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

The in vivo occupancy of brain benzodiazepine binding sites by compounds A and B was measured using a [ 3 H]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 n M , respectively), however, there was little difference in the brain concentrations required (73 and 63 n M ). Both compounds showed a non‐linear relationship between plasma and brain concentrations such that above brain concentrations of ∼100 n M 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 Occ 50 brain concentrations in the α1H101R animals was lower (17 n M ) than in wild type mice (63 n M ), as was the plateau concentration in the brain (105 and 195 n M , 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.