The radiation biology of boron neutron capture therapy

Cerebral ischemia leads to a massive increase in cytoplasmic calcium activity resulting from an influx of calcium ions into cells and a release of calcium from mitochondria and endoplasmic reticulum (ER). It is widely believed that this increase in cytoplasmic calcium activity plays a major role in ischemic cell injury in neurons. Recently, this concept was modified, taking into account that disturbances occurring during ischemia are potentially reversible: it then was proposed that after reversible ischemia, calcium ions are taken up by mitochondria, leading to disturbances of oxidative phosphorylation, formation of free radicals, and deterioration of mitochondrial functions. The current review focuses on the possible role of disturbances of ER calcium homeostasis in the pathologic process culminating in ischemic cell injury. The ER is a subcellular compartment that fulfills important functions such as the folding and processing of proteins, all of which are strictly calcium dependent. ER calcium activity is therefore relatively high, lying in the lower millimolar range (i.e., close to that of the extracellular space). Depletion of ER calcium stores is a severe form of stress to which cells react with a highly conserved stress response, the most important changes being a suppression of global protein synthesis and activation of stress gene expression. The response of cells to disturbances of ER calcium homeostasis is almost identical to their response to transient ischemia, implying common underlying mechanisms. Many observations from experimental studies indicate that disturbances of ER calcium homeostasis are involved in the pathologic process leading to ischemic cell injury. Evidence also has been presented that depletion of ER calcium stores alone is sufficient to activate the process of programmed cell death. Furthermore, it has been shown that activation of the ER-resident stress response system by a sublethal form of stress affords tolerance to other, potentially lethal insults. Also, disturbances of ER function have been implicated in the development of degenerative disorders such as prion disease and Alzheimer's disease. Thus, disturbances of the functioning of the ER may be a common denominator of neuronal cell injury in a wide variety of acute and chronic pathologic states of the brain. Finally, there is evidence that ER calcium homeostasis plays a key role in maintaining cells in their physiologic state, since depletion of ER calcium stores causes growth arrest and cell death, whereas cells in which the regulatory link between ER calcium homeostasis and protein synthesis has been blocked enter a state of uncontrolled proliferation.     

Liposomes containing boronophenylalanine for boron neutron capture therapy

In the present work, liposomes loaded with Boronophenylalanine (BPA), with or without stabilization, were formulated for the application in boron neutron capture therapy. BPA was encapsulated into liposomes as a complex with fructose, but also as a free drug in two different pH buffers. The influence of critical variables (cholesterol content, drug:lipid molar ratio, osmotic stress of liposomes containing hyperosmotic drug solution) on liposome morphology and drug content was evaluated. The drug content and dissolution profile of different BPA loaded liposomes were also studied. The physical stability of liposomes in terms of changes in the size distribution in different osmotic pressure buffers and the chemical oxidation of phospholipids during storing conditions were investigated. The encapsulation efficiencies of all formulations were always satisfactory, being between 20-48%; even when the liposomes were exposed to high osmotic stress, the particle size was below 200 nm. The BPA-fructose complex loaded liposomes showed a slower drug release profile.     

Design for an accelerator-based orthogonal epithermal neutron beam for boron neutron capture therapy

These studies examined whether the small to moderate reductions in body weight gain (< or = 15%) affect mammary carcinogenesis. Beginning 1 week prior to methylnitrosourea (MNU) administration (experiment 1), rats received diets supplemented with 4-hydroxyphenylretinamide (4-HPR) (782 mg/kg of diet) and retinyl acetate (328 mg/kg of diet) or underwent food restrictions. Rats were administered an i.v. dose of MNU (50 mg/kg body wt) at 50 days of age. Although the final body weights were similarly depressed by 4-HPR (8%) and by retinyl acetate (11%) from rats fed ad libitum, the kinetics of inhibition were quite different. Whereas 4-HPR caused an acute decrease in body weight at the time it was administered, the effect of retinyl acetate was more chronic. At 110 days after the administration of MNU, the average number of mammary cancers per rat was 4.9 for rats fed ad libitum, 1.3 for rats fed 4-HPR, 3.1 when body weights were matched to 4-HPR-treated rats, 1.9 for retinyl acetate and 3.2 when body weights were matched to retinyl acetate. Experiment II was performed to determine the minimal degree of acute body weight gain reduction that would alter MNU-induced mammary carcinogenesis. Body weight gain depressions of 3, 6, 9, 12 and 15% were initiated at 43 days of age by dietary restrictions and MNU was administered at 50 days of age. At 120 days after MNU, the percentage decreases in mammary cancer multiplicity in the various groups were 14, 15, 41, 44 and 55%, respectively. These data demonstrate that moderate reductions (9-15%) in body weight gain, in particular when occurring during the initiation and early promotion stages can greatly affect cancer multiplicity.     

Toward a final design for the Birmingham boron neutron capture therapy neutron beam

CD10 (CALLA) antigen is expressed in a wide variety of epithelial and nonepithelial tissues, but its most significant application is in the diagnosis and classification of certain types of malignant lymphoma and leukemia. CD10 is expressed in a high percentage of cases of acute lymphoblastic leukemia (ALL), follicular lymphoma, Burkitt's lymphoma, and some hematopoietic tumors. Although the antigen is not lineage specific, CD10 expression is widely used to define subgroups within B-ALL and is a useful tool for detecting the presence of leukemic blasts in the bloodstream. Currently available monoclonal antibodies to CD10 have been found to be effective only in fresh-frozen tissue and for techniques such as flow cytometry. We have used a recombinant protein corresponding to the whole of CD10 to generate a monoclonal antibody that is effective in paraffin-embedded tissue sections. We have used this antibody to assay for the presence of CD10 on a range of normal and pathological tissues. Strong staining was seen in lymphoid germinal centers, renal tubules, glomeruli, syncytiotrophoblast, hepatic parenchymal canaliculi, B-lineage ALL, follicle center cell lymphoma, and a proportion of cases of large-B-cell lymphoma. We believe that this antibody will be of value in the characterization of malignant lymphoma, in particular the differential diagnosis of small-B-cell lymphoma and subtyping of lymphoblastic leukemia, as well as the investigation of the significance of expression of CD10 in other normal and pathological tissues.     

Applicability of combination with tirapazamine in boron neutron capture therapy

SCC VII tumor-bearing mice were continuously given 5-bromo-2'-deoxyuridine (BrdU) to label all proliferating cells. After injection of tirapazamine (TPZ), a bioreductive agent, combined with sodium borocaptate-10B (BSH) or dl-p-boronophenylalanine-10B (BPA) administration, the tumors were irradiated with thermal neutrons, and then isolated and incubated with cytochalasin-B (a cytokinesis blocker). The micronucleus (MN) frequency in cells without BrdU labeling (quiescent (Q) cells) was determined by means of immunofluorescence staining for BrdU, and that for total cells was obtained from tumors not pretreated with BrdU. Even when no 10B-compound was administered, TPZ increased the MN frequency of tumor cells including Q cells, resulting in reduction of the difference in MN frequency between total and Q cells, mainly by increasing the MN frequency of Q cells. TPZ increased the MN frequency of Q cells when combined with BPA administration, but TPZ showed no apparent effect on each cell population when combined with BSH. Namely, TPZ reduced the difference in MN frequency between total and Q cells caused by 10B-compound administration, especially when BPA was administered. From the viewpoint of the overall cell killing effect in boron neutron capture therapy (BNCT), combination with TPZ appeared to be useful in BPA-BNCT, but not in BSH-BNCT.     

Monte Carlo calculation of dose enhancement by neutron capture of 10B in fast neutron therapy

Since 1978 the Essen Medical Cyclotron Facility has been used for fast neutron therapy. The treatment of deep-seated tumours by d(14) + Be neutron beam therapy (mean energy = 5.8 MeV) is still limited because of the steep decrease in depth-dose distribution. The interactions of fast neutrons in tissue leads to a thermal neutron distribution. These partially thermalized neutrons can be used to produce neutron capture reactions with 10B. Thus incorporation of 10B in tumours treated with fast neutrons will increase the relative local tumour dose due to the reaction 10B (n, alpha) 7Li. The magnitude of dose enhancement by 10B depends on the distribution of the thermal neutron fluence, 10B concentration, field size of the neutron beam, beam energy and the specific phantom geometry. The slowing down of the fast neutrons, resulting in a thermal neutron distribution in a phantom, has been computed using a Monte Carlo model. This model, which includes a deep-seated tumour, was experimentally verified by measurements of the thermal neutron fluence rate in a phantom using neutron activation of gold foil. When non-boronated water phantoms were irradiated with a total dose of 1 Gy at a depth of 6 cm, the thermal fluencies at this depth were found to be 2 x 10(10) cm-2. The absorbed dose in a tumour with 100 ppm 10B, at the same depth, was enhanced by 15%.