Variability in the measurement of hERG potassium channel inhibition: Effects of temperature and stimulus pattern

Introduction: In vitro evaluation of drug effects on hERG K + channels is a valuable tool for identifying potential proarrhythmic side effects in drug safety testing. Patch-clamp recording of hERG K + current in mammalian cells can accurately evaluate drug effects, but the methodology has not been standardized, and results vary widely. Our objective was to evaluate two potential sources of variability: the temperature at which recordings are performed and the voltage pulse protocol used to activate hERG K + channels expressed in HEK293 cells. Methods: A panel of 15 drugs that spanned a broad range of potency for hERG inhibition and pharmacological class was evaluated at both room and near-physiological temperatures using several patch-clamp voltage protocols. Concentration–response analysis was performed with three stimulus protocols: 0.5- and 2-s step pulses, or a step-ramp pattern. Results: Block by 2 of the 15 drugs tested, d,l-sotalol (antiarrhythmic) and erythromycin (antibiotic), was markedly temperature sensitive. hERG inhibition measured using a 2-s step-pulse protocol underestimated erythromycin potency compared with results obtained with a step-ramp protocol. Using conservative acceptance criteria and the step-ramp protocol, the IC50 values for hERG block differed by less than twofold for 15 drugs. Discussion: Data obtained at near-physiological temperatures using a step-ramp pattern are highly repeatable and provide a conservative safety evaluation of hERG inhibition.