Discovery, Development, and Cellular Delivery of Potent and Selective Bicyclic Peptide Inhibitors of Grb7 Cancer Target

Grb7
is a signaling protein with critical roles in tumor cell proliferation
and migration and an established cancer therapeutic target. Here we
explore chemical space to develop a new bicyclic peptide inhibitor,
incorporating thioether and lactam linkers that binds with affinity
(KD = 1.1 μM) and specificity to
the Grb7–SH2 domain. Structural analysis of the Grb7–SH2/peptide
complex revealed an unexpected binding orientation underlying the
binding selectivity by this new scaffold. We further incorporated
carboxymethylphenylalanine and carboxyphenylalanine phosphotyrosine
mimetics and arrived at an optimized inhibitor that potently binds
Grb7–SH2 (KD = 0.13 μM) under
physiological conditions. X-ray crystal structures of these Grb7–SH2/peptide
complexes reveal the structural basis for the most potent and specific
inhibitors of Grb7 developed to date. Finally, we demonstrate that
cell permeable versions of these peptides successfully block Grb7
mediated interactions in a breast cancer cell line, establishing the
potential of these peptides in the development of novel therapeutics
targeted to Grb7.