KRP-109 is a neutrophil elastase (NE) inhibitor known for its ability to alleviate lung inflammation. In mouse models, KRP-109 has been shown to increase survival rates and reduce both the number of neutrophils in alveolar walls and the severity of inflammation. It significantly decreases cell and neutrophil counts, along with cytokine levels (such as interleukin 1β and macrophage inflammatory protein 2) in bronchoalveolar lavage fluid. KRP-109 is applicable for investigating severe pneumonia.