Abstract
Boron Neutron Capture Therapy (BNCT) is a binary cancer therapy that involves boron (10B) drug administration and epithermal/thermal neutron irradiation. The nuclear fission reaction between low-energy thermal neutrons and 10B atoms accumulated in cancer cells generate high linear energy transfer (LET) species (α and 7Li particles). These species are short range (4-10 μm, less than a cell diameter) and therefore offer potential to kill cancer cells selectively and spare surrounding healthy cells if the 10B drug can be targeted specifically and sufficiently to cancer cells. Numerous studies have explored the role of selected targets on the influence of 10B transport to make progress in developing novel, safe and effective 10B carriers. These findings would expand the clinical BNCT services to more patients, with potential to make BNCT a more precise and effective treatment modality for various types of cancer. This review summarizes the current knowledge of a variety of potential targets for 10B transport in BNCT.
1 Introduction
BNCT has proven to be an effective treatment for a range of cancers such as malignant glioma (1), melanoma (2), and head and neck cancer (3), confirming BNCT a promising radiotherapy for precise cancer treatment (4, 5). There are advanced features with BNCT: (i) the capability to treat recurrent tumors at sites where radiation has already been delivered, (ii) treatment of diffuse tumors where cell targeting offers potential benefit to spare healthy tissue, (iii) treatment of radiation resistant tumors by the high LET of the fission products which have potential to be more damaging than conventional radiotherapy.
General requirements for 10B accumulation are tumor concentrations ≥ 20 μg/g and a tumor/normal(T/N) tissue and tumor/blood(T/B) 10B concentration ratios of >3:1 (6). Boron delivery agents should also satisfy essential criteria: low intrinsic cytotoxicity, minimal accumulation in normal tissues adjacent to the tumor irradiation area, and effective retention of 10B inside cancer cells (5–7). Thus far, only two 10B carriers, sodium mercaptoundecahydrododecaborate-10B (sodium borocaptate-10B, BSH, Na210B12H11SH) and boronophenylalanine-10B (BPA, C9H1210BNO4), have been approved for clinical BNCT (6, 8). BPA, structurally similar to phenylalanine, is primarily recognized by L-type amino acid transporter 1 (LAT1) which forms a complex with CD98 and mediates transport of BPA (9), while BSH is difficult to selectively internalize in tumor cells and accumulates in tumor through a passive diffusion (10). It is recognized there opportunity to improve upon 10B uptake concentration and micro-distribution offered by BPA and BSH (11, 12), with many studies exploring the role of selected targets on the influence of 10B delivery, and downstream therapeutic effect, towards identifying potential targets and biomarkers for BNCT (Table 1) (12). In this review, the primary inclusion criterion was demonstrated selective accumulation of a 10B-containing agent in tumor cells or tissues, regardless of whether BNCT (neutron irradiation) was subsequently performed. This approach was intended to provide a focused yet up-to-date overview of potential molecular targets influencing 10B transport.
Table 1
| Target | Uptake/transport mechanism | Boron carrier | Study type | BNCT | Ref |
|---|---|---|---|---|---|
| LAT1 | LAT1/CD98 mediates a Na+ and pH independent antiport of amino acids. | LAT1-overexpressing clones, BCH: a LAT1 inhibitor | In vitro | Yes | (13, 21) |
| LAT1 | Preloading L−DOPA (structurally similar to L−tyrosine and BPA) enhances the antiport mechanism of BPA transporters. | L-DOPA | In vitro and in vivo | No | (11) |
| LAT1 | Reducing intracellular efflux. | PVA-BPA | In vitro and in vivo | Yes | (26) |
| LAT1 | High-dose L-phenylalanine reduced the accumulation of L-BPA in the normal brain relative to tumor tissue. | L-phenylalanine | In vivo | No | (27) |
| LAT2 | N/A | N/A | In vitro | No | (9) |
| ATB0,+ | ATB0,+ transports all amino acids except glutamate and aspartate by using a Na+ and Cl gradient | N/A | In vitro | No | (9) |
| HIF-1α | HIF-1α suppresses LAT1 expression in hypoxic cells. | YC-1 | In vitro and in vivo | Yes | (40, 41) |
| p53 | N/A | N/A | In vitro and in vivo | Yes | (38, 48, 49) |
| mTORC1 | By inhibiting mTORC1 (a downstream target of LAT1), rapamycin reduces tumor cell proliferation and thus 10B loading. | Rapamycin | In vitro | Yes | (56, 59) |
| EGFR | Ligand receptor reaction. | Z33-DB, B-ASOs | In vitro | Yes | (7, 73) |
| HER2 | Tetrazine moieties, attached to the surface of small boron-rich nanoparticles, could undergo the click-reaction with trans-cyclooctene mAb | Tetrazine-functionalized boron-rich carbon dots | In vitro and in vivo | No | (76) |
| HER2 | The DARPin-modified liposomes can bind to HER2-overexpressing cells and undergo effective internalization into the cytoplasm | DARPin-modified liposomes | In vitro | No | (75) |
| Anxa1 | Membrane−expressed Anxa1 mediates active transport of IF7−10B into both tumor vascular endothelial and tumor cells | IF7-BPA/BSH | In vivo | Yes | (83) |
| CD44 | CD44 protein and translational machinery proteins as a major cell surface target and intracellular targets of BSH-polyR, respectively. | BSH-polyR | In vitro | No | (84) |
| CD44 | Ligand receptor reaction. | HA/CBP-H complex | In vitro | Yes | (85) |
| TSPO | Binding at the TSPO. | 1,2-closo-carboranylpyrazolopyrimidine, | In vitro | No | (88) |
| TSPO | Binding at the TSPO. | DPA-BSTPG | In vitro and in vivo | Yes | (90) |
| Integrin αvβ3 | Cyclic RGD (cRGD) peptide-conjugated boronated albumin can directly toward integrin αvβ3 | cRGD-MID-BSA, cRGD-MID-AC | In vitro and in vivo | Yes | (93) |
| FRα | N/A | PBC, PBC-IP | In vitro and in vivo | Yes | (99, 100) |
| LDLR | These nanoparticles were selectively internalized via LDLR-mediated endocytosis. | B4C nanoparticles | In vitro | No | (105) |
| GLUT1 | Binding at the GLUT1. | Ortho-carboranylmethyl-bearing glucoconjugates | In vitro | No | (108) |
| GLUT1 | Binding at the GLUT1. | 6-deoxy-6-thio-carboranyl d-glucoconjugates | In vitro | No | (109) |
| GLUT1 | Binding at the GLUT1. | Carboranylmethyl derivatives of d-Man, d-All, and d-Gal | In vitro | No | (110) |
| GLUT1 | Binding at the GLUT1. | B-Glc | In vitro and in vivo | No | (111) |
| ASCT2 | GluB-2 are selectively transported by ASCT2. | GluB-2 | In vitro and in vivo | Yes | (113) |
Targets for boron transportation in BNCT.
2 L-type amino acid transporter 1 and its regulation
2.1 L-type amino acid transporter 1
Among several amino acid transporters in cells, LAT1 (SLC7A5) is a neutral amino acid carrier highly overexpressed in tumor cells and serves as the primary mediator of BPA transportation (13). LAT1 belongs to the amino acid−polyamine−organocation superfamily and forms a heterodimeric complex with CD98 (SLC3A2) via a conserved disulfide bond. This complex mediates essential amino acid uptake in key barriers such as the placenta and blood−brain barrier (14). Studies in intact cells have shown that the LAT1/CD98 heterodimer operates a Na+− and pH−independent amino acid antiport (15). Functional assays using proteoliposomes reconstituted with recombinant human LAT1 identified histidine as its optimal substrate. Meanwhile, CD98 is dispensable for transport activity and presumably acts to traffic LAT1 to the cell surface (16). LAT1 is closely implicated in multiple human disorders(such as neurological diseases and malignancies) and correlates with tumor proliferation, angiogenesis, immune regulation and unfavorable clinical prognosis (17). Accordingly, LAT1, along with other nutrient transporters, has emerged as an important pharmacological target for therapeutic development (15). LAT1 expression is also regulated by multiple factors, hypoxia alters LAT1 levels via HIF−2α in differentiated neurons (18), while protein kinase C activation promotes robust endocytosis and subsequent degradation of LAT1 in HeLa cells (19).
Previous studies have assessed the relationship between LAT1 expression levels and cellular BPA accumulation. By employing genetically engineered cell lines with LAT1 knockdown or overexpression, Watanabe et al. reported that BPA uptake positively correlates with LAT1 expression (13). By analyzing data from The Cancer Genome Atlas, the relative expression rank of LAT1 in human tumor tissues is malignant melanoma, head and neck tumors, esophageal tumors, squamous cell carcinoma of the lung, cervical tumors, glioblastoma, transitional epithelial carcinoma (such as bladder and urinary tract cancer), adenocarcinoma (such as gastrointestinal, breast and uterine cancer and lung adenocarcinoma) and lymphomas. According to histological type, squamous cell carcinoma was the most common tumor with a high expression of LAT1. Elevated LAT1 expression in tumors correlates with increased proliferation marker levels and unfavorable clinical prognosis (13). In another study, intracellular uptake of BPA was 1.5–5.0 times greater among LAT1-overexpressing clones than that in a control clone, and the LAT1-overexpressing clones and transiently LAT1-transfected cells showed significant improved sensitivity to BNCT (20). In the hypoxic microenvironment, CD133−positive cells exhibit cancer stem cell characteristics, with LAT1 selectively overexpressed in these stem cell−like cells. Tani et al. constructed plasmids to induce tdTomato−tagged LAT1 overexpression on the plasma membrane of CD133−expressing cancer cells (21). Overexpressing LAT1-tdTomato in tumor improved the BPA uptake and efficacy of BNCT (21).
As LAT1 forms a complex with CD98 and mediates transport of L-type amino acid (22), several molecules structurally similar to L-type amino acids or BPA have been investigated. Multiple studies have verified that L-tyrosine treatment elevates intracellular BPA accumulation in tumor cells (23, 24). Similarly, pretreatment with substrates recognized by L or A amino acid transport systems effectively promotes BPA enrichment in gliosarcoma cells (11). These evidence support that such transporters operate via a substrate-coupled antiport exchange mechanism, which can be further activated by preloading specific amino acids. L-3,4-dihydroxyphenylalanine (L-DOPA) shares high structural similarity with L-tyrosine and BPA. Pretreament with L-DOPA can markedly boost intracellular BPA accumulation by accelerating the L-type antiport process (11, 24, 25).
LAT1 mediates BPA influx and efflux of intracellular substrates such as glutamine. When extracellular BPA is low, intracellular BPA may be exchanged for extracellular substrates like tyrosine. Poly(vinyl alcohol) (PVA), a biocompatible polymer, forms reversible boronate esters with BPA in aqueous solution. This complexation does not alter the phenylalanine structure of BPA, allowing continued recognition by LAT1. The PVA-BPA complex is internalized via LAT1-mediated endocytosis and sequestered in endo-/lysosomes, where it avoids antiport-mediated efflux by membrane-bound LAT1, thereby enhancing intracellular retention. In summary, PVA-BPA exhibited efficient accumulation and prolonged retention in tumors with quick clearance from the bloodstream and normal organs (26).
The tumor to normal tissue 10B concentration ratio (T/N ratio) is often used to evaluate the effect of 10B carriers. Instead of enhancing tumor uptake of 10B, decreasing the relative accumulation of 10B in normal tissue can be a useful way to increase the T/N ratio. Decreasing the relative accumulation of 10B in normal tissue may reduce the radiation-induced damage to the healthy tissue. To reduce the accumulation of 10B in the normal brain relative to tumor tissue, Watanabe et al. examined the effects of oral preloading with various analogues of BPA in a xenograft tumor model and found that high-dose L-phenylalanine reduced the accumulation of 10B in the normal brain. The L−phenylalanine group showed a 19.2% reduction in maximum irradiation dose to the normal brain, offering a simple strategy to enhance the therapeutic efficacy of conventional 10B agents for BNCT of brain tumors (27).
Regarding other members of LAT, Wongthai et al. reported that LAT2 can transport BPA whereas LAT3 and LAT4 cannot (9). LAT2 is mainly distributed in normal tissues, and expressed sequence tag analysis has revealed a negative correlation between LAT2 expression and tumor tissues (28, 29). Accordingly, LAT2 is presumed to modulate BPA pharmacokinetics by reducing the tumor-to-normal tissue accumulation ratio of BPA.
2.2 Amino acid transporter B0,+
Amino acid transporter B0,+ (ATB0,+), also known as solute carrier family 6 member 14 (SLC6A14), is upregulated in solid tumors and correlated with several pathological states (30). ATB0,+ transports most amino acids (excluding glutamate and aspartate) via Na+/Cl gradients (31). Its broad substrate selectivity makes ATB0,+ a promising candidate for targeted drug delivery applications (30, 32). A study screened aromatic amino acid transporters and found ATB0,+ was a transporter of BPA depending on its concentration. At 100 μM, BPA uptake was Na+-independent and positively linked to LAT1 protein levels, indicating LAT1 as a major transporter for BPA. At 1000 μM BPA, ATB0,+ measurably transports BPA (9). At a clinical dose of 250 mg BPA/kg, blood BPA reaches 2000 μM, suggesting that ATB0,+ may contribute significantly to BPA uptake in tumors expressing this transporter (33).
2.3 Hypoxia-inducible factor
Tumors develop hypoxic regions characterized by inadequate oxygen supply due to abnormal microvasculature via angiogenesis and rapid growth leading to insufficient blood flow (34). In hypoxic tumor cells, hypoxia-inducible factor 1 (HIF-1) accumulates as an adaptive response to hypoxia. HIF-1 is a heterodimeric transcription factor that comprises HIF-1α and HIF-1β subunits and mediates the cellular hypoxic response (35, 36). HIF-1α is pivotal in regulating tumor progression, metabolic reprogramming, excessive angiogenesis, invasive capacity and therapeutic resistance (37). Accumulated evidence has indicated that hypoxia suppresses cellular uptake of BPA and BSH across multiple cell lines (38, 39). Harada et al. further found that the hypoxia mimetic DFO markedly reduced BPA transportation, whereas the HIF-1α inhibitor YC-1 could sensitize hypoxic tumor cells to BNCT treatment (40).
Sanada et al. demonstrated that hypoxia elevated post−irradiation survival of BPA-treated squamous cell carcinoma (SCC VII) cells by down-regulating the LAT1 expression and inhibiting BPA uptake (41). In HIF−1α−deficient cells, hypoxia failed to alter the BNCT sensitivity of BPA-treated cells and LAT1 expression was not significantly reduced in hypoxia-induced cells, suggesting that hypoxia affect the sensitivity in a HIF-1α dependent manner. Furthermore, hypoxia and HIF−1α did not influence the sensitivity of BSH−treated SCC VII cells. While Masunaga et al. reported that hypoxia reduce the sensitivity of both BPA and BSH treated squamous cell carcinoma cell line to BNCT, more cells survived in the BSH group than BPA group (38). The reason may be hypoxia and HIF-1 α affect the post-irradiation survival of BPA-treated cells more than BSH-treated cells. The underlying mechanism between HIF and LAT1 has been investigated. HIF−2α upregulates LAT1 transcription by binding to its proximal promoter region, whereas direct binding of HIF−1α to the LAT1 promoter has not been detected, implying that HIF−1α does not activate LAT1 gene transcription (42). The mechanism between HIF-1α and LAT1, and the value of HIF as potential target or marker of BNCT need further investigation (41).
2.4 P53
The p53 gene is broadly involved in both cancer progression and suppression, as well as in cellular responses to various stressors, including hypoxia, viral infection, metabolic stress, endoplasmic reticulum stress and oxidative stress (43). p53 maintains genomic stability at G1 and G2/M cell cycle checkpoints, and regulates DNA repair and apoptosis (44, 45). p53 mutations occur frequently in most human solid tumors and regulate cellular responses to DNA−damaging therapies (e.g., radiation, chemotherapy) and hypoxic stress (45). Hypoxic stress triggers the accumulation of p53 protein and induces p53-dependent apoptosis, yet it fails to elicit p53-mediated cell cycle arrest (46). Loss of p53 function leads to resistance against DNA-damaging agents, including ionizing radiation and hypoxic stress (47). Therefore, the genetic and functional status of the p53 gene constitutes a critical factor in cancer therapeutic strategies. Wild-type p53 tumors show higher ¹0B accumulation than p53-mutant tumors following BPA and BSH administration (48, 49). This phenomenon is partially attributed to the higher cellular density in SAS/neo tumors compared with p53-mutant counterparts (50). Another possible explanation is that tumor cells carrying dominant-negative p53 evade genomic stability checkpoint regulation. These cells proliferate continuously without cell cycle arrest or apoptosis, thereby altering cell growth status and ultimately influencing intracellular 10B accumulation efficiency (43, 51).The intratumor distribution of BPA depended on the oxygen pressure in tumor tissues of p53 wild type tumors, whereas BPA uptake capacity was not dependent on oxygen pressure for p53-mutated tumor cells (38).
2.5 Mammalian target of rapamycin complex 1
The mechanistic target of rapamycin (mTOR) is a central regulator of cellular homeostasis and includes two complexes: mTORC1 and mTORC2. mTOR controls translation of mRNAs involved in cell cycle and proliferation (52). mTOR is a key anticancer drug target, and its inhibitors are used in combination chemotherapy (53). Rapamycin specifically inhibits mTORC1, suppresses angiogenesis, and induces autophagy (54, 55). Tatebe et al. found that rapamycin reduced the efficacy of BPA−BNCT, likely because mTOR inhibition slows tumor cell proliferation, thereby decreasing ¹0B loading and antitumor effects (56). Previous studies demonstrated a positive correction between LAT1 and mTORC1. Milkereit et al. reported that LAT1 in lysosomal membranes induce mTOR activation on the lysosomal membranes for the activation of mTORC1 recruited onto the lysosomal membrane (14). LAT1 facilitates mTORC1 activity by supplying cancer cells with leucine that triggers mTORC1 to regulate cancer cell growth (57–59). Inhibitors of LAT1 and knockdown of LAT1 reduced the phosphorylation of p70 S6 kinase and 4E-BP1, downstream targets of mTORC1 (60–62). These results suggest that mTORC1 may be a synergistic biomarker to predict the effect of BPA-BNCT. The underlying role of mTORC1 on BPA transportation, intracellular location and retention need further study.
3 Other potential targets
3.1 Erb-B2 receptor tyrosine kinase family
The Erb-B2 receptor tyrosine kinase family, including epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor 2 (HER2), is overexpressed in various human cancers and has emerged as promising targets for BNCT.
3.1.1 Epidermal growth factor receptor
EGFR, overexpressed in various human cancers and involved in angiogenesis, metastasis, and apoptosis, is considered a potential target for cancer therapy (63). EGFR is also a macropinocytosis-inducing receptor that can induce macropinocytotic cellular uptake pathways which are accompanied by clathrin-independent and actin-dependent plasma membrane ruffling and engulfment of extracellular fluids (64–67). Z33 peptide has been applied for drug delivery systems as it can specifically and effectively bind to an Fc (fragment crystallizable region) of human IgG1. A dodecaborate conjugated to the Z33 peptide (Z33‐DB) could bind to the Fc of the objective antibody, EGFR activation by EGF induced macropinocytosis, and lead to efficient cellular uptake of Z33-DB (7). To avoid EGFR conjugation problems, Rondina et al. established a computational workflow based on cetuximab (68). Cetuximab, a chimeric monoclonal antibody (mAb) for certain cancers (e.g., metastatic colorectal, lung, head/neck), has high T/B, high T/N, and low intrinsic toxicity (68). In another study, the 10B-rich EGFR-inhibitor, hybrid 1, was developed as a potential drug for BNCT. Hybrid 1 concentrated 10B in glioma cells more selectively than BPA and demonstrated a 1.2- and 1.5-times enhancement of antitumor effect of BNCT compared to BPA in glioma F98 and U87 cells, respectively (69).
Couto et al. reported that sunitinib decorated with a 10B cluster against glioma cancer cells was found to be 4 times more cytotoxic than sunitinib and 1.7 times more effective than BPA fructose complex when the cells were irradiated with neutrons (70). This conjugate selectively increased ¹0B accumulation in EGFR−overexpressing F98 glioma cells compared to normal mouse astrocytes. Hybrid erlotinib−10B clusters were more effective against EGFR−overexpressing cancer cells than unmodified erlotinib, with metallacarborane derivatives showing the highest activity (71). Kaniowski et al. developed antisense oligonucleotide−10B cluster composites as functional nanoparticles that downregulate EGFR expression and can enter EGFR−overexpressing cancer cells without a transfection agent (72). They also designed 10B−cluster−decorated antisense oligonucleotides (B−ASOs) targeting EGFR mRNA and confirmed that EGFR expression levels correlated with B−ASO uptake, offering higher tumor selectivity. Antisense−mediated EGFR reduction decreased 10B accumulation, indicating that B−ASO uptake is EGFR−dependent and represents a novel strategy for delivering therapeutic nucleic acids (and potentially other drugs) conjugated to 10B clusters (73).
3.1.2 Human epidermal growth factor receptor 2
Human epidermal growth factor receptor 2 (HER2) overexpression is associated with aggressive tumor behavior and poor prognosis, making it an attractive target for targeted cancer therapy and BNCT (74). Proshkina et al. used HER2-specific designed ankyrin repeat protein (DARPin)_9–29 as a vector molecule on the outer surface of liposomes. These DARPin-modified liposomes were loaded with the 4-L-¹0BPA-D−fructose complex as the 10B delivery agent, which was shown to bind to HER2-overexpressing cells and undergo effective internalization into the cytoplasm in vitro (75). Feiner et al. proposed that tetrazine moieties, attached to the surface of small 10B-rich nanoparticles (NPs), could undergo the click-reaction with trans-cyclooctene mAb (TCO-mAb) (76). This would give a base for the pre-targeting approach that would lead to the desired selectivity of 10B rich NPs and provide a valuable alternative to current nanomaterial-based BNCT agents. They used tetrazine‐functionalized 10B‐rich carbon dots as a potential BNCT agent. Trastuzumab is a well-known FDA-approved humanized mAb with high target specificity and binding affinity to HER2. Pre-targeting with trans-cyclooctene-modified trastuzumab remarkably boosted intracellular 10B accumulation. Meanwhile, 10B carbon dots were found to undergo rapid in vivo clearance and display poor tumor enrichment following intravenous injection in a HER2-positive murine tumor model. This pretargeting strategy can improve the selectivity of tumor-targeted 10B delivery, thereby potentially attenuating off-tumor toxicity to the liver, lung and spleen, while elevating 10B accumulation within tumor tissues. Using of a pre-targeting strategy improved the accumulation of 10B-rich carbon dots in the tumor, with potential to improve the efficacy of BNCT (12).
3.2 Annexin A1
Annexin family proteins localize on the surface of endothelial caveolae and can be internalized via endocytosis (77, 78). Annexin A1 (Anxa1) plays important role in tumor growth and is a promising target for tumor vasculature (79–81). Specific Anxa1-binding carbohydrate mimetic peptide IFLLWQR (IF7) peptide effectively delivers anticancer drugs to tumors (82). For IF7-10B drug, membrane-expressed Anxa1 mediates the active uptake of IF7-10B into both tumor vascular endothelial cells and tumor cells, thereby elevating intracellular 10B accumulation. Neutron irradiation efficiently eliminated IF7-10B-treated tumor cells, while causing no obvious damage to tumor blood vessels. Neutron irradiation effectively killed cancer cells treated with IF7−10B but did not affect tumor vasculature. Moreover, BNCT upregulates Anxa1 in tumor tissue via inflammation or induced immunogenicity, leading to greater 10B accumulation after a second IF7−10B injection. IF7−10B−mediated BNCT may destroy Anxa1−positive tumor vasculature, increasing therapeutic potential. Therefore, multiple ultralow doses may be required to maximize efficacy (83).
3.3 Cluster of differentiation 44
Cluster of differentiation 44 (CD44), a stem cell-associated marker in various cancer types, is essential for the cellular uptake of polyR-conjugated molecules. Additionally, BSH conjugated with poly-arginine peptides has been developed (84). PolyR enhances BSH’s cell permeability and tumor selectivity, with CD44 as the main surface target and translational machinery proteins as intracellular targets. After delivery, BSH−polyR interacted with translational components, including initiation factors, termination factors, and poly(A)−binding protein. Furthermore, BSH-polyR successfully induced BNCT-dependent cell death specifically in CD44 high-expressed cells (84). Yamana et al. used sodium hyaluronate (HA), a major cellular matrix component and tumor-targeting ligand due to its high affinity for CD44 overexpressed on tumor cell surfaces, as a nanocarrier building block to prepare a water-soluble hyaluronic acid/pyrene-substituted o-carborane (HA/CBP-H) complex. After thermal neutron irradiation, this complex produced excellent cytotoxicity, equal to or greater than that of the clinically-used BPA-fructose (85).
3.4 Translocator protein
The translocator protein (TSPO) is an 18 kDa protein with five-transmembrane domain of 169 amino acid residues located in the outer mitochondrial membrane of cells in a variety of tissues (86, 87). Crossley et al. reported a new, 10B-rich carboranylpyrazolopyrimidines, 1,2-closo-carboranylpyrazolopyrimidine, which possesses a unique two-site binding profile at the TSPO and both closo- and nido-carborane derivatives are able to accumulate in T98G human glioma cells at about 13 times higher than BPA and 14 times higher than BSH (88). A 10B compound targeting TSPO, closo-dodecaborate [(B12H11)2-] anion-containing translocator protein ligand, significantly improved the 10B accumulation through convection-enhanced delivery administration compared to BPA intravenous administration in the F98 rat glioma bearing brain tumor model. BNCT using DPA-BSTPG demonstrated more efficacy over the untreated group. Combined BNCT with BPA and DPA−BSTPG achieved markedly prolonged survival compared with BPA monotherapy (89, 90).
3.5 Integrin αvβ3
Serum albumin is an abundant protein that has an extraordinary ligand-binding capacity to carry various endogenous and exogenous compounds in plasma (91). The serum albumin accumulates in tumor due to the combination of leaky and abnormal blood vessels with the absence or defect of the lymphatic drainage system known as the enhanced permeability and retention effect (92). Cyclic RGD (cRGD) peptide-conjugated boronated albumin was developed to direct toward integrin αvβ3, which overexpresses on many cancer cells. A stepwise conjugation of c[RGDfK(Mal)] and maleimide-conjugated closo-dodecaborate (MID) to bovine serum albumin (BSA) afforded cRGD-MID-BSA, which was noncytotoxic to cancer cells. In both in vitro and in vivo models, cRGD-MID-BSA exhibited superior selective accumulation, prolonged retention, and stronger antitumor activity against integrin αvβ3-overexpressing U87MG cells relative to BPA (93). Previously, MID albumin conjugate (MID-AC) was found as an effective 10B carrier. Recently, Kawabata et al. developed cRGD-functionalized MID-AC which is a αvβ3-targeted long-retention-type 10B carrier and highly selective against gliomas (93, 94).
3.6 Folate receptor α
Folate receptors are cell membrane proteins responsible for binding folate, a B−vitamin essential for DNA synthesis. Folate receptor α (FRα) is important in embryogenesis but minimally expressed after birth; however, it is overexpressed in many solid tumors while low in healthy tissues (95). Accordingly, FRα is significantly upregulated in malignant gliomas than in normal tissues (96, 97). Water−soluble pteroyl−closo−dodecaborate conjugates (PBCs) were developed for BNCT, showing low cytotoxicity and selective accumulation in FRα−positive cells (e.g., U87MG), but their in vivo tumor accumulation was limited by poor blood retention (98, 99). To overcome this limitation, the research group synthesized PBC−IP, a pteroyl−closo−dodecaborate derivative conjugated with 4−(p−iodophenyl)butyric acid (100). This molecule integrates three functional domains: an FRα targeting motif, a 10B carrier containing twelve 10B atoms, and an albumin−binding fragment. Conjugation with an albumin ligand enables PBC derivatives to bind endogenous albumin, prolonging circulatory retention and facilitating tumor enrichment (101, 102). PBC−IP was selectively taken up by C6, F98, and U87MG glioma cells, achieving higher 10B accumulation than BPA both in vitro and in vivo. When administered via convection−enhanced delivery (CED) in an F98 orthotopic glioma model, PBC−IP produced T/N and T/B 10B ratios of 37.8 and 94.6, respectively, at 3 h post−CED. Post−BNCT survival at 180 days was 50% with PBC−IP alone and 70% with combined BPA and PBC−IP, with no residual tumors. Unlike BPA, which requires fructose/sorbitol for solubility, PBC−IP is water−soluble without additives. CED allows approximately 100−fold dose reduction of PBC−IP compared to BPA, thereby minimizing adverse effects such as BPA−induced crystalluria (100). It should be noted that CED is technically complex and invasive, with unproven clinical benefit, and remains largely an investigational technique (103).
3.7 Low-density lipoprotein receptor
Low-density lipoprotein receptor (LDLR) is a transmembrane glycoprotein that plays a central role in cholesterol homeostasis by mediating the cellular uptake of low-density lipoproteins via receptor-mediated endocytosis. LDLR is overexpressed in various cancers, including gliomas, hepatocellular carcinoma, prostate cancer, lung cancer, breast cancer, and colorectal cancer, due to the increased demand for cholesterol and lipids required for rapid tumor cell proliferation (104). Rudawska et al. developed functionalized 10B carbide (B4C) nanoparticles as active 10B delivery agents for BNCT. These nanoparticles were functionalized with anti-LDLR antibodies to achieve active targeting of LDLR-overexpressing cancer cells, and they were selectively internalized via LDLR-mediated endocytosis. In vitro studies demonstrated that the B4C anti-LDLR nanoparticles achieved a 10B concentration of 9.58 ± 2.6 mg/L per million cells in SCC-25 cells (105).
3.8 Glucose transporter 1
Glucose transporter 1 (GLUT1) is a facilitative glucose transporter that mediates the uptake of glucose across the plasma membrane. GLUT1 is overexpressed in a wide range of human cancers due to the increased glucose demand associated with the Warburg effect. This distinct overexpression profile of GLUT1 in cancer cells, compared to normal tissues, renders it a highly attractive target for tumor-selective drug delivery strategies (106).
Matović et al. conducted a series of systematic studies to develop glucose-based 10B carriers targeting GLUT1 (107–110). Initially, a library of ortho-carboranylmethyl-bearing glucoconjugates was synthesized, and the substrate specificity of these compounds for GLUT1 was evaluated. In cellular uptake studies using CAL 27 cells (human tongue squamous cell carcinoma), all glucoconjugates delivered significantly higher 10B content than BPA and BSH (107). Next, the entire positional isomer library of ortho-carboranylmethyl-bearing glucoconjugates was synthesized and assessed through a comprehensive in vitro evaluation. The glucoconjugates were found to compete with the natural substrate for GLUT1, to deliver significantly higher 10B content (approximately 1.5- to 3-fold higher) to CAL 27 cells than BPA and BSH, and not to enter the common metabolic routes of D-glucose (108). Subsequently, short and accessible synthetic methods were developed for the construction of a set of 6-deoxy-6-thio-carboranyl d-glucoconjugates, and molecular recognition studies revealed that the atom involved in cluster conjugation has a marked effect on GLUT1 affinity, with species connected to a carbon atom exhibiting significantly higher affinity than those connected to a 10B atom (109). Since hexose transporters, including GLUT1, also recognize monosaccharides other than d-Glc, the corresponding carboranylmethyl derivatives of d-Man, d-All, and d-Gal (epimers of d-Glc) were synthesized. Using docking studies with a GLUT1 model derived from d-xylose-proton symporter crystal structures (PDB ID 4QIQ and 6N3I) and cis-inhibition assays in CAL 27 cells, glycoconjugates 1–3 displayed stronger binding affinity toward GLUT1 than d-Glc and exhibited superior glucose transporter targeting capabilities (IC50 values all < 1 mM) compared to the natural substrate (IC50 > 1 mM), although none matched the affinity of the hit compound 6-oCb-Glc (IC50 = 43.96 μM). The 10B delivery capacity of glycoconjugates 1–3 was found to be superior to that of clinically used agents BPA and BSH, with modification at position 6 emerging as a promising strategy regardless of stereochemical configuration, and all three glycoconjugates exhibited comparable 10B delivery efficiency to the previous hit compound 6-oCb-Glc (110). A recent study extended the in vitro investigations to three additional patient-derived HNSCC cell lines (UT-SCC-14, UT-SCC-28, and UT-SCC-42B) and evaluated in vivo pharmacokinetics in HNSCC tumor xenografts. Positron emission tomography - computed tomography (PET-CT) imaging using [ (18)F]fluoro-2-deoxy-d-glucose ([¹8F]FDG) radiotracer confirmed increased glucose uptake in CAL 27 and UT-SCC-14 tumors in vivo, supported by GLUT1 expression observed in tumor section immunohistochemistry. 6-O-(o-carboranylmethyl)-d-glucopyranose (B-Glc) demonstrated superior uptake and favorable kinetic parameters compared to BPA and BSH across all tested cell lines, with peak 10B accumulation at 15–30 min post-injection (75 mg/kg dose) achieving the required tumor 10B concentration (>20 ppm) for effective BNCT, comparable to the clinical BPA-fructose complex (400 mg/kg dose) at 60 min (111). However, neutron irradiation was not conducted in these studies, leaving BNCT efficacy unvalidated.
3.9 Alanine-serine-cysteine transporter 2
Alanine-serine-cysteine transporter 2 (ASCT2) is a sodium-dependent neutral amino acid transporter that mediates the cellular uptake of glutamine, alanine, serine, and cysteine. ASCT2 is overexpressed in a wide range of human cancers due to the increased demand for glutamine to support tumor cell proliferation (112). This characteristic makes ASCT2 an attractive target for tumor-selective 10B delivery, particularly for tumors that are refractory to L-BPA. Miura et al. developed a water-soluble, ASCT2-targeted small-molecule ¹0B carrier GluB-2. In vitro assessments across multiple cancer cell lines—especially those characterized by low LAT1 and high ASCT2 expression—revealed that GluB-2 possesses high solubility, low cytotoxicity, and selective uptake mediated by ASCT2. In vivo experiments demonstrated that GluB-2, administered either intravenously or intraperitoneally, achieved tumor 10B concentrations exceeding the therapeutic benchmark of >20 μg ¹0B/g tissue. Following thermal neutron irradiation, GluB-2 led to marked tumor suppression in both the CT26 allograft and the BPA-refractory U87MG xenograft models, with no observed systemic toxicity (113).
4 Conclusion and perspectives
BNCT represents a promising treatment modality, with data confirming the safety and efficacy of treatment for a number of tumor types. Nevertheless, clinically used BPA and BSH cannot fulfill all criteria of the treatment. As a result, significant efforts are being made to develop novel 10B carriers as well as optimizing the administration protocol of the currently used drugs. Unfortunately, none of the novel 10B carriers have yet been approved for clinical application. Similar to the development of low selectivity chemotherapy to current high selectivity immune and targeted therapy, creating a selective and effective 10B carrier is a very valuable development. Therefore, finding potential targets influencing 10B transport may be beneficial for developing innovative 10B carriers for BNCT. In addition, clarifying the mechanism by which these potential targets affect the import and export of 10B will help improve 10B uptake and retention. In the review, we discussed the latest research results of potential targets, their mechanisms affecting 10B transport, and related agents. It is hoped that through the efforts of further study, novel 10B carriers will expand BNCT indications, and unlock the potential of BNCT being highly effective and less toxic, benefiting cancer patients.
Statements
Author contributions
YW: Writing – original draft, Writing – review & editing. WZ: Writing – review & editing. QC: Writing – review & editing. HA: Writing – review & editing. YL: Writing – review & editing, Visualization, Conceptualization, Supervision. KR: Supervision, Conceptualization, Writing – review & editing.
Funding
The author(s) declared that financial support was received for this work and/or its publication. The work was supported by National Natural Science Foundation of China (82260612), Fundamental Research Funds for the Central Universities of Lanzhou University (lzujbky-2022-sp08), and Cuiying Scientific and Technological Innovation Program (CY2025-MS-B07) and the Key Incubation Project Funds (2025-22-zdfy-013) of the Second Hospital & Clinical Medical School, Lanzhou University.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Generative AI statement
The author(s) declared that generative AI was not used in the creation of this manuscript.
Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
References
1
KawabataSSuzukiMHiroseKTanakaHKatoTGotoHet al. Accelerator-based BNCT for patients with recurrent glioblastoma: a multicenter phase II study. Neuro-Oncol Adv. (2021) 3:vdab067. doi: 10.1093/noajnl/vdab067. PMID:
2
FukudaH. Boron neutron capture therapy (BNCT) for cutaneous Malignant melanoma using (10)B-p-boronophenylalanine (BPA) with special reference to the radiobiological basis and clinical results. Cells. (2021) 10:2881. doi: 10.3390/cells10112881. PMID:
3
HiroseKKonnoAHiratsukaJYoshimotoSKatoTOnoKet al. Boron neutron capture therapy using cyclotron-based epithermal neutron source and borofalan ((10)B) for recurrent or locally advanced head and neck cancer (JHN002): an open-label phase II trial. Radiother Oncol. (2021) 155:182–7. doi: 10.1016/j.radonc.2020.11.001. PMID:
4
WangSZhangZMiaoLLiY. Boron neutron capture therapy: current status and challenges. Front Oncol. (2022) 12:788770. doi: 10.3389/fonc.2022.788770. PMID:
5
Maliszewska-OlejniczakKKaniowskiDAraszkiewiczMTymińskaKKorgulA. Molecular mechanisms of specific cellular DNA damage response and repair induced by the mixed radiation field during boron neutron capture therapy. Front Oncol. (2021) 11:676575. doi: 10.3389/fonc.2021.676575. PMID:
6
BarthRFMiPYangW. Boron delivery agents for neutron capture therapy of cancer. Cancer Commun (Lond). (2018) 38:35. doi: 10.1186/s40880-018-0299-7. PMID:
7
NakaseIAokiASakaiYHiraseSIshimuraMTakatani-NakaseTet al. Antibody-based receptor targeting using an Fc-binding peptide-dodecaborate conjugate and macropinocytosis induction for boron neutron capture therapy. ACS Omega. (2020) 5:22731–8. doi: 10.1021/acsomega.0c01377. PMID:
8
JalilianARShahiASwainsonIPNakamuraHVenkateshMOssoJA. Potential theranostic boron neutron capture therapy agents as multimodal radiopharmaceuticals. Cancer Biother Radiopharm. (2022) 37:342–54. doi: 10.1089/cbr.2021.0276. PMID:
9
WongthaiPHagiwaraKMiyoshiYWiriyasermkulPWeiLOhgakiRet al. Boronophenylalanine, a boron delivery agent for boron neutron capture therapy, is transported by ATB0,+, LAT1 and LAT2. Cancer Sci. (2015) 106:279–86. doi: 10.1111/cas.12602. PMID:
10
YokoyamaKMiyatakeSKajimotoYKawabataSDoiAYoshidaTet al. Analysis of boron distribution in vivo for boron neutron capture therapy using two different boron compounds by secondary ion mass spectrometry. Radiat Res. (2007) 167:102–9. doi: 10.1667/rr0501.1. PMID:
11
CapuaniSGiliTBozzaliMRussoSPorcariPCamettiCet al. Boronophenylalanine uptake in C6 glioma model is dramatically increased by L-DOPA preloading. Appl Radiat Isot. (2009) 67:S34–6. doi: 10.1016/j.apradiso.2009.03.017. PMID:
12
CoghiPLiJHosmaneNSZhuY. Next generation of boron neutron capture therapy (BNCT) agents for cancer treatment. Med Res Rev. (2023) 43:1809–30. doi: 10.1002/med.21964. PMID:
13
WatanabeTSanadaYHattoriYSuzukiM. Correlation between the expression of LAT1 in cancer cells and the potential efficacy of boron neutron capture therapy. J Radiat Res. (2023) 64:91–8. doi: 10.1093/jrr/rrac077. PMID:
14
MilkereitRPersaudAVanoaicaLGuetgAVerreyFRotinD. LAPTM4b recruits the LAT1-4F2hc Leu transporter to lysosomes and promotes mTORC1 activation. Nat Commun. (2015) 6:7250. doi: 10.1038/ncomms8250. PMID:
15
ScaliseMGalluccioMConsoleLPochiniLIndiveriC. The human SLC7A5 (LAT1): the intriguing histidine/large neutral amino acid transporter and its relevance to human health. Front Chem. (2018) 6:243. doi: 10.3389/fchem.2018.00243. PMID:
16
ScaliseMPochiniLGiangregorioNTonazziAIndiveriC. Proteoliposomes as tool for assaying membrane transporter functions and interactions with xenobiotics. Pharmaceutics. (2013) 5:472–97. doi: 10.3390/pharmaceutics5030472. PMID:
17
CappoliNJenkinsonMDDello RussoCDickensD. LAT1, a novel pharmacological target for the treatment of glioblastoma. Biochem Pharmacol. (2022) 201:115103. doi: 10.1016/j.bcp.2022.115103. PMID:
18
OnishiYHiraiwaMKamadaHIezakiTYamadaTKanedaKet al. Hypoxia affects Slc7a5 expression through HIF-2α in differentiated neuronal cells. FEBS Open Bio. (2019) 9:241–7. doi: 10.1002/2211-5463.12559. PMID:
19
BarthelemyCAndréB. Ubiquitylation and endocytosis of the human LAT1/SLC7A5 amino acid transporter. Sci Rep. (2019) 9:16760. doi: 10.1038/s41598-019-53065-w. PMID:
20
OhnishiKMisawaMSikanoNNakaiKSuzukiM. Enhancement of cancer cell-killing effects of boron neutron capture therapy by manipulating the expression of L-type amino acid transporter 1. Radiat Res. (2021) 196:17–22. doi: 10.1667/rade-20-00214.1. PMID:
21
TaniTFujitaTMisawaMTojoNShikanoNSuzukiMet al. Advanced boron neutron capture therapy targeting cancer stem cells by selective induction of LAT1 overexpression. Radiat Res. (2023) 200:21–31. doi: 10.1667/rade-22-00195.1. PMID:
22
YanRZhaoXLeiJZhouQ. Structure of the human LAT1-4F2hc heteromeric amino acid transporter complex. Nature. (2019) 568:127–30. doi: 10.1038/s41586-019-1011-z. PMID:
23
PapaspyrouMFeinendegenLEMuller-GartnerHW. Preloading with L-tyrosine increases the uptake of boronophenylalanine in mouse melanoma cells. Cancer Res. (1994) 54:6311–4.
24
WittigASauerweinWACoderreJACoderreJA. Mechanisms of transport of p-borono-phenylalanine through the cell membrane in vitro. Radiat Res. (2000) 153:173–80. doi: 10.1667/0033-7587(2000)153[0173:MOTOPB]2.0.CO;2
25
CapuaniSGiliTBozzaliMRussoSPorcariPCamettiCet al. L-DOPA preloading increases the uptake of borophenylalanine in C6 glioma rat model: a new strategy to improve BNCT efficacy. Int J Radiat Oncol Biol Phys. (2008) 72:562–7. doi: 10.1016/j.ijrobp.2008.06.1493. PMID:
26
NomotoTInoueYYaoYSuzukiMKanamoriKTakemotoHet al. Poly(vinyl alcohol) boosting therapeutic potential of p-boronophenylalanine in neutron capture therapy by modulating metabolism. Sci Adv. (2020) 6:eaaz1722. doi: 10.1126/sciadv.aaz1722. PMID:
27
WatanabeTTanakaHFukutaniSSuzukiMHiraokaMOnoK. L-Phenylalanine preloading reduces the (10)B(n, α)(7)Li dose to the normal brain by inhibiting the uptake of boronophenylalanine in boron neutron capture therapy for brain tumours. Cancer Lett. (2016) 370:27–32. doi: 10.1016/j.canlet.2015.10.004. PMID:
28
FuchsBCBodeBP. Amino acid transporters ASCT2 and LAT1 in cancer: partners in crime? Semin Cancer Biol. (2005) 15:254–66. doi: 10.1016/j.semcancer.2005.04.005. PMID:
29
NakadaNMikamiTHanaKIchinoeMYanagisawaNYoshidaTet al. Unique and selective expression of L-amino acid transporter 1 in human tissue as well as being an aspect of oncofetal protein. Histol Histopathol. (2014) 29:217–27.
30
NałęczKA. Amino acid transporter SLC6A14 (ATB(0,+)) - a target in combined anti-cancer therapy. Front Cell Dev Biol. (2020) 8:594464. doi: 10.3389/fcell.2020.594464. PMID:
31
SloanJLMagerS. Cloning and functional expression of a human Na(+) and Cl(-)-dependent neutral and cationic amino acid transporter B(0+). J Biol Chem. (1999) 274:23740–5. doi: 10.1074/jbc.274.34.23740. PMID:
32
GanapathyVThangarajuMPrasadPD. Nutrient transporters in cancer: relevance to Warburg hypothesis and beyond. Pharmacol Ther. (2009) 121:29–40. doi: 10.1016/j.pharmthera.2008.09.005. PMID:
33
CoderreJAElowitzEHChadhaMBerglandRCapalaJJoelDDet al. Boron neutron capture therapy for glioblastoma multiforme using p-boronophenylalanine and epithermal neutrons: trial design and early clinical results. J Neuro-Oncol. (1997) 33:141–52. doi: 10.1023/a:1005741919442. PMID:
34
SingletonDCMacannAWilsonWR. Therapeutic targeting of the hypoxic tumour microenvironment. Nat Rev Clin Oncol. (2021) 18:751–72. doi: 10.1038/s41571-021-00539-4. PMID:
35
WuQYouLNepovimovaEHegerZWuWKucaKet al. Hypoxia-inducible factors: master regulators of hypoxic tumor immune escape. J Hematol Oncol. (2022) 15:77. doi: 10.1186/s13045-022-01292-6. PMID:
36
HarrisAL. Hypoxia--a key regulatory factor in tumour growth. Nat Rev Cancer. (2002) 2:38–47. doi: 10.1038/nrc704. PMID:
37
DeFratesKGFrancoDHeber-KatzEMessersmithPB. Unlocking mammalian regeneration through hypoxia inducible factor one alpha signaling. Biomaterials. (2021) 269:120646. doi: 10.1016/j.biomaterials.2020.120646. PMID:
38
MasunagaSTatebeHNishimuraYTanoKSanadaYMoriwakiTet al. Effect of oxygen pressure during incubation with a (10)B-carrier on (10)B uptake capacity of cultured p53 wild-type and mutated tumor cells: dependency on p53 status of tumor cells and types of (10)B-carriers. Int J Radiat Biol. (2016) 92:187–94. doi: 10.3109/09553002.2016.1137104. PMID:
39
WadaYHiroseKHaradaTSatoMWatanabeTAnbaiAet al. Impact of oxygen status on 10B-BPA uptake into human glioblastoma cells, referring to significance in boron neutron capture therapy. J Radiat Res. (2018) 59:122–8. doi: 10.1093/jrr/rrx080. PMID:
40
HaradaTHiroseKWadaYSatoMIchiseKAokiMet al. YC-1 sensitizes the antitumor effects of boron neutron capture therapy in hypoxic tumor cells. J Radiat Res. (2020) 61:524–34. doi: 10.1093/jrr/rraa024. PMID:
41
SanadaYTakataTTanakaHSakuraiYWatanabeTSuzukiMet al. HIF-1α affects sensitivity of murine squamous cell carcinoma to boron neutron capture therapy with BPA. Int J Radiat Biol. (2021) 97:1441–9. doi: 10.1080/09553002.2021.1956004. PMID:
42
ElorzaASoro-ArnáizIMeléndez-RodríguezFRodríguez-VaelloVMarsboomGde CárcerGet al. HIF2α acts as an mTORC1 activator through the amino acid carrier SLC7A5. Mol Cell. (2012) 48:681–92. doi: 10.1016/j.molcel.2012.09.017. PMID:
43
LaneDLevineA. p53 research: the past thirty years and the next thirty years. Cold Spring Harb Perspect Biol. (2010) 2:a000893. doi: 10.1101/cshperspect.a000893. PMID:
44
HassinOOrenM. Drugging p53 in cancer: one protein, many targets. Nat Rev Drug Discov. (2023) 22:127–44. doi: 10.1038/s41573-022-00571-8. PMID:
45
FeiPEl-DeiryWS. P53 and radiation responses. Oncogene. (2003) 22:5774–83. doi: 10.1038/sj.onc.1206677. PMID:
46
SigalARotterV. Oncogenic mutations of the p53 tumor suppressor: the demons of the guardian of the genome. Cancer Res. (2000) 60:6788–93.
47
HammondEMDorieMJGiacciaAJ. ATR/ATM targets are phosphorylated by ATR in response to hypoxia and ATM in response to reoxygenation. J Biol Chem. (2003) 278:12207–13. doi: 10.1074/jbc.M212360200. PMID:
48
MasunagaSOnoKTakahashiASakuraiYOhnishiKKobayashiTet al. Impact of the p53 status of the tumor cells on the effect of reactor neutron beam irradiation, with emphasis on the response of intratumor quiescent cells. Jpn J Cancer Res. (2002) 93:1366–77. doi: 10.1111/j.1349-7006.2002.tb01246.x. PMID:
49
MasunagaSISanadaYTakataTTanakaHSakuraiYSuzukiMet al. The impact of TP53 status of tumor cells including the type and the concentration of administered 10B delivery agents on compound biological effectiveness in boron neutron capture therapy. J Radiat Res. (2023) 64:399–411. doi: 10.1093/jrr/rrad001. PMID:
50
MasunagaSOnoKTakahashiAOhnishiTKinashiYTakagakiM. Radiobiological characteristics of solid tumours depending on the p53 status of the tumour cells, with emphasis on the response of intratumour quiescent cells. Eur J Cancer. (2002) 38:718–27. doi: 10.1016/s0959-8049(01)00430-0. PMID:
51
LevineAJOrenM. The first 30 years of p53: growing ever more complex. Nat Rev Cancer. (2009) 9:749–58. doi: 10.1038/nrc2723. PMID:
52
Marques-RamosACervantesR. Expression of mTOR in normal and pathological conditions. Mol Cancer. (2023) 22:112. doi: 10.1186/s12943-023-01820-z. PMID:
53
WuHESuCCWangSCLiuPLChengWCYehHCet al. Anticancer effects of morusin in prostate cancer via inhibition of Akt/mTOR signaling pathway. Am J Chin Med. (2023) 51:1019–39. doi: 10.1142/s0192415x23500477. PMID:
54
Meric-BernstamFGonzalez-AnguloAM. Targeting the mTOR signaling network for cancer therapy. J Clin Oncol. (2009) 27:2278–87. doi: 10.1200/jco.2008.20.0766. PMID:
55
HuangJChenLWuJAiDZhangJQChenTGet al. Targeting the PI3K/AKT/mTOR signaling pathway in the treatment of human diseases: current status, trends, and solutions. J Med Chem. (2022) 65:16033–61. doi: 10.1021/acs.jmedchem.2c01070. PMID:
56
TatebeHMasunagaSINishimuraY. Effect of rapamycin on the radio-sensitivity of cultured tumor cells following boron neutron capture reaction. World J Oncol. (2020) 11:158–64. doi: 10.14740/wjon1296. PMID:
57
LiFYinYTanBKongXWuG. Leucine nutrition in animals and humans: mTOR signaling and beyond. Amino Acids. (2011) 41:1185–93. doi: 10.1007/s00726-011-0983-2. PMID:
58
ShigemitsuKTsujishitaYMiyakeHHidayatSTanakaNHaraKet al. Structural requirement of leucine for activation of p70 S6 kinase. FEBS Lett. (1999) 447:303–6. doi: 10.1016/s0014-5793(99)00304-x. PMID:
59
XuGKwonGMarshallCALinTALawrenceJCMcDanielML. Branched-chain amino acids are essential in the regulation of PHAS-I and p70 S6 kinase by pancreatic beta-cells. A possible role in protein translation and mitogenic signaling. J Biol Chem. (1998) 273:28178–84. doi: 10.1074/jbc.273.43.28178. PMID:
60
NagamoriSWiriyasermkulPOkudaSKojimaNHariYKiyonakaSet al. Structure-activity relations of leucine derivatives reveal critical moieties for cellular uptake and activation of mTORC1-mediated signaling. Amino Acids. (2016) 48:1045–58. doi: 10.1007/s00726-015-2158-z. PMID:
61
NicklinPBergmanPZhangBTriantafellowEWangHNyfelerBet al. Bidirectional transport of amino acids regulates mTOR and autophagy. Cell. (2009) 136:521–34. doi: 10.1016/j.cell.2008.11.044. PMID:
62
YamauchiKSakuraiHKimuraTWiriyasermkulPNagamoriSKanaiYet al. System L amino acid transporter inhibitor enhances anti-tumor activity of cisplatin in a head and neck squamous cell carcinoma cell line. Cancer Lett. (2009) 276:95–101. doi: 10.1016/j.canlet.2008.10.035. PMID:
63
LevantiniEMaroniGDel ReMTenenDG. EGFR signaling pathway as therapeutic target in human cancers. Semin Cancer Biol. (2022) 85:253–75. doi: 10.1016/j.semcancer.2022.04.002. PMID:
64
ArakiNHamasakiMEgamiYHataeT. Effect of 3-methyladenine on the fusion process of macropinosomes in EGF-stimulated A431 cells. Cell Struct Funct. (2006) 31:145–57. doi: 10.1247/csf.06029. PMID:
65
DiseRSFreyMRWhiteheadRHPolkDB. Epidermal growth factor stimulates Rac activation through Src and phosphatidylinositol 3-kinase to promote colonic epithelial cell migration. Am J Physiol Gastrointest Liver Physiol. (2008) 294:G276–85. doi: 10.1152/ajpgi.00340.2007. PMID:
66
TanakaGNakaseIFukudaYMasudaROishiSShimuraKet al. CXCR4 stimulates macropinocytosis: implications for cellular uptake of arginine-rich cell-penetrating peptides and HIV. Chem Biol. (2012) 19:1437–46. doi: 10.1016/j.chembiol.2012.09.011. PMID:
67
SwansonJA. Shaping cups into phagosomes and macropinosomes. Nat Rev Mol Cell Biol. (2008) 9:639–49. doi: 10.1038/nrm2447. PMID:
68
RondinaAFossaPOrroAMilanesiLDe PalmaAPericoDet al. A boron delivery antibody (BDA) with boronated specific residues: new perspectives in boron neutron capture therapy from an in silico investigation. Cells. (2021) 10:3225. doi: 10.3390/cells10113225. PMID:
69
AlamónCDávilaBGarcíaMFNievasSDagrosaMAThorpSet al. A potential boron neutron capture therapy agent selectively suppresses high-grade glioma: In vitro and in vivo exploration. Mol Pharm. (2023) 20:2702–13. doi: 10.1021/acs.molpharmaceut.3c00152. PMID:
70
CoutoMAlamónCNievasSPeronaMDagrosaMATeixidorFet al. Bimodal therapeutic agents against glioblastoma, one of the most lethal forms of cancer. Chemistry. (2020) 26:14335–40. doi: 10.1002/chem.202002963. PMID:
71
CoutoMAlamónCSánchezCDávilaBFernándezMLecotNet al. Carboranylanilinoquinazoline EGFR-inhibitors: toward ‘lead-to-candidate’ stage in the drug-development pipeline. Future Med Chem. (2019) 11:2273–85. doi: 10.4155/fmc-2019-0060. PMID:
72
KaniowskiDEbenryter-OlbińskaKKulikKSuwaraJCyprykWJakóbik-KolonAet al. Composites of nucleic acids and boron clusters (C(2)B(10)H(12)) as functional nanoparticles for downregulation of EGFR oncogene in cancer cells. Int J Mol Sci. (2021) 22:4863. doi: 10.3390/ijms22094863. PMID:
73
KaniowskiDSuwaraJEbenryter-OlbińskaKJakóbik-KolonANawrotB. EGFR-targeted cellular delivery of therapeutic nucleic acids mediated by boron clusters. Int J Mol Sci. (2022) 23:14793. doi: 10.3390/ijms232314793. PMID:
74
SeneviratneDAdvaniPTrifilettiDMChumsriSBeltranCJBushAFet al. Exploring the biological and physical basis of boron neutron capture therapy (BNCT) as a promising treatment frontier in breast cancer. Cancers (Basel). (2022) 14:3009. doi: 10.3390/cancers14123009. PMID:
75
ProshkinaGMShramovaEIMirkasymovABZavestovskayaINDeyevSM. Targeted nanoliposomes for the delivery of boronophenylalanine into HER2-positive cells. Acta Naturae. (2025) 17:88–93. doi: 10.32607/actanaturae.27722. PMID:
76
FeinerIVJPulagamKRUribeKBPassannanteRSimóCZamacolaKet al. Pre-targeting with ultra-small nanoparticles: boron carbon dots as drug candidates for boron neutron capture therapy. J Mater Chem B. (2021) 9:410–20. doi: 10.1039/d0tb01880e. PMID:
77
de Souza FerreiraLPda SilvaRAGilCDGeisowMJ. Annexin A1, A2, A5, and A6 involvement in human pathologies. Proteins. (2023) 91:1191–204. doi: 10.1002/prot.26512. PMID:
78
SchnitzerJELiuJOhP. Endothelial caveolae have the molecular transport machinery for vesicle budding, docking, and fusion including VAMP, NSF, SNAP, annexins, and GTPases. J Biol Chem. (1995) 270:14399–404. doi: 10.1074/jbc.270.24.14399. PMID:
79
YamanoiMYamanoiKFujiiCFukudaMNNakayamaJ. Annexin A1 expression is correlated with Malignant potential of renal cell carcinoma. Int J Urol. (2019) 26:284–90. doi: 10.1111/iju.13869. PMID:
80
ChenFXiaoYShaoKZhuBJiangM. Positron emission tomography imaging of a novel Anxa1-targeted peptide (18) F-Al-NODA-Bn-p-SCN-GGGRDN-IF7 in A431 cancer mouse models. J Labelled Comp Radiopharm. (2020) 63:494–501. doi: 10.1002/jlcr.3865. PMID:
81
YuDHLiuYRLuanXLiuHJGaoYGWuHet al. IF7-conjugated nanoparticles target annexin 1 of tumor vasculature against P-gp mediated multidrug resistance. Bioconjug Chem. (2015) 26:1702–12. doi: 10.1021/acs.bioconjchem.5b00283. PMID:
82
NonakaMSuzuki-AnekojiMNakayamaJMabashi-AsazumaHJarvisDLYehJCet al. Overcoming the blood-brain barrier by annexin A1-binding peptide to target brain tumours. Br J Cancer. (2020) 123:1633–43. doi: 10.1038/s41416-020-01066-2. PMID:
83
YoneyamaTHatakeyamaSSutoh YoneyamaMYoshiyaTUemuraTIshizuTet al. Tumor vasculature-targeted (10)B delivery by an annexin A1-binding peptide boosts effects of boron neutron capture therapy. BMC Cancer. (2021) 21:72. doi: 10.1186/s12885-020-07760-x. PMID:
84
FujimuraAYasuiSIgawaKUedaAWatanabeKHanafusaTet al. 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. Cells. (2020) 9:2149. doi: 10.3390/cells9102149. PMID:
85
YamanaKKawasakiRSanadaYTabataABandoKYoshikawaKet al. Tumor-targeting hyaluronic acid/fluorescent carborane complex for boron neutron capture therapy. Biochem Biophys Res Commun. (2021) 559:210–6. doi: 10.1016/j.bbrc.2021.04.037. PMID:
86
PapadopoulosVBaraldiMGuilarteTRKnudsenTBLacapèreJJLindemannPet al. Translocator protein (18kDa): new nomenclature for the peripheral-type benzodiazepine receptor based on its structure and molecular function. Trends Pharmacol Sci. (2006) 27:402–9. doi: 10.1016/j.tips.2006.06.005. PMID:
87
AmmerLMVollmann-ZwerenzARufVWetzelCHRiemenschneiderMJAlbertNLet al. The role of translocator protein TSPO in hallmarks of glioblastoma. Cancers (Basel). (2020) 12:2973. doi: 10.3390/cancers12102973. PMID:
88
CrossleyELIssaFScarfAMKassiouMRendinaLM. Synthesis and cellular uptake of boron-rich pyrazolopyrimidines: exploitation of the translocator protein for the efficient delivery of boron into human glioma cells. Chem Commun (Camb). (2011) 47:12179–81. doi: 10.1039/c1cc14587h. PMID:
89
HattoriYIshimuraMOhtaYTakenakaHKawabataSKirihataM. Dodecaborate conjugates targeting tumor cell overexpressing translocator protein for boron neutron capture therapy. ACS Med Chem Lett. (2022) 13:50–4. doi: 10.1021/acsmedchemlett.1c00377. PMID:
90
KashiwagiHHattoriYKawabataSKayamaRYoshimuraKFukuoYet al. Multi-targeted neutron capture therapy combined with an 18 kDa translocator protein-targeted boron compound is an effective strategy in a rat brain tumor model. Cancers (Basel). (2023) 15:1034. doi: 10.3390/cancers15041034. PMID:
91
FanaliGdi MasiATrezzaVMarinoMFasanoMAscenziP. Human serum albumin: from bench to bedside. Mol Aspects Med. (2012) 33:209–90. doi: 10.1016/j.mam.2011.12.002. PMID:
92
MatsumuraYMaedaH. A new concept for macromolecular therapeutics in cancer chemotherapy: mechanism of tumoritropic accumulation of proteins and the antitumor agent smancs. Cancer Res. (1986) 46:6387–92.
93
KawaiKNishimuraKOkadaSSatoSSuzukiMTakataTet al. Cyclic RGD-functionalized closo-dodecaborate albumin conjugates as integrin targeting boron carriers for neutron capture therapy. Mol Pharm. (2020) 17:3740–7. doi: 10.1021/acs.molpharmaceut.0c00478. PMID:
94
KashiwagiHKawabataSYoshimuraKFukuoYKanemitsuTTakeuchiKet al. Boron neutron capture therapy using dodecaborated albumin conjugates with maleimide is effective in a rat glioma model. Invest New Drugs. (2022) 40:255–64. doi: 10.1007/s10637-021-01201-7. PMID:
95
ScarantiMCojocaruEBanerjeeSBanerjiU. Exploiting the folate receptor α in oncology. Nat Rev Clin Oncol. (2020) 17:349–59. doi: 10.1038/s41571-020-0339-5. PMID:
96
McNeeleyKMKarathanasisEAnnapragadaAVBellamkondaRV. Masking and triggered unmasking of targeting ligands on nanocarriers to improve drug delivery to brain tumors. Biomaterials. (2009) 30:3986–95. doi: 10.1016/j.biomaterials.2009.04.012. PMID:
97
NagaiTTanakaMTsuneyoshiYXuBMichieSAHasuiKet al. Targeting tumor-associated macrophages in an experimental glioma model with a recombinant immunotoxin to folate receptor β. Cancer Immunol Immunother. (2009) 58:1577–86. doi: 10.1007/s00262-009-0667-x. PMID:
98
NakagawaFKawashimaHMoritaTNakamuraH. Water-soluble closo-docecaborate-containing pteroyl derivatives targeting folate receptor-positive tumors for boron neutron capture therapy. Cells. (2020) 9:1615. doi: 10.3390/cells9071615. PMID:
99
KanemitsuTKawabataSFukumuraMFutamuraGHiramatsuRNonoguchiNet al. Folate receptor-targeted novel boron compound for boron neutron capture therapy on F98 glioma-bearing rats. Radiat Environ Biophys. (2019) 58:59–67. doi: 10.1007/s00411-018-0765-2. PMID:
100
NishimuraKKashiwagiHMoritaTFukuoYOkadaSMiuraKet al. Efficient neutron capture therapy of glioblastoma with pteroyl-closo-dodecaborate-conjugated 4-(p-iodophenyl)butyric acid (PBC-IP). J Controlled Release. (2023) 360:249–59. doi: 10.1016/j.jconrel.2023.06.022. PMID:
101
LiuZChenX. Simple bioconjugate chemistry serves great clinical advances: albumin as a versatile platform for diagnosis and precision therapy. Chem Soc Rev. (2016) 45:1432–56. doi: 10.1039/C5CS00158G. PMID:
102
JacobsonOKiesewetterDOChenX. Albumin-binding Evans Blue derivatives for diagnostic imaging and production of long-acting therapeutics. Bioconjugate Chem. (2016) 27:2239–47. doi: 10.1021/acs.bioconjchem.6b00487. PMID:
103
ParvarSJWongCILewisASzychotEMorrisCJShorthouseDet al. Convection-enhanced delivery for brain Malignancies: technical parameters, formulation strategies and clinical perspectives. Adv Drug Delivery Rev. (2025) 224:115657. doi: 10.1016/j.addr.2025.115657. PMID:
104
SchmidtHMJarrettKEde Aguiar VallimTQTarlingEJ. Pathways and molecular mechanisms governing LDL receptor regulation. Circ Res. (2025) 136:902–19. doi: 10.1161/CIRCRESAHA.124.323578. PMID:
105
RudawskaASzermer-OlearnikBSzczygiełAMierzejewskaJWęgierek-CiuraKŻeliszewskaPet al. Functionalized boron carbide nanoparticles as active boron delivery agents dedicated to boron neutron capture therapy. Int J Nanomedicine. (2025) 20:6637–57. doi: 10.2147/ijn.S516534. PMID:
106
RenZZhaoJLiSYuanH. Targeting glucose transporter 1 (GLUT1) in cancer: molecular mechanisms and nanomedicine applications. Int J Nanomedicine. (2025) 20:11859–79. doi: 10.2147/ijn.S534976. PMID:
107
MatovićJJärvinenJBlandHCSokkaIKImlimthanSFerrandoRMet al. Addressing the biochemical foundations of a glucose-based “Trojan horse”-strategy to boron neutron capture therapy: from chemical synthesis to In Vitro assessment. Mol Pharm. (2020) 17:3885–99. doi: 10.1021/acs.molpharmaceut.0c00630. PMID:
108
MatovićJJärvinenJSokkaIKImlimthanSRaitanenJEMontaserAet al. Exploring the biochemical foundations of a successful GLUT1-targeting strategy to BNCT: chemical synthesis and In Vitro evaluation of the entire positional isomer library of ortho-carboranylmethyl-bearing glucoconjugates. Mol Pharm. (2021) 18:285–304. doi: 10.1021/acs.molpharmaceut.0c00917. PMID:
109
MatovićJJärvinenJSokkaIKStockmannPKellertMImlimthanSet al. Synthesis and In Vitro evaluation of a set of 6-deoxy-6-thio-carboranyl d-glucoconjugates shed light on the substrate specificity of the GLUT1 transporter. ACS Omega. (2022) 7:30376–88. doi: 10.1021/acsomega.2c03646. PMID:
110
MatovićJBahramiKStockmannPSokkaIKKhngYCSarparantaMet al. Sweet battle of the epimers─continued exploration of monosaccharide-derived delivery agents for boron neutron capture therapy. Mol Pharm. (2023) 20:3127–39. doi: 10.1021/acs.molpharmaceut.3c00119. PMID:
111
ImlimthanSBahramiKPehkonenHCentanniAMontaserABVäräAet al. Biological evaluation of a glucose-based boron carrier as a potential agent for boron neutron capture therapy. Int J Cancer. (2025) 157:2374–84. doi: 10.1002/ijc.70054. PMID:
112
LopesCPereiraCMedeirosR. ASCT2 and LAT1 contribution to the hallmarks of cancer: from a molecular perspective to clinical translation. Cancers (Basel). (2021) 13:203. doi: 10.3390/cancers13020203. PMID:
113
MiuraKArakiTMoritaTNishimuraKOkadaSSuzukiMet al. Alanine-serine-cysteine transporter-targeted small-molecule boron carriers for neutron capture therapy of L-4-boronophenylalanine-refractory tumors. J Control Release. (2026) 390:114566. doi: 10.1016/j.jconrel.2025.114566. PMID:
Summary
Keywords
boron delivery, boron neutron capture therapy, boron transporters, molecular targets, tumor-selective accumulation
Citation
Wei Y, Zhu W, Chen Q, Alnufaei H, Li Y and Ricketts K (2026) Potential targets for boron transport in boron neutron capture therapy. Front. Oncol. 16:1817038. doi: 10.3389/fonc.2026.1817038
Received
25 February 2026
Revised
30 April 2026
Accepted
05 May 2026
Published
28 May 2026
Volume
16 - 2026
Edited by
Charles A Kunos, National Cancer Institute (NIH), United States
Reviewed by
Kazuki Miura, Tokyo Institute of Technology, Japan
Julia Finogenova, Ministry of Health of Russia, Russia
Updates
Copyright
© 2026 Wei, Zhu, Chen, Alnufaei, Li and Ricketts.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Yumin Li, liym@lzu.edu.cn; Kate Ricketts, k.ricketts@ucl.ac.uk
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.