Human neutrophil elastase (HNE) is a serine protease that is expressed in polymorphonuclear neutrophils. It has been recognized as an important therapeutic target for treating inflammatory diseases, especially related to the respiratory system, but also for various types of cancer. Thus, compounds able to inhibit HNE are of great interest in medicinal chemistry. In the present paper, we report the synthesis and biological evaluation of a new series of HNE inhibitors with an innovative 1,5,6,7-tetrahydro-4H-indazol-4-one core that was developed as a molecular modification of our previously reported indazole-based HNE inhibitors. Since the 1,5,6,7-tetrahydro-4H-indazol-4-one scaffold can occur in two possible tautomeric forms, the acylation/alkylation reactions resulted in a mixture of the two isomers, often widely unbalanced in favor of one form. Using analytical techniques and NMR spectroscopy, we characterized and separated the isomer pairs and confirmed the compounds used in biological testing. Analysis of the compounds for HNE inhibitory activity showed that they were potent inhibitors, with K_i values in the low nanomolar range (6–35 nM). They also had reasonable stability in aqueous buffer, with half-lives over 1 h. Overall, our results indicate that the 1,5,6,7-tetrahydro-4H-indazol-4-one core is suitable for the synthesis of potent HNE inhibitors that could be useful in the development of new therapeutics for treating diseases involving excessive HNE activity.

人中性粒细胞弹性蛋白酶 (HNE) 是一种在多形核中性粒细胞中表达的丝氨酸蛋白酶。它已被认为是治疗炎症性疾病的重要治疗靶点，尤其是与呼吸系统相关的炎症性疾病，也可用于治疗各种类型的癌症。因此，能够抑制 HNE 的化合物在药物化学中引起了极大的兴趣。在本文中，我们报告了一系列新的 HNE 抑制剂的合成和生物学评价，这些抑制剂具有创新的 1,5,6,7-tetrahydro-4H-indazol-4-one 核心，是作为我们之前的分子修饰而开发的报道了基于吲唑的 HNE 抑制剂。由于 1,5,6,7-tetrahydro-4 H-indazol-4-one 骨架可以两种可能的互变异构形式出现，酰化/烷基化反应导致两种异构体的混合物，通常广泛不平衡，有利于一种形式。使用分析技术和核磁共振光谱，我们对异构体对进行了表征和分离，并确认了用于生物测试的化合物。化合物对 HNE 抑制活性的分析表明它们是有效的抑制剂，K _i低纳摩尔范围内的值 (6–35 nM)。它们在水性缓冲液中也具有合理的稳定性，半衰期超过 1 小时。总体而言，我们的结果表明 1,5,6,7-tetrahydro-4H-indazol-4-one 核心适用于合成有效的 HNE 抑制剂，可用于开发治疗涉及过量 HNE 的疾病的新疗法活动。