New Boron Containing Acridines: Synthesis and Preliminary Biological Study.

The synthesis of the first conjugates of acridine with cobalt bis(dicarbollide) are reported. A novel 9-azido derivative of acridine was prepared through the reaction of 9-methoxyacridine with N3CH2CH2NH2, and its solid-state molecular structure was determined via single-crystal X-ray diffraction. The azidoacridine was used in a copper (I)-catalyzed azide-alkyne cycloaddition reaction with cobalt bis(dicarbollide)-based terminal alkynes to give the target 1,2,3-triazoles. DNA interaction studies via absorbance spectroscopy showed the weak binding of the obtained conjugates with DNA. The antiproliferative activity (IC50) of the boronated conjugates against a series of human cell lines was evaluated through an MTT assay. The results suggested that acridine derivatives of cobalt bis(dicarbollide) might serve as a novel scaffold for the future development of new agents for boron neutron capture therapy (BNCT).

Boron-Cluster Embedded Necklace-Shaped Nanohoops.

The combination of carbon-based nanohoops with other functional organic molecular structures should lead to the design of new molecular configurations with interesting properties. Here, necklace-like nanohoops embedded with carborane were synthesized for the first time. The unique deboronization of o-carborane has led to the facile preparation of ionic nanohoop compounds. Nanohoops functionalized by nido-o-carborane show excellent fluorescence emission, with a solution quantum yield of up to 90.0 % in THF and a solid-state quantum efficiency of 87.3 %, which opens an avenue for the applications of the nanohoops in OLEDs and bioimaging.

New Azido Coumarins as Potential Agents for Fluorescent Labeling and Their "Click" Chemistry Reactions for the Conjugation with closo-Dodecaborate Anion.

Novel fluorescent 7-methoxy- and 7-(diethylamino)-coumarins modified with azido-group on the side chain have been synthesized. Their photophysical properties and single crystals structure characteristics have been studied. In order to demonstrate the possibilities of fluorescent labeling, obtained coumarins have been tested with closo-dodecaborate derivative bearing terminal alkynyl group. CuI catalyzed Huisgen 1,3-dipolar cycloaddition reaction has led to fluorescent conjugates formation. The absorption-emission spectra of the formed conjugates have been presented. The antiproliferative activity and uptake of compounds against several human cell lines were evaluated.

Synthesis of Cobalt Bis(Dicarbollide)-Curcumin Conjugates for Potential Use in Boron Neutron Capture Therapy.

A series of novel cobalt bis(dicarbollide)-curcumin conjugates were synthesized. Two conjugates were obtained through the nucleophilic ring-opening reaction of the 1,4-dioxane and tetrahydropyran derivatives of cobalt bis(dicarbollide) with the OH group of curcumin, and using two equiv. of the oxonium derivatives, two other conjugates containing two cobalt bis(dicarbollide) units per molecule were obtained. In contrast to curcumin, the conjugates obtained were found to be non-cytotoxic against both tumor and normal cell lines. The analysis of the intracellular accumulation of the conjugates by flow cytometry showed that all cobalt bis(dicarbollide)-curcumin conjugates entered HCT116 colorectal carcinoma cells in a time-dependent manner. New non-cytotoxic conjugates contain a large amount of boron atoms in the biomolecule and can potentially be used for further biological research into boron neutron capture therapy (BNCT).

Palladium-Catalyzed Hydroboration of Alkynes with Carboranes: Facile Construction of a Library of Boron Cluster-Based AIE-Active Luminogens.

The classical aggregation-induced emission (AIE)-active luminogens (AIEgens) usually include two-dimensional aromatic systems such as tetraphenylethenes, which are synthesized in several steps by using toxic additives. Here, we proposed a new molecular design strategy for the realization of AIE properties by combining three-dimensional aromatic boron clusters of carboranes with vinyl group(s). To obtain a library of the boron cluster-based AIEgens, a Pd-catalyzed hydroboration of alkynes with carboranes is reported. This reaction protocol proceeds in one step under mild conditions with rapid reaction rate, excellent yields and regioselectivity. Photophysical property studies demonstrate that the facile molecular motions in solution can be inhibited in the solid state for these molecules, which leads to interesting AIE properties. This work provides not only a general design principle for AIEgens but also an efficient methodology to synthesize boron cluster-based photo-functional molecules.

Synthesis and Antibacterial Activity Studies of the Conjugates of Curcumin with closo-Dodecaborate and Cobalt Bis(Dicarbollide) Boron Clusters.

A series of novel conjugates of cobalt bis(dicarbollide) and closo-dodecaborate with curcumin were synthesized by copper(I)-catalyzed azide-alkyne cycloaddition. These conjugates were tested for antibacterial activity. It was shown that all derivatives are active when exposed to Bacillus cereus ATCC 10702 and are not active against Gram-negative microorganisms and Candida albicans at the maximum studied concentration of 1000 mg/L. The conjugate of alkynyl-curcumin with azide synthesized from the tetrahydropyran derivative of cobalt bis(dicarbollide) exhibited activity against Gram-positive microorganisms: Staphylococcus aureus ATCC 29213, Enterococcus faecalis ATCC 29212 and the clinical isolate MRSA 17, that surpassed curcumin by 2-4 times.

Nucleophilic addition reactions to nitrilium derivatives [B12H11NCCH3]- and [B12H11NCCH2CH3]-. Synthesis and structures of closo-dodecaborate-based iminols, amides and amidines.

The synthesis of the acetonitrilium and propionitrilium derivatives of closo-dodecaborate [B12H12]2- was discussed. The nucleophilic addition reactions of water, alcohols and secondary amines to the activated triple bond of the nitrilium derivatives yielded the corresponding iminols, imidates and amidines. The obtained results were supported by X-ray diffraction studies and DFT calculations.

Synthesis of Boronated Amidines by Addition of Amines to Nitrilium Derivative of Cobalt Bis(Dicarbollide).

A series of novel cobalt bis(dicarbollide) based amidines were synthesized by the nucleophilic addition of primary and secondary amines to highly activated B-N+≡C-R triple bond of the propionitrilium derivative [8-EtC≡N-3,3'-Co(1,2-C2B9H10)(1',2'-C2B9H11)]. The reactions with primary amines result in the formation of mixtures of E and Z isomers of amidines, whereas the reactions with secondary amines lead selectively to the E-isomers. The crystal molecular structures of E-[8-EtC(NMe2)=HN-3,3'-Co(1,2-C2B9H10)(1',2'-C2B9H11)], E-[8-EtC(NEt2)=HN-3,3'-Co(1,2- C2B9H10)(1',2'-C2B9H11)] and E-[8-EtC(NC5H10)=HN-3,3'-Co(1,2-C2B9H10)(1',2'-C2B9H11)] were determined by single crystal X-ray diffraction.

Synthesis of Zwitter-Ionic Conjugate of Nido-Carborane with Cholesterol.

9-HC≡CCH2Me2N-nido-7,8-C2B9H11, a previously described carboranyl terminal alkyne, was used for the copper(I)-catalyzed azide-alkyne cycloaddition with azido-3ß-cholesterol to form a novel zwitter-ionic conjugate of nido-carborane with cholesterol, bearing a 1,2,3-triazol fragment. The conjugate of nido-carborane with cholesterol, containing a charge-compensated group in the linker, can be used as a precursor for the preparation of liposomes for BNCT (Boron Neutron Capture Therapy). The solid-state molecular structure of a nido-carborane derivative with the 9-Me2N(CH2)2Me2N-nido-7,8-C2B9H11 terminal dimethylamino group was determined by single-crystal X-ray diffraction.

Synthesis and Structure of Nido-Carboranyl Azide and Its "Click" Reactions.

Novel zwitter-ionic nido-carboranyl azide 9-N3(CH2)3Me2N-nido-7,8-C2B9H11 was prepared by the reaction of 9-Cl(CH2)3Me2N-nido-7,8-C2B9H11 with NaN3. The solid-state molecular structure of nido-carboranyl azide was determined by single-crystal X-ray diffraction. 9-N3(CH2)3Me2N-nido-7,8-C2B9H11 was used for the copper(I)-catalyzed azide-alkyne cycloaddition with phenylacetylene, alkynyl-3ß-cholesterol and cobalt/iron bis(dicarbollide) terminal alkynes to form the target 1,2,3-triazoles. The nido-carborane-cholesterol conjugate 9-3ß-Chol-O(CH2)C-CH-N3(CH2)3Me2N-nido-7,8-C2B9H11 with charge-compensated group in a linker can be used as a precursor for preparation of liposomes for Boron Neutron Capture Therapy (BNCT). A series of novel zwitter-ionic boron-enriched cluster compounds bearing a 1,2,3-triazol-metallacarborane-carborane conjugated system was synthesized. Prepared conjugates contain a large amount of boron atom in the biomolecule and potentially can be used for BNCT.

Effects of Linkers on the Development of Liposomal Formulation of Cholesterol Conjugated Cobalt Bis(dicarbollides).

Boron neutron capture therapy (BNCT) remains an important treatment arm for cancer patients with locally invasive malignant tumors. This therapy needs a significant amount of boron to deposit in cancer tissues selectively, sparing other healthy organs. Most of the liposomes contain water-soluble polyhedral boron salts stay in the core of the liposomes and have low encapsulation efficiency. Thus, modifying the polyhedral boron core to make it hydrophobic and incorporating those into the lipid layer could be one of the ways to increase drug loading and encapsulation efficiency. Additionally, a systematic study about the linker-dependent effect on drug encapsulation and drug-release is lacking, particularly for the liposomal formulation of hydrophobic-drugs. To achieve these goals, liposomal formulations of a series of lipid functionalized cobalt bis(dicarbollide) compounds have been prepared, with the linkers of different hydrophobicity. Hydrophobicity of the linkers have been evaluated through logP calculation and its effect on drug encapsulation and release have been investigated. The liposomes have shown high drug loading, excellent encapsulation efficiency, stability, and non-toxic behavior. Release experiment showed minimal release of drug from liposomes in phosphate buffer, ensuring some amount of drug, associated with liposomes, can be available to tumor tissues for Boron Neutron Capture Therapy.

Highly Purified Conjugates of Natural Chlorin with Cobalt Bis(dicarbollide) Nanoclusters for PDT and BNCT Therapy of Cancer.

To combine the neutron-capturing and photodynamic properties of boron nanoclusters and derivatives of natural chlorins, respectively, in one molecule, conjugate of chlorin e6 methyl ester with cyclen and dioxane and nitrile derivatives of cobalt bis(dicarbollide) were synthesized. The conditions for the purification of compounds by HPLC were selected since the work with natural compounds is complicated by the production of closely related impurities.

Boron-Containing Lipids and Liposomes: New Conjugates of Cholesterol with Polyhedral Boron Hydrides.

A series of boron-containing lipids were prepared by reactions of cyclic oxonium derivatives of polyhedron boranes and metallacarboranes (closo-dodecaborate anion, cobalt and iron bis(dicarbollides)) with amine and carboxylic acids which are derived from cholesterol. Stable liposomal formulations, on the basis of synthesized boron-containing lipids, hydrogenated soybean l-α-phosphatidylcholine and (HSPC) 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DSPE-PEG) as excipients, were prepared and then characterized by dynamic light scattering (DLS) that revealed the formation of particles to be smaller than 200 nm in diameter. The resulting liposomal formulations showed moderate to excellent loading and entrapment efficiency, thus justifying the design of the compounds to fit in the lipid bilayer and ensuring ease of in vivo use for future application. The liposomal formulations based on cobalt and iron bis(dicarbollide)-based lipids were found to be nontoxic against both human breast normal epithelial cells MCF-10A and human breast cancer cells MCF-7.

Conjugate of chlorin е6 with iron bis(dicarbollide) nanocluster: synthesis and biological properties.

Background: Efficiency of both photodynamic and boron-neutron capture anticancer therapies (BNCT) depends on the properties of the used photo- and neutronsensitizer. We report on the synthesis and properties of the advanced photo- and neutronsensitizer designed as a conjugate of chlorin e6 with iron bis(dicarbollide) nanocluster. Results: The conjugate is shown to accumulate efficiently in rat glioblastoma C6 cells delivering >109 boron atoms per cell and thus meeting requirements for BNCT agents, to provide photoinduced 50% death of C6 cells at 35 ± 3 nM, to be not toxic for cells without activating stimulus. Conclusions: The conjugate is a prospective theranostic agent for photodynamic, BNCT and fluorescent diagnostics of tumors.

Small-sized gadolinium oxide based nanoparticles for high-efficiency theranostics of orthotopic glioblastoma.

Glioblastoma (GBM) is one of the most malignant tumors with poor prognosis and outcomes. Although smaller particle size can lead to higher blood-brain barrier (BBB)-permeability of the nanomaterials, most of the reported BBB-crossable nanomaterials for targeted GBM therapy are larger than 24 nm. To realize theranostics of GBM, co-loading of therapeutic and diagnostic agents on the same nanomaterials further results in larger particle size. In this study, we developed a kind of novel BBB-transportable nanomaterials smaller than 14 nm for high-efficiency theranostics of GBM (i.e., high contrast magnetic resonance imaging (MRI) and radiosensitization of GBM). Typically, poly(acrylic acid) (PAA) stabilized extremely small gadolinium oxide nanoparticles with modification of reductive bovine serum albumin (ES-GON-rBSA) was synthesized in water phase, resulting in excellent water-dispersibility. RGD dimer (RGD2, Glu-{Cyclo[Arg-Gly-Asp-(D-Phe)-Lys]}2) and lactoferrin (LF) were then conjugated to the ES-GON-rBSA to obtain composite nanoparticle ES-GON-rBSA-LF-RGD2 with extraordinary relaxivities (r1 = 60.8 mM-1 s-1, r2/r1 = 1.1). The maximum signal enhancement (ΔSNR) for T1-weighted MRI of tumors reached up to 423 ± 42% at 12 h post-injection of ES-GON-rBSA-LF-RGD2, which is much higher than commercial Gd-chelates (<80%). ES-GON-rBSA-LF-RGD2 exhibited high biocompatibility and can transport across the in vitro BBB model and the in vivo BBB of mice due to its small particle size (dh = 13.4 nm) and LF receptor mediated transcytosis. Orthotopic GBM studies reinforce that ES-GON-rBSA3-LF-RGD2 can accumulate in the orthotopic GBM and enhance the radiation therapy of GBM as an effective radiosensitizing agent.

Synthesis and study of C-substituted methylthio derivatives of cobalt bis(dicarbollide).

The C-methylthio derivatives of cobalt bis(dicarbollide) were synthesized by reaction of anhydrous CoCl2 with nido-carborane [7-MeS-7,8-C2B9H11]- and isolated as a mixture of rac-[1,1'-(MeS)2-3,3'-Co(1,2-C2B9H10)2]- and meso-[1,2'-(MeS)2-3,3'-Co(1,2-C2B9H10)2]- isomers. The structures of both isomers were studied using DFT quantum chemical calculations. The most preferable geometry of rotamers and the stabilization energy of C-methylthio derivatives of cobalt bis(dicarbolide) were calculated. The (BEDT-TTF)[1,1'-(MeS)2-3,3'-Co(1,2-C2B9H10)2] salt was prepared and its structure was determined by single crystal X-ray diffraction. The cisoid conformation of the rac-[1,1'-(MeS)2-3,3'-Co(1,2-C2B9H10)2]- anion is stabilized by short intramolecular CH⋯S hydrogen and BH⋯S chalcogen bonds between the dicarbollide ligands, that is in good agreement with the data of quantum chemical calculations.

Exceedingly Small Gadolinium Oxide Nanoparticles with Remarkable Relaxivities for Magnetic Resonance Imaging of Tumors.

Gd chelates have occupied most of the market of magnetic resonance imaging (MRI) contrast agents for decades. However, there have been some problems (nephrotoxicity, non-specificity, and low r1 ) that limit their applications. Herein, a wet-chemical method is proposed for facile synthesis of poly(acrylic acid) (PAA) stabilized exceedingly small gadolinium oxide nanoparticles (ES-GON-PAA) with an excellent water dispersibility and a size smaller than 2.0 nm, which is a powerful T1 -weighted MRI contrast agent for diagnosis of diseases due to its remarkable relaxivities (r1 = 70.2 ± 1.8 mM-1 s-1 , and r2 /r1 = 1.02 ± 0.03, at 1.5 T). The r1 is much higher and the r2 /r1 is lower than that of the commercial Gd chelates and reported gadolinium oxide nanoparticles (GONs). Further ES-GON-PAA is developed with conjugation of RGD2 (RGD dimer) (i.e., ES-GON-PAA@RGD2) for T1 -weighted MRI of tumors that overexpress RGD receptors (i.e., integrin αv ß3 ). The maximum signal enhancement (ΔSNR) for T1 -weighted MRI of tumors reaches up to 372 ± 56% at 2 h post-injection of ES-GON-PAA@RGD2, which is much higher than commercial Gd-chelates (<80%). Due to the high biocompatibility and high tumor accumulation, ES-GON-PAA@RGD2 with remarkable relaxivities is a promising and powerful T1 -weighted MRI contrast agent.

Fenton-Reaction-Acceleratable Magnetic Nanoparticles for Ferroptosis Therapy of Orthotopic Brain Tumors.

Cancer is one of the leading causes of morbidity and mortality in the world, but more cancer therapies are needed to complement existing regimens due to problems of existing cancer therapies. Herein, we term ferroptosis therapy (FT) as a form of cancer therapy and hypothesize that the FT efficacy can be significantly improved via accelerating the Fenton reaction by simultaneously increasing the local concentrations of all reactants (Fe2+, Fe3+, and H2O2) in cancer cells. Thus, Fenton-reaction-acceleratable magnetic nanoparticles, i.e., cisplatin (CDDP)-loaded Fe3O4/Gd2O3 hybrid nanoparticles with conjugation of lactoferrin (LF) and RGD dimer (RGD2) (FeGd-HN@Pt@LF/RGD2), were exploited in this study for FT of orthotopic brain tumors. FeGd-HN@Pt@LF/RGD2 nanoparticles were able to cross the blood-brain barrier because of its small size (6.6 nm) and LF-receptor-mediated transcytosis. FeGd-HN@Pt@LF/RGD2 can be internalized into cancer cells by integrin αvß3-mediated endocytosis and then release Fe2+, Fe3+, and CDDP upon endosomal uptake and degradation. Fe2+ and Fe3+ can directly participate in the Fenton reaction, whereas the CDDP can indirectly produce H2O2 to further accelerate the Fenton reaction. The acceleration of Fenton reaction generates reactive oxygen species to induce cancer cell death. FeGd-HN@Pt@LF/RGD2 successfully delivered reactants involved in the Fenton reaction to the tumor site and led to significant inhibition of tumor growth. Finally, the intrinsic magnetic resonance imaging (MRI) capability of the nanoparticles was used to assess and monitor tumor response to FT (self-MRI monitoring).

Pharmacokinetics of Chlorin e6-Cobalt Bis(Dicarbollide) Conjugate in Balb/c Mice with Engrafted Carcinoma.

The necessary precondition for efficient boron neutron capture therapy (BNCT) is control over the content of isotope 10B in the tumor and normal tissues. In the case of boron-containing porphyrins, the fluorescent part of molecule can be used for quantitative assessment of the boron content. Study Objective: We performed a study of the biodistribution of the chlorin e6-Cobalt bis(dicarbollide) conjugate in carcinoma-bearing Balb/c mice using ex vivo fluorescence imaging, and developed a mathematical model describing boron accumulation and release based on the obtained experimental data. Materials and Methods: The study was performed on Balb/c tumor-bearing mice (CT-26 tumor model). A solution of the chlorin e6-Cobalt bis(dicarbollide) conjugate (CCDC) was injected into the blood at a dose of 10 mg/kg of the animal's weight. Analysis of the fluorescence signal intensity was performed at several time points by spectrofluorimetry in blood and by laser scanning microscopy in muscle, liver, and tumor tissues. The boron content in the same samples was determined by mass spectroscopy with inductively coupled plasma. Results: Analysis of a linear approximation between the fluorescence intensity and boron content in the tissues demonstrated a satisfactory value of approximation reliability with a Spearman's rank correlation coefficient of r = 0.938, p < 0.01. The dynamics of the boron concentration change in various organs, calculated on the basis of the fluorescence intensity, enabled the development of a model describing the accumulation of the studied compound and its distribution in tissues. The obtained results reveal a high level of correspondence between the model and experimental data.

Fast flip-flop of halogenated cobalt bis(dicarbollide) anion in a lipid bilayer membrane.

Transmembrane translocation (flip-flop) of cobalt bis(dicarbollide) (COSAN) anions, elicited by application of a voltage-jump across the lipid bilayer membrane, manifested itself in monoexponential electrical current transients in the microsecond time scale. Halogenation of COSAN led to multi-fold acceleration of the flip-flop, the effect increasing with the molecular weight of the halogens. The exception was a fluorinated analog which exhibited slowing of the translocation kinetics. Measurements of the fluorescence ratio of the dye di-4-ANEPPS in lipid vesicles showed significant differences in the adsorption of studied hydrophobic anions. Based on these data, it can be concluded that COSAN and COSAN-F2 were located on the surface of the lipid membrane in the cisoid conformation increasing the dipole potential of the lipid membrane, while other halogenated COSAN analogs were adsorbed in the transoid conformation. Differences in the flip-flop kinetics of COSAN analogs were attributed to variation in the molecular volume of the anions and their orientation on the membrane surface.