The neonatal Fc receptor (FcRn) is expressed in the endosomal compartments of a variety of cell types, including vascular endothelium and antigen-presenting cells (APCs). FcRn binds to the Fc region of immunoglobulin G (IgG) antibodies at acidic pH within endosomes. In utero, FcRn acts to transfer maternal IgG to the developing fetus. In adults, it is involved in recycling of IgG and albumin. Recycling by FcRn is the primary reason for the long half-life (several weeks) of IgG and albumin in serum. Furthermore, a critical factor for the success of therapeutic antibodies is their extended half-life, which contributes to better efficacy and long dosing schedule. Therefore, the FcRn-IgG interaction is a key parameter to optimize and track throughout the antibody drug development process.
The Lumit™ FcRn Binding Immunoassay is a novel homogeneous (no-wash) competition assay to measure the interaction between human FcRn and Fc proteins, including antibodies. NanoLuc® Binary Technology (NanoBiT®) is a structural complementation reporter designed for biomolecular interaction studies. The NanoBiT® system is composed of two subunits, Large BiT (LgBiT; 18kDa) and Small BiT (SmBiT; 11 amino acid peptide), that have been optimized for stability and minimal self-association. In the Lumit™ FcRn Binding Immunoassay, a Human IgG1 labeled with LgBiT (Tracer-LgBiT) is used as the tracer. A C-terminal biotinylated human FcRn bound to Streptavidin-SmBiT (hFcRn-Biotin-SA-SmBiT) is used as the target. In the absence of an antibody analyte sample, Tracer-LgBiT binds to the hFcRn-SmBiT target resulting in maximum luminescence signal. In analyte samples, unlabeled IgG will compete with Tracer-LgBiT for binding to the FcRn target, resulting in a concentration-dependent decrease in luminescent signal.