Current status of boron neutron capture therapy of high grade gliomas and recurrent head and neck cancer

In Vitro Studies to Define the Cell-Surface and Intracellular Targets of Polyarginine-Conjugated Sodium Borocaptate as a Potential Delivery Agent for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) requires pharmaceutical innovations and molecular-based evidence of effectiveness to become a standard cancer therapeutic in the future. Recently, in Japan, 4-borono-L-phenylalanine (BPA) was approved as a boron agent for BNCT against head and neck (H&N) cancers. H&N cancer appears to be a suitable target for BPA-BNCT, because the expression levels of L-type amino acid transporter 1 (LAT1), one of the amino acid transporters responsible for BPA uptake, are elevated in most cases of H&N cancer. However, in other types of cancer including malignant brain tumors, LAT1 is not always highly expressed. To expand the possibility of BNCT for these cases, we previously developed poly-arginine peptide (polyR)-conjugated mercaptoundecahydrododecaborate (BSH). PolyR confers the cell membrane permeability and tumor selectivity of BSH. However, the molecular determinants for the properties are not fully understood. In this present study, we have identified the cluster of differentiation 44 (CD44) protein and translational machinery proteins as a major cell surface target and intracellular targets of BSH-polyR, respectively. CD44, also known as a stem cell-associated maker in various types of cancer, is required for the cellular uptake of polyR-conjugated molecules. We showed that BSH-polyR was predominantly delivered to a CD44^High cell population of cancer cells. Once delivered, BSH-polyR interacted with the translational machinery components, including the initiation factors, termination factors, and poly(A)-biding protein (PABP). As a proof of principle, we performed BSH-polyR-based BNCT against glioma stem-like cells and revealed that BSH-polyR successfully induced BNCT-dependent cell death specifically in CD44^High cells. Bioinformatics analysis indicated that BSH-polyR would be suitable for certain types of malignant tumors. Our results shed light on the biochemical properties of BSH-polyR, which may further contribute to the therapeutic optimization of BSH-BNCT in the future.

Thermal Neutron Relative Biological Effectiveness Factors for Boron Neutron Capture Therapy from In Vitro Irradiations

The experimental determination of the relative biological effectiveness of thermal neutron factors is fundamental in Boron Neutron Capture Therapy. The present values have been obtained while using mixed beams that consist of both neutrons and photons of various energies. A common weighting factor has been used for both thermal and fast neutron doses, although such an approach has been questioned. At the nuclear reactor of the Institut Laue-Langevin a pure low-energy neutron beam has been used to determine thermal neutron relative biological effectiveness factors. Different cancer cell lines, which correspond to glioblastoma, melanoma, and head and neck squamous cell carcinoma, and non-tumor cell lines (lung fibroblast and embryonic kidney), have been irradiated while using an experimental arrangement designed to minimize neutron-induced secondary gamma radiation. Additionally, the cells were irradiated with photons at a medical linear accelerator, providing reference data for comparison with that from neutron irradiation. The survival and proliferation were studied after irradiation, yielding the Relative Biological Effectiveness that corresponds to the damage of thermal neutrons for the different tissue types.

Dosimetric comparison of Boron Neutron Capture Therapy, Proton Therapy and Volumetric Modulated Arc Therapy for Recurrent Anaplastic Meningioma

A patient with Recurrent Anaplastic Meningioma of the left temporal lobe was being planned for treatment. Using the same target volumes and organs-at-risk, we compared the treatment plans for Boron Neutron Capture Therapy, Proton Therapy and Volumetric Modulated Arc Therapy. The results showed that BNCT provides the best dosimetry amongst the three modalities.

Boron neutron capture therapy: Current status and future perspectives

The development of new accelerators has given a new impetus to the development of new drugs and treatment technologies using boron neutron capture therapy (BNCT). We analyzed the current status and future directions of BNCT for cancer treatment, as well as the main issues related to its introduction. This review highlights the principles of BNCT and the key milestones in its development: new boron delivery drugs and different types of charged particle accelerators are described; several important aspects of BNCT implementation are discussed. BCNT could be used alone or in combination with chemotherapy and radiotherapy, and it is evaluated in light of the outlined issues. For the speedy implementation of BCNT in medical practice, it is necessary to develop more selective boron delivery agents and to generate an epithermal neutron beam with definite characteristics. Pharmacological companies and research laboratories should have access to accelerators for large-scale screening of new, more specific boron delivery agents.

The accelerator-based boron neutron capture reaction evaluation system for head and neck cancer

The purpose of this study is to assess accelerator-based boron neutron capture reaction (BNCR) in human tumor cell lines by colony formation assay and modified high density survival assay (HDS assay). The results of post irradiation survival rate in human oral squamous cell carcinoma and osteosarcoma using both assays were similar. Therefore, HDS assay would be efficient to evaluate BNCR in not only tumor cells but also in normal cells as BNCT screening.

Neutron autoradiography to study the microdistribution of boron in the lung

In Argentina, a multi-institutional project has been established to assess the feasibility of applying BNCT ex-situ to the treatment of patients with multiple metastases in both lungs. Within this context, this work aims at applying the neutron autoradiography technique to study boron microdistribution in the lung. A comprehensive analysis of the different aspects for the generation of autoradiographic images of both normal and metastatic BDIX rat lungs was achieved. Histology, boron uniformity, optimal tissue thickness and water content in tissue were explored for the two types of samples. A qualitative and a quantitative analysis were performed. No heterogeneities in uptake were observed in normal lung. Conversely, samples with metastasis showed preferential boron uptake in the tumour areas with respect to surrounding tissue. Surrounding tissue would present a slightly higher uptake of boron than the normal lung. Quantitative results of boron concentration values and ratios determined by neutron autoradiography were obtained. In order to contribute to BNCT dosimetry, further analysis increasing the number of samples is warranted.

Design of a BNCT irradiation room based on proton accelerator and beryllium target

Preliminary studies for the design of an accelerator-based BNCT clinical facility are presented. The Beam Shaping Assembly neutron activation was evaluated experimentally and with Monte Carlo simulations. The activations of patient, air and walls in the room, the absorbed doses by the patient and the in-air dose distributions were evaluated. Based on these calculations, different walls compositions were tested to optimize the environmental conditions. Borated concrete, advantageously reducing the thermal flux in the room, was proven the best choice.

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.

BNCT research activities at the Granada group and the project NeMeSis: Neutrons for medicine and sciences, towards an accelerator-based facility for new BNCT therapies, medical isotope production and other scientific neutron applications

The Granada group in BNCT research is currently performing studies on: nuclear and radiobiological data for BNCT, new boron compounds and a new design for a neutron source for BNCT and other applications, including the production of medical radioisotopes. All these activities are described in this report.

Studies on the Exposure of Gadolinium Containing Nanoparticles with Monochromatic X-rays Drive Advances in Radiation Therapy

While conventional radiation therapy uses white X-rays that consist of a mixture of X-ray waves with various energy levels, a monochromatic X-ray (monoenergetic X-ray) has a single energy level. Irradiation of high-Z elements such as gold, silver or gadolinium with a synchrotron-generated monochromatic X-rays with the energy at or higher than their K-edge energy causes a photoelectric effect that includes release of the Auger electrons that induce DNA damage—leading to cell killing. Delivery of high-Z elements into cancer cells and tumor mass can be facilitated by the use of nanoparticles. Various types of nanoparticles containing high-Z elements have been developed. A recent addition to this growing list of nanoparticles is mesoporous silica-based nanoparticles (MSNs) containing gadolinium (Gd–MSN). The ability of Gd–MSN to inhibit tumor growth was demonstrated by evaluating effects of irradiating tumor spheroids with a precisely tuned monochromatic X-ray.

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.

Synthesis and Evaluation of Dodecaboranethiol Containing Kojic Acid (KA-BSH) as a Novel Agent for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a form of tumor-cell selective particle irradiation using low-energy neutron irradiation of boron-10 (¹⁰B) to produce high-linear energy transfer (LET) alpha particles and recoiling ⁷Li nuclei (¹⁰B [n, alpha] ⁷Li) in tumor cells. Therefore, it is important to achieve the selective delivery of large amounts of ¹⁰B to tumor cells, with only small amounts of ¹⁰B to normal tissues. To develop practical materials utilizing ¹⁰B carriers, we designed and synthesized novel dodecaboranethiol (BSH)-containing kojic acid (KA-BSH). In the present study, we evaluated the effects of this novel ¹⁰B carrier on cytotoxicity, ¹⁰B concentrations in F98 rat glioma cells, and micro-distribution of KA-BSH in vitro. Furthermore, biodistribution studies were performed in a rat brain tumor model. The tumor boron concentrations showed the highest concentrations at 1 h after the termination of administration. Based on these results, neutron irradiation was evaluated at the Kyoto University Research Reactor Institute (KURRI) with KA-BSH. Median survival times (MSTs) of untreated and irradiated control rats were 29.5 and 30.5 days, respectively, while animals that received KA-BSH, followed by neutron irradiation, had an MST of 36.0 days (p = 0.0027, 0.0053). Based on these findings, further studies are warranted in using KA-BSH as a new B compound for malignant glioma.

m-Carborane as a Novel Core for Periphery-Decorated Macromolecules

Closom-C₂B₁₀H₁₂ can perform as a novel core of globular periphery-decorated macromolecules. To do this, a new class of di and tetrabranched m-carborane derivatives has been synthesized by a judicious choice of the synthetic procedure, starting with 9,10-I₂-1,7-closo-C₂B₁₀H₁₀. The 2a-NPA (sum of the natural charges of the two bonded atoms) value for a bond, which is defined as the sum of the NPA charges of the two bonded atoms, matches the order of electrophilic reaction at the different cluster bonds of the icosahedral o-and m- carboranes that lead to the formation of B-I bonds. As for m-carborane, most of the 2a-NPA values of B-H vertexes are positive, and their functionalization is more challenging. The synthesis and full characterization of dibranched 9,10-R₂-1,7-closo-carborane (R = CH₂CHCH₂, HO(CH₂)₃, Cl(CH₂)₃, TsO(CH₂)₃, C₆H₅COO(CH₂)₃, C₆H₅COO(CH₂)₃, N₃(CH₂)₃, CH₃CHCH, and C₆H₅C₂N₃(CH₂)₃) compounds as well as the tetrabranched 9,10-R₂-1,7-R₂-closo-C₂B₁₀H₈ (R = CH₂CHCH₂, HO(CH₂)₃) are presented. The X-ray diffraction of 9,10-(HO(CH₂)₃)₂-1,7-closo-C₂B₁₀H₁₀ and 9,10-(CH₃CHCH)₂-1,7-closo-C₂B₁₀H₁₀, as well as their Hirshfeld surface analysis and decomposed fingerprint plots, are described. These new reported tetrabranched m-carborane derivatives provide a sort of novel core for the synthesis of 3D radially grown periphery-decorated macromolecules that are different to the 2D radially grown core of the tetrabranched o-carborane framework.

Conjugate of chlorin е6 with iron bis(dicarbollide) nanocluster: synthesis and biological properties

Background: Efficiency of both photodynamic and boron-neutron capture anticancer therapies (BNCT) depends on the properties of the used photo- and neutronsensitizer. We report on the synthesis and properties of the advanced photo- and neutronsensitizer designed as a conjugate of chlorin e₆ with iron bis(dicarbollide) nanocluster. Results: The conjugate is shown to accumulate efficiently in rat glioblastoma C6 cells delivering >10⁹ boron atoms per cell and thus meeting requirements for BNCT agents, to provide photoinduced 50% death of C6 cells at 35 ± 3 nM, to be not toxic for cells without activating stimulus. Conclusions: The conjugate is a prospective theranostic agent for photodynamic, BNCT and fluorescent diagnostics of tumors.

Functionalized mesoporous silica nanoparticles for innovative boron-neutron capture therapy of resistant cancers

Treatment resistance, relapse and metastasis remain critical issues in some challenging cancers, such as chondrosarcomas. Boron-neutron capture therapy (BNCT) is a targeted radiation therapy modality that relies on the ability of boron atoms to capture low energy neutrons, yielding high linear energy transfer alpha particles. We have developed an innovative boron-delivery system for BNCT, composed of multifunctional fluorescent mesoporous silica nanoparticles (B-MSNs), grafted with an activatable cell penetrating peptide (ACPP) for improved penetration in tumors and with gadolinium for magnetic resonance imaging (MRI) in vivo. Chondrosarcoma cells were exposed in vitro to an epithermal neutron beam after B-MSNs administration. BNCT beam exposure successfully induced DNA damage and cell death, including in radio-resistant ALDH+ cancer stem cells (CSCs), suggesting that BNCT using this system might be a suitable treatment modality for chondrosarcoma or other hard-to-treat cancers.

Boron Chemistry for Medical Applications

Boron compounds now have many applications in a number of fields, including Medicinal Chemistry. Although the uses of boron compounds in pharmacological science have been recognized several decades ago, surprisingly few are found in pharmaceutical drugs. The boron-containing compounds epitomize a new class for medicinal chemists to use in their drug designs. Carboranes are a class of organometallic compounds containing carbon (C), boron (B), and hydrogen (H) and are the most widely studied boron compounds in medicinal chemistry. Additionally, other boron-based compounds are of great interest, such as dodecaborate anions, metallacarboranes and metallaboranes. The boron neutron capture therapy (BNCT) has been utilized for cancer treatment from last decade, where chemotherapy and radiation have their own shortcomings. However, the improvement in the already existing (BPA and/or BSH) localized delivery agents or new tumor-targeted compounds are required before realizing the full clinical potential of BNCT. The work outlined in this short review addresses the advancements in boron containing compounds. Here, we have focused on the possible clinical implications of the new and improved boron-based biologically active compounds for BNCT that are reported to have in vivo and/or in vitro efficacy.

Boron neutron capture therapy at the crossroads - Where do we go from here?

As elegant as is the concept upon which Boron Neutron Capture Therapy (BNCT) is based, unfortunately it has not gained widespread acceptance by the physicians who are treating cancer patients on a daily basis. The question is why? Very simply put, the clinical results obtained in treating patients with high grade gliomas and recurrent tumors of the head and neck region have not been convincing enough to produce more interest in BNCT as a cancer treatment modality. There are a variety of reasons for this, one of the most important of which has been its dependency on nuclear reactors as neutron sources. With the advent of accelerator based neutron sources (ABNS), this hopefully will be addressed. If the results obtained from ongoing and soon to be initiated clinical trials can at least demonstrate equivalency to those obtained with nuclear reactors, this should address the first problem. The second problem relates to boron delivery agents, and despite the considerable efforts of chemists and biologists over the past 50 years, there are only two drugs that currently are being used clinically, sodium borocaptate (BSH) and boronophenylalanine (BPA). It is widely recognized that these two drugs are less than ideal. Perhaps new and more effective boron delivery agents will finally appear on the scene, but barring that, we will address the question of what can be done now to make BNCT a more effective cancer treatment modality.

On-Demand Biodegradable Boron Nitride Nanoparticles for Treating Triple Negative Breast Cancer with Boron Neutron Capture Therapy

Compared with photon-induced binary cancer therapy, such as photothermal therapy (PTT) and photodynamic therapy (PDT), boron neutron capture therapy (BNCT) emerges as an alternative noninvasive treatment strategy that could overcome the shallow penetration of light. One key factor in performing successful BNCT is to accumulate a sufficient amount of B-10 (>20 ppm) within tumor cells, which has been a long-standing challenge for small-molecule-based boron drugs. Boron nitride nanoparticles (BNNPs) are promising boron carriers due to their high boron content and good biocompatibility, as certain types of BNNPs can undergo rapid degradation under physiological conditions. To design an on-demand degradable boron carrier, BNNPs were coated by a phase-transitioned lysozyme (PTL) that protects BNNPs from hydrolysis during blood circulation and can be readily removed by vitamin C after neutron capture therapy. According to PET imaging, the coated BNNPs exhibited high tumor boron accumulation while maintaining a good tumor to nontumor ratio. Tail-vein injections of vitamin C were followed by neutron irradiation, and BNNPs were found to be rapidly cleared from major organs according to ex vivo ICP-OES analysis. Compared with the control group, animals treated with BNCT showed suppression of tumor growth, while almost negligible side effect was observed. This strategy not only utilized the high boron content of BNNPs but also successfully performed an on-demand degradation of BNNPs to avoid the potential toxicity caused by the long-term accumulation of nanoparticles.

Application of Nitroimidazole-Carbobane-Modified Phenylalanine Derivatives as Dual-Target Boron Carriers in Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) has received extensive attention as noninvasive cell-level oncotherapy for treating solid cancer tumors. However, boron-containing drugs such as l-boronophenylalanine (BPA) and sodium borocaptate have low boron content and/or poor tumor-targeting ability, limiting their application. In this study, we designed and synthesized a series of nontoxic, dual-target boron carriers (B139, B142, and B151) with the ability to accumulate specifically in tumor cells. We found that the B139 uptake into hypoxic tumor regions was high, with a 70-fold boron content compared to BPA. In addition, in vivo observation showed that B139 can be trapped in tumor cells for a prolonged period and maintains an effective therapeutic concentration, with a peak boron concentration of 50.7 μg/g and a high tumor: blood boron ratio of >3, achieving ideal BNCT conditions. Cytotoxicity evaluation in mice further proved that B139 is safe and reliable. Therefore, B139 has great potential for BNCT application as a dual-target, safe, and efficient boron carrier.

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).

PEGylated liposome encapsulating nido-carborane showed significant tumor suppression in boron neutron capture therapy (BNCT)

Boron neutron capture therapy (BNCT) is a binary radiotherapy based on nuclear reactions that occur when boron-10 is irradiated with neutrons, which result in the ejection of high-energy alpha particles. Successful BNCT requires the efficient delivery of a boron-containing compound to effect high concentrations in tumor cells while minimizing uptake in normal tissues. In this study, PEGylated liposomes were employed as boron carriers to maximize delivery to tumors and minimize uptake in the reticuloendothelial system (RES). The water-soluble potassium salt of nido-7,8-carborane, nido-carborane, was chosen as the boron source due to its high boron content per molecule. Nido-carborane was encapsulated in the aqueous cores of PEGylated liposomes by hydrating thin lipid films. Repeated freezing and thawing increased nido-carborane loading by up to 47.5 ± 3.1%. The average hydrodynamic diameter of the prepared boronated liposomes was determined to be 114.5 ± 28 nm through dynamic light scattering (DLS) measurement. Globular liposomes approximately 100 nm in diameter were clearly visible in transmission electron microscope (TEM) images. The viability of tumor cells following BNCT with 70 μM nido-carborane was reduced to 17.1% compared to irradiated control cells, which did not contain boronated liposomes. Confocal microscopy revealed that fluorescently labeled liposomes injected into the tail veins of mice were deeply and evenly distributed in tumor tissues and localized in the cytoplasm of tumor cells. When mice were properly shielded with a 12 mm-thick polyethylene board during in-vivo irradiation at a thermal neutron flux of 1.94 × 10⁴/cm²·sec, almost complete tumor suppression was achieved in tumor models injected with boronated liposomes (21.0 mg ¹⁰B/kg). Two BNCT cycles spaced 10 days apart further enhanced the therapeutic anti-tumor effect, even when the dose was lowered to 10.5 mg ¹⁰B/kg. No notable weight loss was observed in the tumor models during the BNCT study.

Selective Neuropeptide Y Conjugates with Maximized Carborane Loading as Promising Boron Delivery Agents for Boron Neutron Capture Therapy

G-protein-coupled receptors like the human Y₁ receptor (hY₁R) are promising targets in cancer therapy due to their high overexpression on cancer cells and their ability to internalize together with the bound ligand. This mechanism was exploited to shuttle boron atoms into cancer cells for the application of boron neutron capture therapy (BNCT), a noninvasive approach to eliminate cancer cells. A maximized number of carboranes was introduced to the hY₁R-preferring ligand [F⁷,P³⁴]-NPY by solid phase peptide synthesis. Branched conjugates loaded with up to 80 boron atoms per peptide molecule exhibited a maintained receptor activation profile, and the selective uptake into hY₁R-expressing cells was demonstrated by internalization studies. In order to ensure appropriate solubility in aqueous solution, we proved the need for eight hydroxyl groups per carborane. Thus, we suggest the utilization of bis-deoxygalactosyl-carborane building blocks in solid phase peptide synthesis to produce selective boron delivery agents for BNCT.

A Metabolically Stable Boron-Derived Tyrosine Serves as a Theranostic Agent for Positron Emission Tomography Guided Boron Neutron Capture Therapy

Boronophenylalanine (BPA) is the dominant boron delivery agent for boron neutron capture therapy (BNCT), and [¹⁸F]FBPA has been developed to assist the treatment planning for BPA-BNCT. However, the clinical application of BNCT has been limited by its inadequate tumor specificity due to the metabolic instability. In addition, the distinctive molecular structures between [¹⁸F]FBPA and BPA can be of concern as [¹⁸F]FBPA cannot quantitate boron concentration of BPA in a real-time manner. In this study, a metabolically stable boron-derived tyrosine (denoted as fluoroboronotyrosine, FBY) was developed as a theranostic agent for both boron delivery and cancer diagnosis, leading to PET imaging-guided BNCT of cancer. [¹⁸F]FBY was synthesized in high radiochemical yield (50%) and high radiochemical purity (98%). FBY showed high similarity with natural tyrosine. As shown in in vitro assays, the uptake of FBY in murine melanoma B16-F10 cells was L-type amino acid transporter (LAT-1) dependent and reached up to 128 μg/10⁶ cells. FBY displayed high stability in PBS solution. [¹⁸F]FBY PET showed up to 6 %ID/g in B16-F10 tumor and notably low normal tissue uptake (tumor/muscle = 3.16 ± 0.48; tumor/blood = 3.13 ± 0.50; tumor/brain = 14.25 ± 1.54). Moreover, administration of [¹⁸F]FBY tracer along with a therapeutic dose of FBY showed high accumulation in B16-F10 tumor and low normal tissue uptake. Correlation between PET-image and boron biodistribution was established, indicating the possibility of estimating boron concentration via a noninvasive approach. At last, with thermal neutron irradiation, B16-F10 tumor-bearing mice injected with FBY showed significantly prolonged median survival without exhibiting obvious systemic toxicity. In conclusion, FBY holds great potential as an efficient theranostic agent for imaging-guided BNCT by offering a possible solution of measuring local boron concentration through PET imaging.

Preparation of pentagamaboronon-0 and its fructose and sorbitol complexes as boron carrier for boron neutron capture therapy (BNCT) application

Development of specific and selective boron carriers is indispensable for boron neutron capture therapy (BNCT) application. Pentagamaboronon-0 (PGB-0) is a promising candidate as boron carrier compound due to the low but selective cytotoxicity in breast cancer cells. Formerly we reported synthesis of PGB-0 which was ineffective due to its low aqueous solubility. In the present study, we, therefore, introduced the new PGB-0 preparation complexed with sugars to increase its solubility in water. By synthesizing at room temperature and using flash chromatography for the purification, we produced PGB-0 with a yield of 40%. PGB-0 fructose complex (PGB-0-F) and PGB-0 sorbitol complex (PGB-0-Sor) were obtained with smaller particle size compared to PGB-0 suspension in water. Based on the MTT assay, the cytotoxicity of PGB-0-F and PGB-0-Sor were higher than PGB-0 even though still categorized as low cytotoxic agents. In conclusion, we provided PGB-0 with a new method and improved its solubility in water. Further investigations are still needed to develop more efficient PGB-0 as boron carrier for BNCT in various cancers.

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.

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.

Boron neutron capture therapy for locally recurrent head and neck squamous cell carcinoma: An analysis of dose response and survival

BACKGROUND AND PURPOSE

Head and neck squamous cell carcinoma (HNSCC) that recurs locally is a therapeutic challenge. We investigated the efficacy of boron neutron capture therapy (BNCT) in the treatment of such patients and the factors associated with treatment response and survival.

METHODS AND MATERIALS

Seventy-nine patients with inoperable, locally recurred HNSCC were treated with l-boronophenylalanine-mediated BNCT in Espoo, Finland, between February, 2003 and January, 2012. Prior treatments consisted of surgery and conventionally fractionated radiotherapy to a median cumulative dose of 66 Gy (interquartile range [IQR], 59-70 Gy) administered with or without concomitant chemotherapy. Tumor response was assessed using the RECISTv.1.0 criteria.

RESULTS

Forty patients received BNCT once (on 1 day), and 39 twice. The median time between the 2 treatments was 6 weeks. Forty-seven (68%; 95% confidence interval [CI], 57-79%) of the 69 evaluable patients responded; 25 (36%) had a complete response, 22 (32%) a partial response, 17 (25%) a stable disease lasting for a median of 4.2 months, and 5 (7%) progressed. The patients treated with BNCT twice responded more often than those treated once. The median follow-up time after BNCT was 7.8 years. The 2-year locoregional progression-free survival rate was 38% and the overall survival rate 21%. A high minimum tumor dose and a small volume were independently associated with long survival in a multivariable analysis.

CONCLUSIONS

Most patients responded to BNCT. A high minimum tumor dose from BNCT was predictive for response and survival.

Combining magnetic nanoparticles and icosahedral boron clusters in biocompatible inorganic nanohybrids for cancer therapy

The potential biomedical applications of the MNPs nanohybrids coated with m-carboranylphosphinate (1-MNPs) as a theranostic biomaterial for cancer therapy were tested. The cellular uptake and toxicity profile of 1-MNPs from culture media by human brain endothelial cells (hCMEC/D3) and glioblastoma multiform A172 cell line were demonstrated. Prior to testing 1-MNPs' in vitro toxicity, studies of colloidal stability of the 1-MNPs' suspension in different culture media and temperatures were carried out. TEM images and chemical titration confirmed that 1-MNPs penetrate into cells. Additionally, to explore 1-MNPs' potential use in Boron Neutron Capture Therapy (BNCT) for treating cancer locally, the presence of the m-carboranyl coordinated with the MNPs core after uptake was proven by XPS and EELS. Importantly, thermal neutrons irradiation in BNCT reduced by 2.5 the number of cultured glioblastoma cells after 1-MNP treatment, and the systemic administration of 1-MNPs in mice was well tolerated with no major signs of toxicity.

Charged particle spectrometry to measure 10B concentration in bone

Osteosarcoma is the most common primary malignant tumour of bone in young patients. The survival of these patients has largely been improved due to adjuvant and neo-adjuvant chemotherapy in addition to surgery. Boron neutron capture therapy (BNCT) is proposed as a complementary therapy, due to its ability to inactivate tumour cells that may survive the standard treatment and that may be responsible for recurrences and/or metastases. BNCT is based on neutron irradiation of a tumour enriched in ¹⁰B with a boron-loaded drug. Low-energy neutron capture in ¹⁰B creates charged particles that impart a high dose to tumour cells, which can be calculated only knowing the boron concentration. Charged particle spectrometry is a method that can be used to quantify boron concentration. This method requires acquisition of the energy spectra of charged particles such as alpha particles produced by neutron capture reactions in thin tissue sections irradiated with low-energy neutrons. Boron concentration is then determined knowing the stopping power of the alpha particles in the sample material. This paper describes the adaptation of this method for bone, with emphasis on sample preparation, experimental set-up and stopping power assessment of the involved alpha particles. The knowledge of boron concentration in healthy bones is important, because it allows for any dose limitation that might be necessary to avoid adverse effects such as bone fragility. The measurement process was studied through Monte Carlo simulations and analytical calculations. Finally, the boron content of bone samples was measured by alpha spectrometry at the TRIGA reactor in Pavia, Italy, and compared to that obtained by neutron autoradiography. The agreement between the results obtained with these techniques confirms the suitability of alpha spectrometry to measure boron in bone.

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.

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.

Folate receptor-targeted novel boron compound for boron neutron capture therapy on F98 glioma-bearing rats

Folic acid (FA) has high affinity for the folate receptor (FR), which is limited expressed in normal human tissues, but over-expressed in several tumor cells, including glioblastoma cells. In the present work, a novel pteroyl-closo-dodecaborate conjugate (PBC) was developed, in which the pteroyl group interacts with FR, and the efficacy of boron neutron capture therapy (BNCT) using PBC was investigated. Thus, in vitro and in vivo studies were performed using F98 rat glioma cells and F98 glioma-bearing rats. For the in vivo study, boronophenylalanine (BPA) was intravenously administered, while PBC was administered by convection-enhanced delivery (CED)-a method for direct local drug infusion into the brain of rats. Furthermore, a combination of PBC administered by CED and BPA administered by intravenous (i.v.) injection was also investigated. In the biodistribution experiment, PBC administration at 6 h after CED termination showed the highest cellular boron concentrations (64.6 ± 29.6 µg B/g). Median survival time (MST) of untreated controls was 23.0 days (range 21-24 days). MST of rats administered PBC (CED) followed by neutron irradiation was 31 days (range 26-36 days), which was similar to that of rats administered i.v. BPA (30 days; range 25-37 days). Moreover, the combination group [PBC (CED) and i.v. BPA] showed the longest MST (38 days; range 28-40 days). It is concluded that a significant MST increase was noted in the survival time of the combination group of PBC (CED) and i.v. BPA compared to that in the single-boron agent groups. These findings suggest that the combination use of PBC (CED) has additional effects.

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.

Translational boron neutron capture therapy (BNCT) studies for the treatment of tumors in lung

PURPOSE

Boron neutron capture therapy (BNCT) combines selective accumulation of ¹⁰B carriers in tumor tissue with subsequent neutron irradiation. BNCT has been proposed for the treatment of multiple, non-resectable, diffuse tumors in lung. The aim of the present study was to evaluate the therapeutic efficacy and toxicity of BNCT in an experimental model of lung metastases of colon carcinoma in BDIX rats and perform complementary survival studies.

MATERIALS AND METHODS

We evaluated tumor control and toxicity in lung 2 weeks post-BNCT at 2 dose levels, including 5 experimental groups per dose level: T0 (euthanized pre-treatment), Boronophenylalanine-BNCT (BPA-BNCT), BPA + Sodium decahydrodecaborate-BNCT ((BPA + GB-10)-BNCT), Beam only (BO) and Sham (no treatment, same manipulation). Tumor response was assessed employing macroscopic and microscopic end-points. An additional experiment was performed to evaluate survival and oxygen saturation in blood.

RESULTS AND CONCLUSIONS

No dose-limiting signs of short/medium-term toxicity were observed in lung. All end-points revealed statistically significant BNCT-induced tumor control vs Sham at both dose levels. The survival experiment showed a statistically significant 45% increase in post-treatment survival time in the BNCT group (48 days) versus Sham (33 days). These data consistently revealed growth suppression of lung metastases by BNCT with no manifest lung toxicity. Highlights Boron Neutron Capture Therapy suppresses growth of experimental lung metastases No BNCT-induced short/medium-term toxicity in lung is associated with tumor control Boron Neutron Capture Therapy increased post-treatment survival time by 45.

Boron delivery for boron neutron capture therapy targeting a cancer-upregulated oligopeptide transporter

Boron neutron capture therapy (BNCT) is a radiotherapy utilizing the neutron capture and nuclear fission reaction of ¹⁰B taken up into tumor cells. The most commonly used boron agent in BNCT, p-borono-l-phenylalanine (BPA), is accumulated in tumors by amino acid transporters upregulated in tumor cells. Here, by using dipeptides of BPA and tyrosine (BPA-Tyr and Tyr-BPA), we propose a novel strategy of selective boron delivery into tumor cells via oligopeptide transporter PEPT1 upregulated in various cancers. Kinetic analyses indicated that BPA-Tyr and Tyr-BPA are transported by oligopeptide transporters, PEPT1 and PEPT2. The intrinsic oligopeptide transport activity in tumor cells clearly correlated with PEPT1 protein expression level but not with PEPT2, suggesting that PEPT1 is the predominant oligopeptide transporter at least in tumor cell lines. Furthermore, using BPA-Tyr and Tyr-BPA, boron was successfully delivered into PEPT1-expressing pancreatic cancer AsPC-1 cells via a PEPT1-mediated mechanism. Intravenous administration of BPA-Tyr into the mice bearing AsPC-1 xenograft tumors resulted in significant boron accumulation in the tumors. It is proposed that the oligopeptide transporters, especially PEPT1, are promising candidates for molecular targets of boron delivery in BNCT. The BPA-containing dipeptides would have a potential for the development of novel boron carriers targeting PEPT1.

Microarray assisted toxicological investigations of boron carbide nanoparticles on human primary alveolar epithelial cells

It is important to understand the adverse effects of nanoparticles on human health and to prepare risk reports for widely used nanoscale materials. Synthesis, characterization and cytotoxicity evaluation of B₄C nanoparticles were performed on HPAEpiC since, first encounter with nanoparticles would generally happen through lung by inhaling chemicals. B₄C nanoparticles were synthesized via chemical vapor deposition techniques and characterized by using transmission electron microscope (TEM), scanning electron microscope (SEM) and X-ray crystallography (XRD). 3-(4,5-dimethyl-thiazol-2-yl) 2,5-diphenyltetrazolium bromide (MTT), lactate dehydrogenase (LDH) and neutral red (NR) tests were used to analyze cell viability and cytotoxicity against nanoparticles exposure. Microarray analysis was used to discover whole genome effects of B₄C NPs on gene expressions changes of HPAEpiC cells. Then, the database for annotation, visualization and integrated discovery (DAVID) analysis was performed to understand relationships between gene pathways and nanoparticle exposure. Finally, cytotoxicity analysis revealed that IC₂₀ value for boron carbide (B₄C) nanoparticles was 202.525 mg/L. According to microarray analysis 32 genes expression change significantly (FC ≥ 2) over 40,000 genes scanning. The gene pathways analysis showed that boron carbide (B₄C) nanoparticles mostly affect amino acid biosynthesis process, TGF-beta signaling pathway and developmental proteins regulation. In conclusion, our results supported for the first time that boron carbide (B₄C) nanoparticles could be used as a safe nanomaterial in both pharmacological and medical applications.