Neonatal Fc receptor (FcRn) (FcRn)

The Neonatal Fc receptor (FcRn) is a heterodimeric membrane protein, structurally related to MHC class I molecules, composed of an alpha chain (FCGRT) and beta-2-microglobulin [UniProt: P55899]. It is the primary biological mediator of drug half-life extension for IgG-based therapeutics and albumin, protecting these proteins from lysosomal degradation via a pH-dependent salvage pathway [PubMed: 31133071]. In this process, FcRn binds to the Fc region of IgG or the DIII domain of albumin within acidic endosomes, facilitating their recycling back into the vascular space [PubMed: 26116745]. Therapeutically, FcRn is exploited by engineering 'YTE' or 'LS' mutations into the Fc region of antibodies to enhance binding affinity at low pH, thereby significantly extending their serum half-life [PubMed: 28532490]. Conversely, FcRn is a direct target for pharmacological inhibition in autoimmune diseases, where blockers like efgartigimod are used to accelerate the clearance of pathogenic autoantibodies [FDA: Vyvgart Label]. Because the term 'Drug half-life extension' refers to a pharmacokinetic strategy rather than a specific molecule, this entry focuses on FcRn as the relevant biological target.

Other names

FCGRTBrambell receptorIgG receptor FcRn large subunit p51Neonatal Fc receptor for IgG

FcRn facilitates a pH-dependent salvage pathway where it binds the Fc region of IgG and the DIII domain of albumin within acidic endosomes (pH < 6.5), diverting them from lysosomal degradation and recycling them back to the plasma membrane for release at physiological pH (7.4). Drugs can exploit this by enhancing binding affinity to extend half-life or by competitively inhibiting the receptor to clear pathogenic autoantibodies.