In vitro and in vivo properties of 3-tert-butyl-7-(5-methylisoxazol-3-yl)-2-(1-methyl-1H-1,2,4-triazol-5-ylmethoxy)-pyrazolo[1,5-d]-[1,2,4]triazine (MRK-016), a GABAA receptor alpha5 subtype-selective inverse agonist.
3-tert-Butyl-7-(5-methylisoxazol-3-yl)-2-(1-methyl-1H-1,2,4-triazol-5-ylmethoxy)-pyrazolo[1,5-d][1,2,4]triazine (MRK-016) is a pyrazolotriazine with an affinity of between 0.8 and 1.5 nM for the benzodiazepine binding site of native rat brain and recombinant human alpha1-, alpha2-, alpha3-, and alpha5-containing GABA(A) receptors. It has inverse agonist efficacy selective for the alpha5 subtype, and this alpha5 inverse agonism is greater than that of the prototypic alpha5-selective compound 3-(5-methylisoxazol-3-yl)-6-[(1-methyl-1,2,3-triazol-4-hdyl)methyloxy]-1,2,4-triazolo[3,4-a]phthalazine (alpha5IA). Consistent with its greater alpha5 inverse agonism, MRK-016 increased long-term potentiation in mouse hippocampal slices to a greater extent than alpha5IA. MRK-016 gave good receptor occupancy after oral dosing in rats, with the dose required to produce 50% occupancy being 0.39 mg/kg and a corresponding rat plasma EC(50) value of 15 ng/ml that was similar to the rhesus monkey plasma EC(50) value of 21 ng/ml obtained using [(11)C]flumazenil positron emission tomography. In normal rats, MRK-016 enhanced cognitive performance in the delayed matching-to-position version of the Morris water maze but was not anxiogenic, and in mice it was not proconvulsant and did not produce kindling. MRK-016 had a short half-life in rat, dog, and rhesus monkey (0.3-0.5 h) but had a much lower rate of turnover in human compared with rat, dog, or rhesus monkey hepatocytes. Accordingly, in human, MRK-016 had a longer half-life than in preclinical species ( approximately 3.5 h). Although it was well tolerated in young males, with a maximal tolerated single dose of 5 mg corresponding to an estimated occupancy in the region of 75%, MRK-016 was poorly tolerated in elderly subjects, even at a dose of 0.5 mg, which, along with its variable human pharmacokinetics, precluded its further development.
Atack
John R
JR
Merck Sharp & Dohme Research Laboratories, Neuroscience Research Centre, Harlow, Essex, United Kingdom. jatack1@its.jnj.com
Maubach
Karen A
KA
Wafford
Keith A
KA
O'Connor
Desmond
D
Rodrigues
A David
AD
Evans
David C
DC
Tattersall
F David
FD
Chambers
Mark S
MS
MacLeod
Angus M
AM
Eng
Wai-Si
WS
Ryan
Christine
C
Hostetler
Eric
E
Sanabria
Sandra M
SM
Gibson
Raymond E
RE
Krause
Stephen
S
Burns
H Donald
HD
Hargreaves
Richard J
RJ
Agrawal
Nancy G B
NG
McKernan
Ruth M
RM
Murphy
M Gail
MG
Gingrich
Kevin
K
Dawson
Gerard R
GR
Musson
Donald G
DG
Petty
Kevin J
KJ
D000818:Animals; D001007:Anxiety / Q000523:psychology; D001522:Behavior, Animal / Q000187:drug effects; D003292:Convulsants / Q000494:pharmacology; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D004594:Electrophysiology; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects; D005347:Fibroblasts; D005442:Flumazenil / Q000378:metabolism; D018755:GABA Agonists / Q000378:metabolism / Q000493:pharmacokinetics / Q000494:pharmacology*; D018757:GABA Modulators / Q000378:metabolism; D058785:GABA-A Receptor Agonists; D022781:Hepatocytes / Q000378:metabolism; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D007555:Isoxazoles / Q000494:pharmacology*; D008253:Macaca mulatta; D008297:Male; D018782:Maze Learning / Q000187:drug effects; D051379:Mice; D018408:Patch-Clamp Techniques; D004856:Postural Balance / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000378:metabolism; D014227:Triazines / Q000494:pharmacology*; D055815:Young Adult
D003292:Convulsants; D018755:GABA Agonists; D018757:GABA Modulators; D058785:GABA-A Receptor Agonists; C496250:Gabra5 protein, mouse; D007555:Isoxazoles; C494554:MRK 016; D011963:Receptors, GABA-A; D014227:Triazines; D005442:Flumazenil