Ester to amide substitution improves selectivity, efficacy and kinetic behavior of a benzodiazepine positive modulator of GABAA receptors containing the α5 subunit

Abstract

We have synthesized and characterized MP-III-022 ((R)-8-ethynyl-6-(2-fluorophenyl)-N,4-dimethyl-4H-benzo[f]imidazo[1,5-a][1,4]diazepine-3-carboxamide) in vitro and in vivo as a binding- and efficacy-selective positive allosteric modulator of GABAA receptors containing the α5 subunit (α5GABAARs). By approximation of the electrophysiological responses which the estimated free rat brain concentrations can induce, we demonstrated that convenient systemic administration of MP-III-022 in the dose range 1-10 mg/kg may result in a selective potentiation, over a wide range from mild to moderate to strong, of α5βγ2 GABAA receptors. For eliciting a comparable range of potentiation, the widely studied parent ligand SH-053-2′F-R-CH3 containing an ester moiety needs to be administered over a much wider dose range (10-200 mg/kg), but at the price of activating non-α5 GABAARs as well as the desired α5GABAARs at the highest dose. At the dose of 10 mg/kg, which elicits a strong positive modulation of α5GABAARs, MP-III-022 caused mild, but significant muscle relaxation, while at doses 1-10 mg/kg was devoid of ataxia, sedation or an influence on the anxiety level, characteristic for non-selective benzodiazepines. As an amide compound with improved stability and kinetic properties, MP-III-022 may represent an optimized tool to study the influence of α5GABAARs on the neuronal pathways related to CNS disorders such as schizophrenia, Alzheimer's disease, Down syndrome or autism.

Novel Benzodiazepine-Like Ligands with Various Anxiolytic, Antidepressant, or Pro-Cognitive Profiles

“…This suggests that proper cognitive functions require α5 potentiation and reduced, but present, α1 potentiation. Such a mechanism may explain the effects on working memory observed with GL-II-73 and GL-II-75 in our studies, and also explain the lack of effect of compounds with greater selectivity for α5-GABAA-R but no potentiation at α1-GABAA-R, such as SH-053-2’F-R-CH3 or MP-III-022 [31] in the YM (data not shown). As to the lack of pro-cognitive effects of GL-II-74, it may be connected with its more pronounced (relative to GL-II-73 and GL-II-75) potentiation at α5-GABAA-R (online suppl.…”

“…As to the lack of pro-cognitive effects of GL-II-74, it may be connected with its more pronounced (relative to GL-II-73 and GL-II-75) potentiation at α5-GABAA-R (online suppl. Table S7), which may be high enough to cause deterioration of cognitive processing, generally ascribed in studies with knock-in mice to substantial potentiation at this receptor population [31]. Follow-up tests for additional cognitive parameters, such as executive functions, cognitive flexibility, or impulsivity need to be performed to come to a definitive conclusion regarding efficacy on broader cognitive functions of the novel ligands.…”

“…Amides are commonly used as replacements to improve metabolic stability and bioavailability [31]. Hence, a series of amide IBZD ligands were designed [32] that fit the pharmacophore/receptor model [27], with similar or increased preferential binding affinity and efficacy at GABAA-Rs, compared to corresponding esters [33].…”

“…Binding studies were performed on HEK-293 cells following methods described by Stamenić et al [31] (online supplementary Methods). HEK-293 cells were transfected with cDNAs encoding rat GABAAR subunits.…”

“…The ligands’ stabilities were tested in vitro at 37°C, utilizing blank mouse plasma spiked with individual ligand and internal standard, as described by Stamenić et al [31]. …”