Late sodium current inhibitors to treat exercise-induced obstruction in hypertrophic cardiomyopathy: an in vitro study in human myocardium

From Atrial Fibrillation Management to Atrial Myopathy Assessment: The Evolving Concept of Left Atrium Disease in Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most prevalent genetically inherited cardiovascular disorder in adults and a significant cause of heart failure and sudden cardiac death. Historically, atrial fibrillation (AF) has been considered as a critical aspect in HCM patients as it is considered to be a marker of disease progression, escalates the frequency of heart failure hospitalisations, increases the risk of thromboembolic events, and worsens quality of life and outcome. Increasing evidence suggests that AF is the result of a subtle long-standing process that starts early in the history of HCM. The process of left atrial dilation accompanied by morphologic and functional remodelling is the quintessential prerequisite for the onset of AF. This review aims to describe the current understanding of AF pathophysiology in HCM, emphasising the role of left atrial myopathy in its development. In addition, we discuss risk factors and management strategies specific to AF in the context of HCM, providing insights into the complexities and challenges of treating this specific patient population.

Mechanism of Ion Channel Impairment in the Occurrence of Arrhythmia in Patients with Hypertrophic Cardiomyopathy

Sudden cardiac death is the most unpredictable and devastating consequence of hypertrophic cardiomyopathy, most often caused by persistent ventricular tachycardia or ventricular fibrillation. Although myocardial hypertrophy, fibrosis, and microvascular disorders are the main mechanisms of persistent reentrant ventricular arrhythmias in patients with advanced hypertrophic cardiomyopathy, the cardiomyocyte mechanism based on ion channel abnormalities may play an important role in the early stages of the disease.

Inflammation across the spectrum of hypertrophic cardiac phenotypes

The hypertrophic cardiomyopathy phenotype encompasses a heterogeneous spectrum of genetic and acquired diseases characterized by the presence of left ventricular hypertrophy in the absence of abnormal cardiac loading conditions. This "umbrella diagnosis" includes the "classic" hypertrophic cardiomyopathy (HCM), due to sarcomere protein gene mutations, and its phenocopies caused by intra- or extracellular deposits, such as Fabry disease (FD) and cardiac amyloidosis (CA). All these conditions share a wide phenotypic variability which results from the combination of genetic and environmental factors and whose pathogenic mediators are poorly understood so far. Accumulating evidence suggests that inflammation plays a critical role in a broad spectrum of cardiovascular conditions, including cardiomyopathies. Indeed, inflammation can trigger molecular pathways which contribute to cardiomyocyte hypertrophy and dysfunction, extracellular matrix accumulation, and microvascular dysfunction. Growing evidence suggests that systemic inflammation is a possible key pathophysiologic process potentially involved in the pathogenesis of cardiac disease progression, influencing the severity of the phenotype and clinical outcome, including heart failure. In this review, we summarize current knowledge regarding the prevalence, clinical significance, and potential therapeutic implications of inflammation in HCM and two of its most important phenocopies, FD and CA.

Transcriptional Dysregulation Underlies Both Monogenic Arrhythmia Syndrome and Common Modifiers of Cardiac Repolarization

BACKGROUND

Brugada syndrome (BrS) is an inherited arrhythmia syndrome caused by loss-of-function variants in the cardiac sodium channel gene SCN5A (sodium voltage-gated channel alpha subunit 5) in ≈20% of subjects. We identified a family with 4 individuals diagnosed with BrS harboring the rare G145R missense variant in the cardiac transcription factor TBX5 (T-box transcription factor 5) and no SCN5A variant.

METHODS

We generated induced pluripotent stem cells (iPSCs) from 2 members of a family carrying TBX5-G145R and diagnosed with Brugada syndrome. After differentiation to iPSC-derived cardiomyocytes (iPSC-CMs), electrophysiologic characteristics were assessed by voltage- and current-clamp experiments (n=9 to 21 cells per group) and transcriptional differences by RNA sequencing (n=3 samples per group), and compared with iPSC-CMs in which G145R was corrected by CRISPR/Cas9 approaches. The role of platelet-derived growth factor (PDGF)/phosphoinositide 3-kinase (PI3K) pathway was elucidated by small molecule perturbation. The rate-corrected QT (QTc) interval association with serum PDGF was tested in the Framingham Heart Study cohort (n=1893 individuals).

RESULTS

TBX5-G145R reduced transcriptional activity and caused multiple electrophysiologic abnormalities, including decreased peak and enhanced "late" cardiac sodium current (INa), which were entirely corrected by editing G145R to wild-type. Transcriptional profiling and functional assays in genome-unedited and -edited iPSC-CMs showed direct SCN5A down-regulation caused decreased peak INa, and that reduced PDGF receptor (PDGFRA [platelet-derived growth factor receptor α]) expression and blunted signal transduction to PI3K was implicated in enhanced late INa. Tbx5 regulation of the PDGF axis increased arrhythmia risk due to disruption of PDGF signaling and was conserved in murine model systems. PDGF receptor blockade markedly prolonged normal iPSC-CM action potentials and plasma levels of PDGF in the Framingham Heart Study were inversely correlated with the QTc interval (P<0.001).

CONCLUSIONS

These results not only establish decreased SCN5A transcription by the TBX5 variant as a cause of BrS, but also reveal a new general transcriptional mechanism of arrhythmogenesis of enhanced late sodium current caused by reduced PDGF receptor-mediated PI3K signaling.

Multi-modality management of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is an autosomal dominant inherited condition defined by left ventricular wall thickness greater than 15 mm in the absence of other conditions that could explain that degree of hypertrophy. Obstructive HCM associated with left ventricular outflow tract obstruction is defined by an intraventricular systolic pressure gradient greater than or equal to 30 mm Hg. Over the past couple of decades, there has been an expansion of both invasive and pharmacotherapeutic options for patients with HCM, with recent guidelines calling for a melody of invasive and non-invasive treatment strategies. There are several invasive therapies including proven therapies such as alcohol septal ablation and septal myectomy. Novel invasive therapies such as MitraClip, radiofrequency septal ablation and SESAME procedure have more recently been promoted. Pharmacological therapy has also dramatically evolved and includes conventional medications such as beta-blockers, calcium channel blockers, and disopyramide. Mavacamten, a novel cardiac myosin inhibitor, may significantly change management. Other myosin inhibitors and modulators are also being developed and tested in large clinical trials. Given significant phenotypical variability in patients with HCM, clinical management can be challenging, and often requires an individualized approach with a combination of invasive and non-invasive options.

Current and emerging perspectives on pathophysiology, diagnosis, and management of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most common genetically inherited cardiomyopathy with an autosomal dominant inheritance pattern. A disease-causing gene is found between 34% and >60% of the times and the two most frequently mutated genes, which encode sarcomeric proteins, are MYBPC3 and MYH7. HCM is a diagnosis of exclusion since secondary causes of left ventricular hypertrophy should first be ruled out. These include hypertension, aortic stenosis, infiltrative disease, metabolic and endocrine disorders, mitochondrial cardiomyopathies, neuromuscular disorders, malformation syndromes and some chronic drug use. The disease is characterized by great heterogeneity of its clinical manifestations, however diastolic dysfunction and increased ventricular arrhythmogenesis are commonly seen. Current HCM therapies focus on symptom management and prevention of sudden cardiac death. Symptom management includes the use of pharmacological agents, elimination of medication promoting outflow track obstruction, control of comorbid conditions and invasive procedures, whereas in the prevention of sudden cardiac death, implantable cardiac defibrillators and antiarrhythmic drugs are used. A targeted therapy for LVOTO represented by allosteric cardiac myosin inhibitors has been developed. In terms of sport participation, a more liberal approach is recently recommended, after careful evaluation and common-shared decision. The application of the current therapies has lowered HCM mortality rates to <1.0%/year, however it appears to have shifted focus to heart failure and atrial fibrillation, as the predominant causes of disease-related morbidity and mortality and, therefore, unmet treatment need. With improved understanding of the genetic and molecular basis of HCM, the present decade will witness novel treatments for disease prevention and modification.

Remodelling of potassium currents underlies arrhythmic action potential prolongation under beta-adrenergic stimulation in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) patients often present an enhanced arrhythmogenicity that can lead to lethal arrhythmias, especially during exercise. Recent studies have indicated an abnormal response of HCM cardiomyocytes to β-adrenergic receptor stimulation (β-ARS), with prolongation of their action potential rather than shortening. The mechanisms underlying this aberrant response to sympathetic stimulation and its possible proarrhythmic role remain unknown. The aims of this study are to investigate the key ionic mechanisms underlying the HCM abnormal response to β-ARS and the resultant repolarisation abnormalities using human-based experimental and computational methodologies. We integrated and calibrated the latest models of human ventricular electrophysiology and β-ARS using experimental measurements of human adult cardiomyocytes from control and HCM patients. Our major findings include: (1) the developed in silico models of β-ARS capture the behaviour observed in the experimental data, including the aberrant response of HCM cardiomyocytes to β-ARS; (2) the reduced increase of potassium currents under β-ARS was identified as the main mechanism of action potential prolongation in HCM, rather than a more sustained inward calcium current; (3) action potential duration differences between healthy and HCM cardiomyocytes were increased upon β-ARS, while endocardial to epicardial differences in HCM cardiomyocytes were reduced; (4) models presenting repolarisation abnormalities were characterised by downregulation of the rapid delayed rectifier potassium current and the sodium‑potassium pump, while inward currents were upregulated. In conclusion, our results identify causal relationships between the HCM phenotype and its arrhythmogenic response to β-ARS through the downregulation of potassium currents.

Hypertrophic cardiomyopathy: an up-to-date snapshot of the clinical drug development pipeline

INTRODUCTION

Hypertrophic cardiomyopathy (HCM) is a complex cardiac disease with highly variable phenotypic expression and clinical course most often caused by sarcomeric gene mutations resulting in left ventricular hypertrophy, fibrosis, hypercontractility, and diastolic dysfunction. For almost 60 years, HCM has remained an orphan disease and still lacks a disease-specific treatment.

AREAS COVERED

This review summarizes recent preclinical and clinical trials with repurposed drugs and new emerging pharmacological and gene-based therapies for the treatment of HCM.

EXPERT OPINION

The off-label drugs routinely used alleviate symptoms but do not target the core pathophysiology of HCM or prevent or revert the phenotype. Recent advances in the genetics and pathophysiology of HCM led to the development of cardiac myosin adenosine triphosphatase inhibitors specifically directed to counteract the hypercontractility associated with HCM-causing mutations. Mavacamten, the first drug specifically developed for HCM successfully tested in a phase 3 trial, represents the major advance for the treatment of HCM. This opens new horizons for the development of novel drugs targeting HCM molecular substrates which hopefully modify the natural history of the disease. The role of current drugs in development and genetic-based approaches for the treatment of HCM are also discussed.

Ventricular arrhythmia and sudden cardiac death in hypertrophic cardiomyopathy: From bench to bedside

Patients with hypertrophic cardiomyopathy (HCM) mostly experience minimal symptoms throughout their lifetime, and some individuals have an increased risk of ventricular arrhythmias and sudden cardiac death (SCD). How to identify patients with a higher risk of ventricular arrythmias and SCD is the priority in HCM research. The American College of Cardiology/American Heart Association (ACC/AHA) and the European Society of Cardiology (ESC) both recommend the use of risk algorithms to identify patients at high risk of ventricular arrhythmias, to be selected for implantation of implantable cardioverters/defibrillators (ICDs) for primary prevention of SCD, although major discrepancies exist. The present SCD risk scoring systems cannot accurately identify early-stage HCM patients with modest structural remodeling and mild disease manifestations. Unfortunately, SCD events could occur in young asymptomatic HCM patients and even as initial symptoms, prompting the determination of new risk factors for SCD. This review summarizes the studies based on patients' surgical specimens, transgenic animals, and patient-derived induced pluripotent stem cells (hiPSCs) to explore the possible molecular mechanism of ventricular arrhythmia and SCD. Ion channel remodeling, Ca2+ homeostasis abnormalities, and increased myofilament Ca2+ sensitivity may contribute to changes in action potential duration (APD), reentry circuit formation, and trigger activities, such as early aferdepolarization (EAD) or delayed afterdepolarization (DAD), leading to ventricular arrhythmia in HCM. Besides the ICD implantation, novel drugs represented by the late sodium current channel inhibitor and myosin inhibitor also shed light on the prevention of HCM-related arrhythmias. The ideal prevention strategy of SCD in early-stage HCM patients needs to be combined with gene screening, hiPSC-CM testing, machine learning, and advanced ECG studies, thus achieving individualized SCD prevention.

Pharmacological Management of Hypertrophic Cardiomyopathy: From Bench to Bedside

Hypertrophic cardiomyopathy (HCM), the most common inherited heart disease, is still orphan of a specific drug treatment. The erroneous consideration of HCM as a rare disease has hampered the design and conduct of large, randomized trials in the last 50 years, and most of the indications in the current guidelines are derived from small non-randomized studies, case series, or simply from the consensus of experts. Guideline-directed therapy of HCM includes non-selective drugs such as disopyramide, non-dihydropyridine calcium channel blockers, or β-adrenergic receptor blockers, mainly used in patients with symptomatic obstruction of the outflow tract. Following promising preclinical studies, several drugs acting on potential HCM-specific targets were tested in patients. Despite the huge efforts, none of these studies was able to change clinical practice for HCM patients, because tested drugs were proven to be scarcely effective or hardly tolerated in patients. However, novel compounds have been developed in recent years specifically for HCM, addressing myocardial hypercontractility and altered energetics in a direct manner, through allosteric inhibition of myosin. In this paper, we will critically review the use of different classes of drugs in HCM patients, starting from "old" established agents up to novel selective drugs that have been recently trialed in patients.

Paradoxical prolongation of QT interval during exercise in patients with hypertrophic cardiomyopathy: cellular mechanisms and implications for diastolic function

Aims

Ventricular cardiomyocytes from hypertrophic cardiomyopathy (HCM) patient hearts show prolonged action potential duration (APD), impaired intracellular Ca²⁺ homeostasis and abnormal electrical response to beta -adrenergic stimulation. We sought to determine whether this behaviour is associated with abnormal changes of repolarization during exercise and worsening of diastolic function, ultimately explaining the intolerance to exercise experienced by some patients without obstruction.

Methods and results

Non-obstructive HCM patients (178) and control subjects (81) underwent standard exercise testing, including exercise echocardiography. Ventricular myocytes were isolated from myocardial samples of 23 HCM and eight non-failing non-hypertrophic surgical patients. The APD shortening in response to high frequencies was maintained in HCM myocytes, while β-adrenergic stimulation unexpectedly prolonged APDs, ultimately leading to a lesser shortening of APDs in response to exercise. In HCM vs. control subjects, we observed a lesser shortening of QT interval at peak exercise (QTc: +27 ± 52 ms in HCM, −4 ± 50 ms in controls, P < 0.0001). In patients showing a marked QTc prolongation (>30 ms), the excessive shortening of the electrical diastolic period was linked with a limited increase of heart-rate and deterioration of diastolic function at peak effort.

Conclusions

Abnormal balance of Ca²⁺- and K⁺-currents in HCM cardiomyocytes determines insufficient APD and Ca²⁺-transient shortening with exercise. In HCM patients, exercise-induced QTc prolongation was associated with impaired diastolic reserve, contributing to the reduced exercise tolerance. Our results support the idea that severe electrical cardiomyocyte abnormalities underlie exercise intolerance in a subgroup of HCM patients without obstruction.

Sex-Related Differences in Genetic Cardiomyopathies

Cardiomyopathies are a heterogeneous collection of diseases that have in common primary functional and structural abnormalities of the heart muscle, often genetically determined. The most effective categorization of cardiomyopathies is based on the presenting phenotype, with hypertrophic, dilated, arrhythmogenic, and restrictive cardiomyopathy as the prototypes. Sex modulates the prevalence, morpho-functional manifestations and clinical course of cardiomyopathies. Aspects as diverse as ion channel expression and left ventricular remodeling differ in male and female patients with myocardial disease, although the reasons for this are poorly understood. Moreover, clinical differences may also result from complex societal/environmental discrepancies between sexes that may disadvantage women. This review provides a state-of-the-art appraisal of the influence of sex on cardiomyopathies, highlighting the many gaps in knowledge and open research questions.

Genotype-Driven Pathogenesis of Atrial Fibrillation in Hypertrophic Cardiomyopathy: The Case of Different TNNT2 Mutations

Atrial dilation and atrial fibrillation (AF) are common in Hypertrophic CardioMyopathy (HCM) patients and associated with a worsening of prognosis. The pathogenesis of atrial myopathy in HCM remains poorly investigated and no specific association with genotype has been identified. By re-analysis of our cohort of thin-filament HCM patients (Coppini et al. 2014) AF was identified in 10% of patients with sporadic mutations in the cardiac Troponin T gene (TNNT2), while AF occurrence was much higher (25-75%) in patients carrying specific "hot-spot" TNNT2 mutations. To determine the molecular basis of arrhythmia occurrence, two HCM mouse models expressing human TNNT2 variants (a "hot-spot" one, R92Q, and a "sporadic" one, E163R) were selected according to the different pathophysiological pathways previously demonstrated in ventricular tissue. Echocardiography studies showed a significant left atrial dilation in both models, but more pronounced in the R92Q. In E163R atrial trabeculae, in line with what previously observed in ventricular preparations, the energy cost of tension generation was markedly increased. However, no changes of twitch amplitude and kinetics were observed, and there was no atrial arrhythmic propensity. R92Q atrial trabeculae, instead, displayed normal ATP consumption but markedly increased myofilament calcium sensitivity, as previously observed in ventricular preparations. This was associated with reduced inotropic reserve and slower kinetics of twitch contractions and, importantly, with an increased occurrence of spontaneous beats and triggered contractions that represent an intrinsic arrhythmogenic mechanism promoting AF. The association of specific TNNT2 mutations with AF occurrence depends on the mutation-driven pathomechanism (i.e., increased atrial myofilament calcium sensitivity rather than increased myofilament tension cost) and may influence the individual response to treatment.

Effect of Myosin Isoforms on Cardiac Muscle Twitch of Mice, Rats and Humans

To understand how pathology-induced changes in contractile protein isoforms modulate cardiac muscle function, it is necessary to quantify the temporal-mechanical properties of contractions that occur under various conditions. Pathological responses are much easier to study in animal model systems than in humans, but extrapolation between species presents numerous challenges. Employing computational approaches can help elucidate relationships that are difficult to test experimentally by translating the observations from rats and mice, as model organisms, to the human heart. Here, we use the spatially explicit MUSICO platform to model twitch contractions from rodent and human trabeculae collected in a single laboratory. This approach allowed us to identify the variations in kinetic characteristics of α- and β-myosin isoforms across species and to quantify their effect on cardiac muscle contractile responses. The simulations showed how the twitch transient varied with the ratio of the two myosin isoforms. Particularly, the rate of tension rise was proportional to the fraction of α-myosin present, while the β-isoform dominated the rate of relaxation unless α-myosin was >50%. Moreover, both the myosin isoform and the Ca2+ transient contributed to the twitch tension transient, allowing two levels of regulation of twitch contraction.

Quantitative cross-species translators of cardiac myocyte electrophysiology: Model training, experimental validation, and applications

Animal experimentation is key in the evaluation of cardiac efficacy and safety of novel therapeutic compounds. However, interspecies differences in the mechanisms regulating excitation-contraction coupling can limit the translation of experimental findings from animal models to human physiology and undermine the assessment of drugs’ efficacy and safety. Here, we built a suite of translators for quantitatively mapping electrophysiological responses in ventricular myocytes across species. We trained these statistical operators using a broad dataset obtained by simulating populations of our biophysically detailed computational models of action potential and Ca2+ transient in mouse, rabbit, and human. We then tested our translators against experimental data describing the response to stimuli, such as ion channel block, change in beating rate, and β-adrenergic challenge. We demonstrate that this approach is well suited to predicting the effects of perturbations across different species or experimental conditions and suggest its integration into mechanistic studies and drug development pipelines.

Potential Mechanisms of SGLT2 Inhibitors for the Treatment of Heart Failure With Preserved Ejection Fraction

Heart failure with preserved ejection fraction (HFpEF) is an unsolved and growing concern in cardiovascular medicine. While no treatment options that improve prognosis in HFpEF patients has been established so far, SGLT2 inhibitors (SGLT2i) are currently being investigated for the treatment of HFpEF patients. SGLT2i have already been shown to mitigate comorbidities associated with HFpEF such as type 2 diabetes and chronic renal disease, however, more recently there has been evidence that they may also directly improve diastolic function. In this article, we discuss some potential beneficial mechanisms of SGLT2i in the pathophysiology of HFpEF with focus on contractile function.

Hypertrophic Cardiomyopathy: From Phenotype and Pathogenesis to Treatment

Hypertrophic cardiomyopathy (HCM) is a very common inherited cardiovascular disease (CAD) and the incidence is about 1/500 of the common population. It is caused by more than 1,400 mutations in 11 or more genes encoding the proteins of the cardiac sarcomere. HCM presents a heterogeneous clinical profile and complex pathophysiology and HCM is the most important cause of sudden cardiac death (SCD) in young people. HCM also contributes to functional disability from heart failure and stroke (caused by atrial fibrillation). Current treatments for HCM (medication, myectomy, and alcohol septal ablation) are geared toward slowing down the disease progression and symptom relief and implanted cardiac defibrillator (ICD) to prevent SCD. HCM is, however, entering a period of tight translational research that holds promise for the major advances in disease-specific therapy. Main insights into the genetic landscape of HCM have improved our understanding of molecular pathogenesis and pointed the potential targets for the development of therapeutic agents. We reviewed the critical discoveries about the treatments, mechanism of HCM, and their implications for future research.

Ion Channel Impairment and Myofilament Ca2+ Sensitization: Two Parallel Mechanisms Underlying Arrhythmogenesis in Hypertrophic Cardiomyopathy

Life-threatening ventricular arrhythmias are the main clinical burden in patients with hypertrophic cardiomyopathy (HCM), and frequently occur in young patients with mild structural disease. While massive hypertrophy, fibrosis and microvascular ischemia are the main mechanisms underlying sustained reentry-based ventricular arrhythmias in advanced HCM, cardiomyocyte-based functional arrhythmogenic mechanisms are likely prevalent at earlier stages of the disease. In this review, we will describe studies conducted in human surgical samples from HCM patients, transgenic animal models and human cultured cell lines derived from induced pluripotent stem cells. Current pieces of evidence concur to attribute the increased risk of ventricular arrhythmias in early HCM to different cellular mechanisms. The increase of late sodium current and L-type calcium current is an early observation in HCM, which follows post-translation channel modifications and increases the occurrence of early and delayed afterdepolarizations. Increased myofilament Ca²⁺ sensitivity, commonly observed in HCM, may promote afterdepolarizations and reentry arrhythmias with direct mechanisms. Decrease of K⁺-currents due to transcriptional regulation occurs in the advanced disease and contributes to reducing the repolarization-reserve and increasing the early afterdepolarizations (EADs). The presented evidence supports the idea that patients with early-stage HCM should be considered and managed as subjects with an acquired channelopathy rather than with a structural cardiac disease.

Abnormalities in sodium current and calcium homoeostasis as drivers of arrhythmogenesis in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a common inherited monogenic disease with a prevalence of 1/500 in the general population, representing an important cause of arrhythmic sudden cardiac death (SCD), heart failure, and atrial fibrillation in the young. HCM is a global condition, diagnosed in >50 countries and in all continents. HCM affects people of both sexes and various ethnic and racial origins, with similar clinical course and phenotypic expression. The most unpredictable and devastating consequence of HCM is represented by arrhythmic SCD, most commonly caused by sustained ventricular tachycardia or ventricular fibrillation. Indeed, HCM represents one of the main causes of arrhythmic SCD in the young, with a marked preference for children and adults <30 years. SCD is most prevalent in patients with paediatric onset of HCM but may occur at any age. However, risk is substantially lower after 60 years, suggesting that the potential for ventricular tachyarrhythmias is mitigated by ageing. SCD had been linked originally to sports and vigorous activity in HCM patients. However, it is increasingly clear that the majority of events occurs at rest or during routine daily occupations, suggesting that triggers are far from consistent. In general, the pathophysiology of SCD in HCM remains unresolved. While the pathologic and physiologic substrates abound and have been described in detail, specific factors precipitating ventricular tachyarrhythmias are still unknown. SCD is a rare phenomenon in HCM cohorts (<1%/year) and attempts to identify patients at risk, while generating clinically useful algorithms for primary prevention, remain very inaccurate on an individual basis. One of the reasons for our limited understanding of these phenomena is that limited translational research exists in the field, while most efforts have focused on clinical markers of risk derived from pathology, instrumental patient evaluation, and imaging. Specifically, few studies conducted in animal models and human samples have focused on targeting the cellular mechanisms of arrhythmogenesis in HCM, despite potential implications for therapeutic innovation and SCD prevention. These studies found that altered intracellular Ca2+ homoeostasis and increased late Na+ current, leading to an increased likelihood of early and delayed after-depolarizations, contribute to generate arrhythmic events in diseased cardiomyocytes. As an array of novel experimental opportunities have emerged to investigate these mechanisms, including novel 'disease-in-the-dish' cellular models with patient-specific induced pluripotent stem cell-derived cardiomyocytes, important gaps in knowledge remain. Accordingly, the aim of the present review is to provide a contemporary reappraisal of the cellular basis of SCD-predisposing arrhythmias in patients with HCM and discuss the implications for risk stratification and management.

Sex-Specific Classification of Drug-Induced Torsade de Pointes Susceptibility Using Cardiac Simulations and Machine Learning

Torsade de Pointes (TdP), a rare but lethal ventricular arrhythmia, is a toxic side effect of many drugs. To assess TdP risk, safety regulatory guidelines require quantification of hERG channel block in vitro and QT interval prolongation in vivo for all new therapeutic compounds. Unfortunately, these have proven to be poor predictors of torsadogenic risk, and are likely to have prevented safe compounds from reaching clinical phases. Although this has stimulated numerous efforts to define new paradigms for cardiac safety, none of the recently developed strategies accounts for patient conditions. In particular, despite being a well-established independent risk factor for TdP, female sex is vastly under-represented in both basic research and clinical studies, and thus current TdP metrics are likely biased toward the male sex. Here, we apply statistical learning to synthetic data, generated by simulating drug effects on cardiac myocyte models capturing male and female electrophysiology, to develop new sex-specific classification frameworks for TdP risk. We show that (i) TdP classifiers require different features in females vs. males; (ii) male-based classifiers perform more poorly when applied to female data; and (iii) female-based classifier performance is largely unaffected by acute effects of hormones (i.e., during various phases of the menstrual cycle). Notably, when predicting TdP risk of intermediate drugs on female simulated data, male-biased predictive models consistently underestimate TdP risk in women. Therefore, we conclude that pipelines for preclinical cardiotoxicity risk assessment should consider sex as a key variable to avoid potentially life-threatening consequences for the female population.

Pathophysiology and Treatment of Hypertrophic Cardiomyopathy: New Perspectives

PURPOSE OF REVIEW

We provide a state of the art of therapeutic options in hypertrophic cardiomyopathy (HCM), focusing on recent advances in our understanding of the pathophysiology of sarcomeric disease.

RECENT FINDINGS

A wealth of novel information regarding the molecular mechanisms associated with the clinical phenotype and natural history of HCM have been developed over the last two decades. Such advances have only recently led to a number of controlled randomized studies, often limited in size and fortune. Recently, however, the allosteric inhibitors of cardiac myosin adenosine triphosphatase, countering the main pathophysiological abnormality associated with HCM-causing mutations, i.e. hypercontractility, have opened new management perspectives. Mavacamten is the first drug specifically developed for HCM used in a successful phase 3 trial, with the promise to reach symptomatic obstructive patients in the near future. In addition, the fine characterization of cardiomyocyte electrophysiological remodelling has recently highlighted relevant therapeutic targets. Current therapies for HCM focus on late disease manifestations without addressing the intrinsic pathological mechanisms. However, novel evidence-based approaches have opened the way for agents targeting HCM molecular substrates. The impact of these targeted interventions will hopefully alter the natural history of the disease in the near future.

Mexiletine-like cellular electrophysiological effects of GS967 in canine ventricular myocardium

Enhancement of the late Na⁺ current (I_NaL) increases arrhythmia propensity in the heart, while suppression of the current is antiarrhythmic. GS967 is an agent considered as a selective blocker of I_NaL. In the present study, effects of GS967 on I_NaL and action potential (AP) morphology were studied in canine ventricular myocytes by using conventional voltage clamp, action potential voltage clamp and sharp microelectrode techniques. The effects of GS967 (1 µM) were compared to those of the class I/B antiarrhythmic compound mexiletine (40 µM). Under conventional voltage clamp conditions, I_NaL was significantly suppressed by GS967 and mexiletine, causing 80.4 ± 2.2% and 59.1 ± 1.8% reduction of the densities of I_NaL measured at 50 ms of depolarization, and 79.0 ± 3.1% and 63.3 ± 2.7% reduction of the corresponding current integrals, respectively. Both drugs shifted the voltage dependence of the steady-state inactivation curve of I_NaL towards negative potentials. GS967 and mexiletine dissected inward I_NaL profiles under AP voltage clamp conditions having densities, measured at 50% of AP duration (APD), of −0.37 ± 0.07 and −0.28 ± 0.03 A/F, and current integrals of −56.7 ± 9.1 and −46.6 ± 5.5 mC/F, respectively. Drug effects on peak Na⁺ current (I_NaP) were assessed by recording the maximum velocity of AP upstroke (V⁺_max) in multicellular preparations. The offset time constant was threefold faster for GS967 than mexiletine (110 ms versus 289 ms), while the onset of the rate-dependent block was slower in the case of GS967. Effects on beat-to-beat variability of APD was studied in isolated myocytes. Beat-to-beat variability was significantly decreased by both GS967 and mexiletine (reduction of 42.1 ± 6.5% and 24.6 ± 12.8%, respectively) while their shortening effect on APD was comparable. It is concluded that the electrophysiological effects of GS967 are similar to those of mexiletine, but with somewhat faster offset kinetics of V⁺_max block. However, since GS967 depressed V⁺_max and I_NaL at the same concentration, the current view that GS967 represents a new class of drugs that selectively block I_NaL has to be questioned and it is suggested that GS967 should be classified as a class I/B antiarrhythmic agent.

Translational investigation of electrophysiology in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) patients are at increased risk of ventricular arrhythmias and sudden cardiac death, which can occur even in the absence of structural changes of the heart. HCM mouse models suggest mutations in myofilament components to affect Ca²⁺ homeostasis and thereby favor arrhythmia development. Additionally, some of them show indications of pro-arrhythmic changes in cardiac electrophysiology. In this study, we explored arrhythmia mechanisms in mice carrying a HCM mutation in Mybpc3 (Mybpc3-KI) and tested the translatability of our findings in human engineered heart tissues (EHTs) derived from CRISPR/Cas9-generated homozygous MYBPC3 mutant (MYBPC3hom) in induced pluripotent stem cells (iPSC) and to left ventricular septum samples obtained from HCM patients.

We observed higher arrhythmia susceptibility in contractility measurements of field-stimulated intact cardiomyocytes and ventricular muscle strips as well as in electromyogram recordings of Langendorff-perfused hearts from adult Mybpc3-KI mice than in wild-type (WT) controls. The latter only occurred in homozygous (Hom-KI) but not in heterozygous (Het-KI) mouse hearts. Both Het- and Hom-KI are known to display pro-arrhythmic increased Ca²⁺ myofilament sensitivity as a direct consequence of the mutation. In the electrophysiological characterization of the model, we observed smaller repolarizing K⁺ currents in single cell patch clamp, longer ventricular action potentials in sharp microelectrode recordings and longer ventricular refractory periods in Langendorff-perfused hearts in Hom-KI, but not Het-KI. Interestingly, reduced K⁺ channel subunit transcript levels and prolonged action potentials were already detectable in newborn, pre-hypertrophic Hom-KI mice. Human iPSC-derived MYBPC3hom EHTs, which genetically mimicked the Hom-KI mice, did exhibit lower mutant mRNA and protein levels, lower force, beating frequency and relaxation time, but no significant alteration of the force-Ca²⁺ relation in skinned EHTs. Furthermore, MYBPC3hom EHTs did show higher spontaneous arrhythmic behavior, whereas action potentials measured by sharp microelectrode did not differ to isogenic controls. Action potentials measured in septal myectomy samples did not differ between patients with HCM and patients with aortic stenosis, except for the only sample with a MYBPC3 mutation.

The data demonstrate that increased myofilament Ca²⁺ sensitivity is not sufficient to induce arrhythmias in the Mybpc3-KI mouse model and suggest that reduced K⁺ currents can be a pro-arrhythmic trigger in Hom-KI mice, probably already in early disease stages. However, neither data from EHTs nor from left ventricular samples indicate relevant reduction of K⁺ currents in human HCM. Therefore, our study highlights the species difference between mouse and human and emphasizes the importance of research in human samples and human-like models.

•

Sudden cardiac death is threatening hypertrophic cardiomyopathy (HCM) patients.

•

Arrhythmia mechanisms are not well understood.

•

Mouse HCM models showed relevant reduction in K⁺ currents.

•

Human iPSC-EHT model and HCM patient septal myectomies did not display this mechanism.

A Perspective on Personalized Therapies in Hypertrophic Cardiomyopathy

ABSTRACT

A dominant mechanism of sudden cardiac death in the young is the progression of maladaptive responses to genes encoding proteins linked to hypertrophic cardiomyopathy. Most are mutant sarcomere proteins that trigger the progression by imposing a biophysical defect on the dynamics and levels of myofilament tension generation. We discuss approaches for personalized treatments that are indicated by recent advanced understanding of the progression.

Research progress on the role of CaMKII in heart disease

In the heart, Ca²⁺ participates in electrical activity and myocardial contraction, which is closely related to the generation of action potential and excitation contraction coupling (ECC) and plays an important role in various signal cascades and regulates different physiological processes. In the Ca²⁺ related physiological activities, CaMKII is a key downstream regulator, involving autophosphorylation and post-translational modification, and plays an important role in the excitation contraction coupling and relaxation events of cardiomyocytes. This paper reviews the relationship between CaMKII and various substances in the pathological process of myocardial apoptosis and necrosis, myocardial hypertrophy and arrhythmia, and what roles it plays in the development of disease in complex networks. This paper also introduces the drugs targeting at CaMKII to treat heart disease.

T-tubule remodeling in human hypertrophic cardiomyopathy

The highly organized transverse T-tubule membrane system represents the ultrastructural substrate for excitation–contraction coupling in ventricular myocytes. While the architecture and function of T-tubules have been well described in animal models, there is limited morpho-functional data on T-tubules in human myocardium. Hypertrophic cardiomyopathy (HCM) is a primary disease of the heart muscle, characterized by different clinical presentations at the various stages of its progression. Most HCM patients, indeed, show a compensated hypertrophic disease (“non-failing hypertrophic phase”), with preserved left ventricular function, and only a small subset of individuals evolves into heart failure (“end stage HCM”). In terms of T-tubule remodeling, the “end-stage” disease does not differ from other forms of heart failure. In this review we aim to recapitulate the main structural features of T-tubules during the “non-failing hypertrophic stage” of human HCM by revisiting data obtained from human myectomy samples. Moreover, by comparing pathological changes observed in myectomy samples with those introduced by acute (experimentally induced) detubulation, we discuss the role of T-tubular disruption as a part of the complex excitation–contraction coupling remodeling process that occurs during disease progression. Lastly, we highlight how T-tubule morpho-functional changes may be related to patient genotype and we discuss the possibility of a primitive remodeling of the T-tubule system in rare HCM forms associated with genes coding for proteins implicated in T-tubule structural integrity, formation and maintenance.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Human Cell Modeling for Cardiovascular Diseases

The availability of appropriate and reliable in vitro cell models recapitulating human cardiovascular diseases has been the aim of numerous researchers, in order to retrace pathologic phenotypes, elucidate molecular mechanisms, and discover therapies using simple and reproducible techniques. In the past years, several human cell types have been utilized for these goals, including heterologous systems, cardiovascular and non-cardiovascular primary cells, and embryonic stem cells. The introduction of induced pluripotent stem cells and their differentiation potential brought new prospects for large-scale cardiovascular experiments, bypassing ethical concerns of embryonic stem cells and providing an advanced tool for disease modeling, diagnosis, and therapy. Each model has its advantages and disadvantages in terms of accessibility, maintenance, throughput, physiological relevance, recapitulation of the disease. A higher level of complexity in diseases modeling has been achieved with multicellular co-cultures. Furthermore, the important progresses reached by bioengineering during the last years, together with the opportunities given by pluripotent stem cells, have allowed the generation of increasingly advanced in vitro three-dimensional tissue-like constructs mimicking in vivo physiology. This review provides an overview of the main cell models used in cardiovascular research, highlighting the pros and cons of each, and describing examples of practical applications in disease modeling.

Recent updates on novel therapeutic targets of cardiovascular diseases

In recent times cardiovascular diseases (CVDs) are the leading cause of mortality universally, caused more or less 17.7 million casualties with 45% of all illnesses (except communicable ones) in 2015 as per World Health Organization (WHO). According to American National Center for Health Statistics, cardiac disorders are costliest. Moreover, health care expenditures related to cardiac disorders are anticipated to exceed than diabetes and Alzheimer's. Straining of reactive oxygen species with diminished neutralization & inflammation critically adds to atherosclerosis and also proceed to other cardiovascular diseases such as cardiac remodeling and myocardial infarction (MI). In the past few years, researchers revealed multiple drug targets from animal studies and evaluated them in the therapeutics of cardiac disorders, which offered exciting clues for novel therapeutic strategies. Although, only few newer agents approved clinically and actual approaches for treatment are lagging behind. Several novel drugs found effective for the treatment of hypertension, congestive heart failure, cardiac arrhythmia and angina pectoris. Detailed mechanism of action, basic and clinical pharmacology of all novel drugs has been discussed in this review.

Advances in Stem Cell Modeling of Dystrophin-Associated Disease: Implications for the Wider World of Dilated Cardiomyopathy

Familial dilated cardiomyopathy (DCM) is mostly caused by mutations in genes encoding cytoskeletal and sarcomeric proteins. In the pediatric population, DCM is the predominant type of primitive myocardial disease. A severe form of DCM is associated with mutations in the DMD gene encoding dystrophin, which are the cause of Duchenne Muscular Dystrophy (DMD). DMD-associated cardiomyopathy is still poorly understood and orphan of a specific therapy. In the last 5 years, a rise of interest in disease models using human induced pluripotent stem cells (hiPSCs) has led to more than 50 original studies on DCM models. In this review paper, we provide a comprehensive overview on the advances in DMD cardiomyopathy disease modeling and highlight the most remarkable findings obtained from cardiomyocytes differentiated from hiPSCs of DMD patients. We will also describe how hiPSCs based studies have contributed to the identification of specific myocardial disease mechanisms that may be relevant in the pathogenesis of DCM, representing novel potential therapeutic targets.

Contribution of the neuronal sodium channel NaV1.8 to sodium- and calcium-dependent cellular proarrhythmia

OBJECTIVE

In myocardial pathology such as heart failure a late sodium current (I_NaL) augmentation is known to be involved in conditions of arrhythmogenesis. However, the underlying mechanisms of the I_NaL generation are not entirely understood. By now evidence is growing that non-cardiac sodium channel isoforms could also be involved in the I_NaL generation. The present study investigates the contribution of the neuronal sodium channel isoform Na_V1.8 to arrhythmogenesis in a clearly-defined setting of enhanced I_NaL by using anemone toxin II (ATX-II) in the absence of structural heart disease.

METHODS

Electrophysiological experiments were performed in order to measure I_NaL, action potential duration (APD), SR-Ca²⁺-leak and cellular proarrhythmic triggers in ATX-II exposed wild-type (WT) and SCN10A^-/- mice cardiomyocytes. In addition, WT cardiomyocytes were stimulated with ATX-II in the presence or absence of Na_V1.8 inhibitors. I_NCX was measured by using the whole cell patch clamp method.

RESULTS

In WT cardiomyocytes exposure to ATX-II augmented I_NaL, prolonged APD, increased SR-Ca²⁺-leak and induced proarrhythmic triggers such as early afterdepolarizations (EADs) and Ca²⁺-waves. All of them could be significantly reduced by applying Na_V1.8 blockers PF-01247324 and A-803467. Both blockers had no relevant effects on cellular electrophysiology of SCN10A^-/- cardiomyocytes. Moreover, in SCN10A^-/--cardiomyocytes, the ATX-II-dependent increase in I_NaL, SR-Ca²⁺-leak and APD prolongation was less than in WT and comparable to the results which were obtained with WT cardiomyocytes being exposed to ATX-II and Na_V1.8 inhibitors in parallel. Moreover, we found a decrease in reverse mode NCX current and reduced CaMKII-dependent RyR2-phosphorylation after application of PF-01247324 as an underlying explanation for the Na⁺-mediated Ca²⁺-dependent proarrhythmic triggers.

CONCLUSION

The current findings demonstrate that Na_V1.8 is a significant contributor for I_NaL-induced arrhythmic triggers. Therefore, Na_V1.8 inhibition under conditions of an enhanced I_NaL constitutes a promising antiarrhythmic strategy which merits further investigation.

Electrophysiological and Contractile Effects of Disopyramide in Patients With Obstructive Hypertrophic Cardiomyopathy: A Translational Study

•

In patients with HCM and symptomatic LVOT-obstruction, first treatment with disopyramide leads to a marked reduction of LVOT gradients, with a slight decrease of resting ejection fraction and a modest increase of corrected QT interval, highlighting high efficacy and safety.

•

In single cardiomyocytes and intact trabeculae from surgical samples of patients with obstructive HCM, in vitro treatment with 5 μmol/l disopyramide lowered force and Ca2⁺ transients while reducing action potential duration and the rate of arrhythmic afterdepolarizations.

•

These effects are mediated by the combined inhibition of peak and late Na⁺ currents, L-type Ca²⁺ current, delayed-rectifier K⁺ current, and ryanodine receptors.

•

In addition to the negative inotropic effect of disopyramide, in vitro results suggest additional antiarrhythmic actions.

Disopyramide is effective and safe in patients with obstructive hypertrophic cardiomyopathy. However, its cellular and molecular mechanisms of action are unknown. We tested disopyramide in cardiomyocytes from the septum of surgical myectomy patients: disopyramide inhibits multiple ion channels, leading to lower Ca transients and force, and shortens action potentials, thus reducing cellular arrhythmias. The electrophysiological profile of disopyramide explains the efficient reduction of outflow gradients but also the limited prolongation of the QT interval and the absence of arrhythmic side effects observed in 39 disopyramide-treated patients. In conclusion, our results support the idea that disopyramide is safe for outpatient use in obstructive patients.

Optical Investigation of Action Potential and Calcium Handling Maturation of hiPSC-Cardiomyocytes on Biomimetic Substrates

Cardiomyocytes from human induced pluripotent stem cells (hiPSC-CMs) are the most promising human source with preserved genetic background of healthy individuals or patients. This study aimed to establish a systematic procedure for exploring development of hiPSC-CM functional output to predict genetic cardiomyopathy outcomes and identify molecular targets for therapy. Biomimetic substrates with microtopography and physiological stiffness can overcome the immaturity of hiPSC-CM function. We have developed a custom-made apparatus for simultaneous optical measurements of hiPSC-CM action potential and calcium transients to correlate these parameters at specific time points (day 60, 75 and 90 post differentiation) and under inotropic interventions. In later-stages, single hiPSC-CMs revealed prolonged action potential duration, increased calcium transient amplitude and shorter duration that closely resembled those of human adult cardiomyocytes from fresh ventricular tissue of patients. Thus, the major contribution of sarcoplasmic reticulum and positive inotropic response to β-adrenergic stimulation are time-dependent events underlying excitation contraction coupling (ECC) maturation of hiPSC-CM; biomimetic substrates can promote calcium-handling regulation towards adult-like kinetics. Simultaneous optical recordings of long-term cultured hiPSC-CMs on biomimetic substrates favor high-throughput electrophysiological analysis aimed at testing (mechanistic hypothesis on) disease progression and pharmacological interventions in patient-derived hiPSC-CMs.

Interplay Between Sub-Cellular Alterations of Calcium Release and T-Tubular Defects in Cardiac Diseases

Asynchronous Ca²⁺ release promotes non-homogeneous myofilament activation, leading to mechanical dysfunction, as well as initiation of propagated calcium waves and arrhythmias. Recent advances in microscopy techniques have allowed for optical recordings of local Ca²⁺ fluxes and action potentials from multiple sub-cellular domains within cardiac cells with unprecedented spatial and temporal resolution. Since then, sub-cellular local information of the spatio-temporal relationship between Ca²⁺ release and action potential propagation have been unlocked, providing novel mechanistic insights in cardiac excitation-contraction coupling (ECC). Here, we review the promising perspectives arouse from repeatedly probing Ca²⁺ release at the same sub-cellular location while simultaneously probing multiple locations at the same time within a single cardiac cell. We also compare the results obtained in three different rodent models of cardiac diseases, highlighting disease-specific mechanisms. Slower local Ca²⁺ release has been observed in regions with defective action potential conduction in diseased cardiac cells. Moreover, significant increment of Ca²⁺ variability (both in time and in space) has been found in diseased cardiac cells but does not directly correlate with local electrical defects nor with the degree of structural aberrations of the cellular membrane system, suggesting a role for other players of the ECC machinery. We finally explore exciting opportunities provided by the technology for studying different cardiomyocyte populations, as well as for dissecting the mechanisms responsible for subcellular spatio-temporal variability of Ca²⁺ release.

Altered Ca2+ and Na+ Homeostasis in Human Hypertrophic Cardiomyopathy: Implications for Arrhythmogenesis

Hypertrophic cardiomyopathy (HCM) is the most common mendelian heart disease, with a prevalence of 1/500. HCM is a primary cause of sudden death, due to an heightened risk of ventricular tachyarrhythmias that often occur in young asymptomatic patients. HCM can slowly progress toward heart failure, either with preserved or reduced ejection fraction, due to worsening of diastolic function. Accumulation of intra-myocardial fibrosis and replacement scars underlies heart failure progression and represents a substrate for sustained arrhythmias in end-stage patients. However, arrhythmias and mechanical abnormalities may occur in hearts with little or no fibrosis, prompting toward functional pathomechanisms. By studying viable cardiomyocytes and trabeculae isolated from inter-ventricular septum samples of non-failing HCM patients with symptomatic obstruction who underwent myectomy operations, we identified that specific abnormalities of intracellular Ca²⁺ handling are associated with increased cellular arrhytmogenesis and diastolic dysfunction. In HCM cardiomyocytes, diastolic Ca²⁺ concentration is increased both in the cytosol and in the sarcoplasmic reticulum and the rate of Ca²⁺ transient decay is slower, while the amplitude of Ca²⁺-release is preserved. Ca²⁺ overload is the consequence of an increased Ca²⁺ entry via L-type Ca²⁺-current [due to prolongation the action potential (AP) plateau], combined with a reduced rate of Ca²⁺-extrusion through the Na⁺/Ca²⁺ exchanger [due to increased cytosolic (Na⁺)] and a lower expression of SERCA. Increased late Na⁺ current (I_NaL) plays a major role, as it causes both AP prolongation and Na⁺ overload. Intracellular Ca²⁺ overload determines an higher frequency of Ca²⁺ waves leading to delayed-afterdepolarizations (DADs) and premature contractions, but is also linked with the increased diastolic tension and slower relaxation of HCM myocardium. Sustained increase of intracellular [Ca²⁺] goes hand-in-hand with the increased activation of Ca²⁺/calmodulin-dependent protein-kinase-II (CaMKII) and augmented phosphorylation of its targets, including Ca²⁺ handling proteins. In transgenic HCM mouse models, we found that Ca²⁺ overload, CaMKII and increased I_NaL drive myocardial remodeling since the earliest stages of disease and underlie the development of hypertrophy, diastolic dysfunction and the arrhythmogenic substrate. In conclusion, diastolic dysfunction and arrhythmogenesis in human HCM myocardium are driven by functional alterations at cellular and molecular level that may be targets of innovative therapies.

Late sodium current inhibitors to treat exercise-induced obstruction in hypertrophic cardiomyopathy: an in vitro study in human myocardium

Background and Purpose

In 30–40% of hypertrophic cardiomyopathy (HCM) patients, symptomatic left ventricular (LV) outflow gradients develop only during exercise due to catecholamine‐induced LV hypercontractility (inducible obstruction). Negative inotropic pharmacological options are limited to β‐blockers or disopyramide, with low efficacy and tolerability. We assessed the potential of late sodium current (I_NaL)‐inhibitors to treat inducible obstruction in HCM.

Experimental Approach

The electrophysiological and mechanical responses to β‐adrenoceptor stimulation were studied in human myocardium from HCM and control patients. Effects of I_NaL‐inhibitors (ranolazine and GS‐967) in HCM samples were investigated under conditions simulating rest and exercise.

Key Results

In cardiomyocytes and trabeculae from 18 surgical septal samples of patients with obstruction, the selective I_NaL‐inhibitor GS‐967 (0.5 μM) hastened twitch kinetics, decreased diastolic [Ca²⁺] and shortened action potentials, matching the effects of ranolazine (10μM). Mechanical responses to isoprenaline (inotropic and lusitropic) were comparable in HCM and control myocardium. However, isoprenaline prolonged action potentials in HCM myocardium, while it shortened them in controls. Unlike disopyramide, neither GS‐967 nor ranolazine reduced force at rest. However, in the presence of isoprenaline, they reduced Ca²⁺‐transient amplitude and twitch tension, while the acceleration of relaxation was maintained. I_NaL‐inhibitors were more effective than disopyramide in reducing contractility during exercise. Finally, I_NaL‐inhibitors abolished arrhythmias induced by isoprenaline.

Conclusions and Implications

Ranolazine and GS‐967 reduced septal myocardium tension during simulated exercise in vitro and therefore have the potential to ameliorate symptoms caused by inducible obstruction in HCM patients, with some advantages over disopyramide and β‐blockers.