The radiobiology of the dose components in a BNCT exposure is examined. The effect of exposure time in determining the biological effectiveness of γ-rays, due to the repair of sublethal damage, has been largely overlooked in the application of BNCT. Recoil protons from fast neutrons vary in their relative biological effectiveness (RBE) as a function of energy and tissue endpoint. Thus the energy spectrum of a beam will influence the RBE of this dose component. Protons from the neutron capture reaction in nitrogen have not been studied but in practice protons from nitrogen capture have been combined with the recoil proton contribution into a total proton dose. The relative biological effectiveness of the products of the neutron capture reaction in boron is derived from two factors, the RBE of the short range particles and the bio-distribution of boron, referred to collectively as the compound biological effectiveness factor. Caution is needed in the application of these factors for different normal tissues and tumors.

Selective irradiation of the vasculature of the rat spinal cord was used in this study, which was designed specifically to address the question as to whether it is the endothelial cell or the glial progenitor cell that is the target responsible for late white matter necrosis in the CNS. Selective irradiation of the vascular endothelium was achieved by the intraperitoneal (ip) administration of a boron compound known as BSH (Na2B12H11SH), followed by local irradiation with thermal neutrons. The blood-brain barrier is known to exclude BSH from the CNS parenchyma. Thirty minutes after the ip injection of BSH, the boron concentration in blood was 100 mu g B-10/g, while that in the CNS parenchyma was below the detection limit of the boron analysis system, < 1 mu g B-10/g. An ex vivo clonogenic assay of the O2A (oligodendrocyte-type 2 astrocyte) glial progenitor cell survival was performed I week after irradiation and at various times during the latent period before white matter necrosis in the spinal cord resulted in myelopathy. One week after 4.5 Gy of thermal neutron irradiation alone (approximately one-third of the dose required to produce a 50% incidence of radiation myelopathy), the average glial progenitor cell surviving fraction was 0.03. The surviving fraction of glial progenitor cells after a thermal neutron irradiation with BSH for a comparable effect was 0.46. The high level of glial progenitor cell survival after irradiation in the presence of BSH clearly reflects the lower dose delivered to the parenchyma due to the complete exclusion of BSH by the blood-brain barrier. The intermediate response of glial progenitor cells after irradiation with thermal neutrons in the presence of a boron compound known as BPA (p-dihydroxyboryl-phenylalanine), again for a dose that represents one-third the ED50 for radiation-induced myelopathy, reflects the differential partition of boron-10 between blood and CNS parenchyma for this compound, which crosses the blood-brain barrier, at the time of irradiation. The large differences in glial progenitor survival seen 1 week after irradiation were also maintained during the 4-5-month latent period before the development of radiation myelopathy, due to selective white matter necrosis, after irradiation with doses that would produce a high incidence of radiation myelopathy. Glial progenitor survival was similar to control values at 100 days after irradiation with a dose of thermal neutrons in the presence of BSH, significantly greater than the ED100, shortly before the normal time of onset of myelopathy. In contrast, glial progenitor survival was less than 1% of control levels after irradiation with 15 Gy of thermal neutrons alone. This dose of thermal neutrons represents the approximate ED90-100 for myelopathy. The response to irradiation with an equivalent dose of X rays (ED90: 23 Gy) was intermediate between these extremes as it was to thermal neutrons in the presence of BPA at a slightly lower dose equivalent to the approximate ED60 for radiation myelopathy. The conclusions from these studies, performed at dose levels approximately iso-effective for radiation-induced myelopathy as a consequence of white matter necrosis, were that the large differences observed in glial progenitor survival were directly related to the dose distribution in the parenchyma. These observations clearly indicate the relative importance of the dose to the vascular endothelium as the primary event leading to white atter necrosis. (c) 2006 by Radiation Research Society.

Radiation-induced bystander effects have been demonstrated in both normal and tumor cells using a variety of different radiation qualities. Literature reports are contradictory, however, on whether there is an LET dependence of the bystander effect. This study investigated the ability of DU-145 human prostate carcinoma cells irradiated with either a particles or 250 kVp X rays to cause medium-mediated bystander effects in unirradiated populations of DU-145 cells or in AG01522 human fibroblasts. The end points measured in both of the bystander cell lines v ere micronucleus formation, gamma-H2AX focus induction, and the surviving fraction. The incidence of niicronuclei increased 1.5-2.0-fold in both tumor and fibroblast bystander cells after 4 h of co-culture with DU-145 tumor cells that had been directly irradiated with either alpha particles or X rays. Only the AG01522 fibroblasts showed bystander effects for the gamma-H2AX focus (a 1.5-fold increase) and surviving fraction (a decrease to 0.8) end points when co-cultured with X-irradiated tumor cells. Alpha-particle irradiation of DU-145 tumor cells produced no decrease in the surviving fraction and no increase in gamma-H2AX focus induction in co-cultured bystander cells of either cell line. These results indicate that there are LET-dependent differences in the signal released from DU-145 human prostate carcinoma cells and that, for some end points, bystander AG01522 fibroblasts and bystander DU-145 prostate carcinoma cells respond differently to the same medium-mediated signal. (C) 2008 by Radiation Research Society.

To the Editor: Our initial proof-of-principle study in the mouse intestine showed that selective elevation of the (calculated) dose to the vascular endothelium had no effect on the clinically relevant endpoint of crypt stem cell survival at 3.5 days after irradiation ( 1 Schuller B.W. Binns P.J. Riley K.J. et al. Selective irradiation of the vascular endothelium has no effect on the survival of murine intestinal crypt stem cells. Proc Natl Acad Sci USA. 2006; 103: 3787-3792 Crossref PubMed Scopus (41) Google Scholar ). We next began to look for evidence of a direct effect on the vasculature: apoptosis at 4 h. These studies ( 2 Schuller B.W. Rogers A.B. Cormier K.S. et al. No significant endothelial apoptosis in the radiation-induced gastrointestinal syndrome. Int J Radiat Oncol Biol Phys. 2007; 68: 205-210 Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar ) showed only a low level of apoptotic cells in the villus lamina propria and no dose response, even in the control arm, which used the same whole body photon irradiation conditions originally reported by Paris et al.( 3 Paris F. Fuks Z. Kang A. et al. Endothelial apoptosis as the primary lesion initiating intestinal radiation damage in mice. Science. 2001; 293: 293-297 Crossref PubMed Scopus (1084) Google Scholar ). We contacted Drs. Fuks and Kolesnick of the Memorial Sloan-Kettering Cancer Center (MSKCC) in March 2006. We exchanged tissue blocks in May 2006 for the purposes of an interlaboratory comparison. On sections prepared at the Massachusetts Institute of Technology (MIT) from the MSKCC block (from the original study by Paris et al.[ 3 Paris F. Fuks Z. Kang A. et al. Endothelial apoptosis as the primary lesion initiating intestinal radiation damage in mice. Science. 2001; 293: 293-297 Crossref PubMed Scopus (1084) Google Scholar ]), our in situ DNA nick end labeling (TUNEL) staining showed only the same low level of leukocyte apoptosis in the lamina propria as in our own material. These data were communicated to Drs. Fuks and Kolesnick in May 2006. We never received any slides or images of the MSKCC staining on the MIT tissue blocks. In Regard to Schuller et al. (Int J Radiat Oncol Biol Phys 2007;68:205–210)International Journal of Radiation Oncology, Biology, PhysicsVol. 70Issue 3PreviewTo the Editor: In the article written by Schuller et al.(1), the authors assert that apoptosis does not occur in the endothelial cells in the gastrointestinal (GI) tract after whole-body irradiation, which is in direct conflict with the earlier publication of Paris et al. in 2001 in Science (2). Full-Text PDF

The motivation for this work was an unexpected occurrence of lung side effects in two human subjects undergoing cranial boron neutron capture therapy (BNCT). The objectives were to determine experimentally the biological weighting factors in rat lung for the high-LET dose components for a retrospective assessment of the dose to human lung during cranial BNCT. Lung damage after whole-thorax irradiation was assessed by serial measurement of breathing rate and evaluation of terminal lung histology. A positive response was defined as a breathing rate 20% above the control group mean and categorized as occurring either early (<110 days) or late (>110 days). The ED(50) values derived from probit analyses of the early breathing rate dose-response data for X rays and neutrons were 11.4+/-0.4 and 9.2+/-0.6 Gy, respectively, and were similar for the other end points. The ED(50) values for irradiation with neutrons plus p-boronophenylalanine were 8.7+/-1.0 and 6.7+/-0.4 for the early and late breathing rate responses, respectively, and 7.0+/-0.5 Gy for the histological response. The RBEs for thermal neutrons ranged between 2.9+/-0.7 and 3.1+/-1.2 for all end points. The weighting factors for the boron component of the dose differed significantly between the early (1.4+/-0.3) and late (2.3+/-0.3) breathing rate end points. A reassessment of doses in patients during cranial BNCT confirmed that the maximum weighted doses were well below the threshold for the onset of pneumonitis in healthy human lung.

PURPOSE:This report addresses the incidence of vascular endothelial cell apoptosis in the mouse small intestine in relation to the radiation-induced gastrointestinal (GI) syndrome. METHODS AND MATERIALS:Nonanesthetized mice received whole-body irradiation at doses above and below the threshold for death from the GI syndrome with 250 kVp X-rays, (137)Cs gamma rays, epithermal neutrons alone, or a unique approach for selective vascular irradiation using epithermal neutrons in combination with boronated liposomes that are restricted to the blood. Both terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) staining for apoptosis and dual-fluorescence staining for apoptosis and endothelial cells were carried out in jejunal cross-sections at 4 h postirradiation. RESULTS:Most apoptotic cells were in the crypt epithelium. The number of TUNEL-positive nuclei per villus was low (1.62 +/- 0.03, mean +/- SEM) for all irradiation modalities and showed no dose-response as a function of blood vessel dose, even as the dose crossed the threshold for death from the GI syndrome. Dual-fluorescence staining for apoptosis and endothelial cells verified the TUNEL results and identified the apoptotic nuclei in the villi as CD45-positive leukocytes. CONCLUSION:These data do not support the hypothesis that vascular endothelial cell apoptosis is the cause of the GI syndrome.

Boron Neutron Capture Therapy (BNCT) is a binary cancer treatment that exploits the short range particles released from a nuclear fission reaction involving the non-radioactive 10B nucleus and low-energy (thermal) neutrons for the destruction of tumour cells. If boronated agents are targeted towards chromosomal DNA, the efficiency of BNCT is greatly enhanced. This article presents a concise review of DNA-binding compounds that have been functionalised with boron.

Boron neutron capture therapy (BNCT) is based on the nuclear reaction that occurs when non-radioactive boron-10 is irradiated with low-energy thermal neutrons to yield high-linear energy transfer alpha particles and recoiling lithium-7 nuclei. Clinical interest in BNCT has focused primarily on the treatment of high-grade gliomas (HGG), and either cutaneous primaries or cerebral metastases of melanoma. Neutron sources for BNCT currently are limited to nuclear reactors and these are available in the United States, Japan, and several European countries. Accelerators also can be used to produce epithermal neutrons and these are being developed in a number of countries, but at this time none are being used for BNCT. Two boron drugs have been used clinically, sodium borocaptate (BSH) (Na2B12H11SH), and a dihydroxyboryl derivative of phenylalanine, referred to as boronophenylalanine (BPA). The major challenge in the development of boron delivery agents has been the requirement for selective tumor-targeting in order to achieve boron concentrations sufficient to deliver therapeutic doses of radiation to the tumor with minimal normal tissue toxicity. Over the past 20 yr, a wide variety of boron-containing compounds have been designed and synthesized. These include boron containing amino acids, biochemical precursors of nucleic acids, DNA binding molecules, and porphyrin derivatives. In addition, high-molecular-weight delivery agents have been developed, including liposomes and monoclonal antibodies (MAbs) and their fragments, which can recognize a tumor-associated epitope, such as epidermal growth factor. However, it is unlikely that any single agent will target all or even most of cells of brain tumors, and that combinations of agents will be required and their dosage and delivery will have to be optimized. Current or recently completed clinical trials have been carried out in Japan, Europe, and the United States. The vast majority of patients have had HGG. Treatment has consisted, first, of "debulking" surgery to remove as much of the tumor as possible, followed by BNCT at varying times after surgery. BSH and BPA have been used as the boron delivery agents, administered either intravenously or intra-arterially as was the case in the early studies with BSH. The best survival data from these studies are at least comparable to those obtained with surgery and external beam photon irradiation, and the safety of the procedure has been established. Critical issues that must be addressed include the need for more selective and effective boron delivery agents, the development of methods to provide semiquantitative estimates of tumor boron content prior to treatment, improvements in clinical implementation of BNCT, and finally, a need for randomized clinical trials with an unequivocal demonstration of therapeutic efficacy. If these issues are adequately addressed, then BNCT could move forward as a treatment modality.

The underlying mechanisms associated with radiation-induced cognitive impairments remain elusive but may involve changes in hippocampal neural precursor cells. Proliferating neural precursor cells have been shown to be extremely sensitive to X rays, either from damage to the cells themselves and/or through microenvironmental factors, including the anatomical relationship with the microvasculature, which is altered by radiation. The neutron capture reaction in boron was used to determine whether the sensitivity of neural precursor cells was dominated by direct radiation effects or was mediated through changes in the microvasculature. Young adult rats were irradiated with X rays, neutrons only, or neutrons plus either mercapto-undecahydro-dodecaborane (BSH) or p-dihydroxyboryi-phenylaianine (BPA). BSH remains inside cerebral vessels, thereby limiting the neutron capture intravascularly; BPA readily passes into the parenchyma. One month after irradiation, cell proliferation and numbers of immature neurons were determined using immunohistochemistry. Results showed that (1) neural precursor cells and their progeny were decreased in a dose-dependent manner by mixed high- and low-LET radiation, and (2) selective irradiation of the microvasculature resulted in less loss of neural precursor cells than when the radiation dose was delivered uniformly to the parenchyma. This information, and in particular the approach of selectively irradiating the vasculature, may be useful in developing radioprotective compounds for use during therapeutic irradiation. (c) 2006 by Radiation Research Society.

The possible role of vascular endothelial cell damage in the loss of intestinal crypt stem cells and the subsequent development of the gastrointestinal (GI) syndrome is addressed. Mice received whole-body epithermal neutron irradiation at a dose rate of 0.57 +/- 0.04 Gy x min(-1). An additional dose was selectively targeted to endothelial cells from the short-ranged (5-9 microm) particles released from neutron capture reactions in 10B confined to the blood by incorporation into liposomes 70-90 nm in diameter. Different liposome formulations produced 45 +/- 7 or 118 +/- 12 microg/g 10B in the blood at the time of neutron irradiation, which resulted in total absorbed dose rates in the endothelial cells of 1.08 +/- 0.09 or 1.90 +/- 0.16 Gy x min(-1), respectively. At 3.5 d after irradiation, the intestinal crypt microcolony assay showed that the 2- to 3-fold increased doses to the microvasculature, relative to the nonspecific whole-body neutron beam doses, caused no additional crypt stem cell loss beyond that produced by the neutron beam alone. The threshold dose for death from the GI syndrome after neutron-beam-only irradiation was 9.0 +/- 0.6 Gy. There were no deaths from the GI syndrome, despite calculated absorbed doses to endothelial cells as high as 27.7 Gy, in the groups that received neutron beam doses of <9.0 Gy with boronated liposomes in the blood. These data indicate that endothelial cell damage is not causative in the loss of intestinal crypt stem cells and the eventual development of the GI syndrome.

Boron neutron capture therapy (BNCT) is based on the nuclear reaction that occurs when boron-10, is irradiated with low energy-thermal neutrons to yield high linear energy transfer alpha particles and recoiling lithium-7 nuclei. Clinical interest in BNCT has focused primarily on the treatment of high grade gliomas, and either cutaneous primaries or cerebral metastases of melanoma, and more recently head and neck and liver cancer. Neutron sources for BNCT currently are limited to nuclear reactors and these are available in the United States, Japan, several European countries, and Argentina. Accelerators also can be used to produce epithermal neutrons and these are being developed in a number of countries, but none are currently being used for BNCT. Two boron drugs have been used clinically, sodium borocaptate or “BSH” (Na2B12H11SH), and a dihydroxyboryl derivative of phenylalanine, referred to as boronophenylalanine or “BPA”. The major challenge in the development of boron delivery agents has been the requirement for selective tumor-targeting in order to achieve boron concentrations (∼ 20 μg·g−1 tumor) sufficient to deliver therapeutic doses of radiation to the tumor with minimal normal tissue toxicity. Over the past 20 years, other classes of boron-containing compounds have been designed and synthesized that include boron-containing amino acids, biochemical precursors of nucleic acids, DNA binding molecules, and porphyrins derivatives. High molecular weight delivery agents include MoAbs and their fragments, which can recognize a tumor-associated epitope, (e.g., EGF) and liposomes. However, it is unlikely that any single agent will target all or even most of the tumor cells, and most likely combinations of agents will be required and their delivery will have to be optimized. Current or recently completed clinical trials have been carried out in Japan, Europe, and the United States. The vast majority of patients have had high grade gliomas. Treatment has consisted first of “debulking” surgery to remove as much of the tumor as possible followed by BNCT at varying times after surgery. Both BSH and BPA have been used as the boron delivery agents, administered intravenously. The best survival data from these studies are at least comparable to those obtained by current standard therapy for glioblastomas (GBM), and the safety of the procedure has been established. Critical issues that must be addressed include the need for more selective and effective boron delivery agents, the development of methods to provide semiquantitative estimates of tumor boron content prior to treatment, improvements in clinical implementation of BNCT, and finally, a need for randomized clinical trials with an unequivocal demonstration of therapeutic efficacy. If these issues are adequately addressed, then BNCT could move forward as a treatment modality. Keywords: boron neutron; capture theory; brain tumors; targeting agents

Purpose: The purpose of the present study was to evaluate a boronated EGFRvIII-specific monoclonal antibody, L8A4, for boron neutron capture therapy (BNCT) of the receptor-positive rat glioma, F98npEGFRvIII. Experimental Design: A heavily boronated polyamido amine (PAMAM) dendrimer (BD) was chemically linked to L8A4 by two heterobifunctional reagents, N-succinimidyl 3-(2-pyridyldithio)propionate and N-(k-maleimidoundecanoic acid)hydrazide. For in vivo studies, F98 wild-type receptor-negative or EGFRvIII human gene-transfected receptor-positive F98npEGFRvIII glioma cells were implanted i.c. into the brains of Fischer rats. Biodistribution studies were initiated 14 days later. Animals received [125I]BD-L8A4 by either convection enhanced delivery (CED) or direct i.t. injection and were euthanized 6, 12, 24, or 48 hours later. Results: At 6 hours, equivalent amounts of the bioconjugate were detected in receptor-positive and receptor-negative tumors, but by 24 hours the amounts retained by receptor-positive gliomas were 60.1% following CED and 43.7% following i.t. injection compared with 14.6% ID/g by receptor-negative tumors. Boron concentrations in normal brain, blood, liver, kidneys, and spleen all were at nondetectable levels (<0.5 μg/g) at the corresponding times. Based on these favorable biodistribution data, BNCT studies were initiated at the Massachusetts Institute of Technology Research Reactor-II. Rats received BD-L8A4 (∼40 μg 10B/∼750 μg protein) by CED either alone or in combination with i.v. boronophenylalanine (BPA; 500 mg/kg). BNCT was carried out 24 hours after administration of the bioconjugate and 2.5 hours after i.v. injection of BPA for those animals that received both agents. Rats that received BD-L8A4 by CED in combination with i.v. BPA had a mean ± SE survival time of 85.5 ± 15.5 days with 20% long-term survivors (>6 months) and those that received BD-L8A4 alone had a mean ± SE survival time of 70.4 ± 11.1 days with 10% long-term survivors compared with 40.1 ± 2.2 days for i.v. BPA and 30.3 ± 1.6 and 26.3 ± 1.1 days for irradiated and untreated controls, respectively. Conclusions: These data convincingly show the therapeutic efficacy of molecular targeting of EGFRvIII using either boronated monoclonal antibody L8A4 alone or in combination with BPA and should provide a platform for the future development of combinations of high and low molecular weight delivery agents for BNCT of brain tumors.

Emerging evidence points to the importance of deoxyribose oxidation in the toxicity of oxidative DNA damage, including the formation of protein-DNA crosslinks and base adducts. With the goal of understanding the differences in deoxyribose oxidation chemistry known to occur with different oxidants, we have compared the formation of one product of 3'-oxidation of deoxyribose in DNA, 3'-phosphoglycolaldehyde (PGA) residues, in isolated DNA and cells exposed to ionizing radiations. A recently developed gas chromatography/negative chemical ionization mass spectrometry method was used to quantify PGA residues in purified DNA and in human TK6 lymphoblastoid cells exposed to gamma radiation (60Co) and alpha particles (241Am). The level of PGA residues was then correlated with the total quantity of deoxyribose oxidation determined by plasmid topoisomer analysis. Alpha-particle irradiation (0-100 Gy) of purified DNA in 50 mM potassium phosphate (pH 7.4) produced a linear dose response of 0.13 PGA residues per 10(6) nucleotides per gray. When normalized to an estimate of the total number of deoxyribose oxidation events (2.0 per 10(6) nucleotides per gray), PGA formation occurred in 7% (+/-0.5) of deoxyribose oxidation events produced by alpha-particle radiation. In contrast, the efficiency of PGA formation in gamma-irradiated DNA was found to be 1% (+/-0.02), which indicates a shift in the chemistry of deoxyribose oxidation, possibly as a result of the different track structures of the two types of ionizing radiation. Studies with gamma radiation were extended to TK6 cells, in which it was observed that gamma radiation produced a linear dose response of 0.0019 PGA residues per 10(6) nucleotides per gray. This is consistent with an approximately 1000-fold quenching effect in cells, similar to the results of other published studies of oxidative DNA damage in vivo.

Abstract Morris, G. M., Coderre, J. A., Micca, P. L., Nawrocky, M. M., Hopewell, J. W. and Miura, M. Porphyrin-Mediated Boron Neutron Capture Therapy: A Preclinical Evaluation of the Response of the Oral Mucosa. Radiat. Res. 163, 72– 78 (2005). Preclinical studies are in progress to determine the potential of boron neutron capture therapy (BNCT) for the treatment of carcinomas of the head and neck. Recently, it has been demonstrated that various boronated porphyrins can target a variety of tumor types. Of the porphyrins evaluated so far, copper tetracarboranylphenyl porphyrin (CuTCPH) is potentially a strong candidate for clinical use. In the present investigation, the response of the oral mucosa to CuTCPH-mediated boron neutron capture (BNC) irradiation was assessed using the ventral surface of the tongue of adult male Fischer 344 rats, a standard rodent model. CuTCPH was administered by intravenous infusion, at a dose of 200 mg/kg body weight, over a 48-h period. Three days after the end of the administration of CuTCPH, biodistribution studies indicated very low levels of boron (<2 μg/g) in the blood. Levels of boron in tongue tissue were 39.0 ± 3.8 μg/g at this time. This was the time selected for irradiation with single doses of thermal neutrons from the Brookhaven Medical Research Reactor. The estimated level of boron-10 in the oral mucosa was used in the calculation of the physical radiation doses from the 10B(n,α)7Li reaction. This differs from the approach using the present generation of clinical boron carriers, where boron levels in blood at the time of irradiation are used for this calculation. Dose–response curves for the incidence of mucosal ulceration were fitted using probit analysis, and the doses required to produce a 50% incidence of the effect (ED50 ± SE) were calculated. Analysis of the dose–effect data for CuTCPH-mediated BNC irradiation, compared with those for X rays and thermal neutrons alone, gave a compound biological effectiveness (CBE) factor of ∼0.04. This very low CBE factor would suggest that there was relatively low accumulation of boron in the key target epithelial stem cells of the oral mucosa. As a consequence, with low levels of boron (<2 μg/g) in the blood, the response of the oral mucosa to CuTCPH-mediated BNCT will be governed primarily by the radiation effects of the thermal neutron beam and not from the boron neutron capture reaction [10B(n,α)7Li].

Alpha-particle exposures were used to determine whether cells of the human prostate carcinoma cell line DU-145 can produce and respond to a bystander effect signal. An apparatus for alpha-particle irradiation of cells growing as a monolayer on a 1.4-mu m-thick Mylar membrane directly above an Am-241 alpha-particle source was constructed and calibrated. At the cell irradiation position, the alpha-particle fluence was 998 counts/mm(2) s(-1), the average alpha-particle energy was 3.14 MeV, and the average linear energy transfer was 128 keV/mu m. The average dose rate to the cells growing on the Mylar surface was 1.2 Gy/min. A co-culture system was used to examine bystander effects transmitted through the medium from the directly targeted cells to tumor cells growing on an insert well beyond the range of the alpha particles. Alpha-particle doses from 0.1 to 6.0 Gy to the targeted cells on the Mylar membrane, followed by a 2-h co-incubation of the cells on the insert in the irradiated medium above the irradiated cells, all caused an similar to 50% increase in micronucleus formation in the nontargeted co-cultured cells. Addition of the radical scavenger DMSO to the medium during the irradiation and the 2-h postirradiation incubation period completely blocked the bystander effect, whereas addition of a nitric oxide scavenger had no effect. Irradiation of medium containing serum, followed by a 2-h incubation, caused no bystander effect in the co-cultured cells. When the co-cultured cells on the insert were placed into the irradiated medium above the directly targeted cells immediately (similar to 1 min) after the irradiation and co-incubated for 2 h, there was no bystander effect. These data indicate that the observed bystander effect requires that the co-cultured cells be present in the medium during the irradiation of the directly targeted cells and suggest the involvement of a short-lived radical species. (c) 2005 by Radiation Research Society.

Background: Boron neutron capture therapy (BNCT) is based on the nuclear reaction that occurs when boron-10 is irradiated with low-energy thermal neutrons to yield high linear energy transfer alpha particles and recoiling lithium-7 nuclei. Clinical interest in BNCT has focused primarily on the treatment of high-grade gliomas and either cutaneous primaries or cerebral metastases of melanoma, most recently, head and neck and liver cancer. Neutron sources for BNCT currently are limited to nuclear reactors and these are available in the United States, Japan, several European countries, and Argentina. Accelerators also can be used to produce epithermal neutrons and these are being developed in several countries, but none are currently being used for BNCTBoron Delivery Agents: Two boron drugs have been used clinically, sodium borocaptate (Na2B12H11SH) and a dihydroxyboryl derivative of phenylalanine called boronophenyla la nine. The major challenge in the development of boron delivery agents has been the requirement for selective tumor targeting to achieve boron concentrations( similar to 20 mu g/g tumor) sufficient to deliver therapeutic doses of radiation to the tumor with minimal normal tissue toxicity. Over the past 20 years, other classes of boron-containing compounds have been designed and synthesized that include boron-containing amino acids, biochemical precursors of nucleic acids, DNA-binding molecules, and porphyrin derivatives. High molecular weight delivery agents include monoclonal antibodies and their fragments, which can recognize a tumor-associated epitope, such as epidermal growth factor, and liposomes. However, it is unlikely that any single agent will target all or even most of the tumor cells, and most likely, combinations of agents will be required and their delivery will have to be optimized.Clinical Trials: Current or recently completed clinical trials have been carried out in Japan, Europe, and the United States. The vast majority of patients have had high-grade gliomas. Treatment has consisted first of "debulking" surgery to remove as much of the tumor as possible, followed by BNCT at varying times after surgery. Sodium borocaptate and boronophenylalanine administered i.v have been used as the boron delivery agents. The best survival data from these studies are at least comparable with those obtained by current standard therapy for glioblastoma multiforme, and the safety of the procedure has been established.Conclusions: Critical issues that must be addressed include the need for more selective and effective boron delivery agents, the development of methods to provide semiquantitative estimates of tumor boron content before treatment, improvements in clinical implementation of BNCT, and a need for randomized clinical trials with an unequivocal demonstration of therapeutic efficacy. If these issues are adequately addressed, then BNCT could move forward as a treatment modality.