Design, Synthesis, and Biological Evaluation of Novel Spiro Imidazobenzodiazepines to Identify Improved Inhaled Bronchodilators

Novel gamma-aminobutyric acid receptor (GABA_AR) ligands structurally related to imidazobenzodiazepine MIDD0301 were synthesized using spiro-amino acid N-carboxyanhydrides (NCAs). These compounds demonstrated increased resistance to phase 2 metabolism and avoided the formation of a 6H isomer. Compound design was guided by molecular docking using the available crystal structure of the α₁β₃γ₂ GABA_AR and correlated with in vitro binding data. The carboxylic acid containing GABA_AR ligands have high aqueous solubility, low permeability, and low cell toxicity. The inability of GABA_AR ligands to cross the blood-brain barrier was confirmed in vivo by the absence of sensorimotor inhibition. Pharmacological activities at lung GABA_ARs were demonstrated by ex vivo relaxation of guinea pig airway smooth muscle and reduction of methacholine-induced airway hyperresponsiveness (AHR) in conscious mice. We identified bronchodilator 5c with an affinity of 9 nM for GABA_ARs that was metabolically stable in the presence of human and mouse microsomes.