Distribution of BPA and metabolic assessment in glioblastoma patients during BNCT treatment: a microdialysis study

Boron neutron capture therapy (BNCT) is dependent on the selective accumulation of boron-10 in tumour cells. To maximise the radiation effect, the neutrons should be delivered when the ratio between the boron concentration in tumour cells to that in normal tissues reaches maximum. However, the pharmacokinetics of p-boronophenylalanine (BPA) and other boron delivery agents are only partly known. We used microdialysis to investigate the extracellular in vivo kinetics of boron in three intracerebral compartments -- solid tumour, brain adjacent to tumour (BAT), and the normal brain, as well as the subcutaneous tissue before, during, and after BNCT treatment. The findings were compared to the pharmacokinetics of BPA in the blood. We also measured the glucose metabolism and the levels of glutamate and glycerol in those compartments. Four patients were studied, two patients underwent surgical tumour resection and in two a stereotactic biopsy was performed. The patients were given BPA (900 mg/kg body weight) by a 6-h infusion. The infusion was completed approximately 2-3 h before neutron irradiation. In tumour tissue the extracellular concentration of BPA followed that of blood with a maximal concentration of 31.2 ppm and a maximal ratio vs. blood of 1.07. In BAT, the maximal concentration of BPA was 18.0 ppm with the peak level delayed for 4-6 h compared to the peak in blood with a maximal ratio of 1.2. Maximal blood concentration found was 41.0 ppm. The uptake of BPA in the normal brain was considerably lower than that in the blood and tumour tissue. No change in glucose metabolism was observed. The extracellular level of glycerol was increased after treatment in tumour tissue but not in normal brain suggesting a selective acute cytotoxic effect of BNCT on tumour cells.