Efficacy of Flecainide in Catecholaminergic Polymorphic Ventricular Tachycardia Is Mutation-Independent but Reduced by Calcium Overload

Background: The dual Na+ and cardiac Ca2+-release channel inhibitor, Flecainide (FLEC) is effective in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT), a disease caused by mutations in cardiac Ca2+ -release channels (RyR2), calsequestrin (Casq2), or calmodulin. FLEC suppresses spontaneous Ca2+ waves in Casq2-knockout (Casq2(-/-) ) cardiomyocytes, a CPVT model. However, a report failed to find FLEC efficacy against Ca2+ waves in another CPVT model, RyR2-R4496C heterozygous mice (RyR2(R)(4496)(C+/-)), raising the possibility that FLEC efficacy may be mutation dependent. Objective: To address this controversy, we compared FLEC in Casq2(-/-) and RyR2(R)(4496)(C+ /-) cardiomyocytes and mice under identical conditions. Methods: After 30 min exposure to FLEC (6 mu M) or vehicle (VEH), spontaneous Ca2+ waves were quantified during a 40 s pause after 1 Hz pacing train in the presence of isoproterenol (ISO, 1 mu M). FLEC efficacy was also tested in vivo using a low dose (LOW: 3 mg/kg ISO + 60 mg/kg caffeine) or a high dose catecholamine challenge (HIGH: 3 mg/kg ISO + 120 mg/kg caffeine). Results: In cardiomyocytes, FLEC efficacy was dependent on extracellular [Ca2+]. At 2 mM [Ca2+], only Casq2(-/-) myocytes exhibited Ca2+ waves, which were strongly suppressed by FLEC. At 3 mM [Ca2+] both groups exhibited Ca2+ waves that were suppressed by FLEC. At 4 mM [Ca2+], FLEC no longer suppressed Ca2+ waves in both groups. Analogous to the results in myocytes, RyR2(R)(4496C)(+/- )mice (n = 12) had significantly lower arrhythmia scores than Casq2(-/-) mice (n = 9), but the pattern of FLEC efficacy was similar in both groups (i.e., reduced FLEC efficacy after HIGH dose catecholamine challenge). Conclusion: FLEC inhibits Ca2+ waves in RyR2(R4496)(C+/-) cardiomyocytes, indicating that RyR2 channel block by FLEC is not mutation-specific. However, FLEC efficacy is reduced by Ca2+ overload in vitro or by high dose catecholamine challenge in vivo, which could explain conflicting literature reports.