– Neutrophil elastase inhibitor 4 (compound 4f) is a competitive inhibitor of human neutrophil elastase (HNE) with an IC50 of 42.30 nM and a Ki of 8.04 nM. It induces apoptosis in T47D cells and inhibits the proliferation of T47D, RPMI 8226, A549, and HSF cells with respective IC50 values of 21.25, 34.17, 29.93, and 99.11 nM [1].