Genetic polymorphisms, drugs, and proarrhythmia

Influence of the G2677T/C3435T haplotype of MDR1 on P-glycoprotein trafficking and ibutilide-induced block of HERG

The drug efflux pump P-glycoprotein possesses two common and often linked polymorphisms that result in variable drug action. G2677T results in A893S, whereas C3435T is synonymous and has been reported to alter protein folding. We tested the effect of these MDR1 variants on Human Ether-Related A Go-Go (HERG) block by ibutilide in CHO cells 48 h following transient transfection with an IRES-dsRed vector containing MDR1, G2677T MDR1, G2677T/C3435T MDR1 or an empty bicistronic site and an IRES-GFP vector containing HERG (KCNH2). Cotransfection of MDR1 variants had no effect on I(Kr) amplitude at baseline. Cells cotransfected with MDR1-G2677T showed resistance to ibutilide vs HERG alone (IC(50): 105.3+/-1.42 nM vs 27.4+/-2.5 nM; P<0.0001), consistent with the idea that A893S attenuates I(Kr) block by enhancing drug efflux and thus reducing the drug available to interact with the channel binding site. However, G2677T/C3435T cells showed ibutilide sensitivity similar to cells expressing HERG alone (IC(50): 22.2+/-0.9 nM). Immunostaining showed that the C3435T variant did not traffic to the cell surface. Coculture with fexofenadine(1 microM), an MDR1 substrate known to rescue misfolding in other membrane proteins, restored cell surface expression of MDR1 G2677T/C3435T and restored resistance to block HERG by ibutilide 200 nM (98.5+/-0.98% vs 42.3+/-2.2%, P<0.001). The non-synonymous MDR1 variant G2677 T (A893S) confers resistance to ibutilide block of I(Kr), which is mitigated by the C3435T polymorphism through reduced protein expression, an effect that can be restored by coculture with fexofenadine. These data identify ibutilide as an MDR1 substrate and further support the concept that variable drug transport function can modulate the action of HERG blockers.

Cardiotoxic effects of antihistamines: from basics to clinics (...and back)

Drug-induced arrhythmias, particularly those caused by a prolonged QT interval, have become a critical safety issue for compound selection during development by pharmaceutical companies and for health care regulators. The last two decades have witnessed enormous progress in the definition of the clinical conditions that facilitate the occurrence of such serious adverse effects, of its molecular basis, and in the preclinical strategies aimed at early identification of the cardiotoxic liability of compounds undergoing investigation or already used in the clinic. Moreover, despite the fact that acquired factors play an obvious role in drug-induced arrhythmias, it has become evident that the disease is often manifested upon the interaction of strong environmental stressors with specific genetic determinants of the affected individuals; in that sense, few examples can illustrate the existing interaction between acquired and genetic factors in disease manifestation better than drug-induced arrhythmogenesis. Progress in this field has been mainly driven by a strong interaction among various disciplines, including medicinal chemistry, pharmacology, electrophysiology, molecular genetics, and clinical cardiology; such an interdisciplinary approach has often generated unexpected discoveries of great clinical value, allowing clinicians to drive drug selection toward compounds of proven efficacy and safety. Historically, studies on antihistamines have paved the way for much of our current understanding of the mechanisms and problems associated with QT prolongation and drug-induced arrhythmogenesis; therefore, in this perspective, we will attempt to summarize how basic research studies have helped the interpretation of clinically relevant phenomena (from basics to clinics...) and how this information has prompted new emphasis in preclinical studies aimed at predicting the cardiotoxic potential of compounds (...and back). The current availability of several strategies provided with great predictive potential, together with an increased awareness of physicians, pharmaceutical industries, and health care regulators to this potentially serious cardiovascular side effect, has significantly decreased the risk associated with drug-induced arrhythmias caused by drugs newly introduced into the market; nevertheless, given the large number of cases of QT prolongation still occurring during treatment with a wide variety of congeners, it seems appropriate to review the issue of the cardiotoxic actions of antihistamines, as a better comprehension of the underlying mechanisms and risk factors is likely to contribute to the improvement of the risk/benefit ratio for pharmacological treatment in several therapeutic areas.

Drug-induced torsades de pointes: The evolving role of pharmacogenetics

Drug-induced torsades de pointes (TdP) is a rare, but potentially lethal, unwanted effect of drugs, including many commonly prescribed noncardiac drugs. Despite its low frequency, drug-induced TdP has generated a great deal of angst among physicians and pharmaceutical companies as well as tragedy, albeit rare, among patients. Although in retrospect many patients who died suddenly as a result of drug-induced TdP had identifiable risk factors, prediction in individual cases remains problematic. Over the past decade, tremendous progress has been made with respect to elucidating the fundamental pathogenic mechanisms that underlie drug-induced TdP. The vast majority of drugs associated with "QT liability" and the potential for drug-induced TdP, including all of the drugs removed from the market because of this side effect, are "HERG (human ether-á-go-go-related gene) blockers." These drugs inhibit the KCNH2-encoded HERG potassium channel, which is one of the critical repolarizing forces involved in the exquisite orchestration of the heart's action potential. Consequently, myocyte repolarization is potentially delayed as evidenced by prolongation of the QT interval, thus providing the substrate for drug-induced TdP. Rare mutations in KCNH2 provide the pathogenic substrate for type 2 congenital long QT syndrome (LQTS), thus placing this cardiac potassium channel squarely in the intersection between congenital LQTS (the "Rosetta stone" of the heritable channelopathies) and acquired LQTS (drug-induced TdP). In addition, common polymorphisms residing in the LQTS-causing channel genes may confer heightened arrhythmogenic susceptibility and contribute to the makings of a vulnerable host. This review focuses on the present strategy of identifying "at-risk compounds" and the potential future strategy involving pharmacogenetics to pinpoint "at-risk hosts" in an effort to curb this rare, unintended, but potentially life-threatening side effect.

Ciprofloxacin-induced acquired long QT syndrome

Quinolone antibiotics have potentially serious proarrhythmic effects. The effects on intracardiac potassium channels result in QT interval prolongation, leading to torsades de pointes. Evidence suggests fluoroquinolones cause QT-mediated proarrhythmia, and weak evidence links ciprofloxacin with QT-mediated arrhythmias. Ciprofloxacin may be given to select patients because the agent is believed to be safer than other drugs in its class. We report two cases of unexplained cardiac arrest temporally related to ciprofloxacin administration. Two female patients (ages 44 and 67 years) developed marked QTc prolongation (QTc 590 and 680 ms) within 24 hours of ciprofloxacin administration, with recurrent syncope and documented torsades de pointes requiring defibrillation. The patients previously were stable with sotalol and amiodarone therapy for supraventricular arrhythmia without obvious QTc prolongation prior to ciprofloxacin therapy. Marked QTc prolongation and subsequent proarrhythmia became a clinical concern only after initiation of ciprofloxacin. In both cases, the QTc normalized after cessation of ciprofloxacin. Ciprofloxacin may cause QTc prolongation and rarely torsades de pointes. This effect is of particular concern in patients with predisposing factors, such as concomitant medications or underlying heart disease reflecting decreased repolarization reserve.

Genomics in sudden cardiac death

Sudden cardiac death (SCD) remains a public health problem of major magnitude. Contrary to earlier expectations, and despite decreased overall cardiac mortality, SCD rates appear to be rising in concert with escalating global prevalence of coronary disease and heart failure, the two major conditions predisposing to SCD. With the exception of the implantable defibrillator, there are few effective approaches to SCD prevention and even fewer clues concerning patient phenotypes predisposed to life-threatening arrhythmias. Clinical variables such as ejection fraction predict mortality but are not sensitive enough to identify many high SCD risk patients. The predictive power of autonomic dysregulation and markers such as lipid levels, hypertension, diabetes, and smoking is quite low in subclinical heart disease, the population in which the majority of SCDs occur. This review addresses advances in genomic science applicable to the SCD public health problem in both rare and common forms of heart disease. These include novel bioinformatic approaches to both identify candidate genes/pathways and identify previously unknown functional genetic elements, as well as methods to comprehensively screen these elements. We also discuss the possibility of applying high-density genome-wide SNP analyses to examine genetic contributions to arrhythmia susceptibility in community-based, case-control studies of common forms of SCD. The development of novel strategies to identify contributors to susceptibility in common cardiac phenotypes is most likely to lead to new and relevant therapeutic targets for SCD.

Dysrhythmias and the Athlete

Young competitive athletes are perceived by the general population to be the healthiest members of society. The possibility that highly trained high school and college athletes may have a potentially serious cardiac condition that can predispose to life-threatening dysrhythmias or sudden cardiac death (SCD) seems paradoxical. The occurrence of SCD in young athletes from dysrhythmias is an uncommon but highly visible event. Media reports of sudden death in athletes have intensified the public and medical interest in medical, ethical, and legal issues related to cardiac disorders in the athlete. Developing screening strategies to identify conditions associated with sudden death has been the focus of attention of experts in the fields of arrhythmology and sports medicine and has resulted in Consensus Statements and Guidelines for evaluation of athletes. These guidelines provide information and recommendations for detection, evaluation, and management of athletes with cardiovascular disorders and criteria for eligibility and disqualification from participation in high-intensity and competitive sports. Differentiating normal exercise-induced physiologic changes in the heart from pathological conditions associated with sudden death is critical for developing screening strategies to identify athletes at high risk. This article discusses a case report of sudden cardiac death in an athlete followed by a brief review of various causes of cardiac dysrhythmias in young athletes and recommendations for screening and management of athletes with cardiovascular diseases.