Kinase experiment:

L(tk-) cells expressing human recombinant GABAA receptors containing β3- and γ2-subunits in combinatioon with various α-subunits are harvested and binding performed. The displacement of [3H]Ro 15-1788 binding by the test compounds (MRK-016, etc.) is measured in GABAA receptors containing eigher an α1-, α2-, α3-, and α5-subunit and from the IC50 and the Ki is calculated assuming respective Kd values of [3H]Ro 15-1788 binding of 0.92, 1.05, 0.58 and 0.45 nM at the α1-, α2-, α3-, and α5-subtypes. Nonspecific binding is defined by the inclusion of 10 μM flunitrazepam for the α1-, α2-, α3-, and α5-subtypes. The percentage inhibition of [3H]Ro 15-1788, the IC50 and the Ki values are calculated using ActivityBase[1].

Animal experiment:

Mice[1]The proconvulsant potential MRK-016 is determined in this assay. The convulsant is pentylenetetrazole which is dosed at 15 mg/mL with an infusion rate of 0.2 mL/min. This rate is chosen to ensure that drug-naive mince reach the terminal convulsion sign within 1 min. MRK-016 is dosed intraperitoneally (ip) at a concentration of 1, 3, and 10 mg/kg in 70% PEG 300[1].

MRK-016 is a selective, orally bioavailable inverse agonist of GABAA α5 receptor, with an EC50 of 3 nM for GABAA α5, and Kis of 0.83, 0.85, 0.77 and 1.4 nM for human GABAA α1β3γ2, GABAA α2β3γ2, GABAA α3β3γ2, and GABAA α5β3γ2, respectively; MRK-016 also readily penetrates the CNS.

MRK-016 is a selective, orally bioavailable inverse agonist of GABAA α5 receptor, with an EC50 of 3 nM for GABAA α5, and Kis of 0.83, 0.85, 0.77 and 1.4 nM for human GABAA α1β3γ2, GABAA α2β3γ2, GABAA α3β3γ2, and GABAA α5β3γ2, respectively[1][2]. MRK-016 is a full inverse agonist at the α5-subtype, shows very weak affinity at the GABAA α4β3γ2-subtype (Ki 395 ± 173 nM) and is essentially inactive at the GABAA α6β3γ2 receptor (Ki >4000 nM)[1]. MRK-016 shows a weak effect on GABAA α4β3γ2 with a Ki of 400 nM. MRK-016 (100 nM) alao increases long-term potentiation in mouse hippocampal slices[2].

MRK-016 does not enhance pentylenetetrazole-induced convulsions at 10 mg/kg via ip, or cause seizures at 30 mg/kg, via po for 20 days in mice. MRK-016 shows no obvious anxiogenic-like effects in rats at doses that occupy >95% of benzodiazepine (BZ) binding sites. MRK-016 (0.3, 1, and 3 mg/kg, p.o.) dose-dependently improves performance of rats hippocampal-dependent memory task[1]. MRK-016 (0.3-30 mg/kg, p.o.) causes good receptor occupancy in rats. MRK-016 (0.3, 1, or 3 mg/kg p.o.) shows cognition-enhancing activity in the delayed matching-to-position version of the Morris water maze. MRK-016 (1, 3, or 10 mg/kg i.p.) does not produce kindling in mice[2]. MRK-016 (3 mg/kg, i.p.) protects against LPS-induced learning/memory decrements in mice[3].

References:
[1]. Chambers MS, et al. An orally bioavailable, functionally selective inverse agonist at the benzodiazepine site of GABAA alpha5 receptors with cognition enhancing properties. J Med Chem. 2004 Nov 18;47(24):5829-32.
[2]. Atack JR, et al. 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. J Pharmacol Exp Ther. 2009 Nov;331(2):470-84.
[3]. Eimerbrink MJ, et al. Administration of the inverse benzodiazepine agonist MRK-016 rescues acquisition and memory consolidation following peripheral administration of bacterial endotoxin. Behav Brain Res. 2015 Jul 15;288:50-3.