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 UP 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 UP 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 UP. The vast majority of drugs associated with QT liability and the potential for drug-induced UP, including all of the drugs removed from the market because of this side effect, are HERG (human ether- a-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 UP. 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 UP). 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.