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 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.
- DMCM, methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate
- L-655,708, ethyl[S]-11,12,13,13a-tetrahydro-7-methoxy-9-oxo-9H-imidazo[1,5-a]pyrrolo[2,1-c][1,4]benzodiazepine-1-carboxlate
- α5IA, 3-(5-methylisoxazol-3-yl)-6-[(1-methyl-1,2,3-triazol-4-yl)methyloxy]-1,2,4-triazolo[3,4-a]phthalazine
- RO4938581, 3-bromo-10-difluoromethyl-9H-imidazo(1,5-a)(1,2,4)triazolo(1,5-d)(1,4)benzodiazepine
- MRK-016, 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
- FG-7142, N-methyl-β-carboline-3-carboxamide
- Ro 15-1788, [flumazenil,8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5a][1,4]benzodiazepine-3-carboxylic acid, ethyl ester]
- Ro 15-4513, ethyl 8-azido-6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a]-[1,4]benzodiazepine-3-carboxylate
- PPF, paired pulse facilitation
- LTP, long-term potentiation
- fEPSP, field excitatory postsynaptic potential
- ANOVA, analysis of variance
- Occ50, dose required to produce 50% occupancy
- PET, positron emission tomography
- LC-MS/MS, liquid chromatography-tandem mass spectrometry
- ROI, region of interest
- PTZ, pentylenetetrazole
- AUC, area under the curve
- AE, adverse event.
Footnotes
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This work was conducted while all authors were employees of Merck and Co., Inc.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
doi:10.1124/jpet.109.157636
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ABBREVIATIONS:
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↵1 Current affiliation: Johnson and Johnson Pharmaceutical Research and Development, Beerse, Belgium.
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↵2 Current affiliation: BTG International, London, United Kingdom.
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↵3 Current affiliation: Eli-Lilly, Windlesham, Surrey, United Kingdom.
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↵4 Current affiliation: Novartis, Horsham, Surrey, United Kingdom.
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↵5 Current affiliation: Bristol-Myers Squibb Pharmaceutical Research Institute, Princeton, New Jersey.
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↵6 Current affiliation: Pfizer Global Research and Development, Sandwich, Kent, United Kingdom.
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↵7 Current affiliation: BioFocus, Saffron Walden, Essex, United Kingdom.
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↵8 Current affiliation: P1Vital, University of Oxford, Oxford, United Kingdom.
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↵9 Current affiliation: Centocor R&D USA, Malvern, Pennsylvania.
- Received June 12, 2009.
- Accepted August 21, 2009.
- © 2009 by The American Society for Pharmacology and Experimental Therapeutics
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