Perception of validity of clinical and preclinical methods for assessment of torsades de pointes liability

Anticancer drug-induced cardiac rhythm disorders: Current knowledge and basic underlying mechanisms

Significant advances in cancer treatment have resulted in decreased cancer related mortality for many malignancies with some cancer types now considered chronic diseases. Despite these improvements, there is increasing recognition that many cancer patients or cancer survivors can develop cardiovascular diseases, either due to the cancer itself or as a result of anticancer therapy. Much attention has focused on heart failure; however, other cardiotoxicities, notably cardiac rhythm disorders, can occur without underlying cardiomyopathy. Supraventricular tachycardias occur in cancer patients treated with cytotoxic chemotherapy (anthracyclines, gemcitabine, cisplatin and alkylating-agents) or kinase-inhibitors (KIs) such as ibrutinib. Ventricular arrhythmias, with a subset of them being torsades-de-pointes (TdP) favored by QTc prolongation have been reported: this may be the result of direct hERG-channel inhibition or a more recently-described mechanism of phosphoinositide-3-kinase inhibition. The major anticancer drugs responsible for QTc prolongation in this context are KIs, arsenic trioxide, anthracyclines, histone deacetylase inhibitors, and selective estrogen receptor modulators. Anticancer drug-induced cardiac rhythm disorders remain an underappreciated complication even by experienced clinicians. Moreover, the causal relationship of a particular anticancer drug with cardiac arrhythmia occurrence remains challenging due in part to patient comorbidities and complex treatment regimens. For example, any cancer patient may also be diagnosed with common diseases such as hypertension, diabetes or heart failure which increase an individual's arrhythmia susceptibility. Further, anticancer drugs are generally usually used in combination, increasing the challenge around establishing causation. Thus, arrhythmias appear to be an underappreciated adverse effect of anticancer agents and the incidence, significance and underlying mechanisms are now being investigated.

Inter-individual variability and modeling of electrical activity: a possible new approach to explore cardiac safety?

Safety pharmacology aims to predict rare side effects of new drugs. We explored whether rare pro-arrhythmic effects could be linked to the variability of the effects of these drugs on ion currents and whether taking into consideration this variability in computational models could help to better detect and predict cardiac side effects. For this purpose, we evaluated how intra- and inter-individual variability influences the effect of hERG inhibition on both the action potential duration and the occurrence of arrhythmias. Using two computer simulation models of human action potentials (endocardial and Purkinje cells), we analyzed the contribution of two biological parameters on the pro-arrhythmic effects of several hERG channel blockers: (i) spermine concentration, which varies with metabolic status, and (ii) L-type calcium conductance, which varies due to single nucleotide polymorphisms or mutations. By varying these parameters, we were able to induce arrhythmias in 1 out of 16 simulations although conventional modeling methods to detect pro-arrhythmic molecules failed. On the basis of our results, taking into consideration only 2 parameters subjected to intra- and inter-individual variability, we propose that in silico computer modeling may help to better define the risks of new drug candidates at early stages of pre-clinical development.

Influence of steroid hormones on ventricular repolarization

QT interval prolongation, corrected for heart rate (QTc), either spontaneous or drug-induced, is associated with an increased risk of torsades de pointes and sudden death. Women have longer QTc than men and are at higher risk of torsades de pointes, particularly during post-partum and the follicular phase. Men with peripheral hypogonadism have longer QTc than healthy controls. The role of the main sex steroid hormones has been extensively studied with inconsistent findings. Overall, estradiol is considered to promote QTc lengthening while progesterone and testosterone shorten QTc. New findings suggest more complex regulation of QTc by sex steroid hormones involving gonadotropins (i.e. follicle-stimulating hormone), the relative concentrations of sex steroid hormones (which depends on gender, i.e., progesterone/estradiol ratio in women). Aldosterone, another structurally related steroid hormone, can also prolong ventricular repolarization in both sex. Better understanding of pathophysiological hormonal processes which may lead to increased susceptibility of women (and possibly hypogonadic men) to drug-induced arrhythmia may foster preventive treatments (e.g. progesterone in women). Exogenous hormonal intake might offer new therapeutic opportunities or, alternatively, increase the risk of torsades de pointes. Some exogenous sex steroids may also have paradoxical effects on ventricular repolarization. Lastly, variations of QTc in women linked to the menstrual cycle and sex hormone fluctuations are generally ignored in regulatory thorough QT studies. Investigators and regulatory agencies promoting inclusion of women in thorough QT studies should be aware of this source of variability especially when studying drugs over several days of administration.

Optical and electrical recordings from isolated coronary-perfused ventricular wedge preparations

The electrophysiological heterogeneity that exists across the ventricular wall in the mammalian heart has long been recognized, but remains an area that is incompletely understood. Experimental studies of the mechanisms of arrhythmogenesis in the whole heart often examine the epicardial surface in isolation and thereby disregard transmural electrophysiology. Significant heterogeneity exists in the electrophysiological properties of cardiomyocytes isolated from different layers of the ventricular wall, and given that regional heterogeneities of membrane repolarization properties can influence the electrophysiological substrate for re-entry, the diversity of cell types and characteristics spanning the ventricular wall is important in the study of arrhythmogenesis. For these reasons, coronary-perfused left ventricular wedge preparations have been developed to permit the study of transmural electrophysiology in the intact ventricle. Since the first report by Yan and Antzelevitch in 1996, electrical recordings from the transmural surface of canine wedge preparations have provided a wealth of data regarding the cellular basis for the electrocardiogram, the role of transmural heterogeneity in arrhythmogenesis, and differences in the response of the different ventricular layers to drugs and neurohormones. Use of the wedge preparation has since been expanded to other species and more recently it has also been widely used in optical mapping studies. The isolated perfused wedge preparation has become an important tool in cardiac electrophysiology. In this review, we detail the methodology involved in recording both electrical and optical signals from the coronary-perfused wedge preparation and review the advances in cardiac electrophysiology achieved through study of the wedge.

Differentiating electrophysiological effects and cardiac safety of drugs based on the electrocardiogram: a blinded validation

BACKGROUND

The ventricular components (QRS and QT) on the electrocardiogram (ECG) depend on the properties of ventricular action potentials that can be modulated by drugs via specific ion channels. However, the correlation of ECG ventricular waveforms with underlying ion actions is not well established and has been extensively debated.

OBJECTIVE

To conduct a blinded in vitro assessment of the ionic mechanisms for drug-induced ECG changes.

METHODS AND RESULTS

Fourteen cardiac and noncardiac drugs with known effects on cardiac ion channels were selected by the study sponsor, and were tested in the rabbit left ventricular wedge preparation with recording of the ECG and contractility. The investigators who performed the experiments and analyzed the data were blinded to names, concentrations, and molecular weights of the drugs. The compounds were prepared by the sponsor and sent to the investigators as 56 stock solutions. The effects of I(Kr), I(Ks), I(Ca,L), I(Na) blocker, and I(KATP) opener on QRS, QT, and T(p-e), were evaluated. Disclosure of the names and concentrations after completion of the study revealed that there were highly correlated ECG changes with underlying ionic mechanisms and proarrhythmic potential of drugs that, respectively, target I(Kr), I(Ks), I(Ca,L), I(Na), and I(KATP). Among ECG parameters, T(p-e) was more useful in differentiating drugs' actions.

CONCLUSIONS

Specific electrophysiological action and the consequent proarrhythmic potential of a drug can be accurately determined by analysis of drug-induced changes in ECG in the rabbit left ventricular wedge preparation. Change in T(p-e) provides the most relevant information.

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.

Molecular mechanisms of adverse drug reactions in cardiac tissue

The myocardium is the target of toxicity for a number of drugs. Based on pharmacological evidence, cellular targets for drugs that produce adverse reactions can be categorized into a number of sites that include the cell membrane-bound receptors, the second messenger system, ionic channels, ionic pumps, and intracellular organelles. Additionally, interference with the neuronal input to the heart can also present a global site where adverse drug effects can manifest themselves. Simply, a drug can interfere with the normal cardiac action by modifying an ion channel function at the plasma membrane level leading to abnormal repolarization and/or depolarization of the heart cells thus precipitating a disruption in the rhythm and causing dysfunction in contractions and/or relaxations of myocytes. It is now recognized that toxic actions of drugs against the myocardium are not exclusive to the antitumor or the so-called cardiac drugs, and many other drugs with diverse chemical structures, such as antimicrobial, antimalarial, antihistamines, psychiatric, and gastrointestinal medications, seem to be capable of severely compromising myocardium function. At present, great emphasis in terms of drug safety is being placed on the interaction of many classes of drugs with the hERG potassium channel in cardiac tissue. The interest in the latter channel stems from the simplified view that drugs that block the hERG potassium channel cause prolongation of the QT interval, and this can cause life-threatening cardiac arrhythmias. Based on the evidence in the current literature, this concept does not seem to always hold true.