Effect of flecainide derivatives on sarcoplasmic reticulum calcium release suggests a lack of direct action on the cardiac ryanodine receptor

Abstract

Background and PurposeFlecainide is a use‐dependent blocker of cardiac Na+ channels. Mechanistic analysis of this block showed that the cationic form of flecainide enters the cytosolic vestibule of the open Na+ channel. Flecainide is also effective in the treatment of catecholaminergic polymorphic ventricular tachycardia but, in this condition, its mechanism of action is contentious. We investigated how flecainide derivatives influence Ca2 +‐release from the sarcoplasmic reticulum through the ryanodine receptor channel (RyR2) and whether this correlates with their effectiveness as blockers of Na+ and/or RyR2 channels.Experimental ApproachWe compared the ability of fully charged (QX‐FL) and neutral (NU‐FL) derivatives of flecainide to block individual recombinant human RyR2 channels incorporated into planar phospholipid bilayers, and their effects on the properties of Ca2 + sparks in intact adult rat cardiac myocytes.Key ResultsBoth QX‐FL and NU‐FL were partial blockers of the non‐physiological cytosolic to luminal flux of cations through RyR2 channels but were significantly less effective than flecainide. None of the compounds influenced the physiologically relevant luminal to cytosol cation flux through RyR2 channels. Intracellular flecainide or QX‐FL, but not NU‐FL, reduced Ca2 + spark frequency.Conclusions and ImplicationsGiven its inability to block physiologically relevant cation flux through RyR2 channels, and its lack of efficacy in blocking the cytosolic‐to‐luminal current, the effect of QX‐FL on Ca2 + sparks is likely, by analogy with flecainide, to result from Na+ channel block. Our data reveal important differences in the interaction of flecainide with sites in the cytosolic vestibules of Na+ and RyR2 channels.

Multiple targets for flecainide action: implications for cardiac arrhythmogenesis

“…Similarly in WT RyR2 channels exposed to EMD41000, consequently with high open probabilities, flecainide (10 μM) reduced cytoplasmic‐to‐luminal currents, but not the luminal‐to‐cytosolic current even at higher (50 μM) concentrations (Bannister et al , ). The fully charged (QX‐FL) and neutral (NU‐FL) flecainide derivatives were less effective antagonists of cytoplasmic‐to‐luminal currents and similarly did not affect luminal‐to‐cytosolic current (Bannister et al , ).…”

Safety and efficacy of flecainide for patients with catecholaminergic polymorphic ventricular tachycardia

“…[20,21] The precise mechanism underlying the specificity of flecainide to CPVT is unclear, but current data support a model of flecainide action in which Na + -dependent modulation of intracellular Ca 2+ handling attenuates RyR2 dysfunction. [22,23] Further study is therefore required to fully elucidate the anti-arrhythmic mechanism of flecainide in CPVT.…”

Introduction to biological complexity as a missing link in drug discovery

“…However, the harmonization of experimental data to fit to a 'unifying' MOA of flecainide has been precluded by findings from single molecule studies (i.e. an exemplar TBS) that contradict any direct effect on ryanodine receptors [29, 30,31]. This narrative serves to emphasise the potential utility of lower- The above examples attest to the difficulty of resolving MMOA from studies using more complex systems but also reveal the potential for being unable to delineate between 'cause and effect' by using systems of reduced complexity, especially if the data is considered in isolation from those emerging from complementary approaches.…”

“…After all, the study of very low complexity systems (e.g. flecainide interaction with single ryanodine receptors [29,30,31]) is required to unequivocally resolve fundamental mechanisms of drug-protein interaction separate from any confounding complexity (i.e. the true MMOA).…”