Surmounting an impasse of FcRn structure

Study on FcγRn Electrochemical Receptor Sensor and Its Kinetics

Neonatal γ-immunoglobulin (IgG) Fc receptor (FcγRn) is a receptor that transports IgG across the intestinal mucosa, placenta, and mammary gland, ensuring the balance of IgG and albumin in the body. These functions of FcγRn depend on the intracellular signal transduction and activation caused by the combination of its extracellular domain and IgG Fc domain. Nevertheless, there are still no kinetic studies on this interaction. Consequently, in the present study, we successfully constructed the human FcγRn (hFcγRn) electrochemical receptor sensor. The signal amplification system formed by chitosan nanogold-hFcγRn protein and horseradish peroxidase was used to simulate the cell signal amplification system in vivo, and the kinetic effects between seven IgG and hFcγRn receptors from different species were quantitatively measured. The results showed that the interaction of these seven IgGs with hFcγRn was similar to the catalytic kinetics of enzyme and substrate, and there was a ligand-receptor saturation effect. The order of the interconnect allosteric constants (Ka), which is similar to the Michaelis constant (Km), was human IgG < bovine IgG < horse IgG < rabbit IgG < sheep IgG < donkey IgG < quail IgY. The results showed that hFcγRn had the strongest ability to transport human IgG, which was consistent with the evolution of the system. Therefore, our hFcγRn electrochemical receptor sensor can be used to measure and evaluate the interconnected allosteric network. It is also an essential parameter of the interaction between hFcγRn and different IgGs and, thus, provides a new detection and evaluation method for immunoemulsion, therapeutic monoclonal antibody therapy, heteroantibody treatment, and half-life research.

Influence of size and charge of unstructured polypeptides on pharmacokinetics and biodistribution of targeted fusion proteins

Alternative non-IgG binding proteins developed for therapy are small in size and, thus, are rapidly cleared from the circulation by renal filtration. To avoid repeated injection or continuous infusion for the maintenance of therapeutic serum concentrations, extensions of unfolded polypeptides have been developed to prolong serum half-life, but systematic, comparative studies investigating the influence of their size and charge on serum half-life, extravasation, tumor localization and excretion mechanisms have so far been lacking. Here we used a high-affinity Designed Ankyrin Repeat Protein (DARPin) targeting the tumor marker epithelial cell adhesion molecule (EpCAM) in a preclinical tumor xenograft model in mice, and fused it with a series of defined unstructured polypeptides. We used three different sizes of two previously described polypeptides, an uncharged one consisting of only Pro, Ala and Ser (termed PAS) and a charged one consisting of Pro, Ala, Ser, Thr, Gly, Glu (termed XTEN) and performed for the first time a precise comparative localization, distribution and extravasation study. Pharmacokinetic analysis showed a clear linear relationship between hydrodynamic radius and serum half-life across both polypeptides, reaching a half-life of up to 21 h in mice. Tumor uptake was EpCAM-dependent and directly proportional to half-life and size, showing an even tumor penetration for all fusion proteins without unspecific accumulation in non-target tissue. Unexpectedly, charge had no influence on any parameter, neither tumor nor tissue accumulation nor kidney elimination kinetics. Thus, both polypeptide types have a very similar potential for precise half-life modification and tumor targeting.