Benzodiazepine receptor ligands: a patent review (2006 – 2012)

GABAA receptor subtype modulators in medicinal chemistry: an updated patent review (2014-present)

Introduction: Ligands at the benzodiazepine binding site of the GABA_A receptor (GABA_AR) act by modulating the effect of GABA (γ-aminobutyric acid). The benzodiazepine drugs are conventionally categorized as positive allosteric modulators enhancing the chloride ion current GABA-induced. In literature there are also reported ligands that act as negative allosteric modulators, reducing chloride ion current, and silent allosteric modulators not influencing the chloride ion flux.Areas covered: This review covers patents published from 2014 to present on ligands for the benzodiazepine binding site of the GABA_ARs. Patents filed from different companies and research groups report many compounds that may be used in the treatment/prevention of a large variety of diseases.Expert opinion: Since the discovery of the first benzodiazepine about 60 years have passed and about 50 years since the identification of their target, GABA_A receptor. Even if benzodiazepines are the most popular anxiolytic drugs, the research in this field is still very active. From patents/application analysis arises that most of them claim methods for alleviating specific symptoms in different neurodegenerative diseases and their related memory deficits. Noteworthy is the presence of the α4- and α5-GABA_A receptor subtype ligands as new pharmacological tools for airway hyperresponsiveness, inflammation diseases, and asthma.

Insights Into Nicotinic Receptor Signaling in Nicotine Addiction: Implications for Prevention and Treatment

BACKGROUND

Nicotinic acetylcholine receptors (nAChRs) belong to the Cys-loop ligandgated ion-channel (LGIC) superfamily, which also includes the GABA, glycine, and serotonin receptors. Many nAChR subunits have been identified and shown to be involved in signal transduction on binding to them of either the neurotransmitter acetylcholine or exogenous ligands such as nicotine. The nAChRs are pentameric assemblies of homologous subunits surrounding a central pore that gates cation flux, and they are expressed at neuromuscular junctions throughout the nervous system.

METHODS AND RESULTS

Because different nAChR subunits assemble into a variety of pharmacologically distinct receptor subtypes, and different nAChRs are implicated in various physiological functions and pathophysiological conditions, nAChRs represent potential molecular targets for drug addiction and medical therapeutic research. This review intends to provide insights into recent advances in nAChR signaling, considering the subtypes and subunits of nAChRs and their roles in nicotinic cholinergic systems, including structure, diversity, functional allosteric modulation, targeted knockout mutations, and rare variations of specific subunits, and the potency and functional effects of mutations by focusing on their effects on nicotine addiction (NA) and smoking cessation (SC). Furthermore, we review the possible mechanisms of action of nAChRs in NA and SC based on our current knowledge.

CONCLUSION

Understanding these cellular and molecular mechanisms will lead to better translational and therapeutic operations and outcomes for the prevention and treatment of NA and other drug addictions, as well as chronic diseases, such as Alzheimer's and Parkinson's. Finally, we put forward some suggestions and recommendations for therapy and treatment of NA and other chronic diseases.

Different Benzodiazepines Bind with Distinct Binding Modes to GABAA Receptors

Benzodiazepines are clinically relevant drugs that bind to GABAA neurotransmitter receptors at the α+/γ2- interfaces and thereby enhance GABA-induced chloride ion flux leading to neuronal hyperpolarization. However, the structural basis of benzodiazepine interactions with their high-affinity site at GABAA receptors is controversially debated in the literature, and in silico studies led to discrepant binding mode hypotheses. In this study, computational docking of diazepam into α+/γ2- homology models suggested that a chiral methyl group, which is known to promote preferred binding to α5-containing GABAA receptors (position 3 of the seven-membered diazepine ring), could possibly provide experimental evidence that supports or contradicts the proposed binding modes. Thus, we investigated three pairs of R and S isomers of structurally different chemotypes, namely, diazepam, imidazobenzodiazepine, and triazolam derivatives. We used radioligand displacement studies as well as two-electrode voltage clamp electrophysiology in α1β3γ2-, α2β3γ2-, α3β3γ2-, and α5β3γ2-containing GABAA receptors to determine the ligand binding and functional activity of the three chemotypes. Interestingly, both imidazobenzodiazepine isomers displayed comparable binding affinities, while for the other two chemotypes, a discrepancy in binding affinities of the different isomers was observed. Specifically, the R isomers displayed a loss of binding, whereas the S isomers remained active. These findings are in accordance with the results of our in silico studies suggesting the usage of a different binding mode of imidazobenzodiazepines compared to those of the other two tested chemotypes. Hence, we conclude that different chemically related benzodiazepine ligands interact via distinct binding modes rather than by using a common binding mode.

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

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 GABA_A receptors containing the α5 subunit (α5GABA_ARs). 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 1-10mg/kg may result in a selective potentiation, over a wide range from mild to moderate to strong, of α5βγ2 GABA_A 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 (10-200mg/kg), but at the price of activating non-α5 GABA_ARs as well as the desired α5GABA_ARs at the highest dose. At the dose of 10mg/kg, which elicits a strong positive modulation of α5GABA_ARs, MP-III-022 caused mild, but significant muscle relaxation, while at doses 1-10mg/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 α5GABA_ARs on the neuronal pathways related to CNS disorders such as schizophrenia, Alzheimer's disease, Down syndrome or autism.

Negative modulation of α₅ GABAA receptors in rats may partially prevent memory impairment induced by MK-801, but not amphetamine- or MK-801-elicited hyperlocomotion

Reportedly, negative modulation of α5 GABAA receptors may improve cognition in normal and pharmacologically-impaired animals, and such modulation has been proposed as an avenue for treatment of cognitive symptoms in schizophrenia. This study assessed the actions of PWZ-029, administered at doses (2, 5, and 10 mg/kg) at which it reached micromolar concentrations in brain tissue with estimated free concentrations adequate for selective modulation of α5 GABAA receptors, in three cognitive tasks in male Wistar rats acutely treated with the noncompetitive N-methyl-d-aspartate receptor antagonist, MK-801 (0.1 mg/kg), as well in tests of locomotor activity potentiated by MK-801 (0.2 mg/kg) or amphetamine (0.5 mg/kg). In a hormetic-like manner, only 5 mg/kg PWZ-029 reversed MK-801-induced deficits in novel object recognition test (visual recognition memory), whereas in the Morris water maze, the 2 mg/kg dose of PWZ-029 exerted partial beneficial effects on spatial learning impairment. PWZ-029 did not affect recognition memory deficits in social novelty discrimination procedure. Motor hyperactivity induced with MK-801 or amphetamine was not preventable by PWZ-029. Our results show that certain MK-801-induced memory deficits can be ameliorated by negative modulation of α5 GABAA receptors, and point to the need for further elucidation of their translational relevance to cognitive deterioration in schizophrenia.

The history of benzodiazepines

After more than 50 years of experience with benzodiazepines, the American health care system has a love-hate relationship with them. In 1955, Hoffmann-La Roche chemist Leo Sternbach serendipitously identified the first benzodiazepine, chlordiazepoxide (Librium). By 1960, Hoffmann-La Roche marketed it as Librium, and it pursued molecular modifications for enhanced activity. Valium (diazepam) followed in 1963. Hoffmann-La Roche's competitors also began looking for analogues. Initially, benzodiazepines appeared to be less toxic and less likely to cause dependence than older drugs. A specific improvement was their lack of respiratory depression, a safety concern with barbiturates. Medical professionals greeted benzodiazepines enthusiastically at first, skyrocketing their popularity and patient demand. In the mid-to-late 1970s, benzodiazepines topped all "most frequently prescribed" lists. It took 15 years for researchers to associate benzodiazepines and their effect on gamma-aminobutyric acid as a mechanism of action. By the 1980s, clinicians' earlier enthusiasm and propensity to prescribe created a new concern: the specter of abuse and dependence. As information about benzodiazepines, both raising and damning, accumulated, medical leaders and legislators began to take action. The result: individual benzodiazepines and the entire class began to appear on guidelines and in legislation giving guidance on their use. Concurrently, clinicians began to raise concerns about benzodiazepine use by elderly patients, indicating that elders'lesser therapeutic response and heightened sensitivity to side effects demanded prescriber caution. The benzodiazepine story continues to evolve and includes modern-day issues and concerns beyond those ever anticipated.