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Drug-induced long QT syndrome has resulted in many drugs being withdrawn from the market. At the same time, the current regulatory paradigm for screening new drugs causing long QT syndrome is preventing drugs from reaching the market, sometimes inappropriately. In this study, we report the results of a first-of-a-kind clinical trial studying late sodium (mexiletine and lidocaine) and calcium (diltiazem) current blocking drugs to counteract the effects of hERG potassium channel blocking drugs (dofetilide and moxifloxacin). We demonstrate that both mexiletine and lidocaine substantially reduce heart-rate corrected QT (QTc) prolongation from dofetilide by 20 ms. Furthermore, all QTc shortening occurs in the heart-rate corrected J-Tpeak (J-Tpeakc) interval, the biomarker we identified as a sign of late sodium current block. This clinical trial demonstrates that late sodium blocking drugs can substantially reduce QTc prolongation from hERG potassium channel block and assessment of J-Tpeakc may add value beyond only assessing QTc. [ABSTRACT FROM AUTHOR]

Cardiac side-effects are one of the major reasons for failure of drugs during preclinical development. Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) have been proposed as a model for predicting drug-induced arrhythmias under the Comprehensive in vitro Proarrhythmia Assay (CiPA) paradigm. Field potential duration (FPD) in spontaneously beating iPSC-CMs is commonly corrected for beating rate using formulas originally derived from the clinical QT-RR relationship that have not been thoroughly validated for use with iPSC-CMs. In this study, channelrhodopsin-2 was expressed in iPSC-CMs allowing for recordings in both spontaneously beating and optically paced (0.8, 1, and 1.5 Hz pacing rate) iPSC-CMs using a microelectrode array system (Maestro, Axion Biosystems). After optimizing the intensity (>1 mW/mm2), duration (15 ms) and frequency of the stimulating light pulses, we recorded iPSC-CMs' responses to 28 blinded CiPA compounds with clinically characterized risk of causing ventricular arrhythmia (Torsade de Pointes or TdP). Drug-induced FPD prolongation data along with drug-induced arrhythmia-like events were used to build a logistic regression model, separating high or intermediate TdP risk drugs from low-or-no TdP risk drugs. The area under the receiver operator characteristic curve for drug TdP risk prediction was identical for spontaneously beating and 0.8 Hz-paced iPSC-CMs (AUC = 0.96; 95% CI [0.9, 1]), while it was slightly lower for 1 and 1.5 Hz pacing (AUC = 0.88; 95% CI [0.76, 1] and 0.93; 95% CI [0.84, 1], respectively). In this study, optical pacing did not offer substantial improvement in proarrhythmic risk prediction when compared with nonpaced iPSC-CMs in the sample of 28 drugs., (Published by Oxford University Press on behalf of the Society of Toxicology 2019.)

Introduction: Cardiotoxicity assessment using human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) forms a key component of the Comprehensive in Vitro Proarrhythmia Assay (CiPA). A potentially impactful factor on iPSC-CM testing is the presence of serum in the experimental media. Generally, serum-free media is used to most accurately reproduce "free" drug concentration. However, caution is needed; drug solubility and cardiomyocyte electrophysiology could be affected by media formulation, potentially impacting interpretation of drug-induced effects., Methods: Effects of 25 drugs on properties of spontaneous field potentials in iPSC-CMs were assayed using a high-throughput microelectrode array (MEA) in two media formulations: serum-containing and serum-free. Comparative analysis was conducted on rate-corrected field potential duration (FPDc) and prevalence of arrhythmic events. Further MEA experiments were conducted, varying percentages of serum as well as carbon substrate components. Comparative LC-MS/MS analysis was done on two compounds to evaluate drug concentrations., Results: In serum-free media, 9 drugs prolonged FPDc. In serum-containing, 11 drugs prolonged FPDc. Eighteen drugs induced arrhythmias, 8 of these induced arrhythmias at lower concentrations in serum-containing media. At the highest non-arrhythmic concentrations, 13 of 25 drugs exhibited significant differences in FPDc prolongation/shortening between the media. Increasing fractions of serum in media yielded higher FPDc measurements. LC-MS/MS analysis of moxifloxacin and quinidine showed higher concentrations in serum-containing media., Discussion: The present study highlights media formulation as an important consideration for cardiac safety testing with iPSC-CMs. Results described here suggest that media formulation influences both compound availability and baseline electrophysiological properties. Special attention should be paid to media for future iPSC-CM assays., (Published by Elsevier Inc.)

Background: The failing heart exhibits an increased arrhythmia susceptibility that is often attributed to action potential (AP) prolongation due to significant ion channel remodeling. The inwardly rectifying K + current (I K1 ) has been reported to be reduced, but its contribution to shaping the AP waveform and cell excitability in the failing heart remains unclear., Objective: The purpose of this study was to define the effect of I K1 suppression on the cardiac AP and excitability in the normal and failing hearts., Methods: We used electrophysiological and pharmacological approaches to investigate I K1 function in a swine tachy-pacing model of heart failure (HF)., Results: Terminal repolarization of the AP (TRAP; the time constant of the exponential fit to terminal repolarization) was markedly prolonged in both myocytes and arterially perfused wedges from animals with HF. TRAP was increased by 54.1% in HF myocytes (P < .001) and 26.2% in HF wedges (P = .014). The increase in TRAP was recapitulated by the potent and specific I K1 inhibitor, PA-6 (pentamidine analog 6), indicating that I K1 is the primary determinant of the final phase of repolarization. Moreover, we find that I K1 suppression reduced the ratio of effective refractory period to AP duration at 90% of repolarization, permitting re-excitation before full repolarization, reduction of AP upstroke velocity, and likely promotion of slow conduction., Conclusion: Using an objective measure of terminal repolarization, we conclude that I K1 is the major determinant of the terminal repolarization time course. Moreover, suppression of I K1 prolongs repolarization and reduces postrepolarization refractoriness without marked effects on the overall AP duration. Collectively, these findings demonstrate how I K1 suppression may contribute to arrhythmogenesis in the failing heart., (Published by Elsevier Inc.)

Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM) hold promise for assessment of drug-induced arrhythmias and are being considered for use under the comprehensive in vitro proarrhythmia assay (CiPA). We studied the effects of 26 drugs and 3 drug combinations on 2 commercially available iPSC-CM types using high-throughput voltage-sensitive dye and microelectrode-array assays being studied for the CiPA initiative and compared the results with clinical QT prolongation and torsade de pointes (TdP) risk. Concentration-dependent analysis comparing iPSC-CMs to clinical trial results demonstrated good correlation between drug-induced rate-corrected action potential duration and field potential duration (APDc and FPDc) prolongation and clinical trial QTc prolongation. Of 20 drugs studied that exhibit clinical QTc prolongation, 17 caused APDc prolongation (16 in Cor.4U and 13 in iCell cardiomyocytes) and 16 caused FPDc prolongation (16 in Cor.4U and 10 in iCell cardiomyocytes). Of 14 drugs that cause TdP, arrhythmias occurred with 10 drugs. Lack of arrhythmic beating in iPSC-CMs for the four remaining drugs could be due to differences in relative levels of expression of individual ion channels. iPSC-CMs responded consistently to human ether-a-go-go potassium channel blocking drugs (APD prolongation and arrhythmias) and calcium channel blocking drugs (APD shortening and prevention of arrhythmias), with a more variable response to late sodium current blocking drugs. Current results confirm the potential of iPSC-CMs for proarrhythmia prediction under CiPA, where iPSC-CM results would serve as a check to ion channel and in silico modeling prediction of proarrhythmic risk. A multi-site validation study is warranted., (Published by Oxford University Press on behalf of the Society of Toxicology 2016. This work is written by US Government employees and is in the public domain in the US.)

Application of electrical field to the heart during the refractory period of the beat has been shown to increase the force of contraction both in animal models and in heart failure patients (cardiac contractility modulation, or CCM). A direct increase in intracellular calcium during CCM has been suggested to be the mechanism behind the positive inotropic effect of CCM. We studied the effect of CCM on isolated rabbit cardiomyocytes and perfused whole rat hearts. The effect of CCM was observed in single cells via fluorescent measurements of intracellular calcium concentration ([Ca(2+)]i) and cell length (L). Cells were paced once per second throughout these recordings, and CCM stimulation was delivered via biphasic electric fields of 20 ms duration applied during the refractory period. CCM increased the peak amplitude of both [Ca(2+)]i and L for the first beat during CCM compared to control, but then [Ca(2+)]i and L decayed to levels lower than the control. During CCM, all contractions had a faster time to peak for both [Ca(2+)]i and L; after stopping CCM the rise times returned to control levels. In the whole rat heart, the positive inotropic effect of CCM stimulation on left ventricular pressure was completely abolished in the presence of metoprolol, a beta-1 adrenergic blocker. In summary, the CCM-induced changes in intracellular calcium handling by cardiomyocytes did not explain the sustained positive inotropic effect in the whole heart and the β-adrenergic pathway may be involved in the CCM mechanism of action., (Published 2014. This article is a U.S. Government work and is in the public domain in the USA. Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society.)

The innovative arbitrary waveform defibrillator for animal research presented in this paper is based on two power linear amplifiers in bridge configuration. It is capable of delivering 10 J shocks of arbitrary shape and duration. The system can be used to test new waveforms by comparing them to traditional ones, in in-vitro experiments. The system is battery operated, has an isolated output, and is PC controlled. Loads with impedance ranging from 10 to 25 ohms can be connected. A maximum of +/-130 V, 10A can be delivered to the loads. Effective voltage and current are measured and collected in the PC. Examples of waveforms as well as preliminary results from experiments of isolated rabbit hearts are presented.

Female rats were treated with the nonsteroidal estrogen antagonist clomiphene citrate for the first 5 days postpartum. The expectation was that clomiphene would influence the sexual development of these animals as is the case with sexually immature animals treated with steroidal compounds. Analysis showed that treatment with clomiphene completely disrupted the adult sexual behavior of the female. In five consecutive days of testing the experimental females were never observed to mate. They were mounted significantly less often than controls, suggesting that the males were less attracted to them than to normal females. The initial suggestion that clomiphene might affect the sexual behavior of females has been supported; however, further research must be done to identify the underlying mechanism of the change.