Phage Display for Improved Peptide-Based Tumor Targeting and Imaging Agents

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Aggressive forms of cancer are a leading health problem for United States Veterans. While most cancers are treatable in their earliest forms, gender-related cancers including those of the breast and prostate are often fatal when diagnosed at advanced and/or metastatic stages. In 2020, these cancers will account for over 75,400 annual deaths in the general population, with morbidity rates significantly higher in the Veteran population. While most patients initially respond to treatment, they often relapse and develop aggressive and drug resistant tumors. A promising approach to better detect and treat these types of cancers may be through development of agents that target drug resistance biomarkers that associate with cancer stem cells (CSCs). CSCs make up a small proportion of a tumor but drive tumor aggressiveness and drug resistance. There are few validated CSC biomarkers, and most are not cancer-specific. Our laboratory is at the forefront of new biomarker and CSC identification using bacteriophage (phage) display. Peptides and antibody (Ab) fragments that bind the breast and prostate associated Thomsen-Friedenreich carbohydrate antigen, (TF), the lectin galectin-3 (gal-3) that binds TF and inhibits immune response, and CD444v6 have been obtained. These antigens are involved in cell adhesion, angiogenesis, immune regulation, and cell signaling, and are thought to represent CSC biomarkers. Further, we demonstrated that CD44v6 binds gal-3 through TF interaction, suggesting this triad may represent a novel target to probe CSCs, tumor growth and immune surveillance. Both Abs and peptides have been used with mixed success in cancer imaging and therapy. In terms of Abs their large size can cause immunogenic problems and long distribution times that once radiolabeled often leads to organ and tissue damage. Conversely, peptides, particularly those from phage display are usually hydrophobic and often suffer from poor affinity and tumor uptake. Two different tacks will be employed that overcome these drawbacks. A pretargeted “click” chemistry approach which decouples the tumor-targeting step from the radioimaging or radiotherapy step can be employed. This pretargeted approach is based on the rapid and highly specific cycloaddition reaction that occurs between the trans-cyclooctene (TCO) on the tumor by allowing the unlabeled targeting Ab to circulate and bind the target. Subsequently, injection of a small molecule radiolabeled probe with rapid pharmacokinetics and high affinity for the pretargeted Ab, will result in higher tumor to normal tissue uptake ratios compared to direct labeled Abs. The second tack will be to conjugate small peptides to a nanoparticle for multi-peptide display. Cornell prime dots (C' dots) are a new and well-characterized nanoparticle platform that offer both multimodal-targeted imaging and therapy, and multiplexing biomarker interrogation. C' dots are silica core-shell PEG nanoparticles that exhibit favorable clearance from the blood and are primarily excreted through the kidney overcoming extended circulation times. The tunable surface chemistries and favorable in vivo pharmacokinetics allow C' dots to overcome many of the dose limiting toxicities associated with current monomolecular cancer therapeutics. It is hypothesized that the Ab fragments and peptides that target gal-3/TF/CD44v6, coupled with pretargeting and C'dot multivalent display, will produce theranostic agents for aggressive prostate (PCa) and breast cancer (BCa). The objectives of this research are to develop: 1) radioimaging and radiotherapeutic agents of aggressive PCa and BCa tumors, 2) probes that inhibit tumorigenesis leading to reduced tumor growth and metastasis, and 3) bifunctional molecular therapeutics that increase tumor immune response through dual targeting with gal-3 and immune checkpoint inhibitors. Our work will positively impact VA healthcare in that we are developing new targeting agents for detection and treatment of cancer that will be well tolerated in vivo. This work will help improve the lives of our Veterans and coincides with BLR&D general goals and special interests of Women's Health and Genomic Medicine.