Potential Boron Neutron Capture Therapy Research Articles

1,2-Diphenyl-o-carborane and Its Chromium Derivatives: Synthesis, Characterization, X-ray Structural Studies, and Biological Evaluations.

The objective of this study is to design and synthesize substituted η6-chromium(0) tricarbonyl metal complexes carrying o-carborane units as potential boron neutron capture therapy (BNCT) agents. In this study, 1,2-diphenyl-o-carborane (1) units were used as starting materials to generate biologically active species. We investigated how the structural changes of 1 substituted with chromium(0) tricarbonyl affect the biological properties, and 1-(Phenyl-η6-chromium(0) tricarbonyl)-2-phenyl-o-carborane (2) and 1,2-bis(phenyl-η6-chromium(0) tricarbonyl)-o-carborane (3) species were produced in moderate yields. The molecular structures of compounds 1-3 were identified and established by infrared (IR); 1H, 11B, and 13C nuclear magnetic resonance (NMR) and X-ray crystallography analyses. Crystal structures of 1,2-diphenyl-o-carborane and the corresponding chromium complexes 1, 2, and 3 were obtained. In an in vitro study using B16 and CT26 cancer cells containing the triphenyl-o-carboranyl chromium(0) complexes Ph3C2BCr2 and Ph3C2BCr3, which we reported previously, compounds 2 and 3 accumulated at higher levels than compounds Ph3C2BCr2 and Ph3C2BCr3. However, the phenylated o-carboranyl chromium complexes have been found to be more cytotoxic than p-boronophenylalanine (BPA).

A Potential Boron Neutron Capture Therapy Agent Selectively Suppresses High-Grade Glioma: In Vitro and in Vivo Exploration.

Glioblastoma (GBM), as the most central nervous system (CNS) intractable disease, has spoiled millions of lives due to its high mortality. Even though several efforts have been made, the existing treatments have had limited success. In this sense, we studied a lead compound, the boron-rich selective epidermal growth factor receptor (EGFR)-inhibitor hybrid 1, as a potential drug for GBM treatment. For this end, we analyzed the in vitro activity of hybrid 1 in a glioma/primary astrocytes coculture, studying cellular death types triggered by treatment with this compound and its cellular localizations. Additionally, hybrid 1 concentrated boron in glioma cells selectively and more effectively than the boron neutron capture therapy (BNCT)-clinical agent 10B-l-boronophenylalanine and thus displayed a better in vitro-BNCT effect. This encouraged us to analyze hybrid 1 in vivo. Therefore, immunosuppressed mice bearing U87 MG human GBM were treated with both 1 and 1 encapsulated in a modified liposome (recognized by brain-blood barrier peptide transporters), and we observed a potent in vivo per se antitumor activity (tumor size decrease and animal survival increase). These data demonstrate that 1 could be a promising new targeted therapy for GBM.

Bis(CBT)palladium(II) Derivatives (CBT = m-carborane-1-thiolate): Synthesis, Molecular Structure, and Physicochemical Properties of cis-[(bipy)Pd(CBT)2] and trans-[(py)2Pd(CBT)2

The new synthesized PdII complex cis-[(bipy)Pd(CBT)2] (bipy = 2,2'-bipyridyl; CBT = m-carborane-1-thiolate anion), which is a potential BNCT (boron neutron capture therapy) agent and of structure elucidated by single-crystal X-ray work, has been studied by infrared (IR) and ultraviolet-visible light (UV-vis) spectra and its properties compared with those of the previously reported and also the structurally characterized analogue trans-[(py)2Pd(CBT)2]. This trans species, prepared via a direct method, was previously isolated from a pyridine solution, consequent to the occurring releasing of the external Pd(CBT)2 moieties of the porphyrazine macrocycle [{Pd(CBT)2}4LZn]·xH2O (L = tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazinato anion), which is an active photosensitizer in photodynamic therapy (PDT) and a potential bimodal PDT/BNCT agent. The UV-vis spectral behavior of both cis and trans species in CHCl3 solution and in the gas phase has been examined in detail by density functional theory (DFT) and time-dependent density functional theory (TDDFT) studies devoted to explain their distinct behavior observed in the region of 400-500 nm, as determined by the presence in the cis structure of a vicinal arrangement of the two CBT groups, an ensemble of results closely similar to those observed for the macrocycles [{Pd(CBT)2}4LM]·xH2O (M = MgII(H2O), ZnII, PdII). It has also been experimentally proved the tendency of the cis isomer in CHCl3/pyridine solution to be changed to the respective trans analogue, with conversion occurring in two steps, as interpreted by detailed DFT studies.

Synthesis and Characterization of Carborane-Functionalized Hyperbranched Polyester

The purpose of this study is to synthesize new boron containing carrier compound for Boron Neutron Capture Therapy (BNCT) which is two-component radiation therapy method that especially is promising for the treatment of brain tumors and is being actively researched in many countries. For this research hyperbranched polyester (HBP), bifunctional p-carborane and carborane-functionalized HBP were synthesized. Dipentaerythritol was chosen as a core molecule of the HBP and esterified with dimethylol propionic acid. Observed characteristic ester bands and OH stretching band in the FTIR spectrum and the methyl & methylene peaks in the 1H NMR spectrum of HBP indicated that hyperbranched polyester synthesis was done successfully. A bifunctionalized p-carborane containing an acid group and a benzyl ether protected alcohol was prepared in three-step reactions. As a result of these reactions, a bifunctionalized p-carborane compound was produced with 60% yield. Then, the HBP was esterified with the bifunctional p-carborane. The characterization of the synthesized compounds was determined by FTIR and NMR spectra. The synthesis of carborane-functionalized HBP was confirmed by disappearance of HBP's OH groups and B-H stretching band observed in the FTIR spectrum of carborane-functionalized HBP and in addition, appearance of proton signals of HBP core, carborane linker and peripheral-protecting groups in the 1H-NMR spectrum. Finally, synthesized water-soluble carborane-containing HB carrying many boron atoms should be served as potential BNCT agents.

Ru(ii) and Ir(iii) phenanthroline-based photosensitisers bearingo-carborane: PDT agents with boron carriers for potential BNCT

Four novel transition metal-carborane photosensitisers were prepared by Sonogashira cross-coupling of 1-(4-ethynylbenzyl)-2-methyl-o-carborane (A-CB) with halogenated Ru(ii)- or Ir(iii)-phenanthroline complexes. The resulting boron-rich complexes with one (RuCB and IrCB) or two carborane cages (RuCB2 and IrCB2) were spectroscopically characterised, and their photophysical properties investigated. RuCB displayed the most attractive photophysical properties in solution (λem 635 nm, τT 2.53 μs, and φp 20.4%). Nanosecond time-resolved transient absorption studies were used to explore the 3MLCT nature of the triplet excited states, and the highest singlet oxygen quantum yields (ΦΔ) were obtained for the mono-carborane-phenanthroline complexes (RuCB: 52% and IrCB: 25%). None of the complexes produce dark toxicity in SKBR-3 cells after incubation under photodynamic therapy (PDT) conditions. Remarkably, mono-carboranes RuCB and IrCB were the best internalised by the SKBR-3 cells, demonstrating the first examples of tris-bidentate transition metal-carborane complexes acting as triplet photosensitisers for PDT with a high photoactivity; RuCB or IrCB killed ∼50% of SKBR-3 cells at 10 μM after irradiation. Therefore, the high-boron content and the photoactive properties of these photosensitisers make them potential candidates as dual anti-cancer agents for PDT and Boron Neutron Capture Therapy (BNCT).

Modular triazine-based carborane-containing carboxylic acids - synthesis and characterisation of potential boron neutron capture therapy agents made of readily accessible building blocks.

Based on a modular combination of s-triazine, the well-known 9-mercapto-1,7-dicarba-closo-dodecaborane(12) and commercially available carboxylic acids, namely thioglycolic acid, glycine, and Nα-Boc-l-lysine, several carboxylic acid derivatives were synthesised and fully characterised. The thioglycolic acid derivative was introduced into a peptide hormone by solid phase peptide synthesis. High activity and selective internalisation into peptide receptor-expressing cells was observed. With a very high boron content of twenty boron atoms, these derivatives are interesting as selective Boron Neutron Capture Therapy (BNCT) agents.

High Boron-loaded DNA-Oligomers as Potential Boron Neutron Capture Therapy and Antisense Oligonucleotide Dual-Action Anticancer Agents.

Boron cluster-modified therapeutic nucleic acids with improved properties are of interest in gene therapy and in cancer boron neutron capture therapy (BNCT). High metallacarborane-loaded antisense oligonucleotides (ASOs) targeting epidermal growth factor receptor (EGFR) were synthesized through post-synthetic Cu (I)-assisted “click” conjugation of alkyne-modified DNA-oligonucleotides with a boron cluster alkyl azide component. The obtained oligomers exhibited increased lipophilicity compared to their non-modified precursors, while their binding affinity to complementary DNA and RNA strands was slightly decreased. Multiple metallacarborane residues present in the oligonucleotide chain, each containing 18 B-H groups, enabled the use of IR spectroscopy as a convenient analytical method for these oligomers based on the diagnostic B-H signal at 2400–2650 cm−1. The silencing activity of boron cluster-modified ASOs used at higher concentrations was similar to that of unmodified oligonucleotides. The screened ASOs, when used in low concentrations (up to 50 μM), exhibited pro-oxidative properties by inducing ROS production and an increase in mitochondrial activities in HeLa cells. In contrast, when used at higher concentrations, the ASOs exhibited anti-oxidative properties by lowering ROS species levels. In the HeLa cells (tested in the MTT assay) treated (without lipofectamine) or transfected with the screened compounds, the mitochondrial activity remained equal to the control level or only slightly changed (±30%). These findings may be useful in the design of dual-action boron cluster-modified therapeutic nucleic acids with combined antisense and anti-oxidant properties.

Synthesis of Sugar-Boronic Acid Derivatives: A Class of Potential Agents for Boron Neutron Capture Therapy.

To date, sugar analogues that contain boronic acids as substitutes for hydroxyl groups are a class of compounds nearly unknown in the literature. The challenging synthesis of two sugar-boronic acid analogues is described, and data are retrieved on their solution behavior, stability, and toxicity. As these compounds were expected to mimic the behavior of carbohydrates, they were tested in regards to their future development as potential boron neutron capture therapy agents.

Study of the tissue distribution of potential boron neutron-capture therapy agents based on conjugates of chlorin e 6 aminoamide derivatives with boron nanoparticles

Boron neutron-capture therapy of cancer is based on the ability of the 10B isotope to capture thermal neutrons; it is one of the most promising techniques in radiation therapy. The high content and selective accumulation of 10B in the tumor tissue are the most important prerequisites for its efficacy. The purpose of this study was to determine the biodistribution of cobalt bis(dicarbollide) conjugates with chlorin e6 amino amide derivatives. Experiments were carried out in Balb/c mice with transplanted CT-26 murine colon carcinoma. Boron-containing conjugates were injected into the tail vein at a dose of 10 mg/kg. The conjugate accumulation in tumor tissue and organs was studied by laser scanning microscopy. Excitation was performed at the wavelength of 514 nm; the signals were recorded in the range of 560–710 nm in increments of 10 nm. To evaluate the amount of the boron conjugate, we calculated the intensity of the fluorescence signal of the samples under investigation. At 3 h after administration of the agent, a high level of fluorescence was observed in the liver, spleen, and lung. The tumor/muscle accumulation ratio was approximately 3. The study demonstrated that boron derivatives of chlorin e6 are promising agents for boron neutron-capture therapy.

Biological activity of N(4)-boronated derivatives of 2′-deoxycytidine, potential agents for boron-neutron capture therapy

Boron-neutron capture therapy (BNCT) is a binary anticancer therapy that requires boron compound for nuclear reaction during which high energy alpha particles and lithium nuclei are formed. Unnatural, boron-containing nucleoside with hydrophobic pinacol moiety was investigated as a potential BNCT boron delivery agent. Biological properties of this compound are presented for the first time and prove that boron nucleoside has low cytotoxicity and that observed apoptotic effects suggest alteration of important functions of cancer cells. Mass spectrometry analysis of DNA from cancer cells proved that boron nucleoside is inserted into nucleic acids as a functional nucleotide derivative. NMR studies present very high degree of similarity of natural dG–dC base pair with dG–boron nucleoside system.

One Stone Kills Three Birds: Novel Boron-Containing Vesicles for Potential BNCT, Controlled Drug Release, and Diagnostic Imaging

A new conjugate polymer was prepared by an efficient thiol-ene coupling of one carborane with a linear PEG chain (Mn = 2,000 g/mol), and each carborane was further labeled with a fluorescence rhodamine dye. Such a novel polymer can associate in water to form narrowly distributed spherical vesicles, which were characterized using a range of methods, including laser light scattering, confocal laser scanning microscopy, and TEM. The vesicular structure is potentially multifunctional in biomedical applications, namely, serving as a boron neutron capture therapy (BNCT) agent, a hydrophilic drug carrier, and a diagnostic imaging fluorescent probe. As expected, either cleaving the thiol-ene linked PEO chain by esterase or destroying carborane by neutron irradiation results in a dismantlement of such a vesicle structure to release its encapsulated drugs. Its potential biomedical applications have been evaluated in vitro and in vivo. Our preliminary results reveal that these small vesicles can be quickly taken up by cells and have an enhanced stability in the bloodstream so that their targeting to specific cancer cells becomes feasible.

Boronated Carbohydrate Derivatives as Potential Boron Neutron Capture Therapy Reagents

The treatment of cancer remains one of the most challenging problems for humanity. Boron neutron capture therapy is a binary approach for cancer treatment that is particularly attractive in treating high-grade gliomas and metastatic brain tumors. Among the types of boron-containing molecules used as boron neutron capture therapy agents, boronated carbohydrate derivatives have received significant attention because of their preferential uptake by growing tumor cells. This review provides a summary of the recent developments in the chemistry of carborane-containing carbohydrates.

Synthesis and Biological Data of ortho-Carborane Analogs from 4-Aminobenzoic Acid and 4-Hydroxybenzoic Acid as a Potential Boron Neutron Capture Therapy Agent

Department of Chemistry, Faculty of Science, Gakushuin University, Tokyo 171-8588, JapanReceived October 8, 2010, Accepted April 28, 2011Key Words : Boron neutron capture therapy (BNCT), ortho-carborane, 4-Aminobenzoic acid, 4-Hydroxyben-zoic acid, Glutamic acidBoron neutron capture therapy (BNCT) has recentlyreceived considerable attention due to a highly selectivetherapy for treating cancer in clinical trial.

Carborane-Conjugated 2-Quinolinecarboxamide Ligands of the Translocator Protein for Boron Neutron Capture Therapy

Potential boron neutron capture therapy (BNCT) agents have been designed on the basis of the evidence about translocator protein (TSPO) overexpression on the outer mitochondrial membrane of tumor cells. The structure of the first TSPO ligand bearing a carborane cage (compound 2d) has been modified in order to find a suitable candidate for in vivo studies. The designed compounds were synthesized and evaluated for their potential interaction with TSPO and tumor cells. In vitro biological evaluation showed in the case of fluoromethyl derivative 4b a nanomolar TSPO affinity very similar to that of 2d, a significantly lower cytotoxicity, and a slightly superior performance as boron carrier toward breast cancer cells. Moreover, compound 4b could be used as a ¹⁹F magnetic resonance imaging (MRI) agent as well as labeled with ¹¹C or ¹⁸F to obtain positron emission tomography (PET) radiotracers in order to apply the "see and treat" strategy in BNCT.

In vitro neutron irradiation of glioma and endothelial cultured cells

To fully develop its potential boron neutron capture therapy (BNCT) requires the combination of a suitable thermal/epithermal neutron flux together with a selective intake of 10B-boron nuclei in the target tissue. The latter condition is the most critical to be realized as none of the boron carriers used for experimental or clinical purposes proved at the moment an optimal selectivity for cancer cells compared to normal cells. In addition to complex physical factors, the assessment of the intracellular concentration of boron represent a crucial parameter to predict the dose delivered to the cancer cells during the treatment. Nowadays the dosimetry calculation and then the prediction of the treatment effectiveness are made using Monte Carlo simulations, but some of the model assumption are still uncertain: the radiobiological dose efficacy and the probability of tumour cell survival are crucial parameters that needs a more reliable experimental approach. The aim of this work was to evaluate the differential ability of two cell lines to selectively concentrate the boron-10 administered as di-hydroxyboryl-phenylalanine (BPA)-fructose adduct, and the effect of the differential boron intake on the damage produced by the irradiation with thermal neutrons; the two cell lines were selected to be representative one of normal tissues involved in the active/passive transport of boron carriers, and one of the tumour. Recent in vitro studies demonstrated how BPA is taken by proliferating cells, however the mechanism of BPA uptake and the parameters driving the kinetics of influx and the elimination of BPA are still not clarified. In these preliminary studies we analysed the survival of F98 and human umbilical vein endothelial cells (HUVEC) cells line after irradiation, using different thermal fluencies at the same level of density population and boron concentration in the growing medium prior the irradiation. This is first study performed on endothelium model obtained by a primary human cell line (HUVEC). The perspective application of this work is to develop a model able to foresee the effects produced by different combination of boron influx with a thermal neutron fluencies, applying a standardized radiobiological methodology, and in particular to continue the investigation of the radiobiological effects on the endothelium model as the main tissue involved in the transport of boronated molecules.

Neutral and polyanionic carboranylporphyrazines: Synthesis and physico-chemical properties

Among the challenges precluding the widespread use of boron neutron capture therapy (BNCT) has been the difficulty in achieving selective delivery of large quantities of boron to malignant cells. In an attempt to address this issue we have recently developed an effective strategy to synthesize a new family of boronated porphyrazines to be delivered through the membrane of cancerous tissues as such or with the help of liposomes. Herein we describe the synthesis and the basic physico-chemical properties of neutral octa- closo-carboranyl-alkylthio-porphyrazines as well as of their water-soluble counterparts obtained by mild deboronation of the closo-polyedra. Preliminary studies indicate that these compounds show negligible cell toxicity and, compared with BPA, good cellular uptake. This encourages further studies for their evaluation as potential BNCT agents.

Synthesis of stable platinum complexes containing carborane in a carrier group for potential BNCT agents

A series of stable platinum complexes containing closo-carborane cages in a 'carrier group', which could be potentially utilized for boron neutron capture therapy (BNCT), was successfully synthesized and characterized by various spectroscopic methods. Two ortho-closo-carborane cages were connected to the 3,3' or 4,4'-positions of 2,2'-bipyridine through ester bonds to give three carborane-linked bipyridine ligands, 3,3'-(3-(1,2-dicarba-closo-dodecaborane-1-yl)propoxycarbonyl)-2,2'-bipyridine (3) and n,n-(3-(1-methyl-1,2-dicarba-closo-dodecaborane-1-yl)propoxycarbonyl)-2,2-bipyridine (n = 3 4; n = 4 5). The prepared ligands successfully replaced the labile benzonitrile groups of [Pt(PhCN)2Cl2] to afford three cisplatin analogs [Pt(L)Cl2] (L = 3-5). In order to increase their aqueous solubility, three carboplatin analogs, [Pt(L)(CBDCA), L = 4 and 5, CBDCA = 1,1-cyclobutanedicarboxylate] and [Pt( 4)(oxalate)], were synthesized by employing [Pt(DMSO)2(CBDCA)] or [Pt(DMSO)2(oxalate)] as a starting platinum(II) source. Two platinum(IV) complexes [Pt(L)Cl4, L = 4 and 5] were also successfully prepared from Na2[PtCl6] x 6H2O. All of the synthesized platinum complexes containing closo-carboranes exhibited excellent stability in both the solid and solution states and could be stored in air for several months. Even though the three closo-carborane ligands were successfully deboronated to the corresponding nido-mono anions by employing a mild degradation method involving CsF/ethanol, all attempts at the incorporation of nido-carborane-linked bipyridine into the platinum complexes failed, leading to the formation of black precipitates of reduced platinum.

Synthesis of a series of boronated unnatural cyclic amino acids as potential boron neutron capture therapy agents

AbstractNew boronated unnatural cyclic amino acids, 1–6, were synthesized for potential use in neutron capture therapy. In order to understand the effect of molecular lipophilicity on the biological activity, different linkers were introduced between the boronic acid and 1‐aminocycloalkanecarboxylic acid moieties. The key step in the syntheses was the preparation of a series of alkenyl‐substituted cycloalkanones, which were subsequently converted to amino acids via the Bücherer–Strecker reaction. The introduction of the boronic acid function into hydrantoins 19–24 was realized by hydroboration reactions using diisopinocampheylborane (Ipc2BH). The target boronated amino acids were modeled after 1‐aminocyclobutanecarboxylic acid and 1‐amino‐3‐boronocyclopentanecarboxylic acid, which have previously demonstrated high uptake in tumors. Copyright © 2008 John Wiley & Sons, Ltd.

Boronated Thymidine Analogues for Boron Neutron Capture Therapy

Concise synthetic methods for synthesizing 3-carboranyl thymidine analogues (3CTAs) modified with cyclic and acyclic alcohols have been developed. The synthesis of these potential boron neutron capture therapy (BNCT) agents and their preliminary biological evaluation is described.

Synthesis of THIQ Derivatives as Potential Boron Neutron Capture Therapy Agents: N-Functionalizedo-Carboranylmethyl Benzopiperidines

A method for synthesizing o-carborane substituted tetrahydroisoquinolines containing a polar functional group such as sulfonic or phosphoric acid on the nitrogen atom of the piperidine ring, starting from N-(2-arylethyl)sulfamic acid or 2-arylethylamidophosphate, is described. In vitro studies showed that the desired compounds 7a and 10b accumulate to high levels in B-16 melanoma cells despite low cytotoxicity.