Conventional surgery, radiotherapy, and chemotherapy for the treatment of gliomas has failed to provide definitive gains. We propose to synthesize boron-containing nucleosides, related to 5-dihydroxyboryl-2'-deoxyuridine (DBDU), which could be used for boron neutron capture therapy (BNCT) for the treatment of gliomas. BNCT is based on the property of the non- radioactive boron-10 isotope to capture low energy neutrons, thereby producing a localized cell-destroying nuclear reaction. Thus, irradiation of tumor cells with neutrons, following selective incorporation of the boronated nucleoside, would result in the destruction of tumor tissue only. A number of nucleoside require intracellular phosphorylation by nucleoside kinases in order to be activated to their active metabolite, namely the 5'- triphosphates. Once the compounds are phosphorylated, they are trapped in the cell. The nucleotide triphosphates can be incorporated into the cancer cell DNA as false nucleotide precursors. Previous results by our group indicate that DBDU, the first boron- containing nucleotide, is capable of sensitizing hamster V-79 cells to neutrons, and that the drug is incorporated into DNA as counterfeit thymidine. Depending on the experimental conditions, the dose enhancement ranged from 1.5 to 2.4. Cells exposed to DBDU incorporate an amount of boron such that the resultant biological effect is equivalent to a boron concentration of at least 6 mug boron/g cells, an amount approaching that which should be adequate for neutron capture therapy. The percent replacement of DBDU for thymidine is estimated to be >1%. Blocking de novo pyrimidine nucleotide synthesis increases the incorporation of DBDU in DNA and thus the magnitude of the neutron capture reaction. The focus of the work proposed is on the synthesis of novel boron pyrimidines, purines, and their corresponding 2'-deoxynucleosides. The uptake of radioactive nucleoside will be determined in human and rat cells and in a glioma rat model. The compounds proposed will include nucleosides containing an exocyclic and endocyclic boron nuclei which may be selectively taken up in glioma cells, and nucleosides related to 2'- deoxyadenosine and thymidine containing an endocyclic boron function. Enzymatic and hydrolytic stabilization of the glycosidic bond will be achieved by synthesizing 2'-fluoroarabinosyl analogs of the boron nucleosides. Biochemical studies will be performed to determine if the compounds or their nucleotides are substrates or competitive inhibitors of cellular kinases and DNA polymerases obtained from human glioma cells. Preliminary toxicological and pharmacokinetic studies in mice and rats will also be performed on select compounds. In order to increase the lipophylicity of the nucleosides for greater cellular and CNS uptake, the nucleosides will be converted to various boronic esters. The overall aim is to exploit BNCT by utilizing boronated nucleosides which show preferential uptake and localization in tumors. The hope is that sufficient information will be developed on these compounds so as to select one or more compounds for advanced preclinical studies prior to evaluation for BNCT in patients with diagnosed gliomas, and other tumors.
