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


METHODS
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.


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
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.
Atack
John R
JR
Merck Sharp and Dohme Research Laboratories, The Neuroscience Research Centre, Harlow, Essex, UK. JAtack1@its.jnj.com
Eng
Wai-Si
WS
Gibson
Ray E
RE
Ryan
Christine
C
Francis
Barbara
B
Sohal
Bindi
B
Dawson
Gerard R
GR
Hargreaves
Richard J
RJ
Burns
H Donald
HD
D000284:Administration, Oral; D000818:Animals; D001569:Benzodiazepines / Q000008:administration & dosage / Q000097:blood / Q000378:metabolism* / Q000493:pharmacokinetics; D001665:Binding Sites; D001667:Binding, Competitive; D001921:Brain / Q000000981:diagnostic imaging / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D005442:Flumazenil / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000378:metabolism; D058787:GABA-A Receptor Antagonists; D007262:Infusions, Intravenous; D007275:Injections, Intravenous; D007457:Iodine Radioisotopes; D008024:Ligands; D008253:Macaca mulatta; D008297:Male; D011725:Pyridines / Q000008:administration & dosage / Q000097:blood / Q000378:metabolism* / Q000493:pharmacokinetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000378:metabolism*; D015899:Tomography, Emission-Computed, Single-Photon
D058787:GABA-A Receptor Antagonists; D007457:Iodine Radioisotopes; D008024:Ligands; D011725:Pyridines; D011963:Receptors, GABA-A; C510446:alpha5IA-II; D001569:Benzodiazepines; D005442:Flumazenil; C063038:iomazenil