Development of Novel Tumor-Targeted Compounds for Boron Neutron Capture Therapy

Glioblastoma multiforme (GBM) represents the most common primary brain tumor among adults. Despite surgical resection and aggressive chemoradiotherapy regimens, the current 2- and 5-year survival rates are only 27% and 9.8%, respectively. The low survival stems from the poor response to conventional therapy and underscores the critical need to develop new therapeutic approaches for GBM treatment. The high recurrence rate observed in GBM is in part attributed to the hypoxic (poorly oxygenated) tumor microenvironment. Hypoxic tumor conditions have been shown to increase metastasis, promote angiogenesis, and confer resistance to chemotherapy and radiation. Hypoxic tissues are inherently radiation resistant due to a diminished oxygen enhancement effect. Additionally, limited diffusion of oxygen and small molecules to hypoxic tissues mitigates the efficacy of chemotherapeutics. Therefore, due to its unique mechanism of cell death, boron neutron capture therapy (BNCT) has the potential to become an alternative treatment modality for cancer patients where radiation and chemotherapy have fallen short. However, before the full clinical potential of BNCT is realized, there is a dire need to either develop novel tumor-targeted compounds or improve the localized delivery of existing BNCT agents. The work outlined in this dissertation aims to address both these needs. First, a series of novel boronated compounds have been synthesized capable of targeting the hypoxic (and often therapy resistant) tumor microenvironment. Second, the local tumor delivery of several boronated agents has been improved utilizing a thermal sensitive liposome delivery system. BNCT utilizes the nuclear fission reaction that occurs when a boron-10 isotope (10B) captures a neutron. Upon 10B neutron capture, the resulting unstable 11B isotope undergoes a nuclear fission reaction (10B(n,α,γ)7Li ) to release an alpha particle (4He), lithium-7 (7Li) ion and gamma radiation. The generated particles have a limited path length of approximately 5-10 microns, thereby localizing the cytotoxic effect. Therefore, the biggest treatment hurdle for BNCT is the requirement to preferentially deliver boron to the tumor with minimal accumulation in the surrounding normal tissue. Therefore, we hypothesized that the hypoxic tumor microenvironment could be exploited to improve preferential delivery of boronated compounds to the tumor. To begin the dissertation, a novel boronated 2-nitroimidazole derivative (B-381) has been synthesized in a single step reaction. It has long been recognized that 2-nitroimidazole derivatives have preferential retention in hypoxic cells compared to normoxic cells. Therefore, we hypothesized that B-381 would have preferential retention in hypoxic glioma cells by exploiting the unique metabolism and retention of 2-nitroimidazoles in hypoxia. Towards this end, the cellular uptake of B-381 in D54 glioma cells was evaluated in vitro and in vivo compared to 4-borono-L-phenylalanine (BPA), the most commonly investigated agent in BNCT clinical trials. Unlike BPA, B-381 illustrated preferential retention in hypoxic glioma cells compared to normoxic glioma cells in vitro. In vivo, B-381 illustrated significantly higher long-term tumor retention compared to BPA, with 9.5-fold and 6.5-fold higher boron levels at 24 and 48 h, respectively. While these initial studies supported the unique retention of B-381 in hypoxic cells, it was desirable to improve the total boron content delivered to the tumor. To further improve total boron content delivered to the tumor, thermal sensitive liposomes (TSLs) were investigated. A DPPC/DSPC/DSPE-PEG2000 /Cholesterol TSL was designed capable of having a stable drug payload at 37°C while releasing \u3e90% of the drug payload at 42°C. Therefore, by locally inducing mild hyperthermia in vivo (42-43oC), it is possible to trigger a localized release of boronated drug within the tumor vasculature. Using both B-381 and BPA, TSLs can significantly improve tumor boron delivery at 42°C compared to normal tissue temperature (37°C). In summary, B-381 is effectively administered as both a free agent or incorporated into a thermal sensitive liposome formulation. B-381 represents a new class of BNCT agents in which their selectivity to tumors is based on a hypoxic tumor metabolism. Further studies are warranted to evaluate boronated 2-nitroimidazoles as well as boron-containing thermal sensitive liposomes for future BNCT clinical trials