The design of potent and specific peptide inhibitors to therapeutic targets is of enormous utility for both proof-of-concept studies and for the development of potential new therapeutics. Here we describe the development of a specific inhibitor of the Grb7-SH2 domain involved in cancer progression. Grb7 is an adapter protein, aberrantly co-overexpressed with erbB-2 and identified as an independent prognostic marker in breast cancer. Grb7 signals the activation of erbB-2 which plays a key role in disregulated cell growth in cancer. Grb7 also mediates signalling from focal adhesion kinase (FAK) exacerbating cell migration and the metastatic potential of cells. It is thus a prime target for the development of novel anti-cancer therapies.
We have structurally characterised a cyclic peptide (G7-18NATE) that is a specific inhibitor of Grb7 and inhibits cellular growth and migration in cancer cell lines1. Based on this we have developed a series of second generation bicyclic peptides, constrained via O-ally-serine ring-closure metathesis, that show enhanced affinity and maintained specificity for the Grb7-SH2 domain2. Interestingly, structural studies reveal an unexpected involvement of the O-ally-serine linker in target binding. We have also developed cyclic peptides that incorporate carboxymethylphenylalanine and carboxyphenylalanine as phosphotyrosine mimetics, and shown using X-ray crystallography the way in which this also contributes to improved binding. Finally, we have shown that by combining these two strategies we are able to achieve peptides with affinities in the nM range that still maintain target specificity.