Drug-Induced Torsade de Pointes: From Molecular Biology to Bedside

Chinese herbal medicine for the treatment of cardiovascular diseases ─ targeting cardiac ion channels

Cardiovascular disease (CVD) remains the leading cause of morbidity and mortality, imposing an increasing global health burden. Cardiac ion channels (voltage-gated Na_V, Ca_V, K_Vs, and others) synergistically shape the cardiac action potential (AP) and control the heartbeat. Dysfunction of these channels, due to genetic mutations, transcriptional or post-translational modifications, may disturb the AP and lead to arrhythmia, a major risk for CVD patients. Although there are five classes of anti-arrhythmic drugs available, they can have varying levels of efficacies and side effects on patients, possibly due to the complex pathogenesis of arrhythmias. As an alternative treatment option, Chinese herbal remedies have shown promise in regulating cardiac ion channels and providing anti-arrhythmic effects. In this review, we first discuss the role of cardiac ion channels in maintaining normal heart function and the pathogenesis of CVD, then summarize the classification of Chinese herbal compounds, and elaborate detailed mechanisms of their efficacy in regulating cardiac ion channels and in alleviating arrhythmia and CVD. We also address current limitations and opportunities for developing new anti-CVD drugs based on Chinese herbal medicines.

In silico prediction of Antifungal compounds from Natural sources towards Lanosterol 14-alpha demethylase (CYP51) using Molecular docking and Molecular dynamic simulation

An increase in the occurrence of fungal infections throughout the world, as well as the rise of novel fungal strains and antifungal resistance to commercially available drugs, suggests that new therapeutic choices for fungal infections are needed. The purpose of this research was to find new antifungal candidates or leads of secondary metabolites derived from natural sources that could effectively inhibit the enzymatic activity of Candida albicans lanosterol 14-alpha demethylase (CYP51) while also having good pharmacokinetics. In silico prediction of the drug-likeness, chemo-informatics and enzyme inhibition indicate that the 46 compounds derived from fungi, sponges, plants, bacteria and algae sources have a high novelty to meet all five requirements of Lipinski's rules and impede enzymatic function. Among the 15 candidate molecules with strong binding affinity to CYP51 investigated by molecular docking simulation, didymellamide A-E compounds demonstrated the strongest binding energy against the target protein at -11.14, -11.46, -11.98, -11.98, and -11.50 kcal/mol, respectively. Didymellamide molecules bind to comparable active pocket sites of antifungal ketoconazole and itraconazole medicines by hydrogen bonds forming to Tyr132, Ser378, Met508, His377 and Ser507, and hydrophobic interactions with HEM601 molecule. The stability of the CYP51-ligand complexes was further investigated using molecular dynamics simulations that took into account different geometric features and computed binding free energy. Using the pkCSM ADMET descriptors tool, several pharmacokinetic characteristics and the toxicity of candidate compounds were assessed. The findings of this study revealed that didymellamides could be a promising inhibitor against these CYP51 protein. However, there is still a need for further in vivo and in vitro studies to support these findings.

Optimization of the in vitro cardiac safety of hydroxamate-based histone deacetylase inhibitors

Histone deacetylase (HDAC) inhibitors have shown promise in treating various forms of cancer. However, many HDAC inhibitors from diverse structural classes have been associated with QT prolongation in humans. Inhibition of the human ether a-go-go related gene (hERG) channel has been associated with QT prolongation and fatal arrhythmias. To determine if the observed cardiac effects of HDAC inhibitors in humans is due to hERG blockade, a highly potent HDAC inhibitor devoid of hERG activity was required. Starting with dacinostat (LAQ824), a highly potent HDAC inhibitor, we explored the SAR to determine the pharmacophores required for HDAC and hERG inhibition. We disclose here the results of these efforts where a high degree of pharmacophore homology between these two targets was discovered. This similarity prevented traditional strategies for mitigating hERG binding/modulation from being successful and novel approaches for reducing hERG inhibition were required. Using a hERG homology model, two compounds, 11r and 25i, were discovered to be highly efficacious with weak affinity for the hERG and other ion channels.

Drug-induced block of cardiac HERG potassium channels and development of torsade de pointes arrhythmias: the case of antipsychotics

The prolongation of the cardiac repolarization process, a result of the blocking of the Human Ether-a-go-go Related Gene potassium channel, is an undesired accessory property shared by many pharmacological classes of non-cardiovascular drugs. Often the delayed cardiac repolarization process can be identified by a prolongation of the QT interval of the electrocardiograph. In these conditions, premature action potentials can trigger a dangerous polymorphic ventricular tachyarrhythmia, known as torsade de pointes, which occasionally can result in lethal ventricular fibrillation. In this work, brief descriptions of the electrophysiological basis of torsade de pointes and of the several pharmacological classes of torsadogenic drugs are given. Attention is focused on antipsychotics, with a deeper overview on the experimental and clinical reports about their torsadogenic properties.

QTc monitoring during a phase I study: experience with SR271425

OBJECTIVE

SR271425 is a thioxanthone cytotoxic drug that induces dose-related cardiac electrophysiologic changes in preclinical models. A phase I trial was conducted to determine the maximally tolerated dose and safety profile, notably cardiac events.

METHODS

SR271425 was administered weekly as a 2-hour single intravenous infusion with a fixed 30 mg/m2 increment at each dose level (DL). A sustained cardiac evaluation was performed. ECG parameters were evaluated at bedside by an investigator or a cardiologist, as well as by central reading for dose limiting toxicity (DLT) determination.

RESULTS

Sixteen patients were treated. Five DLs were explored, from 75 mg/m2/wk to 195 mg/m2/wk. Fourteen patients (87.5%) experienced noncardiac adverse events related to treatment; only 2 patients presented grade 3 toxicity (nausea/vomiting and GGT increase) and no grade 4 toxicities were reported. Asymptomatic grade 1 or 2 QTcF prolongations were observed in 5 patients during central readings, and in 4 cases at bedside. One QTc-DLT, registered at bedside (grade 2), was unconfirmed at central reading, while another QTc-DLT, not noted at bedside, was highlighted by central reading. No arrhythmias or QRS prolongations were observed.

CONCLUSIONS

The maximum tolerated dose of SR271425 was not reached in this trial due to early termination of the trial, not related to cardiac toxicity, following the termination of the development program by the sponsor. Sustained ECG monitoring is quite feasible in oncology phase I trials, but discrepancies between bedside and central evaluation could lead to conflicting decisions for management of patient care.

Comparison of sensitivity of surrogate markers of drug-induced torsades de pointes in canine hearts

Given a limited information regarding the difference of the sensitivity of surrogate markers of drug-induced torsades de pointes, including early afterdepolarization, ectopic beats, phase 3 repolarization and dispersion of ventricular repolarization, we simultaneously analyzed them in the halothane-anesthetized canine model (n=5). A non-specific IKr channel blocker sparfloxacin, which has been known to induce torsades de pointes in animals and clinical patients, prolonged the repolarization process in a dose-related and reverse use-dependent manner. No significant change was detected in any of the proarrhythmic markers except for the backward parallel shift of phase 3 repolarization in the cardiac cycle with the QT interval prolongation, which would be the most sensitive marker in predicting the potential arrhythmogenic property of sparfloxacin in the "non-remodeled" normal heart.

QTc interval prolongation associated with intravenous methadone

Numerous medications prolong the rate-corrected QT (QTc) interval and induce arrhythmias by blocking ionic current through cardiac potassium channels composed of subunits expressed by the human ether-a-go-go-related gene (HERG). Recent reports suggest that high doses of methadone cause torsades de pointes. To date, no controlled study has described an association between methadone and QTc prolongation. The only commercial formulation of parenteral methadone available in the United States contains the preservative chlorobutanol. The objectives of this study are to determine: (1) whether the administration of intravenous (i.v.) methadone causes QTc prolongation in humans; (2) whether methadone and/or chlorobutanol block cardiac HERG potassium currents (IHERG) in vitro. Over 20 months, we identified every inpatient with at least one electrocardiogram (ECG) performed on i.v. methadone. For each patient, we measured QTc intervals for every available ECG performed on and off i.v. methadone. Concurrent methadone doses were also recorded. Similar data were collected for a separate group of inpatients treated with i.v. morphine. In a separate set of experiments IHERG was evaluated in transfected human embryonic kidney cells exposed to increasing concentrations of methadone, chlorobutanol, and the two in combination. Mean difference (+/- standard error) per patient in QTc intervals on and off methadone was 41.7 (+/- 7.8)ms, p<0.0001. Mean difference in QTc intervals on and off morphine was 9.0 (+/- 6.1)ms, p=0.15. The approximately linear relationship between QTc measurements and log-dose of methadone was significant (p<0.0001). Methadone and chlorobutanol independently block IHERG in a concentration-dependent manner with IC50 values of 20 +/- 2 microM and 4.4 +/- 0.3 mM, respectively. Chlorobutanol potentiates methadone's ability to block IHERG. Methadone in combination with chlorobutanol is associated with QTc interval prolongation. Our data strongly suggest that methadone in combination with chlorobutanol is associated with QTc interval prolongation.

Effects of mexiletine on the canine model of sparfloxacin-induced long QT syndrome

Potential utility of mexiletine for the treatment of sparfloxacin-induced long QT syndrome was assessed using the in vivo halothane-anesthetized canine model. At 30 min after the administration of a supratherapeutic dose of sparfloxacin (30 mg/kg, i.v.), the mean blood pressure and heart rate decreased, whereas repolarization process and effective refractory period of the ventricular muscle were significantly prolonged. Additional administration of a clinically recommended dose of mexiletine (3 mg/kg, i.v.) at this time point increased the mean blood pressure, suppressed ventricular contraction, delayed atrioventricular as well as intraventricular conduction, and shortened repolarization process and effective refractory period. The extent of abbreviation of the repolarization was more prominent than that of the refractoriness, indicating that mexiletine could decrease the electrical vulnerability of the heart during sparfloxacin overdose. Thus, mexiletine may become a promising pharmacological strategy against the drug-induced long QT syndrome.

Optimization of a tertiary alcohol series of phosphodiesterase-4 (PDE4) inhibitors: structure-activity relationship related to PDE4 inhibition and human ether-a-go-go related gene potassium channel binding affinity

A SAR study on the tertiary alcohol series of phosphodiesterase-4 (PDE4) inhibitors related to 1 is described. In addition to inhibitory potency against PDE4 and the lipopolysaccharide-induced production of TNFalpha in human whole blood, the binding affinity of these compounds for the human ether-a-go-go related gene (hERG) potassium channel (an in vitro measure for the potential to cause QTc prolongation) was assessed. Four key structural moieties in the molecule were studied, and the impact of the resulting modifications in modulating these activities was evaluated. From these studies, (+)-3d (L-869,298) was identified as an optimized structure with respect to PDE4 inhibitory potency, lack of binding affinity to the hERG potassium channel, and pharmacokinetic behavior. (+)-3d exhibited good in vivo efficacy in several models of pulmonary function with a wide therapeutic index with respect to emesis and prolongation of the QTc interval.