Tumor imaging using the pacifastin family of peptides

Thesis (Ph.D.)--University of Washington, 2018Cancer treatment involves multiple treatment modalities from the more traditional surgery, chemotherapy, and radiation treatment approaches to relatively modern forays into immunotherapy and the use of molecularly targeted agents. Survival rates continue to increase for many cancers as new therapies are introduced. The use of monoclonal antibodies targeting cancer, for instance, has played a major role in increased survival rates for subtypes of breast cancer as well as certain leukemias and lymphomas. Monoclonal antibodies, however, have limitations and several groups have been exploring peptides as both cancer imaging agents and cancer therapeutics. These peptide agents offer theoretical and practical advantages over antibody-based therapeutics and are showing promise in numerous clinical trials across a range of indications. In this work, I created a novel peptide capable of delivering an imaging agent, attempted to broaden its utility to deliver a chemotherapeutic, and sought to determine its mechanism of binding. I have shown that variants of LCMI-II, a naturally occurring serine protease inhibitor of the pacifastin family, targets multiple types of flank xenograft tumors in mice. We discovered that a modified version of the peptide, THP1, delivered the Alexa Fluor 647 fluorescent dye to tumors in vivo, which subsequently internalized into the cancer cells. The examination of closely related peptides revealed residues critical for tumor accumulation and enabled the engineering of a variant with improved tumor targeting. A THP1 conjugate carrying the microtubule inhibitor MMAE showed limited activity in vitro and no difference in vivo when compared to the vehicle control. While these studies reveal some of the obstacles to developing LCMI-II derived peptides for therapeutic delivery, they highlight the promise they show for cancer imaging. Additionally, this work sets the stage for future testing of next generation therapeutic peptides based on the pacifastin family