Immunotherapy of folate receptor expressing cancers
The folate receptor (FR) is a GPI anchored cell surface glycoprotein that functions to facilitate folic acid uptake and mediate signal transduction. With the introduction of multiple folate-targeted drugs into the clinic, the question has arisen regarding how frequently a patient can be dosed with a FR-targeted drug or antibody, and whether dosing frequency exerts any impact on the availability of FR for subsequent rounds of FR-mediated drug uptake. Although the rate of FR recycling has been examined in murine tumor models, little or no information exists on the impact of FR occupancy on its rate of endocytosis. The studies described in chapter two of this thesis quantitates the number of cell surface FR-&agr; and FR-β following exposure to saturating concentrations of a variety of folate-linked molecules and anti-FR antibodies, including the unmodified vitamin, folate-linked drug mimetics, multi-folate derivatized nanoparticles, and monoclonal antibodies to FR. The collected data indicate that FR occupancy has no impact on the rate of FR internalization. The results also demonstrate that multivalent conjugates that bind and cross-link FRs at the cell surface internalize at the same rate as monovalent folate conjugates that have no impact on FR clustering, even though the multivalent conjugates traffic through a different endocytic pathway. Having described early, fundamental studies regarding the mechanism of action of cell surface folate receptors, the remaining chapters in this thesis will focus on the design and evaluation of combination immunotherapy techniques for the treatment of FR expressing cancers. Cancer immunotherapy relies on harnessing the power of the body's immune system and directing its killing power towards a patient's malignant tumor while avoiding toxicity to healthy tissues. Since physiological folates play a critical role in DNA synthesis and cell division, and since uncontrolled proliferation is a trademark of malignant cancer cells, FR is overexpressed by a variety of tumors in order to meet the demand for larger quantities of the vitamin. Therefore, the folate receptor has been successfully exploited for the delivery of folic acid linked imaging and therapeutic agents to a variety of cancer cell types. Chapters 3 and 4 of this thesis describes experiments aimed towards evaluating immunotherapy combinations that could have clinically translatable effects on FR positive cancers. By using in vivo mouse models of kidney cancer, lung cancer, and lymphoma these studies demonstrate that vascular endothelial growth factor receptor (VEGFR) inhibitors as well as T cell check point inhibitors synergize with folate-hapten mediated immunotherapy to slow tumor progression, reduce tumor cell metastasis, and prolong survival. We then proceed to answer the more challenging question of why this observed synergy occurs by evaluating resected tumor and spleen tissues from treated mice for changes in immune cell components and microenvironment composition. It is apparent from the collected data that the physiological mechanism of synergy is multi-fold and includes the combination's ability to better activate immune effector cells, recruit cytotoxic cells to the tumor site, down regulate immune suppressor cells, and slow the growth of tumor vasculature.
Low, Purdue University.
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