A Straightforward Guide to the Basic Science Behind Arrhythmogenesis

This article describes basic mechanisms behind cardiac arrhythmia. In clinical practice significant arrhythmias are mostly unpredictable, and under some conditions, potentially life-threatening. The question is how basic science can help in understanding better the molecular entities and factors behind the arrhythmia to better develop, adapt or use available medications. Structural heart disease and remodeling (e.g., heart failure, cardiomyopathy), the presence of modulating factors (i.e., diabetes mellitus, autonomic nervous system), genetic predispositions (i.e, channelopathies) considerably precondition, influence the development of an arrhythmia. Cardiac arrhythmia may indeed share common basic mechanisms, while elements and substrates perpetuating these may be different and ultimately manifest as various ECG abnormalities. This article lists cellular and subcellular iatrogenic disorders responsible for abnormal impulse generation or conduction disturbances including latest development in theories and biology research for a better understanding of cellular disorders behind arrhythmogenesi

Oscillations of Cardiac Wave Length and Proarrhythmia

Purpose: Drug-induced APD prolongation was first proposed to be antiarrhythmic, but is now widely presumed to be torsadogenic. To elucidate this paradox we tested the effect of APD upon liability for torsade de pointes. In addition, torsadogenicity is commonly associated with disturbances of repolarization, but at least in theory, it could also result from disturbances of conduction. Methods: These possibilities were tested in female rabbit hearts. Dofetilide, ATX II and sodium channel blockers that did not prolong the action potential duration were used to modulate the APD and induce disturbances of conduction and disturbances of repolarization. Results: Torsadogenicity could be induced by dofetilide and ATX II starting at normal APD (210 ms), reaching a peak incidence around a doubling of APD (400 and 450 ms), to then sharply decline with further APD prolongation, until TdP disappeared above 725 ms. Early afterdepolarizations (EAD) were regular triggers for torsade de pointes; while most of the EADs occurred in the plateau range, their incidence declined with repolarization but their potential for torsadogenicity increased. Sodium channel blockers that shorten the APD, even when devoid of hERG blocking properties, can yield torsade de pointes. Conclusions: Torsade de pointes can occur at normal, prolonged and shortened APD, so that QT prolongation is an incomplete predictor of torsadogenicity. Torsade de pointes cannot only result from disturbances of repolarization, but also from disturbances of conduction

Analysis of unipolar electrograms in rabbit heart demonstrated the key role of ventricular apicobasal dispersion in arrhythmogenicity

INTRODUCTION: Reduced repolarization reserve and increased transmural dispersion of repolarization (TDR) are known risk factors for Torsade de Pointes (TdP) development, but less is known about the role of apex-to-base (apicobasal) repolarization in arrhythmogenesis. METHODS: Three needles were inserted in rabbit left ventricle to record unipolar electrograms from endocardium to epicardium and base to apex. Total repolarization interval (TRI) and peak-to-end repolarization interval (Tp) were assessed after quinidine (n=6) and d,l-sotalol (n=6) perfusion in combination with the IKs inhibitor chromanol 293B. RESULTS: 30 µM d,l-sotalol increased TRI and Tp more at the base (TRI +40 ± 4 %; Tp +89 ± 11 %) relative to the apex (TRI +28 ± 3 %, Tp +30 ± 8 %). Similar results were obtained with quinidine: TRI and Tp increased more at the base compared to the apex. No significant differences were recorded from the endocardium to the epicardium. DISCUSSION: Our results show that combined IKr + IKs block prolonged TRI and Tp significantly more at the ventricular base than at the apex, in the absence of transmural dispersion of refractoriness. Regional changes in TRI and Tp indicate the contribution of apicobasal dispersion to arrhythmogenicity compared to TDR in a rabbit heart model

Cardiotoxic Potential of Hydroxychloroquine, Chloroquine and Azithromycin in adult Human Primary Cardiomyocytes.

Background Clinical communications regarding the efficacy and cardiac safety of Hydroxychloroquine (HCQ), Chloroquine (CQ) alone or in combination with Azithromycin (AZ) in patients with Covid-19 provided conflicting evidence. Although these drugs in general have a good safety record, however, if this is true for patients with COVID-19 is unknown. . Methods We studied both pro-arrhythmic potential as well as inotropic effect of HCQ, CQ and AZ in paced adult human primary cardiomyocytes in vitro, assessing drug concentrations, the effects of electrolyte changes and elevated temperature that are prevalent risk factors among hospitalized COVID-19 patients. Results Concentration-dependent pro-arrhythmia and negative inotropic potential risks of HCQ started within the therapeutic free exposure range (0.1 - 0.3 µM) and were found to be less pronounced compared to CQ. AZ co-administration with HCQ not only altered the pro-arrhythmia profile of HCQ, but also attenuated the negative inotropic effect of HCQ due to mechanisms that need further evaluation. Hypokalemia caused significant indices of pro-arrhythmia at the lower limit of HCQ therapeutic exposure tested, but exposure to high level of Mg2+ significantly reduced all markers of pro-arrhythmia associated with HCQ treatment. When cardiomyocytes were subjected to elevated temperature pro-arrhythmia was observed that was not increased by therapeutic exposure levels of HCQ. Conclusions Our data indicate that high exposures levels of HCQ should be avoided. The clinical environment (e.g., elevated temperature, electrolyte changes) associated with severe COVID-19 modulates cardiotoxicity, reinforces the clinical advice to maintain high normal levels of K+ and Mg2+. These results emphasizes the need to assess drugs under the disease-specific conditions

DISTURBANCES OF λ-TRIAD ARE MORE PREDICTIVE FOR PROARRHTHMIA THAN QT PROLONGATION: A CASE STUDY OF TERFENADINE

BACKGROUND – Terfenadine was withdrawn because of proarrhythmia. Its therapeutic anti-histamine concentration is normally below 1 nM, while IKr and INa block have IC50 > 200 nM. Thus, IKr block singly is an unlikely proarrhythmic cause. METHODS AND RESULTS – Rabbit hearts were perfused with 1-10,000 nM terfenadine for 10-450 min. Terfenadine (1 nM, 450 min) insignificantly shortened APD60; at 10 nM APD60 prolonged 46±11 ms (P < 0.05; n = 6), but 1 hour washout further prolonged APD60. Above 30 nM, APD60 shortening was followed by prolongation (23±7 ms; 300 nM;10 min; P < 0.05; n = 23). In the µM range, APD60 declined and conduction slowed (14±2%; 3 µM; 10 min; P < 0.05; n = 24). Terfenadine disturbed repolarization (TRIaD: triangulation, reverse use depen-dence, instability and dispersion) from 1 to 1000 nM, increasing with concentration (450 min: 1 nM yields 50%, 10 nM 100%) and exposure time (100 nM: 10 min yields 16%, 30 min yields 33%, 90 min 83%). TRIaD with APD prolongation yielded 2 TdP; TRIaD with APD shortening yielded 7 VT and 5 VF. CONCLUSIONS –In the low nM and µM range TRIaD is accompanied by little change or shortening of APD; in the mid nM range APD shortening is followed by prolongation. TRIaD with APD prolongation preferentially induces TdP, but VT/VF in its absence. In patients ter-fenadine causes normally little QT prolongation and according to FDA records, VT/VF ap-pears indeed the primary tachycardia. Thus, TRIaD predicts proarrhythmia, while QT diffe-rentiates between VT/VF or TdP

Secondary Pharmacology: Screening and Interpretation of Off-target Activities – Focus on translation

Secondary pharmacology has become an essential tool for the drug discovery process during the past decade and is used extensively in the pharmaceutical industry for off-target mitigation particularly during the lead optimization phase. This was achieved by increasing the translational value of the approach, based on the recognition of biological target – drug molecule – adverse drug reaction (ADR) associations and integration of the secondary pharmacology data with pharmacokinetic parameters. Accumulation of information obtained from reverse translation of clinical ADRs, from recognition of specific phenotypes of target-based animal models and from hereditary diseases provides increasing regulatory confidence in the target-based approach to side effect prediction and mitigation. Here we review the progress of secondary pharmacology during the past decade, highlight and demonstrate its applications and impact in drug discovery

Blinded contractility analysis in hiPSC-cardiomyocytes in engineered heart tissue format: Comparison with human atrial trabeculae

Objective: Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) may serve as a new assay for drug testing in a human context, but their validity particularly for the evaluation of inotropic drug effects remains unclear. In this blinded analysis, we compared the effects of 10 compounds with known inotropic effects in electrically stimulated (1.5 Hz) hiPSC-CM-derived 3 dimensional engineered heart tissue (EHT) and human atrial trabeculae (hAT) as reference. Methods and results: Human EHTs were prepared from iCell® hiPSC-CM, hAT obtained at routine heart surgery. Variation coefficient in force measurement was 17% for EHT and 49% for hAT. The PDE-inhibitors milrinone and rolipram did not have any effect on EHT contractility, but a positive inotropic effect in hAT for milrinone and decrease in force for rolipram due to run-down phenomena. Citalopram (SSRI), nifedipine (LTCC-blocker) and lidocaine (Na+ channel-blocker) had negative inotropic effects on EHT and hAT. Formoterol (beta-2-agonist) had no effect in EHT, but positive inotropic effects in hAT. Tacrolimus (calcineurin-inhibitor) had a negative inotropic effect in EHTs and hAT. Digoxin (Na+-K+-ATPase-inhibitor) showed a positive inotropic effect only in EHTs, but no effect in hAT due to short incubation time. Acetylsalicylic acid (COX-inhibitor) had no effect in EHT, but hAT force decreased due to run-down. Ryanodine (ryanodine receptor-inhibitor) reduced contraction force in both models. Conclusions: hiPSC-EHTs were more stable over time and less variable than hAT. They faithfully detected cAMP-dependent and independent positive and negative inotropic effects, but no beta-2 adrenergic or PDE3 effects, compatible with an immature CM phenotype

Conformational Refinement of Hydroxamate-Based Histone Deacetylase Inhibitors and Exploration of 3-Piperidin-3-yl-indole Analogues of Dacinostat (LAQ824)

Inspired by natural product HDAC inhibitors, we prepared a series of conformationally restrained HDAC inhibitors based on the hydroxamic acid dacinostat (LAQ824, 7). Several scaffolds with improved biochemical and cellular potency, as well as attenuated hERG inhibition, were identified, suggesting that the introduction of molecular rigidity is a viable strategy to enhance HDAC binding and mitigate hERG liability. Further SAR studies around a 3-piperidin-3-ylindole moiety resulted in the discovery of compound 30, for which a unique conformation was speculated to contribute to overcoming increased lipophilicity and attenuating hERG binding. Separation of racemate 30 afforded 32, the R enantiomer, which demonstrated improved potency in both enzyme and cellular assays compared to dacinostat

Optimization of a Class of Dihydrobenzofurane Analogs Toward Orally Efficacious YAP-TEAD Protein-Protein Interaction Inhibitors

The inhibition of the YAP-TEAD protein-protein interaction constitutes a promising therapeutic approach for the treatment of cancers linked to the dysregulation of the Hippo signaling pathway. The identification of a class of small molecules which potently inhibit the YAP-TEAD interaction by binding tightly to the Ω-loop pocket of TEAD has previously been communicated. This report details the further multi-parameter optimization of this class of compounds resulting in advanced analogs combining nanomolar cellular potency with a balanced ADME and off-target profile, and efficacy of these compounds in tumor bearing mice is demonstrated for the first time