Anti-inflammatory agent 113 (compound B5) is a cathepsin L (CTSL) inhibitor and anti-inflammatory compound with an IC50 for CTSL of 5.52 μM. It hinders the maturation of CTSL, downregulates the activation of NF-κB and p38 MAPK signaling pathways, and binds stably at the CTSL active site through non-covalent interactions with Asp162, Cys25, and Glu63. This compound also inhibits the production of pro-inflammatory cytokines (IL-6, IL-8), reduces pulmonary infiltration of inflammatory cells, and alleviates lung tissue damage. Anti-inflammatory agent 113 is applicable in research on acute lung injury.