Staufen dsRNA-binding domain as modules to design bifunctional antibodies for siRNA delivery

Antibody-small interfering RNA (siRNA) conjugates present an opportunity to expand the siRNA therapy to extrahepatic tissues. However, their investigation is now only confined to a limited number of targets, partially owing to some flaws in structures. Here, we described a modular design of bifunctional antibody that tethers siRNA without conjugation, yielding a diligent one-to-one antibody-siRNA pairing structure feasible for target expansion, charge masking, and further functionalization. Focusing on a noncationic siRNA-recruiting module, Staufen1 dsRBD34, we demonstrated that bifunctional antibodies recruit siRNA independent of base modification and enable target gene silencing on multiple cell types at a stoichiometry (1/1). Notably, by functionalizing siRNA terminus with small-molecule enhancers, the silencing potency of this pairing system can be augmented by seven times (IC50 from 200 to 28 nM) through the endosome-to-cytosol import. Affinity maturation by arginine scanning yields the 32 times higher affinity of dsRBD34 to siRNA, but the augment led to neither stronger silencing nor higher stability in mouse plasma as compared to p19 protein. The competition from sulfated GAGs in circulations can alter the pharmacokinetics of pairs and prevent a practical assessment of their potential in vivo. Altogether, bifunctional antibodies here possess notable properties, but ultrahigh-affinity dsRNA-binding domain is necessary to realize applications.