title: Safety pharmacology assessment of drug-induced QT-prolongation in dogs with reduced repolarization reserve.
abstract: BACKGROUND
  Drug-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 microM). 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 microM; 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 microM; 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 approximately 1.2 mM, IC20 approximately 250 microM) and droperidol (IC50 approximately 0.1 microM, IC20 approximately 0.02 microM). Purkinje fiber APs were concentration-dependently prolonged by D,L-sotalol (APD90:+60% at 30 microM) and droperidol (APD90:+55% at 1 microM). Low droperidol concentrations increased the sensitivity of Purkinje fibers towards D,L-sotalol-mediated AP prolongation.
  
  
  CONCLUSIONS
  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.
authors.affiliation: BHC-PH-GDD-Toxicology, Department Clinical Pathology and Safety Studies, Bayer HealthCare AG, Aprather Weg 18a, D-42096 Wuppertal, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Vormberge
authors.forename: Michael
authors.initials: M
authors.lastname: Hoffmann
authors.forename: Herbert
authors.initials: H
authors.lastname: Himmel
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000319:Adrenergic beta-Antagonists / Q000633:toxicity; D000818:Animals; D000889:Anti-Arrhythmia Agents / Q000633:toxicity; D014150:Antipsychotic Agents / Q000633:toxicity; D002460:Cell Line; D003627:Data Interpretation, Statistical; D004285:Dogs; D004329:Droperidol / Q000633:toxicity; D064420:Drug-Related Side Effects and Adverse Reactions; D004594:Electrophysiology; D005260:Female; D006801:Humans; D007262:Infusions, Intravenous; D008133:Long QT Syndrome / Q000139:chemically induced* / Q000503:physiopathology; D008297:Male; D018408:Patch-Clamp Techniques; D010880:Piperidines / Q000633:toxicity; D011725:Pyridines / Q000633:toxicity; D011817:Rabbits; D013015:Sotalol / Q000633:toxicity; D014162:Transfection
chemical_list: D000319:Adrenergic beta-Antagonists; D000889:Anti-Arrhythmia Agents; D014150:Antipsychotic Agents; D010880:Piperidines; D011725:Pyridines; C063968:E 4031; D013015:Sotalol; D004329:Droperidol