Week 1, April 2025: Emerging Trends in Fc Domain Engineering for Therapeutic Development

The therapeutic potential of monoclonal antibodies (mAbs) has expanded dramatically over the past two decades, driven not only by advances in target discovery but also by innovations in antibody engineering. Among these, the engineering of the fragment crystallizable (Fc) domain has emerged as a powerful strategy to modulate antibody pharmacokinetics, tissue distribution, and effector functions. Originally recognized for its role in mediating immune responses through interactions with Fc gamma receptors (FcγRs) and the neonatal Fc receptor (FcRn), the Fc domain is now a key focal point for optimizing therapeutic antibody performance. From enhancing antibody half-life and brain penetration to fine-tuning immune activation and overcoming manufacturing challenges, Fc domain modifications offer a versatile toolkit to meet diverse clinical needs. In this article, we explore recent studies that exemplify the expanding landscape of Fc domain engineering, highlighting its role in shaping the next generation of antibody therapeutics. 

Fc Mutagenesis for Enhancing ADCC Without Glycoengineering 

ADCC is a key mechanism for antibody-mediated clearance of target cells, and its effectiveness depends on antibody interaction with FcγRIIIa on effector cells. Glycoengineering strategies to reduce fucosylation can enhance FcγRIIIa binding but introduce complexity in manufacturing. 

A recent study by researchers from the Walter and Eliza Hall Institute of Medical Research investigated an alternative approach by introducing point mutations into the Fc domain of anti-RhD mAbs (Brad3 and Fog1) to enhance ADCC. These Fc-modified antibodies were expressed in standard CHO cell lines and maintained RhD binding and high production yield. The Fc variants demonstrated increased FcγRIIIa engagement and ADCC activity comparable to polyclonal anti-RhD immunoglobulin (RhD-pIgG), which is used clinically to prevent hemolytic disease of the fetus and newborn (HDFN). These results support Fc protein engineering as a viable method for improving effector function without altering glycosylation. 

FcRn Binding Modulation for CNS Antibody Delivery 

Therapeutic delivery of antibodies to the central nervous system is limited by the blood-brain barrier. FcRn, expressed at brain endothelial cells, can mediate transcytosis of IgG and affect CNS exposure. A study from the University at Buffalo using human FcRn (hFcRn) transgenic mice examined the impact of Fc mutations on brain disposition of trastuzumab variants with different FcRn binding profiles. 

Mutants with increased FcRn binding at both neutral and acidic pH (YPY, YQAY) exhibited higher brain:plasma and ISF:plasma AUC ratios compared to wild-type. The YTE variant, which binds FcRn only at acidic pH, showed no increase in brain exposure, while the IHH variant, with no FcRn binding, had reduced brain uptake but prolonged ISF retention. These data indicate that FcRn binding at neutral pH is required for effective brain transcytosis, and that modifying the Fc domain to enhance such interactions can improve CNS delivery of antibodies. 

Preclinical Prediction of Pharmacokinetics for Fc-Engineered mAbs 

Accurate prediction of human pharmacokinetics is essential for therapeutic antibody development. Transgenic mice expressing hFcRn (Tg32) are commonly used to evaluate mAb pharmacokinetics. A recent study applied this model to predict human pharmacokinetics of Fc-engineered antibodies with increased FcRn affinity. 

Clearance and distribution parameters were assessed in Tg32 mice after intravenous administration of mAbs, with or without high-dose IVIG as a competitor for FcRn binding. Fc-engineered mAbs maintained low clearance in the presence of IVIG, unlike wild-type antibodies, which exhibited increased clearance. Allometric scaling was used to derive optimal exponents for pharmacokinetic parameters, enabling accurate prediction of human plasma concentration-time profiles. These results validate the use of FcRn transgenic mice to model the pharmacokinetics of Fc-engineered mAbs and support their application in early-stage development.

Impact of Antibody Aggregation on FcγR Binding 

Antibody aggregation can influence FcγR engagement and may affect therapeutic efficacy and safety. Another recent study investigated how forced aggregation of an IgG1 antibody (mAb1) altered binding to FcγRs using surface plasmon resonance (SPR) and cell-based assays. 

Aggregated mAb1 fractions showed increased binding to all FcγRs in avidity-based SPR formats and in solution, with the greatest effect observed for FcγRIIa. However, when binding was measured using an antibody-down SPR format (less sensitive to avidity), FcγRIIa binding was not increased. Functionally, FcγRIIa-mediated reporter activity increased slightly with aggregates, whereas FcγRIIIa activity decreased, likely due to altered glycosylation in aggregates. These findings highlight the need to monitor and control aggregation during manufacturing and formulation, as it can significantly affect FcγR binding and downstream immune signaling.