Chemical Macrocyclization of Peptides Fused to Antibody Fc Fragments

To extend the plasma half-life of a bicyclic peptide antagonist, we chose to link it to the Fc fragment of the long-lived serum protein IgG1. Instead of chemically conjugating the entire bicyclic peptide, we recombinantly expressed its peptide moiety as a fusion protein to an Fc fragment and subsequently cyclized the peptide by chemically reacting its three cysteine residues with tris-(bromomethyl)­benzene. This reaction was efficient and selective, yielding completely modified peptide fusion protein and no side products. After optimization of the linker and the Fc fragment format, the bicyclic peptide was fully functional as an inhibitor (<i>K</i>_i = 76 nM) and showed an extended terminal half-life of 1.5 days in mice. The unexpectedly clean reaction makes chemical macrocyclization of peptide-Fc fusion proteins an attractive synthetic approach. Its good compatibility with the Fc fragment may lend the bromomethylbenzene-based chemistry also for the generation of antibody–drug conjugates