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

A bis-boron boramino acid PET tracer for brain tumor diagnosis

PURPOSE

Boramino acids are a class of amino acid biomimics that replace the carboxylate group with trifluoroborate and can achieve the 18F-labeled positron emission tomography (PET) and boron neutron capture therapy (BNCT) with identical chemical structure.

METHODS

This study reports a trifluoroborate-derived boronophenylalanine (BBPA), a derived boronophenylalanine (BPA) for BNCT, as a promising PET tracer for tumor imaging.

RESULTS

Competition inhibition assays in cancer cells suggested the cell accumulation of [18F]BBPA is through large neutral amino acid transporter type-1 (LAT-1). Of note, [18F]BBPA is a pan-cancer probe that shows notable tumor uptake in B16-F10 tumor-bearing mice. In the patients with gliomas and metastatic brain tumors, [18F]BBPA-PET shows good tumor uptake and notable tumor-to-normal brain ratio (T/N ratio, 18.7 ± 5.5, n = 11), higher than common amino acid PET tracers. The [18F]BBPA-PET quantitative parameters exhibited no difference in diverse contrast-enhanced status (P = 0.115-0.687) suggesting the [18F]BBPA uptake was independent from MRI contrast-enhancement.

CONCLUSION

This study outlines a clinical trial with [18F]BBPA to achieve higher tumor-specific accumulation for PET, provides a potential technique for brain tumor diagnosis, and might facilitate the BNCT of brain tumors.

B-Glycine as a marker for β cell imaging and β cell mass evaluation

PURPOSE

β cell mass (BCM) and function are essential to the diagnosis and therapy of diabetes. Diabetic patients serve β cell loss is, and damage of β cells leads to severe insulin deficiency. Our understanding of the role of BCM in diabetes progression is extremely limited by lacking efficient methods to evaluate BCM in vivo. In vitro methods of labeling islets, including loading of contrast reagent or integration of exogenous biomarker, require artificial manipulation on islets, of which the clinical application is limited. Imaging methods targeting endogenous biomarkers may solve the above problems. However, traditional reagents targeting GLP-1R and VAMT2 result in a high background of adjacent tissues, complicating the identification of pancreatic signals. Here, we report a non-invasive and quantitative imaging technique by using radiolabeled glycine mimics ([18F]FBG, a boron-trifluoride derivative of glycine) to assay islet function and monitor BCM changes in living animals.

METHODS

Glycine derivatives, FBG, FBSa, 2Me-FBG, 3Me-FBG, were successfully synthesized and labeled with 18F. Specificity of glycine derivatives were characterized by in vitro experiment. PET imaging and biodistribution studies were performed in animal models carring GLYT over-expressed cells. In vivo evaluation of BCM with [18F]FBG were performed in STZ (streptozocin) induced T1D (type 1 diabetes) models.

RESULTS

GLYT responds to excess blood glycine levels and transports glycine into islet cells to maintain the activity of the glycine receptor (GLYR). Best PET imaging condition was 80 min after given a total of 240 ~ 250 nmol imaging reagent (a mixture of [18F]FBG and natural glycine) intravenously. [18F]FBG can detect both endogenous and exogenous islets clearly in vivo. When applied to STZ induced T1D mouse models, total uptake of [18F]FBG in the pancreas exhibited a linear correlation with survival BCM.

CONCLUSION

[18F]FBG targeting the endogenous glycine transporter (GLYT), which is highly expressed on islet cells, avoiding extra modification on islet cells. Meanwhile the highly restricted expression pattern of GLYT excluded the background in adjacent tissues. This [18F]FBG-based imaging technique provides a non-invasive method to quantify BCM in vivo, implying a new evaluation index for diabetic assessment.

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.

Amino Acid-Based Boron Carriers in Boron Neutron Capture Therapy (BNCT)

Interest in the design of boronated amino acids has emerged, partly due to the utilization of boronophenylalanine (BPA), one of the two agents employed in clinical Boron Neutron Capture Therapy (BNCT). The boronated amino acids synthesized thus far for BNCT investigations can be classified into two categories based on the source of boron: boronic acids or carboranes. Amino acid-based boron carriers, employed in the context of BNCT treatment, demonstrate significant potential in the treatment of challenging tumors, such as those located in the brain. This review aims to shed light on the developmental journey and challenges encountered over the years in the field of amino acid-based boron delivery compound development. The primary focus centers on the utilization of the large amino acid transporter 1 (LAT1) as a target for boron carriers in BNCT. The development of efficient carriers remains a critical objective, addressing challenges related to tumor specificity, effective boron delivery, and rapid clearance from normal tissue and blood. LAT1 presents an intriguing and promising target for boron delivery, given its numerous characteristics that make it well suited for drug delivery into tumor tissues, particularly in the case of brain tumors.

Bioorthogonal chemistry for prodrug activation in vivo

Prodrugs have emerged as a major strategy for addressing clinical challenges by improving drug pharmacokinetics, reducing toxicity, and enhancing treatment efficacy. The emergence of new bioorthogonal chemistry has greatly facilitated the development of prodrug strategies, enabling their activation through chemical and physical stimuli. This "on-demand" activation using bioorthogonal chemistry has revolutionized the research and development of prodrugs. Consequently, prodrug activation has garnered significant attention and emerged as an exciting field of translational research. This review summarizes the latest advancements in prodrug activation by utilizing bioorthogonal chemistry and mainly focuses on the activation of small-molecule prodrugs and antibody-drug conjugates. In addition, this review also discusses the opportunities and challenges of translating these advancements into clinical practice.

Next generation of boron neutron capture therapy (BNCT) agents for cancer treatment

Boron neutron capture therapy (BNCT) is one of the most promising treatments among neutron capture therapies due to its long-term clinical application and unequivocally obtained success during clinical trials. Boron drug and neutron play an equivalent crucial role in BNCT. Nevertheless, current clinically used l-boronophenylalanine (BPA) and sodium borocaptate (BSH) suffer from large uptake dose and low blood to tumor selectivity, and that initiated overwhelm screening of next generation of BNCT agents. Various boron agents, such as small molecules and macro/nano-vehicles, have been explored with better success. In this featured article, different types of agents are rationally analyzed and compared, and the feasible targets are shared to present a perspective view for the future of BNCT in cancer treatment. This review aims at summarizing the current knowledge of a variety of boron compounds, reported recently, for the application of BCNT.

A Review of Planned, Ongoing Clinical Studies and Recent Development of BNCT in Mainland of China

Boron neutron capture therapy (BNCT) is a promising cancer treatment modality that combines targeted boron agents and neutron irradiation to selectively destroy tumor cells. In mainland China, the clinical implementation of BNCT has made certain progress, primarily driven by the development of compact neutron source devices. The availability, ease of operation, and cost-effectiveness offered by these compact neutron sources make BNCT more accessible to cancer treatment centers. Two compact neutron sources, one being miniature reactor-based (IHNI-1) and the other one being accelerator-based (NeuPex), have entered the clinical research phase and are planned for medical device registration. Moreover, several accelerator-based neutron source devices employing different technical routes are currently under construction, further expanding the options for BNCT implementation. In addition, the development of compact neutron sources serves as an experimental platform for advancing the development of new boron agents. Several research teams are actively involved in the development of boron agents. Various types of third-generation boron agents have been tested and studied in vitro and in vivo. Compared to other radiotherapy therapies, BNCT in mainland China still faces specific challenges due to its limited clinical trial data and its technical support in a wide range of professional fields. To facilitate the widespread adoption of BNCT, it is crucial to establish relevant technical standards for neutron devices, boron agents, and treatment protocols.

Recent Development of Radiofluorination of Boron Agents for Boron Neutron Capture Therapy of Tumor: Creation of 18F-Labeled C-F and B-F Linkages

Boron neutron capture therapy (BNCT) is a binary therapeutic technique employing a boron agent to be delivered to the tumor site followed by the irradiation of neutrons. Biofunctional molecules/nanoparticles labeled with F-18 can provide an initial pharmacokinetic profile of patients to guide the subsequent treatment planning procedure of BNCT. Borono phenylalanine (BPA), recognized by the l-type amino acid transporter, can cross the blood-brain barrier and be accumulated in gliomas. The radiofluoro BNCT agents are reviewed by considering (1) less cytotoxicity, (2) diagnosing and therapeutic purposes, (3) aqueous solubility and extraction route, as well as (4), the trifluoroborate effect. A trifluoroborate-containing amino acid such as fluoroboronotyrosine (FBY) represents an example with both functionalities of imaging and therapeutics. Comparing with the insignificant cytotoxicity of clinical BPA with IC50 > 500 μM, FBY also shows minute toxicity with IC50 > 500 μM. [18F]FBY is a potential diagnostic agent for its tumor to normal accumulation (T/N) ratio, which ranges from 2.3 to 24.5 from positron emission tomography, whereas the T/N ratio of FBPA is greater than 2.5. Additionally, in serving as a BNCT therapeutic agent, the boron concentration of FBY accumulated in gliomas remains uncertain. The solubility of 3-BPA is better than that of BPA, as evidenced by the cerebral dose of 3.4%ID/g vs. 2.2%ID/g, respectively. While the extraction route of d-BPA differs from that of BPA, an impressive T/N ratio of 6.9 vs. 1.5 is noted. [18F]FBPA, the most common clinical boron agent, facilitates the application of BPA in clinical BNCT. In addition to [18F]FBY, [18F] trifluoroborated nucleoside analog obtained through 1,3-dipolar cycloaddition shows marked tumoral uptake of 1.5%ID/g. Other examples using electrophilic and nucleophilic fluorination on the boron compounds are also reviewed, including diboronopinacolone phenylalanine and nonsteroidal anti-inflammatory agents.

Organotrifluoroborate Sugar Conjugates for a Guided Boron Neutron Capture Therapy: From Synthesis to Positron Emission Tomography

Sugars are a versatile tool for targeting malignant cells and have been extensively used for drug delivery and imaging techniques. Their prototype, fluorodeoxyglucose ([¹⁸F]FDG), is currently used for positron emission tomography. Boron neutron capture therapy (BNCT) is a cancer treatment that relies on irradiation with thermal neutrons of cancer cells previously loaded with [¹⁰B]-containing compounds. The recent introduction of accelerators as a neutron source for clinical use prompts the planning of delivery compounds enriched with boron able to be traced in real time. This work describes the first synthesis of a new class of sugar derivatives conjugated to a trifluoroborate moiety as potential theranostic agents. Stability and cytotoxicity studies are reported for all compounds, together with [¹⁸F] radiolabeling optimization and in vivo preliminary positron emission tomography (PET) experiments on a selected compound.

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.

Recent Advances in 18F-Labeled Amino Acids Synthesis and Application

Radiolabeled amino acids are an important class of agents for positron emission tomography imaging that target amino acid transporters in many tumor types. Traditional ¹⁸F-labeled amino acid synthesis strategies are always based on nucleophilic aromatic substitution reactions with multistep radiosynthesis and low radiochemical yields. In recent years, new ¹⁸F-labeling methodologies such as metal-catalyzed radiofluorination and heteroatom (B, P, S, Si, etc.)-¹⁸F bond formation are being effectively used to synthesize radiopharmaceuticals. This review focuses on recent advances in the synthesis, radiolabeling, and application of a series of ¹⁸F-labeled amino acid analogs using new ¹⁸F-labeling strategies.

Tailored hyaluronic acid-based nanogels as theranostic boron delivery systems for boron neutron cancer therapy

Boron-rich nanocarriers possess great potential for advanced boron neutron capture therapy (BNCT) as an effective radiation treatment for invasive malignant tumors. If additionally, they can be imaged in a non-invasive and real-time manner allowing the assessment of local boron concentration, they could serve for dose calculation and image-guided BNCT to enhance tumor treatment efficacy. To meet this challenge, this study describes the design of a theranostic nanogel, enriched in ¹⁰B and fluorescent dye, to achieve selective imaging, and sufficient accumulation of boron at the tumor site. The boron-rich and fluorescent nanogels can be easily obtained via temperature triggered-assembly of hyaluronic acid (HA) modified with a thermoresponsive terpolymer. The latter was specifically designed to enable the efficient encapsulation of the fluorescent dye – an aza‑boron-dipyrromethene (aza-BODIPY) – linked to ¹⁰B-enriched sodium borocaptate (BSH), in addition to induce nanogel formation below room temperature, and to enable their core-crosslinking by hydrazone bond formation. The HA nanogel considerably concentrates aza-BODIPY-BSH into the hydrophobic nanodomains made of the terpolymer chains. Here, we present the detailed synthesis of the HA-terpolymer conjugate, nanogel formation, and characterization in terms of size, morphology, and stability upon storage, as well as the biological behavior of the boron nanocarrier using real-time fluorescence imaging in cells and in vivo. This work suggested the potential of the theranostic HA nanogel as a boron delivery system for the implementation of BNCT in brain cancer and sarcoma.

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.

Clinical Viability of Boron Neutron Capture Therapy for Personalized Radiation Treatment

Simple Summary

Usually, for dose planning in radiotherapy, the tumor is delimited as a volume on the image of the patient together with other clinical considerations based on populational evidence. However, the same prescription dose can provide different results, depending on the patient. Unfortunately, the biological aspects of the tumor are hardly considered in dose planning. Boron Neutron Capture Radiotherapy enables targeted treatment by incorporating boron-10 at the cellular level and irradiating with neutrons of a certain energy so that they produce nuclear reactions locally and almost exclusively damage the tumor cell. This technique is not new, but modern neutron generators and more efficient boron carriers have reactivated the clinical interest of this technique in the pursuit of more precise treatments. In this work, we review the latest technological facilities and future possibilities for the clinical implementation of BNCT and for turning it into a personalized therapy.

Abstract

Boron Neutron Capture Therapy (BNCT) is a promising binary disease-targeted therapy, as neutrons preferentially kill cells labeled with boron (¹⁰B), which makes it a precision medicine treatment modality that provides a therapeutic effect exclusively on patient-specific tumor spread. Contrary to what is usual in radiotherapy, BNCT proposes cell-tailored treatment planning rather than to the tumor mass. The success of BNCT depends mainly on the sufficient spatial biodistribution of ¹⁰B located around or within neoplastic cells to produce a high-dose gradient between the tumor and healthy tissue. However, it is not yet possible to precisely determine the concentration of ¹⁰B in a specific tissue in real-time using non-invasive methods. Critical issues remain to be resolved if BNCT is to become a valuable, minimally invasive, and efficient treatment. In addition, functional imaging technologies, such as PET, can be applied to determine biological information that can be used for the combined-modality radiotherapy protocol for each specific patient. Regardless, not only imaging methods but also proteomics and gene expression methods will facilitate BNCT becoming a modality of personalized medicine. This work provides an overview of the fundamental principles, recent advances, and future directions of BNCT as cell-targeted cancer therapy for personalized radiation treatment.

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

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

Synthesis and Evaluation of 18F-Labeled Boramino Acids as Potential New Positron Emission Tomography Agents for Cancer Management

Boron neutron capture therapy (BNCT) is a promising cancer treatment strategy that utilizes boron-containing ligands. In this report, a series of substituted boramino acids were synthesized and evaluated, aiming to obtain metabolically stable boron-derived agents that could integrate positron emission tomography (PET) with BNCT (a theranostic agent). Based on the phenylalanine (Phe) core structure, the impact of substitution groups on tumor accumulation was studied. The agents were labeled with fluorine-18 in 27.2-66.8% yield via the ¹⁸F-¹⁹F isotope exchange reaction. In B16-F10 tumor-bearing mice, [¹⁸F]-(R)-(1-ammonio-2-(4-methoxyphenyl) ethyl) trifluoroborate (R-[¹⁸F]-5a) demonstrated the best tumor uptake (5.54 ± 2.32% ID/g based on ex vivo biodistribution and 3.5 ± 0.04% ID/g based on PET imaging with the tumor-to-muscle ratio up to 2.6) and stability compared with other tested agents. Together, R-[¹⁸F]-5a is a promising agent that could potentially integrate PET and BNCT, whose treatment efficacy is worth further evaluation in the future.

China’s radiopharmaceuticals on expressway: 2014–2021

This review provides an essential overview on the progress of rapidly-developing China’s radiopharmaceuticals in recent years (2014–2021). Our discussion reflects on efforts to develop potential, preclinical, and in-clinical radiopharmaceuticals including the following areas: (1) brain imaging agents, (2) cardiovascular imaging agents, (3) infection and inflammation imaging agents, (4) tumor radiopharmaceuticals, and (5) boron delivery agents (a class of radiopharmaceutical prodrug) for neutron capture therapy. Especially, the progress in basic research, including new radiolabeling methodology, is highlighted from a standpoint of radiopharmaceutical chemistry. Meanwhile, we briefly reflect on the recent major events related to radiopharmaceuticals along with the distribution of major R&D forces (universities, institutions, facilities, and companies), clinical study status, and national regulatory supports. We conclude with a brief commentary on remaining limitations and emerging opportunities for China’s radiopharmaceuticals.

Boron encapsulated in a liposome can be used for combinational neutron capture therapy

Boron neutron capture therapy (BNCT) is an attractive approach to treat invasive malignant tumours due to binary heavy-particle irradiation, but its clinical applications have been hindered by boron delivery agents with low in vivo stability, poor biocompatibility, and limited application of combinational modalities. Here, we report boronsome, a carboranyl-phosphatidylcholine based liposome for combinational BNCT and chemotherapy. Theoretical simulations and experimental approaches illustrate high stability of boronsome. Then positron emission tomography (PET) imaging with Cu-64 labelled boronsome reveals high-specific tumour accumulation and long retention with a clear irradiation background. In particular, we show the suppression of tumour growth treated with boronsome with neutron irradiation and therapeutic outcomes are further improved by encapsulation of chemotherapy drugs, especially with PARP1 inhibitors. In sum, boronsome may be an efficient agent for concurrent chemoradiotherapy with theranostic properties against malignancies.

Boron neutron capture therapy is a type of cancer therapy but is associated with insufficient boron delivery and with poor biocompatibility. Here, the authors constructed boronated lipids to generate - boronsome - and show the system can reduce tumour growth.

Larger 18F-fluoroboronotyrosine (FBY) active volume beyond MRI contrast enhancement in diffuse gliomas than in circumscribed brain tumors

BACKGROUND

To investigate the relationship between ¹⁸F-fluoroboronotyrosine (FBY) positron emission tomography (PET)- and magnetic resonance imaging (MRI)-defined tumor volumes in contrast-enhanced diffuse gliomas and circumscribed brain tumors.

METHODS

A total of 16 diffuse gliomas and 7 circumscribed brain tumors were included, and two types of three-dimensional regions of interest (ROIs), namely, MRI-based ROI (ROI_MRI) and FBY-based ROI (ROI_FBY), were semiautomatically defined. The overlap volume and DICE score were calculated to reveal the spatial relationship between the ROI_MRI and ROI_FBY.

RESULTS

The ROI_MRI was smaller than the ROI_FBY and was mostly contained by the ROI_FBY with an overlap volume of 0.995 ± 0.006 in the whole population. A significant difference in the DICE score was observed between circumscribed tumors and diffuse tumors (0.886 ± 0.026 vs. 0.684 ± 0.165, p = 0.004), and for the regions that have increased FBY metabolism but not MRI contrast enhancement, diffuse tumors and circumscribed tumors showed similar SUVmean values (0.630 ± 0.19 vs. 0.671 ± 0.18, p = 0.625).

CONCLUSION

FBY uptake beyond contrast enhancement is more significant in diffuse tumors than in circumscribed tumors, which may aid the delineation of active tumor areas and facilitate boron neutron capture therapy.

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

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

The basis and advances in clinical application of boron neutron capture therapy

Boron neutron capture therapy (BNCT) was first proposed as early as 1936, and research on BNCT has progressed relatively slowly but steadily. BNCT is a potentially useful tool for cancer treatment that selectively damages cancer cells while sparing normal tissue. BNCT is based on the nuclear reaction that occurs when ¹⁰B capture low-energy thermal neutrons to yield high-linear energy transfer (LET) α particles and recoiling ⁷Li nuclei. A large number of ¹⁰B atoms have to be localized within the tumor cells for BNCT to be effective, and an adequate number of thermal neutrons need to be absorbed by the ¹⁰B atoms to generate lethal ¹⁰B (n, α)⁷Li reactions. Effective boron neutron capture therapy cannot be achieved without appropriate boron carriers. Improvement in boron delivery and the development of the best dosing paradigms for both boronophenylalanine (BPA) and sodium borocaptate (BSH) are of major importance, yet these still have not been optimized. Here, we present a review of this treatment modality from the perspectives of radiation oncology, biology, and physics. This manuscript provides a brief introduction of the mechanism of cancer-cell-selective killing by BNCT, radiobiological factors, and progress in the development of boron carriers and neutron sources as well as the results of clinical study.

Enzyme-instructed supramolecular assemblies promote intracellular boron accumulation for boron neutron capture therapy

Selective accumulation of boron agents in cancer cells is of critical importance for BNCT. Here we involve enzyme-instructed supramolecular assembly (EISA) to facilitate the accumulation of a typical boron agent borylphenylalanine (BPA) in cancer cells. By covalently conjugating BPA to the phosphorylated assembly precursor, the boron-bearing precursors undergo phosphatase-catalyzed dephosphorylation to yield assembly molecules, which then self-assemble to form nanomaterials. Due to the up-regulated phosphatase activity of cancer cells, kinetic preference allows the EISA to accumulate boron in HeLa cells selectively. Interestingly, by attaching BPA on the backbone or side-chain of precursor, the boron-bearing isomers show different assembly propensity with time-dependent morphology change, which leads to the differentiated accumulation of boron inside cells. Overall, the optimized boron-bearing assembly precursor could significantly improve the boron accumulation compared with BPA in cancer cells. In this study, we have demonstrated a convenient method to introduce boron agents to cancer cells. We envision that the EISA-mediated accumulation of boron will be helpful in the design of boron agents to facilitate BNCT treatment.

Hyaluronic acid-decorated carborane-TAT conjugation nanomicelles: A potential boron agent with enhanced selectivity of tumor cellular uptake

Boron neutron capture therapy (BNCT) has received widespread attention as a new type of radiation therapy. The main problem encountered in BNCT is insufficient tumor cellular uptake of boron agents. In this study, cell-penetrating peptide TAT-conjugated o-carborane was synthesized. The conjugation can self-assemble to form positively charged carborane-TAT micelles, and then adsorb negatively charged hyaluronic acid (HA) to give core-shell structured carborane-TAT@HA micelles. Carborane-TAT@HA micelles exhibits a large amount of boron uptake at the tumor tissue through the enhanced permeability and retention (EPR) effect and the ability of HA to bind to CD44 receptors. Carborane-TAT@HA was wrapped by the HA shell during systemic circulation to avoid non-specific uptake of TAT with normal cells, while tumor microenvironment-responsive shedding of HA shell could expose Carborane-TAT to penetrate the cell membrane into tumor cells. Experiments have proved the enhanced selectivity of tumor cellular uptake of the boron drug, displayed excellent drug delivery potential, and can meet the basic requirements of BNCT.

Theranostics in Boron Neutron Capture Therapy

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

18F-Boramino acid PET/CT in healthy volunteers and glioma patients

PURPOSE

In this work, the safety, biodistribution, and radiation dosimetry of large neutral amino acid transporter type-1 (LAT-1) targeting PET tracer ¹⁸F-trifluorobborate-derived tyrosine (denoted as ¹⁸F-FBY) has been investigated. It is designed as a first-in-human study in healthy volunteers and to assay LAT-1 expression level in glioma patients.

METHODS

Six healthy volunteers (3 M, 3 F) underwent whole-body PET acquisitions at multiple time points after bolus injection of ¹⁸F-FBY. Regions of interest (ROIs) were mapped manually on major organs, and then the time-activity curves (TACs) were obtained. Dosimetry was calculated with the OLINDA/EXM software. Thirteen patients who were suspected of glioma were scanned with PET/CT at 30 min after ¹⁸F-FBY injection. Within 7 days after PET/CT, the tumor was removed surgically, and LAT-1 immunohistochemical staining for LAT-1 was performed on tumor samples and correlated with ¹⁸F-FBY PET imaging.

RESULTS

¹⁸F-FBY was well tolerated by all healthy volunteers, and no adverse symptoms were observed or reported. ¹⁸F-FBY is rapidly cleared from the blood circulation and excreted mainly through the kidneys and urinary tract. The effective dose (ED) was 0.0039 ± 0.0006 mSv/MBq. In 14 surgical confirmed gliomas (one of the patiens had two gliomas), ¹⁸F-FBY uptake increased consistently with tumor grade, with maximum standard uptake values (SUV_max) of 0.28 ± 0.14 and 2.84 ± 0.46 and tumor-to-normal contralateral activity (T/N) ratio of 2.30 ± 1.26 and 24.56 ± 6.32 in low- and high-grade tumors, respectively. In addition to the significant difference in the uptakes between low- and high-grade gliomas (P < 0.001), the immunohistochemical staining confirmed the positive correlations between the SUV_max, LAT-1 expression (r² = 0.80, P < 0.001), and Ki-67 labeling index (r² = 0.79, P < 0.001).

CONCLUSION

¹⁸F-FBY is a PET tracer with favorable dosimetry profile and pharmacokinetics. It has the potential to assay LAT-1 expression in glioma patients and may provide imaging guidance for further boron neutron capture therapy of gliomas.

TRIAL REGISTRATION

clinicaltrials.gov (NCT03980431).

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.

A Boron-10 nitride nanosheet for combinational boron neutron capture therapy and chemotherapy of tumor

Combination cancer therapy (e.g., radiochemotherapy) is widely used to enhance the therapeutic effects and prevent the recurrence of cancer. However, the side effects of monotherapy are also amplified when treating cancer with combination therapy. A locally activated drug delivery strategy that can release the payload in a tumor-selective manner is greatly needed to overcome the side effects of combination therapy. Here, we explore the potential of combining boron neutron capture therapy and chemotherapy as a new type of radiochemotherapy. Two-dimensional (2D) boron-10-rich nanosheets (BNNSs) were fabricated as a dual-functional delivery system: targeted boron-10 delivery systems for boron neutron capture therapy (BNCT) and drug delivery vehicles to load doxorubicin for chemotherapy. Irradiated by low-energy thermal neutron, BNNSs can produce high linear energy transfer (LET) particles to kill tumor cells, and the loaded doxorubicin can be released in situ at the same time. This neutron-triggered radiochemotherapy shows noteworthy efficacy in suppressing tumor growth in triple-negative breast cancer. To the best of our knowledge, this is the first report to combine BNCT with chemotherapy as a new type of radiochemotherapy. We hope this study could inspire additional BNCT-induced combination cancer therapies and provide insight for the further clinical translation of BNCT.

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.

Side Chain Optimization Remarkably Enhances the in Vivo Stability of 18F-Labeled Glutamine for Tumor Imaging

Similar to glycolysis, glutaminolysis acts as a vital energy source in tumor cells, providing building blocks for the metabolic needs of tumor cells. To capture glutaminolysis in tumors, ¹⁸F-(2S,4R)4-fluoroglutamine ([¹⁸F]FGln) and ¹⁸F-fluoroboronoglutamine ([¹⁸F]FBQ) have been successfully developed for positron emission tomography (PET) imaging, but these two molecules lack stability, resulting in undesired yet significant bone uptake. In this study, we found that [¹⁸F]FBQ-C₂ is a stable Gln PET tracer by adding two more methylene groups to the side chain of [¹⁸F]FBQ. [¹⁸F]FBQ-C₂ was synthesized with a good radiochemical yield of 35% and over 98% radiochemical purity. [¹⁸F]FBQ-C₂ showed extreme stability in vitro, and no defluorination was observed after 2 h in phosphate buffered saline at 37 °C. The competitive inhibition assay results indicated that [¹⁸F]FBQ-C₂ enters cells via the system ASC and N, similar to natural glutamine, and can be transported by tumor-overexpressed ASCT2. PET imaging and biodistribution results indicated that [¹⁸F]FBQ-C₂ is stable in vivo with low bone uptake (0.81 ± 0.20% ID/g) and can be cleared rapidly from most tissues. Dynamic scan and pharmacokinetic studies using BGC823-xenograft-bearing mice revealed that [¹⁸F]FBQ-C₂ accumulates specifically in tumors, with a longer half-life (101.18 ± 6.50 min) in tumor tissues than in other tissues (52.70 ± 12.44 min in muscle). Biodistribution exhibits a high tumor-to-normal tissue ratio (4.8 ± 1.7 for the muscle, 2.5 ± 1.0 for the stomach, 2.2 ± 0.9 for the liver, and 17.8 ± 8.4 for the brain). In conclusion, [¹⁸F]FBQ-C₂ can be used to perform high-contrast Gln imaging of tumors and can serve as a PET tracer for clinical research.