Impacts of reducing the number of hERG assay concentration levels on IC50 estimation

Assessing the human ether-a-go-go-related gene (hERG) safety margin is essential for estimating the risk of delayed repolarization and QT interval prolongation prior to a drug’s first administration in humans. The hERG safety margin is usually defined by the half-inhibitory concentration (IC50) normalized to the drug’s estimated clinical exposures. Per the FDA guidance for industry entitled “E14 and S7b Clinical and Nonclinical Evaluation of QT/QTc Interval Prolongation and Proarrhythmic Potential – Question and Answers” (E14 and S7B Q&A), IC50 can be estimated using sufficient replicates and two or more concentrations achieving 20–80% inhibition. In this project, we explored whether two concentrations achieving 20–80% inhibition are sufficient to estimate an accurate IC50 and assessed the potential impact of using only two concentrations. We also investigated whether the current number of replicates at each concentration is adequate for reliable IC50 determination. Our findings demonstrate that two concentrations achieving 20–80% inhibition or two replicates at each concentration may be insufficient to generate accurate IC50 estimates. Based on these results, we recommend alternative approaches: strategically positioning two concentration levels closer to the anticipated IC50 value, or if expanding to three concentration levels, selecting concentrations that are representative of the complete dose–response curve.