The therapeutic efficacy of an antibody drug depends on the variable domains and on the constant crystallizable fragment (Fc). IgG variable domains have been the targets of extensive molecular engineering in search of more specific binders with higher affinities for their targets. Similarly, Fc engineering approaches have led to modulating both the immune effector responses and serum half-lives of therapeutic antibodies. A high-affinity interaction between the IgG Fc and neonatal Fc receptor (FcRn) at a slightly acidic pH can protect IgG molecules from undergoing lysosomal or serum proteinase-induced degradation. Here we describe an optimized protocol for the development of a tailored, synthetic human Fc repertoire to select Fc mutants which show highly pH-restricted FcRn binding with high affinity.
