Sivelestat sodium(ONO5046; LY544349; EI546) is a competitive inhibitor of human neutrophil elastase(IC50 = 44 nM; Ki=200 nM); also inhibited leukocyte elastase obtained from rabbit, rat, hamster and mouse. IC50 value: 44 nM [1] Target: neutrophil elastase ONO-5046 did not inhibit trypsin, thrombin, plasmin, plasma kallikrein, pancreas kallikrein, chymotrypsin and cathepsin G even at 100 microM. In in vivo studies, ONO-5046 suppressed lung hemorrhage in hamster (ID50 = 82 micrograms/kg) by intratracheal administration and increase of skin capillary permeability in guinea pig (ID50 = 9.6 mg/kg) by intravenous administration, both of which were induced by human neutrophil elastase [1]. Sivelestat sodium hydrate is an anti-neutrophil elastase inhibitor and may be one of the treatment options for acute respiratory failure due to pneumocystis pneumonia in AIDS patients [2].