This is the first study to demonstrate differential responses of LQTS patients to interventions targeted to their specific genetic defect. These findings also suggest that LQT3 patients may be more likely to benefit from Na+ channel blockers and from cardiac pacing because they would be at higher risk of arrhythmia at slow heart rates. Conversely, LQT2 patients may be at higher risk to develop syncope under stressful conditions because of the combined arrhythmogenic effect of catecholamines with the insufficient adaptation of their QT interval when heart rate increases.

Background-The current proarrhythmia safety testing paradigm, although highly efficient in preventing new torsadogenic drugs from entering the market, has important limitations that can restrict the development and use of valuable new therapeutics. The CiPA (Comprehensive in vitro Proarrhythmia Assay) proposes to overcome these limitations by evaluating drug effects on multiple cardiac ion channels in vitro and using these data in a predictive in silico model of the adult human ventricular myocyte. A set of drugs with known clinical torsade de pointes risk was selected to develop and calibrate the in silico model. Methods and Results-Manual patch-clamp data assessing drug effects on expressed cardiac ion channels were integrated into the O'Hara-Rudy myocyte model modified to include dynamic drug-hERG channel (human Ether-à-go-go-Related Gene) interactions. Together with multichannel pharmacology data, this model predicts that compounds with high torsadogenic risk are more likely to be trapped within the hERG channel and show stronger reverse use dependency of action potential prolongation. Furthermore, drug-induced changes in the amount of electronic charge carried by the late sodium and L-type calcium currents was evaluated as a potential metric for assigning torsadogenic risk. Conclusions-Modeling dynamic drug-hERG channel interactions and multi-ion channel pharmacology improves the prediction of torsadogenic risk. With further development, these methods have the potential to improve the regulatory assessment of drug safety models under the CiPA paradigm. (Circ Arrhythm Electrophysiol. 2017;10:e004628.

The implementation of the ICH S7B and E14 guidelines has been successful in preventing the introduction of potentially torsadogenic drugs to the market, but it has also unduly constrained drug development by focusing on hERG block and QT prolongation as essential determinants of proarrhythmia risk. The Comprehensive in Vitro Proarrhythmia Assay (CiPA) initiative was established to develop a new paradigm for assessing proarrhythmic risk, building on the emergence of new technologies and an expanded understanding of torsadogenic mechanisms beyond hERG block. An international multi-disciplinary team of regulatory, industry and academic scientists are working together to develop and validate a set of predominantly nonclinical assays and methods that eliminate the need for the thorough-QT study and enable a more precise prediction of clinical proarrhythmia risk. The CiPA effort is led by a Steering Team that provides guidance, expertise and oversight to the various working groups and includes partners from US FDA, HESI, CSRC, SPS, EMA, Health Canada, Japan NIHS, and PMDA. The working groups address the three pillars of CiPA that evaluate drug effects on: 1) human ventricular ionic channel currents in heterologous expression systems, 2) in silico integration of cellular electrophysiologic effects based on ionic current effects, the ion channel effects, and 3) fully integrated biological systems (stem-cell-derived cardiac myocytes and the human ECG). This article provides an update on the progress of the initiative towards its target date of December 2017 for completing validation.

For the past decade, cardiac safety screening to evaluate the propensity of drugs to produce QT interval prolongation and Torsades de Pointes (TdP) arrhythmia has been conducted according to ICH S7B and ICH E14 guidelines. Central to the existing approach are hERG channel assays and in vivo QT measurements. Although effective, the present paradigm carries a risk of unnecessary compound attrition and high cost, especially when considering costly thorough QT (TQT) studies conducted later in drug development. The Comprehensive In Vitro Proarrhythmia Assay (CiPA) initiative is a publicprivate collaboration with the aim of updating the existing cardiac safety testing paradigm to better evaluate arrhythmia risk and remove the need for TQT studies. It is hoped that CiPA will produce a standardized ion channel assay approach, incorporating defined tests against major cardiac ion channels, the results of which then inform evaluation of proarrhythmic actions in silico, using human ventricular action potential reconstructions. Results are then to be confirmed using human (stem cell-derived) cardiomyocytes. This perspective article reviews the rationale, progress of, and challenges for the CiPA initiative, if this new paradigm is to replace existing practice and, in time, lead to improved and widely accepted cardiac safety testing guidelines.

The safe disposal of unused opioid drugs is an area of regulatory concern. While toilet flushing is recommended for some drugs to prevent accidental exposure, there is a need for data that can support a more consistent disposal policy based on an assessment of relative risk. For drugs acting at the Mu-opioid receptor (MOR), published measurements of binding affinity (K(i)) are incomplete and inconsistent due to differences in methodology and assay system, leading to a wide range of values for the same drug thus precluding a simple and meaningful relative ranking of drug potency. Experiments were conducted to obtain K(i)'s for 19 approved opioid drugs using a single binding assay in a cell membrane preparation expressing recombinant human MOR. The K(i) values obtained ranged from 0.1380 nM (sufentanil) to 12.486 μM (tramadol). The drugs were separated into three categories based upon their K(i) values: K(i)> 100 nM (tramadol, codeine, meperidine, propoxyphene and pentazocine), K(i)=1-100 nM (hydrocodone, oxycodone, diphenoxylate, alfentanil, methadone, nalbuphine, fentanyl and morphine) and K(i) < 1 nM (butorphanol, levorphanol, oxymorphone, hydromorphone, buprenorphine and sufentanil). These data add to the understanding of the pharmacology of opioid drugs and support the development of a more consistent labeling policies regarding safe disposal.

Drug-induced Torsade-de-Pointes (TdP) has been responsible for the withdrawal of many drugs from the market and is therefore of major concern to global regulatory agencies and the pharmaceutical industry. The Comprehensive in vitro Proarrhythmia Assay (CiPA) was proposed to improve prediction of TdP risk, using in silico models and in vitro multi-channel pharmacology data as integral parts of this initiative. Previously, we reported that combining dynamic interactions between drugs and the rapid delayed rectifier potassium current (IKr) with multi-channel pharmacology is important for TdP risk classification, and we modified the original O'Hara Rudy ventricular cell mathematical model to include a Markov model of IKr to represent dynamic drug-IKr interactions (IKr-dynamic ORd model). We also developed a novel metric that could separate drugs with different TdP liabilities at high concentrations based on total electronic charge carried by the major inward ionic currents during the action potential. In this study, we further optimized the IKr-dynamic ORd model by refining model parameters using published human cardiomyocyte experimental data under control and drug block conditions. Using this optimized model and manual patch clamp data, we developed an updated version of the metric that quantifies the net electronic charge carried by major inward and outward ionic currents during the steady state action potential, which could classify the level of drug-induced TdP risk across a wide range of concentrations and pacing rates. We also established a framework to quantitatively evaluate a system's robustness against the induction of early afterdepolarizations (EADs), and demonstrated that the new metric is correlated with the cell's robustness to the pro-EAD perturbation of IKr conductance reduction. In summary, in this work we present an optimized model that is more consistent with experimental data, an improved metric that can classify drugs at concentrations both near and higher than clinical exposure, and a physiological framework to check the relationship between a metric and EAD. These findings provide a solid foundation for using in silico models for the regulatory assessment of TdP risk under the CiPA paradigm.

SUMMARY The effects of lidocaine and quinidine on rabbit cardiac Purkinje fibers were compared at 37 °C, using action potential and voltage clamp measurements. At therapeutic concentrations (5 /ig/ml), lidocaine shortened the duration of the action potential while quinidine lengthened it. When membrane potential was held constant between -40 and -50 mV by the application of the two-microelectrode voltage clamp, holding current consistently became more outward with lidocaine (+2.3 ± 0.7 nA, mean ± SEM, n = 5), but it either did not change or became more inward with quinidine (-2.1 ± 1.4 nA, n = 5). Both drugs reduced the outward tails generally associated with deactivation of the delayed rectifier current, i x . The depression of delayed rectification by quinidine (-68 ± 6%, n = 5) was greater than that produced by lidocaine (-12 ± 4%, n = 5), and can explain the observed prolongation of the action potential. To evaluate the possibility that lidocaine blocks steady state sodium channels, experiments were performed with tetrodotoxin (TTX), a specific blocker of sodium channels in a variety of excitable membranes. TTX at concentrations of 0.1-5 /tM shortened the action potential, reducing its duration to 50% of control at 1.6 JIM. Voltage clamp experiments revealed a small TTX-sensitive component of steady state current flowing at membrane potentials positive to -80 mV. In the presence of 10 /IM TTX, lidocaine failed to produce additional steady outward membrane current. These data suggest that (1) lidocaine and quinidine differ substantially in their modification of membrane channels, with quinidine having a strong effect on delayed rectification, (2) a TTX-sensitive "window" current exists in the rabbit Purkinje fiber and helps maintain the action potential plateau, and (3) the effect of lidocaine on the cardiac action potential results primarily from block of TTX-sensitive sodium channels, rather than from an enhancement of potassium conductance. Circ Res 50: 17-27, 1982 LIDOCAINE and quinidine are used widely in the clinical management of ventricular arrhythmias. However, the mechanisms by which these agents exert their therapeutic effects remain unclear (for a recent review, see Hauswirth and Singh, 1979). Available experimental evidence suggests that both lidocaine and quinidine depress the excitatory sodium current underlying the action potential upstroke and delay its reactivation during diastole (Ducouret, 1976;Hondeghem and Katzung, 1977;Lee et al., 1981). Additional effects on the cardiac membrane have also been indicated. At clinically relevant concentrations (2-5 jig/ml), lidocaine is known to shorten the cardiac action potential (Davis and Temte, 1969;Bigger and Mandel, 1970), whereas quinidine generally produces a lengthening September 17, 1981. or no change (Vaughan Williams, 1958;Hoffman, 1958;Mirro et al., 1981). The basis for the quinidine effect on action potential duration has not been defined, whereas an increase of potassium conductance has been proposed to mediate the action of lidocaine...

The Comprehensive in vitro Proarrhythmia Assay (CiPA) is a global initiative intended to improve drug proarrhythmia risk assessment using a new paradigm of mechanistic assays. Under the CiPA paradigm, the relative risk of drug-induced Torsade de Pointes (TdP) is assessed using an in silico model of the human ventricular action potential (AP) that integrates in vitro pharmacology data from multiple ion channels. Thus, modeling predictions of cardiac risk liability will depend critically on the variability in pharmacology data, and uncertainty quantification (UQ) must comprise an essential component of the in silico assay. This study explores UQ methods that may be incorporated into the CiPA framework. Recently, we proposed a promising in silico TdP risk metric (qNet), which is derived from AP simulations and allows separation of a set of CiPA training compounds into Low, Intermediate, and High TdP risk categories. The purpose of this study was to use UQ to evaluate the robustness of TdP risk separation by qNet. Uncertainty in the model parameters used to describe drug binding and ionic current block was estimated using the non-parametric bootstrap method and a Bayesian inference approach. Uncertainty was then propagated through AP simulations to quantify uncertainty in qNet for each drug. UQ revealed lower uncertainty and more accurate TdP risk stratification by qNet when simulations were run at concentrations below 5× the maximum therapeutic exposure (Cmax). However, when drug effects were extrapolated above 10× Cmax, UQ showed that qNet could no longer clearly separate drugs by TdP risk. This was because for most of the pharmacology data, the amount of current block measured was <60%, preventing reliable estimation of IC50-values. The results of this study demonstrate that the accuracy of TdP risk prediction depends both on the intrinsic variability in ion channel pharmacology data as well as on experimental design considerations that preclude an accurate determination of drug IC50-values in vitro. Thus, we demonstrate that UQ provides valuable information about in silico modeling predictions that can inform future proarrhythmic risk evaluation of drugs under the CiPA paradigm.