Rob Towart

Predicting drug-induced changes in QT interval and arrhythmias: QT-shortening drugs point to gaps in the ICHS7B Guidelines

The regulatory guidelines (ICHS7B) recommending inhibition of the delayed rectifier K+ current (IKr), carried by human ether‐a‐go‐go‐related gene (hERG) channels in cardiac cells (the hERG test), as a ‘first line’ test for identifying compounds inducing QT prolongation, have limitations, some of which are outlined here.

Blockade of the IKs potassium channel: An overlooked cardiovascular liability in drug safety screening?

The problem of drug-induced hERG channel blockade, which can lead to acquired long QT syndrome and potentially fatal arrhythmias, has exercised drug developers and regulatory authorities for over 10 years, and exacting guidelines have been put into place to test for this liability both preclinically (ICH S7B) and clinically (ICH E14). However, the I(Ks) channel, which along with the transient outward current (I(to)) is the other main potassium channel affecting cardiac repolarisation and thus the length of the QT interval, has received little attention, and potent I(Ks) blocking drugs with serious side effects could potentially enter into human testing without being detected by the existing regulatory core battery and standard screening strategies. Here we review the pharmacology of cardiac I(Ks) channel blockade and describe the discovery of a potent I(Ks) blocker whose activity was not detected by standard hERG or invitro action potential screens, but subsequently evoked unprovoked torsades de pointes (TdP) invivo in our anaesthetised dog model. We have exploited this molecule to develop a ligand binding assay to detect I(Ks) blockade at an earlier stage in drug discovery, and note that several other laboratories developing new drugs have also developed higher throughput screens to detect I(Ks) blockade (e.g., [Trepakova, E. S., Malik, M. G., Imredy, J. P., Penniman, J. R., Dech, S. J., & Salata, J. J. (2007) Application of PatchXpress planar patch clamp technology to the screening of new drug candidates for cardiac KCNQ1/KCNE1 (I(Ks)) activity. Assay Drug Development Technology 5, 617-627]). Because of the presence of I(Ks) channels in other tissues, including blood vessels and in the epithelia of intestine, kidney, lung and the cochlea, I(Ks) blockade has the potential to cause extensive side effects in addition to QT prolongation and arrhythmias. We therefore suggest that compounds selected for development should also be examined for I(Ks) liability before testing in humans. The possibility of undetected I(Ks) blockade is therefore an additional gap to that identified earlier [Lu, H. R., Vlaminckx, E., Hermans, A. N., Rohrbacher, J., Van Ammel, K., Towart, R., et al. (2008) Predicting drug-induced changes in QT interval and arrhythmias: QT-shortening drugs point to gaps in the ICH S7B Guidelines. British Journal of Pharmacology, 154, 1427-1438] in the ICH S7B regulatory guidelines.

The effect of changes in core body temperature on the QT interval in beagle dogs: a previously ignored phenomenon, with a method for correction

Body core temperature (Tc) changes affect the QT interval, but correction for this has not been systematically investigated. It may be important to correct QT intervals for drug‐induced changes in Tc.

The Electro-Mechanical window: a risk marker for Torsade de Pointes in a canine model of drug induced arrhythmias.

BACKGROUND AND PURPOSE In cardiovascular pharmacology, electrical and mechanical events can be distinguished, and the phrase ‘electro‐mechanical window’ (EMw) describes the temporal difference between these events. We studied whether changes in EMw have potential predictive value for the occurrence of arrhythmias in fentanyl/etomidate‐anaesthetized beagle (FEAB) dogs.

Pharmacological dissection of Kv7.1 channels in systemic and pulmonary arteries

BACKGROUND AND PURPOSE The aim of this study was to characterize the functional impact of KCNQ1‐encoded voltage‐dependent potassium channels (Kv7.1) in the vasculature.

Innovation in safety pharmacology testing

This issue of the Journal of Pharmacological and Toxicological Methods (JPTM) is themed. It is the eighth in a series, arising from the Annual Safety Pharmacology Society (SPS) meeting. The SPS is now in its 10th year as an independent branch of biological sciences (distinct from pharmacology and toxicology) and is the primary forum for driving advances in safety pharmacology. The theme of the meeting and this journal issue is innovation, and the focus is non-clinical safety assessment of new chemical entity (NCEs). The content is informed by regulatory guidance documents (S7A and S7B) prior to first in human (FIH) studies. The manuscripts cover a broad spectrum of safety pharmacology topics from theory to practice, with interrogation of state-of-the-art techniques, and profiling of methods that are in development for safety assessment. Philosophical and strategic issues are addressed, with consideration of the use of novel methods for population pharmacokinetic (PK) analysis, abuse liability, electrocardiogram (ECG) analysis algorithms, in vitro cardiac slice preparations, human pluripotent stem cells, and a brief discussion regarding the assessment of changes in the QRS complex of the ECG indicative of drug-induced blockade of cardiac sodium channels. Safety pharmacology methods continue to evolve.

Non-clinical models: Validation, study design and statistical consideration in safety pharmacology

The current issue of the Journal of Pharmacological and Toxicological Methods (JPTM) focuses exclusively on safety pharmacology methods. This is the 7th year the Journal has published on this topic. Methods and models that specifically relate to methods relating to the assessment of the safety profile of a new chemical entity (NCE) prior to first in human (FIH) studies are described. Since the Journal started publishing on this topic there has been a major effort by safety pharmacologists, toxicologists and regulatory scientists within Industry (both large and small Pharma as well as Biotechnology companies) and also from Contract Research Organizations (CRO) to publish the surgical details of the non-clinical methods utilized but also provide important details related to standard and non-standard (or integrated) study models and designs. These details from core battery and secondary (or ancillary) drug safety assessment methods used in drug development programs have been the focus of these special issues and have been an attempt to provide validation of methods. Similarly, the safety pharmacology issues of the Journal provide the most relevant forum for scientists to present novel and modified methods with direct applicability to determination of drug safety-directly to the safety pharmacology scientific community. The content of the manuscripts in this issue includes the introduction of additional important surgical methods, novel data capture and data analysis methods, improved study design and effects of positive control compounds with known activity in the model.

05 The electro-mechanical window as Torsade de Pointes risk marker in conscious and anaesthetised dogs after IKS blockade

The electro-mechanical window (EMw) is a recently proposed biomarker describing the temporal difference between electrical and mechanical events in beating hearts and is a precursor to identify Torsade de Pointes (TdP) risk in the anaesthetised dog.1 In follow-up studies, conscious dogs (n=6; telemetered) showed comparable baseline values to anaesthetised dogs (n=20): QT=250 vs 252 ms, QLVPend=347 vs 339 ms and EMw=+96 vs +87 ms, respectively. To compare the EMw in conscious and anaesthetised dogs after IKs blockade.2 Six conscious dogs were orally treated with JNJ303 (20 mg/kg) and four anaesthetised dogs received infusions of JNJ303 (cumulative 2.5 mg/kg). In both conditions JNJ303 induced QT prolongation, a large negative EMw and TdP appeared in 50% of the dogs in each condition (pause-dependent and adrenergic-dependent). In the conscious dogs significant differences in maximum plasma levels (PL), QT and EMw were observed between dogs that induced TdP and those without TdP: PL (4950 vs 1473 ng/ml), QT (427 vs 351 ms) and EMw (−150 vs −68 ms), without differences in RR (642 vs 654 ms) and QLVPend (273 vs 287 ms), respectively. Also in anaesthetised dogs long QT (766 ms) and large negative EMw (−445 ms) at similar PL (5743 ng/ml) were observed. In conclusion a potent IKs blocker (JNJ303) can cause TdP in conscious and anaesthetised dogs, coupled with a large negative EMw.

Effects of JNJ-17333030, naratriptan and sumatriptan on human isolated coronary artery

Methods Left anterior descending or right coronary arteries, with either visible or no visible atherosclerosis, were taken from four donors (2 male and 2 female) who died of noncardiac disorders. Arteries were obtained from ethically approved organ procurement organisations. The absence or presence of atherosclerosis was visually determined and subsequently assessed histologically. Vessel rings, ~3mm long, were suspended on hooks in 10ml organ baths containing Krebs’ physiological salt solution, gassed with 95% O2/5% CO2 and maintained at 37°C. Vessels were stretched to a passive tension of 15mN (1.5g) and viability was assessed by exposure to KCl (30mM). Following confirmation of vessel viability, the following protocol was used: