In back-to-back papers published in the Proceedings of the National Academy of Sciences, researchers describe retro-inverted peptide mimetics targeting the vascular endothelial growth factor receptor (VEGFR) and epidermal growth factor receptor (EGFR) pathways. The team begin by screening a phage display library to identify peptides that bind to the target receptor and refining the hits by structural and functional analysis. Since natural peptides are often unstable to proteolysis, the amino acid retro-inversion method (ie substitution of D-amino acids for L-amino acids in conjunction with reversal of chain direction) was used to produce molecules with greater resistance to proteolysis. The side chain topology of the resulting peptide mimetics is similar to that of the original peptide – albeit with the amide bonds and terminal charges reversed.
The formation of new blood vessels has been associated with a number of pathologies including cancer and diseases of the eye such as diabetic retinopathy and age-related macular degeneration. VEGF comprises a family of five growth factors that promote angiogenesis by binding to, and selectively activating, several membrane-bound tyrosine kinase receptors (VEGFR-1, -2, and -3) and neurophilins (NRP-1 and -2). The minimal structural requirement for binding of a small peptide fragment to VEGFR-1 and NRP-1 was determined to be the tripeptide, RPL. The retro-inverted tripeptidomimetic, D-LPR was found to be resistant to proteolysis and to bind effectively to VEGFR-1 and NRP-1. In competition binding experiments, the IC50 values of RPL for VEGFR-1 and NRP-1 were 30 nM and 4 pM and the IC50 values of D-LPR for VEGFR-1 and NRP-1 were 2 pM and 2 pM. D-LPR was shown to be effective in three animal models of angiogenesis, including a mouse model of retinopathy. When administered topically, D-LPR led to a significant (ca 50%) reduction in angiogenesis, suggesting that the peptide mimetic can penetrate the vitreous humour and may provide a lead for the development of soluble and permeable small drug molecules that can be administered in eye drops.
EGFR, a tyrosine kinase receptor, is abnormally activated in many types of epithelial tumours, including including lung, colon, and head and neck cancers. Although, theoretically, the EGFR pathway could be blocked by three drug classes – antibodies, kinase inhibitors or soluble ligand receptor traps/decoys – so far no EGFR decoy has been developed. In the second paper, the team describe small molecules that mimick EGFR and behave as soluble decoys for EGFR ligands, EGF and TGFα. The cysteine-bridged pentapeptide, CVRAC, was found to bind specifically to EGFR ligands and both CVRAC and the retro-inverted derivative, D-CARVC, markedly reduced proliferation of tumour cell lines. In immunocompetent female mice bearing mammary tumours, mice treated with D-CARVC had significantly smaller tumour volumes than control mice.