Progress in Nanomaterial-Based Boron Agents for BNCT

Shi Haojie, Li Fenglin*, Fan Caiyun, Xu Qianlong
(China Institute of Atomic Energy, Beijing 102413)

Abstract

Boron neutron capture therapy (BNCT) is an effective tumor treatment modality, yet its clinical application has been limited by the poor targeting, limited boron loading capacity, and high toxicity of the first two generations of boron delivery agents. Nanoparticles can load various ¹⁰B compounds and conjugate diverse tumor-targeting molecules, enabling efficient and targeted delivery of ¹⁰B to tumor tissues to achieve therapeutically required boron concentrations. These nanomaterial-based boron carriers have attracted considerable attention in preclinical BNCT research. This review systematically analyzes and summarizes recent advances in nanoparticle-based boron delivery agents for BNCT, aiming to provide references for developing novel nanomaterial-based boron carriers and to promote the advancement and clinical translation of BNCT technology.

Keywords: BNCT; nanoparticle; nanomaterial-based boron agents

Boron neutron capture therapy (BNCT) is a binary targeted cancer therapy that delivers ¹⁰B compounds (boron carriers) to tumor tissues through specific means. Upon local irradiation with a neutron beam, ¹⁰B undergoes a nuclear reaction with low-energy neutrons, generating high linear energy transfer (LET) α-particles that effectively kill tumor cells. To ensure therapeutic efficacy, boron carriers must meet stringent criteria: high tumor uptake (¹⁰B concentration >30 µg/g tumor tissue or 10⁹ ¹⁰B atoms/cell), low toxicity, excellent tumor targeting (tumor-to-blood and tumor-to-normal tissue ¹⁰B concentration ratios, i.e., T/B and T/N values >3), and prolonged intratumoral retention. Boron carriers have evolved through three generations. The first two generations (boric acid, sodium mercaptoundecahydrododecaborate (BSH), boronophenylalanine (BPA), etc.) suffered from poor targeting, high toxicity, low boron loading, short tumor retention, and significant inter-individual variability, severely hampering BNCT development and application. The third generation comprises nanoparticle-based boron carriers functionalized with various targeting molecules. These nanocarriers promise to overcome the limitations of low boron loading and short retention, delivering substantial amounts of ¹⁰B compounds to tumor tissues to achieve optimal BNCT efficacy.

Nanoparticle-based boron carriers are primarily categorized into liposomes, polymeric nanoparticles, dendrimers, boron carbide nanoparticles, boron nitride nanomaterials, metal nanoparticles, and carbon-based nanomaterials. Nanoparticles with appropriate sizes can extravasate through the leaky vasculature of tumors and accumulate due to impaired lymphatic drainage—the enhanced permeability and retention (EPR) effect. Functionalization of nanoparticles enables more selective and targeted delivery of ¹⁰B compounds to tumor tissues. Consequently, this research area has garnered significant attention in recent years, demonstrating rapid development and promising potential for clinical translation.

1 Liposome-Based Boron Carriers

Liposomes are spherical vesicles composed of cholesterol and natural phospholipid bilayers, offering excellent biocompatibility, straightforward preparation, and minimal requirements regarding the aqueous or lipid solubility of encapsulated drugs. Liposomes can deliver various ¹⁰B compounds, including boronophenylalanine (BPA), sodium mercaptoundecahydrododecaborate (BSH), and carboranes.

Based on their structure and functionalization, liposome-based boron carriers can be classified into passive-targeting liposomes, long-circulating liposomes, and active-targeting liposomes.

1.1 Passive-Targeting Liposomes

Passive-targeting liposomes are prepared using only modified membrane components without external surface modifications, relying on the EPR effect for tumor accumulation.

Kueffer et al. [1] developed liposomes loaded with borane derivatives in both the hydrophobic bilayer and aqueous core. In a mouse model of breast cancer, multiple injections maintained tumor boron concentrations at 67.8 µg/g after 54 hours, effectively improving spatial utilization and boron loading capacity. Altieri et al. [2] encapsulated carboranes in cationic liposomes and observed boron uptake approximately 30-fold higher than BPA controls after co-incubation with colon and melanoma cancer cells. Unlike conventional passive-targeting liposomes that enter cells via membrane fusion or endocytosis, the electrostatic interaction between negatively charged tumor cell surfaces and cationic liposomes significantly enhanced ¹⁰B uptake. Li et al. [3] incorporated carborane derivatives into phospholipid bilayers and radiolabeled the liposome surface with ⁶⁴Cu for PET imaging. In breast cancer-bearing mice, the measured boron distribution in tissues showed high concordance with PET imaging assessments.

1.2 Long-Circulating Liposomes

Larger liposomes are readily cleared by the reticuloendothelial system (RES). Surface modification with polyethylene glycol (PEG), phosphatidylinositol, chitosan, or sodium alginate extends circulation time and tumor retention while avoiding RES uptake, forming long-circulating liposomes.

Pavanetto et al. [4] used PEGylated liposomes to encapsulate BPA-fructose complexes. In rats with hepatic metastases, tumor boron concentrations at 3 hours post-administration were approximately 50-fold higher than in non-PEGylated controls. Yanagie et al. [5] employed PEGylated liposomes loaded with BSH, achieving tumor boron concentrations in pancreatic cancer-bearing mice at 24 hours that were double those of non-PEGylated controls. These studies demonstrate that PEGylation effectively increases liposomal tumor retention.

1.3 Active-Targeting Liposomes

Active-targeting liposomes (immunoliposomes) conjugate drug-loaded liposomes with tumor-targeting molecules to achieve targeted delivery. Various targeting molecules—including monoclonal antibodies (mAbs), transferrin (TF), folic acid (FA), and epidermal growth factor (EGF)—have been successfully used for liposomal boron delivery.

Yanagie et al. [6] conjugated carcinoembryonic antigen (CEA) monoclonal antibodies to BSH-loaded liposomes, enabling specific targeting of CEA-overexpressing tumor cells. Co-incubation with human pancreatic cancer cells followed by neutron irradiation significantly inhibited cell growth. Transferrin liposomes enter tumor cells via transferrin receptor-mediated endocytosis, which is highly expressed on tumor cells. Maruyama et al. [7] conjugated TF to the distal end of PEG chains on BSH-loaded liposomes, achieving high tumor retention. In colon cancer-bearing mice, tumor boron concentrations remained above 30 µg/g at 72 hours post-injection, with a T/B ratio of 6.0 (compared to 2.5 for non-TF controls).

Yanagie et al. [8] developed TF-conjugated PEGylated liposomes loaded with BSH [FIGURE:1a]. In pancreatic tumor-bearing mice, tumor boron concentration reached approximately 12 ppm at 60 hours, versus 6.5 ppm in non-TF controls and only 2.8 ppm in free BSH-treated groups. Pan et al. [9] conjugated FA to PEGylated liposomes loaded with boronated polyamines, achieving cellular boron uptake approximately 10-fold higher than non-targeted controls in squamous carcinoma cells. Singh et al. [10] linked FA to PEGylated liposomes encapsulating boron nanoparticles [FIGURE:1b], resulting in C6 glioma cell boron uptake of 3.7 pg/cell versus 1.5 pg/cell in non-FA controls. In vitro studies suggested these FA-PEG liposomes (100–120 nm) could potentially cross the blood-brain barrier (BBB). Pan et al. [11] inserted cetuximab into PEGylated liposomes loaded with soluble boron salts via a cholesterol-based anchor, achieving boron concentrations of 509.75±148 µg/g in EGFR(+) F98EGFR glioma cells—approximately 8-fold higher than non-targeted controls. Feng et al. [12] conjugated anti-EGFR antibodies to BSH-loaded nickel-containing liposomes via a protein A (ZZ) antibody-affinity scaffold [FIGURE:1c], effectively delivering BSH to EGFR-overexpressing glioma cells. Koning et al. [13] conjugated RGD peptides to liposomes loaded with disodium dodecahydrododecaborate (DHDB), demonstrating significant inhibition of human umbilical vein endothelial cell viability after 24-hour incubation and neutron irradiation. Multiple targeting molecules have thus enhanced tumor retention and cellular boron uptake in liposomal systems.

Different liposome formulations and functionalization strategies improve boron loading efficiency, extend intratumoral retention, and confer tumor specificity. Passive-targeting liposomes offer simple preparation but suffer from RES clearance when particle sizes are large and exhibit relatively weak tumor-targeting ability. Long-circulating liposomes effectively avoid RES uptake through PEGylation, prolonging blood circulation and tumor retention while increasing intratumoral boron concentration. However, repeated injections can cause liposome aggregation and accelerated blood clearance (ABC). Active-targeting liposomes dramatically enhance specificity and targeting through conjugated ligands, significantly increasing tumor boron content while avoiding toxicity from free boron compounds. Nevertheless, targeting molecule conjugation may alter liposomal properties such as size, requiring case-by-case evaluation.

2 Polymer-Based Boron Carriers

Polymeric nanoparticles self-assemble from block copolymers composed of two or more polymer chains with different hydrophobicities. Advances in polymer chemistry have enabled the creation of precisely controlled, functionalized polymeric nanoparticles with tunable shape, size, internal morphology, and surface charge, making them excellent candidates for BNCT drug delivery [14, 15].

Various polymer nanoparticles loaded or coated with boron compounds have demonstrated exceptional delivery efficiency. Mi et al. [16] conjugated BSH to poly(ethylene glycol)-b-poly(glutamic acid) copolymers, forming [PEG-b-P(Glu-SS-BSH)]. In colon cancer-bearing mice, tumor boron concentrations reached 69 ppm at 24 hours post-administration, compared to 13 ppm and 10 ppm for P(Glu-SS-BSH) and BSH controls, respectively. After neutron irradiation, tumor volumes in the experimental group were only 1/20th of those in the BSH neutron control group at 20 days. Xiong et al. [17] developed DOX@PLMB by conjugating carboranes to amphiphilic copolymers and encapsulating doxorubicin (DOX). After neutron irradiation, tumor volumes in the DOX@PLMB neutron group were one-third those of the non-irradiated DOX@PLMB group at 27 days. Notably, 3 out of 7 mice showed complete tumor regression, with a tumor inhibition rate of 92.9±18.9%. This approach not only reduced DOX's cardiotoxicity and nephrotoxicity but also demonstrated that BNCT-chemotherapy combination therapy far outperformed either modality alone, representing a promising trend in precision oncology.

Takeuchi et al. [18] loaded hydrophobic boron compounds into poly(L-lactide-co-glycolide) nanoparticles. In melanoma-bearing mice, tumor boron concentrations reached 113.9±15.8 µg/g at 8 hours post-injection, 3.5–4.2-fold higher than carborane-albumin complexes, with T/B ratios >5 maintained for 8–12 hours. Dai et al. [19] coated BPA with polydopamine (PDA) to form stable, biocompatible B-PDA nanoparticles. In melanoma-bearing mice, tumor boron concentrations reached 31.485±2.369 ppm at 24 hours after 200 µg/mL administration, with T/B and T/N ratios of approximately 4.922 and 6.32, respectively. Neutron irradiation studies showed median survival of 23.5 days for both the 200 µg/mL B-PDA group and the 100 µg/mL B-PDA plus photodynamic therapy group, compared to less than three weeks for controls. This suggests BNCT-photodynamic therapy combinations may reduce the minimum tumor boron concentration required for BNCT, opening avenues for multi-modal therapeutic strategies.

Evaluating the biodistribution and pharmacokinetics of boron carriers is crucial for BNCT development. Fluorescence imaging and positron emission tomography (PET) have been employed for polymer-based carriers. Ruan et al. [20] synthesized an amphiphilic triblock polymer micelle with polycaprolactone and polycarborane as hydrophobic blocks and poly(ethylene glycol) methyl ether methacrylate as the hydrophilic block, conjugated with a fluorescent dye for imaging [FIGURE:2a]. Boronated porphyrins accumulate in tumors with long retention, but their clinical development was hindered by low tumor-to-blood ratios and direct platelet toxicity observed in Phase I studies. Shi et al. [21] encapsulated tetraboronated porphyrin in methoxy-poly(ethylene glycol)-poly(lactide-co-glycolide) micelles to form boronated porphyrin nanocomplexes (BPN) [FIGURE:2b]. Porphyrins' intrinsic optical imaging properties enabled fluorescence imaging, while ⁶⁴Cu radiolabeling allowed PET imaging to visualize BPN cellular uptake and pharmacokinetics.

Tumor cell specificity can be enhanced by preparing boron-containing polymer micelles with charged surfaces or conjugated targeting molecules. Azab et al. [22] synthesized cationic acrylamide copolymers with varying aminophenylboronic acid (APB) ratios for targeting rat intestinal polyps, achieving T/N ratios of 6.5 and polyp boron concentrations of 88.5±15.1 µg/g at 60 minutes post-administration. The asialoglycoprotein receptor (ASGP-R) is an ideal target for hepatocellular carcinoma-specific drug delivery, with galactose residues showing high receptor affinity. Zhang et al. [23] synthesized PEGylated galactose polymer micelles loaded with carboranes. After 24-hour incubation with human liver cancer cells, intracellular boron concentrations were significantly higher than BSH controls. Neutron irradiation resulted in 36.95% acute cell death in the micelle group, approximately double that of the BSH control.

Reactive oxygen species (ROS) are known intracellular signaling mediators. However, γ-rays produced during BNCT generate substantial ROS, potentially causing oxidative stress, cellular dysfunction, and inflammation. To scavenge BNCT-induced ROS, boron cluster-containing redox nanoparticles (BNPs) have been extensively studied for their ROS-scavenging capacity, high colloidal stability, and tumor-specific accumulation. Gao et al. [24] synthesized two polymers [FIGURE:2c] containing BSH and ROS scavenger monomers, respectively, which were coupled via polyion complexation to form BNPs. Incubation with rectal cancer cells for 18 hours showed significantly higher BNP uptake compared to BSH controls, while normal human aortic endothelial cells showed minimal uptake. In rectal cancer-bearing mice, neutron irradiation with BNPs caused no adverse effects (low leukocyte levels).

Polymeric nanoparticle-based boron carriers offer simple preparation and stable properties, making them widely used in BNCT research. Charged and targeting molecule-conjugated boron-containing polymer micelles effectively target tumors, though cationic micelles may cause in vivo toxicity. ROS-scavenging polymer micelles can mitigate BNCT side effects, potentially improving therapeutic outcomes.

3 Dendrimer-Based Boron Carriers

Dendrimers (DEs) possess well-defined chemical structures, uniform molecular weights, and easily modifiable surface and internal architectures, enabling high drug loading while preventing drug leakage or degradation. Dendrimer-based boron carriers offer high boron content and facile conjugation with multiple tumor-targeting molecules.

Common preparation methods involve encapsulating carboranes within dendrimer interiors or attaching them to surfaces [25]. Parrott et al. [26] conjugated carborane derivatives to dendrimers, creating water-soluble, boron-rich dendrimers capable of encapsulating up to 16 carboranes with tunable solubility [FIGURE:3a]. Dash et al. [27] synthesized phenylene-cored carborane dendrimers via "click" chemistry, loading 3–9 carboranes [FIGURE:3b]. Cabrera-González et al. [28] prepared optically active tetraphenylporphyrin-cored carborane dendrimers with up to 32 carboranes [FIGURE:3c]. However, these dendrimers showed limited improvement in tumor targeting compared to small boron molecules, with only modest enhancements in boron content or biodistribution.

Functionalized boron-containing dendrimers significantly improve tumor specificity and retention. Vascular endothelial growth factor receptor (VEGFR) is overexpressed in tumor neovasculature. Backer et al. [29] conjugated VEGF to boronated dendrimers (BD) containing 1050–1100 boron atoms and labeled them with Cy5 for biodistribution studies (VEGF-BD/Cy5). In breast cancer-bearing mice, VEGF-BD/Cy5 selectively accumulated in peripheral tumor regions with active neovascularization. EGFR represents a potential target for glioma therapy. Capala et al. [30] conjugated EGF to boronated starburst dendrimers (BSD) and radiolabeled them with ¹²⁵I ([¹²⁵I]-EGF-BSD) for evaluating boron uptake in glioma cells. Wu et al. [31] conjugated cetuximab (IMC-C225) to fifth-generation boron-containing dendrimers, forming C225-G5-B1100 [FIGURE:3d]. In F98EGFR glioma-bearing rats, tumor boron concentrations reached 23.3 µg/g at 24 hours, versus only 3.6 µg/g in non-targeted G5-B1100 controls. Folate receptors are overexpressed on many tumor cells. Shukla et al. [32] used PEG to coat folate-modified boronated polyamidoamine (PAMAM) dendrimers to reduce RES uptake, forming boronated G3-DE [FIGURE:3e]. However, in 24JK-FBP sarcoma-bearing mice, high uptake was observed in tumor, liver, and kidney.

Dendrimer-based boron carriers offer high boron content and facile surface modification for enhanced tumor specificity. However, complex synthesis, difficulty in controlling drug release, and unclear in vivo stability and metabolism have limited their further development in BNCT.

4 Boron Carbide Nanoparticle-Based Carriers

Boron carbide particles are synthesized via simple, low-toxicity methods with high boron content, excellent biochemical and thermal stability, large surface area, and unique nanostructures that provide abundant adsorption sites for functionalization and efficient drug loading, making them attractive for BNCT.

Tsuji et al. [33] conjugated transferrin to poly-L-lysine and poly-γ-glutamic acid-coated boron carbide particles, forming spherical transferrin boron carbide nanoparticles (Tf-SBCPs) labeled with fluorescent dyes. After 2-hour incubation with cervical cancer cells, Tf-SBCPs were extensively internalized, whereas albumin-conjugated SBCPs only accumulated on cell surfaces. Free TF competitively inhibited Tf-SBCP internalization. Kaur et al. [34] incubated nanostructured boron carbide (B₄C) with cervical cancer and glioblastoma cells for 3 hours. Neutron irradiation resulted in 61% acute cell death in cervical cancer cells (comparable to BPA) and 78% in glioblastoma cells (significantly higher than BPA's 33%). These findings suggest B₄C may be superior to BPA for BNCT in vitro.

Boron carbide nanoparticle carriers exhibit high boron loading and promising cancer cell uptake in vitro. However, preclinical studies remain limited, and further investigation is needed regarding in vivo biodistribution, pharmacokinetics, and therapeutic efficacy post-neutron irradiation in animal models.

5 Boron Nitride Nanomaterial-Based Carriers

Boron nitride nanoparticles are stable, low-toxicity, biocompatible, and boron-rich, primarily classified as boron nitride nanoparticles (BNNPs) and boron nitride nanotubes (BNNTs), representing promising boron carriers.

BNNPs significantly enhance ¹⁰B delivery efficiency. Li et al. [35] coated BNNPs with phase-transitioned lysozyme (PTL) to form PTL@BNNPs, preventing hydrolysis during delivery. Post-neutron irradiation, on-demand degradation was achieved by administering vitamin C, avoiding long-term accumulation toxicity. Zhang et al. [36] grafted poly(glycerol) onto hexagonal boron nitride particles (h-¹⁰BN-PG) [FIGURE:4a]. In colon cancer-bearing mice, intratumoral ¹⁰B concentrations remained as high as 80 µg/g at 24 hours, with significant tumor volume reduction after neutron irradiation—one of the rare cases where boron-containing drug alone induced tumor shrinkage.

Compared to BNNPs, BNNTs resist decomposition during in vivo delivery. Ciofani et al. [37] coated BNNTs with biocompatible poly-L-lysine (PLL) and functionalized them with fluorescent dyes and folic acid (F-PLL-BNNT) [FIGURE:4b]. After 90-minute incubation with glioblastoma cells, F-PLL-BNNT uptake was significantly higher than PLL-BNNT controls. Nakamura et al. [38] coated BNNTs with DSPE-PEG2000 (BNNT-DSPE-PEG2000). One-hour incubation with melanoma cells yielded intracellular boron accumulation approximately 3-fold higher than BSH controls.

Boron nitride nanoparticles possess high intrinsic boron content, but challenges remain regarding poor water solubility, in vivo decomposition, and potential toxicity from accumulation. Surface functionalization may address these issues.

6 Metal Nanoparticle-Based Boron Carriers

Metal nanoparticles offer facile surface modification and precise size control for drug delivery. Silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) have become popular for boron loading and targeted delivery in BNCT.

6.1 Silver Nanoparticles

AgNPs (<100 nm) are easily functionalized, improving drug pharmacokinetics and biodistribution, facilitating membrane penetration, and enabling controlled release. Kennedy et al. [39] functionalized AgNPs with mercaptocarborane and conjugated anti-EGFR antibodies, achieving ~4.8×10⁸ ¹⁰B atoms per hepatocellular carcinoma cell. The team also demonstrated that AgNPs could absorb energy to generate local hyperthermia, enabling potential BNCT-thermal ablation combination therapy. While promising, AgNPs showed potential toxicity in cell viability assays that warrants attention.

6.2 Gold Nanoparticles

AuNPs exhibit excellent biocompatibility, low toxicity, small size, and optical imaging capability. For BNCT, AuNPs conjugated with targeting molecules can selectively deliver ¹⁰B compounds to tumors while capturing neutrons to emit β-rays, creating synergistic therapeutic effects [40].

Ligand-protected metal nanoclusters are termed monolayer-protected clusters (MPCs), where ligands determine water solubility and function. AuNP boron carriers are typically functionalized with mercapto-o-carborane for ligand protection. Cioran et al. [41] directly functionalized AuNPs with o-carborane, producing 3.2 nm nanoparticles with good water solubility and membrane-crossing ability (i.e., freely crossing membrane boundaries to exist in cytoplasm, nucleus, and mitochondria). The team further investigated ligand exchange between o-carborane and citrate-coated AuNPs to prepare 10 nm AuNPs [FIGURE:5a], finding that adjusting ionic and electronic charges for phase transfer likely correlates with membrane penetration ability [42].

"Click" chemistry enables simple conjugation of o-carborane to macromolecules like dendrimers and polymers, and has been applied to AuNP functionalization. Li et al. [43] synthesized water-soluble o-carborane and PEGylated dendritic AuNPs via "click" chemistry. Liang et al. [44] conjugated o-carborane to polystyrene via "click" chemistry to coat AuNPs (~2.6 nm) and palladium nanoparticles (~1.7 nm). Ciani et al. [45] coupled o-carborane AuNPs to poly(ethylene oxide)-b-poly(caprolactone) diblock copolymer (PEO-b-PCL) via "click" chemistry, significantly improving water solubility and preventing non-specific protein interactions.

Non-invasive imaging facilitates evaluation of AuNP biodistribution and pharmacokinetics. Pulagam et al. [46] PEG-coated AuNPs functionalized with the boron-rich anion [3,3′-Co(1,2-C₂B₉H₁₁)₂]⁻ and dual-radiolabeled them with ¹²⁴I for PET imaging. However, PET imaging in human fibrosarcoma mice revealed low tumor accumulation and poor targeting. Mandal et al. [47] coated AuNPs with FITC-BPA-folate multilayered boronated polyelectrolytes, creating multifunctional AuNPs for boron delivery and imaging. Human epidermal growth factor receptor 2 (Her2) is overexpressed in various cancers. Wu et al. [48] functionalized AuNPs with PEG, carborane, and azide compounds, conjugated them to anti-Her2 antibodies (61 lgG), and radiolabeled them with ¹²³I via "click" chemistry to obtain ¹²³I-61-B-AuNPs [FIGURE:5b]. In gastric cancer-bearing mice, SPECT imaging at 12 hours showed tumor-to-muscle radioactivity ratios of ~12.02.

Metal nanoparticle drug delivery remains a hot research topic. While AgNPs possess intrinsic tumoricidal activity, carborane-functionalized AgNPs exhibit potential toxicity. AuNPs offer low toxicity, high boron loading efficiency, and controlled drug release, while surface coating with fluorescent or radioactive labels enables non-invasive imaging for biodistribution assessment. Despite these advantages, synthesizing uniformly sized, well-dispersed, non-aggregating AuNPs requires extremely cumbersome purification steps.

7 Carbon Nanomaterial-Based Boron Carriers

Carbon nanomaterials—including carbon nanoparticles, carbon nanotubes, and nanodiamonds—are easily functionalized to prolong circulation time and enable long-term drug retention at disease sites, making them suitable for ¹⁰B compound delivery.

7.1 Carbon Nanoparticles

Carbon nanoparticles (CNPs) can be surface-modified to improve biocompatibility and targeting. Environment-sensitive chemical linkers enable controlled drug release kinetics and on-demand release. CNPs also possess intrinsic fluorescence and magnetic resonance imaging capabilities, and can be used for photodynamic and photothermal therapy for synergistic treatment.

Hwang et al. [49] prepared boron-containing carbon nanoparticles (BCo@CNPs) modified with folic acid. After co-incubation with cervical cancer cells and neutron irradiation, acute cell death reached 52% with significantly inhibited proliferation. Dai et al. [50] incubated FA-modified BCo@CNPs with invasive nonfunctional pituitary adenoma (NFPA) cells, demonstrating FR(+)-mediated selective uptake.

7.2 Carbon Nanotubes

Carbon nanotubes offer water solubility and biocompatibility, and surface modification enables efficient drug loading, targeted delivery, and improved stability. Zhu et al. [51] functionalized single-walled carbon nanotubes (SWCNTs) with carboranes to create water-soluble boron-containing nanotubes. In breast cancer-bearing mice, tumor boron concentrations peaked at 22.8 µg/g at 30 hours and remained at 21.5 µg/g at 48 hours, demonstrating prolonged retention though slightly below BNCT requirements. SWCNTs' fluorescence imaging capability facilitates in vivo distribution assessment. Yamagami et al. [52] non-covalently functionalized SWCNTs with BSH-conjugated PAMAM dendrimers, providing an effective imaging method that avoids fluorescence quenching from covalent BSH-SWCNT functionalization.

7.3 Nanodiamonds

Nanodiamonds (NDs) are easily functionalized drug delivery vehicles, though their in vivo biodistribution is difficult to assess. Lin et al. [53] embedded fluorescent ¹⁰B ions into NDs using physical particle injection technology, creating boron-rich, non-toxic B-NDs. This approach provides a novel boron carrier while solving the challenge of tracking ND distribution intracellularly and in vivo.

Carbon nanoparticles are easily functionalized for targeted ¹⁰B delivery, and their potential for photodynamic and photothermal therapy enables synergistic BNCT combinations. SWCNTs' fluorescence imaging facilitates monitoring of in vivo targeting, while fluorescent ¹⁰B-embedded NDs solve distribution tracking challenges. Overall, carbon nanomaterial-based carriers enable efficient drug loading, targeted delivery, and distribution assessment, though complex preparation and potential immune system activation remain challenges.

8 Other Nanomaterial-Based Boron Carriers

Beyond the aforementioned categories, numerous other nanomaterial-based boron carriers have attracted broad research attention due to their unique physicochemical properties and potential applications in biomedicine and materials science.

8.1 Magnetic Nanoparticles

Magnetic nanoparticles (MNPs) can load drugs via physical adsorption or chemical conjugation, enabling magnetically guided delivery and environmentally triggered release. Kuznetsov et al. [54] prepared ferro-carbon (Fe-C) and iron-boron (Fe-B) composite particles that adsorbed borax. Zhu et al. [55] conjugated o-carborane to propargyl-rich starch Fe₃O₄ nanoparticles via "click" chemistry [FIGURE:5a], achieving efficient magnetic targeting with tumor boron concentrations up to ~51.4 µg/g, T/N ratios of 10, and 48-hour intratumoral retention. Iron oxide nanoparticles face challenges of aggregation, low stability, and poor biocompatibility. Tulebayeva et al. [56] used 3-aminopropyltrimethoxysilane-coated Fe₃O₄ nanoparticles with high stability, biocompatibility, low toxicity, and cost-effectiveness as carborane carriers. Oleshkevich et al. [57] coated Fe₃O₄ nanoparticles with meta-carboranyl phosphinate [FIGURE:5b], showing no toxicity in glioma-bearing mice and significantly reduced cell proliferation post-neutron irradiation.

8.2 Inactivated Hemagglutinating Virus of Japan Envelope

Inactivated Hemagglutinating Virus of Japan envelope (HVJ-E) possesses anti-tumor activity, activates anti-tumor immunity, and induces cancer cell death. Fujii et al. [58] combined HVJ-E with cationized gelatin (CG) to reduce hemagglutination activity and cytotoxicity, loading BSH to form CG-HVJ-E-BSH for BNCT. After 30-minute incubation with osteosarcoma cells, intracellular boron concentrations far exceeded BSH controls. In mice with multiple liver tumors, T/N ratios remained higher than BSH controls for 48 hours. Neutron irradiation at 24 hours (CG-HVJ-E-BSH) and 1 hour (BSH) showed prolonged survival and minimal histological damage to surrounding liver tissue in the CG-HVJ-E-BSH group. Yoneoka et al. [59] functionalized HVJ-E with 2-methacryloyloxyethyl phosphorylcholine (MPC) and methacrylamide benzoxaborole (MAAmBO) to form HVJ-E/p[MPC-co-MAAmBO], achieving high ¹⁰B loading and hemolysis inhibition. Fluorescence imaging showed uniform cytoplasmic distribution in liver cancer cells after 45–90 minutes.

8.3 Mesoporous Silica Nanoparticles

Mesoporous silica nanoparticles (MSNs) utilize internal mesoporous structures for drug loading. Yu et al. [60] first covalently linked carborane to mesoporous silica surfaces, creating high boron-content, stable nanoparticles with tunable hydrophilicity/hydrophobicity. Zhang et al. [61] prepared liver cancer-targeted mesoporous silica loaded with carborane, achieving 92.8% acute cell death in human liver cancer cells after neutron irradiation. Tang et al. [62] developed SP94-LB@BA-MSN by coating MSNs with SP94 peptide-modified lipid bilayers (LB) loaded with ¹⁰B-boric acid (¹⁰BA). In liver cancer cells, boron concentrations consistently exceeded ¹⁰BA and BPA controls, with significantly greater viability inhibition post-neutron irradiation.

8.4 Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are emerging materials with high tunability, large loading capacity, good biocompatibility, and facile chemical modification, making them ideal drug carriers. Wang et al. [63] prepared nano-co-crystals (MNCs) from zirconium-meso-tetra(4-carboxyphenyl)porphyrin MOFs and ¹⁰BA, showing excellent stability, colloidal dispersity, and biocompatibility. In orthotopic glioma-bearing mice, tumor ¹⁰B concentrations reached 67.5±4.2 µg/g at 2 hours, with T/N and T/B ratios of ~6.20 and ~3.80, respectively. Fluorescence and ⁸⁹Zr-PET imaging confirmed BBB penetration and tumor accumulation. After neutron irradiation, 75% of mice survived at 5 weeks versus complete mortality in the BPA group within 3 weeks. Wang et al. [64] evaluated the relative biological effectiveness (RBE), finding MNCs' RBE was 4.1-fold higher than ¹⁰BA.

8.5 Other Emerging Carriers

Nanofiber Mats: Nanofiber mats enable precise control of drug release rates through structural parameters and loading. Huang et al. [65] prepared ¹⁰BA-loaded polylactide-polyethylene glycol (PLA-PEO) nanofiber mats for localized administration and controlled release. In liver cancer-bearing mice, optimally releasing PLA80-PEO20 mats achieved T/B ratios of 80.6, with tumor growth to only 3-fold initial volume at 25 days versus 25-fold in ¹⁰BA controls.

Covalent Organic Frameworks: Covalent organic frameworks (COFs) are crystalline porous materials formed by covalently bonded organic molecules. Shao et al. [66] synthesized multifunctional boron-rich COFs that can be surface-modified with peptide targeting molecules and co-loaded with immunoadjuvants, photosensitizers, sonosensitizers, and chemotherapeutics for BNCT-immunotherapy combinations.

Ferritin Nanoparticles: Ferritin nanoparticles (FNP) are attractive protein nanoplatforms for antigen delivery and immune stimulation, naturally targeting transferrin receptor 1 (TfR1)-overexpressing tumors. Zhang et al. [67] loaded BPA into ferritin nanoparticles, achieving tumor-specific accumulation in glioma-bearing mice as confirmed by near-infrared imaging.

Multifunctional Vesicles: Dai et al. [68] developed lipoic acid-boronophenylalanine-derived vesicles encapsulating doxorubicin. Unlike L-BPA's LAT1 transporter-mediated entry, these vesicles rapidly react with sialic acid overexpressed on tumor cells via phenylboronic acid groups, enhancing cellular interaction and uptake. Elevated ROS levels post-uptake induced mitochondrial dysfunction and apoptosis. In pancreatic cancer-bearing mice, 350 mg/kg administration achieved T/B ratios up to 5, with complete tumor suppression at 15 days through BNCT-chemotherapy synergy and mitochondrial apoptosis.

Magnetic nanoparticle carriers enable magnetically guided boron delivery with prolonged intratumoral retention, though complex preparation, premature ¹⁰B release, and stability issues persist. Silane-functionalized MNPs partially address these limitations. HVJ-E carriers leverage membrane fusion for tumor cell internalization and intrinsic anti-tumor activity for synergistic BNCT, though cytotoxicity and hemolysis require monitoring. MOFs, nanofiber mats, COFs, and MSNs achieve therapeutic boron concentrations via active or passive targeting, yet drug leakage, precise release control, and potential accumulation toxicity remain challenges. BPA-loaded ferritin nanoparticles targeting TfR1 and sialic acid-targeting boron vesicles represent novel carriers warranting further investigation.

Summary and Outlook

In summary, nanoparticle-based boron carriers have ushered in a new era of tumor-targeted, high-capacity boron delivery. Despite numerous studies, post-irradiation therapeutic outcomes in animal models remain suboptimal, with few demonstrating tumor volume reduction. This may relate to carrier stability, drug release kinetics, and accelerator neutron beam parameters. Synergistic combinations of BNCT with chemotherapy, immunotherapy, photodynamic therapy, or photothermal therapy show promise, with multiple studies demonstrating tumor regression rather than mere growth inhibition.

Liposome-based boron carriers exhibit distinct advantages: high boron loading capacity via hydrophilic and hydrophobic compartments; prolonged tumor retention through EPR effects and PEGylation; simple preparation and facile surface modification; good biocompatibility without severe side effects; and efficient clearance to avoid long-term toxicity. With multiple liposomal anticancer formulations already approved, liposomal boron carriers may emerge as the next clinically approved BNCT drug, bringing breakthroughs to the field.

Theranostics integration remains a long-sought goal, particularly critical for BNCT to optimize neutron irradiation timing at peak intratumoral ¹⁰B concentration. Radionuclide-mediated PET/SPECT imaging has been applied to nanoparticle-based boron carriers for biodistribution assessment and maximum boron concentration determination, representing a promising future research direction.