Date of Award

May 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

James M. Cook

Committee Members

Alexander E Arnold, Alan W Schwabacher, Arsenio A Pacheco, Guilherme L Indig

Keywords

Airway Hyperresponsiveness, Airway Smooth Muscle Relaxation, Asthma, GABA(A) Receptor, Imidazodiazepine, Neuropathic Pain

Abstract

Part I. Asthma is a major healthcare challenge affecting an estimated 300 million people globally. Over $56 billion in asthma-related healthcare expenses occur in the United States annually. Moreover, asthma accounts for the majority of missed school/work days, Doctor and emergency room visits, and patient hospitalizations in young persons. Consequently, asthma continues to be a significant healthcare burden in terms of morbidity, productivity, and medical costs. Beta 2-adrenergic agonists and inhaled corticosteroids (ICs) are the most commonly prescribed treatments for the acute and chronic management of asthma. Both agents present efficacy, compliance, and adverse side effect concerns.

Hence, there is an unmet need for asthma therapies with novel mechanisms of action to better control the disease with decreased adverse side effects. Previously, it was demonstrated that airway smooth muscle (ASM) cells express GABAA receptors (GABAAR's) of the α4 and α5 subunits. Agonists of these GABAAR subtypes can relax ASM acutely. Targeting the limited and overlapping α subunits with subtype selective GABAAR agonists would effect both ASM relaxation and suppression of inflammation in the absence of any off-target CNS activity. Bz/GABAAergic agents have been proven to be safe and have a long clinical safety record. As a result, targeting Bz/GABAAR in the lung and the peripheral nervous system (PNS) would be a novel and effective strategy in a management of asthma in patients. In this vein, novel GABAAR positive allosteric modulators designed specifically for α4/α6 subunit selectivity were synthesized using iterative computational analyses. In addition, a series of deuterated analogs at key metabolic sites (C-3 and C-8 of the imidazobenzodiazepine scaffold) were synthesized to increase the drugs stability so that the drug stays in the body for a longer time to permit lower doses and still effect its anti-asthmatic properties for a longer duration, presumably with less side effects. Furthermore, a library of α4 subtype selective GABAAR ligands which were more hydrophilic to prevent blood brain barrier (BBB) penetration reduced CNS side effects. To obtain better in vitro and in vivo stability, bioisosteric moieties to replace the labile C-3 ester functional groups were designed and synthesized. Preclinical assays such as microsomal stability, cytotoxicity, and sensorimotor impairment have been studied on these novel analogs. Several ligands exhibited the desired properties required for better management of asthma. The results of studies in several models of asthma in vivo reinforces the novel hypothesis, which rests on relaxation of airway smooth muscle (ASM), a decrease in airway hyperresponsiveness (AHR), and a decrease in airway eosinophils, as well as modulating inflammatory cells. These ligands may be potential treatments for childhood asthma and also for the disease in adults.

Part II. Nonselective ligands of the α1-3,5βγ 2 subtypes of GABAARs, such as diazepam have been used in the clinic for more than five decades for various central nervous system (CNS) disorders. These drugs exhibit various adverse CNS effects including sedation, ataxia, amnesia, tolerance, and addiction, which are believed to be mediated by α1 subtypes of GABAARs. As a result, these drugs are not applicable to all patients and have limited long-term applications. Despite their adverse CNS effects due to non-selective GABAAR efficacy, novel ligands with better subtype selectivity, efficacy and reduced adverse effects are now emerging to be suitable replacements for these benzos. The GABAAR agonists that possess superior α2/α3 subtype selectivity over the α1 and α5 subtypes are considered to be a promising avenue for development of novel GABAAR ligands to treat various CNS disorders including inflammatory pain, anxiety, neuropathic pain, and epilepsy, while avoiding side effects such as, ataxia, amnesia, tolerance and dependence.

Previously, it had been shown the α2/α3 subtype selective Bz/GABAAR positive allosteric modulator (PAM) HZ-166 exhibited anticonvulsant, antihyperalgesic and anxiolytic properties while being devoid of sedation, ataxia, dependence and tolerance. However, the C-3 ester function in HZ-166 was too labile for studies of ADME toxicity. Consequently, research here was carried out to prepare new ligands with better efficacy and stability, which resulted in several new lead compounds including a 1,3-oxazole (KRM-II-81) and a 1,2,4-oxadiazole (MP-III-80). These bioisosteres were synthesized to overcome the problems with the metabolically labile ester functions. Among them KRM-II-81 exhibited prominent anxiolytic, anticonvulsant, antihyperalgesia, and antidepressant activity. An improved synthetic route was developed to better access the key ligand (HZ-166) in gram quantities for further optimization of this "privileged" scaffold. Synthesis of several new ester bioisosteres, importantly, those which contained deuterium in the scaffold at key metabolic sites, resulted in d1-MP-III-80, d3-MP-III-80, d5-MP-III-80, as well as a few 3-alkyl-1,2,4-oxadiazole derivatives. The in vitro and in vivo evaluation of these new ligands look promising and further investigations in vivo are underway. It is felt these new α2/α3 subtype selective ligands will result in novel compounds for development into effective treatments for anxiety disorders, for depression, for pain syndromes and for treatment of epilepsies with no tolerance nor dependence.

Available for download on Thursday, May 20, 2021

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