Peptide vaccines and targeting HER and VEGF proteins may offer a potentially new paradigm in cancer immunotherapy

Abstract

The ErbB family (HER-1, HER-2, HER-3 and HER-4) of receptor tyrosine kinases has been the focus of cancer immunotherapeutic strategies while antiangiogenic therapies have focused on VEGF and its receptors VEGFR-1 and VEGFR-2. Agents targeting receptor tyrosine kinases in oncology include therapeutic antibodies to receptor tyrosine kinase ligands or the receptors themselves, and small-molecule inhibitors. Many of the US FDA-approved therapies targeting HER-2 and VEGF exhibit unacceptable toxicities, and show problems of efficacy, development of resistance and unacceptable safety profiles that continue to hamper their clinical progress. The combination of different peptide vaccines and peptidomimetics targeting specific molecular pathways that are dysregulated in tumors may potentiate anticancer immune responses, bypass immune tolerance and circumvent resistance mechanisms. The focus of this review is to discuss efforts in our laboratory spanning two decades of rationally developing peptide vaccines and therapeutics for breast cancer. This review highlights the prospective benefit of a new, untapped category of therapies biologically targeted to EGF receptor (HER-1), HER-2 and VEGF with potential peptide 'blockbusters' that could lay the foundation of a new paradigm in cancer immunotherapy by creating clinical breakthroughs for safe and efficacious cancer cures.

Figure 1. HER and VEGF family pathways in cancer

The signaling pathways of VEGF and HER family members and the current drugs that target these proteins in cancer are shown. HER-2 can heterodimerize with any of the ligand-activated HER receptors (HER-1, HER-3 or HER-4) and this association leads to intracellular signaling via two major pathways, the MAPK pathway and the PI3K pathway, leading to proliferation, cell survival, metastasis and angiogenesis. On the other hand, VEGF can bind to its main receptor, VEGFR-2 (KDR), and this binding causes intracellular phosphorylation of the receptor, thereby stimulating the PI3K pathway and stimulating angiogenesis. The signaling pathways can be targeted extracellularly using humanized monoclonal antibodies, such as trastuzumab and pertuzumab (HER-2), cetuximab (EGF receptor), as well as bevacizumab or VEGF Trap (VEGF), which can prevent ligand binding and activation of the receptors or can directly block binding of an activated receptor to another. At the intracellular level, small-molecule inhibitors, such as sunitinib (VEGF), lapatinib (HER-1 and HER-2) and erlotinib (HER-2), can disrupt the phosphorylation sites and directly prevent activation of the PI3K or MAPK pathways. P: Phosphate; VEGFR: VEGF receptor.

Figure 2. Combination of HER-2 vaccines and VEGF peptide mimics

HER-2 vaccines and VEGF peptide inhibitors developed in our laboratories to inhibit signaling pathways. Antibodies elicited by immunization with MVF-HER-2 (266–296) vaccine bind to domain II of HER-2 and, similarly, anti-MVF-HER-2 (597–626) binds domain IV of HER-2, providing dual inhibition of homo/heterodimerization, and consequently downstream signaling, shutting down the PI3K and MAPK pathways, thereby preventing cancer growth and metastasis. On the other hand, VEGF peptide mimics P3 and P4, which are designed to directly block VEGF binding to VEGFR-2, inhibit intracellular phosphorylation of the tyrosine kinase domain, which reduces angiogenesis. EGFR: EGF receptor; P: Phosphate; VEGFR: VEGF receptor.

Figure 3. Chimeric B- and T-cell epitopes

B- and T-cell epitopes are colinearly synthesized with a GPSL flexible linker. The linker is flexible, allowing the two epitopes to fold or adopt different conformations independent of each other.

Figure 4. Binding interface of pertuzumab and HER-2

Important binding residues at the interface of pertuzumab and HER-2 show the key amino acid residues that are critical for binding. (I–III) are the HER-2 domains.

Figure 5. Binding interface of trastuzumab to HER-2

The crystal structure of HER-2 (red) in contact with trastuzumab (blue). The three loops that make direct contact with trastuzumab are clearly shown, depicting important binding residues.

Figure 6. Design of retro-inverso peptide mimics

The schematics show the strategy used to design RI peptides. The side-chain conformation structure (mirror image) between the parent peptide and the RI peptide is maintained when D-amino acids are used and the synthesis is carried out in the reverse direction. RI: Retro-inverso.

Figure 7. VEGF peptide mimics

(A) The structure of VEGF shows the critical amino acid residues that are involved in binding of VEGF to its receptor VEGFR-2. (B) The anti-parallel β-sheet structure of VEGF 76–96 connected by loop residues 83–89. (C) Shows the engineered VEGF peptide mimic with two artificial cysteine residues inserted to form a cyclized anti-parallel β-structure. The complex peptide is synthesized from Phe96 to Glu93 followed by Cys and the synthesis continues with Ile80 through Gly92. The other Cys is placed between Gly92 and Gly79 and ends with Ile76.