Boron cluster, boron cluster complex, and methods for producing the same

Abstract

To provide a boron nanocarrier that enables treatment of a primary tumor by boron neutron capture therapy and also enables treatment of a metastatic lesion through immunostimulation.SOLUTION: There is provided a boron cluster which is capable of encapsulating a water-soluble protein and to which boron neutron capture therapy (BNCT) is applicable, wherein ortho-, meta-, or para-carborane derivatives represented by formulae (I-I) to (I-III) form a spherical, rod-like, or plate-like aggregate. In the formulae, a small black dot represents BH, and R1 and R2 each independently represent a hydrogen atom, a linear, branched, or cyclic saturated or unsaturated alkyl group, a phenyl group, a benzyl group, a naphthyl group, an anthracenyl group, a phenanthryl group, or the like, and may be the same as or different from each other.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to boron clusters and boron cluster composites and methods for their manufacture. [Background technology]

[0002] With the advancement of molecular biology, it has long been possible to design and synthesize any protein in a test tube, and the development of protein drugs using this technology is accelerating. In particular, antibody drugs have been shown to be effective in treating diseases such as cancer, and the competition to develop them is fierce. To improve the efficacy of these protein drugs, it is important to develop a delivery system that improves their delivery to the target site.

[0003] Conventionally, antibody drugs have shown longer blood retention compared to general protein drugs, making them excellent for delivery to tumor tissue, but it has been difficult to ensure that their function is expressed only at the target site. Some antibodies have been designed to induce conformational changes in the antibody by binding to ATP in the tumor environment, thereby showing affinity for the target substance, but the technology is still in development.

[0004] Recently, boron compounds have attracted attention as carriers for transporting proteins into cells. For example, Non-Patent Document 1 discloses a boron delivery system using a complex composed of a fructose-modified poly(ethylene glycol)-poly(L-lysine) block copolymer and p-phenylboronic acid. This complex is used in boron neutron capture therapy (BNCT). Non-Patent Document 2 describes a boron compound that mainly uses a dendrimer as a protein delivery system.

[0005] Boron neutron capture therapy is a treatment in which boron compounds are incorporated into cancer cells and then neutron beams are used to destroy the cancer cells locally. In this treatment, it is important to selectively accumulate the boron compounds within the tumor. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] T. Nomoto et al., J. Control. Release, 332 (2021) 184-193 [Non-patent document 2] Q. Ren et al., Adv. Healthcare Mater. 2023, 12, 2202049 Summary of the Invention [Problem to be solved by the invention]

[0007] The boron compound described in Non-Patent Document 1 forms micelle-type nanoparticles in which polymers are highly accumulated. This makes it difficult to stably retain proteins inside the nanoparticles and control their release. Furthermore, although the boron compound described in Non-Patent Document 2 has high structural stability, it is difficult to complex proteins in a stable state and in a state in which they are coated with polymers to impart resistance to degrading enzymes.

[0008] Furthermore, while the boron agents currently used in clinical BNCT demonstrate excellent therapeutic efficacy against primary tumors, their effectiveness in treating metastatic cancer is insufficient. In recent years, it has been discovered that in BNCT, immune activation, triggered by damage to cancer cells due to neutron irradiation, plays an important role in treating metastatic cancer. However, the immunosuppressive environment of cancer cells prevents this immune activation, making it difficult to treat metastatic cancer.

[0009] The technology disclosed here has been developed in light of these issues, and its purpose is to provide a boron nanocarrier that can treat not only primary lesions by boron neutron capture therapy but also metastatic lesions by immunostimulation. It also aims to improve the delivery of water-soluble proteins to target tissues, reduce side effects as a drug, and release water-soluble proteins in response to external stimuli. [Means for solving the problem]

[0010] The present inventors have developed boron clusters that can be stably complexed with water-soluble proteins such as antigens and antibodies, and can be released in response to an external stimulus.

[0011] The boron cluster of the present disclosure is capable of encapsulating a water-soluble protein and is applicable to boron neutron capture therapy, and is a boron cluster in which ortho-, meta-, or para-carborane derivatives represented by the following formulas (II) to (I-III) form spherical, rod-shaped, or plate-shaped aggregates.

[0012] [ka]

[0013] In the formula, ● represents BH, and R 1 and R 2 are each a group selected from a hydrogen atom, a linear, branched, or cyclic saturated or unsaturated alkyl group, a phenyl group, a benzyl group, a naphthyl group, an anthracenyl group, a phenanthryl group, a pyrenyl group, and an acetamido group, and may be the same or different. 1 and R 2 are all hydrogen atoms.

[0014] As a result of extensive research, the inventors of the present invention have found that the boron cluster of the present disclosure can easily and stably complex with proteins through weak supramolecular chemical interactions, efficiently deliver the protein to a target site, and release the complexed protein in response to an external stimulus. Furthermore, the boron cluster of the present disclosure can easily form a complex with a protein by simply mixing it with the protein, and the complex exhibits sufficient behavior to protect the protein from degrading enzymes.

[0015] Furthermore, the present disclosure can provide a boron cluster complex in which a water-soluble protein that functions as an immune checkpoint inhibitor is conjugated to a boron cluster. When an immune checkpoint inhibitor is used as the water-soluble protein, combined therapy with boron neutron capture therapy, a type of radiation therapy, and an immune checkpoint inhibitor not only improves the response rate of primary lesions to boron neutron capture therapy, but also enables the treatment of metastatic lesions by immune activation.

[0016] The carborane derivative is R 1 or R 2 preferably has a phenylboronic acid group, and R 1 or R 2 may be a phenylboronic acid group.

[0017] The phenylboronic acid group in the carborane derivative enables the boron cluster complex to release proteins in response to adenosine triphosphate (ATP).

[0018] The boron clusters are preferably in the form of rod-shaped aggregates.

[0019] The rod-shaped boron clusters are superior to other shapes such as spheres and plates in terms of therapeutic performance and tumor accumulation based on neutron irradiation.

[0020] The method for producing a boron cluster of the present disclosure includes the steps of obtaining an ortho-, meta-, or para-carborane derivative represented by the following formulas (II) to (I-III), which is a boron cluster capable of encapsulating a water-soluble protein and suitable for boron neutron capture therapy, and adjusting the composition of the carborane derivative and cyclodextrin and mixing them in an aqueous solution to obtain aggregates in the form of spheres, rods, or plates.

[0021] [ka]

[0022] In the formula, ● represents BH, and R 1 and R 2 are each a group selected from a hydrogen atom, a linear, branched, or cyclic saturated or unsaturated alkyl group, a phenyl group, a benzyl group, a naphthyl group, an anthracenyl group, a phenanthryl group, a pyrenyl group, and an acetamido group, and may be the same or different. 1 and R 2 are all hydrogen atoms.

[0023] Furthermore, the method for producing a boron cluster complex of the present disclosure includes a step in which the water-soluble protein is an immune checkpoint inhibitor, and the boron cluster obtained by the method is mixed with the immune checkpoint inhibitor to form a complex, thereby obtaining a boron cluster complex.

[0024] The boron cluster of the present disclosure can be easily and stably complexed with a water-soluble protein simply by mixing it with the protein. [Effects of the Invention]

[0025] As described above, the present disclosure makes it possible to provide boron nanocarriers that can not only treat primary lesions through boron neutron capture therapy, but also treat metastatic lesions through immune activation. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a microscope image of boron clusters according to Example 1. [Figure 2] 1 is a microscope image of boron clusters according to Example 1. [Figure 3] 1 is a microscope image of boron clusters according to Example 1. [Figure 4] FIG. 2 is a schematic diagram for explaining the morphology of boron clusters. [Figure 5] 1 is a microscope image of boron clusters according to Example 2. [Figure 6] 1 is a microscope image of boron clusters according to Example 2. [Figure 7] 1 is a microscope image of boron clusters according to Example 3. [Figure 8] 1 is a microscope image of boron clusters according to Example 3. [Figure 9] 1 is a histogram showing the results of single particle analysis of Py-CB rods. [Figure 10] 1 is a histogram showing the results of single particle analysis of Py-CB rods. [Figure 11] 1 is a histogram showing the results of single particle analysis of Py-CB rods. [Figure 12] 1 is a histogram showing the results of single particle analysis of Py-CB rods. [Figure 13] 1 is a histogram showing the results of single particle analysis of PB-CB sph. [Figure 14] 1 is a histogram showing the results of single particle analysis of PB-CB sph. [Figure 15] 1 is a histogram showing the results of single particle analysis of PB-CB sph. [Figure 16] 1 is a histogram showing the results of single particle analysis of PB-CB sph. [Figure 17] 1 is a histogram showing the results of single particle analysis of PB-CB rods. [Figure 18] 1 is a histogram showing the results of single particle analysis of PB-CB rods. [Figure 19]1 is a histogram showing the results of single particle analysis of PB-CB rods. [Figure 20] 1 is a histogram showing the results of single particle analysis of PB-CB rods. [Figure 21] 1 is a graph showing the BNCT activity of boron clusters. [Figure 22] 1 is a graph showing the blood retention of boron clusters. [Figure 23] 1 is a graph showing the tumor accumulation of boron clusters. [Figure 24] 1 is a graph showing the accumulation of boron clusters in each organ. [Figure 25] 1 is a graph showing antibody delivery of boron clusters to tumor tissues. [Figure 26] 1 is a graph showing antibody delivery of boron clusters to various organs. [Figure 27] This is the observation result of tumor growth in the primary lesion. [Figure 28] This is an observation of tumor growth in metastatic lesions. DETAILED DESCRIPTION OF THE INVENTION

[0027] The boron clusters of the present disclosure will be described in detail below.

[0028] The boron clusters disclosed herein are intended to stably encapsulate water-soluble proteins such as antigens and antibodies and deliver the water-soluble proteins to target tissues, making them applicable to boron neutron capture therapy.

[0029] The boron cluster of the present disclosure is formed by assembling ortho-, meta-, or para-carborane derivatives represented by the following formulas (II) to (I-III).

[0030] [ka]

[0031] In the formula, ● represents BH, and R 1 and R 2 are each a group selected from a hydrogen atom, a linear, branched, or cyclic saturated or unsaturated alkyl group, a phenyl group, a benzyl group, a naphthyl group, an anthracenyl group, a phenanthryl group, a pyrenyl group, and an acetamido group, and may be the same or different. 1 and R 2 are all hydrogen atoms.

[0032] R 1 and R 2 R may be any group as long as the carborane derivative can form a spherical, rod-like, or flat aggregate, and more specifically, examples of R include, but are not limited to, the following groups: 1 and R 2 For example, one or both of R may be alkyl chains as in the following compounds a to d, fluorescent π-conjugated systems as in compounds e to f, or alkyl chains connecting two carboranes as in compound h. Furthermore, carborane derivatives may be chiral compounds as in the S-form compound i and the R-form compound j. 1 or R 2 preferably has a phenylboronic acid group, and as in compound g, at least R 1 or R 2 is more preferably a phenylboronic acid group. Carborane derivatives having such two functional groups can be synthesized by various known methods.

[0033] [ka]

[0034] The carborane derivative of the present disclosure can be mixed with cyclodextrin in an aqueous solution at an adjusted blend ratio to obtain spherical, rod-like, or flat aggregates. The boron cluster of the present disclosure is formed by the carborane derivative forming spherical, rod-like, or flat aggregates on the order of nanometers to micrometers. The boron cluster is preferably rod-shaped. It is believed that calixarenes and pillararenes can also be used as alternatives to cyclodextrin.

[0035] The boron cluster of the present disclosure can be easily complexed with a water-soluble protein simply by mixing with the water-soluble protein. A variety of substances, including antigens and antibodies, can be used as water-soluble proteins that can be complexed with a boron cluster. Any water-soluble protein can be used as long as it can interact with the boron cluster of the present disclosure and form a complex. Examples of water-soluble proteins include horseradish peroxidase (HRP), which is used as a labeling enzyme for secondary antibodies, antibodies such as IgG, and immune checkpoint inhibitors.

[0036] The electronic interaction between boron clusters and water-soluble proteins enables the boron clusters to transport the water-soluble proteins in a stable state. By accumulating such boron clusters in the primary tumor, not only can boron neutron capture therapy be performed effectively, but the action of the water-soluble proteins carried as a complex can also enable the treatment of metastatic tumors by immune activation. [Example]

[0037] The present invention will be described in more detail below with reference to examples. These examples are merely illustrative examples for suitably explaining the present invention, and are not intended to limit the present invention.

[0038] [Boron Cluster Synthesis] Example 1 As an example of a carborane derivative, the above compound f(R 1 : pyrenyl group, R 2Py-CB (hereinbelow referred to as Py-CB) was synthesized by a known method. The concentrations of this Py-CB were adjusted to 0.1 mM and 1.0 mM. Hydroxypropyl-β-cyclodextrin (β-CD) was also adjusted to 0.1 mM, 1.0 mM, and 2.0 mM. 0.1 mM Py-CB and 0.1 mM β-CD, 1.0 mM Py-CB and 1.0 mM β-CD, and 1.0 mM Py-CB and 2.0 mM β-CD were mixed and dissolved in water, and the morphology after 24 hours was observed using a transmission electron microscope or a scanning electron microscope. The results are shown in Figures 1 to 3. A schematic diagram illustrating the morphology of the boron cluster is shown in Figure 4.

[0039] Figure 1 shows a complex of 0.1 mM Py-CB and 0.1 mM β-CD, Figure 2 shows a complex of 1.0 mM Py-CB and 1.0 mM β-CD, and Figure 3 shows a complex of 1.0 mM Py-CB and 2.0 mM β-CD. The complex in Figure 1 is a rod-shaped aggregate approximately 0.1 μm in diameter and 3.5 μm in length, the complex in Figure 2 is a plate-shaped aggregate 100 nm wide and 2 μm long, and the complex in Figure 3 is a spherical aggregate approximately 100 nm in diameter. As shown in Figure 4, carborane derivatives can be mixed with cyclodextrin in aqueous solution at different ratios to form spherical, rod-shaped, or flat aggregates. By mixing Py-CB with cyclodextrin in aqueous solution at different ratios, the desired spherical, rod-shaped, or flat aggregates can be obtained. Furthermore, these aggregates stably maintained their morphology even after dilution, suggesting that they are resistant to significant dilution after administration into the blood.

[0040] Example 2 As an example of a carborane derivative, the above compound g (R 1 : phenylboronic acid group, R 2: phenyl group. Hereinafter, referred to as PB-CB.) was synthesized by a known method. The concentrations of this PB-CB were adjusted to 0.1 mM and 1.0 mM. Hydroxypropyl-β-cyclodextrin (β-CD) was also adjusted to 0.1 mM and 1.0 mM. 0.1 mM PB-CB and 0.1 mM β-CD, and 1.0 mM PB-CB and 1.0 mM β-CD were mixed and dissolved in water, respectively, and the morphology after 24 hours was observed using a transmission electron microscope or a scanning electron microscope. The results are shown in Figures 5 and 6. Figure 5 shows a complex of 0.1 mM PB-CB and 0.1 mM β-CD, and Figure 6 shows a complex of 1.0 mM PB-CB and 1.0 mM β-CD. The complex in Figure 5 was a spherical aggregate with a diameter of several micrometers, while the complex in Figure 6 was a rod-shaped aggregate with a length of approximately several micrometers. By adjusting the ratio of PB-CB to cyclodextrin in an aqueous solution, spherical or rod-shaped aggregates could be formed, and these aggregates remained stable even after dilution.

[0041] Example 3 As an example of a carborane derivative, the above compound a(R 1 :C8H 17 alkyl group, R 2 Ak-CB (Ak-CB) was synthesized by a known method. The concentrations of this Ak-CB were adjusted to 1.0 mM and 10 mM. Hydroxypropyl-β-cyclodextrin (β-CD) was also adjusted to 1.0 mM and 10 mM. 1.0 mM Ak-CB and 1.0 mM β-CD, and 10 mM Ak-CB and 10 mM β-CD were mixed and dissolved in water, respectively, and the morphology after 24 hours was observed using a transmission electron microscope or a scanning electron microscope. The results are shown in Figures 7 and 8. Figure 7 shows a complex of 0.1 mM Ak-CB and 0.1 mM β-CD, and Figure 8 shows a complex of 1.0 mM Ak-CB and 1.0 mM β-CD. The complexes in Figures 7 and 8 were plate-like aggregates approximately several μm to 100 μm in size. These aggregates stably maintained their morphology even after dilution.

[0042] [Combination of boron clusters with proteins and evaluation of protein release behavior] Of the assemblies prepared in Examples 1 and 2, Py-CB rod-shaped bodies (Py-CB rods), PB-CB spheres (Py-CB sph), and PB-CB rod-shaped bodies (PB-CB rods) were mixed with antibody molecules (IgG), and their conjugation ability was evaluated. Conjugation was evaluated using single particle analysis using imaging flow cytometry. The results are shown in Figures 9 to 20. Figures 9 to 12 show the results for Py-CB rods, Figures 13 to 16 show the results for PB-CB sph, and Figures 17 to 20 show the results for PB-CB rods.

[0043] Particles showing a fluorescence intensity higher than the right end of the peak of the histogram obtained with a conjugate prepared using a non-fluorescent antibody were determined to be particles with advanced antibody conjugation. When fluorescently labeled antibodies were reacted with boron clusters, the histogram showing fluorescence intensity shifted to the right, indicating an increase in the fluorescence intensity derived from the particles. Figures 9, 13, and 17 show histograms for the boron clusters Py-CB rod, PB-CB sph, and PB-CB rod, respectively. Figures 10, 14, and 18 show histograms for the boron cluster conjugates after antibody mixing (Py-CB rod / IgG, PB-CB sph / IgG, and PB-CB rod / IgG). As shown in Figures 9 and 10, Figures 13 and 14, and Figures 17 and 18, the histograms of Py-CB rod, PB-CB sph, and PB-CB rod all shifted to the right after antibody mixing (Py-CB rod / IgG, PB-CB sph / IgG, PB-CB rod / IgG), indicating that all boron clusters can spontaneously complex with antibodies simply by mixing.

[0044] We also evaluated the stability in blood by adding glucose to the boron cluster complex at a concentration similar to that found in blood. Figures 11, 15, and 19 show histograms of Py-CB rod / IgG, PB-CB sph / IgG, and PB-CB rod / IgG after the addition of glucose. No significant changes were observed in any of the histograms, indicating that the antibody was stably maintained in blood.

[0045] The phenylboronic acid structure is known to exhibit high affinity for ATP, which is present at high concentrations (>1 mM) in tumor tissue. Therefore, we further evaluated whether ATP affinity contributes to the controlled release of antibodies. Specifically, we added ATP (1 mM), equivalent to that present in tumor tissue, to the boron cluster complexes. Figures 12, 16, and 20 show histograms of Py-CB rod / IgG, PB-CB sph / IgG, and PB-CB rod / IgG after the addition of ATP, respectively. As shown in Figures 16 and 20, the histograms of the boron cluster complexes consisting of PB-CB modified with a phenylboronic acid derivative, which has ATP affinity, shifted to the left, indicating antibody release. On the other hand, as shown in Figure 12, the histograms of Py-CB modified with pyrene, which has low ATP affinity, remained unchanged, indicating that the antibody was retained without release. This indicates that these methods can be used depending on the type of protein to be delivered and the site of action.

[0046] [BNCT activity of boron clusters (in vitro)] To compare the therapeutic performance of boron clusters (Py-CB rod, PB-CB sph, PB-CB rod) with that of the clinical drug L-BPA / fructose complex based on neutron irradiation, we evaluated the performance using cultured cells (ovalbumin-strongly expressing mouse lymphoma, EG-7-OVA) using the following method.

[0047] Boron clusters (Py-CB rod, PB-CB sph, PB-CB rod) or L-BPA were added to EG-7-OVA and irradiated with neutrons (1 MW, 70 min) 24 hours later. The cytotoxicity was evaluated using the WST-8 assay. Figure 21 shows the results 24 hours after neutron irradiation. As shown in Figure 21, the cytotoxicity ranking was PB-CB rod > PB-CB sph > BPA > PB-Py rod. This trend in cytotoxicity was also observed when the neutron irradiation was evaluated 2 and 7 days later. Furthermore, the lack of a significant decrease in cytotoxicity over time indicates that the damage to the cells does not induce cancer cell dormancy, which directly leads to cancer regrowth, but rather induces cell death.

[0048] [Evaluation of pharmacokinetics of boron cluster complexes] In addition to the antibody delivery of the boron cluster complex, the pharmacokinetics of boron and antibody in vivo were evaluated using ICP emission spectrometry and an in vivo imaging system, respectively, to estimate the optimal time for neutron irradiation. Tumor-bearing mouse models were generated by subcutaneously injecting E.G7-OVA cells into the thighs of C57BL6 mice.

[0049] Specifically, 100 μL of a sample containing 50 ppm boron was administered, and its behavior was analyzed using an in vivo imaging system at 0, 1, 3, 6, and 24 hours after administration to examine the distribution of fluorescence within the body. After 24 hours, the mice were dissected, and the collected organs (liver, lung, heart, tumor, spleen, kidney, and skin) were analyzed for accumulation in each organ, which was then subjected to an oxidation treatment and analyzed using an ICP analyzer. Blood was collected at each time point, and serum was collected by centrifugation. The blood was then subjected to an oxidation treatment and analyzed using an ICP analyzer.

[0050] The results for blood retention are shown in Figure 22. As shown in Figure 22, the rod-shaped boron cluster (PB-CB rod) had slightly improved blood retention compared to the spherical boron cluster (PB-CB sph). The results for tumor accumulation are shown in Figure 23. As shown in Figure 23, the order of tumor accumulation was PB-CB rod > PB-CB sph > Py-CB rod. This means that PB recognized sialic acid overexpressed on the surface of cancer cells and functioned as an active target.

[0051] Figure 24 shows the accumulation of boron clusters in each organ 24 hours after administration. As shown in Figure 24, while boron clusters partially accumulated in metabolic systems such as the liver and lungs, they accumulated most in tumors in all systems. This is largely due to the EPR effect resulting from the particle size. Furthermore, the tumor selectivity value showed a favorable value exceeding the clinically recommended value of 2.5.

[0052] The results of evaluating antibody accumulation are shown in Figure 25. As shown in Figure 25, these results are generally consistent with the kinetics of boron, and it was revealed that the most antibody could be delivered in the following order: PB-CB rod > Py-CB rod > PB-CB sph. Furthermore, higher accumulation was achieved compared to the administration of antibody alone (IgG) as a control. The results of pharmacokinetics revealed that the majority of the antibody accumulated in the tumor (Figure 26).

[0053] [BNCT activity of PB-CB rod immune checkpoint inhibitor complex] The PB-CB rod, which demonstrated the highest therapeutic potential and tumor accumulation in the above evaluations, was used to verify its efficacy in treating metastatic cancer. The PB-CB rod was conjugated with an immune checkpoint inhibitor (ICI), and the ICI and boron were co-delivered to the tumor, followed by neutron irradiation. A mouse model of metastatic cancer was created by subcutaneously implanting E.G7-OVA cells into the thighs of both legs of C57BL6 mice. Tumor growth graphs are shown in Figures 27 and 28.

[0054] Eight control groups were used: saline (physiological saline, non-neutron irradiated), PB-CB rod cold (non-neutron irradiated), PB-CB rod + ICI cold (each drug administered separately, non-neutron irradiated), PB-CB rod / ICI cold (each drug administered as a complex, non-neutron irradiated), BPA hot (neutron irradiated), PB-CB rod hot (neutron irradiated), PB-CB rod + ICI hot (neutron irradiated), and PB-CB / ICI hot (neutron irradiated). After administration of these drugs via the tail vein, mice were irradiated with neutrons (1MW, 70 min) 24 hours later. The controls, saline, PB-CB rod cold, PB-CB rod + ICI cold, and PB-CB / ICI cold, failed to induce tumor growth inhibition in either primary or metastatic lesions. On the other hand, the clinical drugs BPA hot, PB-CB rod hot, PB-CB rod + ICI hot, and PB-CB / ICI hot significantly suppressed tumor growth in the primary lesion.

Claims

1. A boron cluster capable of encapsulating a water-soluble protein and applicable to boron neutron capture therapy (BNCT), comprising: A boron cluster in which ortho-, meta- or para-carborane derivatives represented by the following formulas (II) to (I-III) form spherical, rod-like or flat-plate-like aggregates. 【Chemistry 1】 [wherein ● represents BH, and R １ and R ２ are each a group selected from a hydrogen atom, a linear, branched, or cyclic saturated or unsaturated alkyl group, a phenyl group, a benzyl group, a naphthyl group, an anthracenyl group, a phenanthryl group, a pyrenyl group, and an acetamido group, and may be the same or different. １ and R ２ are all hydrogen atoms.]

2. The carborane derivative is R １ or R ２ The boron cluster according to claim 1, wherein the phenylboronic acid group is a phenylboronic acid group.

3. The carborane derivative is R １ or R ２ The boron cluster according to claim 1, wherein is a phenylboronic acid group.

4. The boron cluster according to claim 3, which is a rod-shaped aggregate.

5. The water-soluble protein is an immune checkpoint inhibitor, and a boron cluster complex is formed by complexing the boron cluster described in any one of claims 1 to 4 with the immune checkpoint inhibitor.

6. A method for producing a boron cluster that can encapsulate a water-soluble protein and is applicable to boron neutron capture therapy (BNCT), comprising the steps of: obtaining an ortho-, meta-, or para-carborane derivative represented by the following formulas (II) to (I-III); and a step of adjusting the composition of the carborane derivative and cyclodextrin and mixing them in an aqueous solution to obtain aggregates in the form of spheres, rods, or plates. 【Chemistry 2】 [wherein ● represents BH, and R １ and R ２ are each a group selected from a hydrogen atom, a linear, branched, or cyclic saturated or unsaturated alkyl group, a phenyl group, a benzyl group, a naphthyl group, an anthracenyl group, a phenanthryl group, a pyrenyl group, and an acetamido group, and may be the same or different. １ and R ２ are all hydrogen atoms.]

7. A method for producing a boron cluster complex, wherein the water-soluble protein is an immune checkpoint inhibitor, and the method comprises a step of mixing the boron cluster obtained by the method of claim 6 with the immune checkpoint inhibitor to complex the boron cluster, thereby obtaining a boron cluster complex.

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