Direct bioanalysis or indirect calculation of target engagement and free drug exposure: do we apply double standards?

Abstract

Analysis of "free"drug/target concentrations is important to set up appropriate pharmacokinetic-pharmacodynamic models, to evaluate active-drug exposure and target engagement. Such "free-analyte"determination could be done by direct bioanalysis using an appropriate "free-analyte"assay. Development of "free"assays is often considered challenging from a technological and regulatory perspective. The application of a "total-total"approach, where the "free-analyte"concentration is determined mathematically, is considered a more convenient option. In this perspective, we examine and discuss the challenges of this "total-total"approach, from the affinity data, the importance of applying an appropriate "total"assay, the impact of additional binding partners and the variability of the total drug/target assays and their impact on the quality and variability of the final "free-analyte"dataset.