Molecular manipulation of the 1,5,6,7-tetrahydro-4 H -indazol-4-one scaffold to obtain new human neutrophil elastase (HNE) inhibitors

Human neutrophil elastase (HNE) is a globular glycoprotein of about 30 kDa belonging to the chy- motrypsin family, and it has been recognized as an important target for several inflammatory diseases, in particular related to the respiratory system. Here, we report the development of potent HNE inhibitors using a classic medicinal chemistry approach starting from our previously published pyrazolocyclohex- anones. By contraction and enlargement of the cyclohexanone ring, we obtained pyrazolocyclopentanone and pyrazolocycloeptanone derivatives, respectively. Both nuclei were suitable for the synthesis of potent HNE inhibitors, and activity was maintained in the nanomolar range. The inhibitory activity was strongly dependent on the presence of an N-CO function as the point of attachment by HNE Ser195. Spectroscopic techniques, GC-MS, and crystallographic studies allowed us to univocally assign the correct structures to the pairs of isomers (N1 and N2) obtained in the alkylation/acylation reactions. The two isomers N1 and N2 exhibited a comparable potency. Finally, molecular modeling of the most active compound 8e (IC 50 = 26 nM) demonstrated that orientation of the molecule was favorable for formation of a Michaelis complex, and the parameter values were in agreement with its potent inhibitory activity.