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 accelerated dissociation at neutral pH, and EML (L309E/Q311M/M428L), which displayed attenuated binding kinetics. In human FcRn transgenic mice, YML extended the serum half-life of clinically used trastuzumab with a wild-type Fc by 6.1-fold, demonstrating a remarkable improvement over previously reported Fc-engineered variants, including PFc29 (Q311R/M428L) and DHS (L309D/Q311H/N434S), which represent the most effective Fc modifications for prolonging serum persistence to date. This in vivo validation underscores the pivotal role of FcRn kinetic tuning in overcoming endogenous IgG competition and maximizing FcRn-mediated antibody transport. Additionally, YML exhibited potent complement-dependent cytotoxicity (CDC) while maintaining favorable physicochemical properties. Conclusion This study presents a rational Fc engineering framework to optimize FcRn binding kinetics, addressing a previously unconsidered challenge—endogenous IgG competition during intracellular trafficking of therapeutic antibodies. The distinct kinetic behaviors of YML and EML highlight the critical necessity of precise control over pH-dependent association and dissociation rates in FcRn binding. YML represents a next-generation Fc platform, offering enhanced pharmacokinetics and improved effector functions, thus providing a powerful strategy for developing biologics with superior serum persistence and therapeutic efficacy.