Optimisation and validation of a medium-throughput electrophysiology-based hERG assay using IonWorks™ HT

IntroductionRegulatory and competitive pressure to reduce the QT interval prolongation risk of potential new drugs has led to focus on methods to test for inhibition of the human ether-a-go-go-related gene (hERG)-encoded K+ channel, the primary molecular target underlying this safety issue. Here we describe the validation of a method that combines medium-throughput with direct assessment of channel function.MethodsThe electrophysiological and pharmacological properties of hERG were compared using two methods: conventional, low-throughput electrophysiology and planar-array-based, medium-throughput electrophysiology (IonWorks™ HT). A pharmacological comparison was also made between IonWorks™ HT and an indirect assay (Rb+ efflux).ResultsBasic electrophysiological properties of hERG were similar whether recorded conventionally (HEK cells) or using IonWorks™ HT (CHO cells): for example, tail current V½ − 12.1 ± 5.0 mV (32) for conventional and − 9.5 ± 6.0 mV (46) for IonWorks™ HT (mean ± S.D. (n)). A key finding was that as the number of cells per well was increased in IonWorks™ HT, the potency reported for a given compound decreased. Using the lowest possible cell concentration (250,000 cells/ml) and 89 compounds spanning a broad potency range, the pIC50 values from IonWorks™ HT (CHO-hERG) were found to correlate well with those obtained using conventional methodology (HEK-hERG)(r = 0.90; p < 0.001). Further validation using CHO-hERG cells with both methods confirmed the correlation (r = 0.94; p < 0.001). In contrast, a comparison of IonWorks™ HT and Rb+ efflux data with 649 compounds using CHO-hERG cells showed that the indirect assay consistently reported compounds as being, on average, 6-fold less potent, though the differences varied depending on chemical series.DiscussionThe main finding of this work is that providing a relatively low cell concentration is used in IonWorks™ HT, the potency information generated correlates well with that determined using conventional electrophysiology. The effect on potency of increasing cell concentration may relate to a reduced free concentration of test compound owing to partitioning into cell membranes. In summary, the IonWorks™ HT hERG assay can generate pIC50 values based on a direct assessment of channel function in a timeframe short enough to influence chemical design.