ABSTRACT:Currently used methodology for determining unbound drug exposure in brain combines measurement of the total drug concentration in the whole brain in vivo with estimation of brain tissue binding from one of two available in vitro methods: equilibrium dialysis of brain homogenate and the brain slice uptake method. This study of 56 compounds compares the fraction of unbound drug in brain (f u,brain ), determined using the brain homogenate method, with the unbound volume of distribution in brain (V u,brain ), determined using the brain slice method. Discrepancies were frequent and were primarily related to drug pH partitioning, attributable to the preservation of cellular structures in the slice that are absent in the homogenate. A mathematical model for pH partitioning into acidic intracellular compartments was derived to predict the slice V u,brain from measurements of f u,brain and drug pK a .This model allowed prediction of V u,brain from f u,brain within a 2.2-fold error range for 95% of the drugs compared with a 4.5-fold error range using the brain homogenate f u,brain method alone. The greatest discrepancies between the methods occurred with compounds that are actively transported into brain cells, including gabapentin, metformin, and prototypic organic cation transporter substrates. It was concluded that intrabrain drug distribution is governed by several diverse mechanisms in addition to nonspecific binding and that the slice method is therefore more reliable than the homogenate method. As an alternative, predictions of V u,brain can be made from homogenate f u,brain using the pH partition model presented, although this model does not take into consideration possible active brain cell uptake.

A high-throughput method for rapid screening of in vitro drug-brain homogenate binding is presented. The method is based on a straightforward sample pooling approach combining equilibrium dialysis with liquid chromatography mass spectrometry (LCMS). A strong correlation of fraction unbound in brain (fu) between single compound measurements and 25-pooled compounds (R2 = 0.906) was obtained for a selection of structurally diverse CNS compounds with a wide range of fractions unbound. Effects of brain homogenate dilution and dialysis time were investigated. To the best of our knowledge, it was the first time that we have demonstrated consistent fraction unbound in mouse and rat brain homogenate, revealing the drug-tissue partitioning mechanism predominated by hydrophobic interaction. On the basis of this finding, a generic approach to estimate drug binding to various tissues is proposed. A robust and interpretable QSAR for fu prediction is also presented by statistical modeling.

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