Preparing (Metalla)carboranes for Nanomedicine

Antitumor Therapy through Photothermal Performance Synergized with Catalytic Activity Based on the Boron Cluster Supramolecular Frameworks

Chemodynamic therapy (CDT) has received widespread attention as a tumor optical treatment strategy in the field of malignant tumor therapy. Nonmetallic multifunctional nanomaterials as CDT agents, due to their low toxicity, long-lasting effects, and safety characteristics, have promising applications in the integrated diagnosis and treatment of cancer. Here, we modified the supramolecular framework of boron clusters, coupled with a variety of dyes to develop a series of metal-free agent compounds, and demonstrated that these nonmetallic compounds have excellent CDT activities through experiments. Subsequently, the best performing Methylene Blue/[closo-B12H12]2- (MB@B12H12) was used as an example. Through theoretical calculations, electron paramagnetic resonance spectroscopy, and 808 nm light irradiation, we confirmed that MB@B12H12 exhibited photothermal performance and CDT activity further. More importantly, we applied MB@B12H12 to melanoma cells and subcutaneous tumor, demonstrating its effective suppression of melanoma growth in vitro and in vivo through the synergistic effects of photothermal performance and CDT activity. This study emphasizes the generalizability of the coupling of dyes to [closo-B12H12]2- with important clinical translational potential for CDT reagents. Among them, MB@B12H12 may have a brighter future, paving the way for the rapid development of metal-free CDT reagents.

Structural Patterns Enhancing the Antibacterial Activity of Metallacarborane-Based Antibiotics

Healthcare systems heavily rely on antibiotics to treat bacterial infections, but the widespread presence of multidrug-resistant bacteria puts this strategy in danger. Novel drugs capable of overcoming current resistances are needed if our ability to treat bacterial infections is to be maintained. Boron clusters offer a valuable possibility to create a new class of antibiotics and expand the chemical space of antibiotics beyond conventional carbon-based molecules. In this work, we identified two promising structural patterns providing cobalta bis(dicarbollide)(COSAN)-based compounds with potent and selective activity toward Staphylococcus aureus (including clinical strains): introduction of the α-amino acid amide and addition of iodine directly to the metallacarborane cage. Furthermore, we found that proper hydrophilic-lipophilic balance is crucial for the selective activity of the tested compounds toward S. aureus over mammalian cells. The patterns proposed in this paper can be useful in the development of metallacarborane-based antibiotics with potent antibacterial properties and low cytotoxicity.

In Vivo Application of Carboranes for Boron Neutron Capture Therapy (BNCT): Structure, Formulation and Analytical Methods for Detection

Carboranes have emerged as one of the most promising boron agents in boron neutron capture therapy (BNCT). In this context, in vivo studies are particularly relevant, since they provide qualitative and quantitative information about the biodistribution of these molecules, which is of the utmost importance to determine the efficacy of BNCT, defining their localization and (bio)accumulation, as well as their pharmacokinetics and pharmacodynamics. First, we gathered a detailed list of the carboranes used for in vivo studies, considering the synthesis of carborane derivatives or the use of delivery system such as liposomes, micelles and nanoparticles. Then, the formulation employed and the cancer model used in each of these studies were identified. Finally, we examined the analytical aspects concerning carborane detection, identifying the main methodologies applied in the literature for ex vivo and in vivo analysis. The present work aims to identify the current strengths and weakness of the use of carboranes in BNCT, establishing the bottlenecks and the best strategies for future applications.

Chemistry of Carbon-Substituted Derivatives of Cobalt Bis(dicarbollide)(1-) Ion and Recent Progress in Boron Substitution

The cobalt bis(dicarbollide)(1-) anion (1-), [(1,2-C2B9H11)2-3,3'-Co(III)](1-), plays an increasingly important role in material science and medicine due to its high chemical stability, 3D shape, aromaticity, diamagnetic character, ability to penetrate cells, and low cytotoxicity. A key factor enabling the incorporation of this ion into larger organic molecules, biomolecules, and materials, as well as its capacity for "tuning" interactions with therapeutic targets, is the availability of synthetic routes that enable easy modifications with a wide selection of functional groups. Regarding the modification of the dicarbollide cage, syntheses leading to substitutions on boron atoms are better established. These methods primarily involve ring cleavage of the ether rings in species containing an oxonium oxygen atom connected to the B(8) site. These pathways are accessible with a broad range of nucleophiles. In contrast, the chemistry on carbon vertices has remained less elaborated over the previous decades due to a lack of reliable methods that permit direct and straightforward cage modifications. In this review, we present a survey of methods based on metalation reactions on the acidic C-H vertices, followed by reactions with electrophiles, which have gained importance in only the last decade. These methods now represent the primary trends in the modifications of cage carbon atoms. We discuss the scope of currently available approaches, along with the stereochemistry of reactions, chirality of some products, available types of functional groups, and their applications in designing unconventional drugs. This content is complemented with a report of the progress in physicochemical and biological studies on the parent cobalt bis(dicarbollide) ion and also includes an overview of recent syntheses and emerging applications of boron-substituted compounds.

Towards the Application of Purely Inorganic Icosahedral Boron Clusters in Emerging Nanomedicine

Traditionally, drugs were obtained by extraction from medicinal plants, but more recently also by organic synthesis. Today, medicinal chemistry continues to focus on organic compounds and the majority of commercially available drugs are organic molecules, which can incorporate nitrogen, oxygen, and halogens, as well as carbon and hydrogen. Aromatic organic compounds that play important roles in biochemistry find numerous applications ranging from drug delivery to nanotechnology or biomarkers. We achieved a major accomplishment by demonstrating experimentally/theoretically that boranes, carboranes, as well as metallabis(dicarbollides), exhibit global 3D aromaticity. Based on the stability-aromaticity relationship, as well as on the progress made in the synthesis of derivatized clusters, we have opened up new applications of boron icosahedral clusters as key components in the field of novel healthcare materials. In this brief review, we present the results obtained at the Laboratory of Inorganic Materials and Catalysis (LMI) of the Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) with icosahedral boron clusters. These 3D geometric shape clusters, the semi-metallic nature of boron and the presence of exo-cluster hydrogen atoms that can interact with biomolecules through non-covalent hydrogen and dihydrogen bonds, play a key role in endowing these compounds with unique properties in largely unexplored (bio)materials.

Metallacarborane Synthons for Molecular Construction-Oligofunctionalization of Cobalt Bis(1,2-dicarbollide) on Boron and Carbon Atoms with Extendable Ligands

The exploitation of metallacarboranes' potential in various fields of research and practical applications requires the availability of convenient and versatile methods for their functionalization with various functional moieties and/or linkers of different types and lengths. Herein, we report a study on cobalt bis(1,2-dicarbollide) functionalization at 8,8'-boron atoms with different hetero-bifunctional moieties possessing a protected hydroxyl function allowing further modification after deprotection. Moreover, an approach to the synthesis of three and four functionalized metallacarboranes, at boron and carbon atoms simultaneously via additional functionalization at carbon to obtain derivatives carrying three or four rationally oriented and distinct reactive surfaces, is described.

Aqueous solubilization of hydrophobic compounds by inorganic nano-ions: An unconventional mechanism

HYPOTHESIS

Solubilization of hydrophobic compounds in water is commonly performed by using organic solubilizers such as hydrotropes, surfactants, co-solvents, and macrocycles to form host-guest complexes. 3,3'-commo-bis[closo-1,2-dicarba-3-cobaltadodecaborane] derivatives (COSANs) are fully inorganic and non-amphiphilic ionic boron clusters with nanometric size (nano-ions) showing superchaotropic properties as they strongly bind to neutral organic molecules. Therefore, we expect COSANs to act as solubilizers of sparingly water-soluble molecules, but with a mechanism different from all other organic solubilizers known so far.

EXPERIMENTS

The aqueous solubilization efficiency of COSANs towards butanol was evaluated by determining phase diagrams and comparing them to classical solubilizers. Nanostructuration of the mixture was studied using UV spectroscopy, small-angle X-ray, and neutron scattering with contrast variation.

FINDINGS

COSANs act as efficient aqueous solubilizers of medium-chain alcohols (0.6 < log P < 1.5). Unlike surfactants, COSAN is an efficient solubilizer in its monomeric state, at concentrations well below its critical aggregation concentration. Solubilization by COSAN takes place with a bi-dimensional anisotropic growth of COSAN/butanol co-assemblies, whereas solubilization by surfactant occurs via an isotropic swelling of micelles. Appealingly, COSANs/2-butanol co-assemblies efficiently solubilize more hydrophobic compounds with log P values up to around 6, offering new opportunities in many applied fields.

Synthesis and Structural Characterization of p-Carboranylamidine Derivatives

In this contribution, the first amidinate and amidine derivatives of p-carborane are described. Double lithiation of p-carborane (1) with n-butyllithium followed by treatment with 1,3-diorganocarbodiimides, R-N=C=N-R (R = ⁱPr, Cy (= cyclohexyl)), in DME or THF afforded the new p-carboranylamidinate salts p-C₂H₁₀B₁₀[C(NⁱPr)₂Li(DME)]₂ (2) and p-C₂H₁₀B₁₀[C(NCy)₂Li(THF)₂]₂ (3). Subsequent treatment of 2 and 3 with 2 equiv. of chlorotrimethylsilane (Me₃SiCl) provided the silylated neutral bis(amidine) derivatives p-C₂H₁₀B₁₀[C{ⁱPrN(SiMe₃)}(=NⁱPr)]₂ (4) and p-C₂H₁₀B₁₀[C{CyN(SiMe₃)}(=NCy)]₂ (5). The new compounds 3 and 4 have been structurally characterized by single-crystal X-ray diffraction. The lithium carboranylamidinate 3 comprises a rare trigonal planar coordination geometry around the lithium ions.

Development of the High-Affinity Carborane-Based Cannabinoid Receptor Type 2 PET Ligand [18F]LUZ5-d8

The development of cannabinoid receptor type 2 (CB2R) radioligands for positron emission tomography (PET) imaging was intensively explored. To overcome the low metabolic stability and simultaneously increase the binding affinity of known CB2R radioligands, a carborane moiety was used as a bioisostere. Here we report the synthesis and characterization of carborane-based 1,8-naphthyridinones and thiazoles as novel CB2R ligands. All tested compounds showed low nanomolar CB2R affinity, with (Z)-N-[3-(4-fluorobutyl)-4,5-dimethylthiazole-2(3H)-ylidene]-(1,7-dicarba-closo-dodecaboranyl)-carboxamide (LUZ5) exhibiting the highest affinity (0.8 nM). Compound [18F]LUZ5-d8 was obtained with an automated radiosynthesizer in high radiochemical yield and purity. In vivo evaluation revealed the improved metabolic stability of [18F]LUZ5-d8 compared to that of [18F]JHU94620. PET experiments in rats revealed high uptake in spleen and low uptake in brain. Thus, the introduction of a carborane moiety is an appropriate tool for modifying literature-known CB2R ligands and gaining access to a new class of high-affinity CB2R ligands, while the in vivo pharmacology still needs to be addressed.

Biosynthesized Gold, Silver, Palladium, Platinum, Copper, and Other Transition Metal Nanoparticles

Nanomedicine is a potential provider of novel therapeutic and diagnostic routes of treatment. Considering the development of multidrug resistance in pathogenic bacteria and the commonness of cancer, novel approaches are being sought for the safe and efficient synthesis of new nanoparticles, which have multifaceted applications in medicine. Unfortunately, the chemical synthesis of nanoparticles raises justified environmental concerns. A significant problem in their widespread use is also the toxicity of compounds that maintain nanoparticle stability, which significantly limits their clinical use. An opportunity for their more extensive application is the utilization of plants, fungi, and bacteria for nanoparticle biosynthesis. Extracts from natural sources can reduce metal ions in nanoparticles and stabilize them with non-toxic extract components.

Carboranes as unique pharmacophores in antitumor medicinal chemistry

Carborane is a carbon-boron molecular cluster that can be viewed as a 3D analog of benzene. It features special physical and chemical properties, and thus has the potential to serve as a new type of pharmacophore for drug design and discovery. Based on the relative positions of two cage carbons, icosahedral closo-carboranes can be classified into three isomers, ortho-carborane (o-carborane, 1,2-C2B10H12), meta-carborane (m-carborane, 1,7-C2B10H12), and para-carborane (p-carborane, 1,12-C2B10H12), and all of them can be deboronated to generate their nido- forms. Cage compound carborane and its derivatives have been demonstrated as useful chemical entities in antitumor medicinal chemistry. The applications of carboranes and their derivatives in the field of antitumor research mainly include boron neutron capture therapy (BNCT), as BNCT/photodynamic therapy dual sensitizers, and as anticancer ligands. This review summarizes the research progress on carboranes achieved up to October 2021, with particular emphasis on signaling transduction pathways, chemical structures, and mechanistic considerations of using carboranes.

Electrochemistry of Cobalta Bis(dicarbollide) Ions Substituted at Carbon Atoms with Hydrophilic Alkylhydroxy and Carboxy Groups

In this study we explore the effect on the electrochemical signals in aqueous buffers of the presence of hydrophilic alkylhydroxy and carboxy groups on the carbon atoms of cobalta bis(dicarbollide) ions. The oxygen-containing exo-skeletal substituents of cobalta bis(dicarbollide) ions belong to the perspective building blocks that are considered for bioconjugation. Carbon substitution provides wider versatility and applicability in terms of the flexibility of possible chemical pathways. However, until recently, the electrochemistry of compounds substituted only on boron atoms could be studied, due to the unavailability of carbon-substituted congeners. In the present study, electrochemistry in aqueous phosphate buffers is considered along with the dependence of electrochemical response on pH and concentration. The compounds used show electrochemical signals around −1.3 and +1.1 V of similar or slightly higher intensities than in the parent cobalta bis(dicarbollide) ion. The signals at positive electrochemical potential correspond to irreversible oxidation of the boron cage (the C2B9 building block) and at negative potential correspond to the reversible redox process of (CoIII/CoII) at the central atom. Although the first signal is typically sharp and its potential can be altered by a number of substituents, the second signal is complex and is composed of three overlapping peaks. This signal shows sigmoidal character at higher concentrations and may be used as a diagnostic tool for aggregation in solution. Surprisingly enough, the observed effects of the site of substitution (boron or carbon) and between individual groups on the electrochemical response were insignificant. Therefore, the substitutions would preserve promising properties of the parent cage for redox labelling, but would not allow for the further tuning of signal position in the electrochemical window.

Borinostats: solid-phase synthesis of carborane-capped histone deacetylase inhibitors with a tailor-made selectivity profile

The elevated expression of histone deacetylases (HDACs) in various tumor types renders their inhibition an attractive strategy for epigenetic therapeutics. One key issue in the development of improved HDAC inhibitors (HDACis) is the selectivity for single HDAC isoforms over unspecific pan inhibition to minimize off-target toxicity. Utilizing the carborane moiety as a fine-tuning pharmacophore, we herein present a robust solid phase synthetic approach towards tailor-made HDACis meeting both ends of the selectivity spectrum, namely pan inhibition and highly selective HDAC6 inhibition.

Free three-dimensional carborane carbanions

Carbon atom functionalization via generation of carbanions is the cornerstone of carborane chemistry. In this work, we report the synthesis and structural characterization of free ortho-carboranyl [C2B10H11]-, a three-dimensional inorganic analog of the elusive phenyl anion that features a "naked" carbanion center. The first example of a stable, discrete C(H)-deprotonated carborane anion was isolated as a completely separated ion pair with a crown ether-encapsulated potassium cation. An analogous approach led to the isolation and structural characterization of a doubly deprotonated 1,1'-bis(o-carborane) anion [C2B10H10]2 2-, which is the first example of a discrete molecular dicarbanion. These reactive carbanions are key intermediates in carbon vertex chemistry of carborane clusters.

Metallacarborane Derivatives Effective against Pseudomonas aeruginosa and Yersinia enterocolitica

Pseudomonas aeruginosa is an opportunistic human pathogen that has become a nosocomial health problem worldwide. The pathogen has multiple drug removal and virulence secretion systems, is resistant to many antibiotics, and there is no commercial vaccine against it. Yersinia pestis is a zoonotic pathogen that is on the Select Agents list. The bacterium is the deadliest pathogen known to humans and antibiotic-resistant strains are appearing naturally. There is no commercial vaccine against the pathogen, either. In the current work, novel compounds based on metallacarborane cage were studied on strains of Pseudomonas aeruginosa and a Yersinia pestis substitute, Yersinia enterocolitica. The representative compounds had IC₅₀ values below 10 µM against Y. enterocolitica and values of 20–50 μM against P. aeruginosa. Artificial generation of compound-resistant Y. enterocolitica suggested a common mechanism for drug resistance, the first reported in the literature, and suggested N-linked metallacarboranes as impervious to cellular mechanisms of resistance generation. SEM analysis of the compound-resistant strains showed that the compounds had a predominantly bacteriostatic effect and blocked bacterial cell division in Y. enterocolitica. The compounds could be a starting point towards novel anti-Yersinia drugs and the strategy presented here proposes a mechanism to bypass any future drug resistance in bacteria.

Ruthenacarborane and Quinoline: A Promising Combination for the Treatment of Brain Tumors

Gliomas and glioblastomas are very aggressive forms of brain tumors, prone to the development of a multitude of resistance mechanisms to therapeutic treatments, including cytoprotective autophagy. In this work, we investigated the role and mechanism of action of the combination of a ruthenacarborane derivative with 8-hydroxyquinoline (8-HQ), linked via an ester bond (complex 2), in rat astrocytoma C6 and human glioma U251 cells, in comparison with the two compounds alone, i.e., the free carboxylic acid (complex 1) and 8-HQ, and their non-covalent combination ([1 + 8-HQ], in 1:1 molar ratio). We found that only complex 2 was able to significantly affect cellular viability in glioma U251 cells (IC₅₀ 11.4 μM) via inhibition of the autophagic machinery, most likely acting at the early stages of the autophagic cascade. Contrary to 8-HQ alone, complex 2 was also able to impair cellular viability under conditions of glucose deprivation. We thus suggest different mechanisms of action of ruthenacarborane complex 2 than purely organic quinoline-based drugs, making complex 2 a very attractive candidate for evading the known resistances of brain tumors to chloroquine-based therapies.

Preparing (Metalla)carboranes for Nanomedicine

"There's plenty of room at the bottom" (Richard Feynman, 1959): an invitation for (metalla)carboranes to enter the (new) field of nanomedicine. For two decades, the number of publications on boron cluster compounds designed for potential applications in medicine has been constantly increasing. Hundreds of compounds have been screened in vitro or in vivo for a variety of biological activities (chemotherapeutics, radiotherapeutics, antiviral, etc.), and some have shown rather promising potential for further development. However, until now, no boron cluster compounds have made it to the clinic, and even clinical trials have been very sparse. This review introduces a new perspective in the field of medicinal boron chemistry, namely that boron-based drugs should be regarded as nanomedicine platforms, due to their peculiar self-assembly behaviour in aqueous solutions, and treated as such. Examples for boron-based 12- and 11-vertex clusters and appropriate comparative studies from medicinal (in)organic chemistry and nanomedicine, highlighting similarities, differences and gaps in physicochemical and biological characterisation methods, are provided to encourage medicinal boron chemists to fill in the gaps between chemistry laboratory and real applications in living systems by employing bioanalytical and biophysical methods for characterising and controlling the aggregation behaviour of the clusters in solution.