Oxygen chiral phosphate esters

During the past four years, methods have been reported for the syntheses and configurational analyses of virtually any phosphate mono- and diester chiral by virtue of oxygen isotope substitution, and these techniques have already been applied to an impressive number of enzymic and chemical reactions. At present, no experimental information is available that contradicts the simplest interpretations that have been applied to the results obtained for enzymic reactions: inversion of configuration indicating a direct displacement of the leaving group by the attacking group, and retention of configuration implying the formation of a phosphorylated or nucleotidylated enzyme intermediate. However, it does seem necessary to further investigate the mechanisms of at least some of the reactions discussed in this review to ensure that the simplest interpretation is correct. For example, the caveat we have raised about the interpretation of inversions of configurations in phosphohydrolase reactions is chemically reasonable, and these reactions should be reexamined to evaluate the importance of covalent catalysis by carboxylate groups. However, for the vast majority of the enzymic reactions that have been investigated, the stereochemical approach to ascertaining whether catalysis involves the formation of covalent intermediates remains the simplest and most direct method.

The effect of dexamethasone on the uptake of p-boronophenylalanine in the rat brain and intracranial 9L gliosarcoma

The steroid dexamethasone sodium phosphate (DEX) is routinely used to treat edema in brain tumor patients. The objective of the present study was to evaluate the effects of DEX on the uptake of boronophenylalanine (BPA) using the rat 9L gliosarcoma tumor model and surrounding brain tissue. Two steroid dosage protocols were used. The high-dose DEX protocol involved five 3mg/kg intraperitoneal injections at 47, 35, 23, 11 and 1 h prior to the administration of the BPA for a total dose of 15 mg DEX/kg rat. The low-dose DEX administration protocol involved two doses of 1.5mg/kg at 17 h and 1h prior to BPA injection for a total dose of 3mg DEX/kg rat. The control animals received no pretreatment, prior to the administration of BPA. Seventeen days after tumor implantation, rats were injected i.p. with 0.014 ml/g body weight BPA solution (1200 mg BPA/kg; approximately 59 mg (10)B/kg). In all groups, rats were euthanized at 3h after BPA injection. Administration of the steroid had an effect on tumor weight, which decreased to approximately 78% (p > 0.05) of the control weight in the low-dose DEX group, and approximately 48% (p < 0.001) of the control weight in the high-dose DEX group. At 3 h after the administration of BPA, the concentration of boron in tumor was comparable (p > 0.1) in the control and high-dose DEX groups. The lowest mean value (73.8+/-1.6 microg/g) was obtained in the low-dose DEX group. This was significantly lower (p > 0.02) than the tumor boron contents in the high-dose DEX and control groups, which were 81.1+/-1.9 and 79.9+/-1.7 microg/g, respectively. Tumor:blood boron partition ratios for the control, low- and high-dose DEX groups were 2.3, 2.3 and 2.5, respectively. Boron concentrations were also measured in the normal brain and in the zone of brain adjacent to the tumor exhibiting edema. Although treatment with DEX had no appreciable effect on boron uptake in the normal brain of the rat, after the administration of BPA, it did impact on the boron levels in the zone of peritumoral edema. After the high-dose DEX administration protocol, boron levels in the zone of edema were reduced by approximately 14% (p < 0.02). This finding suggests that BPA targeting of tumor cells in the peritumoral zone could be compromised by DEX. These cells appear to play a critical role in tumor recurrence after BNCT or conventional radiotherapy.

Porphyrin-Mediated Boron Neutron Capture Therapy: A Preclinical Evaluation of the Response of the Oral Mucosa

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 microg/g) in the blood. Levels of boron in tongue tissue were 39.0 +/- 3.8 microg/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,alpha)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 approximately 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 microg/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,alpha)7Li].

Tolerance of normal human brain to boron neutron capture therapy

Data from the Harvard-MIT and the BNL Phase I and Phase I/II clinical trials, conducted between 1994 and 1999, have been analyzed and combined, providing the most complete data set yet available on the tolerance of the normal human brain to BPA-mediated boron neutron capture therapy. Both peak (1cm(3)) dose and average whole-brain dose show a steep dose-response relationship using somnolence syndrome as the clinical endpoint. Probit analysis indicates that the doses associated with a 50% incidence for somnolence (ED(50)+/-SE) were 6.2+/-1.0 Gy(w) for average whole-brain dose and 14.1+/-1.8 Gy(w) for peak brain dose.

Effects of boron neutron capture irradiation on the normal lung of rats

The whole lung of rats was irradiated with X-rays, thermal neutrons, or thermal neutrons in the presence of p-boronophenylalanine (BPA). A >/= 20% increase in breathing rate, in the period 40-80 days after irradiation, was indicative of radiation-induced pneumonitis. The ED(50) (+/-SE) for a >/= 20% increase in breathing rate, relative to age-matched controls, was 11.6 +/- 0.13 Gy for X-rays and 9.6 +/- 0.08 Gy for neutrons only. This indicated a thermal neutron beam RBE of 1.2 and an RBE of 2.2 for the high-LET components of the dose, assuming a dose reduction factor of 1.0 for gamma rays. Preliminary data indicate the compound biological effectiveness factor for BPA in the lung is approximately 1.5.

Porphyrin-mediated boron neutron capture therapy: evaluation of the reactions of skin and central nervous system

PURPOSE

Recently, various boronated porphyrins have been shown to preferentially target a variety of tumour types. Of the different porphyrins evaluated, copper tetra-phenyl-carboranyl porphyrin (CuTCPH) is a strong candidate for future preclinical evaluation. In the present study, the responses of two critical normal tissues, skin and central nervous system (CNS), to boron neutron capture (BNC) irradiation in the presence of this porphyrin were evaluated.

MATERIALS AND METHODS

Standard models for the skin and spinal cord of adult male Fischer 344 rats were used. CuTCPH was administered by intravenous infusion at a dose of 200 mg x kg(-1) body weight, over 48 h. The thermal beam at the Brookhaven Medical Research Reactor was used for the BNC irradiations. The 20-mm diameter irradiation field, for both the skin and the spinal cord, was located on the mid-dorsal line of the neck. Dose-response data were fitted using probit analysis and the doses required to produce a 50% incidence rate of early and late skin changes or myeloparesis (ED(50) +/- SE) were calculated from these curves.

RESULTS

Biodistribution studies indicated very low levels of boron (<3 microg x g(-1)) in the blood 3 days after the administration of CuTCPH. This was the time point selected for radiation exposure in the radiobiological studies. Levels of boron in the CNS were also low (2.8 +/- 0.6 microg x g(-1)) after 3 days. However, the concentration of boron in the skin was considerably higher at 22.7 +/- 2.6 microg x g(-1). Single radiation exposures were carried out using a thermal neutron beam. The impact of CuTCPH-mediated BNC irradiation on the normal skin and CNS at therapeutically effective exposure times was minimal. This was primarily due to the very low blood boron levels (from CuTCPH) at the time of irradiation. Analysis of the relevant dose-effect data gave compound biological effectiveness factors of about 1.8 for skin (moist desquamation) and about 4.4 for spinal cord (myeloparesis) for CuTCPH. These values were based on the BNC radiation doses to tissues calculated using the blood boron levels at the time of irradiation.

CONCLUSIONS

CuTCPH-mediated BNC irradiation will not cause significant damage to skin and CNS at clinically relevant radiation doses provided that blood boron levels are low at the time of radiation exposure.

Long-term infusions of p-boronophenylalanine for boron neutron capture therapy: evaluation using rat brain tumor and spinal cord models

Rat 9L gliosarcoma cells infiltrating the normal brain have been shown previously to accumulate only approximately 30% as much boron as the intact tumor after administration of the boronated amino acid p-boronophenylalanine (BPA). Long-term i.v. infusions of BPA were shown previously to increase the boron content of these infiltrating tumor cells significantly. Experiments to determine whether this improved BPA distribution into infiltrating tumor cells after a long-term i.v. infusion improves tumor control after BNCT in this brain tumor model and whether it has any deleterious effects in the response of the rat spinal cord to BNCT are the subjects of the present report. BPA was administered in a fructose solution at a dose of 650 mg BPA/kg by single i.p. injection or by i.v. infusion for 2 h or 6 h, at 330 mg BPA/kg h(-1). At 1 h after the end of either the 2-h or the 6-h infusion, the CNS:blood (10)B partition ratio was 0.9:1. At 3 h after the single i.p. injection, the ratio was 0.6:1. After spinal cord irradiations, the ED(50) for myeloparesis was 14.7 +/- 0.4 Gy after i.p. administration of BPA and 12.9 +/- 0.3 Gy in rats irradiated after a 6-h i.v. infusion of BPA; these values were significantly different (P < 0.001). After irradiation with 100 kVp X rays, the ED(50) was 18.6 +/- 0.1 Gy. The boron compound biological effectiveness (CBE) factors calculated for the boron neutron capture dose component were 1.2 +/- 0.1 for the i.p. BPA administration protocol and 1.5 +/- 0.1 after irradiation using the 6-h i.v. BPA infusion protocol (P < 0.05). In the rat 9L gliosarcoma brain tumor model, the blood boron concentrations at 1 h after the end of the 2-h infusion (330 mg BPA/kg h(-1); n = 15) or after the 6-h infusion (190 mg BPA/kg h(-1); n = 13) were 18.9 +/- 2.2 microg 10B/g and 20.7 +/- 1.8 microg 10B/g, respectively. The irradiation times were adjusted individually, based on the preirradiation blood sample, to deliver a predicted 50% tumor control dose of 8.2 Gy ( approximately 30 photon-equivalent Gy) to all tumors. In the present study, the long-term survival was approximately 50% and was not significantly different between the 2-h and the 6-h infusion groups. The mode of BPA administration and the time between administration and irradiation influence the 10B partition ratio between the CNS and the blood, which in turn influences the measured CBE factor. These findings underline the need for clinical biodistribution studies to be carried out to establish 10B partition ratios as a key component in the evaluation of modified administration protocols involving BPA.

The combination of X-ray-mediated radiosurgery and gene-mediated immunoprophylaxis for advanced intracerebral gliosarcomas in rats

Rats with advanced, imminently lethal, approximately 4 mm diameter, left-sided intracerebral 9L gliosarcoma (9LGS), a well characterized malignant tumor with some similarities to human high-grade astrocytomas, were used as a therapy model 14 days post-implantation of 10(4) cells. Such tumor-bearing rats die within two weeks (median, 6 days) thereafter if untreated. However, if these tumors are exposed on day 14 to 12-25 Gy of an electron-equilibrated 6 MV photon beam (radiosurgery), survival is extended about 5-6 fold to a median of 34 days, but long-term survival (> 1 year) is increased only to approximately 18%. Multiple subcutaneous inoculations of radiation-disabled 9LGS cells post-radiosurgery (immunoprophylaxis) extended lifespan and long-term (> 1 year) survival minimally (median, 37 days; 25%, respectively). In sharp contrast, radiosurgery followed by multiple subcutaneous inoculations of radiation-disabled 9LGS cells that had been transfected with granulocyte macrophage colony stimulating factor (GMCSF), a cytokine with demonstrated immune-enhancing properties (i.e. gene-mediated immunoprophylaxis, GMIMPR) increased long-term survival to approximately 67%. To our knowledge, these results are the first to show that the combination of photon radiosurgery and GMIMPR is effective for an advanced, imminently lethal brain tumor in a mammal. These data raise the possibility that GMIMPR following radiation therapy might prove effective for the treatment of some human malignant gliomas.

Boron microquantification in oral muscosa and skin following administration of a neutron capture therapy agent

Clinical trials of boron neutron capture therapy (BNCT) for intracranial tumours using boronophenylalanine-fructose undertaken at Harvard-MIT and Brookhaven National Laboratory have observed acute normal tissue reactions in the skin and oral mucosa. Because the range of the 10B(n, alpha)7Li reaction products is very short, 10-14 microns combined, knowledge of the 10B microdistribution in tissue is critical for understanding the microdosimetry and radiobiology of BNCT. This paper reports measurements of the microdistribution of 10B in an animal model, rat skin and tongue, using high resolution quantitative autoradiography (HRQAR), a neutron-induced etched track autoradiographic technique. The steep spatial gradient and high absolute value relative to blood of the 10B concentration observed in some strata of the rat tongue epithelium and skin are important for properly evaluating the radiobiology and the biological effectiveness factors for normal tissue reactions such as oral mucositis, which are generally assessed using the blood boron concentration rather than the tissue boron concentration.

Quantitative imaging and microlocalization of boron-10 in brain tumors and infiltrating tumor cells by SIMS ion microscopy: relevance to neutron capture therapy

Boron neutron capture therapy (BNCT) is dependent on the selective accumulation of boron-10 in tumor cells relative to the contiguous normal cells. Ion microscopy was used to evaluate the microdistribution of boron-10 from p-boronophenylalanine (BPA) in the 9L rat gliosarcoma and the F98 rat glioma brain tumor models. Four routes of BPA administration were used: i.p. injection, intracarotid (i.c.) injection [with and without blood-brain barrier disruption (BBB-D)], and continuous timed i.v. infusions. i.p. injection of BPA in the 9L gliosarcoma resulted in a tumor-to-brain (T:Br) boron-10 concentration ratio of 3.7:1 when measured at the tumor-normal brain interface. In the F98 glioma, i.c injection of BPA resulted in a T:Br ratio of 2.9:1, and this increased to 5.4:1 when BBB-D was performed. The increased tumor boron uptake would potentially enhance the therapeutic ratio of BNCT by >25%. At present, ion microscopy is the only technique to provide a direct measurement of the T:Br boron-10 concentration ratio for tumor cells infiltrating normal brain. In the 9L gliosarcoma, this ratio was 2.9:1 after i.p. administration. In the F98 glioma, i.c injection resulted in a ratio of 2.2:1, and this increased to 3.0:1 after BBB-D. Ion microscopy revealed a consistent pattern of boron-10 microdistribution for both rat brain tumor models. The boron-10 concentration in the main tumor mass (MTM) was approximately twice that of the infiltrating tumor cells. One hour after a 2-h i.v. infusion of BPA in rats with the 9L gliosarcoma, tumor boron-10 concentrations were 2.7 times higher than that of infiltrating tumor cells [83 +/- 23 microg/g tissue versus 31 +/- 12 microg/g tissue (mean +/- SD)]. Continuous 3- and 6-h i.v. infusions of BPA in the 9L gliosarcoma resulted in similar high boron-10 concentrations in the MTM. The boron-10 concentration in infiltrating tumor cells was two times lower than the MTM after a 3-h infusion. After 6 h, the boron-10 concentration in infiltrating tumor cells had increased nearly 90% relative to the 2- and 3-h infusions. A 24-h i.v. infusion resulted in similar boron-10 levels between the MTM and the infiltrating tumor cells. Boron concentrations in the normal brain were similar for all four infusion times (approximately 20 microg/g tissue). These results are important for BNCT, because clinical protocols using a 2-h infusion have been performed with the assumption that infiltrating tumor cells contain equivalent amounts of boron-10 as the MTM. The results reported here suggest that this is not the case and that a 6-h or longer infusion of BPA may be necessary to raise boron-10 levels in infiltrating tumor cells to that in the MTM.

Biodistribution of copper carboranyltetraphenylporphyrins in rodents bearing an isogeneic or human neoplasm

The biodistributions of carborane-containing copper porphyrins, CuTCP and CuTCPH, have been studied previously in mice bearing subcutaneously implanted mammary carcinomas. We now report biodistributions of those porphyrins in Fischer 344 rats bearing intracranial and/or multiple subcutaneous isogeneic 9L gliosarcomas (9LGS). The porphyrin was given either by i.v. infusion or by multiple i.p. injections. When 190 mg CuTCPH/kg body weight was given to the rats by i.v. infusion, median tissue boron concentrations (microg/g) 3 days after the end of infusion were: 64 in subcutaneous tumor, 13 in intracranial tumor, 1 in blood and 3 in brain. When 450 mg CuTCPH/kg body weight was given to the rats by serial i.p. injections, the median concentrations (microg B/g) 4 days after the last injection were: 117 in subcutaneous tumor, 50 in intracranial tumor, 4 in blood, and 4 in brain. CuTCPH biodistribution was also studied in xenografts of the human malignant gliomas U87 and U373, and of the murine EMT-6 mammary carcinoma and the rat 9LGS, each grown subcutaneously in mice with severe combined immunodeficiency (SCIDs). In SCIDs, median boron concentrations (microg/g) 2 days after the last s.c. injection of a total of 190 mg CuTCPH/kg body weight were: 251 in U373, 33 in U87, <0.6 in blood and <0.5 in brain. Because there were such high boron levels in the U373, and because xenografted U373 is similar to spontaneous intracerebral human glioblastoma multiforme (GBM) microscopically, CuTCPH could prove useful as a boron carrier for boron neutron-capture therapy (BNCT) of GBM and of other human malignant gliomas.

Boron neutron capture therapy: effects of split dose and overall treatment time

New clinical protocols are being developed that will entail the administration of considerably higher doses of the boron delivery agent boronophenylalanine (BPA) than those in current clinical use. Fractionation (2 or 4 fractions) of BPA mediated boron neutron capture therapy (BNCT) is also under consideration at some clinical centres. Given the considerably higher infusion volumes that will be entailed in the delivery of BPA in the new high dosage protocols, there will be a requirement to extend the gap between fractions to 2 or more days. In order to assess the effects of a 2 fraction protocol on the therapeutic efficacy of BPA mediated BNCT, a series of split dose irradiations (two equal fractions) were undertaken using the rat intracranially implanted 9L gliosarcoma model. A single dose exposure to BPA mediated BNCT of 3.0 Gy resulted in long term survival levels of 50%. Survival levels increased to 71% and 77% with a 3 and 5 day gap between dose fractions (two equal fractions), respectively, using the same total dose. A further increase in the time interval between dose fractions to 7 days resulted in a reduction in survival to 36%. However, there was no significant difference between the single dose and the 3, 5 and 7 day survival data (P > 0.1) The difference between the 5 and 7 day split dose survival data was of border-line significance (P = 0.05). It is anticipated that mucositis, could become a potential problem in future BNCT clinical protocols involving higher doses, larger field sizes or multiple fields. The potential sparing of the oral mucosa, due to repopulation during the interval between the two fractions, was investigated using a series of split dose BPA mediated BNC irradiations. The ventral surface of the rat tongue was utilised as a model for oral mucosa. The ED50 (50% incidence) values for the ulceration end point were 3.0+/-0.1, 3.2+/-0.1, 3.0+/-0.1 and 3.6+/-0.1 Gy, for 3, 5, 7 and 9 day splits between doses, respectively. It is evident from this data that there were no significant changes in the ED50 (p < 0.001) until the 9 day dose split, when the ED50 value was 20% higher than the ED50 value after a 7 day split. It was concluded that the two fraction BNCT protocol, with dose splits of up to 5 days, did not diminish the therapeutic response of the rat 9L gliosarcoma, when compared with a single dose BNCT protocol. Tolerance of the oral mucosa to BNC irradiation was not increased until there was a 9 day gap between fractions. However, the beneficial effects of dose sparing at this time interval between doses, would probably be counteracted by a reduction in the therapeutic effectiveness of the BNCT modality, due to repopulation of tumour clonogens between doses.

Boron neutron capture therapy of a murine mammary carcinoma using a lipophilic carboranyltetraphenylporphyrin

The first control of a malignant tumor in vivo by porphyrin- mediated boron neutron capture therapy (BNCT) is described. In mice bearing implanted EMT-6 mammary carcinomas, boron uptake using a single injection of either p-boronophenylalanine (BPA) or mercaptoundecahydrododecaborane (BSH) was compared with either a single injection or multiple injections of the carboranylporphyrin CuTCPH. The BSH and BPA doses used were comparable to the highest doses of these compounds previously administered in a single injection to rodents. For BNCT, boron concentrations averaged 85 microg (10)B/g in the tumor and 4 microg (10)B/g in blood 2 days after the last of six injections (over 32 h) that delivered a total of 190 microg CuTCPH/g body weight. During a single 15, 20, 25 or 30 MW-min exposure to the thermalized neutron beam of the Brookhaven Medical Research Reactor, a tumor received average absorbed doses of approximately 39, 52, 66 or 79 Gy, respectively. A long-term (>200 days) tumor control rate of 71% was achieved at a dose of 66 Gy with minimal damage to the leg. Equivalent long-term tumor control by a single exposure to 42 Gy X rays was achieved, but with greater damage to the irradiated leg.

Long-term immunological memory in the resistance of rats to transplanted intracerebral 9L gliosarcoma (9LGS) following subcutaneous immunization with 9LGS cells

Glioblastoma multiforme (GBM) is the most common primary human brain tumor. About 7000 new cases are diagnosed yearly in the USA. Despite current neurosurgical and postoperative radiotherapeutic tumor cytoreduction methods, in most cases occult foci of tumor cells infiltrate surrounding edematous brain tissues and cause recurrent disease within one year. GBM is almost invariably fatal within a few years after it is diagnosed. Our goal is to achieve long-term control of GBM by combining immunoprophylaxis with a radiation-based technique, such as boron neutron-capture therapy (BNCT), potentially capable of specifically targeting the infiltrating tumor cells while sparing the surrounding normal brain tissue. It has long been known that the subcutaneous (sc) injection of irradiated cells or untreated cultured cells (and the removal of the resulting tumors) derived from the well characterized, highly immunogenic 9L gliosarcoma (9LGS) rat model into young isogenic rats can prevent tumor growth after subsequent sc or intracranial (ic) injection of untreated, otherwise lethal 9LGS cells. In this study we have confirmed, quantified and extended those findings to study the efficacy of such immunological memory in normal aging rats and in aging rats previously treated for ic 9LGS tumors by BNCT. (1) The sc injection of 5,000,000 untreated 9LGS cells and the surgical removal of the resulting tumors (method A) protected 80% of normal young rats from an ic challenge with 10,000 untreated 9LGS cells, and a single sc injection of 5,000,000 lethally X-irradiated 9LGS cells (method B) protected 66% of them, but multiple sc injections with a crude particulate fraction prepared from 9LGS cells were not protective. Protection is long-lasting since contralateral ic rechallenge of six-month survivors with an injection of 10,000 viable 9LGS cells resulted in 100% survival. (2) Normal one-year-old rats were only slightly less protected than were normal young rats, approximately 70% rather than approximately 80% (method A) and approximately 60% rather than approximately 66% (method B). (3) BNCT treatment alone resulted in partial immunological protection, as 30% of one-year post-BNCT survivors of ic 9LGS tumors prevailed after contralateral ic rechallenge with 10,000 viable 9LGS cells. Moreover a single sc immunization with 5,000,000 untreated 9LGS cells prior to ic rechallenge boosted survival from 30% to 100%. The relevance of these observations to strategies of preclinical experimentation for immunoprophylaxis of malignant gliomas is discussed.

The combination of boron neutron-capture therapy and immunoprophylaxis for advanced intracerebral gliosarcomas in rats

Glioblastoma multiforme (GBM) is the most common primary human brain tumor. About 7000 new cases are diagnosed yearly in the USA and GBM is almost invariably fatal within a few years after it is diagnosed. Despite current neurosurgical and radiotherapeutic tumor cytoreduction methods, in most cases occult foci of tumor cells infiltrate surrounding brain tissues and cause recurrent disease. Therefore the combination of neurosurgical and radiotherapeutic debulking methods with therapies to inhibit occult GBM cells should improve prognosis. In this study we have combined boron neutron-capture therapy (BNCT), a novel binary radiotherapeutic treatment modality that selectively irradiates tumor tissue and largely spares normal brain tissue, with immunoprophylaxis, a form of active immunization initiated soon after BNCT treatment, to treat advanced, clinically relevantly-sized brain tumors in rats. Using a malignant rat glioma model of high immunogenicity, the 9L gliosarcoma, we have shown that about half of the rats that would have died after receiving BNCT debulking alone, survived after receiving BNCT plus immunoprophylaxis. Further, most of the surviving rats display immunological-based resistance to recurrent 9LGS growth six months or more after treatment. To our knowledge this study represents the first time BNCT and immunoprophylaxis have been combined to treat advanced brain tumors in rats.

Determination of the MRI contrast agent concentration time course in vivo following bolus injection: effect of equilibrium transcytolemmal water exchange

For bolus-tracking studies, it is commonly assumed that CR concentration bears a linear relationship with the measured (usually longitudinal) (1)H(2)O relaxation rate constant, R*(1) identical with(T(1) *)(-1). This requires that equilibrium transcytolemmal water exchange be in the fast exchange limit (FXL). However, though systems remain in fast exchange, the FXL will not usually obtain. Here, the consequences are considered: 1) the measurement of R(1) * itself can be affected, 2) the resultant non-linear [CR]-dependence causes significant error by assuming FXL, 3) the thermodynamic [CR] (based on the space in which CR is actually distributed) can be determined, 4) transcytolemmal water permeability may be estimated, and 5) the pharmacokinetic parameters can be factored. For a 30-sec, 0.17 mmol/kg dose of GdDTPA(2-), the FXL assumption underestimates the [CR] maximum in rat thigh muscle by a factor of almost two. Similar results are obtained for a rat brain GS-9L gliosarcoma tumor model.

A conducting plastic simulating brain tissue

A new conducting plastic has been composed which accurately simulates the photon and neutron absorption properties of brain tissue. This tissue-equivalent (TE) plastic was formulated to match the hydrogen and nitrogen constituents recommended by ICRU Report #44 for brain tissue. Its development was initiated by the inability of muscle tissue-equivalent plastic to closely approximate brain tissue with respect to low-energy neutron interactions. This new plastic is particularly useful as an electrode in TE dosimetry devices for boron neutron capture therapy (BNCT), which utilizes low-energy neutrons for radiotherapy of the brain. Absorbed dose measurements in a clinical BNCT beam using a proportional counter constructed from this TE plastic show good agreement with Monte Carlo calculations.

Models for estimation of the (10)B concentration after BPA-fructose complex infusion in patients during epithermal neutron irradiation in BNCT

PURPOSE

To create simple and reliable models for clinical practice for estimating the blood (10)B time-concentration curve after p-boronophenylalanine fructose complex (BPA-F) infusion in patients during neutron irradiation in boron neutron capture therapy (BNCT).

METHODS AND MATERIALS

BPA-F (290 mg BPA/kg body weight) was infused i.v. during two hours to 10 glioblastoma multiforme patients. Blood samples were collected during and after the infusion. Compartmental models and bi-exponential function fit were constructed based on the (10)B blood time-concentration curve. The constructed models were tested with data from six additional patients who received various amounts of infused BPA-F and data from one patient who received a one-hour infusion of 170 mg BPA/kg body weight.

RESULTS

The resulting open two-compartment model and bi-exponential function estimate the clearance of (10)B after 290 mg BPA/kg body weight infusion from the blood with satisfactory accuracy during the first irradiation field (1 ppm, i.e., 7%). The accuracy of the two models in predicting the clearance of (10)B during the second irradiation field are for two-compartment model 1.0 ppm (8%) and 0.2 ppm (2%) for bi-exponential function. The models predict the average blood (10)B concentration with an increasing accuracy as more data points are available during the treatment.

CONCLUSION

By combining the two models, a robust and practical modeling tool is created for the estimation of the (10)B concentration in blood after BPA-F infusion.

Determination of the MRI contrast agent concentration time course in vivo following bolus injection: effect of equilibrium transcytolemmal water exchange

For bolus-tracking studies, it is commonly assumed that CR concentration bears a linear relationship with the measured (usually longitudinal) (1)H(2)O relaxation rate constant, R*(1) identical with(T(1) *)(-1). This requires that equilibrium transcytolemmal water exchange be in the fast exchange limit (FXL). However, though systems remain in fast exchange, the FXL will not usually obtain. Here, the consequences are considered: 1) the measurement of R(1) * itself can be affected, 2) the resultant non-linear [CR]-dependence causes significant error by assuming FXL, 3) the thermodynamic [CR] (based on the space in which CR is actually distributed) can be determined, 4) transcytolemmal water permeability may be estimated, and 5) the pharmacokinetic parameters can be factored. For a 30-sec, 0.17 mmol/kg dose of GdDTPA(2-), the FXL assumption underestimates the [CR] maximum in rat thigh muscle by a factor of almost two. Similar results are obtained for a rat brain GS-9L gliosarcoma tumor model.

Improved survival after boron neutron capture therapy of brain tumors by Cereport-mediated blood-brain barrier modulation to enhance delivery of boronophenylalanine

OBJECTIVE

Cereport (Alkermes, Inc., Cambridge, MA), or, as it has been previously called, RMP-7 (receptor-mediated permeabilizer-7), is a bradykinin analog that has been shown to produce a transient, pharmacologically mediated opening of the blood-brain barrier. The purpose of the present study was to determine whether the efficacy of boron neutron capture therapy (BNCT) could be enhanced by means of intracarotid (i.c.) infusion of Cereport, in combination with intravenous (i.v.) injection or i.c. infusion of boronophenylalanine (BPA) in the F98 rat glioma model.

METHODS

For biodistribution studies, Fischer rats bearing intracerebral implants of the F98 glioma received i.v. or i.c. injections of 300 or 500 mg/kg body weight (b.w.) of BPA with or without i.c. infusion of 1.5 microg/kg b.w. of Cereport. For therapy studies, BNCT was initiated 14 days after intracerebral implantation of 10(3) F98 cells. The i.v. or i.c. injection of BPA (500 mg/kg b.w.) was given with or without Cereport, and the animals were irradiated 2.5 hours later at the Brookhaven Medical Research Reactor with a collimated beam of thermal neutrons delivered to the head.

RESULTS

At a BPA dose of 500 mg/kg b.w., tumor boron concentrations (mean +/- standard deviation) were 55.7 +/- 9.6 microg/g with Cereport versus 33.6 +/- 3.9 microg/g without Cereport at 2.5 hours after i.c. infusion of BPA, and concentrations were 29.4 +/- 9.9 microg/g with Cereport versus 15.4 +/- 3.5 microg/g without Cereport (P < 0.05) after i.v. injection of BPA. After i.c. administration of BPA and Cereport, the tumor-to-blood ratio was 5.4 +/- 0.6, and the tumor-to-brain ratio was 5.2 +/- 2.4. After BNCT with BPA at a dose of 500 mg/kg, the survival time was 50 +/- 16 days for i.c. administration of BPA with Cereport versus 40 +/- 6 days without Cereport (P = 0.05), 38 +/- 4 days for i.v. administration of BPA with Cereport versus 34 +/- 3 days without Cereport (P = 0.02), 28 +/- 5 days for irradiated controls, and 23 +/- 3 days for untreated controls. Compared with untreated controls, there was a 117% increase in lifespan in rats that received an i.c. infusion of Cereport and then BPA, and an 86% increase in lifespan in rats that received i.c. administration of BPA without Cereport.

CONCLUSION

These studies have established that i.c. administration of Cereport can not only increase tumor uptake of BPA, but also enhance the efficacy of BNCT.

Boron microlocalization in oral mucosal tissue: implications for boron neutron capture therapy

Clinical studies of the treatment of glioma and cutaneous melanoma using boron neutron capture therapy (BNCT) are currently taking place in the USA, Europe and Japan. New BNCT clinical facilities are under construction in Finland, Sweden, England and California. The observation of transient acute effects in the oral mucosa of a number of glioma patients involved in the American clinical trials, suggests that radiation damage of the oral mucosa could be a potential complication in future BNCT clinical protocols, involving higher doses and larger irradiation field sizes. The present investigation is the first to use a high resolution surface analytical technique to relate the microdistribution of boron-10 (10B) in the oral mucosa to the biological effectiveness of the 10B(n,alpha)7Li neutron capture reaction in this tissue. The two boron delivery agents used clinically in Europe/Japan and the USA, borocaptate sodium (BSH) and p-boronophenylalanine (BPA), respectively, were evaluated using a rat ventral tongue model. 10B concentrations in various regions of the tongue mucosa were estimated using ion microscopy. In the epithelium, levels of 10B were appreciably lower after the administration of BSH than was the case after BPA. The epithelium:blood 10B partition ratios were 0.2:1 and 1:1 for BSH and BPA respectively. The 10B content of the lamina propria was higher than that measured in the epithelium for both BSH and BPA. The difference was most marked for BSH, where 10B levels were a factor of six higher in the lamina propria than in the epithelium. The concentration of 10B was also measured in blood vessel walls where relatively low levels of accumulation of BSH, as compared with BPA, was demonstrated in blood vessel endothelial cells and muscle. Vessel wall:blood 10B partition ratios were 0.3:1 and 0.9:1 for BSH and BPA respectively. Evaluation of tongue mucosal response (ulceration) to BNC irradiation indicated a considerably reduced radiation sensitivity using BSH as the boron delivery agent relative to BPA. The compound biological effectiveness (CBE) factor for BSH was estimated at 0.29 +/- 0.02. This compares with a previously published CBE factor for BPA of 4.87 +/- 0.16. It was concluded that variations in the microdistribution profile of 10B, using the two boron delivery agents, had a significant effect on the response of oral mucosa to BNC irradiation. From a clinical perspective, based on the findings of the present study, it is probable that potential radiation-induced oral mucositis will be restricted to BNCT protocols involving BPA. However, a thorough high resolution analysis of 10B microdistribution in human oral mucosal tissue, using a technique such as ion microscopy, is a prerequisite for the use of experimentally derived CBE factors in clinical BNCT.

Tolerance of the normal canine brain to epithermal neutron irradiation in the presence of p-boronophenylalanine

Twelve normal dogs underwent brain irradiation in a mixed-radiation, mainly epithermal neutron field at the Brookhaven Medical Research Reactor following intravenous infusion of 950 mg of 10B-enriched BPA/kg as its fructose complex. The 5 x 10 cm irradiation aperture was centered over the left hemisphere. For a subgroup of dogs reported previously, we now present more detailed analyses including dose-volume relationships, longer follow-ups, MRIs, and histopathological observations. Peak doses (delivered to 1 cm3 of brain at the depth of maximum thermal neutron flux) ranged from 7.6 Gy (photon-equivalent dose: 11.8 Gy-Eq) to 11.6 Gy (17.5 Gy-Eq). The average dose to the brain ranged from 3.0 Gy (4.5 Gy-Eq) to 8.1 Gy (11.9 Gy-Eq) and to the left hemisphere, 6.6 Gy (10.1 Gy-Eq) to 10.0 Gy (15.0 Gy-Eq). Maximum tolerated 'threshold' doses were 6.7 Gy (9.8 Gy-Eq) to the whole brain and 8.2 Gy (12.3 Gy-Eq) to one hemisphere. The threshold peak brain dose was 9.5 Gy (14.3 Gy-Eq). At doses below threshold, some dogs developed subclinical MRI changes. Above threshold, all dogs developed dose-dependent MRI changes, neurological deficits, and focal brain necrosis.

Boron neutron capture therapy of the rat 9L gliosarcoma: evaluation of the effects of shark cartilage

A number of anti-angiogenic substances are now under evaluation, both experimentally and clinically, as potential agents for the treatment of cancer. It has recently been demonstrated that anti-angiogenic agents can increase the therapeutic potential of photon irradiation in a range of tumour models. In the present communication a preliminary assessment is made of the effects of shark cartilage on the response of the rat 9L gliosarcoma to boron neutron capture therapy (BNCT). Shark cartilage was administered orally as an aqueous suspension at a daily dose of approximately 2000 mg kg-1 body weight. The mean survival time of rats receiving no treatment was 20.7 +/- 0.5 days post intracranial tumour implantation. Administration of shark cartilage alone extended the survival time. Two of the rats treated with shark cartilage were healthy and fully active at the end of the evaluation period (43 days post implantation). At autopsy the brain tumours of these animals were a factor of approximately 4 smaller than controls. In a repeat study with shark cartilage alone the survival time was extended by approximately 30%. After boronophenylalanine-mediated BNCT, with or without shark cartilage, the survival time of rats that eventually became moribund was increased by a factor of approximately 2 relative to controls. In both treatment groups approximately 20% of rats were healthy at 1 year after BNCT. There was no evidence of residual tumour at post-mortem. It was concluded that shark cartilage, when given alone, significantly increased the survival time of tumour-bearing rats, presumably owing to an anti-angiogenic effect. However, the survival data suggested that boronophenylalanine-mediated BNCT did not appear to be enhanced by the administration of shark cartilage.

Mechanisms of transport of p-borono-phenylalanine through the cell membrane in vitro

The mechanisms of transport of p-(dihydroxyboryl)-phenylalanine (BPA) through the cell membrane were investigated in vitro, evaluating especially the systems responsible for the transport of neutral amino acids as potential carriers for BPA. Rat 9L gliosarcoma cells and Chinese hamster V79 cells were exposed to BPA under controlled conditions and in a defined medium that was free of amino acids. The time course of (10)B (delivered by BPA) uptake and efflux was measured under different conditions. To analyze the intracellular boron content, direct-current plasma atomic emission spectroscopy (DCP-AES) was used after separating the cells from extracellular boron in the cell medium using an oil filtration technique. The dependence of factors such as cell type, temperature, composition and concentration of amino acids and specific substrates for amino acid transport systems in the culture medium or in intracellular compartments on BPA uptake and efflux were studied. The results of this study support the hypothesis that BPA is transported by the L system and that transport can be further stimulated by amino acids preaccumulated in the cell by either the L or A amino acid transport system.

Boron neutron capture therapy for malignant gliomas

Boron neutron capture therapy (BNCT) represents a promising modality for a relatively selective radiation dose delivery to the tumour tissue. Boron-10 nuclei capture slow 'thermal' neutrons preferentially and, upon capture, promptly undergo 10B(n,alpha)7Li reaction. The ionization tracks of energetic and heavy lithium and helium ions resulting from this reaction are only about one cell diameter in length (approximately 14 microm). Because of their high linear energy transfer (LET) these ions have a high relative biological effectiveness (RBE) for controlling tumour growth. The key to effective BNCT of tumours, such as glioblastoma multiforme (GBM), is the preferential accumulation of boron-10 in the tumour, including the infiltrating GBM cells, as compared with that in the vital structures of the normal brain. Provided that a sufficiently high tumour boron-10 concentration (approximately 10(9) boron-10 atoms/cell) and an adequate thermal neutron fluence (approximately 10(12) neutrons/cm2) are achieved, it is the ratio of the boron-10 concentration in tumour cells to that in the normal brain cells that will largely determine the therapeutic gain of BNCT.

Treatment of isografted 9L rat brain tumors with beta-5-o-carboranyl-2'-deoxyuridine neutron capture therapy

beta-5-o-Carboranyl-2'-deoxyuridine (D-CDU) is a nontoxic pyrimidine nucleoside analogue designed for boron neutron capture therapy of brain tumors. In vitro studies indicated that D-CDU accumulates to levels 92- and 117-fold higher than the extracellular concentration in rat 9L and human U-251 glioma cells, respectively, and persists for several hours at levels 5-fold higher than the extracellular concentration. Furthermore, D-CDU was not toxic to rats injected i.p. with up to 150 mg/kg. On the basis of these studies, D-CDU was evaluated as a neutron capture therapy agent using rats bearing stereotactically implanted intracranial 9L tumors at single i.p. doses of 30 mg/kg and 150 mg/kg of D-CDU (20% 10B enriched), given 2 h before irradiation with thermal neutrons. Boron concentrations in tumors 2 h after dosing were 2.3 +/- 1.6 and 7.4 +/- 1.3 micrograms boron/g tissue (mean +/- SD), corresponding to tumor/brain ratios of 11.5 +/- 3.6 and 6.8 +/- 2.0 micrograms boron/g tissue for the low and high doses, respectively. All untreated animals died within 28 days, whereas half survived at days 32, 55, and 38 for groups receiving neutrons only, 30 mg/kg D-CDU, and 150 mg/kg D-CDU, respectively. Odds ratios of all treatment groups differed significantly from the untreated group (P < 0.002; logrank test). The median survival time for the 30 mg/kg-treated group but not for the 150 mg/kg-treated group was significantly longer than for rats treated with neutrons only (P = 0.036), which may correlate with the decreased tumor selectivity for D-CDU observed at the higher dose. Additional pharmacodynamic studies are warranted to determine optimal dosing strategies for D-CDU.

Autosomal dominant hypoparathyroidism associated with short stature and premature osteoarthritis

Familial hypoparathyroidism is an unusual and genetically heterogeneous group of disorders that may be isolated or may be associated with congenital or acquired abnormalities in other organs or glands. We have evaluated a family with a novel syndrome of autosomal dominant hypoparathyroidism, short stature, and premature osteoarthritis. A 74-yr-old female (generation I) presented with hypoparathyroidism, a movement disorder secondary to ectopic calcification of the cerebellum and basal ganglia, and a history of knee and hip replacements for osteoarthritis. Two members of generation II and one member of generation III were also documented with hypoparathyroidism, short stature, and premature osteoarthritis evident as early as 11 yr. Because of the known association between autosomal dominant hypoparathyroidism and activating mutations of the calcium-sensing receptor (CaR) gene, further studies were performed. Sequencing of PCR-amplified genomic DNA revealed a leucine to valine substitution at position 616 in the first transmembrane domain of the CaR, which cosegregated with the disorder. However, this amino acid sequence change did not affect the total accumulation of inositol phosphates as a function of extracellular calcium concentrations in transfected HEK-293 cells. In conclusion, a sequence alteration in the coding region of the CaR gene was identified, but is not conclusively involved in the etiology of this novel syndrome. The cosegregation of hypoparathyroidism, short stature, and osteoarthritis in this kindred does suggest a genetic abnormality involving a common molecular mechanism in parathyroid, bone, and cartilage.

Fission converter and metal-oxide-semiconductor field effect transistor study of thermal neutron flux distribution in an epithermal neutron therapy beam

The depth distribution of the thermal neutron flux is a major factor in boron neutron capture therapy (BNCT) in determining the efficiency of cell sterilization. In this paper the fission detector method is developed and applied to measure the in-phantom thermal neutron flux depth distribution. Advantages of the fission detector include small size, direct measurement of thermal neutron flux in a mixed radiation field of BNCT beam, self-calibration, and the possibility of on-line measurement. The measurements were performed at epithermal a BNCT facility. The experimental results were compared with the thermal neutron flux calculated by the Monte Carlo method and found to be in good agreement.

The effects of boron neutron capture irradiation on oral mucosa: evaluation using a rat tongue model

The ventral surface of the tongue of male Fisher 344 rats was used to evaluate the response of oral mucosa to boron neutron capture irradiation. Three hours after i.p. injection of 700 mg/kg of the boron delivery agent p-boronophenylalanine (BPA), the boron concentrations in blood and tongue mucosal epithelium were approximately 21 and 23 microgram (10)B/g, respectively. The doses required to produce a 50% incidence of ulceration with X rays, the Brookhaven Medical Research Reactor thermal neutron beam alone, or the thermal neutron beam in the presence of BPA were 13.4 +/- 0.2, 4. 2 +/- 0.1, and 3.0 +/- 0.1 Gy, respectively. Ulceration of the tongue was evident by 6 to 7 days after irradiation, irrespective of the irradiation modality; healing was related to dose and was relatively rapid (</=19 days). Compared to 100 kVp X rays, the relative biological effectiveness factors were 3.2 for the thermal neutron beam and 4.9 for the products of the boron neutron capture reaction, (10)B(n,alpha)(7)Li. Oral mucosa is highly sensitive to BPA-mediated BNC irradiation and could be a dose-limiting normal tissue in BNCT of brain tumors, or if BPA-based BNCT is applied to the treatment of head and neck tumors.

Effect of dose and infusion time on the delivery of p-boronophenylalanine for neutron capture therapy

Clinical trials of boron neutron capture therapy (BNCT) for glioblastoma multiforme are currently in progress using p-boronophenylalanine (BPA) as the 10B delivery agent. Enhancement of tumor boron uptake and/or the tumor-to-blood (T:B) boron concentration ratio would have the potential of significantly improving the therapeutic gain of BNCT. The effects of total dose, infusion time, and route of administration of BPA on tumor and blood boron concentrations were studied in rats bearing the 9L gliosarcoma. Increasing the total dose of BPA from 250 to 1000 mg/kg, administered intravenously over a 2-h infusion period, resulted in an increase in tumor boron concentration from approximately 30 to approximately 70 microg 10B/g, with a constant T:B boron concentration ratio of about 3.7:1. Similarly, extension of the infusion time from 2 to 6 h, at a constant dose-rate of 125 mg BPA/kg/h, resulted in an increase in tumor boron concentration from approximately 30 to approximately 80 microg 10B/g, while, again, maintaining a constant T:B ratio of about 3.7:1. In contrast, intracarotid infusion of BPA for 1 h at a dose rate of 125 mg BPA/kg resulted in an increase in the tumor boron concentration from approximately 26 to approximately 38 microg 10B/g with a corresponding increase in the T:B ratio from 3.5:1 to 5.0:1. The effects of these results on the therapeutic gain potentially achievable with BNCT are discussed.

Calvarial doughnut lesions associated with high-turnover osteoporosis presenting in childhood

Osteogenesis imperfecta and juvenile osteoporosis are two well-described syndromes of osteoporosis presenting in childhood. There are also several references in the radiology literature to calvarial doughnut lesions (CDLs), areas of radiolucency surrounded by a dense and well-defined area of sclerotic bone, either as an incidental finding or associated with childhood fracture. We have characterized the metabolic abnormalities in a 13-yr-old boy with CDLs and multiple fractures and followed him during his progression through puberty. The patient's paternal grandmother; father; and paternal aunt, uncle, and first cousin were similarly affected, and a mandibular lesion in the uncle was pathologically described as fibrous dysplasia. The subject's physical examination was significant for bony protuberances of the skull and normal hearing, sclearal hue, dentition, and joint flexibility. Radiographs revealed calvarial CDLs and osteopenia which was confirmed by bone mineral density (BMD) testing. Biochemical markers of bone formation and resorption were elevated compared to normal adult and a transiliac crest bone biopsy confirmed high-turnover osteoporosis. Over 6 yr, with no specific therapy, BMD gradually normalized, but the CDLs increased in size, bone turnover remained elevated by biochemical markers, and he continued to fracture. The subject's affected father and maternal grandmother had normal BMD and no history of adult fracture. CDLs with high-turnover osteoporosis should be considered in the differential diagnosis of pediatric osteoporosis. During puberty the BMD normalizes but the high-turnover state persists, and the propensity to fracture eventually decreases in older affected adults. The CDLs may be a variant of fibrous dysplasia, and further study is necessary in order to elucidate the stimulus for increased bone turnover and the familial nature of this syndrome.

The radiation biology of boron neutron capture therapy

Boron neutron capture therapy (BNCT) is a targeted radiation therapy that significantly increases the therapeutic ratio relative to conventional radiotherapeutic modalities. BNCT is a binary approach: A boron-10 (10B)-labeled compound is administered that delivers high concentrations of 10B to the target tumor relative to surrounding normal tissues. This is followed by irradiation with thermal neutrons or epithermal neutrons which become thermalized at depth in tissues. The short range (5-9 microm) of the alpha and 7Li particles released from the 10B(n,alpha)7Li neutron capture reaction make the microdistribution of 10B of critical importance in therapy. The radiation field in tissues during BNCT consists of a mixture of components with differing LET characteristics. Studies have been carried out in both normal and neoplastic tissues to characterize the relative biological effectiveness of each radiation component. The distribution patterns and radiobiological characteristics of the two 10B delivery agents in current clinical use, the amino acid p-boronophenylalanine (BPA) and the sulfhydryl borane (BSH), have been evaluated in a range of normal tissues and tumor types. Considered overall, BSH-mediated BNCT elicits proportionately less damage to normal tissue than does BNCT mediated with BPA. However, BPA exhibits superior in vivo tumor targeting and has proven much more effective in the treatment of brain tumors in rats. In terms of fractionation effects, boron neutron capture irradiation modalities are comparable with other high-LET radiation modalities such as fast-neutron therapy. There was no appreciable advantage in increasing the number of daily fractions of thermal neutrons beyond two with regard to sparing of normal tissue in the rat spinal cord model. The experimental studies described in this review constitute the radiobiological basis for the new BNCT clinical trials for glioblastoma at Brookhaven National Laboratory, at the Massachusetts Institute of Technology, and at the High Flux Reactor, Petten, The Netherlands. The radiobiology of experimental and clinical BNCT is discussed in detail.

Boron neutron capture therapy: re-irradiation response of the rat spinal cord

PURPOSE

To evaluate the retreatment response of the CNS to BNC irradiation using a rat spinal cord model.

MATERIALS AND METHODS

Fischer 344 rats were irradiated with single doses of 6 MeV X-rays which were 22, 40 or 80% of a total effect (TE). An additional group of rats was irradiated with a single exposure of thermal neutrons in the presence of the neutron capture agent boronophenylalanine (BPA) to a dose that represented 82% of the TE. After an interval of 26 weeks, animals were re-irradiated using various single doses of thermal neutrons in combination with BPA.

RESULTS

The re-irradiation ED50 doses represented 77, 80 or 50% of the TE after an initial X-ray dose of 22, 40 or 80% of the TE, respectively. The re-irradiation ED50 dose was 55% of the TE after an initial BNC irradiation dose representing 82% of the TE.

CONCLUSION

The level of the initial radiation damage had a direct bearing on the re-irradiation response. Recovery following initial treatment with BNC irradiation was similar to that after initial irradiation with X-rays.

Effects of human lymphocyte-conditioned medium on MG-63 human osteosarcoma cell function

Lymphocytes are implicated in the pathogenesis of bone disease in chronic inflammation, osteoporosis, transplantation and osteopetrosis. The effects of lymphocytes and lymphocyte-conditioned medium on bone-resorbing activity and osteoclast function have been well studied, but there are few studies of the effects of LCM on bone formation and osteoblast function. The effects of LCM on the function of the MG-63 human osteosarcoma cell line were studied, which, when stimulated with 1,25-(OH)2D3, demonstrates many of the properties of the mature human osteoblast. Lymphocytes contain oestrogen receptors and the model was also used to test the hypothesis that the effects of oestrogen on bone cells may be mediated indirectly via lymphokines. Lymphokines were measured by ELISA in human lymphocyte conditioned medium (LCM) collected following incubation of mixed lymphocytes with or without stimulation for 72 h. Unstimulated LCM increased proliferation of MG-63 cells and this increase was not affected by neutralization of interleukin 1 (IL-1), IL-3, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), tumour necrosis factor (TNF), lymphotoxin alpha, or interferon gamma (IFN-gamma). Phytohaemagglutinin-stimulated LCM decreased proliferation of MG-63 cells, as well as induced expression of IL-6 mRNA, increased alkaline phosphatase production, and inhibited osteocalcin production. The decrease in proliferation was abolished by neutralization of IFN-gamma but was unaffected by neutralization of IL-1, IL-2, IL-3, IL-4, IL-6, GM-CSF, TNF, or lymphotoxin alpha. Neutralization of IFN-gamma in stimulated LCM also partially inhibited the increase in alkaline phosphatase production but had no effects on the decrease in osteocalcin production. Although oestrogen inhibited lymphocyte proliferation, the effects of LCM collected from lymphocytes in the presence of oestrogen on MG-63 cell proliferation and function was no different than the effects of LCM collected in the absence of oestrogen. LCM has multiple effects on MG-63 cell function and gene expression. Lymphocyte stimulation during the preparation of LCM further modulates these effects. Although partially mediated by IFN-gamma, the effects of LCM on these cells cannot be completely explained by individual component lymphokines. This may have implications for understanding the pathophysiology of bone loss in inflammatory disorders as well as possible feedback loops of locally generated cytokines in bone.

Clinical reporting to primary care physicians leads to increased use and understanding of bone densitometry and affects the management of osteoporosis. A randomized trial

BACKGROUND

A major barrier to wider use of bone densitometry has been a lack of reports that are comprehensible to primary care physicians.

OBJECTIVE

To compare the effect of short technical reports and longer clinical reports on use, understanding, and acceptance of bone densitometry by primary care physicians and on management of osteoporosis.

DESIGN

Randomized trial.

SETTING

Osteoporosis center of a community teaching hospital.

SUBJECTS

57 primary care physicians ordering bone mineral density tests with dual x-ray absorptiometry.

INTERVENTION

Physicians were randomly assigned to receive short technical reports or long clinical reports written by endocrinologists with access to clinical information.

MEASUREMENTS

Physicians were interviewed by telephone after receiving at least two reports.

RESULTS

Before being interviewed, physicians receiving short reports ordered a mean +/- SD of 0.72 +/- 0.71 tests per month; those receiving long reports ordered 1.30 +/- 1.21 tests per month (P = 0.002). At the first interview, 30% of physicians receiving short reports and 86% of those receiving long reports understood the bone mineral density definition of osteoporosis (P < 0.001). Receiving long reports led to more modifications in the pharmacologic treatment of osteoporosis by gynecologists (19% of patients whose reports were short and 61% of patients whose reports were long; P = 0.021) and less confusion about reports by all physicians (36% of physicians receiving short reports and 1% of those receiving long reports; P = 0.003).

CONCLUSIONS

Clinical reporting of bone densitometry to primary care physicians increased use and understanding of bone densitometry, changed management of osteoporosis, and was well accepted. It may help achieve appropriate use of bone densitometry and may allow convenient dissemination of information on osteoporosis.

Boron neutron-capture therapy (BNCT) for glioblastoma multiforme (GBM) using the epithermal neutron beam at the Brookhaven National Laboratory

OBJECTIVE

Boron neutron-capture therapy (BNCT) is a binary form of radiation therapy based on the nuclear reactions that occur when boron (10B) is exposed to thermal neutrons. Preclinical studies have demonstrated the therapeutic efficacy of p-boronophenylalanine (BPA)-based BNCT. The objectives of the Phase I/II trial were to study the feasibility and safety of single-fraction BNCT in patients with GBM.

MATERIALS AND METHODS

The trial design required (a) a BPA biodistribution study performed at the time of craniotomy; and (b) BNCT within approximately 4 weeks of the biodistribution study. From September 1994 to July 1995, 10 patients were treated. For biodistribution, patients received a 2-hour intravenous (i.v.) infusion of BPA-fructose complex (BPA-F). Blood samples, taken during and after infusion, and multiple tissue samples collected during surgical debulking were analyzed for 10B concentration. For BNCT, all patients received a dose of 250 mg BPA/kg administered by a 2-hour i.v. infusion of BPA-F, followed by neutron beam irradiation at the Brookhaven Medical Research Reactor (BMRR). The average blood 10B concentrations measured before and during treatment were used to calculate the time of reactor irradiation that would deliver the prescribed dose.

RESULTS

10B concentrations in specimens of scalp and tumor were higher than in blood by factors of approximately 1.5 and approximately 3.5, respectively. The 10B concentration in the normal brain was < or = that in the blood; however, for purposes of estimating radiation doses to normal brain endothelium, it was always assumed to be equal to blood. BNCT doses are expressed as gray-equivalent (Gy-Eq), which is the sum of the various physical dose components multiplied to appropriate biologic effectiveness factors. The dose to a 1-cm3 volume where the thermal flux reached a maximum was 10.6 +/- 0.3 Gy-Eq in 9 patients and 13.8 Gy-Eq in 1 patient. The minimum dose in tumor ranged from 20 to 32.3 Gy-Eq. The minimum dose in the target volume (tumor plus 2 cm margin) ranged from 7.8 to 16.2 Gy-Eq. Dose to scalp ranged from 10 to 16 Gy-Eq. All patients experienced in-field alopecia. No CNS toxicity attributed to BNCT was observed. The median time to local disease progression following BNCT was 6 months (range 2.7 to 9.0). The median time to local disease progression was longer in patients who received a higher tumor dose. The median survival time from diagnosis was 13.5 months.

CONCLUSION

It is feasible to safely deliver a single fraction of BPA-based BNCT. At the dose prescribed, the patients did not experience any morbidity. To further evaluate the therapeutic efficacy of BNCT, a dose-escalation study delivering a minimum target volume dose of 17 Gy-Eq is in progress.

Biodistribution of p-boronophenylalanine in patients with glioblastoma multiforme for use in boron neutron capture therapy

OBJECTIVE

The success of boron neutron capture therapy depends on the safety and specificity of the boron delivery agent. As a preface to clinical boron neutron capture therapy of glioblastoma multiforme, a biodistribution study of intravenous p-boronophenylalanine (BPA) in patients undergoing craniotomy for resection of glioblastoma was performed.

METHODS

Varying doses of intravenously administered BPA-fructose (130-250 mg BPA per kilogram of body weight) were given to patients 2 to 3 hours prior to the start of craniotomy for either suspected or known glioblastoma multiforme. Blood samples were collected over a 48-hour period for boron assay. At surgery, multiple samples of tumor, brain, and scalp were obtained for boron and histological analysis.

RESULTS

Seventeen patients were studied; all but one had glioblastoma multiforme. No adverse effects from the BPA infusions were noted. The boron concentration in the blood reached a maximum at the end of the BPA infusion and was proportional to the administered dose of BPA. Normal brain concentrations of boron generally were equal to or less than that in blood. Tumor-blood boron ratios were highly variable: 1.6 +/- 0.8 (mean +/- standard deviation; n = 187; range, 0.3-3.5). The observed heterogeneity of BPA uptake in glioblastoma samples appears to correlate with the degree of cellularity observed on histological examination.

CONCLUSION

Intravenous BPA administration up to a dose of 250 mg/kg is safe and well tolerated. BPA uptake in surgical samples of glioblastoma tissue is variable and may depend on the fraction of viable tumor cells in the individual sample. Further clinical studies using BPA as a boron delivery agent for boron neutron capture therapy of glioblastoma multiforme appear warranted.

Biodistribution of boronophenylalanine in patients with glioblastoma multiforme: boron concentration correlates with tumor cellularity

Boron-10 (10B) concentrations were measured in 107 surgical samples from 15 patients with glioblastoma multiforme who were infused with 95 atom% 10B-enriched p-boronophenylalanine (BPA) intravenously for 2 h just prior to surgery at doses ranging from 98 to 290 mg BPA/kg body weight. The blood 10B concentration reached a maximum at the end of the infusion (ranging from 9.3 to 26.0 microg 10B/g) and was proportional to the amount of BPA infused. The boron concentrations in excised tumor samples ranged from 2.7 to 41.3 microg 10B/g over the range of administered BPA doses and varied considerably among multiple samples from individual patients and among patients at the same BPA dose. A morphometric index of the density of viable-appearing tumor cells in histological sections obtained from samples adjacent to, and macroscopically similar to, the tumor samples used for boron analysis correlated linearly with the boron concentrations. From that correlation it is estimated that 10B concentrations in glioblastoma tumor cells were over four times greater than concurrent blood 10B concentrations. Thus, in the dose range of 98 to 290 mg BPA/kg, the accumulation of boron in tumor cells is a linear function of BPA dose and the variations observed in boron concentrations of tumor specimens obtained surgically are largely due to differences in the proportion of nontumor tissue (i.e. necrotic tissue, normal brain) present in the samples submitted for boron analysis. The tumor:blood 10B concentration ratio derived from this analysis provides a rationale for estimating the fraction of the radiation dose to viable tumor cells resulting from the boron neutron capture reaction based on measured boron concentrations in the blood at the time of BNCT without the need for analysis of tumor samples from individual patients.

A review: CNS effects and normal tissue tolerance in dogs

Large animal studies have been utilized to define tolerance of normal brain to irradiation and verify treatment planning programs with two recently installed epithermal neutron beams. The normal brain tolerance studies utilized two biological endpoints, magnetic resonance visible damage only and neurologic signs progressing to death. The studies focused on defining the proton RBE for the contaminant fast neutrons, and from nitrogen capture of thermal neutrons and boron capture reaction biologic effect. The proton RBE was approximately 3.0 to 6.7, depending on whether a dose reduction factor for the low gamma dose rate was employed. The microscopic distribution of the boron compounds, coupled with the extremely short length of the fission fragments from thermal neutron capture by 10B yields an observed biologic effect much less than would be expected from such high LET irradiation. This observed biologic effect, which is a product of the microdistribution of the boron atom and the relative biologic effect of the fission fragments has been termed compound factor. The compound factor was based on the calculated physical dose from the fission fragment in blood based on measured blood 10B concentration. The approximate compound factor for BSH was studied at the two institutions and it ranged from 0.27 to 0.55, depending on the site and the endpoint chosen. The mean compound factor for BPA was only studied at one site and was found to be 1.1 for both endpoints. The increase in the compound factor for BPA is in keeping with previous calculations based on the differences in compound distribution. Results of these studies has helped the initiation of phase I and phase II clinical trials at Brook haven National Laboratory and the planned European clinical trials at Petten, The Netherlands.

Boron neutron capture therapy for glioblastoma multiforme using p-boronophenylalanine and epithermal neutrons: trial design and early clinical results

A Phase I/II clinical trial of boron neutron capture therapy (BNCT) for glioblastoma multiforme is underway using the amino acid analog p-boronophenylalanine (BPA) and the epithermal neutron beam at the Brook-haven Medical Research Reactor. Biodistribution studies were carried out in 18 patients at the time of craniotomy using an i.v. infusion of BPA, solubilized as a fructose complex (BPA-F). There were no toxic effects related to the BPA-F administration at doses of 130, 170, 210, or 250 mg BPA/kg body weight. The tumor/ blood, brain/blood and scalp/blood boron concentration ratios were approximately 3.5:1, 1:1 and 1.5:1, respectively. Ten patients have received BNCT following 2-hr infusions of 250 mg BPA/kg body weight. The average boron concentration in the blood during the irradiation was 13.0 +/- 1.5 micrograms 10B/g. The prescribed maximum dose to normal brain (1 cm3 volume) was 10.5 photon-equivalent Gy (Gy-Eq). Estimated maximum and minimum doses (mean +/- sd, n = 10) to the tumor volume were 52.6 +/- 4.9 Gy-Eq (range: 64.4-47.6) and 25.2 +/- 4.2 Gy-Eq (range: 32.3-20.0), respectively). The estimated minimum dose to the target volume (tumor +2 cm margin) was 12.3 +/- 2.7 Gy-Eq (range: 16.2-7.8). There were no adverse effects on normal brain. The scalp showed mild erythema, followed by epilation in the 8 cm diameter field. Four patients developed recurrent tumor, apparently in the lower dose (deeper) regions of the target volume, at post-BNCT intervals of 7,5,3.5 and 3 months, respectively. The remaining patients have had less than 4 months of post-BNCT follow-up. BNCT, at this starting dose level, appears safe. Plans are underway to begin the dose escalation phase of this protocol.

Response of the central nervous system to fractionated boron neutron capture irradiation: studies with borocaptate sodium

The response of the central nervous system (CNS) to fractionated doses of boron neutron capture (BNC) irradiation was assessed using a rat spinal cord model. The thermal neutron beam at the Brookhaven Medical Research Reactor (BMRR) was used for the spinal cord irradiations, with borocaptate sodium (BSH) as the neutron capture agent. Irradiations were given as a single dose or as two or four equal fractions. The ED50 for radiation-induced myeloparesis, as indicated by limb paralysis within 7 months, after a single exposure to thermal neutrons in the presence of BSH (blood boron-10 content approximately 70 micrograms/g) was 27.2 +/- 0.9 Gy. This was expressed as the total physical dose to the blood. Dividing the radiation dose into two consecutive daily fractions or four fractions given over 1 week, resulted in ED50 = 32.0 +/- 1.4 and 31.5 +/- 0.4 Gy respectively. Although there was no significant dose sparing in moving from two to four fractions, there was a dose increment of approximately 17% as compared with single-dose irradiation. The variation in the relative biological effectiveness of the thermal neutron beam, with dose per fraction, was established using data from a previous study with single and fractionated doses of thermal neutrons in the absence of a neutron capture agent. This varied from 1.40 to 3.74 for thermal neutron dose per fraction in the range 13.6-1.5 Gy. Previously published CBE factors for both BSH and BPA have been recalculated in the present report to take into account the change in the RBE of the thermal neutron beam with dose. In all cases the recalculated CBE factors were lower than those obtained previously. Values for this parameter increased with fraction number. In the case of BSH, the CBE factor increased from 0.36 +/- 0.03 after a single-dose to 0.51 +/- 0.06 after four fractions.

A kindred with a variant of multiple endocrine neoplasia type 1 demonstrating frequent expression of pituitary tumors but not linked to the multiple endocrine neoplasia type 1 locus at chromosome region 11q13

Acromegaly is uncommon in kindreds with multiple endocrine neoplasia type 1 (MEN1), whereas primary hyperparathyroidism (PHP) has the highest penetrance of any endocrinopathy. We report an unusual MEN1 kindred with frequent expression of pituitary tumors and a low penetrance of PHP. Four members were found to have disease: PHP in generation I, acromegaly (2 cases) in generation II, and hyperprolactinemia associated with a pituitary tumor in generation III. There was no evidence for PHP in 1 patient with acromegaly (age 60 yr), the patient with hyperprolactinemia and the pituitary tumor (age 22 yr), and 1 asymptomatic obligate carrier (age 50 yr). Screening of 26 members revealed the possible diagnosis of PHP in 1 family member in generation II and possible early acromegaly in 2 members of generation III with elevated serum concentrations of insulin-like growth factor I and insulin-like growth factor-binding protein-3 but normal patterns of pulsatile GH release. Although the predisposing genetic defect in typical MEN1 families has previously been mapped to chromosome location 11q13 without evidence of heterogeneity among the 87 families analyzed, linkage of disease in this family to the MEN1 region is unlikely based on haplotype analysis. Localization of the gene(s) responsible for disease in such atypical families may aid in the understanding of the pathogenesis of MEN1. In addition, further study of the earliest changes in patterns of pulsatile GH release in familial acromegaly may allow more insight into the pathogenesis and natural history of this disease.

Single-and dual-energy CT with monochromatic synchrotron x-rays

We explored the potential for clinical research of computed tomography (CT) with monochromatic x-rays using the preclinical multiple energy computed tomography (MECT) system at the National Synchrotron Light Source. MECT has a fixed, horizontal fan beam with a subject apparatus rotating about a vertical axis; it will be used for imaging the human head and neck. Two CdWO4-photodiode array detectors with different spatial resolutions were used. A 10.5 cm diameter acrylic phantom was imaged with MECT at 43 keV and with a conventional CT (CCT) at 80 kVp: spatial resolution approximately equal to 6.5 line pairs (lp)/cm for both; slice height, 2.6 mm for MECT against 3.0 mm for CCT; surface dose, 3.1 cGy for MECT against 2.0 cGy for CCT. The resultant image noise was 1.5 HU for MECT against 3 HU for CCT. Computer simulations of the same images with more precisely matched spatial resolution, slice height and dose indicated an image-noise ratio of 1.4:1.0 for CCT against MECT. A 13.5 cm diameter acrylic phantom imaged with MECT at approximately 0.1 keV above the iodine K edge and with CCT showed, for a 240 micrograms I ml-1 solution, an image contrast of 26 HU for MECT and 13 and 9 HU for the 80 and 100 kVp CCT, respectively. The corresponding numbers from computer simulation of the same images were 26, 12, and 9 HU, respectively. MECT's potential for use in clinical research is discussed.

Central nervous system tolerance to boron neutron capture therapy with p-boronophenylalanine

A rat spinal cord model was used to evaluate the effects of boron neutron capture irradiation on the central nervous system (CNS), using a range of doses of the boron delivery agent p-boronophenylalanine (BPA). Three doses of BPA 700, 1000 and 1600 mg kg(-1) were used to establish the biodistribution of boron-10 (10B) in blood, spinal cord and brain over a 3-h period after intraperitoneal (i.p.) administration. At the lowest dose of BPA used, blood 10B levels remained relatively stable over the 3-h sampling period. With the two higher doses of BPA, blood 10B concentrations were greatest at 1 h after BPA administration, and thereafter exhibited a biphasic clearance profile. The largest decline in blood 10B levels occurred between 1 and 2 h after i.p. injection and was most pronounced (approximately 45%) in the highest BPA dose group. Considered overall, 10B concentrations were marginally lower in the spinal cord than in the brain. Levels of 10B in both of these organs showed a slow but progressive increase with time after administration of BPA. The 10B concentration ratio for blood relative to CNS tissue increased with BPA dosage and reached a peak value of approximately 10:1 in the highest BPA dose group, at 1 h after i.p. injection. However, at 3 h after injection the 10B concentration ratios had decreased to approximately 3:1 in all of the BPA dose groups. After irradiation with thermal neutrons in combination with BPA at blood 10B concentrations of approximately 42 and approximately 93 microg g(-1), myelopathy developed after latent intervals of 20.0 +/- 0.6 and 20.0 +/- 1.2 weeks respectively. ED50 values (+/- s.e.) for the incidence of myelopathy were calculated from probit-fitted curves, and were 17.5 +/- 0.7 and 25.0 +/- 0.6 Gy after irradiation with thermal neutrons at blood 10B levels of approximately 42 and approximately 93 microg g(-1) respectively. The compound biological effectiveness (CBE) factor values, estimated from these data, were 0.67 +/- 0.23 and 0.48 +/- 0.18 respectively. This compared with a previous estimate of 0.88 +/- 0.14 at a blood 10B concentration of approximately 19 microg g(-1). It was concluded that the value of the CBE factor was not influenced by the level of 10B in the blood, but by the blood:CNS 10B concentration ratio. In effect, the CBE factor decreases as the concentration ratio increases. Simulations using boron neutron capture therapy (BNCT) treatment planning software indicate a significant therapeutic advantage could be obtained in moving to higher BPA doses than those in current clinical use.

Accumulation of boron in malignant and normal cells incubated in vitro with boronophenylalanine, mercaptoborane or boric acid

The short (< 10 microns) ranges of alpha and 7Li particles produced during boron neutron capture therapy (BNCT) make the partitioning of the boronated drug within and without the cell of critical importance. The evaluation of the potential usefulness of a boron-containing substance for BNCT requires information about its intracellular accumulation. In the present report, an in vitro method is described for direct measurement of intracellular boron based on rapid centrifugation of cells through a layer of mineral oil and silicon oil to strip away extracellular growth medium. The intracellular concentrations of boronophenylalanine (BPA), mercaptoborane (BSH) and horic acid in malignant cells and in normal cells have been compared. The accumulation ratio is defined as the ratio of the intracellular to the extracellular boron concentration. Boric acid showed an accumulation ratio of 1 while the ratios for BSH and BPA were dependent on cell type and tended to be greater for BPA than for BSH in malignant but not in normal cells.

Ion microscopy imaging of 10B from p-boronophenylalanine in a brain tumor model for boron neutron capture therapy

Boron neutron capture therapy (BNCT), a binary treatment modality that can potentially irradiate tumor tissue within cellular dimensions, is critically dependent on the preferential delivery of 10B to individual neoplastic cells. In this study, ion microscopy was used to quantitatively evaluate the selectivity of p-boronophenylalanine-fructose (BPA-F) in the rat 9L gliosarcoma brain tumor model. With a spatial resolution of approximately 0.5 microm, ion microscopy images show that BPA-F delivers 3.5 times more boron to the main tumor mass [99 +/- 36 microg/g tissue (mean +/- SD)] than to the contiguous normal brain (27 +/- 12 microg/g tissue). A similar, but lower, accumulation was observed away from the main tumor mass in small clusters of neoplastic cells (47 +/- 15 microg/g tissue) invading the surrounding brain (16 +/- 8 microg/g tissue). These findings establish for the first time the selectivity of BPA-F to the neoplastic cells invading the normal brain and provide a much-needed baseline measurement of the distribution of a clinically approved BNCT drug. Given the propensity for malignant brain tumors to infiltrate the surrounding normal brain, these observations have particular significance for clinical trials of BNCT for human glioblastoma multiforme using the drug BPA-F.

Boron neutron capture irradiation of the rat spinal cord: histopathological evidence of a vascular-mediated pathogenesis

A histopathological study was carried out on the spinal cord of rats after boron neutron capture (BNC) irradiation. Rats were irradiated with thermal neutrons alone or in combination with borocaptate sodium (BSH) or p-boronophenylalanine (BPA). Spinal cords were examined 1 year after irradiation, or at earlier times in rats developing myelopathy. Considered overall, the pathology of the spinal cord after BNC irradiation was comparable with that reported previously after X irradiation of the spinal cord in the identical strain of rat. When BSH was used as the neutron capture agent, the biologically effective dose of radiation delivered to the CNS parenchyma was a factor of -2.7 lower than that delivered to the vascular endothelium. In effect, the blood vessels were selectively irradiated using this BNC modality. The resultant pathology was similar to that observed after irradiation with thermal neutrons alone or in the presence of BPA, situations in which the CNS vasculature was not selectively irradiated. This points to the vascular endothelium as being the critical target cell population, damage to which results in the development of the lesions seen in the spinal cord after BNC irradiation and, by inference, after irradiation with more conventional modalities.

Boron neutron capture irradiation of the rat spinal cord: effects of variable doses of borocaptate sodium

The Fischer 344 rat spinal cord model has been used to evaluate the response of the central nervous system to boron neutron capture irradiation with variable doses of the neutron capture agent, borocaptate sodium (BSH). Three doses of BSH, 190, 140 and 80 mg/kg body weight, administered by i.p. injection, were used to establish the time course of 10B accumulation in and removal from the blood. After administration of the two lower doses of BSH, blood 10B levels peaked at 0.5 h after injection, with no significant (P > 0.1) change at 1 h after injection. Beyond this time point, levels of 10B in the blood began to decrease after a dose of 80 mg/kg BSH, but remained constant until 3 h after administration after the two higher doses of BSH. Myelopathy developed after latent intervals of 20.4 +/- 0.1, 20.8 +/- 1.4, 15.0 +/- 0.8, 15.4 +/- 0.4 and 15.6 +/- 0.4 weeks, following irradiation with thermal neutrons in combination with BSH at doses of 20, 40, 80, 140 and 190 mg/kg body weight, respectively. The radiation-induced lesion in the spinal cord was white matter necrosis. ED50 values for myelopathy were calculated from probit-fitted dose-effect curves. Expressed as total physical absorbed doses, these values were 20.7 +/- 1.9, 24.9 +/- 1.2, 27.2 +/- 0.9, 28.4 +/- 0.6 and 32.4 +/- 1.9 Gy after irradiation with thermal neutrons in the presence of 20, 40, 80, 140 and 190 mg/kg body weight of BSH, respectively. The compound biological effectiveness (CBE) factor values, estimated from this data, were in the range 0.49-0.55. There was no significant (P > 0.1) variation in the CBE factor for BSH as a function of increasing 10B concentration in the blood. It was concluded that there was no significant synergistic interaction between the low and high linear energy transfer (LET) components of the boron neutron capture (BNC) radiation field.