engineered affibody molecule with pH-dependent binding to FcRn mediates extended circulatory half-life of a fusion protein

Proteins endocytosed from serum are degraded in the lysosomes. However, serum albumin (SA) and IgG, through its Fc part, bind to the neonatal Fc receptor (FcRn) at low pH in the endosome after endocytosis, and are transported back to the cellular surface, where they are released into the bloodstream, resulting in an extended serum circulation time. Association with Fc or SA has been used to prolong the in vivo half-life of biopharmaceuticals, using the interaction with FcRn to improve treatment regimens. This has been achieved either directly, by fusion or conjugation to Fc or SA, or indirectly, using SA-binding proteins. The present work takes this principle one step further, presenting small affinity proteins that bind directly to FcRn, mediating extension of the serum half-life of fused biomolecules. Phage display technology was used to select affibody molecules that can bind to FcRn in the pH-dependent manner required for rescue by FcRn. The biophysical and binding properties were characterized in vitro, and the affibody molecules were found to bind to FcRn more strongly at low pH than at neutral pH. Attachment of the affibody molecules to a recombinant protein, already engineered for increased half-life, resulted in a nearly threefold longer half-life in mice. These tags should have general use as fusion partners to biopharmaceuticals to extend their half-lives in vivo. Significance One of the central questions in biopharmaceutical development is how to extend circulation residence times of protein-based drugs to improve efficacy and patient comfort. Immunoglobulin G and serum albumin are rescued from lysosomal degradation by the pH-dependent binding to the neonatal Fc receptor and consequently have naturally long circulation half-lives. Here we describe the development and characterization of small affinity proteins (affibody molecules) that can hitchhike on the neonatal Fc receptor rescue system. We find that addition of the affibody molecules to an already half-life–extended recombinant protein leads to a nearly threefold longer circulation residence time in mice. Such affibody molecules may have a general use as fusion partners with biopharmaceuticals to extend their circulation residence times.