Estimation of potency of HERG channel blockers: impact of voltage protocol and temperature.
BACKGROUND
The HERG channel is widely used for the assessment of proarrhythmic risk for new drugs. HERG channel blockers obstruct channel functions through various mechanisms, which usually show time dependence, voltage dependence, and state dependence. The voltage protocol and temperature may affect the estimation of drug potency, but limited information is available in this regard. The purpose of this study was to evaluate the influence of voltage protocol and temperature on predicting the potency of HERG channel blockers, and to determine electrophysiological approaches for new drugs screening studies.


METHODS
Whole-cell patch-clamp electrophysiology was carried out by utilizing different voltage step protocols to examine the potency of compounds known to preferentially block the channel in the closed (ketoconazole and BeKm-1), open, and/or inactivated states (E-4031, astemizole, and terfenadine) in HEK293 cells transfected with HERG cDNA at room temperature and near-physiological temperature.


RESULTS
Drug potency determined using different voltage protocols varied dependent on the mechanisms of drug actions. For most compounds, the IC(50) values obtained with a long pulse step protocol at room temperature were close to those determined with the voltage protocols designed to disclose their intrinsic potency. Relative to room temperature, the potency of E-4031, terfenadine, and ketoconazole was not changed at approximately 35 degrees C, but potency of astemizole was reduced.


CONCLUSIONS
The long pulse step protocol with room temperature can be selected for HERG channel safety screening studies. Alternative voltage protocols or temperatures should be considered if HERG study results are not consistent with other cardiac safety assessments.
Yao
Jian-An
JA
Department of Safety Pharmacology, Quintiles, Kansas City, MO 64137, USA. jianan.yao@quintiles.com
Du
Xiaoyi
X
Lu
Daniel
D
Baker
Robert L
RL
Daharsh
Eric
E
Atterson
Philip
P
D016589:Astemizole / Q000009:adverse effects; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D004353:Drug Evaluation, Preclinical / Q000379:methods*; D064420:Drug-Related Side Effects and Adverse Reactions; D006358:Hot Temperature; D006801:Humans; D020128:Inhibitory Concentration 50; D007654:Ketoconazole / Q000009:adverse effects; D018408:Patch-Clamp Techniques / Q000379:methods*; D004364:Pharmaceutical Preparations / Q000145:classification; D010880:Piperidines / Q000009:adverse effects; D026902:Potassium Channel Blockers / Q000009:adverse effects*; D024642:Potassium Channels, Voltage-Gated / Q000037:antagonists & inhibitors* / Q000187:drug effects*; D011725:Pyridines / Q000009:adverse effects; D012604:Scorpion Venoms / Q000009:adverse effects; D016593:Terfenadine / Q000009:adverse effects; D014162:Transfection
C425305:BeKm-1 toxin; D004364:Pharmaceutical Preparations; D010880:Piperidines; D026902:Potassium Channel Blockers; D024642:Potassium Channels, Voltage-Gated; D011725:Pyridines; D012604:Scorpion Venoms; C063968:E 4031; D016593:Terfenadine; D016589:Astemizole; D007654:Ketoconazole