Engineering FcRn binding kinetics dramatically extends antibody serum half-life and enhances therapeutic potential

Abstract

Background: Optimizing the IgG Fc domain for neonatal Fc receptor (FcRn) binding is crucial for enhancing antibody pharmacokinetics. The prolonged serum half-life of IgG antibody is governed by its pH-dependent interaction with FcRn, enabling efficient binding at acidic endosomal pH, intracellular trafficking, and release at neutral serum pH. However, a critical yet previously unrecognized challenge in Fc engineering for extending the serum half-life of therapeutic antibodies is the intense competition with endogenous IgG for FcRn binding during intracellular trafficking, which limits FcRn-mediated transport and reduces the serum persistence of therapeutic antibodies. To address this, we developed an Fc variant that precisely modulates pH-dependent FcRn binding kinetics, accelerates FcRn association at acidic pH, and promotes rapid dissociation at neutral pH, thereby enhancing FcRn-driven intracellular transport, outcompeting endogenous IgG, and achieving unprecedented improvement in the serum half-life of therapeutic antibodies. Results: Using comprehensive site-directed saturation mutagenesis coupled with functional screening, we generated a diverse panel of Fc variants and identified two with distinct FcRn binding kinetics: YML (L309Y/Q311M/M428L), which exhibited superior FcRn association at acidic pH and ac