Safety pharmacology assessment of drug-induced QT-prolongation in dogs with reduced repolarization reserve

IntroductionDrug-induced QT-prolongation, often based on hERG K+ current inhibition, has become a major safety concern during drug development. Hence, regulatory guidelines require combined in vitro and in vivo assays to assess the potential of new chemical entities to delay ventricular repolarization. Here, results of a pharmacological validation study with the torsadogenic compound sotalol are presented. Methods: Alteration of ECG parameters was investigated in both conscious and anesthetized Beagle dogs (cumulative infusions of d,l-sotalol; n = 6). The repolarization reserve of the latter was reduced by neurolept anesthesia using the hERG blocker droperidol (0.25 mg/kg/h yielding mean plasma concentrations of 0.5 μM). Furthermore, hERG K+ current and action potentials (AP; rabbit Purkinje fibers) were measured in vitro. Results: The Fridericia corrected QT interval, QTcF, in conscious dogs (control: 254 ± 15 ms), was dose-dependently prolonged by d,l-sotalol (+ 42 ms at plasma levels of 261 μM; dose 30 mg/kg). In anesthetized dogs, baseline QTcF (337 ± 35 ms) was already prolonged compared to conscious dogs. In addition, QTcF-increase (+ 90 ms) was more pronounced at lower d,l-sotalol plasma levels (181 μM; dose 10 mg/kg), and proarrhythmic markers Tpeak–Tend and short term variability of QT were increased. These in vivo findings are supported by in vitro data. The hERG K+ current was blocked by d,l-sotalol (IC50 ∼ 1.2 mM, IC20 ∼ 250 μM) and droperidol (IC50 ∼ 0.1 μM, IC20 ∼ 0.02 μM). Purkinje fiber APs were concentration-dependently prolonged by d,l-sotalol (APD90 : + 60% at 30 μM) and droperidol (APD90 : + 55% at 1 μM). Low droperidol concentrations increased the sensitivity of Purkinje fibers towards d,l-sotalol-mediated AP prolongation. Discussion: In conclusion, the higher sensitivity of anesthetized dogs towards sotalol-induced QT-prolongation is due to a reduced cardiac repolarization reserve caused by the hERG blocker droperidol. Hence, the droperidol-/fentanyl-/N2O-anesthetized dog is a particularly sensitive animal model for the detection of drug-induced QT-prolongation in safety pharmacology studies.