Discovery and Characterization Of a Highly Specific Antibody Inhibitor Of Plasma Kallikrein

The plasma-kallikrein kinin (contact) system contributes to the physiological and pathophysiological reactions of vascular biology. Activation of this pathway causes the release of the potent nonapeptide vasodilator bradykinin following proteolytic cleavage of high-molecular weight kininogen (HMWK) by the serine protease plasma kallikrein (pKal). Normal vascular homeostasis requires regulation of pKal activity by interactions with the C1-inhibitor (C1-INH). This is most apparent in individuals with hereditary angioedema (HAE), a disease characterized by a genetic deficiency in C1-INH that results in persistent pKal activity and consequent bradykinin release. These events can ultimately manifest as unpredictable and potentially fatal attacks of subcutaneous and mucosal edema. Inhibition of pKal proteolytic activity has proven to be a viable therapeutic option for HAE, however there remains an unmet medical need for a long-lasting prophylactic treatment for this disease. Given the potential for target specificity and long serum half-life with antibody therapeutics, we used phage display to select a fully human antibody inhibitor (DX-2930) specific for pKal. In vitro enzyme inhibition and affinity assays demonstrate that DX-2930 is a potent antibody inhibitor of pKal (Ki = 125 pM) that binds the active form of pKal, but not the proenzyme form (prekallikrein) or any other serine protease tested. DX-2930 binding consequently prohibits pKal from cleaving bradykinin out of HMWK and thereby prevents the activation of the bradykinin receptor B₂. A 2.1Å resolution X-ray crystallographic structure of pKal complexed to a DX-2930 Fab construct supports these findings, demonstrating that the pKal proteolytic active site is intimately bound - and thereby occluded - by the Fab. This structural analysis provides both a rationale for the potency and specificity of DX-2930, and demonstrates the utility of using antibodies to specifically target an antigen among a family of related proteins (e.g. serine proteases). To further address the functional activity of DX-2930, we demonstrate that subcutaneous dosing of DX-2930 effectively reduces carrageenan-induced paw edema in vivo in rats when injected 24 hours prior to challenge. Combined with our finding that DX-2930 has a prolonged serum residence time in cynomolgus monkeys (t_1/2 = 301 hours, SC), the data presented here demonstrates the potential of DX-2930 for the prophylactic inhibition of pKal-mediated diseases, such as HAE.