Terfenadine-Induced Torsade de Pointes: Risk Factors and Mechanisms

Abstract

Objective:

To review the risk factors and mechanisms of terfenadine-induced torsade de pointes and to discuss how this adverse reaction might be avoided.

Data Sources:

Previous reports of terfenadine-induced torsade de pointes and studies of the underlying mechanisms were identified by a MEDLINE search or from the reference lists of pertinent articles.

Study Selection and Data Extraction:

All relevant articles were included in the review. Pertinent information was selected for discussion.

Data Synthesis:

Terfenadine is extensively (99%) metabolized by CYP3A4 to an active acid metabolite (terfenadine carboxylate), and with therapeutic dosages, unchanged terfenadine is usually undetectable in plasma. A review of all the reported cases of torsade de pointes indicated that most patients had one or more factors that would be expected to cause excessively high concentrations of unchanged terfenadine, such as overdose; use of supratherapeutic dosages; concurrent use of CYP3A4 inhibitors such as ketoconazole, itraconazole, erythromycin, and troleandomycin; and liver dysfunction. Many patients had one or more factors known to predispose to drug-induced torsade de pointes (e.g., preexisting prolonged QT interval, ischemic heart disease, hypokalemia). Pharmacokinetic studies in healthy volunteers have shown that ketoconazole, itraconazole, erythromycin, and clarithromycin can alter the metabolism of terfenadine and result in the accumulation of unchanged terfenadine, which is associated with significant prolongation of the QT interval. In vitro studies have shown that the proarrhythmic effects of terfenadine are secondary to the blockade of cardiac potassium channels. Terfenadine carboxylate does not have such an effect.

Conclusions:

Supratherapeutic dosages of terfenadine should never be used. The concurrent use of CYP3A4 inhibitors should be avoided. Terfenadine should be avoided in patients with liver dysfunction or factors known to predispose to drug-induced torsade de pointes.