Discovery and Identification of Novel 5‑Hydroxy‑4H‑benzo[1,4]oxazin-3-one Derivatives as Potent β₂‑Adrenoceptor Agonists through Structure-Based Design, Synthesis, and Biological Evaluation

Although β₂-agonists are crucial for treatment of chronic respiratory diseases, optimizing β₂-agonistic activity and selectivity remains essential for achieving favorable therapeutic outcomes. A structure-based molecular design workflow was employed to discover a novel class of β₂ agonists featuring a 5-hydroxy-4H-benzo­[1,4]­oxazin-3-one scaffold, which potently stimulated β₂ adrenoceptors (β₂-ARs). Screening for the β₂-agonistic activity and selectivity led to the identification of compound A19 (EC₅₀ = 3.7 pM), which functioned as a partial β₂-agonist in HEK-293 cells containing endogenous β₂-ARs. Compound A19 exhibited significant relaxant effects, rapid onset time (Ot₅₀ = 2.14 min), and long duration of action (>12 h) on isolated guinea pig tracheal strips, as well as advantageous pharmacokinetic characteristics in vivo, rendering A19 suitable for inhalation administration. Moreover, A19 suppressed the upregulation of inflammatory cytokines and leukocytes and improved lung function in a rat model of COPD, thereby indicating that A19 is a potential β₂ agonist candidate for further study.