Pharmacokinetics and boron uptake of BSH (Na2B12H11SH) in patients with intracranial tumors

We evaluated retrospectively the pharmacokinetics and boron uptakeof BSH (mercaptoundecahydrododecarborate) for Boron Neutron Capture Therapy(BNCT) in 123 patients undergoing craniotomy for intracranialtumors. The pharmacokinetics revealed that BSH could moveeasily from blood to the peripheral organs; itwas retained there and elimination was very slow.BSH after intra-arterial infusion (IA) was found tomove into the peripheral organs more easily thanafter intra-venous (IV) infusion.In patients with malignant glioma, the average valuesof boron concentration in tumor and the tumorto blood ratio (T/B ratio) after IA infusionwere 26.8 ± 19.5 g/g (range, 6.1–104.7 g/g)and 1.77 ± 1.30 (range, 0.47–6.65) respectively. Onthe other hand, after IV infusion the valueswere 20.9 ± 12.2 g/g (range, 7.0–39.7 g/g)and 1.30 ± 0.65 (range, 0.61–2.94) respectively. Thedifferences are not statistically significant. Boron uptake inmalignant glioma was about three times higher thanlow grade glioma. We found a good correlationbetween boron uptake and time interval from BSHinfusion, and 15–20 hours after BSH infusion theboron concentration in tumor was above 20 g/g¹⁰B in 69% of the malignant glioma patients;T/B ratio was above one in 75%, andabove two in 44% of them.We recommend intra-venous infusion of BSH clinically sinceit is safer, and results in sufficient boronconcentration in tumor, and the planned irradiation mightbe optimal around 15–20 hours after the BSHinfusion for treating malignant glioma.     

Tissue Uptake of BSH in Patients with Glioblastoma in the EORTC 11961 Phase I BNCT Trial

Purpose: The uptake of the boron compound Na₂B₁₂H₁₀-SH (BSH) in tumor and normal tissues was investigated in the frame of the EORTC phase I trial Postoperative treatment of glioblastoma with BNCT at the Petten Irradiation Facility (protocol 11961).Methods and Materials: The boron concentration in blood, tumor, normal brain, dura, muscle, skin and bone was detected using inductively coupled plasma-atomic emission spectroscopy in 13 evaluable patients. In a first group of 10 patients 100mg BSH/kg bodyweight (BW) were administered; a second group of 3 patients received 22.9mg BSH/kg BW. The toxicity due to BSH was evaluated.Results: The average boron concentration in the tumor was 19.9±9.1ppm (1 standard deviation (SD)) in the high dose group and 9.8±3.3ppm in the low dose group, the tumor/blood ratios were 0.6±0.2 and 0.9±0.2, respectively. The highest boron uptake has been detected in the dura, very low uptake was found in the bone, the cerebro-spinal fluid and especially in the brain (brain/blood ratio 0.2±0.02 and 0.4±0.2). No toxicity was detected except flush-like symptoms in 2 cases during a BSH infusion at a much higher speed than prescribed.Conclusion: BSH proved to be safe for clinical application at a dose of 100mg BSH/kg infused and at a dose rate of 1mg/kg/min. The study underlines the importance of a further investigation of BSH uptake in order to obtain enough data for significant statistical analysis. The boron concentration in blood seems to be a quite reliable parameter to predict the boron concentration in other tissues.     

Assessment of the Results from the Phase I/II Boron Neutron Capture Therapy Trials at the Brookhaven National Laboratory from a Clinician's Point of View

Boron neutron capture therapy (BNCT) represents a promising modality for a relatively selective radiation dose delivery to the tumor tissue. The key to effective BNCT of tumors such as glioblastoma multiforme (GBM) is the homogeneous preferential accumulation of ¹⁰B in the tumor, including the infiltrating GBM cells, as compared to that in the vital structures of the normal brain. Provided that sufficiently high tumor ¹⁰B concentration (10⁹ boron-10 atoms/cell) and an adequate thermal neutron fluence (10⁹ neutrons/cm²⁾ are achieved, it is the ratio of the ¹⁰B concentration in tumor cells to that in the normal brain cells and the blood that will largely determine the therapeutic gain of BNCT.     

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 neutroncapture therapy (BNCT) for glioblastoma multiforme is underwayusing the amino acid analog p-boronophenylalanine (BPA) andthe epithermal neutron beam at the Brookhaven MedicalResearch Reactor. Biodistribution studies were carried out in18 patients at the time of craniotomy usingan i.v. infusion of BPA, solubilized as afructose complex (BPA-F). There were no toxic effectsrelated to the BPA-F administration at doses of130, 170, 210, or 250 mg BPA/kg bodyweight. The tumor/blood, brain/blood and scalp/blood boron concentrationratios were approximately 3.5:1, 1:1 and 1.5:1, respectively.Ten patients have received BNCT following 2-hr infusionsof 250 mg BPA/kg body weight. The averageboron concentration in the blood during the irradiationwas 13.0 ± 1.5 g ¹⁰B/g. The prescribedmaximum dose to normal brain (1 cm³ volume)was 10.5 photon-equivalent Gy (Gy-Eq). Estimated maximum andminimum doses (mean ± sd, n=10)to the tumor volume were 52.6 ± 4.9Gy-Eq (range: 64.4–47.6) and 25.2 ± 4.2 Gy-Eq(range: 32.3–20.0), respectively). The estimated minimum dose tothe target volume (tumor + 2 cm margin)was 12.3 ± 2.7 Gy-Eq (range: 16.2–7.8). Therewere no adverse effects on normal brain. Thescalp showed mild erythema, followed by epilation inthe 8 cm diameter field. Four patients developedrecurrent tumor, apparently in the lower dose (deeper)regions of the target volume, at post-BNCT intervalsof 7, 5, 3.5 and 3 months, respectively.The remaining patients have had less than 4months of post-BNCT follow-up. BNCT, at this startingdose level, appears safe. Plans are underway tobegin the dose escalation phase of this protocol.     

Cationized gelatin-HVJ envelope with sodium borocaptate improved the BNCT efficacy for liver tumors <Emphasis Type="Italic">in vivo</Emphasis>

Background

Boron neutron capture therapy (BNCT) is a cell-selective radiation therapy that uses the alpha particles and lithium nuclei produced by the boron neutron capture reaction. BNCT is a relatively safe tool for treating multiple or diffuse malignant tumors with little injury to normal tissue. The success or failure of BNCT depends upon the ¹⁰B compound accumulation within tumor cells and the proximity of the tumor cells to the body surface. To extend the therapeutic use of BNCT from surface tumors to visceral tumors will require ¹⁰B compounds that accumulate strongly in tumor cells without significant accumulation in normal cells, and an appropriate delivery method for deeper tissues.Hemagglutinating Virus of Japan Envelope (HVJ-E) is used as a vehicle for gene delivery because of its high ability to fuse with cells. However, its strong hemagglutination activity makes HVJ-E unsuitable for systemic administration.In this study, we developed a novel vector for ¹⁰B (sodium borocaptate: BSH) delivery using HVJ-E and cationized gelatin for treating multiple liver tumors with BNCT without severe adverse events.

Methods

We developed cationized gelatin conjugate HVJ-E combined with BSH (CG-HVJ-E-BSH), and evaluated its characteristics (toxicity, affinity for tumor cells, accumulation and retention in tumor cells, boron-carrying capacity to multiple liver tumors in vivo, and bio-distribution) and effectiveness in BNCT therapy in a murine model of multiple liver tumors.

Results

CG-HVJ-E reduced hemagglutination activity by half and was significantly less toxic in mice than HVJ-E. Higher ¹⁰B concentrations in murine osteosarcoma cells (LM8G5) were achieved with CG-HVJ-E-BSH than with BSH. When administered into mice bearing multiple LM8G5 liver tumors, the tumor/normal liver ratios of CG-HVJ-E-BSH were significantly higher than those of BSH for the first 48 hours (p < 0.05). In suppressing the spread of tumor cells in mice, BNCT treatment was as effective with CG-HVJ-E-BSH as with BSH containing a 35-fold higher ¹⁰B dose. Furthermore, CG-HVJ-E-BSH significantly increased the survival time of tumor-bearing mice compared to BSH at a comparable dosage of ¹⁰B.

Conclusion

CG-HVJ-E-BSH is a promising strategy for the BNCT treatment of visceral tumors without severe adverse events to surrounding normal tissues.

     

Subcellular Biodistribution of Sodium Borocaptate (BSH: Na2B12H11SH) in a Rat Glioma Model in Boron Neutron Capture Therapy

Mercaptoundecahydrododecaborate (Na₂B₁₂H₁₁SH, sodium borocaptate or BSH) has been used clinically as a boron compound for boron neutron capture therapy (BNCT) in patients with malignant glioma in Japan and Europe. Boron-10 is known to accumulate selectively only in brain tumor cells. This work was aimed to clarify the subcellular biodistribution of BSH in a rat glioma model using immunohistochemical approach.Wistar rats were used for this experiment. An intracerebral injection of 5.0 × 10⁶ C6 glioma cells was introduced into the region of cerebral hemisphere. Fifty milligrams of ¹⁰B/kg BSH was infused intravenously two weeks after implantation. Host rats were divided into six groups according to the sampling time: 1, 4, 8, 16, 24 and 48 h after the start of BSH infusion. Immunohistochemical study was carried out using anti-BSH antibody.Boron was already found in a whole cell 1 h after BSH infusion, and then seemed to collect in a cell nuclei around 8–16 h after infusion. It was still recognized in tumor cell 48 h after infusion.This study supports the following hypothesis on selective boron uptake in a tumor. BSH can pass through the disrupted blood–brain barrier (BBB) easily and can come in contact with tumor cells; there, BSH can bind on the extracellular surface of plasma membrane to choline residues. After binding to the plasma membrane, boron with choline residues may be internalized into the cell by endocytic pathways and eventually travel to cell nuclei, and then stay there for a long time.     

Tumor-specific targeting of sodium borocaptate (BSH) to malignant glioma by transferrin-PEG liposomes: a modality for boron neutron capture therapy

Object Boron neutron capture therapy (BNCT) requires selective delivery of a high concentration of boron-10 (¹⁰B) to tumor tissue. To improve a drug delivery in BNCT, we devised transferrin-conjugated polyethylene-glycol liposome encapsulating sodium borocaptate (TF-PEG-BSH). Methods ¹⁰B concentrations of U87Δ human glioma cells from three boron delivery systems (BDS) (bare BSH, PEG-BSH, and TF-PEG-BSH) were analyzed in vitro by use of inductively coupled plasma-atomic emission spectrometry (ICP-AES). A colony-forming assay (CFA) was performed using this cell line with the three BDS and neutron irradiation. Subcellular localization of BSH in tumor cells was analyzed in vitro by immunocytochemistry using monoclonal antibodies raised for BSH. Brain tumor models were made and the ¹⁰B concentrations of the tumor, blood, and normal brain from each BDS were analyzed in vivo by use of ICP-AES. The tumor-to-blood and the tumor-to-normal brain ratios were evaluated. BNCT for the brain tumor models was performed and survival was analyzed. Results In CFA, TF-PEG-BSH showed the most prominent effects by neutron irradiation among the three BDS. TF-PEG-BSH showed highly selective and highly efficient ¹⁰B delivery in tumor tissue. The survival rate in the treatment experiment was best in the TF-PEG-BSH group. Immunocytochemically, TF-PEG-BSH delivered BSH efficiently not only into the cytoplasm but also into the nucleus. Conclusion TF-PEG-BSH is a potent BDS for BNCT not only in terms of delivering a high concentration of ¹⁰B into tumor tissue, but also the selective delivery of ¹⁰B into the tumor cells.     

Randomized Comparison of Intra-arterial Versus Intravenous Infusion of ACNU for Newly Diagnosed Patients with Glioblastoma

The purpose of the present study was to assess the ability of technetium-99m-tetrofosmin (^99mTc-TF) to predict tumor malignancy and to compare its uptake with that of thallium-201 (²⁰¹Tl), technetium-99m-hexakis-2-methoxyisobutyl isonitrile (^99mTc-MIBI) and fluorine-18-fluorodeoxyglucose (¹⁸F-FDG) in brain tumors. ^99mTc-TF single-photon emission computed tomography (SPECT) imaging was performed in 22 patients with brain tumors and 3 healthy controls. Some of the patients underwent ²⁰¹Tl (n = 12) and ^99mTc-MIBI SPECT (n = 14) and ¹⁸F-FDG positron emission tomography (PET) (n = 12). The radioactivity ratio of tumor to contralateral normal tissue (T/N) and the ratio of tumor to contralateral white matter (T/WM) were calculated in SPECT and PET images, respectively. In healthy controls, ^99mTc-TF uptake was seen only in scalp, in the choroid plexus and pituitary gland, but not in normal cerebral parenchyma. TF T/N in low grade gliomas (2.8 ± 0.4) was significantly lower than that in high grade gliomas (22.5 ± 29.8) and malignant non-gliomas (8.3 ± 2.8) without overlap of values (p = 0.003 and p = 0.014, respectively). TF T/N was significantly correlated with MIBI T/N ( = 0.92, p = 0.001), Tl T/N ( = 0.72, p = 0.017), and FDG T/WM ( = 0.65, p = 0.031). There was an excellent agreement between TF T/N and MIBI T/N values on linear regression analysis (MIBI T/N = –0.63 + 0.97 × TF T/N). These preliminary results indicate that SPECT imaging with ^99mTc-TF may be useful for the non-invasive grading of brain tumors. They also suggest that ^99mTc-TF and ^99mTc-MIBI may accumulate in brain tumors by a similar mechanism or in relation to a similar process of tumor cell proliferation.     

Assessment of the results from the phase I/II boron neutron capture therapy trials at the Brookhaven National Laboratory from a clinician's point of view

Summary Boron neutron capture therapy (BNCT) represents a promising modality for a relatively selective radiation dose delivery to the tumor tissue. The key to effective BNCT of tumors such as glioblastoma multiforme (GBM) is the homogeneous preferential accumulation of¹⁰B in the tumor, including the infiltrating GBM cells, as compared to that in the vital structures of the normal brain. Provided that sufficiently high tumor¹⁰B concentration (∼10⁹ boron-10 atoms/cell) and an adequate thermal neutron fluence (∼ 10⁹ neutrons/cm²) are achieved, it is the ratio of the¹⁰B concentration in tumor cells to that in the normal brain cells and the blood that will largely determine the therapeutic gain of BNCT.     

Tissue uptake of BSH in patients with glioblastoma in the EORTC 11961 phase I BNCT trial

Summary Purpose: The uptake of the boron compound Na₂B₁₂H₁₀-SH (BSH) in tumor and normal tissues was investigated in the frame of the EORTC phase I trial ‘Postoperative treatment of glioblastoma with BNCT at the Petten Irradiation Facility’ (protocol 11961). Methods and Materials: The boron concentration in blood, tumor, normal brain, dura, muscle, skin and bone was detected using inductively coupled plasma-atomic emission spectroscopy in 13 evaluable patients. In a first group of 10 patients 100 mg BSH/kg bodyweight (BW) were administered; a second group of 3 patients received 22.9 mg BSH/kg BW. The toxicity due to BSH was evaluated. Results: The average boron concentration in the tumor was 19.9 ± 9.1 ppm (1 standard deviation (SD)) in the high dose group and 9.8 ± 3.3 ppm in the low dose group, the tumor/blood ratios were 0.6 ± 0.2 and 0.9 ± 0.2, respectively. The highest boron uptake has been detected in the dura, very low uptake was found in the bone, the cerebro-spinal fluid and especially in the brain (brain/blood ratio 0.2 ± 0.02 and 0.4 ± 0.2). No toxicity was detected except flush-like symptoms in 2 cases during a BSH infusion at a much higher speed than prescribed. Conclusion: BSH proved to be safe for clinical application at a dose of 100 mg BSH/kg infused and at a dose rate of 1 mg/kg/min. The study underlines the importance of a further investigation of BSH uptake in order to obtain enough data for significant statistical analysis. The boron concentration in blood seems to be a quite reliable parameter to predict the boron concentration in other tissues.     

Effects of Boron Neutron Capture Therapy Using Borocaptate Sodium in Combination with a Tumor-selective Vasoactive Agent in Mice

Boron neutron capture therapy (BNCT) destroys tumor cells by means of α particles and recoil protons emitted by ¹⁰B(n,α)⁷Li reaction. For BNCT to be effective, the tumor/normal tissue concentration ratio of ¹⁰B must be larger than 1.0, because neutron distribution is not selective. We examined the combination of ¹⁰B-enriched borocaptate sodium (BSH) with flavone acetic acid (FAA) as a model compound which causes vascular collapse in squamous cell carcinoma in mice (SCCVII tumors) and would increase the tumor/normal tissue concentration ratio of ¹⁰B. FAA (200 mg/kg, i.p.) was injected, and 5 min later BSH (75 mg/kg, i.v.) was administered, followed 15 to 180 min later by irradiation with thermal neutrons. The ¹⁰B concentrations were measured by prompt gamma ray spectrometry. Without FAA, tumor ¹⁰B concentrations were less than or equal to normal tissue concentrations at all time intervals, except that the concentrations were 1.7- to 2.7-fold greater in tumor than muscle at 15 and 180 min after injection of BSH. With FAA, ¹⁰B concentrations 2.1- to 6.9-fold greater in tumor than in muscle were achieved at all intervals tested. For blood and skin, significant differential accumulations were found in tumors at 120 and 180 min. Tumor/liver ratios were less than 1 at all times. Cell survival was determined by in vivo/in vitro colony assay, and increasing radiosensitization correlated with increasing tumor ¹⁰B concentrations, whether or not they were achieved with FAA. Tumor control rates, determined at 180 days after BNCT, similarly appeared to depend only on ¹⁰B levels at the time of irradiation. Because ¹⁰B levels correlate with the radiation response of tissues, a therapeutic gain would be expected whenever the tumor levels exceed normal tissue levels, such as in tumors located in muscle irradiated at 15–180 min after FAA+BSH, or in those in skin irradiated at 120 and 180 min.     

Rat Brain Tumor Models to Assess the Efficacy of Boron Neutron Capture Therapy: A Critical Evaluation

Development of any therapeutic modality can be facilitated by the use of the appropriate animal models to assess its efficacy. This report primarily will focus on our studies using the F98 and 9L rat glioma models to evaluate the effectiveness of boron neutron capture therapy (BNCT) of brain tumors. Following intracerebral implantation the biological behavior of each tumor resembles that of human high grade gliomas in a number of ways. In both models, glioma cells were implanted intracerebrally into syngeneic Fischer rats and 10–14 days later BNCT was initiated at the Brookhaven National Laboratory Medical Research Reactor. Two low molecular weight (M _r < 210Da) ¹⁰B-containing drugs, boronophenylalanine (BPA) and/or sodium borocaptate (BSH) were used as capture agents, either alone or in combination with each other. The 9L gliosarcoma, which has been difficult to cure by means of either chemo- or radiotherapy alone, was readily curable by BNCT. The best survival data were obtained using BPA at a dose of 1200mg/kg (64.8mg ¹⁰B), administered intraperitoneally (i.p.), with a 100% survival rate at 8 months. In contrast, the F98 glioma has been refractory to all therapeutic modalities. Tumor bearing animals, which had received 500mg/kg (27mg ¹⁰B) of BPA, or an equivalent amount of BSH i.v., had mean survival time (MST) of 37 and 33 days, respectively, compared to 29 days for irradiated controls. The best survival data with the F98 glioma model were obtained using BPA + BSH in combination, administered intra-arterially via the internal carotid artery (i.c.) with hyperosmotic mannitol induced blood–brain barrier disruption (BBB-D). The MST was 140 days with a cure rate of 25%, compared to a MST of 73 days with a 5% cure rate without BBB-D, and 41 days following i.v. administration of both drugs. A modest but significant increase in MST also was observed in rats that received intracarotid (i.c.) BPA in combination with Cereport (RMP-7), which produced a pharmacologically mediated opening of the BBB. Studies also have been carried out with the F98 glioma to determine whether an X-ray boost could enhance the efficacy of BNCT, and it was shown that there was a significant therapeutic gain. Finally, molecular targeting of the epidermal growth factor receptor (EGFR) has been investigated using F98 glioma cells, which had been transfected with the gene encoding EGFR and, intratumoral injection of boronated EGF as the delivery agent, followed by BNCT. These studies demonstrated that there was specific targeting of EGFR and provided proof of principle for the use of high molecular weight, receptor targeting-boron delivery agents. Finally, a xenograft model for melanoma metastatic to the brain has been developed using a human melanoma (MRA27), stereotactically implanted into the brains of nude rats, and these studies demonstrated that BNCT either cured or significantly prolonged the survival of tumor-bearing rats. It remains to be determined, which, if any, of these experimental approaches will be translated into clinical studies. Be that as it may, rat brain tumor models already have made a significant contribution to the design of clinical BNCT protocols, and should continue to do so in the future.     

Enhanced survival of glioma bearing rats following boron neutron capture therapy with blood-brain barrier disruption and intracarotid injection of boronophenylalanine

Boronophenylalanine (BPA) has been used for boron neutron capture therapy (BNCT) of brain tumors in both experimental animals and humans. The purpose of the present study was to determine if the efficacy of BNCT could be enhanced by means of intracarotid (i.c.) injection of BPA with or without blood-brain barrier disruption (BBB-D) and neutron irradiation using a rat brain tumor model. For biodistribution studies, F98 glioma cells were implanted stereotactically into the brains of Fischer rats, and12 days later BBB-D was carried out by i.c. infusion of 25% mannitol (1.373 mOsmol/ml), followed immediately by i.c. administration of 300, 500 or 800 mg of BPA/kg body weight (b.w.). At the 500 mg dose a fourfold increase in tumor boron concentration (94.5 g/g) was seen at 2.5 hours after BBB-D, compared to 20.8 g/g in i.v. injected animals. The best composite tumor to normal tissue ratios were observed at 2.5 hours after BBB-D, at which time the tumor: blood (T: Bl) ratio was10.9, and the tumor: brain (T: Br) ratio was 7.5, compared to 3.2 and 5.0 respectively for i.v. injected rats. In contrast, animals that had received i.c. BPA without BBB-D had T: Bl and T: Br ratios of 8.5 and 5.9, respectively, and the tumor boron concentration was 42.7g/g. For therapy experiments, initiated 14 days after intracerebral implantation of F98 glioma cells, 500 mg/kg b.w. of BPA were administered i.v. or i.c. with or without BBB-D, and the animals were irradiated 2.5 hourslater at the Brookhaven Medical Research Reactor with a collimated beam of thermal neutrons delivered to the head. The mean survival time for untreated control rats was 24 ± 3 days, 30 ± 2 days for irradiated controls, 37 ± 3 days for those receiving i.v. BPA, 52 ± 15 days for rats receiving i.c. BPA without BBB-D, and 95 ± 95 days for BBB-D followed by i.c. BPA and BNCT. The latter group had a 246% increase in life span (ILS) compared to untreated controls and a 124% ILS compared to that of i.v. injected animals. These survival data are the best ever obtained with the F98 glioma model and suggest that i.c. administration of BPA with or without BBB-D may be useful as a means to increase the efficacy of BNCT.     

Pharmacokinetics of Sodium Borocaptate: A Critical Assessment of Dosing Paradigms for Boron Neutron Capture Therapy

The pharmacokinetics of sodium borocaptate (BSH), a drug that has been used clinically for boron neutron capture therapy (BNCT) of malignant brain tumors, have been characterized by measuring boron concentrations by direct current plasma-atomic emission spectroscopy (DCP-AES) in a group of 23 patients with high-grade gliomas. The disposition of BSH following intravenous (i.v.) infusion, which was determined by measuring plasma boron concentrations by DCP-AES, was consistent with a three-compartment open model with zero-order input and first-order elimination from the central compartment. Boron disposition was linear over the dose range of 26.5–88.2mg BSH/kg body weight (b.w.), corresponding to 15–50mg boron/kg b.w. Mean total body boron plasma clearance was 14.4±3.5ml/min and the harmonic mean half-lives (range) were 0.6 (0.3–3.7), 6.5 (4.8–10.1) and 77.8 (49.6–172.0)h for the ,, and disposition phases, respectively. Using an empirically determined plasma:blood boron concentration ratio of 1.3±0.2, the calculated total body boron blood clearance was 18.5±4.5ml/min. In order to develop a model for selecting the optimum dosing paradigm, a pharmacokinetic correlation was established between the boron content of normal brain, solid tumor, and infiltrating tumor to the shallow tissue pharmacokinetic compartment (C₂). Based on our model, it was concluded that although multiple i.v. infusions of BSH might increase absolute tumor boron concentrations, they will not improve the tumor:plasma boron concentration ratios over those attainable by a single i.v. infusion. The results from our study are confirmatory of those previously reported by others when blood sampling has been carried out for a sufficient period of time to adequately characterize the pharmacokinetics.     

Effect of Electroporation on Cell Killing by Boron Neutron Capture Therapy Using Borocaptate Sodium (10B-BSH)

The cell membrane permeability of ¹⁰B-enriched borocaptate sodium (BSH) and the extent to which BSH is accumulated in cells are controversial. To elucidate these points and to enhance the accumulation of BSH in cells, the effect of electroporation on boron neutron capture therapy (BNCT) using BSH was investigated. The first group of SCCVII tumor cells was incubated in culture medium with ¹⁰B-BSH or ¹⁰B-enriched boric acid, and exposed to neutrons from the heavy water facility of the Kyoto University Reactor. More than 99% of neutrons were thermal neutrons at flux base. The second group was pretreated with electroporation in combination with ¹⁰B-BSH, and thereafter the cells were irradiated with neutrons. The cell-killing effect of BNCT was measured by colony formation assay. The surviving cell fraction decreased exponentially with neutron fluence, and addition of BSH significantly enhanced the cell-killing effect of NCT depending on ¹⁰B concentration and the preincubation time of cells in the BSH-containing culture medium. The electroporation of cells with BSH markedly enhanced the BNCT effect in comparison with that obtained with preincubation alone. The effect of BSH-BNCT with electroporation was almost equal to that of BNCT using ¹⁰B-boric acid at the same ¹⁰B concentration. The effect of BNCT on cells pretreated with BSH and electroporation was not reduced by repeated washing of the cells before neutron irradiation. Decrease of the effect of BSH-BNCT plus electroporation with increase in the waiting time between the electroporation and the neutron irradiation could be explained in terms of the extent of cell growth during that time. These data suggest that BSH penetrates the cells slowly and remains after washing. Electroporation can introduce BSH into the cells very efficiently, and BSH thus introduced stays in the cells and is not lost in spite of the intensive washing of the cells. Therefore, if electroporation is applied to tumors after BSH injection, ¹⁰B would remain in the tumors but be cleared from normal tissues, and selective accumulation of ¹⁰B in tumors will be achieved after an appropriate waiting time.     

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 1000mg/kg, administered intravenously over a 2-h infusion period, resulted in an increase in tumor boron concentration from 30 to 70µg 10B/g, with a constant T:B boron concentration ratio of about 3.7:1. Similarly, extension of the infusion time from 2 to 6h, at a constant dose-rate of 125mg BPA/kg/h, resulted in an increase in tumor boron concentration from 30 to 80µg 10B/g, while, again, maintaining a constant T:B ratio of about 3.7:1. In contrast, intracarotid infusion of BPA for 1h at a dose rate of 125mg BPA/kg resulted in an increase in the tumor boron concentration from 26 to 38µg 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.     

Dose-dependent disposition kinetics and tissue accumulation of boron after intravenous injections of sodium mercaptoundecahydrododecaborate in rabbits

Summary Kinetics of boron disposition after single intravenous injections of two different doses (25 and 50 mg/kg) of mercaptoundecahydrododecaborate sodium (Na₂B₁₂H₁₁SH; BSH) was studied in rabbits. Residual boron concentrations in various organs and tissues (heart, lungs, liver, spleen, kidney, adrenals, and brain) were also determined after seven daily injections of the same doses of BSH. Boron blood and tissue concentrations were measured by atomic emission spectrometry. In the majority of animals, the decline of boron blood concentrations after a single intravenous injection of either dose was biphasic, being consistent with a two-compartment model of boron disposition in the body. Although mean boron blood concentrations were roughly proportional to the BSH dose delivered, the mean total body clearance of boron from the body was 3 times lower (6.5±1.9 ml min^–1 kg^–1) after a dose of 50 mg/kg than after the injection of 25 mg/kg (22.4±7.9 ml min^–1 kg^–1), the difference between the means being statistically significant (P<0.05). Moreover, the mean terminal half-life of boron in blood was prolonged after the injection of 50 mg/kg (14.5±5.5 h) as compared with that found after the 25-mg/kg dose (3.5±0.9 h). On the other hand, the different BSH doses did not result in marked differences in the mean values obtained for the volume parameters—the volume of the central compartment (1.3±0.4 vs 1.3±0.5 l kg^–1) and the volume of distribution at steady state (4.7±1.3 vs 6.0±4.0 l kg^–1)—both of which were high, indicating extensive binding of the compound not only in the blood but also in tissues. Residual concentrations of boron found after seven daily injections of both doses of BSH were highest in the kidneys, the difference in the mean values being relatively small (33.6±6.1 vs 39.0±10.7 g/g tissue). In the majority of other organs (heart, lung, liver, spleen, brain, adrenals), the residual concentrations after a dose of 50 mg/kg were disproportionately higher than those measured after the injection of 25 mg/kg, and the mean values corresponded to the reduced total body clearance rather than to the increased BSH dose. The saturability of BSH binding to blood and tissue proteins is suggested as a possible explanation for the dose dependency of the total clearance of boron from the body and the accumulation of BSH in organs and tissues.     

Pharmacokinetic study of BSH and BPA in simultaneous use for BNCT

Summary In order to improve the effectiveness of boron neutron capture therapy (BNCT) for malignant gliomas, we examined the optimization of the administration of boron compounds in brain tumor animal model. We analyzed the concentration of boron atoms in intracranial C6 glioma -bearing rats using inductively coupled plasma atomic emission spectrometry. Each tumor-bearing rat received one of two different amounts of sodium borocaptate (BSH) and/or 500 mg/kg of boronophenylalanine (BPA) via intraperitoneal injection. We compared the boron concentrations of the tumor, the contralateral normal brain and the blood in rats of 3 different treatment groups (BSH alone, BPA alone and a combination of both BSH and BPA). Our results show that the tumor boron concentration increased much more than 30 μg/g by the coadministration of both compounds. Additionally, the blood boron concentration remained below 30 μg/g and the boron concentration in the normal brain was low (mean 4.7±1.1 μg/g). Even in comparison with the administration of BPA alone, coadministration of BPA and BSH shows an improved tumor/normal brain ratio of boron concentrations.     

Boron concentrations in brain during boron neutron capture therapy: in vivo measurements from the phase I trial EORTC 11961 using a gamma-ray telescope

PURPOSE: Gamma-ray spectroscopic scans to measure boron concentrations in the irradiated volume were performed during treatment of 5 patients suffering from brain tumors with boron neutron capture therapy (BNCT). In BNCT, the dose that is meant to be targeted primarily to the tumor is the dose coming from the reaction 10B(n,alpha)7Li, which is determined by the boron concentration in tissue and the thermal neutron fluence rate. The boron distribution throughout the head of the patient during the treatment is therefore of major interest. The detection of the boron distribution during the irradiation was until now not possible. METHODS AND MATERIALS: Five patients suffering from glioblastoma multiforme and treated with BNCT in a dose escalation study were administered the boron compound, boron sulfhydryl (BSH; Na(2)B(12)H(11)SH). Boron concentrations were reconstructed from measurements performed with the gamma-ray telescope which detects locally the specific gamma rays produced by neutron capture in 10B and 1H. RESULTS: For all patients, at a 10B concentration in blood of 30 ppm, the boron concentration in nonoperated areas of the brain was very low, between 1 and 2.5 ppm. In the target volume, which included the area where the tumor had been removed and where remaining tumor cells have to be assumed, much higher boron concentrations were measured with large variations from one patient to another. Superficial tissue contained a higher concentration of 10B than the nonoperated areas of the brain, ranging between 8 and 15 ppm. CONCLUSIONS: The measured results correspond with previous tissue uptake studies, confirming that normal brain tissue hardly absorbs the boron compound BSH. Gamma-ray telescope measurements seem to be a promising method to provide information on the biodistribution of boron during therapy. Furthermore, it also opens the possibility of in vivo dosimetry.     

Delivery of (10)boron to oral squamous cell carcinoma using boronophenylalanine and borocaptate sodium for boron neutron capture therapy

Boron neutron capture therapy (BNCT) is a unique radiation therapy in which boron compounds are trapped into tumor cells. To determine the biodistribution of boronophenylalanine (BPA) in nude mice carrying oral squamous cell carcinoma (SCC), BPA was administered at a dose of 250 mg/kg body weight intraperitoneally. Two hours later, (10)B concentration in the tumor was 15.96 ppm and tumor/blood, tumor/tongue, tumor/skin and tumor/bone (10)B concentration ratios were 6.44, 4.19, 4.68 and 4.56, respectively. Two hours after the administration of borocaptate sodium (BSH) at a dose of 75 mg/kg body weight, (10)B concentration in the tumor was 3.61 ppm, and tumor/blood, tumor/tongue, tumor/skin and tumor/bone (10)B concentration ratios were 0.77, 1.05, 0.60 and 0.59, respectively. When cultured oral SCC cells were incubated with BPA or BSH for 2 h and then exposed to thermal neutrons, the proportion of survival cells that were capable of forming cell colonies decreased exponentially, depending on (10)B concentration. BPA-mediated BNCT was more efficient than BSH-mediated BNCT. Addition of boron compounds in the cell suspension during neutron irradiation enhanced the cell-killing effect of the neutrons. These results indicate that BPA is more selectively incorporated into human oral SCC as compared with normal oral tissues, and that both extra- and intra-cellular BPA contribute to the cell-killing effect of BNCT. BPA may be a useful boron carrier for BNCT in the treatment of advanced oral SCC.