Generation of mixed-valency, modular multispecific antibodies using disulfide-linked Fc–FcγR complexes

There is a strong need for multispecific antibodies that possess favorable clinical properties and can be generated through a simple and efficient process. Here, we repurpose the native IgG Fc–FcγR interaction into a universal, covalent docking site using only a single engineered disulfide bond, providing a simple solution to bypass the complex, bespoke engineering typically required for multispecific antibody design. The resulting Fc–FcγR complex forms a homodimeric Fc with one ligand, with the FcγR offering both N- and C-termini for independent functionalization, enabling single- or dual-payload formats, including masked designs for conditional activation. Introducing the disulfide between Fc (A330C) and FcγRIIIa (I106C) yields stable, covalently linked complexes that do not require post-expression modification, are compatible with standard mammalian expression, and support payloads such as anti-CD3, anti-CD28, IL-2, and protease-activated IL-2. These compounds exhibit potent, antigen-selective cytotoxicity in vitro and in vivo, with tunable avidity and reduced off-target activity. This plug-and-play platform overcomes key developability bottlenecks and enables rapid, scalable creation of next-generation antibody therapeutics.