mRNA display selection and solid-phase synthesis of Fc-binding cyclic peptide affinity ligands.

Cyclic peptides are attractive candidates for synthetic affinity ligands due to their favorable properties, such as resistance to proteolysis, and higher affinity and specificity relative to linear peptides. Here we describe the discovery, synthesis and characterization of novel cyclic peptide affinity ligands that bind the Fc portion of human Immunoglobulin G (IgG; hFc). We generated an mRNA display library of cyclic pentapeptides wherein peptide cyclization was achieved with high yield and selectivity, using a solid-phase crosslinking reaction between two primary amine groups, mediated by a homobifunctional linker. Subsequently, a pool of cyclic peptide binders to hFc was isolated from this library and chromatographic resins incorporating the selected cyclic peptides were prepared by on-resin solid-phase peptide synthesis and cyclization. Significantly, this approach results in resins that are resistant to harsh basic conditions of column cleaning and regeneration. Further studies identified a specific cyclic peptide--cyclo[Link-M-WFRHY-K]--as a robust affinity ligand for purification of IgG from complex mixtures. The cyclo[Link-M-WFRHY-K] resin bound selectively to the Fc fragment of IgG, with no binding to the Fab fragment, and also bound immunoglobulins from a variety of mammalian species. Notably, while the recovery of IgG using the cyclo[Link-M-WFRHY-K] resin was comparable to a Protein A resin, elution of IgG could be achieved under milder conditions (pH 4 vs. pH 2.5). Thus, cyclo[Link-M-WFRHY-K] is an attractive candidate for developing a cost-effective and robust chromatographic resin to purify monoclonal antibodies (mAbs). Finally, our approach can be extended to efficiently generate and evaluate cyclic peptide affinity ligands for other targets of interest.