title: 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 [(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 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 approximately 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 alpha1H101R mice, in which the alpha1 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 alpha1H101R 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.
authors.affiliation: Department of Medicinal Chemistry, Merck Sharp and Dohme Research Laboratories, Neuroscience Research Centre, Terlings Park, Harlow, UK. a.pike@astex-therapeutics.com
authors.forename: Andrew
authors.initials: A
authors.lastname: Pike
authors.forename: Susan M
authors.initials: SM
authors.lastname: Cook
authors.forename: Alan P
authors.initials: AP
authors.lastname: Watt
authors.forename: Paul
authors.initials: P
authors.lastname: Scott-Stevens
authors.forename: Thomas W
authors.initials: TW
authors.lastname: Rosahl
authors.forename: Ruth M
authors.initials: RM
authors.lastname: McKernan
authors.forename: Luanda J
authors.initials: LJ
authors.lastname: Pym
authors.forename: Alec
authors.initials: A
authors.lastname: Guiblin
authors.forename: Christopher
authors.initials: C
authors.lastname: Moyes
authors.forename: Bindi
authors.initials: B
authors.lastname: Sohal
authors.forename: John R
authors.initials: JR
authors.lastname: Atack
mesh_terms: D000284:Administration, Oral; D000818:Animals; D014151:Anti-Anxiety Agents / Q000008:administration & dosage / Q000378:metabolism / Q000493:pharmacokinetics; D000927:Anticonvulsants / Q000008:administration & dosage / Q000378:metabolism / Q000493:pharmacokinetics; D001569:Benzodiazepines / Q000008:administration & dosage / Q000378:metabolism* / Q000493:pharmacokinetics; D001570:Benzodiazepinones / Q000008:administration & dosage / Q000378:metabolism / Q000493:pharmacokinetics; D001665:Binding Sites; D001667:Binding, Competitive / Q000187:drug effects; D001921:Brain / Q000187:drug effects / Q000378:metabolism*; D001923:Brain Chemistry; D004305:Dose-Response Relationship, Drug; D005442:Flumazenil / Q000008:administration & dosage / Q000378:metabolism / Q000493:pharmacokinetics; D005445:Flunitrazepam / Q000008:administration & dosage / Q000378:metabolism / Q000493:pharmacokinetics; D018757:GABA Modulators / Q000008:administration & dosage / Q000378:metabolism / Q000493:pharmacokinetics; D058785:GABA-A Receptor Agonists; D007275:Injections, Intravenous; D008024:Ligands; D008297:Male; D051379:Mice; D008817:Mice, Mutant Strains; D011724:Pyridazines / Q000008:administration & dosage / Q000378:metabolism / Q000493:pharmacokinetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000235:genetics / Q000378:metabolism*; D014018:Tissue Distribution; D014227:Triazines / Q000008:administration & dosage / Q000378:metabolism / Q000493:pharmacokinetics
chemical_list: D014151:Anti-Anxiety Agents; D000927:Anticonvulsants; D001570:Benzodiazepinones; D018757:GABA Modulators; D058785:GABA-A Receptor Agonists; D008024:Ligands; D011724:Pyridazines; D011963:Receptors, GABA-A; D014227:Triazines; D001569:Benzodiazepines; D005442:Flumazenil; D005445:Flunitrazepam; C054626:bretazenil