Freselestat quarterhydrate (ONO-6818 quarterhydrate) is a potent, orally active inhibitor of neutrophil elastase (Ki: 12.2 nM). quarterhydrate is more than 100-fold less active than other proteases such as trypsin, proteinase 3, pancreatic elastase, thrombin, fibrin, collagenase, histone G and murine macrophage elastase, and exhibits potent anti-inflammatory effects.