Boron neutron capture therapy demonstrated in mice bearing EMT6 tumors following selective delivery of boron by rationally designed liposomes

Design and synthesis of boron containing monosaccharides by the hydroboration of d-glucal for use in boron neutron capture therapy (BNCT)

Boron neutron capture therapy (BNCT) is one of the radiotherapies that involves the use of boron-containing compounds for the treatment of cancer. Boron-10 (¹⁰B) containing compounds that can accumulate in tumor tissue are expected to be suitable agents for BNCT. We report herein on the design and synthesis of some new BNCT agents based on a d-glucose scaffold, since glycoconjugation has been recognized as a useful strategy for the specific targeting of tumors. To introduce a boryl group into a d-glucose scaffold, we focused on the hydroboration of d-glucal derivatives, which have a double bond between the C1 and C2 positions. It was hypothesized that a C-B bond could be introduced at the C2 position of d-glucose by the hydroboration of d-glucal derivatives and that the products could be stabilized by conversion to the corresponding boronic acid ester. To test this hypothesis, we prepared some 2-boryl-1,2-dideoxy-d-glucose derivatives as boron carriers and evaluated their cytotoxicity and cellular uptake activity to cancer cells, especially under hypoxic conditions.

The Current Status and Perspectives of Delivery Strategy for Boron-based Drugs

Boron-containing compounds are essential micronutrients for animals and plants despite their low-level natural occurrence. They can strengthen the cell walls of the plants and they play important role in supporting bone health. However, surprisingly, boron-containing compounds are seldom found in pharmaceutical drugs. In fact, there are no inherent disadvantages reported so far in terms of the incorporation of boron into medicines. Indeed, drugs based on boron-containing compounds, such as tavaborole (marked name Kerydin) and bortezomib (trade name Velcade) have been investigated and they are used in clinical treatment. In addition, following the advanced development of boron neutron capture therapy and a new emerging proton boron fusion therapy, more boron-containing medicinals are to be expected. This review discusses the current status and perspectives of delivery strategy for boron-containing drugs.

Boron delivery agents for neutron capture therapy of cancer

Boron neutron capture therapy (BNCT) is a binary radiotherapeutic modality based on the nuclear capture and fission reactions that occur when the stable isotope, boron-10, is irradiated with neutrons to produce high energy alpha particles. This review will focus on tumor-targeting boron delivery agents that are an essential component of this binary system. Two low molecular weight boron-containing drugs currently are being used clinically, boronophenylalanine (BPA) and sodium borocaptate (BSH). Although they are far from being ideal, their therapeutic efficacy has been demonstrated in patients with high grade gliomas, recurrent tumors of the head and neck region, and a much smaller number with cutaneous and extra-cutaneous melanomas. Because of their limitations, great effort has been expended over the past 40 years to develop new boron delivery agents that have more favorable biodistribution and uptake for clinical use. These include boron-containing porphyrins, amino acids, polyamines, nucleosides, peptides, monoclonal antibodies, liposomes, nanoparticles of various types, boron cluster compounds and co-polymers. Currently, however, none of these have reached the stage where there is enough convincing data to warrant clinical biodistribution studies. Therefore, at present the best way to further improve the clinical efficacy of BNCT would be to optimize the dosing paradigms and delivery of BPA and BSH, either alone or in combination, with the hope that future research will identify new and better boron delivery agents for clinical use.

Demonstration of the bactericidal effects of the boron neutron capture reaction

This pilot study represents a paradigm shift, using BNCT for the treatment of bacterial overgrowth on surgically implanted medical devices. In this study, titanium diboride disks were inoculated with S. aureus and irradiated in a thermal neutron beam. After a delivery of 2.6 × 10¹² n/cm² the surviving fraction of S. aureus on an irradiated disk was 3.1 × 10^-5 when compared with non-irradiated controls. This pilot study demonstrates proof of principle of boron neutron capture therapy for infection control (BNCIC).

Cyclic-RGDyC functionalized liposomes for dual-targeting of tumor vasculature and cancer cells in glioblastoma: An in vitro boron neutron capture therapy study

The efficacy of boron neutron capture therapy depends on the selective delivery of ¹⁰B to the target. Integrins αvβ3 are transmembrane receptors over-expressed in both glioblastoma cells and its neovasculature. In this study, a novel approach to dual-target glioblastoma vasculature and tumor cells was investigated. Liposomes (124 nm) were conjugated with a αvβ3 ligand, cyclic arginine-glycine-aspartic acid-tyrosine-cysteine peptide (c(RGDyC)-LP) (1% molar ratio) through thiol-maleimide coupling. Expression of αvβ3 in glioblastoma cells (U87) and human umbilical vein endothelial cells (HUVEC), representing tumor angiogenesis, was determined using Western Blotting with other cells as references. The results showed that both U87 and HUVEC had stronger expression of αvβ3 than other cell types, and the degree of cellular uptake of c(RGDyC)-LP correlated with the αvβ3-expression levels of the cells. In contrast, control liposomes without c(RGDyC) showed little cellular uptake, regardless of cell type. In an in vitro boron neutron capture therapy study, the c(RGDyC)-LP containing sodium borocaptate generated more rapid and significant lethal effects to both U87 and HUVEC than the control liposomes and drug solution. Interestingly, neutron irradiated U87 and HUVEC showed different types of subsequent cell death. In conclusion, this study has demonstrated the potential of a new dual-targeting strategy using c(RGDyC)-LP to improve boron neutron capture therapy for glioblastoma.

Mechanism of generation of closo-decaborato amidrazones. Intramolecular non-covalent B–H⋯π(Ph) interaction determines stabilization of the configuration around the amidrazone CN bond

Three types of N(H)-nucleophiles were used to study the nucleophilic addition to the CN group of the 2-propanenitrilium closo-decaborate cluster giving N-closo-decaborato amidrazones.

Multiplexed imaging for diagnosis and therapy

Complex molecular and metabolic phenotypes depict cancers as a constellation of different diseases with common themes. Precision imaging of such phenotypes requires flexible and tunable modalities capable of identifying phenotypic fingerprints by using a restricted number of parameters while ensuring sensitivity to dynamic biological regulation. Common phenotypes can be detected by in vivo imaging technologies, and effectively define the emerging standards for disease classification and patient stratification in radiology. However, for the imaging data to accurately represent a complex fingerprint, the individual imaging parameters need to be measured and analysed in relation to their wider spatial and molecular context. In this respect, targeted palettes of molecular imaging probes facilitate the detection of heterogeneity in oncogene-driven alterations and their response to treatment, and lead to the expansion of rational-design elements for the combination of imaging experiments. In this Review, we evaluate criteria for conducting multiplexed imaging, and discuss its opportunities for improving patient diagnosis and the monitoring of therapy.

Validation and Comparison of the Therapeutic Efficacy of Boron Neutron Capture Therapy Mediated By Boron-Rich Liposomes in Multiple Murine Tumor Models

Boron neutron capture therapy (BNCT) was performed at the University of Missouri Research Reactor in mice bearing CT26 colon carcinoma flank tumors and the results were compared with previously performed studies with mice bearing EMT6 breast cancer flank tumors. Mice were implanted with CT26 tumors subcutaneously in the caudal flank and were given two separate tail vein injections of unilamellar liposomes composed of cholesterol, 1,2-distearoyl-sn-glycer-3-phosphocholine, and K[nido-7-CH₃(CH₂)₁₅–7,8-C₂B₉H₁₁] in the lipid bilayer and encapsulated Na₃[1-(2`-B₁₀H₉)-2-NH₃B₁₀H₈] within the liposomal core. Mice were irradiated 30 hours after the second injection in a thermal neutron beam for various lengths of time. The tumor size was monitored daily for 72 days. Despite relatively lower tumor boron concentrations, as compared to EMT6 tumors, a 45 minute neutron irradiation BNCT resulted in complete resolution of the tumors in 50% of treated mice, 50% of which never recurred. Median time to tumor volume tripling was 38 days in BNCT treated mice, 17 days in neutron-irradiated mice given no boron compounds, and 4 days in untreated controls. Tumor response in mice with CT26 colon carcinoma was markedly more pronounced than in previous reports of mice with EMT6 tumors, a difference which increased with dose. The slope of the dose response curve of CT26 colon carcinoma tumors is 1.05 times tumor growth delay per Gy compared to 0.09 times tumor growth delay per Gy for EMT6 tumors, indicating that inherent radiosensitivity of tumors plays a role in boron neutron capture therapy and should be considered in the development of clinical applications of BNCT in animals and man.

Lipid-membrane-incorporated arylboronate esters as agents for boron neutron capture therapy

An arylboronate ester bearing a methyl group at both of its ortho positions was stably incorporated into lipid membranes at high concentrations without hydrolysis.

Novel Convenient Synthesis of (10)B-Enriched Sodium Borohydride

A convenient and efficient synthesis of (10)B-enriched sodium borohydride [Na(10)BH4] from commercially available (10)B-enriched boric acid [(10)B(OH)3] is described. The reaction sequence (10)B(OH)3 → (10)B(On-Bu)3 → (10)BH3·Et3N → Na(10)BH4 afforded the product in 60-80% yield. The reaction was successfully scaled to hundreds of gram per run.

Hybrid Calcium Phosphate-Polymeric Micelles Incorporating Gadolinium Chelates for Imaging-Guided Gadolinium Neutron Capture Tumor Therapy

Gadolinium (Gd) chelates-loaded nanocarriers have high potential for achieving magnetic resonance imaging (MRI)-guided Gd neutron capture therapy (GdNCT) of tumors. Herein, we developed calcium phosphate micelles hybridized with PEG-polyanion block copolymers, and incorporated with the clinical MRI contrast agent Gd-diethylenetriaminepentaacetic acid (Gd-DTPA/CaP). The Gd-DTPA/CaP were nontoxic to cancer cells at the concentration of 100 μM based on Gd-DTPA, while over 50% of the cancer cells were killed by thermal neutron irradiation at this concentration. Moreover, the Gd-DTPA/CaP showed a dramatically increased accumulation of Gd-DTPA in tumors, leading to the selective contrast enhancement of tumor tissues for precise tumor location by MRI. The enhanced tumor-to-blood distribution ratio of Gd-DTPA/CaP resulted in the effective suppression of tumor growth without loss of body weight, indicating the potential of Gd-DTPA/CaP for safe cancer treatment.

Nanoscaled boron-containing delivery systems and therapeutic agents for cancer treatment

Significant efforts have recently been made to develop nanoscaled boron-containing delivery systems for improving drug delivery in cancer therapy. On one hand, borate ester chemistry has shown importance in ligand-mediated tumor targeting owing to the recognition ability of boronic acid to polyol residues in cell membranes. In particular, the phenylboronic acid-functionalized nanocarriers for specific targeting to sialic acid groups which are overexpressed on tumor cells have made great achievements. On the other hand, nanoscaled boron neutron capture therapy agents show growing potential in efficiently transporting boron to tumor. The current review outlines the recent developments in the application of borate ester chemistry in tumor targeting by nanoparticles, then summarizes recent work on the development of boron-based nanomaterials as boron neutron capture therapy agents.

Surface activity and molecular organization of metallacarboranes at the air-water interface revealed by nonlinear optics

Because of their amphiphilic structure, surfactants adsorb at the water-air interface with their hydrophobic tails pointing out of the water and their polar heads plunging into the liquid phase. Unlike classical surfactants, metallabisdicarbollides (MCs) do not have a well-defined amphiphilic structure. They are nanometer-sized inorganic anions with an ellipsoidal shape composed of two carborane semicages sandwiching a metal ion. However, MCs have been shown to share many properties with surfactants, such as self-assembly in water (formation of micelles and vesicles), formation of lamellar lyotropic phases, and surface activity. By combining second harmonic generation and surface tension measurement, we show here that cobaltabis(dicarbollide) anion {[(C2B9H11)2Co](-) also named [COSAN](-)} with H(+) as a counterion, the most representative metallacarborane, adsorbs vertically at the water surface with its long axis normal to the surface. This vertical molecular orientation facilitates the formation of intermolecular and nonconventional dihydrogen bonds such as the B-H(δ-)···(δ+)H-C bond that has recently been proven to be at the origin of the self-assembly of MCs in water. Therefore, it appears here that lateral dihydrogen bonds are also involved in the surface activity of MCs.

Biological interaction of living cells with COSAN-based synthetic vesicles

Cobaltabisdicarbollide (COSAN) [3,3'-Co(1,2-C2B9H11)2](-), is a complex boron-based anion that has the unusual property of self-assembly into membranes and vesicles. These membranes have similar dimensions to biological membranes found in cells, and previously COSAN has been shown to pass through synthetic lipid membranes and those of living cells without causing breakdown of membrane barrier properties. Here, we investigate the interaction of this inorganic membrane system with living cells. We show that COSAN has no immediate effect on cell viability, and cells fully recover when COSAN is removed following exposure for hours to days. COSAN elicits a range of cell biological effects, including altered cell morphology, inhibition of cell growth and, in some cases, apoptosis. These observations reveal a new biology at the interface between inorganic, synthetic COSAN membranes and naturally occurring biological membranes.

A theranostic approach based on the use of a dual boron/Gd agent to improve the efficacy of Boron Neutron Capture Therapy in the lung cancer treatment

This study aims at developing an innovative theranostic approach for lung tumor and metastases treatment, based on Boron Neutron Capture Therapy (BNCT). It relies on to the use of low density lipoproteins (LDL) as carriers able to maximize the selective uptake of boron atoms in tumor cells and, at the same time, to quantify the in vivo boron distribution by magnetic resonance imaging (MRI). Tumor cells uptake was initially assessed by ICP-MS and MRI on four types of tumor (TUBO, B16-F10, MCF-7, A549) and one healthy (N-MUG) cell lines. Lung metastases were generated by intravenous injection of a Her2+ breast cancer cell line (i.e. TUBO) in BALB/c mice and transgenic EML4-ALK mice were used as primary tumor model. After neutron irradiation, tumor growth was followed for 30-40 days by MRI. Tumor masses of boron treated mice increased markedly slowly than the control group. From the clinical editor: In this article, the authors described an improvement to existing boron neutron capture therapy. The dual MRI/BNCT agent, carried by LDLs, was able to maximize the selective uptake of boron in tumor cells, and, at the same time, quantify boron distribution in tumor and in other tissues using MRI. Subsequent in vitro and in vivo experiments showed tumor cell killing after neutron irradiation.

Synthesis of oligo-closo-dodecaborates by Hüisgen click reaction as encapsulated agents for the preparation of high-boron-content liposomes for neutron capture therapy

High boron-content molecules and their encapsulated liposomes.

Harnessing medically relevant metals onto water-soluble subphthalocyanines: towards bimodal imaging and theranostics

Water-soluble subphthalocyanine (SubPc) - chelating ligands (DOTA, DTPA) conjugates were complexed with six metals relevant to medical imaging/therapies (MRI, PET, SPECT, RIT, NCT). Magneto-optical properties of the ditopic Gd complex and cellular microscopy studies were reported.

Synthesis of a carborane-containing cholesterol derivative and evaluation as a potential dual agent for MRI/BNCT applications

In this study the synthesis and characterization of a new dual, imaging and therapeutic, agent is proposed with the aim of improving the efficacy of Boron Neutron Capture Therapy (BNCT) in cancer treatment. The agent (Gd-B-AC01) consists of a carborane unit (ten boron atoms) bearing a cholesterol unit on one side (to pursue the incorporation into the liposome bi-layer) and a Gd(iii)/1,4,7,10-tetraazacyclododecane monoamide complex on the other side (as a MRI reporter to attain the quantification of the B/Gd concentration). In order to endow the BNCT agent with specific delivery properties, the liposome embedded with the MRI/BNCT dual probes has been functionalized with a pegylated phospholipid containing a folic acid residue at the end of the PEG chain. The vector allows the binding of the liposome to folate receptors that are overexpressed in many tumor types, and in particular, in human ovarian cancer cells (IGROV-1). An in vitro test on IGROV-1 cells demonstrated that Gd-B-AC01 loaded liposomes are efficient carriers for the delivery of the MRI/BNCT probes to the tumor cells. Finally, the BNCT treatment of IGROV-1 cells showed that the number of surviving cells was markedly smaller when the cells were irradiated after internalization of the folate-targeted GdB10-AC01/liposomes.

Therapeutic efficacy of boron neutron capture therapy mediated by boron-rich liposomes for oral cancer in the hamster cheek pouch model

The application of boron neutron capture therapy (BNCT) mediated by liposomes containing (10)B-enriched polyhedral borane and carborane derivatives for the treatment of head and neck cancer in the hamster cheek pouch oral cancer model is presented. These liposomes are composed of an equimolar ratio of cholesterol and 1,2-distearoyl-sn-glycero-3-phosphocholine, incorporating K[nido-7-CH3(CH2)15-7,8-C2B9H11] (MAC) in the bilayer membrane while encapsulating the hydrophilic species Na3[ae-B20H17NH3] (TAC) in the aqueous core. Unilamellar liposomes with a mean diameter of 83 nm were administered i.v. in hamsters. After 48 h, the boron concentration in tumors was 67 ± 16 ppm whereas the precancerous tissue contained 11 ± 6 ppm, and the tumor/normal pouch tissue boron concentration ratio was 10:1. Neutron irradiation giving a 5-Gy dose to precancerous tissue (corresponding to 21 Gy in tumor) resulted in an overall tumor response (OR) of 70% after a 4-wk posttreatment period. In contrast, the beam-only protocol gave an OR rate of only 28%. Once-repeated BNCT treatment with readministration of liposomes at an interval of 4, 6, or 8 wk resulted in OR rates of 70-88%, of which the complete response ranged from 37% to 52%. Because of the good therapeutic outcome, it was possible to extend the follow-up of BNCT treatment groups to 16 wk after the first treatment. No radiotoxicity to normal tissue was observed. A salient advantage of these liposomes was that only mild mucositis was observed in dose-limiting precancerous tissue with a sustained tumor response of 70-88%.

Hemorrhage in mouse tumors induced by dodecaborate cluster lipids intended for boron neutron capture therapy

The potential of boron-containing lipids with three different structures, which were intended for use in boron neutron capture therapy, was investigated. All three types of boron lipids contained the anionic dodecaborate cluster as the headgroup. Their effects on two different tumor models in mice following intravenous injection were tested; for this, liposomes with boron lipid, distearoyl phosphatidylcholine, and cholesterol as helper lipids, and containing a polyethylene glycol lipid for steric protection, were administered intravenously into tumor-bearing mice (C3H mice for SCCVII squamous cell carcinoma and BALB/c mice for CT26/WT colon carcinoma). With the exception of one lipid (B-THF-14), the lipids were well tolerated, and no other animal was lost due to systemic toxicity. The lipid which led to death was not found to be much more toxic in cell culture than the other boron lipids. All of the lipids that were well tolerated showed hemorrhage in both tumor models within a few hours after administration. The hemorrhage could be seen by in vivo magnetic resonance and histology, and was found to occur within a few hours. The degree of hemorrhage depended on the amount of boron administered and on the tumor model. The observed unwanted effect of the lipids precludes their use in boron neutron capture therapy.

High yielding synthesis of carboranes under mild reaction conditions using a homogeneous silver(I) catalyst: direct evidence of a bimetallic intermediate

Methods used to prepare functionalized carboranes generally require heating to high temperatures, and thus limits the range of derivatives which can be prepared directly from alkynes. We show here that by using a homogeneous silver(I) catalyst it is now possible to prepare carboranes in good to excellent yield at temperatures below 40 °C, including at room temperature. The process is general and provides an important new synthetic strategy for the preparation of functionalized boron clusters.

Preparation and evaluation of carborane-derived inhibitors of prostate specific membrane antigen (PSMA)

Carborane-derived inhibitors of prostate specific membrane antigen are reported. Compounds were prepared from C-hydroxy-carboranes and screened in vitro and in vivo.

Spermidinium closo-dodecaborate-encapsulating liposomes as efficient boron delivery vehicles for neutron capture therapy

Significant anti-tumor effects and longer survivals were observed in the tumor-bearing mice treated with spermidinium closo-dodecaborate-encapsulating liposomes after thermal neutron irradiation.

[Development of high boron content liposomes and their promising antitumor effect for neutron capture therapy]

  High accumulation and selective delivery of boron into tumor tissue are the most important requirements to achieve the efficient cell-killing effect of boron neutron capture therapy (BNCT) that relies on the nuclear reaction of two essentially nontoxic species, boron-10 ((10)B) and thermal neutrons in boron-loaded tissues. Recent development of boron cluster lipids and their liposomal boron delivery system (BDS) are summarized in this article. Boron compounds that have no affinity to tumor can potentially be delivered to tumor tissues by liposomes, therefore, liposomal BDS would be one of the most attractive approaches for efficient BNCT of various cancers. There are two approaches for BDS: encapsulation of boron compounds into liposomes and incorporation of boron-conjugated lipids into the liposomal bilayer. The combination of both approaches has a potential for reduction of the total dose of liposomes without reducing the efficacy of BNCT.