LAUSR

Antibodies, also known as immunoglobulins, are proteins secreted by immune cells to specifically bind foreign antigens, often from invading species. Immunoglobulin G (IgG), one of the major classes of antibodies, is composed of two heavy chains and two light chains that are interlinked via disulfide bonds, resulting in two identical fragment antigen-binding (Fab) domains for recognizing biological targets, and one fragment crystallizable (Fc) domain that mediates immune reaction and improves stability. In recent years, chemically producing and/or modifying antibodies for improved therapeutic functions have attracted attention across the fields of chemistry and medicine. In this issue of ACS Central Science, Thoreau et al. report a bioorthogonal chemical ligation technique to construct synthetic antibodies, dubbed SynAbs. The overall structure of SynAbs mimics the naturally occurring IgG, but they differ in one key aspect: unlike IgGs that specifically recognize only one epitope per antibody, SynAbs can be made to contain two distinct antigen-binding sites, enabling them to bind two biological targets at a time. In other words, they are bispecific. Bispecific antibodies (bsAbs) are attractive as they are capable of recruiting immune cells to attack tumors or increasing binding specificity. Owing to recent advances in antibody conjugation methodologies, more than 200 types of bsAbs have been made for clinical and preclinical trials. However, many of these synthetic strategies are incapable of incorporating the Fc domain that could confer bsAbs with higher stability, better solubility, longer half-life, and enhancement of tumor killing capacity due to Fc-mediated antibody-dependent cell-mediated cytotoxicity (ADCC) and complement dependent cytotoxicity (CDC) effects. Existing methods for producing Fc-containing IgG-like bsAbs are almost exclusively protein bioengineering, including quadroma cell line technology, “knobs-into-holes”, CrossMab, etc. Despite great progress made by these strategies, bioengineering methods are often hampered by low yield, complicated purification processes, and low modularity. In this study, the authors developed a pure chemical ligation method to generate Fc-containing antibodies based on disulfide rebridging and click chemistry. Previous studies reported a subset of cysteine reactive disulfide rebridging reagents, which could reduce disulfide bridges and then covalently rebridge the cysteines via small molecules. By adding click handles to the reagents, protein−protein conjugates could be easily generated through click chemistry. To construct IgG-like bsAbs, the authors started by producing HER2/HER2 bivalent monospecific antibodies. After the site-selective disulfide rebridging reaction of CD20 Fc and HER2 Fab fragments, the dually clickable-Fc and mono