Background: Ibrutinib is a Bruton's tyrosine kinase (BTK) inhibitor with remarkable efficacy against B-cell cancers. Ibrutinib also increases the risk of atrial fibrillation (AF) which remains poorly understood. Methods: We performed electrophysiology studies on mice treated with ibrutinib to assess inducibility of AF. Chemoproteomic analysis of cardiac lysates identified candidate ibrutinib targets, which were further evaluated in genetic mouse models and additional pharmacologic experiments. The pharmacovigilance database, VigiBase ® was queried to determine whether drug inhibition of an identified candidate kinase was associated with increased reporting of AF. Results: We demonstrate that treatment of mice with ibrutinib for four weeks results in inducible AF, left atrial enlargement, myocardial fibrosis and inflammation. This effect was reproduced in mice lacking BTK but not in mice treated with four-weeks of acalabrutinib, a more specific BTK inhibitor, demonstrating that AF is an off-target side effect. Chemoproteomic profiling identified a short list of candidate kinases that was narrowed by additional experimentation leaving C-terminal src kinase (CSK) as the strongest candidate for ibrutinib induced AF. Cardiac specific Csk knockout in mice led to increased AF, left atrial enlargement, fibrosis, and inflammation, phenocopying ibrutinib treatment. Disproportionality analyses in Vigibase ® confirmed increased reporting of AF associated with kinase inhibitors blocking Csk vs non-Csk inhibitors, with a reporting odds-ratio of 8.0 [95% CI 7.3-8.7, p<0.0001]. Conclusions: These data identify Csk inhibition as the mechanism by which ibrutinib leads to AF. Clinical Trial Registration: URL: www.clinicaltrials.gov Unique Identifier: NCT03530215

Background Hypokalemia is known to promote ventricular arrhythmias, especially in combination with Class III antiarrhythmic drugs like dofetilide. Here we evaluated the underlying molecular mechanisms. Methods and Results Arrhythmias were recorded in isolated rabbit and rat hearts or patch-clamped ventricular myocytes exposed to hypokalemia (1.0-3.5 mmol/l) in the absence or presence of dofetilide (1 μmol/l). Spontaneous early afterdepolarizations (EADs) and ventricular tachycardia/fibrillation (VF/VF) occurred in 50% of hearts at 2.7 mmol/l [K] in the absence of dofetilide, and 3.3 mmol/l [K] in its presence. Pre-treatment with the CaMKII inhibitor KN-93, but not its inactive analogue KN-92, abolished EADs and hypokalemia-induced VT/VF, as did the selective late Na current (INa) blocker GS-967. In intact hearts, moderate hypokalemia (2.7 mmol/l) significantly increased tissue CaMKII activity. Computer modeling revealed that EAD generation by hypokalemia (with or without dofetilide) required Na-K pump inhibition to induce intracellular Na and Ca overload with consequent CaMKII activation enhancing late INa and the L-type Ca current. K current suppression by hypokalemia and/or dofetilide alone in the absence of CaMKII activation were ineffective at causing EADs. Conclusions We conclude that Na-K pump inhibition by even moderate hypokalemia plays a critical role in promoting EAD-mediated arrhythmias by inducing a positive feedback cycle activating CaMKII and enhancing late INa. Class III antiarrhythmic drugs like dofetilide sensitize the heart to this positive feedback loop.

Atrial fibrillation (AF) is the most common sustained arrhythmia encountered in humans and is a significant source of morbidity and mortality. Despite its prevalence, our mechanistic understanding is incomplete, the therapeutic options have limited efficacy, and are often fraught with risks. A better biological understanding of AF is needed to spearhead novel therapeutic avenues. Although “natural” AF is nearly nonexistent in most species, animal models have contributed significantly to our understanding of AF and some therapeutic options. However, the impediments of animal models are also apparent and stem largely from the differences in basic physiology as well as the complexities underlying human AF; these preclude the creation of a “perfect” animal model and have obviated the translation of animal findings. Herein, we review the vast array of AF models available, spanning the mouse heart (weighing 1/1000th of a human heart) to the horse heart (10× heavier than the human heart). We attempt to highlight the features of each model that bring value to our understanding of AF but also the shortcomings and pitfalls. Finally, we borrowed the concept of a SWOT analysis from the business community (which stands for strengths, weaknesses, opportunities, and threats) and applied this introspective type of analysis to animal models for AF. We identify unmet needs and stress that is in the context of rapidly advancing technologies, these present opportunities for the future use of animal models.