Therapeutic Efficacy of Mexiletine for Long QT Syndrome Type 2: Evidence From Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes, Transgenic Rabbits, and Patients.

BACKGROUND: Despite major advances in the clinical management of long QT syndrome, some patients are not fully protected by beta-blocker therapy. Mexiletine is a well-known sodium channel blocker, with proven efficacy in patients with sodium channel-mediated long QT syndrome type 3. Our aim was to evaluate the efficacy of mexiletine in long QT syndrome type 2 (LQT2) using cardiomyocytes derived from patient-specific human induced pluripotent stem cells, a transgenic LQT2 rabbit model, and patients with LQT2. METHODS: Heart rate-corrected field potential duration, a surrogate for QTc, was measured in human induced pluripotent stem cells from 2 patients with LQT2 (KCNH2-p.A561V, KCNH2-p.R366X) before and after mexiletine using a multiwell multi-electrode array system. Action potential duration at 90% repolarization (APD90) was evaluated in cardiomyocytes isolated from transgenic LQT2 rabbits (KCNH2-p.G628S) at baseline and after mexiletine application. Mexiletine was given to 96 patients with LQT2. Patients were defined as responders in the presence of a QTc shortening ≥40 ms. Antiarrhythmic efficacy of mexiletine was evaluated by a Poisson regression model. RESULTS: After acute treatment with mexiletine, human induced pluripotent stem cells from both patients with LQT2 showed a significant shortening of heart rate-corrected field potential duration compared with dimethyl sulfoxide control. In cardiomyocytes isolated from LQT2 rabbits, acute mexiletine significantly shortened APD90 by 113 ms, indicating a strong mexiletine-mediated shortening across different LQT2 model systems. Mexiletine was given to 96 patients with LQT2 either chronically (n=60) or after the acute oral drug test (n=36): 65% of the patients taking mexiletine only chronically and 75% of the patients who performed the acute oral test were responders. There was a significant correlation between basal QTc and ∆QTc during the test (r= -0.8; P<0.001). The oral drug test correctly predicted long-term effect in 93% of the patients. Mexiletine reduced the mean yearly event rate from 0.10 (95% CI, 0.07-0.14) to 0.04 (95% CI, 0.02-0.08), with an incidence rate ratio of 0.40 (95% CI, 0.16-0.84), reflecting a 60% reduction in the event rate (P=0.01). CONCLUSIONS: Mexiletine significantly shortens cardiac repolarization in LQT2 human induced pluripotent stem cells, in the LQT2 rabbit model, and in the majority of patients with LQT2. Furthermore, mexiletine showed antiarrhythmic efficacy. Mexiletine should therefore be considered a valid therapeutic option to be added to conventional therapies in higher-risk patients with LQT2.

Long QT syndrome: importance of reassessing arrhythmic risk after treatment initiation.

BACKGROUND AND AIMS: Risk scores are proposed for genetic arrhythmias. Having proposed in 2010 one such score (M-FACT) for the long QT syndrome (LQTS), this study aims to test whether adherence to its suggestions would be appropriate. METHODS: LQT1/2/3 and genotype-negative patients without aborted cardiac arrest (ACA) before diagnosis or cardiac events (CEs) below age 1 were included in the study, focusing on an M-FACT score ≥2 (intermediate/high risk), either at presentation (static) or during follow-up (dynamic), previously associated with 40% risk of implantable cardioverter defibrillator (ICD) shocks within 4 years. RESULTS: Overall, 946 patients (26 ± 19 years at diagnosis, 51% female) were included. Beta-blocker (ßB) therapy in 94% of them reduced the rate of those with a QTc ≥500 ms from 18% to 12% (P < .001). During 7 ± 6 years of follow-up, none died; 4% had CEs, including 0.4% with ACA. A static M-FACT ≥2 was present in 110 patients, of whom 106 received ßBs. In 49/106 patients with persistent dynamic M-FACT ≥2, further therapeutic optimization (left cardiac sympathetic denervation in 55%, mexiletine in 31%, and ICD at 27%) resulted in just 7 (14%) patients with CEs (no ACA), with no CEs in the remaining 57. Additionally, 32 patients developed a dynamic M-FACT ≥2 but, after therapeutic optimization, only 3 (9%) had CEs. According to an M-FACT score ≥2, a total of 142 patients should have received an ICD, but only 22/142 (15%) were implanted, with shocks reported in 3. CONCLUSIONS: Beta-blockers often shorten QTc, thus changing risk scores and ICD indications for primary prevention. Yearly risk reassessment with therapy optimization leads to fewer ICD implants (3%) without increasing life-threatening events.