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.