Structure–function analysis of empasiprubart, a calcium- and pH-dependent clinical phase complement C2 blocking antibody

Empasiprubart (ARGX-117) is a humanized recycling antibody that prevents binding of C2 to C4b, blocking downstream classical and lectin pathways of complement activation. Empasiprubart binds to the CCP2 domain of complement component C2 in a calcium- and pH-dependent manner, leveraging physiological differences between blood and endosomal environments to facilitate the release and subsequent degradation of bound C2. The molecule incorporates Fc region mutations (H433K and N434F) that enhance its affinity for the neonatal Fc receptor (FcRn) under acidic endosomal conditions, thereby prolonging its in vivo half-life and supporting its recycling capacity. However, despite the earlier description of the complex structure, the molecular mechanism underlying these dependencies has remained elusive. Here, we further explored the crystal structure of the empasiprubart fragment antigen-binding (Fab) complexed to a C2 fragment, and provide a molecular rationale for its unique properties, while recognizing that not all contributing factors have been fully elucidated. Our observations indicate that the pH-dependent target release is rooted in a subtle intramolecular complementarity-determining region (CDR) destabilization, rather than direct modulation of the binding interface, and highlight the interplay between framework residues and CDRs. Collectively, our results not only lead to a better understanding of the mode of action of empasiprubart but also demonstrate the pivotal role of framework residues in the orchestration of antibody CDR function for non-trivial target binding.