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

Development of Brain-Tumor-Targeted Benzothiazole-Based Boron Complex for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a precision treatment technology that ideally damages only boron-accumulating cells. The effectiveness of BNCT depends on the amount of boron in the tumor cells and the concentration ratio between normal and tumor tissues. Therefore, for successful brain-tumor treatment using BNCT, it is essential to develop a drug with high blood-brain barrier (BBB) permeability and high tumor accumulation. The benzothiazole-based boron complex 4-(benzo[d]thiazol-2-yl)phenylboronic acid (BTPB) is a hydrophobic, low-molecular-weight compound that has shown high BBB permeability and brain accumulation. The highest boron concentration of BTPB is 36.11 ± 2.73 μg/g (at 1 h post-injection) in the brain, and the highest brain/blood ratio is 3.94 ± 0.46 (at 2 h post-injection), which is sufficient for the BNCT drug condition. In addition, BTPB showed good tumor-targeting ability in vivo in a U87MG glioma tumor model. In this study, we conducted a biological evaluation of BTPB compared to boronophenylalanine as a novel drug for BNCT.

Evaluation of the Key Advantages between Two Modalities of Boronophenylalanine Administration for Clinical Boron Neutron Capture Therapy Using an Animal Model

In clinical boron neutron capture therapy (BNCT), boronophenylalanine (BPA) administrations through one-step infusion (OSI) and two-step infusion (TSI) are the most widely used. This study compared the advantages of OSI and TSI using a human oral squamous cell carcinoma-bearing animal model. OSI was administered at a high-dose rate of 20 mg/kg/min for 20 min (total dose: 400 mg/kg) as the first step infusion. TSI was a prolonged infusion at a low-dose rate of 1.67 mg/kg/min for 15, 30, 45, and 60 min (total dose: 25, 50, 75, and 100 mg/kg) following the first step infusion. The sigmoid Emax model was used to evaluate the boron accumulation effect in the tumor. The advantages of TSI were observed to be greater than those of OSI. The observed advantages of TSI were as follows: a stable level of boron concentration in blood; tumor to blood boron ratio (T/B); tumor to muscle boron ratio (T/M); and skin to blood boron ratio (S/B). The boron accumulation effect in tumors increased to 68.98%. Thus, effective boron concentration in these tumor cells was achieved to enhance the lethal damage in BNCT treatment. Boron concentration in the blood was equal to that in the skin. Therefore, the equivalent dose was accurately estimated for the skin.

Importance of radiobiological studies for the advancement of boron neutron capture therapy (BNCT)

Boron neutron capture therapy (BNCT) is a tumour selective particle radiotherapy, based on the administration of boron carriers incorporated preferentially by tumour cells, followed by irradiation with a thermal or epithermal neutron beam. BNCT clinical results to date show therapeutic efficacy, associated with an improvement in patient quality of life and prolonged survival. Translational research in adequate experimental models is necessary to optimise BNCT for different pathologies. This review recapitulates some examples of BNCT radiobiological studies for different pathologies and clinical scenarios, strategies to optimise boron targeting, enhance BNCT therapeutic effect and minimise radiotoxicity. It also describes the radiobiological mechanisms induced by BNCT, and the importance of the detection of biomarkers to monitor and predict the therapeutic efficacy and toxicity of BNCT alone or combined with other strategies. Besides, there is a brief comment on the introduction of accelerator-based neutron sources in BNCT. These sources would expand the clinical BNCT services to more patients, and would help to make BNCT a standard treatment modality for various types of cancer. Radiobiological BNCT studies have been of utmost importance to make progress in BNCT, being essential to design novel, safe and effective clinical BNCT protocols.

Molecular design of a high-performance polymeric carrier for delivery of a variety of boronic acid-containing drugs

Because of their many useful and unique properties, boronic acids are well suited for biomedical applications such as antitumor chemotherapy and boron neutron capture therapy (BNCT). Bortezomib, a boronic acid derivative, has drawn a lot of attention as a potent proteasome inhibitor. Nevertheless, because of rapid excretion and off-target effects, the clinical translation of boronic acid-containing drugs is limited. To this end, we employed a polymeric carrier to stably encapsulate boronic acid-containing drugs and achieve superior pharmacokinetics with an on-target drug release capability. Accordingly, to construct a supramolecular polymeric nanoparticle, we took advantage of the facile, stable, and pH-sensitive conjugation between boronic acids and diethanolamine-installed polymeric carriers. We demonstrated the feasibility of our molecular design by generating and applying bortezomib-loaded nanoparticles to a subcutaneous tumor-bearing mouse model. Stable encapsulation and pH-sensitive release of bortezomib facilitated antitumor efficacy and alleviated hepatotoxicity. We also verified the versatility of our approach through biological evaluations of the nanoparticles encapsulating benzo(b)thiophene-2-boronic acid, phenylboronic acid, and p-phenylene-diboronic acid.

Non-isotope enriched phenylboronic acid-decorated dual-functional nano-assembles for an actively targeting BNCT drug

The feasibility of boron neutron capture therapy (BNCT) greatly depends on the selective accumulation of ¹⁰B in tumors. The p-boronophenylalanine-fructose (BPA-f) complex has been established as a conventional BNCT agent due to its preferential uptake into tumors, which is driven by amino acid transporters. However, the retention of BPA-f in tumors is highly limited because of an antiport mechanism, which is regulated by a gradient of amino acid concentration across the cancer cell membrane. Thus, to preserve a high ¹⁰B concentration in tumors, patients are inevitably subjected to a constant intravenous infusion. To this end, we employed a phenylboronic acid (PBA)-decorated polymeric nanoparticle (Nano^PBA) as a sialic acid-targeting BNCT agent. In this manner, the PBA can exhibit dual functionalities, i.e., exhibiting a neutron capture capacity and hypersialyated cancer cell targeting effect. Our developed Nano^PBA possesses a supramolecular structure composed of a core and shell comprised of poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) segments, respectively. The PBA moiety is installed at the PEG end, providing an unusually strong targeting effect, supposedly via multivalent binding onto the cancer cell membrane. As in BNCT, we verified the feasibility of Nano^PBA against a B16 melanoma-bearing mouse model. By virtue of efficient tumor targeting, even at a 100-fold lower dose than BPA-f, the Nano^PBA achieved a potent antitumor effect.

Accelerator-based boron neutron capture therapy for malignant glioma: a pilot neutron irradiation study using boron phenylalanine, sodium borocaptate and liposomal borocaptate with a heterotopic U87 glioblastoma model in SCID mice

Purpose: To evaluate the efficacy of boron neutron capture therapy (BNCT) for a heterotopic U87 glioblastoma model in SCID mice using boron phenylalanine (BPA), sodium borocaptate (BSH) and liposomal BSH as boron compounds at a unique, accelerator-based neutron source.Materials and methods: Glioblastoma models were obtained by subcutaneous implantation of U87 cells in the right thighs of SCID mice before administration of 350 mg/kg of BPA (BPA-group), 100 mg/kg of BSH (BSH-group) or 100 mg/kg of BSH in PEGylated liposomes (liposomal BSH-group) into the retroorbital sinus. Liposomes were prepared by reverse-phase evaporation. Neutron irradiation was carried out at a proton accelerator with a lithium target developed for BNCT at the Budker Institute of Nuclear Physics, Novosibirsk, Russian Federation. A proton beam current integral of 3 mA/h and energy of 2.05 MeV were used for neutron generation.Results: Boron compound accumulation in tumor tissues at the beginning of irradiation was higher in the BPA group, followed by the Liposomal BSH and BSH groups. Tumor growth was significantly slower in all irradiated mice from the 7th day after BNCT compared to untreated controls (p < .05). Tumor growth in all treated groups showed no large variation, apart from the Irradiation only group and the BPA group on the 7th day after BNCT. The overall trend of tumor growth was clear and the differences between treatment groups became significant from the 50th day after BNCT. Tumor growth was significantly slower in the Liposomal BSH group compared to the Irradiation only group on the 50th (p = .012), 53rd (p = .005), and the 57th (p = .021) days after treatment. Tumor growth in the Liposomal BSH group was significantly different from that in the BPA group on the 53rd day after BNCT (p = .021) and in the BSH group on the 50th (p = .024), 53rd (p = .015), and 57th (p = .038) days after BNCT. Skin reactions in the form of erosions and ulcers in the tumor area developed in treated as well as untreated animals with further formation of fistulas and necrotic decay cavities in most irradiated mice.Conclusions: We observed a tendency of BNCT at the accelerator-based neutron source to reduce or suspend the growth of human glioblastoma in immunodeficient animals. Liposomal BSH showed better long-term results compared to BPA and non-liposomal BSH. Further modifications in liposomal boron delivery are being studied to improve treatment outcomes.

Focused Ultrasonography-Mediated Blood-Brain Barrier Disruption in the Enhancement of Delivery of Brain Tumor Therapies

Glioblastoma is the most common intracranial malignancy in adults and carries a poor prognosis. Chemotherapeutic treatment figures prominently in the management of primary and recurrent disease. However, the blood-brain barrier presents a significant and formidable impediment to the entry of oncotherapeutic compounds to target tumor tissue. Several strategies have been developed to effect disruption of the blood-brain barrier and in turn enhance the efficacy of cytotoxic chemotherapy, as well as newly developed biologic agents. Focused ultrasonography is one such treatment modality, using acoustic cavitation of parenterally administered microbubbles to mechanically effect disruption of the vascular endothelium. We review and discuss the preclinical and clinical studies evaluating the biophysical basis for, and efficacy of, focused ultrasonography in the enhancement of oncotherapeutic agent delivery. Further, we provide some perspectives regarding future directions for the role of focused ultrasound in facilitating and improving the safe and effective delivery of oncotherapeutic agents in the treatment of glioblastoma.

Towards carborane-functionalised structures for the treatment of brain cancer

Boron neutron capture therapy (BNCT) is a promising targeted chemoradiotherapeutic technique for the management of invasive brain tumors, such as glioblastoma multiforme (GBM). A prerequisite for effective BNCT is the selective targeting of tumour cells with ¹⁰B-rich therapeutic moieties. To this end, polyhedral boranes, especially carboranes, have received considerable attention because they combine a high boron content with relative low toxicity and metabolic inertness. Here, we review progress in the molecular design of recently investigated carborane derivatives in light of the widely accepted performance requirements for effective BNCT.

Boronophenylalanine, a boron delivery agent for boron neutron capture therapy, is transported by ATB0,+, LAT1 and LAT2

The efficacy of boron neutron capture therapy relies on the selective delivery of boron carriers to malignant cells. p-Boronophenylalanine (BPA), a boron delivery agent, has been proposed to be localized to cells through transporter-mediated mechanisms. In this study, we screened aromatic amino acid transporters to identify BPA transporters. Human aromatic amino acid transporters were functionally expressed in Xenopus oocytes and examined for BPA uptake and kinetic parameters. The roles of the transporters in BPA uptake were characterized in cancer cell lines. For the quantitative assessment of BPA uptake, HPLC was used throughout the study. Among aromatic amino acid transporters, ATB^0,+, LAT1 and LAT2 were found to transport BPA with Km values of 137.4 ± 11.7, 20.3 ± 0.8 and 88.3 ± 5.6 μM, respectively. Uptake experiments in cancer cell lines revealed that the LAT1 protein amount was the major determinant of BPA uptake at 100 μM, whereas the contribution of ATB^0,+ became significant at 1000 μM, accounting for 20–25% of the total BPA uptake in MCF-7 breast cancer cells. ATB^0,+, LAT1 and LAT2 transport BPA at affinities comparable with their endogenous substrates, suggesting that they could mediate effective BPA uptake in vivo. The high and low affinities of LAT1 and ATB^0,+, respectively, differentiate their roles in BPA uptake. ATB^0,+, as well as LAT1, could contribute significantly to the tumor accumulation of BPA at clinical dose.

Intracellular boron accumulation in CHO-K1 cells using amino acid transport control

BPA used in BNCT has a similar structure to some essential amino acids and is transported into tumor cells by amino acid transport systems. Previous study groups have tried various techniques of loading BPA to increase intracellular boron concentration. CHO-K1 cells demonstrate system L (LAT1) activity and are suitable for specifying the transport system of a neutral amino acid. In this study, we examined the intracellular accumulation of boron in CHO-K1 cells by amino acid transport control, which involves co-loading with L-type amino acid esters. Intracellular boron accumulation in CHO-K1 cells showed the greatest increased upon co-loading 1.0mM BPA, with 1.0mM l-Tyr-O-Et and incubating for 60min. This increase is caused by activation of a system L amino acid exchanger between BPA and l-Tyr. The amino acid esters are metabolized to amino acids by intracellular hydrolytic enzymes that increase the concentrations of intracellular amino acids and stimulate exchange transportation. We expect that this amino acid transport control will be useful for enhancing intracellular boron accumulation.

Enhancing drug delivery for boron neutron capture therapy of brain tumors with focused ultrasound

BACKGROUND

Glioblastoma is a notoriously difficult tumor to treat because of its relative sanctuary in the brain and infiltrative behavior. Therapies need to penetrate the CNS but avoid collateral tissue injury. Boron neutron capture therapy (BNCT) is a treatment whereby a (10)B-containing drug preferentially accumulates in malignant cells and causes highly localized damage when exposed to epithermal neutron irradiation. Studies have suggested that (10)B-enriched L-4-boronophenylalanine-fructose (BPA-f) complex uptake can be improved by enhancing the permeability of the cerebrovasculature with osmotic agents. We investigated the use of MRI-guided focused ultrasound, in combination with injectable microbubbles, to noninvasively and focally augment the uptake of BPA-f.

METHODS

With the use of a 9L gliosarcoma tumor model in Fisher 344 rats, the blood-brain and blood-tumor barriers were disrupted with pulsed ultrasound using a 558 kHz transducer and Definity microbubbles, and BPA-f (250 mg/kg) was delivered intravenously over 2 h. (10)B concentrations were estimated with imaging mass spectrometry and inductively coupled plasma atomic emission spectroscopy.

RESULTS

The tumor to brain ratio of (10)B was 6.7 ± 0.5 with focused ultrasound and only 4.1 ± 0.4 in the control group (P < .01), corresponding to a mean tumor [(10)B] of 123 ± 25 ppm and 85 ± 29 ppm, respectively. (10)B uptake in infiltrating clusters treated with ultrasound was 0.86 ± 0.10 times the main tumor concentration, compared with only 0.29 ± 0.08 in controls.

CONCLUSIONS

Ultrasound increases the accumulation of (10)B in the main tumor and infiltrating cells. These findings, in combination with the expanding clinical use of focused ultrasound, may offer improvements in BNCT and the treatment of glioblastoma.

Pharmaceutical development and medical applications of porphyrin-type macrocycles

The current state of pharmaceutical development of porphyrin-type macrocycles in medicine is highlighted. Currently, several porphyrinoid-based drugs are under various stages of development as phototherapeutic agents, X-ray radiation enhancers and boron neutron capture agents. These compounds represent a burgeoning class of pharmacological agents that are potentially useful in an array of treatment areas.

Selective uptake of boronophenylalanine by glioma stem/progenitor cells

The success of boron neutron capture therapy (BNCT) depends on the amount of boron in cells and the tumor/blood and tumor/(normal tissue) boron concentration ratios. For the first time, measurements of boron uptake in both stem/progenitor and differentiated glioma cells were performed along with measurements of boron biodistribution in suitable animal models. In glioma stem/progenitor cells, the selective accumulation of boronophenylalanine (BPA) was lower, and retention of boron after BPA removal was longer than in differentiated glioma cells in vitro. However, boron biodistribution was not statistically significantly different in mice with xenografts.

Boron neutron capture therapy for newly diagnosed glioblastoma: a pilot study in Tsukuba

Neutron capture therapy (NCT) theoretically allows an unique tumor-cell-selective high-LET particle radiotherapy. The survival benefits and safety of NCT were evaluated in 15 patients with newly diagnosed glioblastoma multiforme (GBM). Seven patients received intra-operative (IO-) NCT and eight patients received external beam (EB-) NCT. Sulfhydryl borane (BSH, 5 g/body) was administered intravenously 12 h before neutron irradiation. Additionally, p-dihydroxyboryl-phenylalanine (BPA, 250 mg/kg) was given 1 h before irradiation to the eight patients who underwent EB-NCT. EB-NCT was combined with fractionated photon irradiation. Five of 15 patients were alive at analysis for a mean follow-up time of 20.3 M. In 11 of 15 patients followed up for more than 1-year, eight (72.7%) maintained their Karnofsky performance status (KPS; 90 in 6 and 100 in 2). The median overall survival (OS) and time to magnetic resonance (MR) change (TTM) for all patients were 25.7 and 11.9 M, respectively. There was no difference in TTM between the IO-NCT (12.0 M) and EB-NCT (11.9 M) groups. The 1- and 2-year survival rates were 85.7% and 45.5%, respectively. This NCT pilot study in 15 patients with newly diagnosed GBM showed survival benefits, suggesting that the neutron capture reaction may function sufficiently to control tumors locally, and that further optimized studies in large series of patients are warranted.

Boron neutron capture therapy for newly diagnosed glioblastoma

We evaluate the clinical results of a form of tumor selective particle radiation known as boron neutron capture therapy (BNCT) for newly-diagnosed glioblastoma (NDGB) patients, especially in combination with X-ray treatment (XRT). Between 2002 and 2006, we treated 21 patients of NDGB with BNCT utilizing sodium borocaptate and boronophenylalanine simultaneously. The first 10 were treated with only BNCT (protocol 1), and the last 11 were treated with BNCT followed by XRT of 20 to 30 Gy (protocol 2) to reduce the possibility of local tumor recurrence. No chemotherapy was applied until tumor progression was observed. The patients treated with BNCT (protocol 1 plus 2) showed a significant survival prolongation compared with the institutional historical controls. BNCT also showed favorable results in correspondence with the RTOG- and EORTC-RPA subclasses. The median survival time (MST) was 15.6 months for protocols 1 and 2 together. For protocol 2, the MST was 23.5 months. The main causes of death were cerebrospinal fluid dissemination as well as local recurrence. Our modified BNCT protocol showed favorable results of patients with NDGB not only for those with good prognoses but also for those with poor prognoses.

Neuropathological postmortem evaluation of BNCT for GBM

BACKGROUND

Thirty patients with glioblastoma multiforme (GBM) were treated by boron neutron capture therapy (BNCT) at the Studsvik facility in Sweden, in a clinical trial exploring a procedure in which 900 mg p-boronophenylalanine (BPA) per kilo body weight was infused in 6 h.

OBJECTIVE

The present study was designed to assess tumor efficacy and radiation damage to the brain for the seven patients in the Studsvik trial that were available for postmortem neuropathological examination.

METHOD

Whole brain slices containing the initial tumor site and other regions showing pathological changes were chosen for microscopy and selected areas were studied by immunological methods.

RESULTS

Local control of GBM was observed in all cases. Conclusive evidence for radiation induced brain damage was not found.

CONCLUSION

Using a novel procedure for BPA infusion, BNCT achieves local control of GBM for minimum tumor doses as low as 15 wGy, allowing treatment with very low concomitant doses to surrounding healthy tissues.

Early effects of boron neutron capture therapy on rat glioma models

Early effects of boron neutron capture therapy (BNCT) on malignant glioma are characterized by reduction of the enhancement area and regression of the peritumoral edema radiologically. The aim of this study was to investigate the early histological changes of tumors and inflammatory cells after BNCT in the rat brain. Rats were treated with BNCT using boronophenylalanine (BPA) 7 days after implantation of C6 glioma cells. The tumors were assessed with magnetic resonance imaging and histopathological examination at 4 days after BNCT. The mean tumor volumes were 39 +/- 2 mm3 in the BNCT group and 134 +/- 18 mm3 in the control group. In the BNCT group, tumor cells showed a less pleomorphic appearance with atypical nuclei and mitotic figures. The Ki-67 labeling index was 6.5% +/- 4.7% in the BNCT and 35% +/- 3.8% in the control group. The reactions of the inflammatory cells were examined with ED-1 as macrophage marker and OX42 as microglia marker. ED-1- and OX-42-positive cells were reduced both in the core and the marginal area of the tumor in the BNCT group. It is suggested that BNCT reduced tumor progression by suppression of proliferation. Inhibition of the activated macrophages may relate to reduced peritumoral edema in the early phase.

Pharmacokinetic study of BSH and BPA in simultaneous use for BNCT

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 microg/g by the coadministration of both compounds. Additionally, the blood boron concentration remained below 30 microg/g and the boron concentration in the normal brain was low (mean 4.7+/-1.1 microg/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 neutron capture therapy of cancer: current status and future prospects

BACKGROUND

Boron neutron capture therapy (BNCT) is based on the nuclear reaction that occurs when boron-10 is irradiated with low-energy thermal neutrons to yield high linear energy transfer alpha particles and recoiling lithium-7 nuclei. Clinical interest in BNCT has focused primarily on the treatment of high-grade gliomas and either cutaneous primaries or cerebral metastases of melanoma, most recently, head and neck and liver cancer. Neutron sources for BNCT currently are limited to nuclear reactors and these are available in the United States, Japan, several European countries, and Argentina. Accelerators also can be used to produce epithermal neutrons and these are being developed in several countries, but none are currently being used for BNCT.

BORON DELIVERY AGENTS

Two boron drugs have been used clinically, sodium borocaptate (Na(2)B(12)H(11)SH) and a dihydroxyboryl derivative of phenylalanine called boronophenylalanine. The major challenge in the development of boron delivery agents has been the requirement for selective tumor targeting to achieve boron concentrations ( approximately 20 microg/g tumor) sufficient to deliver therapeutic doses of radiation to the tumor with minimal normal tissue toxicity. Over the past 20 years, other classes of boron-containing compounds have been designed and synthesized that include boron-containing amino acids, biochemical precursors of nucleic acids, DNA-binding molecules, and porphyrin derivatives. High molecular weight delivery agents include monoclonal antibodies and their fragments, which can recognize a tumor-associated epitope, such as epidermal growth factor, and liposomes. However, it is unlikely that any single agent will target all or even most of the tumor cells, and most likely, combinations of agents will be required and their delivery will have to be optimized.

CLINICAL TRIALS

Current or recently completed clinical trials have been carried out in Japan, Europe, and the United States. The vast majority of patients have had high-grade gliomas. Treatment has consisted first of "debulking" surgery to remove as much of the tumor as possible, followed by BNCT at varying times after surgery. Sodium borocaptate and boronophenylalanine administered i.v. have been used as the boron delivery agents. The best survival data from these studies are at least comparable with those obtained by current standard therapy for glioblastoma multiforme, and the safety of the procedure has been established.

CONCLUSIONS

Critical issues that must be addressed include the need for more selective and effective boron delivery agents, the development of methods to provide semiquantitative estimates of tumor boron content before treatment, improvements in clinical implementation of BNCT, and a need for randomized clinical trials with an unequivocal demonstration of therapeutic efficacy. If these issues are adequately addressed, then BNCT could move forward as a treatment modality.

Distribution of BPA and metabolic assessment in glioblastoma patients during BNCT treatment: a microdialysis study

Boron neutron capture therapy (BNCT) is dependent on the selective accumulation of boron-10 in tumour cells. To maximise the radiation effect, the neutrons should be delivered when the ratio between the boron concentration in tumour cells to that in normal tissues reaches maximum. However, the pharmacokinetics of p-boronophenylalanine (BPA) and other boron delivery agents are only partly known. We used microdialysis to investigate the extracellular in vivo kinetics of boron in three intracerebral compartments -- solid tumour, brain adjacent to tumour (BAT), and the normal brain, as well as the subcutaneous tissue before, during, and after BNCT treatment. The findings were compared to the pharmacokinetics of BPA in the blood. We also measured the glucose metabolism and the levels of glutamate and glycerol in those compartments. Four patients were studied, two patients underwent surgical tumour resection and in two a stereotactic biopsy was performed. The patients were given BPA (900 mg/kg body weight) by a 6-h infusion. The infusion was completed approximately 2-3 h before neutron irradiation. In tumour tissue the extracellular concentration of BPA followed that of blood with a maximal concentration of 31.2 ppm and a maximal ratio vs. blood of 1.07. In BAT, the maximal concentration of BPA was 18.0 ppm with the peak level delayed for 4-6 h compared to the peak in blood with a maximal ratio of 1.2. Maximal blood concentration found was 41.0 ppm. The uptake of BPA in the normal brain was considerably lower than that in the blood and tumour tissue. No change in glucose metabolism was observed. The extracellular level of glycerol was increased after treatment in tumour tissue but not in normal brain suggesting a selective acute cytotoxic effect of BNCT on tumour cells.

Biodistribution studies of boronophenylalanine-fructose in melanoma and brain tumor patients in Argentina

A study of the (10)B-enriched p-boronophenylalanine-fructose complex ((10)BPA-F) infusion procedure in potential BNCT patients, including four melanoma of extremities and two high-grade gliomas (glioblastoma and ganglioglioma) was performed. T/B and S/B ratios for (10)B concentrations in tumor (T), blood (B) and skin (S) were determined. The T/B ratio for the glioblastoma was in the 1.8-3.4 range. The ganglioglioma did not show any significant boron uptake. For the nodular metastasic melanoma T/B values were between 1.5 and 2.6 (average 2.1+/-0.4), corresponding to the lower limit of the mean values reported for different melanoma categories. This result might suggest a lower boron uptake for nodular metastasic melanomas. S/B was 1.5+/-0.4. An open two-compartment pharmacokinetic model was applied to predict the boron concentration during the course and at the end of a BNCT irradiation.

Boron biodistribution in Beagles after intravenous infusion of 4-dihydroxyborylphenylalanine–fructose complex

Boron biodistribution after intravenous infusion of 4-dihydroxyborylphenylalanine-fructose (BPA-F) complex was investigated in six dogs. Blood samples were evaluated during and following doses of 205 and 250 mg/kgbw BPA in a 30 min infusion, and 500 mg/kgbw in a 1h infusion. Samples from whole blood, urine, brain and other organs were analysed for boron content after varying times following the onset of infusion. The whole blood boron concentrations declined from 27 to 8.4 ppm over the period of 39-165 min after the onset of infusion and the levels increased from 1.9 to 12 ppm in the grey matter of the brain over the same period. The boron concentrations in whole blood decreased steadily, whereas the boron values in brain tissue rose steadily with time. It was concluded that whole blood boron concentrations do not seem to reflect accurately the boron concentration in brain tissue at respective time points.

Biodistribution of GB-10 (Na(2)(10)B10H10 compound for boron neutron capture therapy (BNCT) in an experimental model of oral cancer in the hamster cheek pouch

OBJECTIVE

We previously proposed the hamster cheek pouch model of oral cancer for BNCT studies. We herein present the biodistribution of a non-toxic boron compound, GB-10 (Na(2)(10)B10H10), in this model to assess its potential for BNCT or BNCT enhanced Fast Neutron Therapy.

MATERIALS AND METHODS

We evaluated the uptake and retention of GB-10 in tumour and precancerous tissue and in potentially dose-limiting, clinically relevant normal tissues.

RESULTS

Mean tumour boron concentration delivered by GB-10 (50mgB/kg) peaked to 77.7+/-28.0 ppm at 20min post-administration and remained at therapeutically useful values of 31.9+/-21.4 ppm at 3h. The clearance rate for normal tissues was faster than for tumour tissue. The consistently low brain and spinal cord values would preclude normal tissue toxicity. The uptake of GB-10 by precancerous tissue may be of potential use in the treatment of field cancerized areas. GB-10 was deposited homogeneously in different tumour areas, an asset when treating heterogenous tumours. The data suggests that the joint administration of BPA and GB-10 may improve the therapeutic efficacy of BNCT.

CONCLUSIONS

GB-10 is a potential boron carrier for BNCT of head and neck tumours and for BNCT-FNT.

Boron neutron capture therapy: cellular targeting of high linear energy transfer radiation

Boron neutron capture therapy (BNCT) is based on the preferential targeting of tumor cells with (10)B and subsequent activation with thermal neutrons to produce a highly localized radiation. In theory, it is possible to selectively irradiate a tumor and the associated infiltrating tumor cells with large single doses of high-LET radiation while sparing the adjacent normal tissues. The mixture of high- and low-LET dose components created in tissue during neutron irradiation complicates the radiobiology of BNCT. Much of the complexity has been unravelled through a combination of preclinical experimentation and clinical dose escalation experience. Over 350 patients have been treated in a number of different facilities worldwide. The accumulated clinical experience has demonstrated that BNCT can be delivered safely but is still defining the limits of normal brain tolerance. Several independent BNCT clinical protocols have demonstrated that BNCT can produce median survivals in patients with glioblastoma that appear to be equivalent to conventional photon therapy. This review describes the individual components and methodologies required for effect BNCT: the boron delivery agents; the analytical techniques; the neutron beams; the dosimetry and radiation biology measurements; and how these components have been integrated into a series of clinical studies. The single greatest weakness of BNCT at the present time is non-uniform delivery of boron into all tumor cells. Future improvements in BNCT effectiveness will come from improved boron delivery agents, improved boron administration protocols, or through combination of BNCT with other modalities.

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 approximately 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 1200 mg/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 500 mg/kg (27 mg 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.

Boron neutron capture therapy for glioblastoma multiforme: clinical studies in Sweden

A boron neutron capture therapy (BNCT) facility has been constructed at Studsvik, Sweden. It includes two filter/moderator configurations. One of the resulting neutron beams has been optimized for clinical irradiations with a filter/moderator system that allows easy variation of the neutron spectrum from the thermal to the epithermal energy range. The other beam has been designed to produce a large uniform field of thermal neutrons for radiobiological research. Scientific operations of the Studsvik BNCT project are overseen by the Scientific Advisory Board comprised of representatives of major universities in Sweden. Furthermore, special task groups for clinical and preclinical studies have been formed to facilitate collaboration with academia. The clinical Phase II trials for glioblastoma are sponsored by the Swedish National Neuro-Oncology Group and, presently, involve a protocol for BNCT treatment of glioblastoma patients who have not received any therapy other than surgery. In this protocol, p-boronophenylalanine (BPA), administered as a 6-h intravenous infusion, is used as the boron delivery agent. As of January 2002, 17 patients were treated. The 6-h infusion of 900 mg BPA/kg body weight was shown to be safe and resulted in the average blood-boron concentration of 24 microg/g (range: 15-32 microg/g) at the time of irradiation (approximately 2-3 h post-infusion). Peak and average weighted radiation doses to the brain were in the ranges of 8.0-15.5 Gy(W) and 3.3-6.1 Gy(W), respectively. So far, no severe BNCT-related acute toxicities have been observed. Due to the short follow-up time, it is too early to evaluate the efficacy of these studies.

Cell cycle dependence of boron uptake from two boron compounds used for clinical neutron capture therapy

In neutron capture therapy, it is important that the boron is selectively uptaken by tumor cells. In the present study, we used flow cytometry to sort the cells in the G0/G1 phase and those in the G2/M phase, and the boron concentration in each fraction was measured with inductively coupled plasma atomic emission spectroscopy. The results revealed that sodium borocaptate and boronophenylalanine (BPA), were associated with higher rates of boron uptake in the G2/M than in the G0/G1 phase. However, the difference was more prominent in the case of BPA. The G2/M:G0/G1 ratio decreased as a function of exposure time in BPA containing culture medium, thereby indicating the cell cycle dependency of BPA uptake. Such heterogeneity of boron uptake by tumor cells should be considered for microdosimetry.

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.

Improved treatment planning for boron neutron capture therapy for glioblastoma multiforme using fluorine-18 labeled boronophenylalanine and positron emission tomography

Boron neutron capture therapy (BNCT) is a cancer brachytherapy based upon the thermal neutron reaction: 10B(n,alpha)7Li. The efficacy of the treatment depends primarily upon two conditions being met: (a) the preferential concentration of a boronated compound in the neoplasm and (b) an adequate fluence of thermal neutrons delivered to the neoplasm. The boronated amino acid, para-boronophenylalanine (BPA), is the agent widely used in clinical trials to deliver 10B to the malignancy. Positron emission tomography (PET) can be used to generate in vivo boron distribution maps by labeling BPA with the positron emitting nuclide fluorine-18. The incorporation of the PET-derived boron distribution maps into current treatment planning protocols is shown to provide improved treatment plans. Using previously established protocols, six patients with glioblastoma had 18BPA PET scans. The PET distribution maps obtained were used in the conventional BNCT treatment codes. The isodose curves derived from the PET data are shown to differ both qualitatively and quantitatively from the conventional isodose curves that were derived from calculations based upon the assumption of uniform uptake of the pharmaceutical in tumor and normal brain regions. The clinical course of each of the patients who eventually received BNCT (five of the six patients) was compared using both sets of isodose calculations. The isodose contours based upon PET derived distribution data appear to be more consistent with the patients' clinical course.

Atomic Emission Method for Total Boron in Blood during Neutron-Capture Therapy

Background: Boron neutron-capture therapy (BNCT) is a drug-targeted binary radiotherapy for cancer. The 10B capture of thermal neutrons induces secondary radiation within cells during irradiation. The most widely used boron carrier is 4-dihydroxyborylphenylalanine (BPA). The duration and timing of the irradiation is adjusted by monitoring 10B concentrations in whole blood.

Methods: We developed a new method for boron determination that uses inductively coupled plasma atomic emission spectrometry (ICP-AES) and protein removal with trichloroacetic acid before analysis. This method was compared with the established but tedious inductively coupled plasma mass spectrometry (ICP-MS), which uses wet ashing as sample pretreatment. Erythrocyte boron concentrations were determined indirectly on the basis of plasma and whole blood boron concentrations and the hematocrit. The hematocrit was determined indirectly by measuring calcium concentrations in plasma and whole blood.

Results: Within- and between-day CVs were &lt;5%. The recoveries for boron in whole blood were 95.6–96.2%. A strong correlation was found between results of the ICP-AES and ICP-MS (r = 0.994). Marked differences in plasma and erythrocyte boron concentrations were observed during and after infusion of BPA fructose complex.

Conclusions: The present method is feasible, accurate, and one of the fastest for boron determination during BNCT. Our results indicate that it is preferable to determine boron in plasma and in whole blood. Indirect erythrocyte-boron determination thus becomes possible and avoids the impact of preanalytical confounding factors, such as the influence of the hematocrit of the patient. Such an approach enables a more reliable estimation of the irradiation dose.

Boronated dipeptide borotrimethylglycylphenylalanine as a potential boron carrier in boron neutron capture therapy for malignant brain tumors

Takagaki, M., Ono, K., Masunaga, S-I., Kinashi, Y., Oda, Y., Miyatake, S-I., Hashimoto, N., Powell, W., Sood, A. and Spielvogel, B. F. Boronated Dipeptide Borotrimethylglycylphenylalanine as a Potential Boron Carrier in Boron Neutron Capture Therapy for Malignant Brain Tumors. Radiat. Res. 156, 118-122 (2001).A boronated dipeptide, borotrimethylglycylphenylalanine (BGPA), was synthesized as a possible boron carrier for boron neutron capture therapy (BNCT) for malignant brain tumors. In vitro, at equal concentrations of (10)B in the extracellular medium, BGPA had the same effect in BNCT as p-boronophenylalanine (BPA). Boron analysis was carried out using prompt gamma-ray spectrometry and track-etch autoradiography. The tumor:blood and tumor:normal brain (10)B concentration ratios were 8.9 +/- 2.1 and 3.0 +/- 1.2, respectively, in rats bearing intracranial C6 gliosarcomas using alpha-particle track autoradiography. The IC(50), i.e. the dose capable of inhibiting the growth of C6 gliosarcoma cells by 50% after 3 days of incubation, was 5.9 x 10(-3) M BGPA, which is similar to that of 6.4 x 10(-3) M for BPA. The amide bond of BGPA is free from enzymatic attack, since it is protected from hydrolysis by the presence of a boron atom at the alpha-carbon position of glycine. These results suggest promise for the use of this agent for BNCT of malignant brain tumors. Further preclinical studies of BGPA are warranted, since BGPA has advantages over both BPA and BSH.

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 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.

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.