Hapten‐directed spontaneous disulfide shuffling: a universal technology for site‐directed covalent coupling of payloads to antibodies

ABSTRACT Humanized hapten‐binding IgGs were designed with an accessible cysteine close to their binding pockets, for specific covalent payload attachment. Individual analyses of known structures of digoxigenin (Dig)‐ and fluorescein (Fluo) binding antibodies and a new structure of a biotin (Biot)‐binder, revealed a “universal” coupling position (52+2) in proximity to binding pockets but without contributing to hapten interactions. Payloads that carry a free thiol are positioned on the antibody and covalently linked to it via disulfides. Covalent coupling is achieved and driven toward complete (95‐100%) payload occupancy by spontaneous redox shuffling between antibody and payload. Attachment at the universal position works with different haptens, antibodies, and payloads. Examples are the haptens Fluo, Dig, and Biot combined with various fluorescent or peptidic payloads. Disulfide‐bonded covalent antibody‐payload complexes do not dissociate in vitro and in vivo. Coupling requires the designed cysteine and matching payload thiol because payload or antibody without the Cys/thiol are not linked (