Cellular electrophysiological effect of terikalant in the dog heart

Multichannel modulation of depolarizing and repolarizing ion currents increases the positive rate-dependent action potential prolongation

Prolongation of the action potential duration (APD) could prevent reentrant arrhythmias if prolongation occurs at the fast excitation rates of tachycardia with minimal prolongation at slow excitation rates (i.e., if prolongation is positive rate-dependent). APD prolongation by current anti-arrhythmic agents is either reverse (larger APD prolongation at slow rates than at fast rates) or neutral (similar APD prolongation at slow and fast rates), which may not result in an effective anti-arrhythmic action. In this report we show that, in computer models of the human ventricular action potential, the combined modulation of both depolarizing and repolarizing ion currents results in a stronger positive rate-dependent APD prolongation than modulation of repolarizing potassium currents. A robust positive rate-dependent APD prolongation correlates with an acceleration of phase 2 repolarization and a deceleration of phase 3 repolarization, which leads to a triangulation of the action potential. A positive rate-dependent APD prolongation decreases the repolarization reserve with respect to control, which can be managed by interventions that prolong APD at fast excitation rates and shorten APD at slow excitation rates. For both computer models of the action potential, I_CaL and I_K1 are the most important ion currents to achieve a positive rate-dependent APD prolongation. In conclusion, multichannel modulation of depolarizing and repolarizing ion currents, with ion channel activators and blockers, results in a robust APD prolongation at fast excitation rates, which should be anti-arrhythmic, while minimizing APD prolongation at slow heart rates, which should reduce pro-arrhythmic risks.

Positive rate-dependent action potential prolongation by modulating potassium ion channels

Pharmacological agents that prolong action potential duration (APD) to a larger extent at slow rates than at the fast excitation rates typical of ventricular tachycardia exhibit reverse rate dependence. Reverse rate dependence has been linked to the lack of efficacy of class III agents at preventing arrhythmias because the doses required to have an antiarrhythmic effect at fast rates may have pro-arrhythmic effects at slow rates due to an excessive APD prolongation. In this report, we show that, in computer models of the ventricular action potential, APD prolongation by accelerating phase 2 repolarization (by increasing I_Ks ) and decelerating phase 3 repolarization (by blocking I_Kr and I_K1 ) results in a robust positive rate dependence (i.e., larger APD prolongation at fast rates than at slow rates). In contrast, APD prolongation by blocking a specific potassium channel type results in reverse rate dependence or a moderate positive rate dependence. Interventions that result in a strong positive rate dependence tend to decrease the repolarization reserve because they require substantial I_K1 block. However, limiting I_K1 block to ~50% results in a strong positive rate dependence with moderate decrease in repolarization reserve. In conclusion, the use of a combination of I_Ks activators and I_Kr and I_K1 blockers could result in APD prolongation that potentially maximizes antiarrhythmic effects (by maximizing APD prolongation at fast excitation rates) and minimizes pro-arrhythmic effects (by minimizing APD prolongation at slow excitation rates).

Linked biaromatic compounds as cardioprotective agents

Cardiovascular diseases (CVDs) are widespread in the modern world, and their number is constantly growing. For a long time, CVDs have been the leading cause of morbidity and mortality worldwide. Drugs for the treatment of CVD have been developed almost since the beginning of the 20th century, and a large number of effective cardioprotective agents of various classes have been created. Nevertheless, the need for the design and development of new safe drugs for the treatment of CVD remains. Literature data indicate that a huge number of cardioprotective agents of various generations and mechanisms correspond to a single generalized pharmacophore model containing two aromatic nuclei linked by a linear linker. In this regard, we put forward a concept for the design of a new generation of cardioprotective agents with a multitarget mechanism of action within the indicated pharmacophore model. This review is devoted to a generalization of the currently known compounds with cardioprotective properties and corresponding to the pharmacophore model of biaromatic compounds linked by a linear linker. Particular attention is paid to the history of the creation of these drugs, approaches to their design, and analysis of the structure-action relationship within each class.

Analysis of arrhythmogenic profile in a canine model of chronic atrioventricular block by comparing in vitro effects of the class III antiarrhythmic drug nifekalant on the ventricular action potential indices between normal heart and atrioventricular block heart

The chronic atrioventricular block dog is a useful model for predicting the future onset of drug-induced long QT syndrome in clinical practice. To better understand the arrhythmogenic profile of this model, we recorded the action potentials of the isolated ventricular tissues in the presence and absence of the class III antiarrhythmic drug nifekalant. The action potential durations of the Purkinje fiber and free wall of the right ventricle were longer in the chronic atrioventricular block dogs than in the dogs with normal sinus rhythm. Nifekalant in concentrations of 1 and 10 microM prolonged the action potential durations of Purkinje fiber and the free wall in a concentration-dependent manner. The extent of prolongation was greater in the chronic atrioventricular block dogs than in the normal dogs. However, increase of temporal dispersion of ventricular repolarization including early afterdepolarization was not detected by nifekalant in either group of dogs, indicating lack of potential to trigger arrhythmias in vitro. These results suggest that the ventricular repolarization delay in the chronic atrioventricular block model by nifekalant may largely depend on the decreased myocardial repolarization reserve, whereas the trigger for lethal arrhythmia was not generated in the in vitro condition in contrast to the in vivo experiment.

Thorough QT/QTc not so thorough: removes torsadogenic predictors from the T-wave, incriminates safe drugs, and misses profibrillatory drugs

Drug-induced QT prolongation has such a strong correlation with torsade de pointes (TdP) that it comes to serve as a surrogate for TdP. As a result, drugs that prolong QT by as little as a few ms, even without any evidence of TdP, may get dropped from development or blocked from approval. However, measurement of QT with ms accuracy may be impossible to achieve. Worse, some drugs that lengthen the QT interval are not only not proarrhythmic, they may even be antiarrhythmic; while some that shorten the QT can be strongly proarrhythmic. Indeed, proarrhythmia related to repolarization disturbances is caused by triangulation, reverse use dependence, instability, and dispersion (TRIaD). When TRIaD is present with QT prolongation it commonly yields TdP, but when TRIaD is combined with QT shortening it preferentially leads to VF instead. While TdP is lethal in less than 20% of instances, VF is much more morbid. Worse, available evidence suggests that there is more death from drug-induced fibrillation than TdP. Thus, QT prolongation alone is not very useful. Instead, the T-wave should be used in alternate ways: extraction of TRIaD.

Predictive, non-GLP models of secondary pharmacodynamics: putting the best compounds forward

Secondary pharmacodynamic studies of new chemical entities (NCEs) play a critical role in support of efficient drug discovery. In an era in which speed and efficiency are the norm for pharmaceutical discovery, the need to identify NCEs with greater patient tolerability continues to increase. Early use of secondary pharmacodynamic models (in vivo and in vitro) provides the foundation for critical, early decisions regarding lead molecules. Scientifically robust, non-GLP (good laboratory practices) secondary pharmacodynamic studies can eliminate compounds or structural series with undesirable profiles early, and may prove useful in defining structure-activity relationships (SARs) with regards to off-target effects.