hERG potency estimates based upon dose solution analysis: What have we learned?

INTRODUCTION: Measurement of drug-induced inhibition of potassium current flow through the hERG channel is used to determine potency at the channel, which is used as an in vitro risk assessment for QTc interval prolongation in vivo. In the hERG assay, test solutions of varying strength are prepared to construct a concentration-response curve based upon the nominal drug concentration (NOM). Dose-solution analysis (DSA) is an analytical approach to confirm the test concentration achieved in an in vitro assay (Herron, Towers, & Templeton, 2004), and can be included as a component of hERG channel study to confirm drug concentration in the assay buffer to determine potency using the "actual" drug level in solution (ACT). Thus, DSA could be helpful in confirming test article concentrations. This study examined whether inclusion of DSA improved the accuracy of potency estimates based upon the ACT compared to the NOM concentration during hERG voltage clamp assays (non-GLP) for 99 diverse agents.

METHODS: We examined the correlation of hERG IC(50) derived from NOM with hERG IC(50) derived from ACT, and analyzed potential mechanisms of deviation between ACT and NOM potency values, including solubility, cLogP, PKa, and molecular weights.

RESULTS: Seventy-four (74) of 99 agents (73.7%) had NOM- and ACT-derived IC(50) values within 3-fold, 87 of 99 (87.8%) had an IC(50) ratio within 10-fold, and 12 (12.1%) had a >10-fold difference in their NOM IC(50) and ACT IC(50) values. On average, these 12 compounds had less soluble, more lipophilic (high cLogP values), and more basic characters (high pKa values).

DISCUSSION: Our investigation indicated that DSA did not alter hERG potency estimation for the majority of compounds in this dataset, i.e., DSA confirmed the NOM concentration within 3-fold. For poorly soluble agents or agents with high cLogP and pKa values, however, DSA did not clarify or improve hERG potency estimates.