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

Active control of pharmacokinetics using light-responsive polymer-drug conjugates for boron neutron capture therapy

In boron neutron capture therapy (BNCT), boron drugs should exhibit high intratumoral boron concentrations during neutron irradiation, while being cleared from the blood and normal organs. However, it is usually challenging to achieve such tumor accumulation and quick clearance simultaneously in a temporally controlled manner. Here, we developed a polymer-drug conjugate that can actively control the clearance of the drugs from the blood. This polymer-drug conjugate is based on a biocompatible polymer that passively accumulates in tumors. Its side chains were conjugated with the low-molecular-weight boron drugs, which are immediately excreted by the kidneys, via photolabile linkers. In a murine subcutaneous tumor model, the polymer-drug conjugate could accumulate in the tumor with the high boron concentration ratio of the tumor to the surrounding normal tissue (∼10) after intravenous injection while a considerable amount remained in the bloodstream as well. Photoirradiation to blood vessels through the skin surface cleaved the linker to release the boron drug in the blood, allowing for its rapid clearance from the bloodstream. Meanwhile, the boron concentration in the tumor which was not photoirradiated could be maintained high, permitting strong BNCT effects. In clinical BNCT, the dose of thermal neutrons to solid tumors is determined by the maximum radiation exposure to normal organs. Thus, our polymer-drug conjugate may enable us to increase the therapeutic radiation dose to tumors in such a practical situation.

Fluorinated BPA derivatives enhanced 10B delivery in tumors

Boron neutron capture therapy (BNCT) is an emerging approach for treating malignant tumors with binary targeting. However, its clinical application has been hampered by insufficient 10B accumulation in tumors and low 10B concentration ratios of tumor-to-blood (T/B) and tumor-to-normal tissue (T/N). Herein, we developed fluorinated BPA derivatives with different fluorine groups as boron delivery agents for enabling sufficient 10B accumulation in tumors and enhancing T/B and T/N ratios. Our findings demonstrated that fluorinated BPA derivatives had good biological safety. Furthermore, fluorinated BPA derivatives showed improved 10B accumulation in tumors and enhanced T/B and T/N ratios compared to the clinical boron drug fructose-BPA (f-BPA). In particular, in B16-F10 tumor-bearing mice, fluorinated BPA derivatives met the requirements for clinical BNCT even at half of the clinical dose. Thus, fluorinated BPA derivatives are potentially effective boron delivery agents for clinical BNCT in melanoma.

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.

Targeted strategies to deliver boron agents across the blood-brain barrier for neutron capture therapy of brain tumors

Boron neutron capture therapy (BNCT), as an innovative radiotherapy technology, has demonstrated remarkable outcomes when compared to conventional treatments in the management of recurrent and refractory brain tumors. However, in BNCT of brain tumors, the blood-brain barrier is a main stumbling block for restricting the transport of boron drugs to brain tumors, while the tumor targeting and retention of boron drugs also affect the BNCT effect. This review focuses on the recent development of strategies for delivering boron drugs crossing the blood-brain barrier and targeting brain tumors, providing new insights for the development of efficient boron drugs for the treatment of brain tumors.

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.

In vivo investigation of boron-rich nanodrugs for treating triple-negative breast cancers via boron neutron capture therapy

Triple-negative breast cancer (TNBC) is characterized by highly proliferative cancer cells and is the only subtype of breast cancer that lacks a targeted therapy. Boron neutron capture therapy (BNCT) is an approach that combines chemotherapy with radiotherapy and can potentially offer beneficial targeted treatment for TNBC patients owing to its unique ability to eradicate cancer cells selectively while minimizing damage to the surrounding healthy cells. Since BNCT relies on specific delivery of a high loading of B10 to the tumor site, there is growing research interest to develop more potent boron-based drugs for BNCT that can overcome the limitations of small-molecule boron compounds. In this study, polyethylene-glycol-coated boron carbon oxynitride nanoparticles (PEG@BCNO) of size 134.2±23.6nm were prepared as a promising drug for BNCT owing to their high boron content and enhanced biocompatibility. The therapeutic efficiency of PEG@BCNO was compared with a state-of-the-art 10BPA boron drug in mice bearing MDA-MB-231 tumor. In the orthotopic mouse model, PEG@BCNO showed higher B10 accumulation in the tumor tissues (6 μg 10B/g tissue compared to 3 μg 10B/g tissue in mice administered B10-enriched 10BPA drug) despite using the naturally occurring 11B/10B boron precursor in the preparation of the BCNO nanoparticles. The in vivo biodistribution of PEG@BCNO in mice bearing MDA-MB-231 showed a tumor/blood ratio of ~3.5, which is comparable to that of the state-of-the-art 10BPA-fructose drug. We further demonstrated that upon neutron irradiation, the mice bearing MDA-MB-231 tumor cells treated with PEG@BCNO and 10BPA showed tumor growth delay times of 9 days and 1 day, respectively, compared to mice in the control group after BNCT. The doubling times (DTs) for mice treated with PEG@BCNO and 10BPA as well as mice in the control group were calculated to be 31.5, 19.8, and 17.7 days, respectively. Immunohistochemical staining for the p53 and caspase-3 antibodies revealed that mice treated with PEG@BCNO showed lower probability of cancer recurrence and greater level of cellular apoptosis than mice treated with 10BPA and mice in the control group. Our study thus demonstrates the potential of pegylated BCNO nanoparticles in effectively inhibiting the growth of TNBC tumors compared to the state-of-the-art boron drug 10BPA.

Proposal of recommended experimental protocols for in vitro and in vivo evaluation methods of boron agents for neutron capture therapy

Recently, boron neutron capture therapy (BNCT) has been attracting attention as a minimally invasive cancer treatment. In 2020, the accelerator-based BNCT with L-BPA (Borofalan) as its D-sorbitol complex (Steboronine®) for head and neck cancers was approved by Pharmaceutical and Medical Devices Agency for the first time in the world. As accelerator-based neutron generation techniques are being developed in various countries, the development of novel tumor-selective boron agents is becoming increasingly important and desired. The Japanese Society of Neutron Capture Therapy believes it is necessary to propose standard evaluation protocols at each stage in the development of boron agents for BNCT. This review summarizes recommended experimental protocols for in vitro and in vivo evaluation methods of boron agents for BNCT based on our experience with L-BPA approval.

Investigation of the Impact of L-Phenylalanine and L-Tyrosine Pre-Treatment on the Uptake of 4-Borono-L-Phenylalanine in Cancerous and Normal Cells Using an Analytical Approach Based on SC-ICP-MS

Boron has gained significant attention in medical research due to its B-10 isotope's high cross section for the reaction with thermal neutrons, generating ionizing particles that can eliminate cancer cells, propelling the development of boron neutron capture therapy (BNCT) for cancer treatment. The compound 4-borono-L-phenylalanine (BPA) has exhibited potential in BNCT clinical trials. Enhancing BPA uptake in cells involves proposing L-amino acid preloading. This study introduces a novel analytical strategy utilizing ICP-MS and single cell ICP-MS (SC-ICP-MS) to assess the effectiveness of L-tyrosine and L-phenylalanine preloading on human non-small cell lung carcinoma (A549) and normal Chinese hamster lung fibroblast (V79-4) models, an unexplored context. ICP-MS outcomes indicated that L-tyrosine and L-phenylalanine pre-treatment increased BPA uptake in V79-4 cells by 2.04 ± 0.74-fold (p = 0.000066) and 1.46 ± 0.06-fold (p = 0.000016), respectively. Conversely, A549 cells manifested heightened BPA uptake solely with L-tyrosine preloading, with a factor of 1.24 ± 0.47 (p = 0.028). BPA uptake remained higher in A549 compared to V79-4 regardless of preloading. SC-ICP-MS measurements showcased noteworthy boron content heterogeneity within A549 cells, signifying diverse responses to BPA exposure, including a subset with notably high BPA uptake. This study underscores SC-ICP-MS's utility in precise cellular boron quantification, validating cellular BPA uptake's heterogeneity.

Calculation of the DNA damage yield and relative biological effectiveness in boron neutron capture therapy via the Monte Carlo track structure simulation

Objective.Boron neutron capture therapy (BNCT) is an advanced cellular-level hadron therapy that has exhibited remarkable therapeutic efficacy in the treatment of locally invasive malignancies. Despite its clinical success, the intricate nature of relative biological effectiveness (RBE) and mechanisms responsible for DNA damage remains elusive. This work aims to quantify the RBE of compound particles (i.e. alpha and lithium) in BNCT based on the calculation of DNA damage yields via the Monte Carlo track structure (MCTS) simulation.Approach. The TOPAS-nBio toolkit was employed to conduct MCTS simulations. The calculations encompassed four steps: determination of the angle and energy spectra on the nuclear membrane, quantification of the database containing DNA damage yields for ions with specific angle and energy, accumulation of the database and spectra to obtain the DNA damage yields of compound particles, and calculation of the RBE by comparison yields of double-strand break (DSB) with the reference gamma-ray. Furthermore, the impact of cell size and microscopic boron distribution was thoroughly discussed.Main results. The DSB yields induced by compound particles in three types of spherical cells (radius equal to 10, 8, and 6μm) were found to be 13.28, 17.34, 22.15 Gy Gbp-1for boronophenylalanine (BPA), and 1.07, 3.45, 8.32 Gy Gbp-1for sodium borocaptate (BSH). The corresponding DSB-based RBE values were determined to be 1.90, 2.48, 3.16 for BPA and 0.15, 0.49, 1.19 for BSH. The calculated DSB-based RBE showed agreement with experimentally values of compound biological effectiveness for melanoma and gliosarcoma. Besides, the DNA damage yield and DSB-based RBE value exhibited an increasing trend as the cell radius decreased. The impact of the boron concentration ratio on RBE diminished once the drug enrichment surpasses a certain threshold.Significance. This work is potential to provide valuable guidance for accurate biological-weighted dose evaluation in BNCT.

Redox Mechanisms Underlying the Cytostatic Effects of Boric Acid on Cancer Cells-An Issue Still Open

Boric acid (BA) is the dominant form of boron in plasma, playing a role in different physiological mechanisms such as cell replication. Toxic effects have been reported, both for high doses of boron and its deficiency. Contrasting results were, however, reported about the cytotoxicity of pharmacological BA concentrations on cancer cells. The aim of this review is to briefly summarize the main findings in the field ranging from the proposed mechanisms of BA uptake and actions to its effects on cancer cells.

Boron uptake of boronophenylalanine and the effect of boron neutron capture therapy in cervical cancer cells

There are few studies about boron neutron capture therapy (BNCT) for cervical cancer. The present study evaluated the biodistribution of boronophenylalanine (BPA) and the effect of BNCT on cervical cancer cell lines. BPA exposure and neutron irradiation of cervical cancer cell lines resulted in decreased survival fraction compared to irradiation only. In vivo cervical cancer tumor boron concentration was highest at 2.5 h after BPA intraperitoneal administration, and higher than in the other organs. BNCT may be effective against cervical carcinoma.

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.

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.

Polymeric ligands comprising sulfur-containing amino acids for targeting tumor-associated amino acid transporters

Various cancer cells overexpress L-type amino acid transporter 1 (LAT1) to take up a large number of neutral amino acids such as phenylalanine and methionine, and LAT1 transporter should be a promising target for cancer diagnosis and therapy. However, only a few studies reported drug delivery systems targeting LAT1 probably due to limited knowledge about the interaction between LAT1 and its substrate. Here, we developed polymers having methionine (Met)- or cysteine (Cys)-like structures on their side chains to examine their affinity with LAT1. While both the Met- and Cys-modified polymers exhibited efficient cellular uptake selectively in cancer cells, the Met-modified polymers exhibited higher cellular uptake efficiency in an LAT1-selective manner than the Cys-modified polymers. In the in vivo study, the intraperitoneally injected Met-modified polymers showed appreciable tumor-selective accumulation in the peritoneal dissemination model, and importantly, Met-modified polymers conjugated with photosensitizers exhibited significant therapeutic effects upon photoirradiation with reduced photochemical damage to normal organs. Our results may provide important knowledge about the polymer-LAT1 interaction, and the Met-modified polymers should offer a new concept for designing LAT1-targeting drug delivery systems.

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.

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.

The Anti-Tumor Effect of Boron Neutron Capture Therapy in Glioblastoma Subcutaneous Xenograft Model Using the Proton Linear Accelerator-Based BNCT System in Korea

Boron neutron capture therapy (BNCT) is a radiation therapy that selectively kills cancer cells and is being actively researched and developed around the world. In Korea, development of the proton linear accelerator-based BNCT system has completed development, and its anti-cancer effect in the U-87 MG subcutaneous xenograft model has been evaluated. To evaluate the efficacy of BNCT, we measured 10B-enriched boronophenylalanine (BPA) uptake in U-87 MG, FaDu, and SAS cells and evaluated cell viability by clonogenic assays. In addition, the boron concentration in the tumor, blood, and skin on the U-87 MG xenograft model was measured, and the tumor volume was measured for 4 weeks after BNCT. In vitro, the intracellular boron concentration was highest in the order of SAS, FaDu, and U-87 MG, and cell survival fractions decreased depending on the BPA treatment concentration and neutron irradiation dose. In vivo, the tumor volume was significantly decreased in the BNCT group compared to the control group. This study confirmed the anti-cancer effect of BNCT in the U-87 MG subcutaneous xenograft model. It is expected that the proton linear accelerator-based BNCT system developed in Korea will be a new option for radiation therapy for cancer treatment.

A Review of Boron Neutron Capture Therapy: Its History and Current Challenges

Mechanism of Action

External beam, whether with photons or particles, remains as the most common type of radiation therapy. The main drawback is that radiation deposits dose in healthy tissue before reaching its target. Boron neutron capture therapy (BNCT) is based on the nuclear capture and fission reactions that occur when ¹⁰B is irradiated with low-energy (0.0025 eV) thermal neutrons. The resulting ¹⁰B(n,α)⁷Li capture reaction produces high linear energy transfer (LET) α particles, helium nuclei (⁴He), and recoiling lithium-7 (⁷Li) atoms. The short range (5-9 μm) of the α particles limits the destructive effects within the boron-containing cells. In theory, BNCT can selectively destroy malignant cells while sparing adjacent normal tissue at the cellular levels by delivering a single fraction of radiation with high LET particles.

History

BNCT has been around for many decades. Early studies were promising for patients with malignant brain tumors, recurrent tumors of the head and neck, and cutaneous melanomas; however, there were certain limitations to its widespread adoption and use.

Current Limitations and Prospects

Recently, BNCT re-emerged owing to several developments: (1) small footprint accelerator-based neutron sources; (2) high specificity third-generation boron carriers based on monoclonal antibodies, nanoparticles, among others; and (3) treatment planning software and patient positioning devices that optimize treatment delivery and consistency.

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.

The Effect of Low-Dose Gamma Irradiation on the Uptake of Boronophenylalanine to Enhance the Efficacy of Boron Neutron Capture Therapy in an Orthotopic Oral Cancer Model

The success of boron neutron capture therapy (BNCT) mainly depends on the boron concentration in the tumor and a high tumor/normal tissue (T/N) boron ratio or a high tumor/blood (T/B) boron ratio. Therefore, the effective enhancement of boron ratios is the first priority. Our study investigated whether a low-dose of γ-radiation (LDR) could improve boron ratios and enhance the therapeutic effects of BNCT in an orthotopic human oral squamous cell carcinoma-bearing animal model. SAS/luc cells were used to establish the orthotopic tumor-bearing model. The pharmacokinetics of boronophenylalanine (BPA) administration with 400 mg/kg of body weight both alone and in combination with LDR (0.1 Gy) was evaluated, and BNCT was performed at the Tsing Hua Open-pool Reactor (THOR). The radiation doses were evaluated using a treatment planning system. Moreover, tumor growth and metastasis were monitored via bioluminescence imaging (BLI). The therapeutic effects after BNCT were evaluated using BLI, histopathological findings and the overall survival rate. LDR increased the BPA accumulation in tumors by 52.2%. T/N and T/B ratios were enhanced from 3.77 to 5.31 and from 3.47 to 4.46, respectively. Radiation dose was increased by 44.3%. Notably, tumor recurrence and cervical lymph node metastasis were observed in the BNCT group, which had a survival rate of 50%. Complete responses were found in the combined-treatment group, which had a survival rate of 100%. No toxicity was found according to the histopathological findings. Conclusively, LDR increased BPA accumulation in the tumor and the T/N and T/B ratios, resulting in BNCT efficacy improvement and the overall survival rate extension.

Fructose-functionalized polymers to enhance therapeutic potential of p-boronophenylalanine for neutron capture therapy

In boron neutron capture therapy (BNCT), boron drugs should accumulate selectively within a tumor and be quickly cleared from blood and normal organs. However, it is usually challenging to achieve the efficient tumor accumulation and the quick clearance simultaneously. Here we report the complex composed of a fructose-modified poly(ethylene glycol)-poly(l-lysine) block copolymer and p-boronophenylalanine, termed PEG-P[Lys/Lys(fructose)]-BPA, as a boron delivery system permitting selective accumulation within the target tumor with quick clearance from normal organs as well as blood. Our PEG-P[Lys/Lys(fructose)]-BPA could be internalized into tumor cells through LAT1 amino acid transporter-mediated endocytosis and retain in the targeted cells, thereby accomplishing more efficient accumulation and retention in a subcutaneous tumor than clinically used fructose-BPA complexes. Importantly, the moderately cationic property of the polymer facilitated renal clearance and PEG-P[Lys/Lys(fructose)]-BPA exhibited high accumulation contrast between the target tumor and the blood/normal organ. Finally, upon thermal neutron irradiation, PEG-P[Lys/Lys(fructose)]-BPA significantly inhibited the tumor growth in mice. PEG-P[Lys/Lys(fructose)]-BPA may be a promising boron delivery system for BNCT.

Theranostics in Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) has the potential to specifically destroy tumor cells without damaging the tissues infiltrated by the tumor. BNCT is a binary treatment method based on the combination of two agents that have no effect when applied individually: ¹⁰B and thermal neutrons. Exclusively, the combination of both produces an effect, whose extent depends on the amount of ¹⁰B in the tumor but also on the organs at risk. It is not yet possible to determine the ¹⁰B concentration in a specific tissue using non-invasive methods. At present, it is only possible to measure the ¹⁰B concentration in blood and to estimate the boron concentration in tissues based on the assumption that there is a fixed uptake of ¹⁰B from the blood into tissues. On this imprecise assumption, BNCT can hardly be developed further. A therapeutic approach, combining the boron carrier for therapeutic purposes with an imaging tool, might allow us to determine the ¹⁰B concentration in a specific tissue using a non-invasive method. This review provides an overview of the current clinical protocols and preclinical experiments and results on how innovative drug development for boron delivery systems can also incorporate concurrent imaging. The last section focuses on the importance of proteomics for further optimization of BNCT, a highly precise and personalized therapeutic approach.

Nanocząstki o wysokiej zawartości boru jako potencjalne nośniki w terapii borowo-neutronowej

Podstawą terapii borowo-neutronowej (boron neutron capture therapy, BNCT) jest selektywne dostarczenie boru do komórek nowotworowych, a następnie napromienienie zmienionego chorobowo miejsca wiązką neutronów. W wyniku tego procesu dochodzi do rozszczepienia jądra izotopu 10B, co powoduje uwolnienie energii niszczącej komórki nowotworowe. Mimo że badania związane z BNCT trwają od lat 50. XX wieku, pozostaje ona wciąż terapią eksperymentalną. Jest to związane m.in. z brakiem nośników umożliwiających szybkie i skuteczne wprowadzanie 10B do środowiska nowotworu. Tak więc często podnoszonym zagadnieniem i jednym z głównych wyzwań dla rozwoju BNCT, jest poszukiwanie selektywnych związków dostarczających wymaganą ilość tego pierwiastka. Istotnym aspektem są badania nad nanometrycznymi strukturami, takimi jak liposomy zawierające związki bogate w bor lub nieorganiczne nanocząstki – węglik boru czy azotek boru. Ze względu na dużą zawartość boru oraz możliwość modyfikacji powierzchni tych nanocząstek, mogą się one okazać wyjątkowo atrakcyjnym narzędziem w celowanej BNCT. Równie ważnym problemem tej terapii jest opracowanie precyzyjnych powiązań między źródłem neutronów, specyfiką wiązki a rodzajem zastosowanego nośnika. W artykule wskazujemy na wysoki potencjał związków bogatych w bor jako nośników w celowanej terapii borowo-neutronowej.

The Therapeutic Effects of Dodecaborate Containing Boronophenylalanine for Boron Neutron Capture Therapy in a Rat Brain Tumor Model

Simple Summary

We have developed a new boron compound for application in boron neutron capture therapy (BNCT) named boronophenylalanine–amide alkyl dodecaborate (BADB). It is characterized by a larger amount of ¹⁰B per molecule, linking boronphenylalanine (BPA) and dodecaborate, and we conducted various experiments on its efficacy. Its high accumulation at the cellular level made it a promising novel drug, but it did not sufficiently accumulate in brain tumor tissue when intravenously administered. However, in neutron irradiation experiments, the drug showed remarkably high compound biological effectiveness and significantly prolonged the survival time in rat brain tumor models. We confirmed the antitumor efficacy of BADB in BNCT and its additional efficacy when administered in combination with BPA. Though this drug showed poor results when administered as a single agent, it was superior to BPA alone when administered in combination with BPA, making it a drug that we have been waiting for in our clinical practice.

Abstract

Background: The development of effective boron compounds is a major area of research in the study of boron neutron capture therapy (BNCT). We created a novel boron compound, boronophenylalanine–amide alkyl dodecaborate (BADB), for application in BNCT and focused on elucidating how it affected a rat brain tumor model. Methods: The boron concentration of F98 rat glioma cells following exposure to boronophenylalanine (BPA) (which is currently being utilized clinically) and BADB was evaluated, and the biodistributions in F98 glioma-bearing rats were assessed. In neutron irradiation studies, the in vitro cytotoxicity of each boron compound and the in vivo corresponding therapeutic effect were evaluated in terms of survival time. Results: The survival fractions of the groups irradiated with BPA and BADB were not significantly different. BADB administered for 6 h after the termination of convection-enhanced delivery ensured the highest boron concentration in the tumor (45.8 μg B/g). The median survival time in the BADB in combination with BPA group showed a more significant prolongation of survival than that of the BPA group. Conclusion: BADB is a novel boron compound for BNCT that triggers a prolonged survival effect in patients receiving BNCT.

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.

Glioma Stem-Like Cells Can Be Targeted in Boron Neutron Capture Therapy with Boronophenylalanine

As glioma stem cells are chemo- and radio-resistant, they could be the origins of recurrent malignant glioma. Boron neutron capture therapy (BNCT) is a tumor-selective particle radiation therapy. ¹⁰B(n,α)⁷Li capture reaction produces alpha particles whose short paths (5-9 µm) lead to selective killing of tumor cells. P-boronophenylalanine (BPA) is a chemical compound used in clinical trials for BNCT. Here, we used mass cytometry (Cytof) to investigate whether glioma stem-like cells (GSLCs) take up BPA or not. We used GSLCs, and cells differentiated from GSLCs (DCs) by fetal bovine serum. After exposure to BPA for 24 h at 25 ppm in 5% CO₂ incubator, we immune-stained them with twenty stem cell markers, anti-Ki-67, anti-BPA and anti-CD98 (heterodimer that forms the large BPA transporter) antibodies and analyzed them with Cytof. The percentage of BPA⁺ or CD98⁺ cells with stem cell markers (Oct3/4, Nestin, SOX2, Musashi-1, PDGFRα, Notch2, Nanog, STAT3 and C-myc, among others) was 2-4 times larger among GSLCs than among DCs. Analyses of in vivo orthotopic tumor also indicated that 100% of SOX2⁺ or Nestin⁺ GSLCs were BPA⁺, whereas only 36.9% of glial fibrillary acidic protein (GFAP)⁺ DCs were BPA⁺. Therefore, GSLCs may take up BPA and could be targeted by BNCT.

Clinical Veterinary Boron Neutron Capture Therapy (BNCT) Studies in Dogs with Head and Neck Cancer: Bridging the Gap between Translational and Clinical Studies

Simple Summary

Boron Neutron Capture Therapy (BNCT) is a treatment for cancer based on the selective accumulation in tumor of boron compounds, followed by external irradiation with neutrons. The interaction between boron-10 and a neutron gives rise to very energetic particles that travel only a very short distance (approximately the diameter of a cell) and are lethal for the cell. In this way, BNCT damages tumor tissue selectively while preserving normal tissue. BNCT has proved effective to treat certain tumors in clinical trials worldwide, with room for improvement. Our group has worked on animal models to improve the efficacy of BNCT, in particular for head and neck cancer. Herein we performed clinical veterinary BNCT studies in five terminal dog patients with head and neck cancer with no other therapeutic option. In all cases we observed partial tumor response, clinical benefit, and extension of estimated survival time at recruitment with excellent quality of life. Toxicity associated to the treatment was mild/moderate and reversible. These studies contribute towards preparation for clinical BNCT trials for head and neck cancer in Argentina and suggest a potential role for BNCT in veterinary medicine.

Abstract

Translational Boron Neutron Capture Therapy (BNCT) studies performed by our group and clinical BNCT studies worldwide have shown the therapeutic efficacy of BNCT for head and neck cancer. The present BNCT studies in veterinary patients with head and neck cancer were performed to optimize the therapeutic efficacy of BNCT, contribute towards exploring the role of BNCT in veterinary medicine, put in place technical aspects for an upcoming clinical trial of BNCT for head and neck cancer at the RA-6 Nuclear Reactor, and assess the feasibility of employing the existing B2 beam to treat large, deep-seated tumors. Five dogs with head and neck cancer with no other therapeutic option were treated with two applications of BNCT mediated by boronophenyl-alanine (BPA) separated by 3–5 weeks. Two to three portals per BNCT application were used to achieve a potentially therapeutic dose over the tumor without exceeding normal tissue tolerance. Clinical and Computed Tomography results evidenced partial tumor control in all cases, with slight-moderate mucositis, excellent life quality, and prolongation in the survival time estimated at recruitment. These exploratory studies show the potential value of BNCT in veterinary medicine and contribute towards initiating a clinical BNCT trial for head and neck cancer at the RA-6 clinical facility.

Effect of Rapamycin on the Radio-Sensitivity of Cultured Tumor Cells Following Boron Neutron Capture Reaction

Background

Mammalian target of rapamycin (mTOR) signaling pathway has been implicated in multiple mechanisms of resistance to anticancer drugs and poor treatment outcomes in various human cancers. Meanwhile, clinical boron neutron capture therapy (BNCT) has been carried out for patients with malignant gliomas, melanomas, inoperable head and neck tumors and oral cancers. This study aimed to evaluate the effect of mTOR inhibition on radio-sensitivity of cultured tumor cells in BNCT, employing p-boronophenylalanine-¹⁰B (BPA) as a ¹⁰B-carrier.

Methods

Cultured SAS cells had been incubated for 48 h at RPMI medium with mTOR inhibitor, rapamycin at the dose of 1 µM, and then continuously incubated for 2 more hours at RPMI medium containing both BPA at the ¹⁰B concentration of 10 ppm and rapamycin (1 µM). Subsequently, the SAS cells received reactor neutron beams, and then surviving fraction and micronucleus frequency were determined.

Results

SAS cells incubated with rapamycin showed resistance to γ-rays compared with no treatment with rapamycin. The efficiency of delivery of ¹⁰B from BPA into cultured SAS cells was reduced through combining with rapamycin, leading to reduced sensitivity following boron neutron capture reaction.

Conclusions

Since many tumors are characterized by deregulated PI3K/AKT/mTOR pathway, rapamycin is thought to inhibit the pathway and tumor growth. However, it was revealed that rapamycin can also inhibit the transport of ¹⁰B for BNCT into tumor cells. When BNCT is combined with mTOR inhibitor, the efficiency as cancer treatment can be reduced by repression of distributing ¹⁰B in tumor cells, warranting precaution when the two strategies are combined.

Boron neutron capture therapy for clear cell sarcoma

Clear cell sarcoma of tendons and aponeuroses (CCS) is a rare, malignant tumor arising in lower extremities with no effective treatment other than wide surgical resection. Here described is a case of primary CCS in the peroneal tendon of the right foot of a 54-year-old woman enrolled to undergo BNCT. The tumor mass post-BNCT disappeared totally without damage to other normal tissue, demonstrating, for the first time, the potential efficacy of BNCT in complete local control of CCS.

Peptide-Drug Conjugates and Their Targets in Advanced Cancer Therapies

Cancer became recently the leading cause of death in industrialized countries. Even though standard treatments achieve significant effects in growth inhibition and tumor elimination, they cause severe side effects as most of the applied drugs exhibit only minor selectivity for the malignant tissue. Hence, specific addressing of tumor cells without affecting healthy tissue is currently a major desire in cancer therapy. Cell surface receptors, which bind peptides are frequently overexpressed on cancer cells and can therefore be considered as promising targets for selective tumor therapy. In this review, the benefits of peptides as tumor homing agents are presented and an overview of the most commonly addressed peptide receptors is given. A special focus was set on the bombesin receptor family and the neuropeptide Y receptor family. In the second part, the specific requirements of peptide-drug conjugates (PDC) and intelligent linker structures as an essential component of PDC are outlined. Furthermore, different drug cargos are presented including classical and recent toxic agents as well as radionuclides for diagnostic and therapeutic approaches. In the last part, boron neutron capture therapy as advanced targeted cancer therapy is introduced and past and recent developments are reviewed.

Water-Soluble closo-Docecaborate-Containing Pteroyl Derivatives Targeting Folate Receptor-Positive Tumors for Boron Neutron Capture Therapy

Water-soluble pteroyl-closo-dodecaborate conjugates (PBCs 1-4), were developed as folate receptor (FRα) targeting boron carriers for boron neutron capture therapy (BNCT). PBCs 1-4 had adequately low cytotoxicity with IC₅₀ values in the range of 1~3 mM toward selected human cancer cells, low enough to use as BNCT boron agents. PBCs 1-3 showed significant cell uptake by FRα positive cells, especially U87MG glioblastoma cells, although the accumulation of PBC 4 was low compared with PBCs 1-3 and L-4-boronophenylalanine (L-BPA). The cellular uptake of PBC 1 and PBC 3 by HeLa cells was arrested by increasing the concentration of folate in the medium, indicating that the major uptake mechanisms of PBC 1-3 are primarily through FRα receptor-mediated endocytosis.

Folate-appended cyclodextrin improves the intratumoral accumulation of existing boron compounds

In this study, the tumor accumulation and antitumor effect of folate-modified cyclodextrin (ND201) purified with folate receptor (FR) connotated with BSH were examined. ND201 and BSH were stably bound in blood, and the mixing ratio 1:1 was most efficient. ND-BSH showed higher boron concentration (38.5 ppm) than BSH alone (11.25 ppm). The maximum ND-BSH tumor/blood ratio was also markedly higher (6.58) than that of BSH alone (1.04). ND-BSH showed a significant antitumor effect compared with BSH after neutron irradiation.

Poly(vinyl alcohol) boosting therapeutic potential of p-boronophenylalanine in neutron capture therapy by modulating metabolism

In the current clinical boron neutron capture therapy (BNCT), p-boronophenylalanine (BPA) has been the most powerful drug owing to its ability to accumulate selectively within cancers through cancer-related amino acid transporters including LAT1. However, the therapeutic success of BPA has been sometimes compromised by its unfavorable efflux from cytosol due to their antiport mechanism. Here, we report that poly(vinyl alcohol) (PVA) can form complexes with BPA through reversible boronate esters in aqueous solution, and the complex termed PVA-BPA can be internalized into cancer cells through LAT1-mediated endocytosis, thereby enhancing cellular uptake and slowing the untoward efflux. In in vivo study, compared with clinically used fructose-BPA complexes, PVA-BPA exhibited efficient tumor accumulation and prolonged tumor retention with quick clearance from bloodstream and normal organs. Ultimately, PVA-BPA showed critically enhanced antitumor activity in BNCT. The facile technique proposed in this study offers an approach for drug delivery focusing on drug metabolism.

Evaluation of the total distribution volume of 18F-FBPA in normal tissues of healthy volunteers by non-compartmental kinetic modeling

OBJECTIVE

Boron neutron capture therapy (BNCT) is a noninvasive radiation therapy method for cancer treatment. In BNCT, 4-borono-2-[¹⁸F]-fluoro-L-phenylalanine (¹⁸F-FBPA) PET has been employed to estimate ¹⁰B accumulation in target tumors and normal tissues if ¹⁰B borono-L-phenylalanine (¹⁰B-BPA) is used as a boron carrier. The purpose of the current study was to evaluate the total distribution volume (Vt) of ¹⁸F-FBPA in normal organs of healthy volunteers by kinetic analysis and to estimate boron concentration in normal organs for the therapeutic dose of ¹⁰B-BPA using obtained Vt values.

METHODS

Six healthy volunteers were injected with ¹⁸F-FBPA (3-5 MBq/kg), and 7 PET-CT scans were performed subsequently. ¹⁸F-FBPA radioactivity in whole blood and plasma was measured before, and eight times after the injection. PET images were analyzed by PMOD software. Twelve volumetric regions of interest including the brain, heart, right lung, spleen, liver, parotid salivary glands, esophagus, stomach, pancreas, intestines, and bone marrow were drawn manually for each subject and analyzed with the Logan plot and two Ichise multilinear analyses (MA1 and MA2). The better model was defined by several goodness-of-fit parameters and residual distribution. After Vt values had been derived, boron concentration was estimated in ppm for the ¹⁰B-BPA-fructose (¹⁰B-BPA-fr) dose 30 g 1 and 2 h post-injection using Vt and interpolated plasma activity data.

RESULTS

The Ichise MA2 model showed the best fit among all models. Akaike Information Criterion (AIC) was the lowest for the Ichise's MA2 in all regions (mean AIC value - 14.0) comparing to the other models (Logan plot mean AIC 31.4; Ichise MA1 model mean AIC - 4.2). Mean Vt values of the Ichise MA2 model ranged from 0.94 ± 0.14 ml/ml in the pancreas to 0.16 ± 0.02 ml/ml in the right lung. Estimated boron concentration for ¹⁰B-BPA-fr had the highest value in the pancreas (14.0 ± 1.9 ppm 1 h after, and 5.7 ± 1.7 ppm 2 h after the ¹⁸F-FBPA administration) and the lowest value in the right lung (2.4 ± 0.3 ppm 1 h, and 1.0 ± 0.3 ppm 2 h post-injection).

CONCLUSION

The ¹⁰B concentration in normal tissues was best estimated using Vt values of ¹⁸F-FBPA with the Ichise multilinear analysis 2 (MA2).

TRAIL REGISTRY

The UMIN clinical trial number: UMIN000022850.

Carrier proteins-based boron delivery to tumor

The conjugation of maleimide-functionalized closo-dodecaborate (MID) to transferrin (TF), which has no free SH cysteine residue, proceeded at room temperature for 12 h in PBS buffer (pH = 7.4). MID-TF conjugates were accumulated into the cells in the time- and concentration-dependent manners through the TF receptor mediated mechanism. In vivo biodistribution study of MID-TF conjugates in colon 26-bearing mice revealed the time-dependent selective accumulation of MID-TF conjugates into tumor very similar to that of MID-BSA. The tumor boron concentration of MID-BSA was higher than that of MID-TF conjugates (62 ppm vs. 20 ppm at a dose of 30 mg [B]/kg) 12 h after administration, suggesting that the EPR effect is not remarkably observed in the case of MID-TF conjugates because TF has smaller molecular weight than BSA (48 kDa vs. 68 kDa).

Microdistribution and quantification of the boron neutron capture therapy drug BPA in primary cell cultures of human glioblastoma tumour by NanoSIMS

The ability of secondary ion mass spectrometry (SIMS) to provide high sensitivity imaging of elements and small-medium mass molecules in biological tissues and cells, makes it a very powerful tool for drug distribution studies. Here we report on the application of a high-resolution dynamic SIMS instrument for the quantification and localisation of therapeutic levels of the BNCT agent l-para-(dihydroxyboryl)-phenylalanine (BPA) in primary cell cultures from human patients exhibiting glioblastoma multiform tumours. Boron uptake and distribution was determined quantitatively as a function of cell-sampling location and different treatment regimes. Importantly, BPA was found to accumulate in cancer cells invading the 'brain around tumour' tissue in addition to the main tumour site. Pre-treatment of samples with l-tyrosine was found not to increase the uptake of BPA, nor change the intracellular drug distribution. In cultured cells from the tumour core and brain around tumour, with and without l-tyrosine pre-treatment, normalised boron-related signals were higher from cell nuclei than from cytoplasm. An efflux treatment was found to reduce BPA levels, but at a rate slower than the original uptake, and did not affect the intracellular drug distribution. To the best of our knowledge, these data represent the first published study of BPA uptake and l-amino acid pre-treatment in cultured primary human cells using dynamic SIMS, and the most detailed, subcellular distribution study of a BNCT agent in any cellular system.

Cellular uptake evaluation of pentagamaboronon-0 (PGB-0) for boron neutron capture therapy (BNCT) against breast cancer cells

Pentagamaboronon-0 (PGB-0), a curcumin analog compound, has been synthesized as a candidate of boron-carrier pharmaceutical (BCP) for boron neutron capture therapy (BNCT); however, this compound is poorly soluble in water. To improve its solubility, aqueous formulations of PGB-0 with a monosaccharide, fructose or sorbitol, were successfully synthesized, namely PGB-0-F and PGB-0-So, respectively. The cytotoxicity study showed that PGB-0-F and PGB-0-So exerted low cytotoxicity against MCF-7 and MDA-MB 231 breast cancer cells. The cellular uptake study using inductively coupled plasma optical emission spectrometry (ICP-OES) and DAHMI live-cell imaging indicated that these compounds were accumulated and distributed within the cytoplasm and cell nuclei. The cellular uptake mechanism was also evaluated to clarify the contribution of the glucose transporter, and the results demonstrated that these compounds entered through active transport into MCF-7 cells but through passive diffusion into MDA-MB 231 cells. In conclusion, the sugar formulations of PGB-0 only improved PGB-0 solubility but had no role in its cellular uptake.

4-Borono-2-18F-fluoro-L-phenylalanine PET for boron neutron capture therapy-oriented diagnosis: overview of a quarter century of research

4-¹⁰B-Borono-2-¹⁸F-fluoro-L-phenylalanine (¹⁸F-FBPA) was developed for monitoring the pharmacokinetics of 4-¹⁰B-borono-L-phenylalanine (¹⁰B-BPA) used in boron neutron capture therapy (BNCT) with positron emission tomography (PET). The tumor-imaging potential of ¹⁸F-FBPA was demonstrated in various animal models. Accumulation of ¹⁸F-FBPA was higher in melanomas than in non-melanoma tumors in animal models and cell cultures. ¹⁸F-FBPA was incorporated into tumors mediated mainly by L-type amino acid transporters in in vitro and in vivo models. Tumoral distribution of ¹⁸F-FBPA was primarily related to the activity of DNA synthesis. ¹⁸F-FBPA is metabolically stable but is incorporated into melanogenesis non-enzymatically. These in vitro and in vivo characteristics of ¹⁸F-FBPA corresponded well to those of ¹⁰B-BPA. Nuclear magnetic resonance and other studies using non-radioactive ¹⁹F-^10/11B-FBPA also contributed to characterization. The validity and reliability of ^18/19F-FBPA as an in vivo probe of ¹⁰B-BPA were confirmed by comparison of the pharmacokinetics of ¹⁸F-FBPA and ¹⁰B-BPA and direct measurement of both ¹⁸F and ¹⁰B in tumors with various doses of both probes administered by different routes and methods. Clinically, based on the kinetic parameters of dynamic ¹⁸F-FBPA PET, the estimated ¹⁰B-concentrations in tumors with continuous ¹⁰B-BPA infusion were similar to those measured directly in surgical specimens. The significance of ¹⁸F-FBPA PET was verified for the estimation of ¹⁰B-concentration and planning of BNCT. Later ¹⁸F-FBPA PET has been involved in ¹⁰B-BPA BNCT of patients with intractable tumors such as malignant brain tumors, head and neck tumors, and melanoma. Usually a static PET scan is used for screening patients for BNCT, prediction of the distribution and accumulation of ¹⁰B-BPA, and evaluation of treatment after BNCT. In some clinical trials, a tumor-to-normal tissue ratio of ¹⁸F-FBPA > 2.5 was an inclusion criterion for BNCT. Apart from BNCT, ¹⁸F-FBPA was demonstrated to be a useful PET probe for tumor diagnosis in nuclear medicine: better tumor-to-normal brain contrast compared with ¹¹C-methionine, differentiation of recurrent and radiation necrosis after radiotherapy, and melanoma-preferential uptake. Further progress in ¹⁸F-FBPA studies is expected for more elaborate evaluation of ¹⁰B-concentrations in tumors and normal tissues for successful ¹⁰B-BPA BNCT and for radiosynthesis of ¹⁸F-FBPA to enable higher ¹⁸F-activity amounts and higher molar activities.

Design and synthesis of boron containing monosaccharides by the hydroboration of d-glucal for use in boron neutron capture therapy (BNCT)

Boron neutron capture therapy (BNCT) is one of the radiotherapies that involves the use of boron-containing compounds for the treatment of cancer. Boron-10 (¹⁰B) containing compounds that can accumulate in tumor tissue are expected to be suitable agents for BNCT. We report herein on the design and synthesis of some new BNCT agents based on a d-glucose scaffold, since glycoconjugation has been recognized as a useful strategy for the specific targeting of tumors. To introduce a boryl group into a d-glucose scaffold, we focused on the hydroboration of d-glucal derivatives, which have a double bond between the C1 and C2 positions. It was hypothesized that a C-B bond could be introduced at the C2 position of d-glucose by the hydroboration of d-glucal derivatives and that the products could be stabilized by conversion to the corresponding boronic acid ester. To test this hypothesis, we prepared some 2-boryl-1,2-dideoxy-d-glucose derivatives as boron carriers and evaluated their cytotoxicity and cellular uptake activity to cancer cells, especially under hypoxic conditions.

Non-invasive estimation of 10 B-4-borono-L-phenylalanine-derived boron concentration in tumors by PET using 4-borono-2-18 F-fluoro-phenylalanine

In boron neutron capture therapy (BNCT), 10 B-4-borono-L-phenylalanine (BPA) is commonly used as a 10 B carrier. PET using 4-borono-2-18 F-fluoro-phenylalanine (18 F-FBPA PET) has been performed to estimate boron concentration and predict the therapeutic effects of BNCT; however, the association between tumor uptake of 18 F-FBPA and boron concentration in tumors remains unclear. The present study investigated the transport mechanism of 18 F-FBPA and BPA, and evaluated the utility of 18 F-FBPA PET in predicting boron concentration in tumors. The transporter assay revealed that 2-aminobicyclo-(2.2.1)-heptane-2-carboxylic acid, an inhibitor of the L-type amino acid transporter, significantly inhibited 18 F-FBPA and 14 C-4-borono-L-phenylalanine (14 C-BPA) uptake in FaDu and LN-229 human cancer cells. 18 F-FBPA uptake strongly correlated with 14 C-BPA uptake in 7 human tumor cell lines (r = .93; P < .01). PET experiments demonstrated that tumor uptake of 18 F-FBPA was independent of the administration method, and uptake of 18 F-FBPA by bolus injection correlated well with BPA uptake by continuous intravenous infusion. The results of this study revealed that evaluating tumor uptake of 18 F-FBPA by PET was useful for estimating 10 B concentration in tumors.

Impact of oxygen status on 10B-BPA uptake into human glioblastoma cells, referring to significance in boron neutron capture therapy

Boron neutron capture therapy (BNCT) can potentially deliver high linear energy transfer particles to tumor cells without causing severe damage to surrounding normal tissue, and may thus be beneficial for cases with characteristics of infiltrative growth, which need a wider irradiation field, such as glioblastoma multiforme. Hypoxia is an important factor contributing to resistance to anticancer therapies such as radiotherapy and chemotherapy. In this study, we investigated the impact of oxygen status on 10B uptake in glioblastoma cells in vitro in order to evaluate the potential impact of local hypoxia on BNCT. T98G and A172 glioblastoma cells were used in the present study, and we examined the influence of oxygen concentration on cell viability, mRNA expression of L-amino acid transporter 1 (LAT1), and the uptake amount of 10B-BPA. T98G and A172 glioblastoma cells became quiescent after 72 h under 1% hypoxia but remained viable. Uptake of 10B-BPA, which is one of the agents for BNCT in clinical use, decreased linearly as oxygen levels were reduced from 20% through to 10%, 3% and 1%. Hypoxia with <10% O2 significantly decreased mRNA expression of LAT1 in both cell lines, indicating that reduced uptake of 10B-BPA in glioblastoma in hypoxic conditions may be due to reduced expression of this important transporter protein. Hypoxia inhibits 10B-BPA uptake in glioblastoma cells in a linear fashion, meaning that approaches to overcoming local tumor hypoxia may be an effective method of improving the success of BNCT treatment.

closo-Dodecaborate-conjugated human serum albumins: preparation and in vivo selective boron delivery to tumor

Maleimide-functionalized closo-dodecaborate (MID) and isothiocyanate-functionalized closo-dodecaborate (ISD) were synthesized from closo-dodecaborate via ring opening reaction of 1,4-dioxane-closo-dedecaborate complex 1 with ammonia. MID was found to possess highest conjugation efficacy to bovine serum albumin among three closo-dodecaborate derivatives, MID, ISD, and 1. The conjugation reaction of MID to human serum albumin (HSA) proceeded under PBS buffer conditions (pH 7.4). Boron distribution studies in colon 26 tumor-bearing mice revealed that HSA-MID was highly accumulated in tumor (23 ppm B), whereas boron concentrations in other organs such as liver, kidney and spleen were low (3~8 ppm B).

Preloading with L-BPA, L-tyrosine and L-DOPA enhances the uptake of [18F]FBPA in human and mouse tumour cell lines

Aim of this study was to investigate if cellular [¹⁸F]FBPA uptake can be increased upon preloading with amino acids. [¹⁸F]FBPA uptake was assessed in HuH-7, CaCo-2 and B16-F1 cells pretreated with different concentrations or incubation times of L-BPA, L-tyrosine or L-DOPA. Without preloading, highest uptake of [¹⁸F]FBPA was observed in B16-F1 cells, followed by CaCo-2 cells and HuH-7 cells. In all cell lines higher [¹⁸F]FBPA accumulation (up to 1.65-fold) was obtained with increasing L-BPA, L-DOPA and L-tyrosine concentrations.

Maleimide-functionalized closo-dodecaborate albumin conjugates (MID-AC): Unique ligation at cysteine and lysine residues enables efficient boron delivery to tumor for neutron capture therapy

Maleimide-conjugating closo-dodecaborate sodium form 5c (MID) synthesized by the nucleophilic ring-opening reaction of closo-dodecaborate-1,4-dioxane complex 2 with tetrabutylammonium (TBA) azide was found to conjugate to free SH of cysteine and lysine residues in BSA under physiological conditions, forming highly boronated BSA that showed high and selective accumulation in tumor and significant tumor growth inhibition in colon 26 tumor-bearing mice subjected to thermal neutron irradiation.

The Anti-Proliferative Effect of Boron Neutron Capture Therapy in a Prostate Cancer Xenograft Model

Purpose

Boron neutron capture therapy (BNCT) is a selective radiation treatment for tumors that preferentially accumulate drugs carrying the stable boron isotope, ¹⁰B. BNCT has been evaluated clinically as an alternative to conventional radiation therapy for the treatment of brain tumors, and more recently, recurrent advanced head and neck cancer. Here we investigated the effect of BNCT on prostate cancer (PCa) using an in vivo mouse xenograft model that we have developed.

Materials and Methods

Mice bearing the xenotransplanted androgen-independent human PCa cell line, PC3, were divided into four groups: Group 1: untreated controls; Group 2: Boronophenylalanine (BPA); Group 3: neutron; Group 4: BPA-mediated BNCT. We compared xenograft growth among these groups, and the body weight and any motility disturbance were recorded. Immunohistochemical (IHC) studies of the proliferation marker, Ki-67, and TUNEL staining were performed 9 weeks after treatment.

Results

The in vivo studies demonstrated that BPA-mediated BNCT significantly delayed tumor growth in comparison with the other groups, without any severe adverse events. There was a significant difference in the rate of freedom from gait abnormalities between the BPA-mediated BNCT group and the other groups. The IHC studies revealed that BNCT treatment significantly reduced the number of Ki-67-positive cells in comparison with the controls (mean±SD 6.9±1.5 vs 12.7±4.0, p<0.05), while there was no difference in the number of apoptotic cells, suggesting that BPA-mediated BNCT reduced PCa progression without affecting apoptosis at 9 weeks post-treatment.

Conclusions

This study has provided the first preclinical proof-of-principle data to indicate that BPA-mediated BNCT reduces the in vivo growth of PCa. Although further studies will be necessary, BNCT might be a novel potential treatment for PCa.