The medicinal chemistry of carboranes

Carboranyl-Chlorin e6 as a Potent Antimicrobial Photosensitizer

Antimicrobial photodynamic inactivation is currently being widely considered as alternative to antibiotic chemotherapy of infective diseases, attracting much attention to design of novel effective photosensitizers. Carboranyl-chlorin-e₆ (the conjugate of chlorin e₆ with carborane), applied here for the first time for antimicrobial photodynamic inactivation, appeared to be much stronger than chlorin e₆ against Gram-positive bacteria, such as Bacillus subtilis, Staphyllococcus aureus and Mycobacterium sp. Confocal fluorescence spectroscopy and membrane leakage experiments indicated that bacteria cell death upon photodynamic treatment with carboranyl-chlorin-e₆ is caused by loss of cell membrane integrity. The enhanced photobactericidal activity was attributed to the increased accumulation of the conjugate by bacterial cells, as evaluated both by centrifugation and fluorescence correlation spectroscopy. Gram-negative bacteria were rather resistant to antimicrobial photodynamic inactivation mediated by carboranyl-chlorin-e₆. Unlike chlorin e₆, the conjugate showed higher (compared to the wild-type strain) dark toxicity with Escherichia coli ΔtolC mutant, deficient in TolC-requiring multidrug efflux transporters.

Carborane-based design of a potent vitamin D receptor agonist

The development of a promising clinical antitumor vitamin D analog possessing a side-chain o-carborane cluster that efficiently binds to VDR by unconventional dihydrogen bonding (BH···HN) is described.

The vitamin D nuclear receptor (VDR) is a potential target for cancer therapy. It is expressed in many tumors and its ligand shows anticancer actions. To combine these properties with the application of boron neutron capture therapy (BNCT), we design and synthesize a potent VDR agonist based on the skeleton of the hormone 1α,25-dihydroxyvitamin D₃ (1,25D) and an o-carborane (dicarba-o-closo-1,2-dodecaborane) at the end of its side chain. The present ligand is the first secosteroidal analog with the carborane unit that efficiently binds to VDR and functions as an agonist with 1,25D-like potency in transcriptional assay on vitamin D target genes. Moreover it exhibits similar antiproliferative and pro-differentiating activities but is significantly less hypercalcemic than 1,25D. The crystal structure of its complex with VDR ligand binding domain reveals its binding mechanism involving boron-mediated dihydrogen bonds that mimic vitamin D hydroxyl interactions. In addition to the therapeutic interest, this study establishes the basis for the design of new unconventional vitamin D analogs containing carborane moieties for specific molecular recognition, and drug research and development.

One-pot synthesis of an indole-substituted 7,8-dicarba-nido-dodecahydroundecaborate(-1)

Carbaboranes are increasingly used as pharmacophores to replace phenyl substituents in established drug molecules. In contrast to traditional organic chemistry, elaborate procedures to introduce functionality frequently fail in the case of carbaboranes and their chemistry is often hampered by degradation of the cluster. Herein, the development of a one-pot synthesis of a water-soluble N-nido-dicarbaborato indole is presented, including a proposed mechanism for the reaction sequence. These studies provide useful synthetic tools for the conjugation of two important pharmacophores, indoles and carbaboranes.

Metal-induced B-H bond activation: reactions between half-sandwich Ir and Rh complexes with carboranylthioamide

Novel half-sandwich metal (Ir, Rh) complexes constructed from carboranylthioamide ligands containing an unexpected metal-boron bond were synthesized and characterized. The strong base n-butyllithium is demonstrated to be necessary in the reaction process.

Precious metal carborane polymer nanoparticles: characterisation of micellar formulations and anticancer activity

We report the encapsulation of highly hydrophobic 16-electron organometallic ruthenium and osmium carborane complexes [Ru/Os(p-cymene)(1,2-dicarba-closo-dodecarborane-1,2-dithiolate)] ( and ) in Pluronic® triblock copolymer P123 core-shell micelles. The spherical nanoparticles and , dispersed in water, were characterized by dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), and synchrotron small-angle X-ray scattering (SAXS; diameter ca. 15 and 19 nm, respectively). Complexes and were highly active towards A2780 human ovarian cancer cells (IC(50) 0.17 and 2.50 μM, respectively) and the encapsulated complexes, as and nanoparticles, were less potent (IC(50) 6.69 μM and 117.5 μM, respectively), but more selective towards cancer cells compared to normal cells.

Synthesis and 11C-Radiolabelling of 2-Carboranyl Benzothiazoles

Dicarba-closo-dodecaboranes, commonly known as carboranes, possess unique physico-chemical properties and can be used as hydrophobic moieties during the design of new drugs or radiotracers. In this work, we report the synthesis of two analogues of 2-(4-aminophenyl)benzothiazole (a compound that was found to elicit pronounced inhibitory effects against certain breast cancer cell lines in vitro) in which the phenyl ring has been substituted by a m-carborane cage. Two different synthetic strategies have been used. For the preparation of 1-(9-amino-1,7-dicarba-closo-dodecaboran-1-yl)-benzo-thiazole, the benzothiazole group was first introduced on one of the cluster carbon atoms of m-carborane and the amine group was further attached in three steps. For the synthesis of 1-(9-amino-1,7-dicarba-closo-dodecaboran-1-yl)-6-hydroxybenzothiazole, iodination was performed before introducing the benzothiazole group, and the amino group was subsequently introduced in six steps. Both compounds were radiolabelled with carbon-11 using [¹¹C]CH₃OTf as the labelling agent. Radiolabelling yields and radiochemical purities achieved should enable subsequent in vitro and in vivo investigations.

Carbaboranes – more than just phenyl mimetics

Dicarba-closo-dodecaboranes(12) (C2B10H12, carbaboranes) are highly hydrophobic and stable icosahedral carbon-containing boron clusters. The cage framework of these clusters can be modified with a variety of substituents, both at the carbon and at the boron atoms. Substituted carbaboranes are of interest in medicine as boron neutron capture therapy (BNCT) agents or as pharmacophores. High and selective accumulation in tumour cells is an important requirement for a BNCT agent and is achieved by incorporating boron-rich, water-soluble carbaborane derivatives into breast tumour-selective modified neuropeptide Y, [F7, P34]-NPY. Preliminary studies showed that the receptor binding affinity and signal transduction of the boron-modified peptides were very well retained. Use of carbaboranes as pharmacophores was shown by replacement of Bpa32 (Bpa=benzoylphenylalanine) in the reduced-size NPY analogue [Pro30, Nle31, Bpa32, Leu34]-NPY 28–36 by ortho-carbaboranyl propanoic acid. The inclusion of the carbaborane derivative resulted in a short NPY agonist with an interesting hY2R/hY4R preference. This might be a promising approach in the field of anti-obesity drug development.

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.

Palladium catalyzed selective mono-arylation of o-carboranes via B–H activation

Palladium catalyzed selective mono-arylation of o-carborane on B(8) and B(9).

The electronic structures and photophysical properties of platinum complexes with C^N^N ligands: the influence of the carborane substituent

Introduction ofo-carborane leads to a significant blue-shifted emission band and an obviously-higher electroluminescence efficiencyviasystemic and in-depth theoretical studies.

Syntheses and reductions of C-dimesitylboryl-1,2-dicarba-closo-dodecaboranes

An investigation ofC-dimesitylboryl-ortho-carboranes, 1-(BMes₂)-2-R-1,2-C₂B₁₀H₁₀(1and2), reveals that the carborane is the electron-acceptor and the mesityl group is the electron-donor in these dyads.

Palladium catalyzed regioselective mono-alkenylation of o-carboranes via Heck type coupling reaction of a cage B–H bond

Palladium catalyzed electrophilic B–H activation for regioselective mono-alkenylation of o-carborane on B(8) and B(9).

The hydroboration reaction as a key for a straightforward synthesis of new MRI-NCT agents

A new lipophilic NCT/MRI agent has been synthesised in only four steps and characterised from the relaxometric point of view. This compound shows a high affinity for LDLs that can be loaded with 300 complexes per particle.

Solid state, thermal synthesis of site-specific protein-boron cluster conjugates and their physicochemical and biochemical properties

BACKGROUND

Boron clusters represent a vast family of boron-rich compounds with extraordinary properties that provide the opportunity of exploitation in different areas of chemistry and biology. In addition, boron clusters are clinically used in boron neutron capture therapy (BNCT) of tumors. In this paper, a novel, in solid state (solvent free), thermal method for protein modification with boron clusters has been proposed.

METHODS

The method is based on a cyclic ether ring opening in oxonium adduct of cyclic ether and a boron cluster with nucleophilic centers of the protein. Lysozyme was used as the model protein, and the physicochemical and biological properties of the obtained conjugates were characterized.

RESULTS

The main residues of modification were identified as arginine-128 and threonine-51. No significant changes in the secondary or tertiary structures of the protein after tethering of the boron cluster were found using mass spectrometry and circular dichroism measurements. However, some changes in the intermolecular interactions and hydrodynamic and catalytic properties were observed.

CONCLUSIONS

To the best of our knowledge, we have described the first example of an application of cyclic ether ring opening in the oxonium adducts of a boron cluster for protein modification. In addition, a distinctive feature of the proposed approach is performing the reaction in solid state and at elevated temperature.

GENERAL SIGNIFICANCE

The proposed methodology provides a new route to protein modification with boron clusters and extends the range of innovative molecules available for biological and medical testing.

Regioselective insertion of o-carborynes into the α-C-H bond of tertiary amines: synthesis of α-carboranylated amines

o-Carboryne can undergo α-CH bond insertion with tertiary amines, thus affording α-carboranylated amines in very good regioselectivity and isolated yields. In this process, the nucleophilic addition of tertiary amines to the multiple bond of o-carboryne generates a zwitterionic intermediate. An intramolecular proton transfer, followed by a nucleophilic attack leads to the formation of the final product. Thus, regioselectivity is highly dependent upon the acidity of α-CH proton of tertiary amines. This approach serves as an efficient methodology for the preparation of a series of 1-aminoalkyl-o-carboranes.

Curing behaviour and thermal properties of epoxy resin cured by aromatic diamine with carborane

A new kind of carborane-containing aromatic diamine, 1,2-bis (4-aminophenyl)-1,2-dicarba-closo-dodecaborane (HPPA), as a curing agent for epoxy resins was synthesized and confirmed by proton, carbon and boron nuclear magnetic resonance imaging, Fourier transform infrared spectroscopy and elemental analysis. The compatibility, thermal curing behaviour and thermal properties of HPPA/epoxy resin E51 system were investigated. HPPA exhibited good compatibility with E51. Compared with 4,4′-diaminodiphenylsulphone (DDS)/E51, HPPA/E51 system had higher reaction activity and lower curing temperature. When the ratio of amine protons versus epoxy groups was 1:1 in resin systems, the epoxy resin cured by HPPA exhibited the highest glass transition temperature of 207°C. The residual weight ratios of cured HPPA/E51 at 800°C under argon and in air atmospheres were 43.7% and 50.5%, respectively, which were 28.6% and 47.2% higher than that of DDS/E51, respectively, indicating that the HPPA/E51 system had good heat resistance.

Tumor targeting, trifunctional dendritic wedge

We report in vitro and in vivo evaluation of a newly designed trifunctional theranostic agent for targeting solid tumors. This agent combines a dendritic wedge with high boron content for boron neutron capture therapy or boron MRI, a monomethine cyanine dye for visible-light fluorescent imaging, and an integrin ligand for efficient tumor targeting. We report photophysical properties of the new agent, its cellular uptake and in vitro targeting properties. Using live animal imaging and intravital microscopy (IVM) techniques, we observed a rapid accumulation of the agent and its retention for a prolonged period of time (up to 7 days) in fully established animal models of human melanoma and murine mammary adenocarcinoma. This macromolecular theranostic agent can be used for targeted delivery of high boron load into solid tumors for future applications in boron neutron capture therapy.

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.

A new approach to the synthesis of functional derivatives of nido-carborane: alkylation of [9-MeS-nido-7,8-C2B9H11]⁻

A series of asymmetrically substituted sulfonium derivatives of nido-carborane [9-R(Me)S-nido-7,8-C2B9H11] (R = Et, Pr, Bu, Bn, CH=CH2, CH2CH=CH2, CH2C≡CH, CH=C=CH2) were prepared by alkylation of the 9-methylthio-nido-7,8-carborane. The synthesized compounds are the first examples of diastereomers combining nido-carborane and sulfonium chiral centers.

Syntheses and structural characterization of o-carboranylamides with direct cage-amide bond

Reactions of lithio-o-carborane with isocyanates under various conditions were studied, and the structural features of the resulting carboranylamides are described. The reactions of o-carborane (o-C2B10H12), n-BuLi (two equiv.) and two equiv. of (substituted) phenylisocyanate, pentylisocyanate and p-ethylphenylthioisocyanate in diethyl ether, respectively, led, after workup, to the corresponding mono-substituted carboranylamide 2a-g and carboranylthioamide 5 in low to moderate yields, and only with RNCO (R = Ph, m-MeOC6H4, pentyl) could disubstituted products 3a-c be isolated. The reaction with phenylisocyanate afforded the mono-amide and di-amide products in a ratio of approximately 1 : 2, whereas in the other two reactions the ratios are approximately 4 : 1 and 3 : 2, respectively. In tetrahydrofuran all the reactions attempted with RNCO (R = Ph, p-IC6H4, m-NCC6H4 and pentyl) gave more monoamide products than those in diethyl ether. With phenylisocyanate no diamide product was isolated and with pentylisocyanate the ratio between monoamide and diamide is approximately 3.5 : 1. The new carboranylamides were characterized by means of elemental analyses, IR and NMR spectroscopy and mass spectrometry, as well as single-crystal X-ray diffraction analyses of 2a-f, 3a and 5.

Carborane-based carbonic anhydrase inhibitors: insight into CAII/CAIX specificity from a high-resolution crystal structure, modeling, and quantum chemical calculations

Carborane-based compounds are promising lead structures for development of inhibitors of carbonic anhydrases (CAs). Here, we report structural and computational analysis applicable to structure-based design of carborane compounds with selectivity toward the cancer-specific CAIX isoenzyme. We determined the crystal structure of CAII in complex with 1-methylenesulfamide-1,2-dicarba-closo-dodecaborane at 1.0 Å resolution and used this structure to model the 1-methylenesulfamide-1,2-dicarba-closo-dodecaborane interactions with CAIX. A virtual glycine scan revealed the contributions of individual residues to the energy of binding of 1-methylenesulfamide-1,2-dicarba-closo-dodecaborane to CAII and CAIX, respectively.

Utilization of Boron Compounds for the Modification of Suberoyl Anilide Hydroxamic Acid as Inhibitor of Histone Deacetylase Class II Homo sapiens

Histone deacetylase (HDAC) has a critical function in regulating gene expression. The inhibition of HDAC has developed as an interesting anticancer research area that targets biological processes such as cell cycle, apoptosis, and cell differentiation. In this study, an HDAC inhibitor that is available commercially, suberoyl anilide hydroxamic acid (SAHA), has been modified to improve its efficacy and reduce the side effects of the compound. Hydrophobic cap and zinc-binding group of these compounds were substituted with boron-based compounds, whereas the linker region was substituted with p-aminobenzoic acid. The molecular docking analysis resulted in 8 ligands with ΔG_binding value more negative than the standards, SAHA and trichostatin A (TSA). That ligands were analyzed based on the nature of QSAR, pharmacological properties, and ADME-Tox. It is conducted to obtain a potent inhibitor of HDAC class II Homo sapiens. The screening process result gave one best ligand, Nova2 (513246-99-6), which was then further studied by molecular dynamics simulations.