Cardiac ion channels and antihistamines: Possible mechanisms of cardiotoxicity

Despite the enormous success of second generation antihistamines, in the mid-1980s, about 10 years after their introduction in the market, several reports appeared in the literature indicating the rare occurrence of a form of polymorphic ventricular dysrhythmia, the 'torsade de pointes', after the administration of astemizole or terfenadine. This cardiac side-effect has been interpreted as a consequence of the interference of these drugs with cardiac K⁺ channels involved in action potential repolarization, and in particular with the IKr component of the cardiac repolarizing current. As the K⁺ channels encoded by the human ether-a-gogo-related gene (HERG) seem to represent the molecular basis of IKr, this cardiac K⁺ channel was soon recognized as a primary target for second generation antihistamine-induced proarrhythmic effects. In fact, both terfenadine and astemizole have been shown to block HERG K⁺ channels in a concentration range similar to that found in the plasma of subjects with cardiotoxic manifestations. However, no correlation can be found between the ability to prolong the cardiac action potential duration and the H₁-antagonistic activity by several antihistamines, suggesting that HERG blockade and cardiotoxic potential are not class properties of second generation antihistamines. In fact, other molecules such as cetirizine, loratadine, acrivastine, and fexofenadine seem to lack both cardiotoxic potential and HERG-blocking ability at therapeutically relevant concentrations. The marked heterogeneity displayed by second generation antihistamines in their ability to prolong the cardiac action potential duration and to block HERG K⁺ channels might be of considerable therapeutical significance for those patients at risk of developing cardiac dysrhythmias and in need of therapy with H₁-receptor blockers; it also emphasizes the importance of an evaluation of the possible blockade of HERG K⁺ channels during the early developmental phases of novel compounds belonging to this therapeutical class.