Date of Award
Doctor of Philosophy in Physics
Yana K. Reshetnyak
After heart disease, cancer is the leading cause of death in the U.S. According to the National Cancer Institute, approximately 38% of the population will be diagnosed with cancer at some point over the course of their life. This number is only expected to increase with the increased age of the general public due to medical progress, leading to an increased importance in finding effective and convenient treatment that can be used across a wide array of cancers.
In theory, any tumor can be destroyed using enough of a cytotoxic method, whether it be heating, radiation, surgery, or chemotherapy. However, because cancer cells are derived from a person’s own healthy cells mutating, all of these treatments involve destroying healthy tissue in the process. Because of this, the most important aspect of any treatment modality is its tumor specificity: the ability of the treatment to target cancerous tissue vs. healthy tissue. The higher the ratio of cancer cell death to healthy cell death a treatment contains, the more effectively it can be used in clinical practice.
A common method of targeting cancerous tissues is by targeting biomarkers that are overexpressed in cancer cells. These can include utilizing antibody, receptor, or vitamin bindings. While these methods can increase the specificity of treatment, there are also associated shortcomings with them. They tend to be cell-line specific, so a treatment that may work for one strain of cancer may not have the desired effect across other lines. Even within a single cancer strain, there exists heterogeneity a tumor, resulting in some cells not expressing the biomarkers to the extent needed for adequate targeting. This not only results in some cells surviving damage, but those cells then go on to reproduce, passing on their traits, and causing the tumor to adapt to become more resistant to that treatment. Within this arises the need to target a more general biomarker presented throughout all cancer cells and strains.
In 1931, the Nobel Prize in Physiology was awarded to Dr. Otto Heinrich Warburg for his discovery of what is known as the Warburg effect. In his research, Dr. Warburg discovered that even in the presence of oxygen, cancerous tissue produces the majority of its energy via anaerobic glycolysis instead of the aerobic oxidative phosphorylation. Glycolysis is a much less efficient, but faster, process, resulting in an excess of positively charged hydrogen ion byproducts. These are pumped outside of the cell membrane to maintain a normal pH within the cell, which results in a low pH environment immediately extracellularly to cancerous tissue. This acidity at the surface of cancer cells is ubiquitous across solid tumors, making it an ideal biomarker to target for cancer treatment.
pH-Low Insertion Peptide (pHLIP) is a pH-dependent peptide, whose pH-dependent action is based on the protonation of the aspartic (Asp) and glutamic (Glu) acid residues at tge C-terminal end of the peptide. In the presence of a low pH environment, the normally negatively charged Asp and Glu residues of pHLIP become protonated, increasing the overall hydrophobicity of the polypeptide. In the presence of a lipid bilayer, it triggers the insertion of the peptide across the bilayer to form a transmembrane alpha helix. Because of the Warburg effect described above, the acidity surrounding a cancerous tissue therefor promotes selective insertion of pHLIP across the cell membrane of tumor cells. As a result, different cargoes can be attached to the peptide, consequently being either tethered or translocated across membrane of cells in acidic diseased tissue at much higher proportions than in healthy tissue.
The main goal of this work was to evaluate the efficacy of different pHLIP variants to target tumors and deliver cargo across the membrane for therapeutic and diagnostic/imaging applications. In therapeutic applications, the attached cargo would directly induce cell death via external radiation enhancement, cell heating, or radioactive emission. For this, gold nanoparticles (GNPs) were studied, as gold is an inert and biocompatible. The focus of this work was to create GNPs coated with pHLIP to allow for enhancement of radiation via Auger electron emission, and to explore the possibility of creating gold-coated bicelles for heating via plasmon resonance. For diagnostic and imaging applications, the attached cargo would be used to visually differentiate the cancerous and healthy tissue, and be used in coordination with therapeutic strategies such as surgery for treatment. For this, the near-infrared dye indocyanine green (ICG) was conjugated to pHLIP to make ICG-pHLIP. ICG is already used in clinics for fluorescence guided surgery for imaging of lymph nodes and blood flow, making ICG-pHLIPs transition to clinics more straightforward and efficient. The focus of this study was to find the ICG-pHLIPs selectivity to cancerous tissue in vivo in balb-c and nude mice, exploring proof of concept for its continuation into human trials.
Crawford, Troy M., "ICG- AND GNP-PHLIP: NOVEL AGENTS FOR IMAGING AND THERAPY" (2020). Open Access Dissertations. Paper 1150.