No proarrhythmic properties of the antibiotics Moxifloxacin or Azithromycin in anaesthetized dogs with chronic‐AV block

Background & purpose: The therapeutically available quinolone antibiotic moxifloxacin has been used as a positive control for prolonging the QT interval in both clinical and non‐clinical studies designed to assess the potential of new drugs to delay cardiac repolarization. Despite moxifloxacin prolonging QT, it has not been shown to cause torsades de pointes arrhythmias (TdP). Azithromycin is a macrolide antibiotic that has rarely been associated, clinically, with cases of proarrhythmia. As there is a lack of clinical data available, the cardiac safety of these drugs was assessed in a TdP‐susceptible animal model by evaluating their repolarization and proarrhythmia effects. Experimental approach & Key results: In transfected HEK cells, the IC 50 s for I hERG were 45±6 and 856±259 μg ml ‐1 for moxifloxacin and azithromycin, respectively. Intravenous administration of 2 and 8 mg kg ‐1 moxifloxacin (total peak‐plasma concentrations 4.6±1.5 and 22.9±6.8 μg ml ‐1 ) prolonged the QT c in 6 anaesthetized dogs with chronic AV block by 7±3 and 21±19%, respectively. Similar intravenous doses of azithromycin (total peak‐plasma concentrations 5.4±1.3 and 20.8±4.9 μg ml ‐1 ) had no electrophysiological effects in the same dogs. The reference compound, dofetilide (25 μg kg ‐1 i.v.) caused QT c prolongation (29±15%) and TdP in all dogs. Beat‐to‐beat variability of repolarization (BVR), quantified as short‐term variability of the left ventricular monophasic action potential duration, was only increased after dofetilide (1.8±0.7 to 3.8±1.5 ms; P <0.05). Conclusion & implications: As neither moxifloxacin nor azithromycin caused TdP or an increase in the BVR, we conclude that both drugs can be used safely in clinical situations. British Journal of Pharmacology (2006) 149 , 1039–1048. doi: 10.1038/sj.bjp.0706900