Paradoxical effect of mexiletine treatment in lqt3: In vitro and in silico analysis.

PARADOXICAL EFFECT OF MEXILETINE TREATMENT IN LQT3: IN VITRO AND IN SILICO ANALYSIS INTRODUCTION Sodium channel blockers are used as gene-specific treatment in Long-QT syndrome type 3 (LQT3) that is caused by mutations in the sodium channel gene (SCN5A). Response to treatment varies based on mutation characters. The aim of this study is to investigate the unexpected deleterious effect of the sodium channel blocker mexiletine observed in a LQT3 patient with F1473S mutation: after the treatment, corrected QT interval increased and the patient experienced more torsade de points. METHODS Genetic analysis of the patient was performed [1]. The SCN5A mutation F1473S was expressed in human embryonic kidney (HEK293) cells [2] and membrane currents were measured using whole-cell patch clamp procedures at room temperature. The effect of 48 h incubation with 10 μM mexiletine were tested. Consequences of the biophysical properties of the mutation on action potential were investigated in silico. A Markovian model of the Na+ channel (see [3] for details) was implemented. Parameters defining transition rates, which are functions of the membrane potential (Vm), were identified in order to fit data from experimental registrations. This Markovian description replaced the sodium current Hodgin-Huxley formulation in an action potential model of human ventricular myocyte [4, 5], in which all the membrane currents are described by ordinary differential equations. The effects of mutation and of mexiletine treatment were analyzed. RESULTS AND DISCUSSION In comparison with wild-type (WT), mutation expressed in HEK293 cells presented rightward shift of steady-state inactivation, enlarged window current and huge sustained sodium current. Unexpectedly, however, it also reduced the peak sodium current by 80%: immunostaining showed that clusters of Na+ channel were retained in the cytoplasm. Incubation with 10 μM mexiletine rescued the trafficking defect of F1473S, causing a significant increase in peak current, as rescue effect overwhelmed the block effect. On the contrary, sustained current was unchanged since rescued trafficking simply offset the block effect. Simulation results showed that exposure of F1473S to mexiletine increases action potential duration (APD) over a wide range of pacing frequencies. At fast rates adaptation of APD is impaired resulting in capture failure. In silico analysis pointed out that the detrimental effect of mexiletine is mainly due to the window current, which is very large in mutant channels and further increased by the drug. Sodium channel blockers may be deleterious in selected SCN5A mutations. Clinical phenotype is not enough for predicting mutation character and response to sodium channel blockers, in vitro and in silico analysis may help to choose the proper treatment. REFERENCES [1] Napolitano C. et al., JAMA 2005; 294:2975-2980. [2] Ruan Y. et al., Circulation 2007; 116:1137-1144. [3] Grandi E. et al., Biophys J 2007; 93:3835-3847. [4] Ten Tusscher K. H. et al., Am J Physiol Heart Circ Physiol 2006; 291(3):H1088-H1100. [5] Severi S. et al., Phil Trans 2009; 367:2203-2223.