The plasma-occupancy relationship of the novel GABAA receptor benzodiazepine site ligand, alpha5IA, is similar in rats and primates

Br J Pharmacol. 2009 Jul;157(5):796-803. doi: 10.1111/j.1476-5381.2009.00216.x. Epub 2009 Apr 30.


Background and purpose: alpha5IA (3-(5-methylisoxazol-3-yl)-6-[(1-methyl-1,2,3-triazol-4-yl)methyloxy]-1,2,4-triazolo[3,4-a]phthalazine) is a triazolophthalazine with subnanomolar affinity for alpha1-, alpha2-, alpha3- and alpha5-containing GABA(A) receptors. Here we have evaluated the relationship between plasma alpha5IA concentrations and benzodiazepine binding site occupancy in rodents and primates (rhesus monkey).

Experimental approach: In awake rats, occupancy was measured at various times after oral dosing with alpha5IA (0.03-30 mgxkg(-1)) using an in vivo {[(3)H]flumazenil (8-fluoro 5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester)} binding assay. In anaesthetized rhesus monkeys, occupancy was measured using {[(123)I]iomazenil (ethyl 5,6-dihydro-7-iodo-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester)} gamma-scintigraphy and a bolus/infusion paradigm. In both rat and rhesus monkey, the plasma drug concentration corresponding to 50% occupancy (EC(50)) was calculated.

Key results: In rats, alpha5IA occupancy was dose- and time-dependent with maximum occupancy occurring within the first 2 h. However, rat plasma EC(50) was time-independent, ranging from 42 to 67 ngxmL(-1) over a 24 h time course with the average being 52 ngxmL(-1) (i.e. occupancy decreased as plasma drug concentrations fell). In rhesus monkeys, the EC(50) for alpha5IA displacing steady-state [(123)I]iomazenil binding was 57 ngxmL(-1).

Conclusions and implications: Rat plasma EC(50) values did not vary as a function of time indicating that alpha5IA dissociates readily for the GABA(A) receptor in vivo. These data also suggest that despite the different assays used (terminal assays of [(3)H]flumazenil in vivo binding in rats and [(123)I]iomazenil gamma-scintigraphy in anaesthetized rhesus monkeys), these techniques produced similar plasma alpha5IA EC(50) values (52 and 57 ngxmL(-1) respectively) and that the plasma-occupancy relationship for alpha5IA translates across these two species.

MeSH terms

  • Administration, Oral
  • Animals
  • Benzodiazepines / administration & dosage
  • Benzodiazepines / blood
  • Benzodiazepines / metabolism*
  • Benzodiazepines / pharmacokinetics
  • Binding Sites
  • Binding, Competitive
  • Brain / diagnostic imaging
  • Brain / metabolism*
  • Dose-Response Relationship, Drug
  • Flumazenil / administration & dosage
  • Flumazenil / analogs & derivatives
  • Flumazenil / metabolism
  • GABA-A Receptor Antagonists
  • Infusions, Intravenous
  • Injections, Intravenous
  • Iodine Radioisotopes
  • Ligands
  • Macaca mulatta
  • Male
  • Pyridines / administration & dosage
  • Pyridines / blood
  • Pyridines / metabolism*
  • Pyridines / pharmacokinetics
  • Rats
  • Rats, Sprague-Dawley
  • Receptors, GABA-A / metabolism*
  • Tomography, Emission-Computed, Single-Photon


  • GABA-A Receptor Antagonists
  • Iodine Radioisotopes
  • Ligands
  • Pyridines
  • Receptors, GABA-A
  • alpha5IA-II
  • Benzodiazepines
  • Flumazenil
  • iomazenil