Neutrophil elastase inhibitor 4 (compound 4f) is a competitive human neutrophil elastase (HNE) inhibitor ( IC 50 : 42.30 nM, K i : 8.04 nM). Neutrophil elastase inhibitor 4 induces T47D cell apoptosis. Neutrophil elastase inhibitor 4 inhibits cell proliferation with IC 50 s of 21.25, 34.17, 29.93, 99.11 nM for T47D, RPMI 8226, A549, and HSF cells.