Characterization of cyclic peptides binding serum albumin

Serum Albumin (SA) is a non-glycosylated globular protein composed of three homologous domains in a heart-shaped globular form. SA is the most abundant protein in plasma and lymphatic system with a a maximum circulatory half-life of 19 days in humans. The long half-life of SA is attributed to its structural properties and its ability to bind the the neonatal Fc receptor (FcRn), which mediates endocytic pH-dependent recycling and rescue of SA from lysosomal degradation. Moreover, SA is characterised by excellent solubility and stability, as well as by the ability to bind and transport a wide range of hydrophobic endogenous and exogenous ligands such as hormones, fatty acids and drugs. Furthermore, SA shows low immunogenicity and the propensity to accumulate in inflamed and malignant tissues. Thanks to all these properties, SA is a natural carrier and pharmacokinetics modulator of a broad range of molecules, including drugs, nucleic acids and peptides. So far, two major approaches have been investigated: i) non-covalent tethering of molecule to SA using a specific binding moiety and ii) covalent fusion or chemical conjugation of the molecule to SA. In this study, I characterised the binding properties of some cyclic peptides capable of interacting with SA derived from different species that have the potential to extend the half-life of therapeutic molecules to several days. By applying yeast display technology coupled to flow cytometry, with one fluorescent probe to monitor expression of cyclic peptides on the cell surface of yeast cells and another probe to detect binding of cyclic peptides to biotinylate SA, we were able to determine the binding affinity and specificity of four distinct cyclic peptide families. The characterised cyclic peptides are currently being used to enhance the in vivopharmacological properties of some therapeutic molecules.