Tumor-specific delivery of BSH-3R for boron neutron capture therapy and positron emission tomography imaging in a mouse brain tumor model

Microneedle-adapted PAMAM-BSH delivery facilitates spatiotemporal matching for melanoma boron neutron capture therapy

Boron Neutron Capture Therapy (BNCT) is a binary radiotherapy based on the combination of 10B drugs and thermal neutrons. Coordinating 10B with neutron irradiation will optimize the efficacy of BNCT, but further improvements are still required. Here, we develop a delivery system incorporating microneedles (MNs) and a modified boron agent PAMAM-BSH, enabling precise spatiotemporal matching of 10B with neutron irradiation for melanoma BNCT. The PAMAM-BSH is a cationic polymer 10B agent with amphiphilicity, forming nanomicelles in an aqueous solution. It exhibits high boron content and demonstrates good uptake and retention capabilities in cancer cells. When administering PAMAM-BSH-loaded MNs to melanoma-bearing mice, the duration for which the 10B content in tumors exceeds 20 ppm can extend up to 2 h. Notably, the system facilitates the precise administration of 10B into tumor cells, achieving concentrations exceeding 35 ng 10-6 cells. These contribute to an excellent alignment of spatiotemporal matching of 10B with neutron irradiation in BNCT. Meanwhile, the T/N (tumor-to-normal tissue) and T/B (tumor-to-blood) ratios can reach 30 and 100, respectively, which are much higher than the clinical BPA (T/N is 1.1-2.9, T/B is 1.1-3.6). Utilizing the system ultimately demonstrated a significant enhancement in melanoma BNCT, along with improved safety.

A Bis-Boron Amino Acid for Positron Emission Tomography and Boron Neutron Capture Therapy

Trifluoroborate boronophenylalanine (BBPA) is a boron amino acid analog of 4-boronophenylalanine (BPA) but with a trifluoroborate group (-BF3 -) instead of a carboxyl group (-COOH). Clinical studies have shown that 18F-labeled BBPA ([18F]BBPA) can produce high-contrast tumor images in positron emission tomography (PET). Beyond PET imaging, BBPA is a theranostic agent for boron neutron capture therapy (BNCT). Because BBPA possesses an identical chemical structure to BNCT and PET, it can potentially predict the boron concentration for BNCT using [18F]BBPA-PET. The synthesis of BBPA was achieved by selectively fluorinating the α-aminoborate compound, taking advantage of the varying rates of solvolysis of the B-F bond. The study showcased the high-contrast [18F]BBPA-PET imaging in various tumor models, highlighting its broad applicability for both [18F]BBPA-PET and BBPA-BNCT. [18F]BBPA-PET tumor uptake remains consistent across various doses, including those used in BNCT. This enables accurate estimation of the boron concentration in tumors using [18F]BBPA-PET. With its dual boron structure, BBPA increases boron concentration in tumor cells and tumor tissues compared to BPA. Thus, less boron carrier is needed. This study introduces a new theranostic boron carrier that enhances boron accumulation in tumors, predicts boron concentration, and enhances the accuracy and effectiveness of BNCT.

Current research trends and hotspots of boron neutron capture therapy: a bibliometric and visualization analysis

Purpose

This study aimed to describe the trends, current hotspots, and future directions in boron neutron capture therapy (BNCT) through a bibliometric analysis.

Methods

Articles related to BNCT published before 2023-12-31 were retrieved from the Web of Science Core Collection database. VOSviewer, R, and CiteSpace were used for bibliometric analysis and visualization.

Results

A total of 3347 related publications from 1975 to 2023 were retrieved. Since a burst of published documents in 1992, the past three decades have witnessed continuous investigations into BNCT-related studies. Japan was the most productive country (794, 23.72%), followed by the USA (792, 23.66%), while the latter had the most citations. Kyoto University was the most influential institution. Ono K was the most prolific author, and Applied Radiation and Isotopes was the most popular journal. Ono K was the author that had the most total citations, followed by Barth RF. "Carborane", "boronophenylalanine", "glioblastoma", "sodium borocaptate", "cancer" and "drug delivery" were the most frequent keywords. The article "Dendrimers and dendritic polymers in drug delivery" had the most citations, whereas "Boron delivery agents for neutron capture therapy of cancer" had the highest outbreak value.

Conclusion

Over the past three decades, research on BNCT has expanded significantly, with the development of novel boron carriers with improved medicinal characteristics being the most extensively investigated area. Future research will likely focus on the validation and modification of current BNCT treatment modalities using conventional boron agents in brain tumors, accelerator-based neutron sources and the application of BNCT in more clinical scenarios.

Recent advances in spatio-temporally controllable systems for management of glioma

Malignant glioma remains one of the most aggressive intracranial tumors with devastating clinical outcomes despite the great advances in conventional treatment approaches, including surgery and chemotherapy. Spatio-temporally controllable approaches to glioma are now being actively investigated due to the preponderance, including spatio-temporal adjustability, minimally invasive, repetitive properties, etc. External stimuli can be readily controlled by adjusting the site and density of stimuli to exert the cytotoxic on glioma tissue and avoid undesired injury to normal tissues. It is worth noting that the removability of external stimuli allows for on-demand treatment, which effectively reduces the occurrence of side effects. In this review, we highlight recent advancements in drug delivery systems for spatio-temporally controllable treatments of glioma, focusing on the mechanisms and design principles of sensitizers utilized in these controllable therapies. Moreover, the potential challenges regarding spatio-temporally controllable therapy for glioma are also described, aiming to provide insights into future advancements in this field and their potential clinical applications.

Overcoming immunotherapy resistance and inducing abscopal effects with boron neutron immunotherapy (B-NIT)

Immune checkpoint inhibitors (ICIs) are effective against many advanced malignancies. However, many patients are nonresponders to immunotherapy, and overcoming this resistance to treatment is important. Boron neutron capture therapy (BNCT) is a local chemoradiation therapy with the combination of boron drugs that accumulate selectively in cancer and the neutron irradiation of the cancer site. Here, we report the first boron neutron immunotherapy (B-NIT), combining BNCT and ICI immunotherapy, which was performed on a radioresistant and immunotherapy-resistant advanced-stage B16F10 melanoma mouse model. The BNCT group showed localized tumor suppression, but the anti-PD-1 antibody immunotherapy group did not show tumor suppression. Only the B-NIT group showed strong tumor growth inhibition at both BNCT-treated and shielded distant sites. Intratumoral CD8+ T-cell infiltration and serum high mobility group box 1 (HMGB1) levels were higher in the B-NIT group. Analysis of CD8+ T cells in tumor-infiltrating lymphocytes (TILs) showed that CD62L- CD44+ effector memory T cells and CD69+ early-activated T cells were predominantly increased in the B-NIT group. Administration of CD8-depleting mAb to the B-NIT group completely suppressed the augmented therapeutic effects. This indicated that B-NIT has a potent immune-induced abscopal effect, directly destroying tumors with BNCT, inducing antigen-spreading effects, and protecting normal tissue. B-NIT, immunotherapy combined with BNCT, is the first treatment to overcome immunotherapy resistance in malignant melanoma. In the future, as its therapeutic efficacy is demonstrated not only in melanoma but also in other immunotherapy-resistant malignancies, B-NIT can become a new treatment candidate for advanced-stage cancers.

BNCT pancreatic cancer treatment strategy with glucose-conjugated boron drug

Multidisciplinary therapy centered on radical surgery for resectable pancreatic cancer is expected to prolong prognosis, but relies on CA19-9 biomarker levels to determine treatment strategy. Boron neutron capture therapy (BNCT) is a chemoradiotherapy using tumor hyperaccumulator boron drugs and neutron irradiation. The purpose of this study is to investigate novel boron drug agents for BNCT for pancreatic cancer. Bioinformatics was used to evaluate the uptake of current boron amino acid (BPA) drugs for BNCT into pancreatic cancer. The expression of the amino acid transporter LAT1, a BPA uptake transporter, was low in pancreatic cancer and even lower in high CA19-9 pancreatic cancer. In contrast, the glucose transporter was high in high CA19-9 pancreatic cancers and inversely correlated with LAT1 expression. Considering the low EPR effect in pancreatic cancer, we synthesized a small molecule Glucose-BSH, which is boron BSH bound to glucose, and confirmed its specific uptake in pancreatic cancer. uptake of Glucose-BSH was confirmed in an environment compatible with the tumor microenvironment. The therapeutic efficacy and safety of Glucose-BSH by therapeutic neutron irradiation were confirmed with BNCT. We report Glucose-BSH boron drug discovery study of a Precision Medicine BNCT with application to high CA19-9 pancreatic cancer.

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

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

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.

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.

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

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

The Dawn of a New Era: Tumor-Targeting Boron Agents for Neutron Capture Therapy

Cancer is widely recognized as one of the most devastating diseases, necessitating the development of intelligent diagnostic techniques, targeted treatments, and early prognosis evaluation to ensure effective and personalized therapy. Conventional treatments, unfortunately, suffer from limitations and an increased risk of severe complications. In light of these challenges, boron neutron capture therapy (BNCT) has emerged as a promising approach for cancer treatment with unprecedented precision to selectively eliminate tumor cells. The distinctive and promising characteristics of BNCT hold the potential to revolutionize the field of oncology. However, the clinical application and advancement of BNCT technology face significant hindrance due to the inherent flaws and limited availability of current clinical drugs, which pose substantial obstacles to the practical implementation and continued progress of BNCT. Consequently, there is an urgent need to develop efficient boron agents with higher boron content and specific tumor-targeting properties. Researchers aim to address this need by integrating tumor-targeting strategies with BNCT, with the ultimate goal of establishing BNCT as an effective, readily available, and cutting-edge treatment modality for cancer. This review delves into the recent advancements in integrating tumor-targeting strategies with BNCT, focusing on the progress made in developing boron agents specifically designed for BNCT. By exploring the current state of BNCT and emphasizing the prospects of tumor-targeting boron agents, this review provides a comprehensive overview of the advancements in BNCT and highlights its potential as a transformative treatment option for cancer.

Multi-Functional Boron-Delivery Agents for Boron Neutron Capture Therapy of Cancers

Boron neutron capture therapy (BNCT) is a binary cancer treatment that involves the irradiation of ¹⁰B-containing tumors with low-energy neutrons (thermal or epithermal). The alpha particles and recoiling Li nuclei that are produced in the ¹⁰B-capture nuclear reaction are high-linear-energy transfer particles that destroy boron-loaded tumor cells; therefore, BNCT has the potential to be a localized therapeutic modality. Two boron-delivery agents have been used in clinical trials of BNCT in patients with malignant brain tumors, cutaneous melanoma, or recurrent tumors of the head and neck region, demonstrating the potential of BNCT in the treatment of difficult cancers. A variety of potentially highly effective boron-delivery agents have been synthesized in the past four decades and tested in cells and animal models. These include boron-containing nucleosides, peptides, proteins, polyamines, porphyrins, liposomes, monoclonal antibodies, and nanoparticles of various types. The most promising agents are multi-functional boronated molecules and nanoparticles functionalized with tumor cell-targeting moieties that increase their tumor selectivity and contain a radiolabel or fluorophore to allow quantification of ¹⁰B-biodistribution and treatment planning. This review discusses multi-functional boron agents reported in the last decade, but their full potential can only be ascertained after their evaluation in BNCT clinical trials.

New Boron Delivery Agents

This proceeding article compiles current research on the development of boron delivery drugs for boron neutron capture therapy that was presented and discussed at the National Cancer Institute (NCI) Workshop on Neutron Capture Therapy that took place on April 20-22, 2022. The most used boron sources are icosahedral boron clusters attached to peptides, proteins (such as albumin), porphyrin derivatives, dendrimers, polymers, and nanoparticles, or encapsulated into liposomes. These boron clusters and/or carriers can be labeled with contrast agents allowing for the use of imaging techniques, such as PET, SPECT, and fluorescence, that enable quantification of tumor-localized boron and their use as theranostic agents.

Carborane-Containing Folic Acid bis-Amides: Synthesis and In Vitro Evaluation of Novel Promising Agents for Boron Delivery to Tumour Cells

The design of highly selective low-toxic, low-molecular weight agents for boron delivery to tumour cells is of decisive importance for the development of boron neutron capture therapy (BNCT), a modern efficient combined method for cancer treatment. In this work, we developed a simple method for the preparation of new closo- and nido-carborane-containing folic acid bis-amides containing 18-20 boron atoms per molecule. Folic acid derivatives containing nido-carborane residues were characterised by high water solubility, low cytotoxicity, and demonstrated a good ability to deliver boron to tumour cells in in vitro experiments (up to 7.0 µg B/10⁶ cells in the case of U87 MG human glioblastoma cells). The results obtained demonstrate the high potential of folic acid-nido-carborane conjugates as boron delivery agents to tumour cells for application in BNCT.

New Aspects of the Synthesis of closo-Dodecaborate Nitrilium Derivatives [B12H11NCR]− (R = n-C3H7, i-C3H7, 4-C6H4CH3, 1-C10H7): Experimental and Theoretical Studies

The preparation of novel nitrilium derivatives of closo-dodecaborate anion [B12H11NCR]−, R = n-C3H7, i-C3H7, 4-C6H4CH3, 1-C10H7 is described. Target compounds were obtained in good yields (up to 73%). The synthesis of target borylated nitrilium derivatives was characterised by the simplicity of the chemical apparatus and the absence of the necessity for the purification of desired compounds. The crystal structures of previously obtained [B12H11NCCH3]− and novel [B12H11NCC3H7]− were established with the help of X-ray structure analysis. DFT-analysis of several nitrilium derivatives [B12H11NCR]−, R = CH3, C3H7, 4-CH3C6H4 was carried out. The main peculiarities of the C≡N bond of the exo-polyhedral substituent were revealed in terms of bond lengths, bond orders and atomic charges. The LUMO orbitals of the systems considered were examined for understanding of the electrophilic nature of the nitrilium derivatives of the closo-dodecaborate anion.

Reaction of Chloroacetyl-Modified Peptides with Mercaptoundecahydrododecaborate (BSH) Is Accelerated by Basic Amino Acid Residues in the Peptide

We assessed a reactivity of chloroacetyl-modified tripeptides consisting of various amino acid residues (Cl-3X) and mercaptoundecahydrododecaborate (BSH) by converting Cl-3X to its reactant (BS-3X). We showed that the Cl-3X consisting of basic amino acid residues (e.g., Arg) reacted with BSH effectively and its conversion decreased as the number of Arg residues in the Cl-3X decreased. Furthermore, a reactivity of the peptides with introduction of an alkyl linker between the triarginine and the chloroacetyl group (Cl-Cn-3R) with BSH decreased with increasing alkyl linker length. These results indicate that an electrostatic attraction of positively charged amino acid residues in the tripeptides and negatively charged BSH causes BSH to gather in a vicinity of the chloroacetyl group, resulting in an accelerated reaction. This work should aid a development of new boron agents using BSH in boron neutron capture therapy.

Multimodal Applications of Zinc Gallate-Based Persistent Luminescent Nanoparticles in Cancer Treatment: Tumor Margining, Diagnosis, and Boron Neutron Capture Therapy

On the basis of the boron neutron capture therapy (BNCT) modality, we have designed and synthesized a zinc gallate (ZnGa₂O₄)-based nanoformulation for developing an innovative theranostic approach for cancer treatment. Initially, the (ZnGa_1.995Cr_0.005O₄ or ZnGa₂O₄:(0.5%)Cr persistent luminescence nanoparticles (PLNPs) embedded on silica matrix were synthesized. Their surface functionalization was performed using organic synthesis strategies to attach the amine functional moieties which were further coupled with poly(vicinal diol). These diols were helpful for conjugation with ¹⁰B(OH)₃, which subsequently served to couple with an in-house-synthesized variant of pH-(low)-insertion peptide (pHLIP) finally giving a tumor-targeting nanoformulation. Most importantly, the polymeric diols helped in conjugation of a substantial number of ¹⁰B to provide the therapeutic dose required for effective BNCT. This nanoformulation internalized substantially (∼80%) to WEHI-164 cancer cells within 6 h. Tumor homing studies indicated that the accumulation of this formulation at the acidic tumor site was within 2 h. The in vitro evaluation of the formulation against WEHI-164 cancer cells followed by neutron irradiation revealed its potent cytotoxicity with IC₅₀ ∼ 25 μM. In the case of studies on animal models, the melanoma-induced C57BL/6 and fibrosarcoma-induced BALB/c mice were treated with formulations through intratumoral and intravenous injections, respectively, followed by neutron irradiation, leading to a significant killing of the cancer cells, which was evidenced by a reduction in tumor volume (75-80%) as compared with a control tumor. Furthermore, the histopathological studies confirmed a damaging effect only on tumor cells, while there was no sign of damage to the vital organs in treated mice as well as in controls.

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.

Radiotherapy for glioblastoma: clinical issues and nanotechnology strategies

Glioblastoma multiforme (GBM) is the most common primary brain cancer in adults with poor prognosis. Despite the current state of knowledge on its genetic characteristics, relatively little progress has been made in improving the treatment of patients with this fatal disease. Radiotherapy (RT) has been identified as a crucial treatment for GBM following surgical resection to improve both local control and survival. Unfortunately, radiotherapy resistance is frequently observed in GBM patients, which is the major reason for the high mortality rate of cancer patients. Radioresistance of GBM is often multifactorial and heterogeneous, and associated with the recurrence of GBM after surgery. Nanotechnology has gained increasing attention and has already been investigated for optimization of radiosensitization due to the unique properties of nanobiomaterials, such as photoelectric decay characteristics or potential as carriers for drug delivery to the central nervous system. A large body of preclinical data has accumulated over the past several years, in which nanotechnology-based strategies exhibit promising potential to enhance the radiosensitivity of GBM, both in cellular and animal models. In this review, we summarize the mechanisms of GBM radioresistance, including tumor cell-intrinsic factors as well as tumor microenvironment (TME). We further discuss current nano-biotechnology-based radiosensitizer in the treatment of GBM, summarize the latest findings, highlight challenges, and put forward prospects for the future of nano-radiosensitizers. These data suggest that nanotechnology has the potential to address many of the clinical challenges and nanobiomaterials would become promising next-generation radiotherapy sensitizers for GBM treatment.

Combined BNCT and PET for theranostics

This short review summarizes the issue of boron distribution monitoring in boron neutron capture therapy (BNCT), which remains a serious drawback of this powerful oncological treatment. Here we present the monitoring methods that are presently used with particular emphasis on the positron emission tomography (PET) which has the highest potential to be used for the real-time monitoring of boron biodistribution. We discuss the possibility of using present PET scanners to determine the boron uptake in vivo before the BNCT treatment with the use of p-boronphenylalanine (BPA) labeled with 18F isotope. Several examples of preclinical studies and clinical trials performed with the use of [18F]FBPA are shown. We also discuss shortly the perspectives of using other radiotracers and boron carriers which may significantly improve the boron imaging with the use of the state-of-the-art Total-Body PET scanners providing a theranostic approach in the BNCT.

Molecular Mechanisms of Drug Resistance in Glioblastoma

Glioblastoma multiforme (GBM) is the most common and fatal primary brain tumor, is highly resistant to conventional radiation and chemotherapy, and is not amenable to effective surgical resection. The present review summarizes recent advances in our understanding of the molecular mechanisms of therapeutic resistance of GBM to already known drugs, the molecular characteristics of glioblastoma cells, and the barriers in the brain that underlie drug resistance. We also discuss the progress that has been made in the development of new targeted drugs for glioblastoma, as well as advances in drug delivery across the blood-brain barrier (BBB) and blood-brain tumor barrier (BBTB).

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.

Reprogramming the Tumor Microenvironment through Second-Near-Infrared-Window Photothermal Genome Editing of PD-L1 Mediated by Supramolecular Gold Nanorods for Enhanced Cancer Immunotherapy

A photothermal genome-editing strategy is described to improve immune checkpoint blockade (ICB) therapy by CRISPR/Cas9-mediated disruption of PD-L1 and mild-hyperthermia-induced activation of immunogenic cell death (ICD). This strategy relies on a supramolecular cationic gold nanorod that not only serves as a carrier to deliver CRISPR/Cas9 targeting PD-L1, but also harvests the second near-infrared-window (NIR-II) light and converts into mild hyperthermia to induce both ICD and gene expression of Cas9. The genomic disruption of PD-L1 significantly augments ICB therapy by improving the conversion of dendritic cells to T cells, followed by promoting the infiltration of cytotoxic T lymphocytes into tumors, thereby reprogramming immunosuppressive tumor microenvironment into immunoactive one. Such a therapeutic modality greatly inhibits the activity of primary and metastatic tumors and exhibits long-term immune memory effects against both rechallenged and recurrent tumors. The current therapeutic strategy for synergistic PD-L1 disruption and ICD activation represents an appealing way for cancer immunotherapy.

Improvement of Water Solubility of Mercaptoundecahydrododecaborate (BSH)-Peptides by Conjugating with Ethylene Glycol Linker and Interaction with Cyclodextrin

We previously developed a conjugate consisting of 10B cluster BSH and tri-arginine peptide (BSH-3R). This could potentially be used as a boron agent for boron neutron capture therapy; however, it possesses poor water solubility and thus needs to be improved for use as medicine. In this study, we devised several means of improving the water solubility of BSH-3R. As one of them, we used cyclodextrin (CD), which was expected to improve the water solubility resulting from interaction of the BSH-3R with CD. We evaluated the solubility of BSH-3R in aqueous CD solution by using reverse-phase high-performance liquid chromatography. As we expected, the solubility of BSH-3R was increased in a manner dependent on the addition of β-CD and γ-CD in aqueous solution. Furthermore, we synthesized BSH conjugated to oligoarginine having various chain lengths (BSH-nR) and BSH-3R with ethylene glycol linkers introduced between BSH and 3R (BSH-nEg-3R). The water solubility of these BSH peptides was also evaluated and the results showed that the introduction of nEg to BSH-3R markedly improved the water solubility. Furthermore, we found that the water solubility of these peptides can be further improved by also applying CD.

Tumor vasculature-targeted 10B delivery by an Annexin A1-binding peptide boosts effects of boron neutron capture therapy

Background

p-Boronophenylalanine (¹⁰BPA) is a powerful ¹⁰B drug used in current clinical trials of BNCT. For BNCT to be successful, a high (500 mg/kg) dose of ¹⁰BPA must be administered over a few hours. Here, we report BNCT efficacy after rapid, ultralow-dose administration of either tumor vasculature-specific annexin A1-targeting IFLLWQR (IF7)-conjugated ¹⁰BPA or borocaptate sodium (¹⁰BSH).

Methods

(1) IF7 conjugates of either ¹⁰B drugs intravenously injected into MBT2 bladder tumor-bearing mice and biodistribution of ¹⁰B in tumors and normal organs analyzed by prompt gamma-ray analysis. (2) Therapeutic effect of IF7-¹⁰B drug-mediated BNCT was assessed by either MBT2 bladder tumor bearing C3H/He mice and YTS-1 tumor bearing nude mice.

Results

Intravenous injection of IF7C conjugates of either ¹⁰B drugs into MBT2 bladder tumor-bearing mice promoted rapid ¹⁰B accumulation in tumor and suppressed tumor growth. Moreover, multiple treatments at ultralow (10–20 mg/kg) doses of IF7-¹⁰B drug-mediated BNCT significantly suppressed tumor growth in a mouse model of human YTS-1 bladder cancer, with increased Anxa1 expression in tumors and infiltration by CD8-positive lymphocytes.

Conclusions

We conclude that IF7 serves as an efficient ¹⁰B delivery vehicle by targeting tumor tissues via the tumor vasculature and could serve as a relevant vehicle for BNCT drugs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-020-07760-x.

A periodic development of BPA and BSH based derivatives in boron neutron capture therapy (BNCT)

A schematic representation of various judicious approaches for the synthesis of BPA and BSH modified compounds for effective BNCT.

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 CD44High 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 CD44High 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.

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.

A Selective Carborane-Functionalized Gastrin-Releasing Peptide Receptor Agonist as Boron Delivery Agent for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) allows the selective elimination of malignant tumor cells without affecting healthy tissue. Although this binary radiotherapy approach has been known for decades, BNCT failed to reach the daily clinics to date. One of the reasons is the lack of selective boron delivery agents. Using boron loaded peptide conjugates, which address G protein-coupled receptors overexpressed on tumor cells allow the intracellular accumulation of boron. The gastrin-releasing peptide receptor (GRPR) is a well-known target in cancer diagnosis and can potentially be used for BNCT. Here, we present the successful introduction of multiple bis-deoxygalactosyl-carborane building blocks to the GRPR-selective ligand [d-Phe⁶, β-Ala¹¹, Ala¹³, Nle¹⁴]Bn(6-14) (sBB2L) generating peptide conjugates with up to 80 boron atoms per molecule. Receptor activation was retained, metabolic stability was increased, and uptake into PC3 cells was proven without showing any intrinsic cytotoxicity. Furthermore, undesired uptake into liver cells was suppressed by using l-deoxygalactosyl modified carborane building blocks. Due to its high boron loading and excellent GRPR selectivity, this conjugate can be considered as a promising boron delivery agent for BNCT.

Tuning a modular system - synthesis and characterisation of a boron-rich s-triazine-based carboxylic acid and amine bearing a galactopyranosyl moiety

Introduction of a bis(isopropylidene)-protected galactopyranosyl moiety in s-triazine-based boron-rich carboxylic acids and amines results in soluble and suitable coupling partners for tumour-selective biomolecules with applications as selective agents for boron neutron capture therapy (BNCT). Bearing either a carboxylic acid or primary amine as a functional group, these compounds are highly versatile and thus largely extend the possible coupling strategies with suitable biomolecules. Modification of the gastrin-releasing peptide receptor (GRPR) selective agonist [d-Phe⁶, β-Ala¹¹, Ala¹³, Nle¹⁴]Bn(6-14) with the carboxylic acid derivative yielded a bioconjugate with an optimal receptor activation and internalisation profile. This demonstrates the great potential of this approach for the development of novel boron delivery agents.

Enlargement of a Modular System-Synthesis and Characterization of an s-Triazine-Based Carboxylic Acid Ester Bearing a Galactopyranosyl Moiety and an Enormous Boron Load

The amount of boron accumulated in tumor tissue plays an important role regarding the success of the boron neutron capture therapy (BNCT). In this article, we report a modular system, combining readily available starting materials, like glycine, 1,3,5-triazine and the well-known 9-mercapto-1,7-dicarba-closo-dodecaborane(12), as well as α-d-galactopyranose for increased hydrophilicity, with a novel boron-rich tris-meta-carboranyl thiol.

Glioblastoma Treatment Modalities besides Surgery

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

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

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

Modular triazine-based carborane-containing carboxylic acids - synthesis and characterisation of potential boron neutron capture therapy agents made of readily accessible building blocks

Based on a modular combination of s-triazine, the well-known 9-mercapto-1,7-dicarba-closo-dodecaborane(12) and commercially available carboxylic acids, namely thioglycolic acid, glycine, and N_α-Boc-l-lysine, several carboxylic acid derivatives were synthesised and fully characterised. The thioglycolic acid derivative was introduced into a peptide hormone by solid phase peptide synthesis. High activity and selective internalisation into peptide receptor-expressing cells was observed. With a very high boron content of twenty boron atoms, these derivatives are interesting as selective Boron Neutron Capture Therapy (BNCT) agents.

The Current Status and Perspectives of Delivery Strategy for Boron-based Drugs

Boron-containing compounds are essential micronutrients for animals and plants despite their low-level natural occurrence. They can strengthen the cell walls of the plants and they play important role in supporting bone health. However, surprisingly, boron-containing compounds are seldom found in pharmaceutical drugs. In fact, there are no inherent disadvantages reported so far in terms of the incorporation of boron into medicines. Indeed, drugs based on boron-containing compounds, such as tavaborole (marked name Kerydin) and bortezomib (trade name Velcade) have been investigated and they are used in clinical treatment. In addition, following the advanced development of boron neutron capture therapy and a new emerging proton boron fusion therapy, more boron-containing medicinals are to be expected. This review discusses the current status and perspectives of delivery strategy for boron-containing drugs.

A stable meta-carborane enables the generation of boron-rich peptide agonists targeting the ghrelin receptor

Boron neutron capture therapy (BNCT) is a binary cancer therapy, which combines the biochemical targeting of a boron-containing drug with the regional localization of radiation treatment. Although the concept of BNCT has been known for decades, the selective delivery of boron into tumor cells remains challenging. G protein-coupled receptors that are overexpressed on cancer cells in combination with peptidic ligands can be potentially used as shuttle system for a tumor-directed boron uptake. In this study, we present the generation of short, boron-rich peptide conjugates that target the ghrelin receptor. Expression of the ghrelin receptor on various cancer cells makes it a viable target for BNCT. We designed a novel hexapeptide super-agonist that was modified with different specifically synthesized carborane monoclusters and tested for ghrelin receptor activation. A meta-carborane building block with a mercaptoacetic acid linker was found to be optimal for peptide modification, owing to its chemical stability and a suitable activation efficacy of the conjugate. The versatility of this carborane for the development of peptidic boron delivery agents was further demonstrated by the generation of highly potent, boron-loaded conjugates using the backbone of the known ghrelin receptor ligands growth hormone releasing peptide 6 and Ipamorelin.

Targeting glioma stem cells enhances anti-tumor effect of boron neutron capture therapy

The uptake of (10)boron by tumor cells plays an important role for cell damage in boron neutron capture therapy (BNCT). CD133 is frequently expressed in the membrane of glioma stem cells (GSCs), resistant to radiotherapy and chemotherapy, and represents a potential therapeutic target. To increase (10)boron uptake in GSCs, we created a polyamido amine dendrimer, conjugated CD133 monoclonal antibodies, encapsulating mercaptoundecahydrododecaborate (BSH) in void spaces, and monitored the uptake of the bioconjugate nanoparticles by GSCs in vitro and in vivo. Fluorescence microscopy showed the specific uptake of the bioconjugate nanoparticles by CD133-positive GSCs. Treatment with the biconjugate nanoparticles resulted in a significant lethal effect after neutron radiation due to efficient and CD133-independent cellular targeting and uptake in CD133-expressing GSCs. A significantly longer survival occurred in combination with the biconjugate nanoparticles and BSH compared with BSH alone in human intracranial GBM models employing CD133-positive GSCs xenografts. Our data demonstrated that this bioconjugate nanoparticle targets human CD133-positive GSCs and is a potential boron agent in BNCT.

Zebrafish: A complete animal model to enumerate the nanoparticle toxicity

Presently, nanotechnology is a multi-trillion dollar business sector that covers a wide range of industries, such as medicine, electronics and chemistry. In the current era, the commercial transition of nanotechnology from research level to industrial level is stimulating the world’s total economic growth. However, commercialization of nanoparticles might offer possible risks once they are liberated in the environment. In recent years, the use of zebrafish (Danio rerio) as an established animal model system for nanoparticle toxicity assay is growing exponentially. In the current in-depth review, we discuss the recent research approaches employing adult zebrafish and their embryos for nanoparticle toxicity assessment. Different types of parameters are being discussed here which are used to evaluate nanoparticle toxicity such as hatching achievement rate, developmental malformation of organs, damage in gill and skin, abnormal behavior (movement impairment), immunotoxicity, genotoxicity or gene expression, neurotoxicity, endocrine system disruption, reproduction toxicity and finally mortality. Furthermore, we have also highlighted the toxic effect of different nanoparticles such as silver nanoparticle, gold nanoparticle, and metal oxide nanoparticles (TiO₂, Al2O₃, CuO, NiO and ZnO). At the end, future directions of zebrafish model and relevant assays to study nanoparticle toxicity have also been argued.

New generation non-stationary portable neutron generators for biophysical applications of Neutron Activation Analysis

BACKGROUND

Neutron sources are increasingly employed in a wide range of research fields. For some specific purposes an alternative to existing large-scale neutron scattering facilities, can be offered by the new generation of portable neutron devices.

SCOPE OF REVIEW

This review reports an overview for such recently available neutron generators mainly addressed to biophysics applications with specific reference to portable non-stationary neutron generators applied in Neutron Activation Analysis (NAA).

MAJOR CONCLUSIONS

The review reports a description of a typical portable neutron generator set-up addressed to biophysics applications.

GENERAL SIGNIFICANCE

New generation portable neutron devices, for some specific applications, can constitute an alternative to existing large-scale neutron scattering facilities. Deuterium-Deuterium pulsed neutron sources able to generate 2.5MeV neutrons, with a neutron yield of 1.0×10⁶n/s, a pulse rate of 250Hz to 20kHz and a duty factor varying from 5% to 100%, when combined with solid-state photon detectors, show that this kind of compact devices allow rapid and user-friendly elemental analysis. "This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo".