METHODOLOGICAL DEVELOPMENTS FOR APPLICATION TO THE STUDY OF PHYSIOLOGICAL BORON AND TO BORON NEUTRON CAPTURE THERAPY

Abstract

The combination of immunogold labelling with electron microscopy or the direct detection of boron by electron energy loss spectrometry have the best lateral resolution for the imaging of boron or boron binding sites in tissues at the sub-cellular level. However these methods do not discriminate the boron isotopes. A number of physical methods make it possible to combine analytical imaging with isotopic labelling for boron studies in biological material. Secondary ion mass spectrometry has the potential to isotopically localise virtually any element with a resolution of ∼250nm with conventional instruments and 20–50nm with prototype instruments or with the NanoSIMS50; although SIMS has a relatively poor sensitivity for boron detection in biological matrices, boron imaging in plant samples is possible. Laser microprobe mass analysis also has the potential to detect boron isotopes with a lateral resolution of 3 to 5 μm and a detection limit of a few tens of μg/g with the conventional instruments and of the order of 1ng/g with the new LARIMP system; although mass resolution of LMMS is in general not very good, the risk of interference by other ions at the level of boron masses is limited. Neutron capture radiography is probably the easiest technique for boron imaging and boron isotopic labelling studies in tissues and sometimes at the sub-cellular level, although it detects only 10B isotopes. Nuclear reactions with charged particles (nuclear reaction analysis) have the potential to detect both isotopes of boron and carry out absolute boron concentration measurements with minimal matrix effects, limited risk of interference by other nuclides, a lateral resolution of a few μm at the best, a detection limit better than 1 μg/g for 11B, of the order of 10 μg/g for 10B and an accuracy of 1 to 2% in the determination of 10B/11B isotopic ratios. Preventing the diffusion of possibly mobile forms of boron during the preparation of the biological specimens is still a difficult problem for most techniques. The appropriate application of those methods, or their mutual combination or combination with other methods has made it possible: i) to yield information about the boron concentrations and fluxes in sub-cellular compartments and support the view that the cellular transport of boron was mainly passive under the experimental conditions under consideration; ii) to image the distribution of boron and of boron binding sites in tissues and sometimes at the sub-cellular level; iii) to study the short-distance diffusion and the long-distance transport of boron in plants and to assess the role of the phloem in the long-distance transport in various plant species; iv) to determine the origin (seed reserves vs uptake by roots) of the boron present in different sub-cellular compartments. For boron neutron capture therapy of cancers, invasive techniques of boron detection and imaging are comparable to the techniques described above for the study of physiological boron; for clinical applications, non-invasive techniques to follow 10B-compounds in vivo are being developed, especially by targeting of such compound by 18F and the use of positron emission tomography or by direct detection of 10B by magnetic resonance imaging.