Abstract Body (Do not enter title and authors here): Introduction: Class IC antiarrhythmic agents are widely used to treat arrhythmias. However, in the CAST trial these drugs increased arrhythmic mortality in patients with recent myocardial infarction/systolic heart failure. Some of the underlying molecular mechanisms remain poorly understood.

Research Question: Does chronic exposure to flecainide promote electrical remodeling that is reflected in the sodium channel composition of the murine heart?

Methods: C57BL/6 mice, with or without engineered NaV1.5 sensitivity to acyl/arylsulfonamides (GX drugs) by knock-in (KI) of a chimeric SCN5A construct incorporating a GX drug binding site into VSD IV (GX mice), were treated with flecainide via drug pellets (20 mg/kg/day) for 21–28 days. Surface ECGs were recorded on days 0, 1, 3, 7, 14, and 21 in flecainide-treated (n > 11) and placebo (n = 2) mice. After 21 days, ventricular cardiomyocytes (CMs) were isolated via Langendorff perfusion and manually patch-clamped under voltage-clamp to measure sodium currents (I_Na). Isoform-specific toxins and small-molecule inhibitors were sequentially applied to identify different isoforms contributions to I_Na. GX-674 assessed NaV1.5 in GX mice and NaV1.2/1.6/1.7 in wild-type mice. μ-conotoxin GIIIB (NaV1.4), low-dose TTX (NaV1.1/1.3), and voltage sensitive toxin 3 (NaV1.8) probed other isoforms (n>8 all isoforms). Control data were obtained from untreated cardiomyocytes using the same protocols (n > 7).

Results: Flecainide treatment caused bradycardia and prolongation of the PR interval and QRS duration at day 1 compared to untreated mice (HR 483.0±60.7 vs. 598±33.9 BPM p<.05, PR 62.1±10.7 vs. 43.5±0.7 ms p<.01; QRS 28.2±6.4 vs 14.9±0.8 ms p<.0001). There were no EKG differences at day 3. Patch-clamp (Fig. 2) showed reduced NaV1.5 contributions in CMs from flecainide treated mice (75.9±10.4% vs 92.6±4.9%, p<.0001) with increased NaV1.1/1.3 (11.8±4.8% vs 0.4±0.2%, p<.0001) and NaV1.2/1.6/1.7 (9.0±6.3% vs 0.7±0.9%, P<.0005) contributions. There was no difference in NaV1.4 or NaV1.8/1.9 contributions.

Conclusions: Flecainide effectively treats arrhythmias but increases arrhythmic deaths in patients with structural heart disease/ischemia. We show that flecainide treatment increases the contributions of neuronal sodium channel isoforms to I_Na. These isoforms have distinct biophysical properties which may promote arrhythmogenesis, potentially explaining the adverse outcomes in the CAST trial and offering potential therapeutic strategies.