IN-VITRO DETERMINATION OF TOXICITY, BINDING, RETENTION, SUBCELLULAR-DISTRIBUTION AND BIOLOGICAL EFFICACY OF THE BORON NEUTRON-CAPTURE AGENTDAC-1

In boron neutron capture therapy (ENCT), B-13 is delivered selectively to the tumour sells and the nuclide then forms high-LET radiation (He -4(2+) and Li-7(3+)) upon neutron capture. Today much research is focu sed on development of a variety of boron compounds aimed for BNCT. The compounds must be thoroughly analysed in preclinical tests regarding basic characteristics such as binding and subcellular distribution to enable accurate estimations of dose-modifying factors. DAC-1, 2-[2-(3- amino-propyl)-1,2- loso-dodecaboran(12)-1-yl-methoxy]-1,3-propanediol aas synthesized at our laboratories and the human colon carcinoma cell s LS-174T were used as an in vitro Model. The boron compound showed a remarkable intracellular accumulation, 20-100 times higher than the bo ron content in the culture medium, in cultured cells and was not remov ed by extensive washes. Approximately half of the boron taken up also remained within the cells for at least 4 days. The DAC-1 compound alon e was not toxic at boron concentrations below 2.5 mu g B/g. The intrac ellular distribution of he boron compound was investigated by subcellu lar fractionation experiments and low pH treatments. It is possible th at DAC-1 binds to some intracellular molecules or to membranes connect ed with organelles in the cytoplasm or even to the inside of the outer cell membrane. Another possibility is that the compound, due to the s omewhat lipophilic properties, is embedded in the membranes, Thermal n eutron irradiations were carried out at the Brookhaven Medical Researc h Reactor (BMRR). At a survival level of 0.1, DAC-1 + thermal neutrons were about 10.5 limes more effective in cell inactivation than the th ermal neutrons alone. Monte Carlo calculations gave a mean value of. t he B-10-dependent specific energy, the dose, of 0.22 Gy, The total phy sical dose during irradiation of DAC-1-containing cells with a neutron fluence of 0.18 x 10(12) n/cm(2) was 0.39 Gy. The dose-modifying fact or, at survival level 0.1, when comparing irradiation with thermal neu trons with and without DAC-1 was 3.4, while the dose-modifying factor when comparing neutron irradiations of cells with DAC-1 and irradiatio n of the cells with Co-60-gamma was 7.3. The results are encouraging a nd in vivo tests of tissue distributions and tumour uptake should now be carried out.