An inhibitor of neutrophil elastase (IC50 = 12 nM); greater than 600-fold selective for neutrophil elastase over several other serine proteases; inhibits neutrophil elastase activity in zymosan-stimulated isolated whole blood and in cell-associated assays (IC50s = 46 and 48 nM, respectively); decreases the number of neutrophils in BALF in a mouse model of smoke-induced airway inflammation at 6 mg/kg