MMP-8 Inhibitor I is a selective inhibitor of the neutrophil collagenase matrix metalloproteinase-8 (MMP-8) with an IC50 value of 4 nM. This inhibitor does not target the activities of other MMPs in vitro. MMP-8 cleaves interstitial collagens and has exhibited activity in atherosclerotic plaques, angiogenesis, and stem cell mobilization. Additionally, MMP-8 expression is observed in normal mammary epithelial cells, whereas a loss of expression is observed in human ductal carcinoma in situ and the deletion of MMP-8 accelerates tumor onset in a mouse model of aggressive breast cancer.