A peptoid antagonist of VEGF receptor 2 recognizes a 'hotspot' in the extracellular domain distinct from the hormone-binding site

Antagonists of VEGF-mediated angiogenesis are of great interest clinically for the treatment of solid tumors and certain forms of macular degeneration. We recently described a novel peptoid antagonist of VEGF Receptor 2 (VEGFR2) that binds to the extracellular domain of the receptor and inhibits VEGF-mediated autophosphorylation and subsequent downstream signaling. Given the structural similarities between peptides and peptoids, an obvious model for the mode of action of the peptoid is that it competes with VEGF for binding to VEGFR2. However, we present evidence here that this is not the case and that VEGF and the peptoid antagonist recognize non-overlapping surfaces located within the first three immunoglobulin-like subdomains of the receptor. These data argue that the peptoid inhibits receptor-mediated autophosphorylation by a novel allosteric mechanism that may prevent the receptor from acquiring the conformation necessary to propagate downstream signals.     

A VEGFR2 Antagonist and Other Peptoids Evade Immune Recognition

We have recently reported a peptoid (N-alkyl-oligoglycine) molecule that binds to the Vascular Endothelial Growth Factor Receptor 2 (VEGFR2) with high affinity and specificity. Moreover, this peptoid is capable of inhibiting VEGFR2 function in vivo (Udugamasooriya et al. J Am Chem Soc 130:5744–5745, 2008) and thus is a lead compound for anti-angiogenic agents. Moreover, the assay developed to identify this VEGFR2 inhibitor is likely to be a general route to peptoid antagonists or agonists of integral membrane receptors. Therefore, it is important to determine whether the VEGFR2-targeted peptoid, and indeed peptoids in general, are inherently immunogenic since an anti-peptoid immune response would significantly complicate their development as therapeutic candidates. In this study, the VEGFR2-targeted peptoid as well as other peptoids of varying lengths were injected into mice along with an immunostimulatory agent. We demonstrate that no significant anti-peptoid immune response is induced. It is further shown that this is not a trivial result of the lack of immunogenicity of a particular peptoid sequence, since conjugation of the peptoids to carrier proteins such as KLH prior to injection induces a robust anti-peptoid immune response. We conclude that free peptoid molecules are not immunogenic, probably due to a lack of T cell epitopes and that peptoid-based therapeutics are therefore not likely to be hindered by anti-peptoid antibody production in most cases.     

Effects of including an N-terminal insertion region and arginine-mimetic side chains in helical peptoid analogues of lung surfactant protein B

Surfactant protein B (SP-B) is one of two helical, amphipathic proteins critical for the biophysical functioning of lung surfactant (LS) and hence is an important therapeutic protein. This small, complex 79mer has three internal disulfide bonds and homodimerizes via another disulfide bridge. A helical, amphipathic 25mer from the amino terminus (SP-B(1-25)) exhibits surface-active properties similar to those of full-length, synthetic SP-B. In previous work, we created helical, non-natural mimics of SP-B(1-25) based on sequence-specific peptoid 17mers and demonstrated their biomimetic surface activity. Like SP-B(1-25), the peptoids were designed to adopt helical structures with cationic and nonpolar faces. Here, we compare the surface activities of six different helical peptoid analogues of SP-B(1-25) to investigate the importance of mimicking its N-terminal insertion domain as well as its two arginine residues, both thought to be important for the peptide's proper function. Although the peptoid analogues of SP-B(1-25) studied here share many similar features and all functionally mimic SP-B(1-25) to some degree, it is notable that small differences in their sequences and side chain chemistries lead to substantial differences in their observed interactions with a lipid film. A peptoid comprising a hydrophobic, helical insertion region with aromatic side chains shows more biomimetic surface activity than simpler peptoids, and even better activity, by comparison to natural LS, than SP-B(1-25). However, the substitution of lysine-like side chains for arginine-like side chains in the peptoid has little effect on biomimetic surface activity, indicating that interactions of the guanidino groups with lipids may not be critical for the function of these SP-B mimics.