Since HGF is the only known ligand of MET, formation of a HGF:MET complex blocks MET biological activity. For this purpose, truncated HGF, anti-HGF neutralizing antibodies, and an uncleavable form of HGF have been utilized as HGF inhibitor so far. The major limitation of HGF inhibitors is that they block only HGF-dependent MET activation.
NK4 competes with HGF as it binds MET without inducing receptor activation, thus behaving as a full antagonist. NK4 is a molecule bearing the N-terminal hairpin and the four kringle domains of HGF. Moreover, NK4 is structurally similar to angiostatins, which is why it possesses anti-angiogenic activity.
Neutralizing anti-HGF antibodies were initially tested in combination and it was shown that at least three antibodies, acting on different HGF epitopes, are necessary to prevent MET tyrosine kinase activation. More recently, it has been demonstrated that fully human monoclonal antibodies can individually bind and neutralize human HGF, leading to regression of tumors in mouse models. Two anti-HGF antibodies are currently available: the humanized AV299 (AVEO), and the fully human AMG102 (Amgen).
Uncleavable HGF is an engineered form of pro-HGF carrying a single amino-acid substitution, which prevents the maturation of the molecule. Uncleavable HGF is capable of blocking MET-induced biological responses by binding MET with high affinity and displacing mature HGF. Moreover, uncleavable HGF competes with the wild-type endogenous pro-HGF for the catalytic domain of proteases that cleave HGF precursors. Local and systemic expression of uncleavable HGF inhibits tumor growth and, more importantly, prevents metastasis.