Multivalent Antibody-Recruiters Targeting Fibroblast Activation Protein for Innate Immune Killing.

Fibroblast activation protein (FAP) is selectively overexpressed on cancer-associated fibroblasts (CAFs), making it a prime target for reshaping the tumor microenvironment. Antibody-recruiting molecules (ARMs) offer a monoclonal antibody alternative by enabling endogenous antibodies binding to eliminate target cells. Here, we report the development of multivalent FAP-targeting ARMs based on a truncated derivative of the high-affinity small-molecule OncoFAP. Initial attempts to generate monovalent ARMs by conjugating the FAPL to dinitrophenol (DNP) failed to enable antibody recruitment, despite efficient ligand binding. To overcome steric limitations, FAPL and DNP were coconjugated onto a 150 kDa dextran scaffold for multivalent display of FAP and antibody binding motifs. These dextran-based ARMs efficiently recruited anti-DNP antibodies, triggered robust complement-dependent cytotoxicity (CDC) and antibody-dependent cellular phagocytosis (ADCP) of FAP-expressing cells in vitro. Our findings demonstrate that multivalent macromolecular scaffolds effectively enable small-molecule-mediated antibody recruitment to noninternalizing targets like FAP, highlighting their potential for therapeutic tumor stroma remodeling.