxidative stress, abnormal FKBP12.6, NADPH oxidase p67phox and PKCε in the myocardium. CPU228 was more effective in attenuating these changes than dofetilide in vivo. Dofetilide produced a prolonged QTc to replace a shortened one. In primary neonatal cardiomyocytes, cultured with isoproterenol and treated with either CPU228 or dofetilide at 10−8, 10−7 and 10−6 mol/l, isoproterenol produced a hyperadrenergic state characterized by downregulated FKBP12.6, upregulated NADPH oxidase p67phox and PKCε in vitro. CPU228 was more effective than dofetilide in recovering these changes in a dose-dependent manner without a prolonged QTc.

Conclusions

CPU228 was more effective than dofetilide in attenuating heart failure by normalizing isoproterenol-induced changes, including downregulation of FKBP12.6, upregulation of NADPH oxidase and PKCε hyperphosphorylation in vivo and in vitro.

Preclinical Torsades-de-Pointes screens: advantages and limitations of surrogate and direct approaches in evaluating proarrhythmic risk

The successful development of novel drugs requires the ability to detect (and avoid) compounds that may provoke Torsades-de-Pointes (TdeP) arrhythmia while endorsing those compounds with minimal torsadogenic risk. As TdeP is a rare arrhythmia not readily observed during clinical or post-marketing studies, numerous preclinical models are employed to assess delayed or altered ventricular repolarization (surrogate markers linked to enhanced proarrhythmic risk). This review evaluates the advantages and limitations of selected preclinical models (ranging from the simplest cellular hERG current assay to the more complex in vitro perfused ventricular wedge and Langendorff heart preparations and in vivo chronic atrio-ventricular (AV)-node block model). Specific attention is paid to the utility of concentration-response relationships and "risk signatures" derived from these studies, with the intention of moving beyond predicting clinical QT prolongation and towards prediction of TdeP risk. While the more complex proarrhythmia models may be suited to addressing questionable or conflicting proarrhythmic signals obtained with simpler preclinical assays, further benchmarking of proarrhythmia models is required for their use in the robust evaluation of safety margins. In the future, these models may be able to reduce unwarranted attrition of evolving compounds while becoming pivotal in the balanced integrated risk assessment of advancing compounds.

Augmentation of late sodium current unmasks the proarrhythmic effects of amiodarone

AIM

Clinical use of amiodarone is associated with occasional development of torsade de pointes (TdP). However, preclinical models have failed to demonstrate the proarrhythmic potential of amiodarone. The objective of this study was to reveal and explain the pro- and anti-arrhythmic effects of acute exposure to amiodarone in an animal model.

METHODS AND RESULTS

Endo- and epicardial monophasic action potentials (MAPs) and 12-lead electrocardiogram were recorded in female rabbit isolated hearts. Ion channel currents were measured in human embryonic kidney cells expressing SCN5A Na+ and HERG K+ channels. Acute amiodarone alone caused an insignificant increase in duration of MAP (MAPD90) without causing TdP. In the presence of 3 nM sea anemone toxin (ATX-II), amiodarone (1-30 nM) prolonged MAPD90 from 217 +/- 5 to 250 +/- 8 ms (n = 16, P < 0.01), increased transmural dispersion of repolarization (TDR) from 59 +/- 9 to 70 +/- 10 ms and beat-to-beat variability (BVR) of MAPD(90) from 0.75 +/- 0.03 to 1.06 +/- 0.13 ms (P < 0.05). At 30-300 nM, amiodarone induced TdP in 16 out of 17 hearts. A further increase of amiodarone concentration to 1-10 microM abbreviated MAPD(90) to 211 +/- 9 ms, decreased BVR to 0.5 +/- 0.01 ms, decreased TDR (n = 7, P < 0.05), and suppressed TdP. Amiodarone inhibited HERG K+ and late Na+ currents with IC50s of 0.8 +/- 0.1 and 3.0 +/- 0.9 microM, respectively.

CONCLUSION

In hearts in which late INa is augmented to mimic congenital or acquired pathological conditions, amiodarone has a concentration-dependent biphasic effect to induce and then suppress arrhythmic activity, secondary to inhibition of HERG K+ and late Na+ currents. This is the first preclinical model demonstrating the potential for amiodarone to induce TdP.