The 27th Annual Symposium of The Protein Society: Program & Abstracts

Ligands based on bicyclic peptides can combine favourable properties of antibodies (good\udbinding affinity and target specificity) and small molecule ligands (stability, access to\udchemical synthesis, diffusion properties) and might be suitable molecular structures for the\uddevelopment of therapeutics1. By using a combinatorial methodology based on phage display\udand a chemical cyclisation reaction2, we isolated a potent (Ki = 53 nM) and selective inhibitor\udof human urokinase-type plasminogen activator (uPA), a trypsin-like serine protease that\udparticipates in the turnover of extracellular matrix (ECM) proteins and is implicated in tumor\udgrowth and invasion3. X-ray structure determination of the bicyclic peptide bound to uPA\udrevealed that both peptide loops engage the target to form a large interaction surface of 701\udÅ2 with multiple hydrogen bonds and complementary charge interactions, explaining the high\udaffinity and specificity of the inhibitor. The interface resembles that between two proteins and\udsuggests that these constrained peptides have the potential to act as small protein mimics.\udMoreover, further study revealed that the in vitro-evolved bicyclic peptide are stable in vivo\udand remain active for several days overcoming a limitation faced by many in vitro-evolved\udpeptide leads and promises to be suitable for the generation of long-acting peptide\udtherapeutics4,5. Its therapeutic effect is currently being tested in vivo