Abstract:Dysregulated matriptase activity has been established as a key contributor to cancer progression through its activation of growth factors including the hepatocyte growth factor (HGF). Despite its critical role and prevalence in many human cancers, limitations to developing an effective matriptase inhibitor include weak binding affinity, poor selectivity, and short circulating half-life. We applied rational and combinatorial approaches to engineer a potent inhibitor based on the hepatocyte growth factor activator inhibitor type 1 (HAI-1), a natural matriptase inhibitor. The first Kunitz domain (KD1) of HAI-1 has been well established as a minimal matriptase binding and inhibition domain, while the second Kunitz domain (KD2) is inactive and involved in negative regulation. Here, we replaced the inactive KD2 domain of HAI-1 with an engineered chimeric variant of KD2/KD1 domains, and fused the resulting construct to an antibody Fc domain to increase valency and circulating serum half-life. The final protein variant contains 4 stoichiometric binding sites that we showed were needed to effectively inhibit matriptase with a Ki =70 ± 5 pM, an increase of 120-fold compared to the natural HAI-1 inhibitor, to our knowledge making it one of the most potent matriptase inhibitors identified to date. Furthermore, the engineered inhibitor demonstrates a protease selectivity profile similar to wild-type KD1 but distinct from HAI-1. It also inhibits activation of the natural pro-HGF substrate and matriptase expressed on cancer cells with at least an order of magnitude greater efficacy than KD1. ________________________________________ High expression and dysregulated activity of the type II, membrane-anchored serine protease matriptase in the local tumor environment has been shown to correlate with poor patient prognosis in many human cancers including breast, colorectal, pancreatic, cervical, and prostate cancers (1-10). This dysregulation is partly driven by the high proteolytic processing and turnover of pro-hepatocyte growth factor (Pro-HGF) (9, 11) to a form of HGF that activates its cognate receptor, c-Met (12) (Fig. 1A). Matriptase is also known to activate other proteases and growth factors including urokinase plasminogen activator (uPA) (11), pro-macrophage stimulating protein (Pro-MSP) (13), and platelet derived growth factor-D (PDGF-D) (14), all of which play key roles in cancer growth and metastasis. Furthermore, matriptase has been identified as a critical driver of other diseases, including iron overload disease (15) and osteoarthritis (16), and has been shown to activate the human airway influenza virus (H1N1) (17) and human immunodeficiency virus (HIV) (18 (19)(20)(21)(22)(23). The activity of matriptase is regulated in healthy tissue by the serine protease inhibitor hepatocyte growth factor activator inhibitor type-1 (HAI-1) (Fig. 1A). HAI-1 is primarily expressed on the surface of epithelial cells and naturally blocks the substrate-activating properties of matriptase (24)(25)(26), as well as other struc...