Characterization of a hERG Screen Using the IonWorks HT: Comparison to a hERG Rubidium Efflux Screen

The introduction of parallel patch clamp instruments offers the promise of moderate-throughput, high-fidelity voltage clamp for drug screening assays. One such device, the IonWorks^™ HT (Molecular Devices, Sunnyvale, CA), was evaluated and compared to conventional human ethera- go-go-related gene (hERG) patch clamp data and an alternative functional screen based on rubidium flux. Data generated by the IonWorks HT and rubidium assays were compared to determine if either offered superior predictive value compared to conventional patch clamp. Concentration–effect curves for a panel of known hERG blockers were shifted to higher concentrations on the IonWorks HT compared to conventional voltage clamp determinations. The magnitude of the potency shifts was compound-specific and ranged from no shift (e.g., quinidine) to over 200-fold (astemizole). When the extreme value for astemizole was disregarded, the potency shift for 13 other known reference standards was 12-fold or less, with an average shift of fivefold. The same subset of compounds in the rubidium efflux assay exhibited an average potency shift of 12-fold. To provide a simulation of how the IonWorks HT assay might perform in a single concentration screening mode, a panel of test compounds was evaluated. The IonWorks HT screen did not outperform the rubidium efflux screen in predicting conventional voltage clamp measurements. The most likely explanation appears to rest with variable and compound-specific potency shifts in the IonWorks HT assay. The variable potency shifts make it difficult to select a screening concentration that meets the criterion of a high positive predictive value while avoiding false-positives.