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

Novel gamma-aminobutyric acid receptor (GABAAR) 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 GABAAR and correlated
with in vitro binding data. The carboxylic acid containing GABAAR ligands have high aqueous solubility, low permeability,
and low cell toxicity. The inability of GABAAR ligands
to cross the blood–brain barrier was confirmed in vivo by the
absence of sensorimotor inhibition. Pharmacological activities at
lung GABAARs 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 GABAARs that was metabolically stable in the presence of human
and mouse microsomes.