Bicyclic Peptide Ligands Pulled out of Cysteine-Rich Peptide Libraries

Bicyclic peptide
ligands were found to have good binding affinity
and target specificity. However, the method applied to generate bicyclic
ligands based on phage-peptide alkylation is technically complex and
limits its application to specialized laboratories. Herein, we report
a method that involves a simpler and more robust procedure that additionally
allows screening of structurally more diverse bicyclic peptide libraries.
In brief, phage-encoded combinatorial peptide libraries of the format
XmCXnCXoCXp are oxidized
to connect two pairs of cysteines (C). This allows the generation
of 3 × (m + n + o + p) different peptide topologies because the fourth
cysteine can appear in any of the (m + n + o + p) randomized amino acid
positions (X). Panning of such libraries enriched strongly peptides
with four cysteines and yielded tight binders to protein targets.
X-ray structure analysis revealed an important structural role of
the disulfide bridges. In summary, the presented approach offers facile
access to bicyclic peptide ligands with good binding affinities.