Development of GABAA Receptor Subtype-Selective Imidazobenzodiazepines as Novel Asthma Treatments

Recent studies have demonstrated that subtype selective GABA(A) receptor modulators are able to relax precontracted human airway smooth muscle ex vivo and reduce airway hyper-responsiveness in mice upon aerosol administration. Our goal in this study was to investigate systemic administration of subtype-selective GABAA receptor modulators to alleviate bronchoconstriction in a mouse model of asthma. Expression of GABA(A) receptor subunits was identified in mouse lungs, and the effects of alpha 4-subunit selective GABA(A)R modulators, XHE-III-74EE and its metabolite XHE-III-74A, were investigated in a murine model of asthma (ovalbumin sensitized and challenged BALB/c mice). We observed that chronic treatment with XHE-III-74EE significantly reduced airway hyper-responsiveness. In addition, acute treatment with XHE-III-74A but not XHE-III-74EE decreased airway eosinophilia. Immune suppressive activity was also shown in activated human T-cells with a reduction in IL-2 expression and intracellular calcium concentrations [Ca2+](i) in the presence of GABA or XHE-III-74A, whereas XRE-III-74EE showed only partial reduction of [Ca2+](i) and no inhibition of IL-2 secretion. However, both compounds significantly relaxed precontracted tracheal rings ex vivo. Overall, we conclude that the systemic delivery of a alpha 4-subunit-selective GABA(A)R modulator shows good potential for a novel asthma therapy; however, the pharmacokinetic properties of this class of drug candidates have to be improved to enable better beneficial systemic pharmacodynamic effects.