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 α5 subtype-selective inverse agonist

John R. Atack, Karen A. Maubach, Keith A. Wafford, Desmond O'Connor, A. David Rodrigues, David C. Evans, F. David Tattersall, Mark S. Chambers, Angus M. MacLeod, Wai Si Eng, Christine Ryan, Eric Hostetler, Sandra M. Sanabria, Raymond E. Gibson, Stephen Krause, H. Donald Burns, Richard J. Hargreaves, Nancy G B Agrawal, Ruth M. McKernan, M. Gail MurphyKevin Gingrich, Gerard R. Dawson, Donald G. Musson, Kevin J. Petty

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53 Scopus citations


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 α1-, α2-, α3-, and α5-containing GABAA receptors. It has inverse agonist efficacy selective for the α5 subtype, and this α5 inverse agonism is greater than that of the prototypic α5-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 (α5IA). Consistent with its greater α5 inverse agonism, MRK-016 increased long-term potentiation in mouse hippocampal slices to a greater extent than α5IA. 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 EC50 value of 15 ng/ml that was similar to the rhesus monkey plasma EC50 value of 21 ng/ml obtained using [ 11C]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 (∼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.

Original languageEnglish (US)
Pages (from-to)470-484
Number of pages15
JournalJournal of Pharmacology and Experimental Therapeutics
Issue number2
StatePublished - Nov 2009

ASJC Scopus subject areas

  • Molecular Medicine
  • Pharmacology


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