title: 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.
abstract: 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.
authors.affiliation: Merck Sharp & Dohme Research Laboratories, Neuroscience Research Centre, Harlow, Essex, United Kingdom. jatack1@its.jnj.com
authors.forename: John R
authors.initials: JR
authors.lastname: Atack
authors.forename: Karen A
authors.initials: KA
authors.lastname: Maubach
authors.forename: Keith A
authors.initials: KA
authors.lastname: Wafford
authors.forename: Desmond
authors.initials: D
authors.lastname: O'Connor
authors.forename: A David
authors.initials: AD
authors.lastname: Rodrigues
authors.forename: David C
authors.initials: DC
authors.lastname: Evans
authors.forename: F David
authors.initials: FD
authors.lastname: Tattersall
authors.forename: Mark S
authors.initials: MS
authors.lastname: Chambers
authors.forename: Angus M
authors.initials: AM
authors.lastname: MacLeod
authors.forename: Wai-Si
authors.initials: WS
authors.lastname: Eng
authors.forename: Christine
authors.initials: C
authors.lastname: Ryan
authors.forename: Eric
authors.initials: E
authors.lastname: Hostetler
authors.forename: Sandra M
authors.initials: SM
authors.lastname: Sanabria
authors.forename: Raymond E
authors.initials: RE
authors.lastname: Gibson
authors.forename: Stephen
authors.initials: S
authors.lastname: Krause
authors.forename: H Donald
authors.initials: HD
authors.lastname: Burns
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Hargreaves
authors.forename: Nancy G B
authors.initials: NG
authors.lastname: Agrawal
authors.forename: Ruth M
authors.initials: RM
authors.lastname: McKernan
authors.forename: M Gail
authors.initials: MG
authors.lastname: Murphy
authors.forename: Kevin
authors.initials: K
authors.lastname: Gingrich
authors.forename: Gerard R
authors.initials: GR
authors.lastname: Dawson
authors.forename: Donald G
authors.initials: DG
authors.lastname: Musson
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Petty
mesh_terms: 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
chemical_list: 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