Structural basis for pH-insensitive inhibition of immunoglobulin G recycling by an anti-neonatal Fc receptor antibody.

The neonatal Fc receptor FcRn plays a critical role in the trafficking of IgGs across tissue barriers and in retaining high circulating concentrations of both IgG and albumin. Although generally beneficial from an immunological perspective in maintaining IgG populations, FcRn can contribute to the pathogenesis of autoimmune disorders when an abnormal immune response targets normal biological components. We previously described a monoclonal antibody (DX-2507) that binds to FcRn with high affinity at both neutral and acidic pH, prevents the simultaneous binding of IgG, and reduces circulating IgG levels in preclinical animal models. Here, we report a 2.5 Å resolution X-ray crystal structure of an FcRn-DX-2507 Fab complex, revealing a nearly complete overlap of the IgG-Fc binding site in FcRn by complementarity-determining regions in DX-2507. This overlap explains how DX-2507 blocks IgG binding to FcRn and thereby shortens IgG half-life by preventing IgGs from recycling back into circulation. Moreover, the complex structure explains how the DX-2507 interaction is pH-insensitive unlike normal Fc interactions and how serum albumin levels are unaffected by DX-2507 binding. These structural studies could inform antibody-based therapeutic approaches for limiting the effects of IgG-mediated autoimmune disease.

Inhibition of plasma kallikrein by a highly specific active site blocking antibody.

Plasma kallikrein (pKal) proteolytically cleaves high molecular weight kininogen to generate the potent vasodilator and the pro-inflammatory peptide, bradykinin. pKal activity is tightly regulated in healthy individuals by the serpin C1-inhibitor, but individuals with hereditary angioedema (HAE) are deficient in C1-inhibitor and consequently exhibit excessive bradykinin generation that in turn causes debilitating and potentially fatal swelling attacks. To develop a potential therapeutic agent for HAE and other pKal-mediated disorders, we used phage display to discover a fully human IgG1 monoclonal antibody (DX-2930) against pKal. In vitro experiments demonstrated that DX-2930 potently inhibits active pKal (Ki = 0.120 ± 0.005 nM) but does not target either the zymogen (prekallikrein) or any other serine protease tested. These findings are supported by a 2.1-Å resolution crystal structure of pKal complexed to a DX-2930 Fab construct, which establishes that the pKal active site is fully occluded by the antibody. DX-2930 injected subcutaneously into cynomolgus monkeys exhibited a long half-life (t½ ∼ 12.5 days) and blocked high molecular weight kininogen proteolysis in activated plasma in a dose- and time-dependent manner. Furthermore, subcutaneous DX-2930 reduced carrageenan-induced paw edema in rats. A potent and long acting inhibitor of pKal activity could be an effective treatment option for pKal-mediated diseases, such as HAE.

Evaluation of protein engineering and process optimization approaches to enhance antibody drug manufacturability.

A potent single digit picomolar fully human monoclonal antibody (hMAb) inhibitor with a high degree of specificity to the antigen of interest was identified from a phage display library. The hMAb, however, exhibited a high degree of hydrophobicity and easily formed insoluble aggregates when purified using a Protein A based generic process. Strategies were designed using both protein engineering and process development approaches to optimize the molecule's amino acid sequence and its behavior in process conditions. The insoluble aggregation issue was brought under control by one single amino acid mutation in CDR region or by switching to non-ProA based purification process. Our study therefore presents the rational manufacturability design for future monoclonal antibody product and its purification process under the quality by design concept by either engineering the drug molecule to adapt existing platform process or optimizing the process to fit the specific properties of the drug product.