X-ray structural study of human neutrophil elastase inhibition with a series of azaindoles, azaindazoles and isoxazolones

•Human neutrophil elastase (HNE)•Porcine Pancreatic Elastase (PPE)•Study of inhibition mechanism of HNE inhibitors through X-ray crystallography•Formation of acyl-enzyme complex involving a nucleophilic attack by Ser195 Our purpose was to structurally investigate through X-ray crystallography the inhibition mechanism of small molecule inhibitors of human neutrophil elastase (HNE), a serine protease associated with some serious chronic inflammatory diseases, especially pulmonary pathologies. In particular, we selected some of the compounds previously synthesized by our research group, such as 5/4-azaindole, azaindazole and isoxazolone derivatives, which are promising therapeutic agents with an inhibition potency in the nanomolar range. In this work, we adopted Porcine Pancreatic Elastase (PPE) as a structural model for its high homology with the human enzyme, especially in the active site. The crystallographic study allowed us to gain information about the binding mode and inhibition mechanism of these new HNE inhibitors. In fact, we obtained the structures of the complexes of PPE with a fragment of the inhibitors resulted from the nucleophilic attack by the catalytic residue Ser195. In this way we confirmed an acyl enzyme inhibition mechanism for all the inhibitor scaffolds used. Furthermore, for the isoxazolone derivatives which possess two carbonyl groups we identified the exocyclic CO group as the one susceptible to the Ser attack. [Display omitted]