Metallacarboranes in Medicinal Chemistry: Current Advances and Future Perspectives.

Metallacarboranes, exemplified by cobalt bis(dicarbollide) ([COSAN]-), have excelled their historical metallocene analogue label to become promising in drug design, medical studies, and fundamental biological research. Serving as a unique platform for conjugation with biomolecules, they also constitute an auspicious building block for biologically active derivatives and a carrier for cellular transport of membrane-impermeable cargos. Modified [COSAN]- exhibits specific antimicrobial, antiviral, and anticancer actions showing promise for preclinical trials. Contributing to the ongoing development in medicinal chemistry, metallacarboranes offer desirable physicochemical properties and low acute toxicity. This article presents a critical look at metallacarboranes in the context of their application in medicinal chemistry, emphasizing [COSAN]- as a potential game-changer in drug design and biomedical sciences. As medicinal chemistry seeks innovative building blocks, metallacarboranes emerge as an important novelty with versatile solutions and promising implications.

Composites of Nucleic Acids and Boron Clusters (C2B10H12) as Functional Nanoparticles for Downregulation of EGFR Oncogene in Cancer Cells.

Epidermal growth factor receptor (EGFR) is one of the most promising molecular targets for anticancer therapy. We used boron clusters as a platform for generation of new materials. For this, functional DNA constructs conjugated with boron clusters (B-ASOs) were developed. These B-ASOs, built from 1,2-dicarba-closo-dodecaborane linked with two anti-EGFR antisense oligonucleotides (ASOs), form with their complementary congeners torus-like nanostructures, as previously shown by atomic force microscope (AFM) and transmission electron cryo-microscopy (cryo-TEM) imaging. In the present work, deepened studies were carried out on B-ASO's properties. In solution, B-ASOs formed four dominant complexes as confirmed by non-denaturing polyacrylamide gel electrophoresis (PAGE). These complexes exhibited increased stability in cell lysate comparing to the non-modified ASO. Fluorescently labeled B-ASOs localized mostly in the cytoplasm and decreased EGFR expression by activating RNase H. Moreover, the B-ASO complexes altered the cancer cell phenotype, decreased cell migration rate, and arrested the cells in the S phase of cell cycle. The 1,2-dicarba-closo-dodecaborane-containing nanostructures did not activate NLRP3 inflammasome in human macrophages. In addition, as shown by inductively coupled plasma mass spectrometry (ICP MS), these nanostructures effectively penetrated the human squamous carcinoma cells (A431), showing their potential applicability as anticancer agents.

Ligand Conformational Flexibility Enables Enantioselective Tertiary C-B Bond Formation in the Phosphonate-Directed Catalytic Asymmetric Alkene Hydroboration.

Conformationally flexible ancillary ligands have been widely used in transition metal catalysis. However, the benefits of using flexible ligands are often not well understood. We performed density functional theory (DFT) and experimental studies to elucidate the mechanisms and the roles of conformationally flexible α,α,α',α'-tetraaryldioxolane-4,5-dimethanol (TADDOL)-derived ligands on the reactivity and selectivity in the Rh-catalyzed asymmetric hydroboration (CAHB) of alkenes. DFT calculations and deuterium labeling studies both indicated that the most favorable reaction pathway involves an unusual tertiary C-B bond reductive elimination to give high levels of regio- and enantioselectivities. Here, the asymmetric construction of the fully substituted carbon center is promoted by the flexibility of the TADDOL backbone, which leads to two ligand conformations with distinct steric environments in different steps of the catalytic cycle. A pseudo-chair ligand conformation is preferred in the rate-determining tertiary benzylic C-B reductive elimination. The less hindered steric environment with this conformation allows the benzylic group to bind to the Rh center in an η3 fashion, which stabilizes the C-B reductive elimination transition state. On the other hand, a pseudo-boat ligand conformation is involved in the selectivity-determining alkene migratory insertion step, where the more anisotropic steric environment leads to greater ligand-substrate steric interactions to control the π-facial selectivity. Thus, using a conformationally flexible ligand is beneficial for enhancing both reactivity and enantioselectivity by controlling ligand-substrate interactions in two different elementary steps.

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.

Substituted 2-Phenacylbenzoxazole Difluoroboranes: Synthesis, Structure and Properties.

Novel fluorescent dyes such as benzoxazole-boron complexes, bearing ß-ketoiminate ligands, have been synthesized and characterized with a focus on the influence of a substituent on the basic photophysical properties. 1H, 11B, 13C, 15N, and 19F nuclear magnetic resonance (NMR) spectra of substituted 2-phenacylbenzoxazole difluoroboranes have been recorded and discussed. It is worth mentioning that a high correlation coefficient was found between 15N-NMR parameters and substituent constants. The photophysical properties of these new dyes have been investigated by fluorescence and ultraviolet-visible (UV-Vis) absorption spectroscopy. The geometry optimization, vibrational spectra, and the HOMO and LUMO energies were calculated based on density functional theory with the use of the B3LYP functional and 6-311++G(d,p) basis set.

m-Carborane as a Novel Core for Periphery-Decorated Macromolecules.

Closo m-C2B10H12 can perform as a novel core of globular periphery-decorated macromolecules. To do this, a new class of di and tetrabranched m-carborane derivatives has been synthesized by a judicious choice of the synthetic procedure, starting with 9,10-I2-1,7-closo-C2B10H10. The 2a-NPA (sum of the natural charges of the two bonded atoms) value for a bond, which is defined as the sum of the NPA charges of the two bonded atoms, matches the order of electrophilic reaction at the different cluster bonds of the icosahedral o-and m- carboranes that lead to the formation of B-I bonds. As for m-carborane, most of the 2a-NPA values of B-H vertexes are positive, and their functionalization is more challenging. The synthesis and full characterization of dibranched 9,10-R2-1,7-closo-carborane (R = CH2CHCH2, HO(CH2)3, Cl(CH2)3, TsO(CH2)3, C6H5COO(CH2)3, C6H5COO(CH2)3, N3(CH2)3, CH3CHCH, and C6H5C2N3(CH2)3) compounds as well as the tetrabranched 9,10-R2-1,7-R2-closo-C2B10H8 (R = CH2CHCH2, HO(CH2)3) are presented. The X-ray diffraction of 9,10-(HO(CH2)3)2-1,7-closo-C2B10H10 and 9,10-(CH3CHCH)2-1,7-closo-C2B10H10, as well as their Hirshfeld surface analysis and decomposed fingerprint plots, are described. These new reported tetrabranched m-carborane derivatives provide a sort of novel core for the synthesis of 3D radially grown periphery-decorated macromolecules that are different to the 2D radially grown core of the tetrabranched o-carborane framework.

Leucettamine B analogs and their carborane derivative as potential anti-cancer agents: Design, synthesis, and biological evaluation.

Leucettamine B is a natural product found in marine sponge Leucetta microraphis. Several of analogs of its family, such as aplysinopsine and clathridine, are medicinally active molecules which have applications in many pharmaceuticals and healthcare products; however, thus far, leucettamine B has not been studied. In this report, we describe the synthesis of a new class of analogs of leucettamine B obtained by Knoevenagel condensation using a microwave reactor. The 25 newly synthesized compounds were tested against MDA-MB-468, SW480, and Mahlavu cell lines for anticancer activity. Among them, the carborane-based compound (Z)-5-(benzo[d][1,3]dioxol-5-ylmethylene)-3-(1-closo-carboranyl)-2-thioxo -thiazolidin-4-one (49) and (Z)-5-(benzo[d][1,3]dioxol-5-ylmethylene)-3-(2-(pyrrolidin-1-yl)ethyl)-2-thioxothiazolidin-4-one (31) derivatives were found to have the most potential for use against tumor cells. The carborane derivative 49 had the lowest IC50 value against the SW480 cell line (4.7 µM) and the Mahlavu (6.6 µM) cell line. Furthermore, compound 31 also had a low IC50 value against SW480 (7.5 µM). Our research shows that leucettamine B analogs might have potential for use in cancer chemotherapy.

Exopolyhedral Ligand Orientation Controls Diastereoisomer in Mixed-Metal Bis(carboranes).

Heterobimetallic derivatives of a bis(carborane), [µ7,8-(1',3'-3'-Cl-3'-PPh3-closo-3',1',2'-RhC2B9H10)-2-(p-cymene)-closo-2,1,8-RuC2B9H10] (1) and [µ7,8-(1',3'-3'-Cl-3'-PPh3-closo-3',1',2'-RhC2B9H10)-2-Cp-closo-2,1,8-CoC2B9H10] (2) have been synthesised and characterised, including crystallographic studies. A minor co-product during the synthesis of compound 2 is the new species [8-{8'-2'-H-2',2'-(PPh3)2-closo-2',1',8'-RhC2B9H10}-2-Cp-closo-2,1,8-CoC2B9H10] (3), isolated as a mixture of diastereoisomers. Although, in principle, compounds 1 and 2 could also exist as two diastereoisomers, only one (the same in both cases) is formed. It is suggested that the preferred exopolyhedral ligand orientation in the rhodacarboranes in the non-observed diastereoisomers would lead to unacceptable steric crowding between the PPh3 ligand and either the p-cymene (compound 1) or Cp (compound 2) ligand of the ruthenacarborane or cobaltacarborane, respectively.

Comparative study of the effects of ortho-, meta- and para-carboranes (C2B10H12) on the physicochemical properties, cytotoxicity and antiviral activity of uridine and 2'-deoxyuridine boron cluster conjugates.

In this study, a series of uridine (U) and 2'-deoxyuridine (dU) conjugates containing an isomeric ortho-, meta- or para-carborane cluster (C2B10H12) attached at C-5 through an ethynyl linker were synthesized. The effect of carborane cluster isomerism on the conjugate syn/anti conformation, molar extinction coefficient, lipophilicity, susceptibility to phosphorylation (by TK1, TK2 and dCK), cytotoxicity and antiviral activity was evaluated. A strong effect of the boron cluster modification on the syn/anti equilibrium of the modified nucleosides was observed. An increase in lipophilicity compared with unmodified U and dU, especially for conjugates bearing a para-carborane cluster, was detected. Furthermore a pronounced and differential influence of the boron cluster modification on the electronic properties of the nucleobase chromophore was observed. The obtained conjugates have low or medium toxicity toward several cell lines, are phosphorylated fairly well by TK1 and are poor or not substrates for dCK. Furthermore, the conjugates preferentially inhibit HCMV replication with an SI index as high as 22 for the ortho-carborane derivative of U and more than 180 for the para-carborane derivative of dU.

Boron clusters as a platform for new materials: composites of nucleic acids and oligofunctionalized carboranes (C2B10H12) and their assembly into functional nanoparticles.

Nucleic acids are key biomolecules in all life forms. These biomolecules can encode and transfer information via Watson-Crick base-pairing interactions and can form double-stranded structures between complementary sequences with high precision. These properties make nucleic acids extremely successful in applications in materials science as nanoconstruction materials. Herein, we describe a method for the automated synthesis of "oligopeds", which are building blocks based on the boron cluster structure equipped with short DNA adapters; these building blocks assemble into functional nanoparticles. The obtained, well defined, torus-like structures are the first DNA nanoconstructs based on a boron cluster scaffold. The results indicate the potential of boron clusters in DNA nanoconstruction and open the way for the design of entirely new types of buildings blocks based on polyhedral heteroborane geometry and its unique properties. The use of antisense oligonucleotides as DNA adapters illustrates one of the possible applications of the obtained nanoconstructs as vectors for therapeutic nucleic acids.

Soluble epoxide hydrolase inhibitors with carboranes as non-natural 3-D pharmacophores.

In the present article we describe the creation of a small carboranylcarboxamide compound library followed by a screening campaign at the soluble epoxide hydrolase (sEH). We identified meta-carboranyl alkylamides, -anilides, and -benzylamides as potent sEH inhibitors. Furthermore, we optimized the scaffolds and we derived structure-activity relationships. The most potent benzylamide 33 (MS1) was similar to a previously reported adamantane derivative and gave an IC50 value of 0.07 µM for meta- and 0.08 µM for para-carborane at isolated sEH. The ortho-derivative suffered deboronation. The results underline the potential of carboranes as non-natural 3-D pharmacophores to extend the chemical space in drug discovery.

Polyborane-encapsulated PEGylated Liposomes Prepared Using Post-insertion Technique for Boron Neutron Capture Therapy.

PEGylated liposomes are one of the useful boron carriers for boron neutron capture therapy (BNCT). Recently, a method of adding PEG after liposome formation (post-insertion) was reported. In this study, we prepared polyborane-encapsulated PEGylated liposomes for BNCT with half the amount of DSPE-PEG of the conventional method using post-insertion technique (post-PEG liposomes), and their usefulness were evaluated in comparison with conventional PEGylated liposomes (pre-PEG liposomes). From the results of physicochemical property measurements, it was confirmed that particle size distributions, surface charge densities, and fixed aqueous layer thicknesses of these liposomes were equivalent. In vitro cytotoxicity and cell uptake tests were also carried out using B16 melanoma and RAW264.7 cells. Polyborane solution and bare liposomes were used for comparison. From the results of these tests, we confirmed that post-PEG liposomes and pre-PEG liposomes have the same influence of PEGylation. To evaluate biodistribution properties at 24 h post-administration, these liposomes and polyborane solution were injected into the tail veins of tumor-bearing mice. Boron concentration and tumor/blood ratios of PEGylated liposomes were 73.2-77.6 µg/g of tumor tissue and 5.5-5.8, respectively. From these results, it was found that by using post-insertion technique, liposomes for BNCT having same effect as the liposome prepared using the conventional method can be prepared with half amount of DSPE-PEG.

Efficient blue light emitting materials based on m-carborane-anthracene dyads. Structure, photophysics and bioimaging studies.

Efficient monosubstitution of the non-iodinated, mono-iodinated and di-iodinated m-carborane cluster at one Ccluster has led to the preparation of three single organic molecule-carborane dyads (4-6), which exhibited exceptional fluorescence properties with quantum yield values of 100% in solution, for all of them, with maxima around 415 nm, which correspond to the locally excited state (LE) emission. These results suggest that simply linking the m-carborane fragment to one anthracene unit through a CH2 spacer produces a significant enhancement of the fluorescence in the final fluorophore, probably due to the free rotation of the anthracene linked to the Ccluster. Besides, the presence of one or two iodine atoms linked to boron atoms does not cause any influence on the photophysical properties of the dyads, as it is confirmed by TD-DFT calculations. Notably, the three conjugates show good fluorescence efficiency in the aggregate state with quantum yields in the range of 19-23%, which could be ascribed to the presence of CH2, particularly for 4, and the iodine atoms in 5 and 6, which prevent π-π stacking. All these results indicate that our dyads are extremely good emitters in solution while maintaining the emission properties in the aggregate state. Crystal packing, fingerprint plot analysis, and TD-DFT calculations for the three compounds support these results. Confocal microscopy studies show that 6 is the best-internalized compound by HeLa cells via endocytosis, although 4 and 5 also presented a high fluorescence intensity emission. Moreover, due to the blue emission, this compound is an excellent candidate to be applied as a fluorescent dye in bioimaging studies.

A multimeric MRI contrast agent based on a closo-borane scaffold bearing modified AAZTA chelates on the periphery.

A multimeric MRI contrast agent based on the closo-borane motif is reported. Twelve copies of a modified AAZTA chelate with an alkyne end group are appended on an azide-functionalized closo-borane motif using Cu(i) catalyzed click chemistry. The presence of two water molecules on the Gd-bound AAZTA chelate results in high relaxivity for the closomer in vitro/in vivo.

ER subtype selectivity of m-carborane-containing phenols: C-alkyl groups on the m-carborane cage enhance ERα selectivity.

Estrogen receptor (ER) exhibits two subtypes, ERα and ERß, whose biological functions are quite different despite expression in the same tissues. We developed diiodo-m-carborane derivative 3a, which showed 14-fold selectivity for ERß with high binding affinity toward ERß. Interestingly, introduction of an alkyl group into the carbon atom of the m-carborane cage of 3a markedly enhanced the binding affinity toward ERα and decreased affinity toward ERß. C-n-propyl derivative 3d showed 28-fold selectivity for ERα in an ER binding assay and promoted proliferation of MCF-7 breast cancer cells. Docking simulation studies suggest that the directions of the n-propyl group and the diiodo substituent introduced on the m-carborane cage play important roles for the control of ER subtype selectivity. As 3a and 3d showed ERß and ERα selectivity with high binding affinity, respectively, these ligands may be useful as biological tools to aid in understanding the different roles of ER subtypes.

Cationic Co(I)-Intermediates for Hydrofunctionalization Reactions: Regio- and Enantioselective Cobalt-Catalyzed 1,2-Hydroboration of 1,3-Dienes.

Much of the recent work on catalytic hydroboration of alkenes has focused on simple alkenes and styrene derivatives with few examples of reactions of 1,3-dienes, which have been reported to undergo mostly 1,4-additions to give allylic boronates. We find that reduced cobalt catalysts generated from 1,n- bis-diphenylphosphinoalkane complexes [Ph2P-(CH2) n-PPh2]CoX2; n = 1-5) or from (2-oxazolinyl)phenyldiarylphosphine complexes [(G-PHOX)CoX2] (G = 4-substituent on oxazoline ring) effect selective 1,2-, 1,4-, or 4,3-additions of pinacolborane (HBPin) to a variety of 1,3-dienes depending on the ligands chosen. Conditions have been found to optimize the 1,2-additions. The reactive catalysts can be generated from the cobalt(II)-complexes using trimethylaluminum, methyl aluminoxane, or activated zinc in the presence of sodium tetrakis[(3,5-trifluoromethyl)phenyl]borate (NaBARF). The complex, (dppp)CoCl2, gives the best results (ratio of 1,2- to 1,4-addition >95:5) for a variety of linear terminal 1,3-dienes and 2-substituted 1,3-dienes. The [(PHOX)CoX2] (X = Cl, Br) complexes give mostly 1,4-addition with linear unsubstituted 1,3-dienes, but, surprisingly, selective 1,2-additions with 2-substituted or 2,3-disubstituted 1,3-dienes. Isolated and fully characterized (X-ray crystallography) Co(I)-complexes, (dppp)3Co2Cl2 and [( S,S)-BDPP]3Co2Cl2, do not catalyze the reaction unless activated by a Lewis acid or NaBARF, suggesting a key role for a cationic Co(I) species in the catalytic cycle. Regio- and enantioselective 1,2-hydroborations of 2-substituted 1,3-dienes are best accomplished using a catalyst prepared via activation of a chiral phosphinooxazoline-cobalt(II) complex with zinc and NaBARF. A number of common functional groups, among them, -OBn, -OTBS, -OTs, N-phthalimido- groups, are tolerated, and er's > 95:5 are obtained for several dienes including 1-alkenylcycloalk-1-enes. This operationally simple reaction expands the realm of asymmetric hydroboration to provide direct access to a number of nearly enantiopure homoallylic boronates, which are not readily accessible by current methods. The resulting boronates have been converted into the corresponding alcohols, potassium trifluororoborate salts, N-BOC amines, and aryl derivatives by C-BPin to C-aryl transformation.

Isomeric Carborane Neuromuscular Blocking Agents.

We synthesized a family of neuromuscular blocking agents (NMB) based on decamethonium, but containing a carborane cluster in the methylene chain between the two quaternary ammonium groups. The carborane cluster isomers o-NMB, m-NMB, and p-NMB were tested in animals for neuromuscular block and compared with agents used clinically: rocuronium and decamethonium. All three isomers caused reversible muscle weakness in mice as determined by grip strength and inverted screen tests, with a potency rank of p-NMB > rocuronium > decamethonium > m-NMB > o-NMB. The mechanism of action of the compounds was determined by using the in vitro rat phrenic nerve hemi-diaphragm preparation and electrophysiologic measurements in cells. Neostigmine reversed hemi-diaphragm weakness caused by the three isomers and rocuronium, but not succinylcholine. In electrophysiologic recordings of currents through acetylcholine receptor channels, the carborane compounds did not activate channel activity but did inhibit channel activation by acetylcholine. These results demonstrate that the carborane neuromuscular blocking agents are non-depolarizers in contrast to the depolarizing action of the parent compound.

Optimization of Aqueous Stability versus π-Conjugation in Tetracationic Bis(triarylborane) Chromophores: Applications in Live-Cell Fluorescence Imaging.

The stability of tetracationic triarylboranes in dilute aqueous solution was investigated by tuning the steric demand of the linker in a (para-(N,N,N-trimethylammonio)xylyl)2 B-(linker)-B(para-(N,N,N-trimethylammonio)xylyl)2 structure. With increasing steric bulk of the linker, namely 1,4-phenylene, 2,2'''-(3,3'''-dimethyl)-5,2':5',2'':5'',5'''-quaterthiophene, 9,10-anthracenylene, and 4,4'''-(5'-(3,5-dimethylphenyl))(5''-(3''',5'''-dimethylphenyl))-2',2''-bithiophene, the stability of the compounds increased. The anthracene-based chromophore, compound 3M is water-stable for at least 48 h, is nontoxic to cells and exhibits an exceedingly high fluorescence quantum yield of 0.86 in water making it an ideal candidate for confocal live-cell imaging of lysosomes.

Endoplasmic Reticulum-Targeted Ratiometric N-Heterocyclic Carbene Borane Probe for Two-Photon Microscopic Imaging of Hypochlorous Acid.

The naphthoimidazolium borane 4 is shown to be a selective probe for HOCl over other reactive oxygen species. Unlike other boronate-reactive oxygen species (ROS) fluorogenic probes that are oxidized by HOCl through a nucleophilic borono-Dakin oxidation mechanism, probe 4 is distinguished by its electrophilic oxidation mechanism involving B-H bond cleavage. Two-photon microscopy experiments in living cells and tissues with the probe 4 demonstrate the monitoring of endogenous HOCl generation and changes in HOCl concentrations generated in the endoplasmic reticulum during oxidative stress situations.

Phospholyl(borane) Amino Acids and Peptides: Stereoselective Synthesis and Fluorescent Properties with Large Stokes Shift.

The synthesis of phospholyl(borane) amino acids was stereoselectively achieved by reaction of phospholide anion with iodo α-amino ester derived from l-aspartic acid or l-serine, followed by in situ complexation with borane. Phospholyl(borane) amino acids are easy to store and can be subjected to direct transformation into the corresponding free phospholyl, gold complex, oxide or sulfur derivatives as well as phospholinium salts, thus offering a variety of side chains. After selective deprotection of carboxylic function or amine, C- or N- peptide coupling with an alanine moiety proved the possible incorporation into peptides. Such phospholyl amino acid and peptide derivatives exhibit fluorescent properties with a large Stokes shift (160 nm) and fluorescence up to 535 nm, depending on the phosphole aromaticity and the chemical environment. These phospholyl(borane) amino acids constitute a new class of unnatural amino acids useful for structure-activities relationship studies and appear to be promising fluorophores for the development of labeled peptides.