Nanostructured boron compounds for cancer therapy

BNCT agents of functionalized nanoparticles

The functional nanoparticles, suitable for pharmaceutic applications, are generally organic surfaces that are smaller than 500 nm size. Concept of nanoparticle was first introduced by Peter Speiser in 1976 for pharmaceutical applications [14]. Following the development of biodegradable polymers, nanoparticles found broad clinical applications, primarily in oncology. Some nanoparticles-based drugs, such as Abraxane^® for the treatment breast cancer, are already in clinical use [15]. In the area of BNCT, our report on the functionalized nanoparticles was the first one that was later followed by magnetic nanoparticles incorporating carborane containing matrix as shown in Fig. 2a [16]. A high boron concentration of 35.2 μg (B)/g breast tumor was achieved over a period of 30 h with a selectivity of five for tumor to blood ratio in the presence of an external magnet. It is well recognized that magnetic nanoparticles have important applications in pharmacy and oncology diagnosis, especially using as contrast agents in magnetic resonance imaging (MRI) [17], [18], [19], [20], [21], [22], [23], [24]. Magnetic nanoparticle-based drug carriers show the potential for improvement in delivering the drug amount with the help of an external magnetic field [17], [18], [19], [20], [21], [22], [23], [24]. Application of magnetic nanoparticles in BNCT treatment shows many advantages as there is no drug releasing issue when compared to other drug delivery systems. Therefore, the concept is particularly practical for BNCT application. It has been reported that boron enriched magnetic nanoparticles provided a high performing boron delivery [16]. Since the composite material contains the magnetic nanoparticles, it is potentially active for MRI technology, and thus MRI method can be used to monitor the delivery status. Thus, the magnetic nanocomposite is highly expected to show promising BNCT results with neutron irradiation in clinic treatments.

Fig. 2:

(a) Synthesis, (b) TEM image and (c) biodistribution of the encapsulated magnetic nanocomposites.

Inorganic nanoparticles of multiple components, Fe¹⁰BO₃/Fe₃O₄/SiO₂ and GdFeO₃/Fe₃O₄/SiO₂ have also been reported [25]. The nanoparticles were synthesized by a convenient pyrolysis method followed by a commonly used SiO₂ coating procedure as shown in Fig. 3 [25]. The nanoparticles are spherical with a diameter of around 60 nm based on TEM results. The composites could be further functionalized by chemical modification of the silicon shell to improve their tumor selectivity. Unlike boron clusters and boron oxide nanoparticles, pure boron nanoparticles are potentially a promising boron carrying agent due to their significant boron content. Although boron nanoparticles can be made by different synthetic approaches, the commonly employed methods include chemical vapor deposition (CVD), pyrolysis, reduction in solution, thermal plasma, arc discharge and ball milling techniques [26], [27], [28], [29]. Recently, we have reported the synthesis of magnetic dopamine-functionalized boron nanoparticles using the ball milling method as shown in Fig. 4 [30]. The magnetic nanocomposites show a size range of 100–700 nm with an iron content of about 27 %, as weight percent, which are produced directly from the ball milling material, that is steel [30]. Accordingly, the boron powder was ball-milled in undecylenic acid and the resulting nanoparticles were coated with organic functionalities. Thus, the technology has the advantage of forming alloy and surface functionalization of commercially available boron powder, simultaneously [30]. Consequently, ball-milling can accomplish both the synthesis of boron nanoparticles and their functionalization, simultaneously in one step. However, the issues of controlling iron content and particle size distribution need to be addressed. Further investigation of this research led to construct magnetic nanocomposite comprising Fe₃O₄, polyethylene glycol (PEG, 1500 Da) and mono or bis(ascorbatoborate) as shown in Fig. 5 [31]. The nanocomposite showed a spherical morphology with an average size of 10–15 nm and exhibited a super paramagnetic behavior at 300 K [31]. Since the composites with combined ascorbic acid species are known to be useful as radical scavenging and anti-tumor agents, our newly prepared material has the potential application in magnetic biomedicine.

Fig. 3:

Synthesis of nanocomposites of Fe¹⁰BO₃/Fe₃O₄/SiO₂.

Fig. 4:

Synthesis of magnetic boron nanocomposites by ball-milling method.

Fig. 5:

Synthesis of magnetic boron nanocomposites, functionalized by ascorbatoborate.

Gold nanoparticles (Au-NPs) are one type of the potential materials for various biomedical applications such as drug delivery [32]. Cioran et al. [33] prepared gold nanoparticles protected by a monolayer mercaptocarborane ligand (carboranyl thiol) with a mean particle diameter of 3.2 nm, [Au-NPs]@[1-S-1,2-closo-C₂B₁₀H₁₁]⁻_n. The nanocomposite showed a switchable solubility between water and nonpolar solvents by adjusting the electronic and ionic charge of the gold clusters. The composite could be uptaken by a human cancer line of HeLa cells and accumulated in vesicles within membranes [33]. However, the composite was relatively toxic and killed all cells after 24 h of incubation with them. The carboranylphosphinate ligand (Na[1-OPH(O)-1,7-closo-C₂B₁₀H₁₁]) was used to stabilize CdSe quantum dots (CDs) and assembled a core-canopy nanostructure of {[CdSe-NPs]@[1-OPH(O)-1,7-closo-C₂B₁₀H₁₁]⁻_n}@Cl⁻ /[(Caprylyl)₃N]⁺ [34]. The structure was designed to trap Cl⁻ ions and it exhibited a kinetic fluorescence switching optical property [34].

BNCT agents of functionalized nanotubes

Single and multi-walled carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs) and boron nanotubes (BNTs) are the new nanomaterials with a tubular morphology. The CNTs have found pharmaceutical applications in drug delivery [35], [36], [37], [38] as they possess high surface area and enhanced cellular uptake and can be functionalized with various bioactive components, such as nucleic acids and proteins. The functionalized CNTs are recognized and used as biocompatible and low toxicity drug carrier [35], [36], [37], [38]. Zhu et al. [39] first reported the application of CNTs for BNCT. Accordingly, the carborane clusters were linked to the surface of the single-walled CNTs through a short alkyl bridge as shown in Fig. 6a. According to the report, a high boron concentration of 21.5 μg/g tumor was reached within the 48-h period after administration (Fig. 6b) with a selectivity of >3:1 ratio of tumor-to-blood [39]. It was also reported that retention in tumor tissue was higher than in normal tissues. The resulting nanocomposite also demonstrated good water solubility, an important property for BNCT drug delivery [7], [8]. Cabana et al. [40] functionalized single-walled carbon nanotubes with cobaltabisdicarbollide anions by forming a short link of (SWCNT-C(O)-[O(CH₂)₂]₂-O-metallaborane). This material showed an outstanding water dispersibility. Same strategy was used to prepare graphene oxide-cobaltabisdicabollide and graphene oxide-closo-dodecaborate nanohybrides [41]. The nanohybrids were reported to be well-dispersed in water without decomposition.

Fig. 6:

Synthesis (a) and biodistribution (b) of carborane-linked CNTs.

Boron nitride nanotube (BNNT) is a structural analog of carbon nanotube. It represents a type of innovative and extremely attractive nanomaterials with comparable thermal conductivity and mechanical stiffness of CNTs. However, BNNT shows significantly different properties in comparison with CNTs. A pristine BNNT is an electrical insulator and is more thermally and chemically stable [42], [43], [44], [45]. The BNNTs may have promising aerospace applications, particularly of the ¹⁰B-enriched BNNTs, due to their IR strength and radiation-shielding of spallation neutrons from cosmic rays [46]. Applications of the BNNTs in nanomedicine have been investigated and well developed in the last decades [47] to confirm that the BNNTs are potentially useful materials in biomedical and nanomedicinal applications, including drug delivery, therapeutics and tissue regeneration [47].

The BNNT is also a potentially promising BNCT agent due to its relatively simple chemical composition (B and N) that can be functionalized for specific tumors [48], [49], [50], [51]. In addition, the synthesis of the ¹⁰B-enriched BNNTs is experimentally feasible. Menichetti and coworkers have synthesized Fe-containing BNNTs by a convenient milling process with the Fe content of about 1.5 wt% [51]. Subsequently, the BNNT nanocomposite was employed as contrast agents in magnetic resonance imaging (MRI) by encapsulating with poly(L-lysine) and measured in homogeneous aqueous dispersions at 3T with different concentration. It was reported that the composite has a good potential for negative MRI contrast agent, particularly as T(2) contrast-enhancement agents [51]. However, the controlling of the iron content in the nanocomposite remained a big challenge. Nakamura’s group has demonstrated that BNNT showed antitumor effects towards B16 melanoma cells in the presence of thermal neutron irradiation [52]. The measurement was carried out in an aqueous suspension stabilized by polymer DSPE-PEG2000. It was reported that the composite of BNNT-DSPE-PEG2000 selectively accumulated in B16 cells approximately three times higher than current clinically used sodium borocaptate (BSH). The neutron-dose dependent surviving fractions are shown in Fig. 7 [52]. It was observed that BNNT-DSPE-PEG2000 nanocomposite effectively killed more B16 cells than those with BSH alone, probably due to the higher boron accumulation in B16 cells [52]. The results suggested that the BNNT-DSPE-PEG2000 composite is a potentially important BNCT delivery agent.

Fig. 7:

Clonogenic cell-survivals after irradiation in vitro using thermal neutron beams. BNNT-PEG2000 (100 ppm natural boron); BSH (sodium borocaptate, 100 ppm natural B).

A highly water dispersible nanostructured boron nitride (BN) in the transient phase from two dimensional hexagonal sheets to nanotubes was synthesized at a relatively low temperature. The nanostructured BN shows low cytotoxicity on various cell lines [Hela(cervical cancer), human embryonic kidney (HEK-293) and human breast adenocarcinoma (MCF-7)]. Therefore, the BN can be used for BNCT applications [53]. Ferreira and coworkers [54] have investigated the potential application of BNNT for BNCT in cancer treatment. It was found that BNNTs were selectively accumulated in the HeLa cell line (ATCC CCL-2) and demonstrated high cell killing effect in the presence of a thermal neutron flux of around 6.6×10⁸ n·cm⁻²·s⁻¹ as shown in Fig. 8 [54]. Thus, the results provided the evidence for the suitability of BNNTs for BNCT applications.

Fig. 8:

Cytotoxic effects of BNNTs on HeLa cells with quantification of dead cells (% of total cells).

Although the boron nanotubes have been synthesized [55], [56], their metallic conductivities, irrespective of its diameters and chirality, have been investigated and could also be a useful material for BNCT applications due to their extremely high boron contents, their high-yield synthetic approach still needs to be improved to make their use very practical either for biomedical applications or as a dopant in semiconductor devises.

BNCT agents of functionalized nanospherical polymers and dendrimers

Both naturally occurring and artificially functionalized polymers have been commonly used in medicinal chemistry as drug delivery agents [57], [58], [59], [60]. While dendrimers have been well explored as nanocarriers in drug delivery and diagnosis to reduce the drug-dose, toxicity and tumor resistance [60], various functionalized polymers, such as liposome and dendrimers, have been employed as boron carriers for BNCT applications [7], [8]. Accordingly, Chen and coworkers have reported a multifunctional nanocomposite, comprising polyethylene glycol (PEG 2000) and carborane clusters, labeled or tagged with fluorescence rhodamine dye [61]. It was reported that the nanocomposite could form spherical particles in aqueous solution and quickly absorbed by the cells and have a high stability in the bloodstream. The unique properties of the composite materials enable them to be used as a targeting vehicle. Subsequently, Teixidor’s group has employed other methodologies to prepare water-soluble nanoscale BNCT agent by integrating metallacarborane anion in the targeted molecules [61]. A globular dendritic macromolecule, [NMe₄]₆[1,2,8,9,10,12{(3,3′-Co-{8-(C₄H₈O₂-1,2-C₂B₉H₁₀-1′,2′-C₂B₉H₁₁}-O(CH₂)₃}₆-1,2-closo-C₂B₁₀H₆], containing multiple anionic metallacarborane clusters at the o-carborane periphery has been synthesized. This water-soluble high boron rich molecule is expected to be a useful BNCT carrier in the near future [62].

The liposomes (LP) are natural nanomaterials comprising of phospholipids and cholesterol with enormous diversity and flexibility. They are considered as “green vehicles” in drug delivery due to their extremely biocompatible, biodegradable and non-toxic characteristics. In addition, liposomes can be conveniently functionalized to engineer different chemical and physical characteristics. It has been consistently demonstrated that the uptake of drugs, encapsulated in liposomes, is higher in tumor cells when compared to that in the surrounding normal cells [7], [8]. Koganei et al. [63] have reported the synthesis of encapsulated mercaptoundecahydrododecaborate (BSH) with 10 % distearoyl boron lipid (DSBL) liposomes with a high boron content (boron/phosphorus ratio is 2.6) and, subsequently, used as BNCT carrier (Fig. 9a), thus demonstrating a high performance in boron delivery to tumor with the boron concentration of 174 ppm reaching at a dose of 50 mg B/kg. It was also reported that the functionalized liposomes exhibited excellent antitumor efficacy combining with a thermal neutron irradiation [(1.5~1.8)×10¹² neutrons/cm²] [63]. As shown in Fig. 9b, the tumors in mice (Balb/c, female, 6 weeks old, 14–20 g) bearing colon 26 solid tumors have completely disappeared 3 weeks after thermal neutron irradiation at an injection dose of 15 mg B/kg. It has been claimed that the encapsulated BSH with 10 % DSBL liposomes is potentially a promising BNCT candidate based on these results.

Fig. 9:

Structures of distearoylphosphatidylcholine (DSPC) and boron ion cluster lipids (a), and tumor volumes in mice bearing colon 26 solid tumor (b).

Kang et al. [64] demonstrated the conjugation of liposomes (LP, 124 nm) with a αvβ3 ligand and a cyclic arginine-glycine-aspartic acid-tyrosine-cysteine peptide (c(RGDyC)-LP) (1 % molar ratio) through thiol-maleimide coupling as shown in Fig. 10. When these functionalized liposomes are linked to a boron cage, they exhibited high potential for specific boron delivery to glioblastoma multiforme (GBM). Accordingly, A 70–80 % cell death was achieved with a thermal neutron flux of 4.5×10⁷ n·cm⁻²·s⁻¹ in 3 h [64]. A nanocomposite of the boron compound-attached poly(L-lactide-co-glycolide) (PLLGA) was reported by Takeuchi et al. [65]. The nanoparticles have the diameters of 100–150 nm in which the 100-nm particles reached a boron concentration of 113.9±15.8 μg/g of tissue in the tumor-bearing mice after 8 h of composite drug administration [65]. The tumor/blood ratio of boron concentration was greater than 5 after 8–12 h of injection. In addition, the boron atoms were excreted mainly in the urine. These results indicated that such type of nanoparticles is extremely useful for BNCT applications [65].

Fig. 10:

TEM images (a) and drug release profiles (b) from liposomes (LP) and c(RGDyC)-LP formulations. Release studies were performed at 37 °C in isotonic PBS (pH 7.4).