Contribution of specific binding to the central benzodiazepine site to the brain concentrations of two novel benzodiazepine site ligands

Abstract

The in vivo occupancy of brain benzodiazepine binding sites by compounds A and B was measured using a [³H]Ro 15-1788 binding assay and related to plasma and brain drug concentrations. The plasma concentration associated with 50% occupancy was higher for compound A than compound B (73 and 3.7 nM, respectively), however, there was little difference in the brain concentrations required (73 and 63 nM). Both compounds showed a non-linear relationship between plasma and brain concentrations such that above brain concentrations of ∼100 nM increasing plasma concentrations did not result in a concomitant increase in brain concentrations. This is consistent with brain concentrations being dependent on a saturable compartment which was postulated to be the benzodiazepine binding site-containing GABA_A receptors. This hypothesis was tested in α1H101R mice, in which the α1 subunit of the GABA_A receptor is rendered insensitive to benzodiazepine binding resulting in an approximate 50% reduction in the total benzodiazepine-containing GABA_A receptor population. It was shown that the Occ₅₀ brain concentrations in the α1H101R animals was lower (17 nM) than in wild type mice (63 nM), as was the plateau concentration in the brain (105 and 195 nM, respectively). These data suggest measured concentrations of compounds A and B in brain tissue are dependent on receptor expression with a minimal contribution from unbound and non-specifically bound compound. Copyright © 2007 John Wiley & Sons, Ltd.