Boron neutron capture therapy (BNCT) for glioblastoma multiforme: A phase II study evaluating a prolonged high-dose of boronophenylalanine (BPA)

Translational research of boron neutron capture therapy for spinal cord gliomas using rat model

Boron neutron capture therapy (BNCT) is a type of targeted particle radiation therapy with potential applications at the cellular level. Spinal cord gliomas (SCGs) present a substantial challenge owing to their poor prognosis and the lack of effective postoperative treatments. This study evaluated the efficacy of BNCT in a rat SCGs model employing the Basso, Beattie, and Bresnahan (BBB) scale to assess postoperative locomotor activity. We confirmed the presence of adequate in vitro boron concentrations in F98 rat glioma and 9L rat gliosarcoma cells exposed to boronophenylalanine (BPA) and in vivo tumor boron concentration 2.5 h after intravenous BPA administration. In vivo neutron irradiation significantly enhanced survival in the BNCT group when compared with that in the untreated group, with a minimal BBB scale reduction in all sham-operated groups. These findings highlight the potential of BNCT as a promising treatment option for SCGs.

64Cu-Labeled Boron-Containing Cyclic RGD Peptides for BNCT and PET Imaging

The burgeoning interest in developing boron neutron capture therapy (BNCT) tracers and their accompanying diagnostics for the treatment of recalcitrant tumors has prompted this investigation. Our study aims to devise a tumor treatment strategy utilizing BNCT to target the αvβ3 integrin. To this end, we propose a pioneering boron-infused cyclic Arg-Gly-Asp (RGD) peptide, cRGD(d-BPA)K, designed as an efficacious BNCT tracer. Additionally, we introduce its diagnostic complement, DOTA-cRGD(d-BPA)K, tailored for positron emission tomography (PET) to visualize αvβ3 expressed tumors. Radiolabeling [64Cu]Cu-DOTA-cRGD(d-BPA)K (64Cu-1) resulted in a high radiochemical yield and purity. The radiotracer exhibited exceptional in vitro stability and demonstrated significant uptake in U87MG tumors via PET imaging. Biodistribution analysis using compound 2 showed a 7.0 ppm accumulation of boron in the U87MG tumor 1 h post-intravenous injection. Furthermore, compound 2 displayed superior tumor/blood (2.41) and tumor/muscle (2.46) ratios compared to the clinically approved l-BPA-fructose. Both compound 2 and its diagnostic counterpart 64Cu-1 hold potential for BNCT and cancer diagnosis, respectively, via molecular imaging.

Integrating Physiotherapy for Enhancing Functional Recovery in Glioblastoma Multiforme: A Case Report

Glioblastoma is the most prevalent primary brain tumor. Because glioblastomas are very vascular, they may worsen the disease's neurologic symptoms by causing vasogenic brain edema and mass effects with a wide range of other symptoms. In this case report, a 42-year-old male complaining of severe headache, generalized weakness, and forgetfulness was brought to a territory care hospital, where a detailed neurological examination and investigations with magnetic resonance imaging (MRI) revealed a grade IV (high-grade) glioma at the right frontotemporal and capsuloganglionic regions of the brain, and was suggested for surgery. Postoperatively, the patient was referred for chemotherapy, but due to severe weakness, fatigue, and motor deficits, he was referred for physiotherapy. Follow-up was conducted to monitor the patient's progression using various outcome measures. These measures included the Functional Independence Measure (FIM), the Intensive Care Unit (ICU) Mobility Scale, the Glasgow Coma Scale (GCS), the modified Rankin Scale (mRS), and the Karnofsky Performance Status (KPS) Scale. Significant improvement was observed in the patient's symptoms, as tracked by these outcome measures. Therefore, it is important that a tailored rehabilitation protocol of six weeks was planned, focusing on palliative care and some symptoms of weakness, reduced strength, tone, and breathlessness to prevent secondary complications like deep vein thrombosis, irritability, anxiety, forgetfulness, decreased balance, and coordination in sitting. Since the prognosis of grade IV glioblastoma is poor, the goal-oriented rehabilitation program will help improve the palliative status and the overall quality of life of the patient.

Nonclinical pharmacodynamics of boron neutron capture therapy using direct intratumoral administration of a folate receptor targeting novel boron carrier

Background

Boron neutron capture therapy (BNCT) is a precise particle radiation therapy known for its unique cellular targeting ability. The development of innovative boron carriers is crucial for the advancement of BNCT technologies. Our previous study demonstrated the potential of PBC-IP administered via convection-enhanced delivery (CED) in an F98 rat glioma model. This approach significantly extended rat survival in neutron irradiation experiments, with half achieving long-term survival, akin to a cure, in a rat brain tumor model. Our commitment to clinical applicability has spurred additional nonclinical pharmacodynamic research, including an investigation into the effects of cannula position and the time elapsed post-CED administration.

Methods

In comprehensive in vivo experiments conducted on an F98 rat brain tumor model, we meticulously examined the boron distribution and neutron irradiation experiments at various sites and multiple time intervals following CED administration.

Results

The PBC-IP showed substantial efficacy for BNCT, revealing minimal differences in tumor boron concentration between central and peripheral CED administration, although a gradual decline in intratumoral boron concentration post-administration was observed. Therapeutic efficacy remained robust, particularly when employing cannula insertion at the tumor margin, compared to central injections. Even delayed neutron irradiation showed notable effectiveness, albeit with a slightly reduced survival period. These findings underscore the robust clinical potential of CED-administered PBC-IP in the treatment of malignant gliomas, offering adaptability across an array of treatment protocols.

Conclusions

This study represents a significant leap forward in the quest to enhance BNCT for the management of malignant gliomas, opening promising avenues for clinical translation.

Boron in cancer therapeutics: An overview

Boron has become a crucial weapon in anticancer research due to its significant intervention in cell proliferation. Being an excellent bio-isosteric replacement of carbon, it has modulated the anticancer efficacy of various molecules in the development pipeline. It has elicited promising results through interactions with various therapeutic targets such as HIF-1α, steroid sulfatase, arginase, proteasome, etc. Since boron liberates alpha particles, it has a wide-scale application in Boron Neutron Capture therapy (BNCT), a radiotherapy that demonstrates selectivity towards cancer cells due to high boron uptake capacity. Significant advances in the medicinal chemistry of boronated compounds, such as boronated sugars, natural/unnatural amino acids, boronated DNA binders, etc., have been reported over the past few years as BNCT agents. In addition, boronated nanoparticles have assisted the field of bio-nano medicines by their usage in radiotherapy. This review exclusively focuses on the medicinal chemistry aspects, radiotherapeutic, and chemotherapeutic aspects of boron in cancer therapeutics. Emphasis is also given on the mechanism of action along with advantages over conventional therapies.

In Vivo Application of Carboranes for Boron Neutron Capture Therapy (BNCT): Structure, Formulation and Analytical Methods for Detection

Carboranes have emerged as one of the most promising boron agents in boron neutron capture therapy (BNCT). In this context, in vivo studies are particularly relevant, since they provide qualitative and quantitative information about the biodistribution of these molecules, which is of the utmost importance to determine the efficacy of BNCT, defining their localization and (bio)accumulation, as well as their pharmacokinetics and pharmacodynamics. First, we gathered a detailed list of the carboranes used for in vivo studies, considering the synthesis of carborane derivatives or the use of delivery system such as liposomes, micelles and nanoparticles. Then, the formulation employed and the cancer model used in each of these studies were identified. Finally, we examined the analytical aspects concerning carborane detection, identifying the main methodologies applied in the literature for ex vivo and in vivo analysis. The present work aims to identify the current strengths and weakness of the use of carboranes in BNCT, establishing the bottlenecks and the best strategies for future applications.

Improved Boron Neutron Capture Therapy Using Integrin αvβ3-Targeted Long-Retention-Type Boron Carrier in a F98 Rat Glioma Model

Integrin αvβ3 is more highly expressed in high-grade glioma cells than in normal tissues. In this study, a novel boron-10 carrier containing maleimide-functionalized closo-dodecaborate (MID), serum albumin as a drug delivery system, and cyclic arginine-glycine-aspartate (cRGD) that can target integrin αvβ3 was developed. The efficacy of boron neutron capture therapy (BNCT) targeting integrin αvβ3 in glioma cells in the brain of rats using a cRGD-functionalized MID-albumin conjugate (cRGD-MID-AC) was evaluated. F98 glioma cells exposed to boronophenylalanine (BPA), cRGD-MID-AC, and cRGD + MID were used for cellular uptake and neutron-irradiation experiments. An F98 glioma-bearing rat brain tumor model was used for biodistribution and neutron-irradiation experiments after BPA or cRGD-MID-AC administration. BNCT using cRGD-MID-AC had a sufficient cell-killing effect in vitro, similar to that with BNCT using BPA. In biodistribution experiments, cRGD-MID-AC accumulated in the brain tumor, with the highest boron concentration observed 8 h after administration. Significant differences were observed between the untreated group and BNCT using cRGD-MID-AC groups in the in vivo neutron-irradiation experiments through the log-rank test. Long-term survivors were observed only in BNCT using cRGD-MID-AC groups 8 h after intravenous administration. These findings suggest that BNCT with cRGD-MID-AC is highly selective against gliomas through a mechanism that is different from that of BNCT with BPA.

Construction of targeted 10B delivery agents and their uptake in gastric and pancreatic cancer cells

Boron Neutron Capture Therapy (BNCT) is a new binary radiation therapy for tumor tissue, which kills tumor cells with neutron capture reaction. Boron neutron capture therapy has become a technical means for glioma, melanoma, and other diseases has been included in the clinical backup program. However, BNCT is faced with the key problem of developing and innovating more efficient boron delivery agents to solve the targeting and selectivity. We constructed a tyrosine kinase inhibitor-L-p-boronophenylalanine (TKI-BPA) molecule, aiming to improve the selectivity of boron delivery agents by conjugating targeted drugs while increasing the molecular solubility by adding hydrophilic groups. It shows excellent selectivity in differential uptake of cells, and its solubility is more than 6 times higher than BPA, leading to the saving of boron delivery agents. This modification method is effective for improving the efficiency of the boron delivery agent and is expected to become a potential alternative with high clinical application value.

Multi-Targeted Neutron Capture Therapy Combined with an 18 kDa Translocator Protein-Targeted Boron Compound Is an Effective Strategy in a Rat Brain Tumor Model

BACKGROUND

Boron neutron capture therapy (BNCT) has been adapted to high-grade gliomas (HG); however, some gliomas are refractory to BNCT using boronophenylalanine (BPA). In this study, the feasibility of BNCT targeting the 18 kDa translocator protein (TSPO) expressed in glioblastoma and surrounding environmental cells was investigated.

METHODS

Three rat glioma cell lines, an F98 rat glioma bearing brain tumor model, DPA-BSTPG which is a boron-10 compound targeting TSPO, BPA, and sodium borocaptate (BSH) were used. TSPO expression was evaluated in the F98 rat glioma model. Boron uptake was assessed in three rat glioma cell lines and in the F98 rat glioma model. In vitro and in vivo neutron irradiation experiments were performed.

RESULTS

DPA-BSTPG was efficiently taken up in vitro. The brain tumor has 16-fold higher TSPO expressions than its brain tissue. The compound biological effectiveness value of DPA-BSTPG was 8.43 to F98 rat glioma cells. The boron concentration in the tumor using DPA-BSTPG convection-enhanced delivery (CED) administration was approximately twice as high as using BPA intravenous administration. BNCT using DPA-BSTPG has significant efficacy over the untreated group. BNCT using a combination of BPA and DPA-BSTPG gained significantly longer survival times than using BPA alone.

CONCLUSION

DPA-BSTPG in combination with BPA may provide the multi-targeted neutron capture therapy against HG.

Evaluation of a Tumor-Targeting Oligosaccharide Nanosystem in BNCT on an Orthotopic Hepatocellular Carcinoma Model

Boron neutron capture therapy (BNCT) is becoming a promising radiation treatment technique dealing with tumors due to its cellular targeting specificity. In this article, based on the biocompatible chitosan oligosaccharide (COS), we designed a boron delivery system using carborane (CB) as a boron drug with cRGD peptide modification and paclitaxel (PTX) loaded in the hydrophobic core. The nanoparticles (cRGD-COS-CB/PTX) realized the boron delivery into tumor sites with an enhanced permeability and retention (EPR) effect and an active targeting effect achieved by the cRGD-integrin interaction on the surface of tumor cells. The uniform spherical nanoparticles can be selectively taken by hepatoma cells rather than normal hepatocytes. In vivo experiments showed that the nanoparticles had a targeting effect on tumor sites in both subcutaneous and orthotopic tumor models, which was an encouraging result for radiotherapy for liver cancer. To sum up, the nanoparticles we produced proved to be promising dual-functionalized nanoparticles for radiotherapy and chemotherapy.

Analyzing spatial distribution between 18F-fluorodeoxyglucose and 18F-boronophenylalanine positron emission tomography to investigate selection indicators for boron neutron capture therapy

BACKGROUND

¹⁸F-FDG PET is often utilized to determine BNCT selection due to the limited availability of ¹⁸F-BPA PET, which is performed by synthesizing ¹⁸F into the boron drug used for BNCT, although the uptake mechanisms between those are different. Additionally, only a few non-spatial point parameters, such as maximum SUV (SUV_max), have reported a correlation between those in previous studies. This study aimed to investigate the spatial accumulation pattern between those PET images in tumors, which would be expected to either show higher uptake on ¹⁸F-BPA PET or be utilized in clinical, to verify whether ¹⁸F-FDG PET could be used as a selection indicator for BNCT.

METHODS

A total of 27 patients with 30 lesions (11 squamous cell carcinoma, 9 melanoma, and 10 rhabdomyosarcoma) who received ¹⁸F-FDG and ¹⁸F-BPA PET within 2 weeks were enrolled in this study. The ratio of metabolic tumor volumes (MTVs) to GTV, histogram indices (skewness/kurtosis), and the correlation of total lesion activity (TLA) and non-spatial point parameters (SUV_max, SUV_peak, SUV_min, maximum tumor-to-normal tissue ratio (T_max/N), and T_min/N) were evaluated. After local rigid registration between those images, distances of locations at SUV_max and the center of mass with MTVs on each image and similarity indices were also assessed along its coordinate.

RESULTS

In addition to SUV_max, SUV_peak, and T_max/N, significant correlations were found in TLA. The mean distance in SUV_max was [Formula: see text] and significantly longer than that in the center of mass with MTVs. The ratio of MTVs to GTV, skewness, and kurtosis were not significantly different. However, the similarities of MTVs were considerably low. The similarity indices of Dice similarity coefficient, Jaccard coefficient, and mean distance to agreement for MTV40 were [Formula: see text], [Formula: see text], and [Formula: see text] cm, respectively. Furthermore, it was worse in MTV50. In addition, spatial accumulation patterns varied in cancer types.

CONCLUSIONS

Spatial accumulation patterns in tumors showed low similarity between ¹⁸F-FDG and ¹⁸F-BPA PET, although the various non-spatial point parameters were correlated. In addition, the spatial accumulation patterns were considerably different in cancer types. Therefore, the selection for BNCT using ¹⁸F-FDG PET should be compared carefully with using ¹⁸F-FBPA PET.

Boron Vehiculating Nanosystems for Neutron Capture Therapy in Cancer Treatment

Boron neutron capture therapy is a low-invasive cancer therapy based on the neutron fission process that occurs upon thermal neutron irradiation of ¹⁰B-containing compounds; this process causes the release of alpha particles that selectively damage cancer cells. Although several clinical studies involving mercaptoundecahydro-closo-dodecaborate and the boronophenylalanine-fructose complex are currently ongoing, the success of this promising anticancer therapy is hampered by the lack of appropriate drug delivery systems to selectively carry therapeutic concentrations of boron atoms to cancer tissues, allowing prolonged boron retention therein and avoiding the damage of healthy tissues. To achieve these goals, numerous research groups have explored the possibility to formulate nanoparticulate systems for boron delivery. In this review. we report the newest developments on boron vehiculating drug delivery systems based on nanoparticles, distinguished on the basis of the type of carrier used, with a specific focus on the formulation aspects.

Boron Neutron Capture Therapy: Clinical Application and Research Progress

Boron neutron capture therapy (BNCT) is a binary modality that is used to treat a variety of malignancies, using neutrons to irradiate boron-10 (¹⁰B) nuclei that have entered tumor cells to produce highly linear energy transfer (LET) alpha particles and recoil ⁷Li nuclei (¹⁰B [n, α] ⁷Li). Therefore, the most important part in BNCT is to selectively deliver a large number of ¹⁰B to tumor cells and only a small amount to normal tissue. So far, BNCT has been used in more than 2000 cases worldwide, and the efficacy of BNCT in the treatment of head and neck cancer, malignant meningioma, melanoma and hepatocellular carcinoma has been confirmed. We collected and collated clinical studies of second-generation boron delivery agents. The combination of different drugs, the mode of administration, and the combination of multiple treatments have an important impact on patient survival. We summarized the critical issues that must be addressed, with the hope that the next generation of boron delivery agents will overcome these challenges.

DNA damage and biological responses induced by Boron Neutron Capture Therapy (BNCT)

Boron Neutron Capture Therapy (BNCT) is a tumor cell selective high LET (linear energy transfer) particle beam therapy. The patient is administrated a boron (¹⁰B) compound via intravenous injection or infusion, and when ¹⁰B is sufficiently accumulated in the tumor, neutron beams containing epithermal neutrons as the main component are irradiated. Epithermal neutrons lose energy in the body and become thermal neutrons. The captured ¹⁰B undergoes a (n, α) reaction with thermal neutrons, and the resulting α particles and ⁷Li nuclei have short ranges of 9-10μm and 4-5μm, respectively, and do not reach the surrounding cells in normal tissues. Therefore, these high LET-heavy charged particles can selectively kill cancer cells. The cell-killing effect of these heavy charged particles is thought to be triggered by DNA damage. It is known that DNA damage caused by heavy charged particles is more serious and difficult to repair than DNA damage caused by Low LET radiation such as X-rays and γ-rays. This review focuses on DNA damage, e.g., DNA strand breaks and DNA damage repair caused by BNCT and describes the resulting biological response.

Strategies for Preclinical Studies Evaluating the Biological Effects of an Accelerator-Based Boron Neutron Capture Therapy System

This review discusses the strategies of preclinical studies intended for accelerator-based (AB)-boron neutron capture therapy (BNCT) clinical trials, which were presented at the National Cancer Institute (NCI) Workshop on Neutron Capture Therapy held from April 20 to 22, 2022. Clinical studies of BNCT have been conducted worldwide using reactor neutron sources, with most targeting malignant brain tumors, melanoma, or head and neck cancer. Recently, small accelerator-based neutron sources that can be installed in hospitals have been developed. AB-BNCT clinical trials for recurrent malignant glioma, head and neck cancers, high-grade meningioma, melanoma, and angiosarcoma have all been conducted in Japan. The necessary methods, equipment, and facilities for preclinical studies to evaluate the biological effects of AB-BNCT systems in terms of safety and efficacy are described, with reference to two examples from Japan. The first is the National Cancer Center, which is equipped with a vertical downward neutron beam, and the other is the University of Tsukuba, which has a horizontal neutron beam. The preclinical studies discussed include cell-based assays to evaluate cytotoxicity and genotoxicity, in vivo cytotoxicity and efficacy of BNCT, and radioactivation measurements.

Evaluation of 3-Borono-l-Phenylalanine as a Water-Soluble Boron Neutron Capture Therapy Agent

Although 4-borono-l-phenylalanine (4-BPA) is currently the only marketed agent available for boron neutron capture therapy (BNCT), its low water solubility raises concerns. In this study, we synthesized 3-borono-l-phenylalanine (3-BPA), a positional isomer of 4-BPA, with improved water solubility. We further evaluated its physicochemical properties, tumor accumulation, and biodistribution. The water solubility of 3-BPA was 125 g/L, which is more than 100 times higher than that of 4-BPA. Due to the high water solubility, we prepared the administration solution of 3-BPA without a solubilizer sugar, which is inevitably added to 4-BPA preparation and has adverse effects. In in vitro and in vivo experiments, boron accumulation in cancers after administration was statistically equivalent in both sugar-complexed 3-BPA and 4-BPA. Furthermore, the biodistribution of 3-BPA was comparable with that of sugar-complexed 3-BPA. Since 3-BPA has high water solubility and tumor targetability equivalent to 4-BPA, 3-BPA can replace 4-BPA in future BNCT.

The use of radiosensitizing agents in the therapy of glioblastoma multiforme-a comprehensive review

BACKGROUND

Glioblastoma is the most common malignant brain tumor in human adults. Despite several improvements in resective as well as adjuvant therapy over the last decades, its overall prognosis remains poor. As a means of improving patient outcome, the possibility of enhancing radiation response by using radiosensitizing agents has been tested in an array of studies.

METHODS

A comprehensive review of clinical trials involving radiation therapy in combination with radiosensitizing agents on patients diagnosed with glioblastoma was performed in the National Center for Biotechnology Information's PubMed database.

RESULTS

A total of 96 papers addressing this matter were published between 1976 and 2021, of which 63 matched the subject of this paper. All papers were reviewed, and their findings discussed in the context of their underlining mechanisms of radiosensitization.

CONCLUSION

In the history of glioblastoma treatment, several approaches of optimizing radiation-effectiveness using radiosensitizers have been made. Even though several different strategies and agents have been explored, clear evidence of improved patient outcome is still missing. Tissue-selectiveness and penetration of the blood-brain barrier seem to be major roadblocks; nevertheless, modern strategies try to circumvent these obstacles, using novel sensitizers based on preclinical data or alternative ways of delivery.

Key biological mechanisms involved in high-LET radiation therapies with a focus on DNA damage and repair

DNA damage and repair studies are at the core of the radiation biology field and represent also the fundamental principles informing radiation therapy (RT). DNA damage levels are a function of radiation dose, whereas the type of damage and biological effects such as DNA damage complexity, depend on radiation quality that is linear energy transfer (LET). Both levels and types of DNA damage determine cell fate, which can include necrosis, apoptosis, senescence or autophagy. Herein, we present an overview of current RT modalities in the light of DNA damage and repair with emphasis on medium to high-LET radiation. Proton radiation is discussed along with its new adaptation of FLASH RT. RT based on α-particles includes brachytherapy and nuclear-RT, that is proton-boron capture therapy (PBCT) and boron-neutron capture therapy (BNCT). We also discuss carbon ion therapy along with combinatorial immune-based therapies and high-LET RT. For each RT modality, we summarise relevant DNA damage studies. Finally, we provide an update of the role of DNA repair in high-LET RT and we explore the biological responses triggered by differential LET and dose.

Therapeutic nucleus-access BNCT drug combined CD47-targeting gene editing in glioblastoma

Glioblastoma is the most common brain primary malignant tumor with the highest mortality. Boron neutron capture therapy (BNCT) can efficiently kill cancer cells on the cellular scale, with high accuracy, short course and low side-effects, which is regarded as the most promising therapy for malignant brain tumors like glioma. As the keypoint of BNCT, all boron delivery agents currently in clinical use are beset by insufficient tumor uptake, especially in the tumor nucleus, which limits the clinical application of BNCT. In this study, nuclear targeting of boron is achieved by DOX-CB, consisting of doxorubicin (DOX) and carborane (CB) utilizing the nuclear translocation property of DOX. The nucleus of GL261 cells takes up almost three times the concentration of boron required for BNCT. To further kill glioma and inhibit recurrence, a new multifunctional nanoliposome delivery system DOX-CB@lipo-pDNA-iRGD is constructed. It combines DOX-CB with immunotherapy strategy of blocking macrophage immune checkpoint pathway CD47-SIRPα by CRISPR-Cas9 system, coupling BNCT with immunotherapy simultaneously. Compared with clinical drug Borocaptate Sodium (BSH), DOX-CB@lipo-pDNA-iRGD significantly enhances the survival rate of tumor-bearing mice, reduces tumor stemness, and improves the prognosis. The excellent curative effect of this nanoliposome delivery system provides an insight into the combined treatment of BNCT.

Boron neutron capture therapy for urological cancers

Boron neutron capture therapy is based on a nuclear reaction between the nonradioactive isotope boron-10 and either low-energy thermal neutrons or high-energy epithermal neutrons, which generate high linear energy transfer α particles and a recoiled lithium nucleus (⁷ Li) that selectively destroys the DNA helix in tumor cells. Boron neutron capture therapy is an emerging procedure aimed at improving the therapeutic ratio for the traditional treatment of various malignancies, which has been studied clinically in a variety of diseases, including glioblastoma, head and neck cancer, cutaneous melanoma, hepatocellular carcinoma, lung cancer, and extramammary Paget's disease. However, boron neutron capture therapy has not been clinically performed for urological cancers, excluding genital extramammary Paget's disease that appeared at the scrotum to penis area. In this review, we aimed to provide an updated summary of the current clinical literature of patients treated with boron neutron capture therapy and to focus on the future prospects of boron neutron capture therapy for urological cancers.

Sweet Boron: Boron-Containing Sugar Derivatives as Potential Agents for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a binary type of radiotherapy for the treatment of cancer. Due to recent developments of neutron accelerators and their installation in some hospitals, BNCT is on the rise worldwide and is expected to have a significant impact on patient treatments. Therefore, there is an increasing need for improved boron delivery agents. Among the many small molecules and delivery systems developed, a significant amount of recent research focused on the synthesis of boron-containing sugar and amino acid derivatives to exploit specific transport proteins, as d-glucose transporter 1 (GLUT1) and large neutral amino acid transporter (LAT1), overexpressed by tumor cells. This review will discuss the last year’s achievements in the synthesis and some biological evaluation of boronated sugars derivatives. The compounds described in this review are intrinsically asymmetric due to the presence of chiral sugar moieties, often joined to boron clusters, which are structural elements with high symmetry.

In Vivo Accelerator-Based Boron Neutron Capture Therapy for Spontaneous Tumors in Large Animals: Case Series

(1) Background: accelerator-based neutron sources are a new frontier for BNCT but many technical issues remain. We aimed to study such issues and results in larger-animal BNCT (cats and dogs) with naturally occurring, malignant tumors in different locations as an intermediate step in translating current research into clinical practice. (2) Methods: 10 pet cats and dogs with incurable, malignant tumors that had no treatment alternatives were included in this study. A tandem accelerator with vacuum insulation was used as a neutron source. As a boron-containing agent, 10B-enriched sodium borocaptate (BSH) was used at a dose of 100 mg/kg. Animal condition as well as tumor progression/regression were monitored. (3) Results: regression of tumors in response to treatment, improvements in the overall clinical picture, and an increase in the estimated duration and quality of life were observed. Treatment-related toxicity was mild and reversible. (4) Conclusions: our study contributes to preparations for human BNCT clinical trials and suggests utility for veterinary oncology.

Boron neutron capture therapy using dodecaborated albumin conjugates with maleimide is effective in a rat glioma model

Introduction Boron neutron capture therapy (BNCT) is a biologically targeted, cell-selective particle irradiation therapy that utilizes the nuclear capture reaction of boron and neutron. Recently, accelerator neutron generators have been used in clinical settings, and expectations for developing new boron compounds are growing. Methods and Results In this study, we focused on serum albumin, a well-known drug delivery system, and developed maleimide-functionalized closo-dodecaborate albumin conjugate (MID-AC) as a boron carrying system for BNCT. Our biodistribution experiment involved F98 glioma-bearing rat brain tumor models systemically administered with MID-AC and demonstrated accumulation and long retention of boron. Our BNCT study with MID-AC observed statistically significant prolongation of the survival rate compared to the control groups, with results comparable to BNCT study with boronophenylalanine (BPA) which is the standard use of in clinical settings. Each median survival time was as follows: untreated control group; 24.5 days, neutron-irradiated control group; 24.5 days, neutron irradiation following 2.5 h after termination of intravenous administration (i.v.) of BPA; 31.5 days, and neutron irradiation following 2.5 or 24 h after termination of i.v. of MID-AC; 33.5 or 33.0 days, respectively. The biological effectiveness factor of MID-AC for F98 rat glioma was estimated based on these survival times and found to be higher to 12. This tendency was confirmed in BNCT 24 h after MID-AC administration. Conclusion MID-AC induces an efficient boron neutron capture reaction because the albumin contained in MID-AC is retained in the tumor and has a considerable potential to become an effective delivery system for BNCT in treating high-grade gliomas.

Response of Normal Tissues to Boron Neutron Capture Therapy (BNCT) with 10B-Borocaptate Sodium (BSH) and 10B-Paraboronophenylalanine (BPA)

Boron neutron capture therapy (BNCT) is a cancer-selective radiotherapy that utilizes the cancer targeting ¹⁰B-compound. Cancer cells that take up the compound are substantially damaged by the high liner energy transfer (LET) particles emitted mainly from the ¹⁰B(n, α⁷Li reaction. BNCT can minimize the dose to normal tissues, but it must be performed within the tolerable range of normal tissues. Therefore, it is important to evaluate the response of normal tissues to BNCT. Since BNCT yields a mixture of high and low LET radiations that make it difficult to understand the radiobiological basis of BNCT, it is important to evaluate the relative biological effectiveness (RBE) and compound biological effectiveness (CBE) factors for assessing the responses of normal tissues to BNCT. BSH and BPA are the only ¹⁰B-compounds that can be used for clinical BNCT. Their biological behavior and cancer targeting mechanisms are different; therefore, they affect the CBE values differently. In this review, we present the RBE and CBE values of BPA or BSH for normal tissue damage by BNCT irradiation. The skin, brain (spinal cord), mucosa, lung, and liver are included as normal tissues. The CBE values of BPA and BSH for tumor control are also discussed.

Novel Radiation Approaches

The standard of care treatment for glioblastoma is surgical resection followed by radiotherapy to 60 Gy with concurrent and adjuvant temozolomide with or without tumor-treating fields. Advanced imaging techniques are under evaluation to better guide radiotherapy target volume delineation and allow for dose escalation. Particle therapy, in the form of protons, carbon ions, and boron neutron capture therapy, are being assessed as strategies to improve the radiotherapeutic ratio. Stereotactic, hypofractionated, pulsed-reduced dose-rate, and particle radiotherapy are re-irradiation techniques each uniquely suited for different clinical scenarios. Novel radiotherapy approaches, such as FLASH, represent promising advancements in radiotherapy for glioblastoma.

Boron Neutron Capture Therapy: A Review of Clinical Applications

Boron neutron capture therapy (BNCT) is an emerging treatment modality aimed at improving the therapeutic ratio for traditionally difficult to treat tumors. BNCT utilizes boronated agents to preferentially deliver boron-10 to tumors, which, after undergoing irradiation with neutrons, yields litihium-7 and an alpha particle. The alpha particle has a short range, therefore preferentially affecting tumor tissues while sparing more distal normal tissues. To date, BNCT has been studied clinically in a variety of disease sites, including glioblastoma multiforme, meningioma, head and neck cancers, lung cancers, breast cancers, hepatocellular carcinoma, sarcomas, cutaneous malignancies, extramammary Paget's disease, recurrent cancers, pediatric cancers, and metastatic disease. We aim to provide an up-to-date and comprehensive review of the studies of each of these disease sites, as well as a review on the challenges facing adoption of BNCT.

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.

Boron analysis and imaging of cells with 2-hr BPA exposure by using micro-proton particle-induced gamma-ray emission (PIGE)

Particle-induced gamma-ray emission (PIGE), which measures prompt gamma rays at 428 keV from ¹⁰B (p, p'γ) ⁷Be, was used to confirm the boron distribution within 2hr-BPA-exposed cells. Distribution images of potassium, phosphate, and boron and the whole spectrum showed the ratios of boron counts to total (%) as follows: control group: 1.35 ± 0.073%; 2hr boron exposure group: 2.33 ± 0.35%; and boron exposure/wash group: 1.58 ± 0.095%. Micro-beam PIXE/PIGE can be a promising tool for visualization of intracellular Boron.

Assessment of long-term risks of secondary cancer in paediatric patients with brain tumours after boron neutron capture therapy

This study firstly explored the risks of secondary cancer in healthy organs of Chinese paediatric patients with brain tumours after boron neutron capture therapy (BNCT). Three neutron beam irradiation geometries (i.e. right lateral, top to bottom, posterior to anterior) were adopted in treating patients with brain tumours under the clinical environment of BNCT. The concerned organs in this study were those with high cancer morbidity in China (e.g. lung, liver and stomach). The equivalent doses for these organs were calculated using Monte Carlo and anthropomorphic paediatric phantoms with Chinese physiological features. The risk of secondary cancer, characterised by the lifetime attributable risk (LAR) factor given in the BEIR VII report, was compared among the three irradiation geometries. The results showed that the LAR was lower with the PA irradiation geometry than with the two other irradiation geometries when the 2 cm diameter tumour was at a depth of 6 cm on the right side of the brain. Under the PA irradiation geometry, the LAR in the organs increased with increasing tumour volume and depth because of the long irradiation time. As the patients aged from 10-15 years old, the LAR decreased, which was related to the increased patient height and shortened life expectancy. Female patients had a relatively higher risk of secondary cancer than male patients in this study, which could be due to the thinner body thickness and the weaker protective effect on the internal organs of the female patients. In conclusion, the risks of secondary cancer in organs were related to irradiation geometries, gender, and age, indicating that the risk of secondary cancer is a personalised parameter that needs to be evaluated before administering BNCT, especially in patients with large or deep tumours.

Glioblastoma Treatment Modalities besides Surgery

Glioblastoma multiforme (GBM) is commonly known as the most aggressive primary CNS tumor in adults. The mean survival of it is 14 to 15 months, following the standard therapy from surgery, chemotherapy, to radiotherapy. Efforts in recent decades have brought many novel therapies to light, however, with limitations. In this paper, authors reviewed current treatments for GBM besides surgery. In the past decades, only radiotherapy, temozolomide (TMZ), and tumor treating field (TTF) were approved by FDA. Though promising in preclinical experiments, therapeutic effects of other novel treatments including BNCT, anti-angiogenic therapy, immunotherapy, epigenetic therapy, oncolytic virus therapy, and gene therapy are still either uncertain or discouraging in clinical results. In this review, we went through current clinical trials, underlying causes, and future therapy designs to present neurosurgeons and researchers a sketch of this field.

Synthesis and Initial Biological Evaluation of Boron-Containing Prostate-Specific Membrane Antigen Ligands for Treatment of Prostate Cancer Using Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a therapeutic modality which has been used for the treatment of cancers, including brain and head and neck tumors. For effective treatment via BNCT, efficient and selective delivery of a high boron dose to cancer cells is needed. Prostate-specific membrane antigen (PSMA) is a target for prostate cancer imaging and drug delivery. In this study, we conjugated boronic acid or carborane functional groups to a well-established PSMA inhibitor scaffold to deliver boron to prostate cancer cells and prostate tumor xenograft models. Eight boron-containing PSMA inhibitors were synthesized. All of these compounds showed a strong binding affinity to PSMA in a competition radioligand binding assay (IC₅₀ from 555.7 to 20.3 nM). Three selected compounds 1a, 1d, and 1f were administered to mice, and their in vivo blocking of ⁶⁸Ga-PSMA-11 uptake was demonstrated through a positron emission tomography (PET) imaging and biodistribution experiment. Biodistribution analysis demonstrated boron uptake of 4-7 μg/g in 22Rv1 prostate xenograft tumors and similar tumor/muscle ratios compared to the ratio for the most commonly used BNCT compound, 4-borono-l-phenylalanine (BPA). Taken together, these data suggest a potential role for PSMA targeted BNCT agents in prostate cancer therapy following suitable optimization.

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.

Boron neutron capture therapy (BNCT): a unique role in radiotherapy with a view to entering the accelerator-based BNCT era

Boron neutron capture therapy (BNCT) has a unique property of tumor-cell-selective heavy-particle irradiation. BNCT can form large dose gradients between cancer cells and normal cells, even if the two types of cells are mingled at the tumor margin. This property makes it possible for BNCT to be used for pre-irradiated locally recurrent tumors. Shallow-seated, locally recurrent lesions have been treated with BNCT because of the poor penetration of neutrons in the human body. BNCT has been used in clinical studies for recurrent malignant gliomas and head and neck cancers using neutron beams derived from research reactors, although further investigation is warranted because of the small number of patients. In the latter part of this review, the development of accelerator-based neutron sources is described. BNCT for common cancers will become available at medical institutes that are equipped with an accelerator-based BNCT system. Multiple metastatic lung tumors have been investigated as one of the new treatment candidates because BNCT can deliver curative doses of radiation to the tumors while sparing normal lung tissue. Further basic and clinical studies are needed to move toward an era of accelerator-based BNCT when more patients suffering from refractory cancers will be treated.

Fe₃O₄ Nanoparticles for Complex Targeted Delivery and Boron Neutron Capture Therapy

Magnetic Fe₃O₄ nanoparticles (NPs) and their surface modification with therapeutic substances are of great interest, especially drug delivery for cancer therapy, including boron-neutron capture therapy (BNCT). In this paper, we present the results of boron-rich compound (carborane borate) attachment to previously aminated by (3-aminopropyl)-trimethoxysilane (APTMS) iron oxide NPs. Fourier transform infrared spectroscopy with Attenuated total reflectance accessory (ATR-FTIR) and energy-dispersive X-ray analysis confirmed the change of the element content of NPs after modification and formation of new bonds between Fe₃O₄ NPs and the attached molecules. Transmission (TEM) and scanning electron microscopy (SEM) showed Fe₃O₄ NPs’ average size of 18.9 nm. Phase parameters were studied by powder X-ray diffraction (XRD), and the magnetic behavior of Fe₃O₄ NPs was elucidated by Mössbauer spectroscopy. The colloidal and chemical stability of NPs was studied using simulated body fluid (phosphate buffer—PBS). Modified NPs have shown excellent stability in PBS (pH = 7.4), characterized by XRD, Mössbauer spectroscopy, and dynamic light scattering (DLS). Biocompatibility was evaluated in-vitro using cultured mouse embryonic fibroblasts (MEFs). The results show us an increasing of IC₅₀ from 0.110 mg/mL for Fe₃O₄ NPs to 0.405 mg/mL for Fe₃O₄-Carborane NPs. The obtained data confirm the biocompatibility and stability of synthesized NPs and the potential to use them in BNCT.

Boron-incorporating hemagglutinating virus of Japan envelope (HVJ-E) nanomaterial in boron neutron capture therapy

Combining immunotherapeutic and radiotherapeutic technique has recently attracted much attention for advancing cancer treatment. If boron-incorporated hemagglutinating virus of Japan-envelope (HVJ-E) having high membrane fusion ability can be used as a boron delivery agent in boron neutron capture therapy (BNCT), a radical synergistic improvement of boron accumulation efficiency into tumor cells and antitumor immunity may be induced. In this study, we aimed to develop novel boron-containing biocompatible polymers modified onto HVJ-E surfaces. The copolymer consisting of 2-methacryloyloxyethyl phosphorylcholine (MPC) and methacrylamide benzoxaborole (MAAmBO), poly[MPC-co-MAAmBO], was successfully synthesized by using a simple free radical polymerization. The molecular structures and molecular weight of the poly[MPC-co-MAAmBO] copolymer were characterized by nuclear magnetic resonance and matrix-assisted laser desorption ionization time-of-flight mass spectrometry, respectively. The poly[MPC-co-MAAmBO] was coated onto the HVJ-E surface via the chemical bonding between the MAAmBO moiety and the sugar moiety of HVJ-E. DLS, AFM, UV-Vis, and fluorescence measurements clarified that the size of the poly[MPC-co-MAAmBO]-coated HVJ-E, HVJ-E/p[MPC-MAAmBO], to be about 130 ~ 150 nm in diameter, and that the polymer having 9.82 × 106 ~ 7 boron atoms was steadily coated on a single HVJ-E particle. Moreover, cellular uptake of poly[MPC-co-MAAmBO] could be demonstrated without cytotoxicity, and the hemolysis could be successfully suppressed by 20%. These results indicate that the HVJ-E/p[MPC-MAAmBO] may be used as boron nanocarriers in a combination of immunotherapy with BNCT.

Impact on Glioblastoma U87 Cell Gene Expression of a Carborane Cluster-Bearing Amino Acid: Implications for Carborane Toxicity in Mammalian Cells

Carboranes have been extensively investigated as potential drugs for the treatment of malignant human brain tumors by boron neutron capture therapy (BNCT). This noninvasive treatment modality utilizes compounds containing the nonradioactive isotope ¹⁰B which has a high propensity to capture slow neutrons. In response, it emits high energy α-particles that kill the cell. We have successfully synthesized a boron delivery agent by installing a boron-rich m-carborane within the amino acid cysteine. Rapid uptake of this compound into U87 glioblastoma cells within 5 min of exposure was observed, and fluorescence microscopy studies showed that it was retained intracellularly after 48 h. In the absence of thermal neutrons, a cytostatic effect in U87 cells was observed at exposures ranging from 1 μM to 1 mM relative to the control, while no change was observed at 1-0.01 μM. Microarray studies unveiled a wide range of unique changes in the gene expression profile of the U87 cells, particularly for the genes associated with cell cycle, which were observed to be greatly suppressed after treatment with the compound. These results were validated by qPCR studies. Although the compound was designed for BNCT, its distinctive impacts on gene regulation reveal that it and other carborane-containing cluster molecules may exert unique heretofore unknown effects on the transcriptome, even in the absence of applied radiation.

Synthesis and Evaluation of Thermoresponsive Boron-Containing Poly(N-isopropylacrylamide) Diblock Copolymers for Self-Assembling Nanomicellar Boron Carriers

Development of new boron nanocarriers has been a crucial issue to be solved for advancing boron neutron capture therapy (BNCT) as an effective radiation treatment for cancers. The present study aimed to create a novel double-thermoresponsive boron-containing diblock copolymer based on poly(N-isopropylacrylamide) [poly(NIPAAm)], which exhibits two-step phase transitions (morphological transitions) at the temperature region below human body temperature. The boronated diblock copolymer considerably concentrates boron atoms into the water-dispersible (i.e., intravenous-administration possible) nanomicelles self-assembled by the first phase transition, and furthermore the properly controlled size and hydrophobicity of the second phase-transitioned nanoparticles are expected to make a significant contribution to the selective delivery and long-term retention of boron atoms into tumor tissues. Here we present the detailed synthesis of the strategic NIPAAm-based diblock copolymer with 3-acrylamidophenylboronic acid (PBA), i.e., poly(NIPAAm-block-NIPAAm-co-PBA), through a reversible addition-fragmentation chain transfer polymerization. Furthermore, the stepwise phase transition behavior of the obtained boronic-acid diblock copolymers was characterized in detail by temperature-variable ¹H and 11B-nuclear magnetic resonance spectroscopy. The phase-transition-induced molecular structural changes, including the structural compositions and sizes of nanomicelles and nanoparticles, are also discussed here.

Tracing Boron with Fluorescence and Positron Emission Tomography Imaging of Boronated Porphyrin Nanocomplex for Imaging-Guided Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) induces high-energy radiation within cancer cells while avoiding damage to normal cells without uptake of BNCT drugs, which is holding great promise to provide excellent control over locally invasive malignant tumors. However, lack of quantitative imaging technique to determine local boron concentration has been a great challenge for nuclear physicians to apply accurate neutron irradiation during the treatment, which is a key factor that has limited BNCT's application in clinics. To meet this challenge, this study describes coating boronated porphyrins with a biocompatible poly(lactide- co-glycolide)-monomethoxy-poly(polyethylene-glycol) (PLGA-mPEG) micelle for selective tumor accumulation and reduced toxicity comparing with the previously reported boronated porphyrin drugs. Fluorescence imaging and positron emission tomography (PET) imaging were performed, unveiling the potential imaging properties of this boronated porphyrin nanocomplex (BPN) to locate tumor region and to determine tissue-localized boron concentration which facilitates treatment planning. By studying the pharmacokinetics of BPN with Cu-64 PET imaging, the treatment plan was adjusted from single bolus injection to multiple times of injections of smaller doses. As expected, high tumor uptake of boron (125.17 ± 13.54 ppm) was achieved with an extraordinarily high tumor to normal tissue ratio: tumors to liver, muscle, fat, and blood were 3.24 ± 0.22, 61.46 ± 20.26, 31.55 ± 10.30, and 33.85 ± 5.73, respectively. At last, neutron irradiation with BPN showed almost complete tumor suppression, demonstrating that BPN holds a great potential for being an efficient boron delivery agent for imaging-guided BNCT.

Design and synthesis of novel organometallic complexes using boronated phenylalanine derivatives as potential anticancer agents

Drug design and discovery studies are important because of the prevalence of diseases without available medical cures. New anticancer agents are particularly urgent because of the high mortality rate associated with cancer. A series of mononuclear gold (III) and platinum (II) complexes based on boronated phenylalanine (BPA) were designed and synthesized using 4,4'-dimethyl-2,2'-dipyridyl (L1) or 1,10-phenanthroline-5,6-dion (L2) ligands to obtain promising anticancer drug candidates. Proton nuclear magnetic resonance, infrared, mass spectrometry, and elemental analyses were utilized for chemical characterizations. Cell viability, cancer cell colony formation, endothelial tube formation, and cytoskeleton staining assays were performed using A549 lung adenocarcinoma and human umbilical vein endothelial cells (HUVECs) to investigate preliminary pharmacological activities. L1-based platinum (II) complex (BPA-L1-Pt) was the most promising complex, and has similar activity with the approved chemotherapy drug cis-platinum. Half maximal inhibitory concentration values for BPA-L1-Pt were 9.15 µM on A549s and 16.61 µM on HUVECs; the values for cis-platinum were 5.24 µM on A549s and 23.14 µM on HUVECs. Consequently, further synthesis studies should be performed to boost the cancer cell selectivity feature of BPA by varying metal and ligand types.

Physical, dosimetric and clinical aspects and delivery systems in neutron capture therapy

Neutron capture therapy (NCT) is a targeted radiotherapy for cancer treatment. In this method, neutrons with a spectra/specific energy (depending on the type of agent used for NCT) are captured with an agent that has a high cross-section with these neutrons. There are some agents that have been proposed in NCT including 10B, 157Gd and 33S. Among these agents, only 10B is used in clinical trials. Application of 157Gd is limited to in-vivo and in-vitro research. In addition, 33S has been applied in the field of Monte Carlo simulation. In BNCT, the only two delivery agents which are presently applied in clinical trials are BPA and BSH, but other delivery systems are being developed for more effective treatment in NCT. Neutron sources used in NCT are fission reactors, accelerators, and 252Cf. Among these, fission reactors have the most application in NCT. So far, BNCT has been applied to treat various cancers including glioblastoma multiforme, malignant glioma, malignant meningioma, liver, head and neck, lung, colon, melanoma, thyroid, hepatic, gastrointestinal cancer, and extra-mammary Paget's disease. This paper aims to review physical, dosimetric and clinical aspects as well as delivery systems in NCT for various agents.

Dynamic changes in 18F-borono-L-phenylalanine uptake in unresectable, advanced, or recurrent squamous cell carcinoma of the head and neck and malignant melanoma during boron neutron capture therapy patient selection

Background

We evaluated dynamic changes in ¹⁸F–borono-L-phenylalanine (¹⁸F–BPA) uptake in unresectable, advanced, or recurrent squamous cell carcinoma of the head and neck (SCC) and malignant melanoma (MM) during boron neutron capture therapy (BNCT) patient selection.

Methods

Dynamic changes in the maximum standardized uptake value (SUVmax), tumor-to-normal tissue ratio (TNR), and tumor-to-blood pool ratio (TBR) for ¹⁸F–BPA were evaluated in 20 patients with SCC and 8 patients with MM.

Results

SUVmax in SCC tumors decreased significantly from 30 to 120 min. There was a non-statistically significant decrease in SUVmax for SCC tumors from 30 to 60 min and from 60 to 120 min. Patients with MM had nonsignificant SUVmax changes in ¹⁸F–BPA uptake on delayed imaging. Nonsignificant ¹⁸F–BPA TNR and TBR changes were seen in patients with SCC and MM.

Conclusions

Dynamic changes in SUVmax for ¹⁸F–BPA uptake had a washout pattern in SCC and a persistent pattern in MM. Dynamic ¹⁸F–BPA -PET studies should be performed to investigate the pharmacokinetics of ¹⁸F–BPA in humans and select appropriate candidates who may benefit from BNCT.

Radiation absorbed dose estimates for 18F-BPA PET

Background Boron neutron capture therapy (BNCT) is a molecular radiation therapy approach based on the ¹⁰B (n, α) ⁷Li nuclear reaction in cancer cells. In BNCT, delivery of ¹⁰B in the form of 4-borono-phenylalanine conjugated with fructose (BPA-fr) to the cancer cells is important. The PET tracer 4-borono-2-18F-fluoro-phenylalanine (FBPA) has been used to predict the accumulation of BPA-fr before BNCT. Purpose To determine the biodistribution and dosimetric parameters in 18F-BPA PET/CT studies. Material and Methods Human biokinetic data were obtained during clinical 18F-BPA PET studies between February and June 2015 at one institution. Nine consecutive patients were studied prospectively. The internal radiation dose was calculated on the basis of radioactivity data from blood, urine, and normal tissue of the heart, liver, spleen, kidney, and other parts of the body at each time point using OLINDA/EXM1.1 program. We compared our calculations with published 18F-FDG data. Results Adult patients (3 men, 3 women; age range, 28-68 years) had significantly smaller absorbed doses than pediatric patients (3 patients; age range, 5-12 years) ( P = 0.003). The mean effective dose was 57% lower in adult patients compared with pediatric patients. Mean effective doses for 18F-BPA were 25% lower than those for 18F-FDG presented in International Commission of Radiation Protection (ICRP) publication 106. Conclusion We found significant differences in organ absorbed doses for 18F-BPA against those for 18F-FDG presented in ICRP publication 106. Mean effective doses for 18F-BPA were smaller than those for 18F-FDG in the publication by 0.5-38% (mean difference, 25%).

Validation and Comparison of the Therapeutic Efficacy of Boron Neutron Capture Therapy Mediated By Boron-Rich Liposomes in Multiple Murine Tumor Models

Boron neutron capture therapy (BNCT) was performed at the University of Missouri Research Reactor in mice bearing CT26 colon carcinoma flank tumors and the results were compared with previously performed studies with mice bearing EMT6 breast cancer flank tumors. Mice were implanted with CT26 tumors subcutaneously in the caudal flank and were given two separate tail vein injections of unilamellar liposomes composed of cholesterol, 1,2-distearoyl-sn-glycer-3-phosphocholine, and K[nido-7-CH₃(CH₂)₁₅–7,8-C₂B₉H₁₁] in the lipid bilayer and encapsulated Na₃[1-(2`-B₁₀H₉)-2-NH₃B₁₀H₈] within the liposomal core. Mice were irradiated 30 hours after the second injection in a thermal neutron beam for various lengths of time. The tumor size was monitored daily for 72 days. Despite relatively lower tumor boron concentrations, as compared to EMT6 tumors, a 45 minute neutron irradiation BNCT resulted in complete resolution of the tumors in 50% of treated mice, 50% of which never recurred. Median time to tumor volume tripling was 38 days in BNCT treated mice, 17 days in neutron-irradiated mice given no boron compounds, and 4 days in untreated controls. Tumor response in mice with CT26 colon carcinoma was markedly more pronounced than in previous reports of mice with EMT6 tumors, a difference which increased with dose. The slope of the dose response curve of CT26 colon carcinoma tumors is 1.05 times tumor growth delay per Gy compared to 0.09 times tumor growth delay per Gy for EMT6 tumors, indicating that inherent radiosensitivity of tumors plays a role in boron neutron capture therapy and should be considered in the development of clinical applications of BNCT in animals and man.