Boron chemicals in diagnosis and therapeutics. Future Med Chem. 2013;5:653-676. [PMID: 23617429 DOI: 10.4155/fmc.13.38]

α-Aminoboronates: recent advances in their preparation and synthetic applications

α-Aminoboronic acids and their derivatives are useful as bioactive agents. Thus far, three compounds containing an α-aminoboronate motif have been approved by the Food and Drug Administration (FDA) as protease inhibitors, and more are currently undergoing clinical trials. In addition, α-aminoboronic acids and their derivatives have found applications in organic synthesis, e.g. as α-aminomethylation reagents for the synthesis of chiral nitrogen-containing molecules, as nucleophiles for preparing valuable vicinal amino alcohols, and as bis-nucleophiles in the construction of valuable small molecule scaffolds. This review summarizes new methodology for the preparation of α-aminoboronates, including highlights of asymmetric synthetic methods and mechanistic explanations of reactivity. Applications of α-aminoboronates as versatile synthetic building blocks are also discussed.

Recent developments in the medicinal chemistry of single boron atom-containing compounds

Various boron-containing drugs have been approved for clinical use over the past two decades, and more are currently in clinical trials. The increasing interest in boron-containing compounds is due to their unique binding properties to biological targets; for example, boron substitution can be used to modulate biological activity, pharmacokinetic properties, and drug resistance. In this perspective, we aim to comprehensively review the current status of boron compounds in drug discovery, focusing especially on progress from 2015 to December 2020. We classify these compounds into groups showing anticancer, antibacterial, antiviral, antiparasitic and other activities, and discuss the biological targets associated with each activity, as well as potential future developments.

Novel inhibitors of eukaryotic elongation factor 2 kinase: In silico, synthesis and in vitro studies

Eukaryotic elongation factor 2 kinase (eEF2K) is an unusual alpha kinase whose expression is highly upregulated in various cancers and contributes to tumor growth, metastasis, and progression. More importantly, eEF2K expression is associated with poor clinical outcome and shorter patient survival in breast, lung and ovarian cancers. Therefore, eEF2K is an emerging molecular target for development of novel targeted therapeutics and precision medicine in solid cancers. Currently, there are not any available potent and specific eEF2K inhibitors for clinical translation. In this study, we designed and synthesized a series of novel compounds with coumarin scaffold with various substitutions and investigated their effects in inhibiting eEF2K activity using in silico approaches and in vitro studies in breast cancer cells. We utilized an amide substitution at position 3 on the coumarin ring with their pharmacologically active groups containing pyrrolidine, piperidine, morpholine and piperazine groups with (CH₂)₂ bridged for aliphatic amides. Due to their ability to form covalent binding to the target enzyme, we also investigated the effects of boron containing groups on functionalized coumarin ring (3 compounds) and designed novel aliphatic and aromatic derivatives of coumarin scaffolds (10 compounds) and phenyl ring with boron groups (4 compounds). The Glide/SP module of the Maestro molecular modeling package was used to perform in silico analysis and molecular docking studies. According to our combined results, structure activity relationship (SAR) was performed in detail. Among the newly designed, synthesized, and tested compounds, our in vitro findings revealed that several compounds displayed a highly effective eEF2K inhibition at submicromolar concentrations in in vitro breast cancer cells. In conclusion, we identified novel compounds that can be used as eEF2K inhibitors and that they should be further evaluated by in vivo preclinical tumor models studies for antitumor efficacy and clinical translation.

Boronic acid with high oxidative stability and utility in biological contexts

Despite their desirable attributes, boronic acids have had a minimal impact in biological contexts. A significant problem has been their oxidative instability. At physiological pH, phenylboronic acid and its boronate esters are oxidized by reactive oxygen species at rates comparable to those of thiols. After considering the mechanism and kinetics of the oxidation reaction, we reasoned that diminishing electron density on boron could enhance oxidative stability. We found that a boralactone, in which a carboxyl group serves as an intramolecular ligand for the boron, increases stability by 10⁴-fold. Computational analyses revealed that the resistance to oxidation arises from diminished stabilization of the p orbital of boron that develops in the rate-limiting transition state of the oxidation reaction. Like simple boronic acids and boronate esters, a boralactone binds covalently and reversibly to 1,2-diols such as those in saccharides. The kinetic stability of its complexes is, however, at least 20-fold greater. A boralactone also binds covalently to a serine side chain in a protein. These attributes confer unprecedented utility upon boralactones in the realms of chemical biology and medicinal chemistry.

Redox-dependent cytotoxicity of ferrocene derivatives and ROS-activated prodrugs based on ferrocenyliminoboronates

Four ferrocene derivatives - ferrocenecarboxylic acid, ferrocenium salt, ferroceneboronic acid, and aminoferrocene - were characterized electrochemically, and their cytotoxicity was probed using cancer cells (line MG-63). We related the observed cytotoxicity with the determined redox potentials of these four ferrocenes - aminoferrocene with its lowest redox potential exhibited the highest cytotoxicity. Thus, we synthesized four derivatives consisting of aminoferrocene and phenylboronic acid residue with the intent to employ them as ROS-activated prodrugs (ROS - reactive oxygen species). We characterized them and studied their time-dependent stability in aqueous environments. Then, we performed electrochemical measurements at oxidative conditions to confirm ROS-responsivity of the synthesized molecules. Finally, the cytotoxicity of the synthesized molecules was tested using cancer MG-63 cells and noncancerous NIH-3T3 cells. The experiments revealed sought behaviour, especially for para-regioisomers of synthesized ferrocenyliminoboronates.

Development of structurally extended benzosiloxaboroles - synthesis and in vitro biological evaluation

The synthesis of potassium 6-hydroxy-7-chloro-1,1-dimethyl-3,3-difluorobenzo-1,2,3-siloxaborolate 5b from readily available 4-bromo-2-chlorophenol was developed. This compound proved useful in various derivatizations resulting in a wide range of O-functionalized benzosiloxaboroles. Reactions of 5b with selected substituted benzoyl chlorides gave rise to a series of respective derivatives with 6-benzoate side groups attached to the benzosiloxaborole core. Furthermore, treatment of 5b with substituted benzenesufonyl chlorides afforded several benzosiloxaboroles bearing functionalized benzenesulfonate moieties at the 6 position. The synthesis of related chloropyridine-2-yloxy substituted benzosiloxaboroles was accomplished by a standard approach involving silylation/boronation of appropriate heterodiaryl ethers. Investigation of biological activity of obtained compounds revealed that some benzoate and most benzenesulfonate derivatives exhibit high activity against Gram-positive cocci such as methicillin-sensitive Staphylococcus aureus ATCC 6538P as well as methicillin-resistant S. aureus ATCC 43300 with the MIC values in the range of 0.39–3.12 mg L⁻¹. Some benzenesulfonate derivatives showed also potent activity against Enterococcus faecalis ATCC 29212 and E. faecium ATCC 6057 with MIC = 6.25 mg L⁻¹. Importantly, for the most promising cocci-active benzenesulfonate derivatives the obtained MIC values were far below the cytotoxicity limit determined with respect to human normal lung fibroblasts (MRC-5). For those derivatives, the obtained IC₅₀ values were higher than 12.3 mg L⁻¹. The results of antimicrobial activity and cytotoxicity indicate that the tested compounds can be considered as potential antibacterial agents.

The synthesis of potassium 6-hydroxy-7-chloro-1,1-dimethyl-3,3-difluorobenzo-1,2,3-siloxaborolate 5b from readily available 4-bromo-2-chlorophenol was developed.

Organoboron Compounds: Effective Antibacterial and Antiparasitic Agents

The unique electron deficiency and coordination property of boron led to a wide range of applications in chemistry, energy research, materials science and the life sciences. The use of boron-containing compounds as pharmaceutical agents has a long history, and recent developments have produced encouraging strides. Boron agents have been used for both radiotherapy and chemotherapy. In radiotherapy, boron neutron capture therapy (BNCT) has been investigated to treat various types of tumors, such as glioblastoma multiforme (GBM) of brain, head and neck tumors, etc. Boron agents playing essential roles in such treatments and other well-established areas have been discussed elsewhere. Organoboron compounds used to treat various diseases besides tumor treatments through BNCT technology have also marked an important milestone. Following the clinical introduction of bortezomib as an anti-cancer agent, benzoxaborole drugs, tavaborole and crisaborole, have been approved for clinical use in the treatments of onychomycosis and atopic dermatitis. Some heterocyclic organoboron compounds represent potentially promising candidates for anti-infective drugs. This review highlights the clinical applications and perspectives of organoboron compounds with the natural boron atoms in disease treatments without neutron irradiation. The main topic focuses on the therapeutic applications of organoboron compounds in the diseases of tuberculosis and antifungal activity, malaria, neglected tropical diseases and cryptosporidiosis and toxoplasmosis.

Effects of boron-containing compounds in the fungal kingdom

BACKGROUND

The number of known boron-containing compounds (BCCs) is increasing due to their identification in nature and innovative synthesis procedures. Their effects on the fungal kingdom are interesting, and some of their mechanisms of action have recently been elucidated.

METHODS

In this review, scientific reports from relevant chemistry and biomedical databases were collected and analyzed.

RESULTS

It is notable that several BCC actions in fungi induce social and economic benefits for humans. In fact, boric acid was traditionally used for multiple purposes, but some novel synthetic BCCs are effective antifungal agents, particularly in their action against pathogen species, and some were recently approved for use in humans. Moreover, most reports testing BCCs in fungal species suggest a limiting effect of these compounds on some vital reactions.

CONCLUSIONS

New BCCs have been synthesized and tested for innovative technological and biomedical emerging applications, and new interest is developing for discovering new strategic compounds that can act as environmental or wood protectors, as well as antimycotic agents that let us improve food acquisition and control some human infections.

How macrophages respond to two-dimensional materials: a critical overview focusing on toxicity

With wider use of graphene-based materials and other two-dimensional (2 D) materials in various fields, including electronics, composites, biomedicine, etc., 2 D materials can trigger undesired effects at cellular, tissue and organ level. Macrophages can be found in many organs. They are one of the most important cells in the immune system and they are relevant in the study of nanomaterials as they phagocytose them. Nanomaterials have multi-faceted effects on phagocytic immune cells like macrophages, showing signs of inflammation in the form of pro-inflammatory cytokine or reactive oxidation species production, or upregulation of activation markers due to the presence of these foreign bodies. This review is catered to researchers interested in the potential impact and toxicity of 2 D materials, particularly in macrophages, focusing on few-layer graphene, graphene oxide, graphene quantum dots, as well as other promising 2 D materials containing molybdenum, manganese, boron, phosphorus and tungsten. We describe applications relevant to the growing area of 2 D materials research, and the possible risks of ions and molecules used in the production of these promising 2 D materials, or those produced by the degradation and dissolution of 2 D materials.

One tool to bring them all: Au-catalyzed synthesis of B,O- and B,N-doped PAHs from boronic and borinic acids

The isoelectronic replacement of C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds with ⁻BN⁺ bonds in polycyclic aromatic hydrocarbons (PAHs) is a widely used tool to prepare novel optoelectronic materials. Far less well explored are corresponding B,O-doped PAHs, although they have a similarly high application potential. We herein report on the modular synthesis of B,N- and B,O-doped PAHs through the [Au(PPh₃)NTf₂]-catalyzed 6-endo-dig cyclization of BN–H and BO–H bonds across suitably positioned C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds in the key step. Readily available, easy-to-handle o-alkynylaryl boronic and borinic acids serve as starting materials, which are either cyclized directly or first converted into the corresponding aminoboranes and then cyclized. The reaction even tolerates bulky mesityl substituents on boron, which later kinetically protect the formed B,N/O-PAHs from hydrolysis or oxidation. Our approach is also applicable for the synthesis of rare doubly B,N/O-doped PAHs. Specifically, we prepared 1,2-B,E-naphthalenes and -anthracenes, 1,5-B₂-2,6-E₂-anthracenes (E = N, O) as well as B,O₂-containing and unprecedented B,N,O-containing phenalenyls. Selected examples of these compounds have been structurally characterized by X-ray crystallography; their optoelectronic properties have been studied by cyclic voltammetry, electron spectroscopy, and quantum-chemical calculations. Using a new unsubstituted (B,O)₂-perylene as the substrate for late-stage functionalization, we finally show that the introduction of two pinacolatoboryl (Bpin) substituents is possible in high yield and with perfect regioselectivity via an Ir-catalyzed C–H borylation approach.

Singly and doubly B,E-doped PAHs were synthesized using a protocol that starts from easy-to-handle boronic and borinic acids and offers the possibility to choose between the preparation of B,O- and B,N-PAHs in the final reaction step.

Photoinduced Borylation for the Synthesis of Organoboron Compounds

Organoboron compounds have important synthetic value and can be applied in numerous transformations. The development of practical and convenient ways to synthesize boronate esters has thus attracted significant interest. Photoinduced borylations originated from stoichiometric reactions of alkanes and arenes with well-defined metal-boryl complexes. Now, photoredox-initiated borylations, catalyzed by either transition metal or organic photocatalysts, and photochemical borylations with high efficiency have become a burgeoning area of research. In this Focus Review, we summarize research on photoinduced borylations, especially emphasizing recent developments and trends. This includes the photoinduced borylation of arenes, alkanes, aryl/alkyl halides, activated carboxylic acids, amines, alcohols, and so on based on transition metal catalysis, metal-free organocatalysis, and direct photochemical activation. We focus on reaction mechanisms involving single-electron transfer, triplet-energy transfer, and other radical processes.

Merging Boron with Nitrogen-Oxygen Bonds: A Review on BON Heterocycles

Cyclic boronate esters play important roles in organic synthesis, pharmacology, supramolecular chemistry and materials science owing to their stability in air and versatile reactivity. Most of these compounds contain a B-O-C linkage with an alkoxy- or carboxylate group bound to the boron atom (e.g. boronate-diol esters, MIDA boronates). Boron chelates comprising a B-O-N motif (BON heterocycles) are much less explored, although first representatives of this class were prepared in the early 1960s. In recent years, there has been a growing interest in BON heterocycles as new chemotypes for drug design. The exocyclic B-O-N linkage, which is readily formed under mild conditions, shows surprising hydrolytic and thermal resistance. This allows the formation of BON heterocycles to be used as click-type reactions for the preparation of bioconjugates and functionally modified polymers. We believe that BON heterocycles are promising yet underrated organoboron derivatives. This review summarizes the scattered information about known types of BON heterocycles, including their synthesis, reactivity and structural data. Available applications of BON heterocycles in materials science and medicinal chemistry, along with their prospects, are also discussed. The bibliography contains 289 references.

Reactivity of a magnesium diboranate with organic nitriles

A series of complexes generated through reactions of the β-diketiminato magnesium diboranate species, [(BDI)Mg{(n-Bu)pinB-Bpin}] (BDI = HC{(Me)CNDipp}2; Dipp = 2,6-di-iso-propylphenyl), and a variety of organic nitriles are reported. Although, in every case, the diboranate anion acts as a surrogate source of the {Bpin} nucleophile, resulting in B-C bond formation at the electrophilic sp-hydridised nitrile carbon, the resultant compounds display a variable propensity to undergo subsequent reaction with additional nitrile equivalents. This behaviour is rationalised to be a consequence of substituent-dependent modulation in the basicity and resultant electrophilicity of magnesium-coordinated nitrile intermediates.

Light-induced borylation: developments and mechanistic insights

Organoboron compounds are very important derivatives because of their profound impacts on medicinal, biological as well as industrial applications. The development of several novel borylation methodologies has achieved momentous interest among synthetic chemists. In this scenario, eco-friendly light-induced borylation is progressively becoming one of the best synthetic tools in recent days to prepare organoboronic ester and acid derivatives based on green chemistry rules. In this article, we have discussed all the UV- and visible-light-induced borylation strategies developed in the last decade. Furthermore, special attention is given to the mechanisms of these borylation methodologies for better understanding of reaction insights.

Samarium(iii) catalyzed synthesis of alkenylboron compounds via hydroboration of alkynes

The homoleptic lanthanide complex Sm[N(TMS)₂]₃ is an efficient rare-earth catalyst for the hydroboration of alkynes to the corresponding alkenylboron compounds.

Boronic Acids of Pharmaceutical Importance Affect the Growth and Photosynthetic Apparatus of Cyanobacteria in a Dose-Dependent Manner

The dynamic increase in the commercial application of antimicrobial derivatives of boronic acids, and potential impact of their presence in aquatic systems, supports the necessity to study the toxicity of these substances towards microorganisms of crucial meaning in the environment. One example of the mentioned derivatives is tavaborole (5-fluoro-substituted benzoxaborole), a pharmaceutical agent with antifungal activity. Cyanobacteria were used as model organisms, which are photoautotrophic prokaryotes, as representative aquatic bacteria and photoautotrophs associated with the plant kingdom. To the best of our knowledge, we investigated this issue for the first time. In order to recognize the under-stress response of those microorganisms, the concentration of photopigments—a key factor in the activity of photosynthetic apparatus—was measured spectrophotometrically. We found that the 3-piperazine bis(benzoxaborole) significantly suppressed the growth of halophilic and freshwater cyanobacteria, at a concentration 3.0 mM and 0.3 mM, respectively. Our results also showed that the tested substances at micromolar concentrations stimulated the growth of cyanobacteria, particularly in the freshwater strain Chroococcidiopsis thermalis. The tested substances acted with various strengths, depending on their structure and concentration; nevertheless, they had a greater influence on the synthesis of phycobiliproteins (e.g., lowered their concentration) than on the formation of chlorophyll and carotenoids.

Biochemical and histomorphological findings in Swiss Wistar rats treated with potential boron-containing therapeutic - K2[B3O3F4OH]

BACKGROUND

Boron and boron containing compounds are known for their biological and protective roles being non-toxic and non-mutagenic in low concentrations. Male rats were exposed to halogenated boroxine (HB), dipotassium-trioxohydroxytetrafluorotriborate K₂[B₃O₃F₄OH], a potential new boron-containing therapeutic, aiming to determine concentrations with no adverse effects on selected serum biochemical parameters and histomorphological features.

METHODS

HB was prepared by reacting potassium hydrofluoride (KHF₂) with boric acid in molar ratios 2:3 at room temperature and its primary structure contains 4 fluorine atoms substituted in 6-membered ring. In concentrations of 10, 25, 35 and 45 mg/kg, HB was administered intraperitoneally as a single dose. Biochemical parameters were observed 24 and 96 h following the treatment. Effects of HB on biochemical blood parameters were also observed 24 h following continuous nine days application in concentrations of 10 mg/kg intraperitoneally and 50 mg/kg per os. Histomorphological observation of kidneys, liver, spleen, lungs and heart was performed for all treated animals.

RESULTS

Administration of single high dose of HB (35 mg/kg-45 mg/kg) effected high levels of urea and creatinine, which indicated renal injury that appeared to be temporary. Possible cause of concern is pancreatic injury indicated by elevated levels of serum amylase in the groups of animals that received the highest dosages of the substance. Histopathological examination of selected tissues revealed mild to moderate lesions in the kidneys and livers associated with administration of HB.

CONCLUSION

Observation of biochemical serum parameters or histopathology of examined tissues revealed no adverse effects of HB either after the administration of single dose lower than 35 mg/kg or following repeated administration at 10 mg/kg. These dosages should be further considered for potential therapeutic applications.

Synthesis and structure-activity studies of novel anhydrohexitol-based Leucyl-tRNA synthetase inhibitors

Leucyl-tRNA synthetase (LeuRS) is a clinically validated target for the development of antimicrobials. This enzyme catalyzes the formation of charged tRNA^Leu molecules, an essential substrate for protein translation. In the first step of catalysis LeuRS activates leucine using ATP, forming a leucyl-adenylate intermediate. Bi-substrate inhibitors that mimic this chemically labile phosphoanhydride-linked nucleoside have proven to be potent inhibitors of different members of the aminoacyl-tRNA synthetase family but, to date, they have demonstrated poor antibacterial activity. We synthesized a small series of 1,5-anhydrohexitol-based analogues coupled to a variety of triazoles and performed detailed structure-activity relationship studies with bacterial LeuRS. In an in vitro assay, K_i^app values in the nanomolar range were demonstrated. Inhibitory activity differences between the compounds revealed that the polarity and size of the triazole substituents affect binding. X-ray crystallographic studies of N. gonorrhoeae LeuRS in complex with all the inhibitors highlighted the crucial interactions defining their relative enzyme inhibitory activities. We further examined their in vitro antimicrobial properties by screening against several bacterial and yeast strains. While only weak antibacterial activity against M. tuberculosis was detected, the extensive structural data which were obtained could make these LeuRS inhibitors a suitable starting point towards further antibiotic development.

Ameliorative Properties of Boronic Compounds in In Vitro and In Vivo Models of Alzheimer's Disease

Alzheimer's disease (AD) is characterized by amyloid (Aβ) aggregation, hyperphosphorylated tau, neuroinflammation, and severe memory deficits. Reports that certain boronic compounds can reduce amyloid accumulation and neuroinflammation prompted us to compare trans-2-phenyl-vinyl-boronic-acid-MIDA-ester (TPVA) and trans-beta-styryl-boronic-acid (TBSA) as treatments of deficits in in vitro and in vivo models of AD. We hypothesized that these compounds would reduce neuropathological deficits in cell-culture and animal models of AD. Using a dot-blot assay and cultured N₂a cells, we observed that TBSA inhibited Aβ42 aggregation and increased cell survival more effectively than did TPVA. These TBSA-induced benefits were extended to C. elegans expressing Aβ42 and to the 5xFAD mouse model of AD. Oral administration of 0.5 mg/kg dose of TBSA or an equivalent amount of methylcellulose vehicle to groups of six- and 12-month-old 5xFAD or wild-type mice over a two-month period prevented recognition- and spatial-memory deficits in the novel-object recognition and Morris-water-maze memory tasks, respectively, and reduced the number of pyknotic and degenerated cells, Aβ plaques, and GFAP and Iba-1 immunoreactivity in the hippocampus and cortex of these mice. These findings indicate that TBSA exerts neuroprotective properties by decreasing amyloid plaque burden and neuroinflammation, thereby preventing neuronal death and preserving memory function in the 5xFAD mice.

Potent Inhibition of Mandelate Racemase by Boronic Acids: Boron as a Mimic of a Carbon Acid Center

Boronic acids have been successfully employed as inhibitors of hydrolytic enzymes. Typically, an enzymatic nucleophile catalyzing hydrolysis adds to the electrophilic boron atom forming a tetrahedral species that mimics the intermediate(s)/transition state(s) for the hydrolysis reaction. We show that para-substituted phenylboronic acids (PBAs) are potent competitive inhibitors of mandelate racemase (MR), an enzyme that catalyzes a 1,1-proton transfer rather than a hydrolysis reaction. The K_i value for PBA was 1.8 ± 0.1 μM, and p-Cl-PBA exhibited the most potent inhibition (K_i = 81 ± 4 nM), exceeding the binding affinity of the substrate by ∼4 orders of magnitude. Isothermal titration calorimetric studies with the wild-type, K166M, and H297N MR variants indicated that, of the two Brønsted acid-base catalysts Lys 166 and His 297, the former made the greater contribution to inhibitor binding. The X-ray crystal structure of the MR·PBA complex revealed the presence of multiple H-bonds between the boronic acid hydroxyl groups and the side chains of active site residues, as well as formation of a His 297 N^ε2-B dative bond. The dramatic upfield change in chemical shift of 27.2 ppm in the solution-phase ¹¹B nuclear magnetic resonance spectrum accompanying binding of PBA by MR was consistent with an sp³-hybridized boron, which was also supported by density-functional theory calculations. These unprecedented findings suggest that, beyond substituting boron at carbon centers participating in hydrolysis reactions, substitution of boron at the acidic carbon center of a substrate furnishes a new approach for generating inhibitors of enzymes catalyzing the deprotonation of carbon acid substrates.

3-Pyridinylboronic acid normalizes the effects of 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine exposure in zebrafish embryos

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a neurotoxin that damages dopaminergic neurons. Zebrafish has been shown to be a suitable model organism to investigate the molecular pathways in the pathogenesis of Parkinson's disease and also for potential therapeutic agent research. Boron has been shown to play an important role in the neural activity of the brain. Boronic acids are used in combinatorial approaches in drug design and discovery. The effect of 3-pyridinylboronic acid which is an important sub-class of heterocyclic boronic acids has not been evaluated in case of MPTP exposure in zebrafish embryos. Accordingly, this study was designed to investigate the effects of 3-pyridinylboronic acid on MPTP exposed zebrafish embryos focusing on the molecular pathways related to neurodegeneration and apoptosis by RT-PCR. Zebrafish embryos were exposed to MPTP (800 μM); MPTP + Low Dose 3-Pyridinylboronic acid (50 μM) (MPTP + LB) and MPTP + High Dose 3-Pyridinylboronic acid (100 μM) (MPTP + HB) in well plates for 72 hours post fertilization. Results of our study showed that MPTP induced a P53 dependent and Bax mediated apoptosis in zebrafish embryos and 3-pyridinylboronic acid restored the locomotor activity and gene expressions related to mitochondrial dysfunction and oxidative stress due to the deleterious effects of MPTP, in a dose-dependent manner.

High Reactivity of α-Boryl Radical of Potassium Alkyltrifluoroborate in Atom-Transfer Radical Addition

We found that the α-boryl radical of potassium alkyltrifluoroborate shows higher reactivity compared to the α-boryl radicals of alkylboronic acid pinacol ester and alkyl N-methyl imidodiacetic acid (MIDA) boronate in the halogen atom abstraction step of atom-transfer radical addition (ATRA) between alkyl bromide and vinylborons. In this research, an ATRA of alkyl halides with potassium vinyltrifluoroborate furnished unique alkylborons, which are difficult to synthesize by other methods.

Amino Acid Transporters as Potential Therapeutic Targets in Thyroid Cancer

Thyroid cancer cells have a high amino acid demand for proliferation, invasion, and metastasis. Amino acids are taken up by thyroid cancer cells, both thyroid follicular cell and thyroid parafollicular cells (commonly called "C-cells"), via amino acid transporters. Amino acid transporters up-regulate in many cancers, and their expression level associate with clinical aggressiveness and prognosis. This is the review to discuss the therapeutic potential of amino acid transporters and as molecular targets in thyroid cancer.

Recent Advances in Metal-Catalyzed Alkyl–Boron (C(sp3)–C(sp2)) Suzuki-Miyaura Cross-Couplings

Boron chemistry has evolved to become one of the most diverse and applied fields in organic synthesis and catalysis. Various valuable reactions such as hydroborylations and Suzuki–Miyaura cross-couplings (SMCs) are now considered as indispensable methods in the synthetic toolbox of researchers in academia and industry. The development of novel sterically- and electronically-demanding C(sp3)–Boron reagents and their subsequent metal-catalyzed cross-couplings attracts strong attention and serves in turn to expedite the wheel of innovative applications of otherwise challenging organic adducts in different fields. This review describes the significant progress in the utilization of classical and novel C(sp3)–B reagents (9-BBN and 9-MeO-9-BBN, trifluoroboronates, alkylboranes, alkylboronic acids, MIDA, etc.) as coupling partners in challenging metal-catalyzed C(sp3)–C(sp2) cross-coupling reactions, such as B-alkyl SMCs after 2001.

Novel, Self-Assembling Dimeric Inhibitors of Human β Tryptase

β-Tryptase, a homotetrameric serine protease, has four identical active sites facing a central pore, presenting an optimized setting for the rational design of bivalent inhibitors that bridge two adjacent sites. Using diol, hydroxymethyl phenols or benzoyl methyl hydroxamates, and boronic acid chemistries to reversibly join two [3-(1-acylpiperidin-4-yl)phenyl]methanamine core ligands, we have successfully produced a series of self-assembling heterodimeric inhibitors. These heterodimeric tryptase inhibitors demonstrate superior activity compared to monomeric modes of inhibition. X-ray crystallography validated the dimeric mechanism of inhibition, and compounds demonstrated high selectivity against related proteases, good target engagement, and tryptase inhibition in HMC1 xenograft models. Screening 3872 possible combinations from 44 boronic acid and 88 diol derivatives revealed several combinations that produced nanomolar inhibition, and seven unique pairs produced greater than 100-fold improvement in potency over monomeric inhibition. These heterodimeric tryptase inhibitors demonstrate the power of target-driven combinatorial chemistry to deliver bivalent drugs in a small molecule form.

Advances in Discovery of PDE10A Inhibitors for CNS-Related Disorders. Part 1: Overview of the Chemical and Biological Research

Phosphodiesterase 10A (PDE10A) is a double substrate enzyme that hydrolyzes second messenger molecules such as cyclic-3',5'-adenosine monophosphate (cAMP) and cyclic-3',5'-guanosine monophosphate (cGMP). Through this process, PDE10A controls intracellular signaling pathways in the mammalian brain and peripheral tissues. Pharmacological, biochemical, and anatomical data suggest that disorders in the second messenger system mediated by PDE10A may contribute to impairments in the central nervous system (CNS) function, including cognitive deficits as well as disturbances of behavior, emotion processing, and movement. This review provides a detailed description of PDE10A and the recent advances in the design of selective PDE10A inhibitors. The results of preclinical studies regarding the potential utility of PDE10A inhibitors for the treatment of CNS-related disorders, such as schizophrenia as well as Huntington's and Parkinson's diseases are also summarized.

Boron Chemistry for Medical Applications

Boron compounds now have many applications in a number of fields, including Medicinal Chemistry. Although the uses of boron compounds in pharmacological science have been recognized several decades ago, surprisingly few are found in pharmaceutical drugs. The boron-containing compounds epitomize a new class for medicinal chemists to use in their drug designs. Carboranes are a class of organometallic compounds containing carbon (C), boron (B), and hydrogen (H) and are the most widely studied boron compounds in medicinal chemistry. Additionally, other boron-based compounds are of great interest, such as dodecaborate anions, metallacarboranes and metallaboranes. The boron neutron capture therapy (BNCT) has been utilized for cancer treatment from last decade, where chemotherapy and radiation have their own shortcomings. However, the improvement in the already existing (BPA and/or BSH) localized delivery agents or new tumor-targeted compounds are required before realizing the full clinical potential of BNCT. The work outlined in this short review addresses the advancements in boron containing compounds. Here, we have focused on the possible clinical implications of the new and improved boron-based biologically active compounds for BNCT that are reported to have in vivo and/or in vitro efficacy.

A MetAP2 inhibitor blocks adipogenesis, yet improves glucose uptake in cells

Adipose tissue expansion involves angiogenesis to remodel its capillary network. The enzymemethionine aminopeptidase 2(MetAP2) promotes angiogenesis.MetAP2 inhibitors suppress angiogenesis and have potential anti-obesity effect. However, impairment in adipose tissue expansion is also linked with impaired glycemic control.This study investigated the effect of BL6, a MetAP2 inhibitor, on adipogenesis and glucose disposal.To test effect on angiogenesis, Human Umbilical Vein Endothelial Cells(HUVECs) were treated with BL6 for 24h to determine tube formation. Further, to test effect on adipogenesis and glucose disposal,3T3-L1 pre-adipocytes were treated with BL6(0 µM, 20µM, 50 µM or 100µM) during differentiation. Differentiated cells were stained with Oil Red O for determining lipid accumulation, and glucose uptake assay. Protein levels and RNA expression for key genes involved in the adipogenic cascade were determined.BL6 treatment of HUVECs dose dependently blocked angiogenesis. During differentiation of pre-adipocytes, 50μM and 100µM BL6 significantly reduced lipid accumulation. Treatment with 100µM BL6 significantly decreased expression of adipogenic genes. Interestingly, BL6 treatment dose dependently increased glucose uptake by 3T3-L1 cells.MetAP2 inhibitor blocks angiogenesis, attenuates adipogenesis, yet increases cellular glucose uptake. Collectively this proof of concept study supports a possible role for MetAP2 inhibitor BL6, as a putative anti-obesity therapeutic agent.

Recent development of leucyl-tRNA synthetase inhibitors as antimicrobial agents

Aminoacyl-tRNA synthetases (aaRSs) widely exist in organisms and mediate protein synthesis. Inhibiting these synthetases can lead to the termination of protein synthesis and subsequently achieve antibacterial and antiparasitic purposes. Moreover, the structures of aaRSs found in eukaryotes have considerable structural differences compared to those in prokaryotes, based on which it is possible to develop highly selective inhibitors. Leucyl-tRNA synthetase (LeuRS) with unique synthesis and editing sites is one of 20 kinds of aaRSs. Many inhibitors targeting LeuRS have been designed and synthesized, some of which have entered clinical use. For example, the benzoxaborole compound AN2690 has been approved by the FDA for the treatment of onychomycosis. AN3365 is suspended in the phase II clinical trial due to the rapid development of AN3365 resistance, but it may be used in combination with other antibiotics. The aaRSs, especially LeuRS, are being considered as targets of new potential anti-infective drugs for the treatment of not only bacterial or fungal infections but also infections by trypanosomes and malaria parasites. This review mainly describes the development of LeuRS inhibitors, focusing on their mechanisms of action, structure-activity relationships (SARs), and in vitro and in vivo activities.

Arylboronic acids inhibit P2X7 receptor function and the acute inflammatory response

The P2X7 receptor (P2X7R) is an ion channel which is activated by interactions with the extracellular ATP molecules. The molecular complex P2X7R/ATP induces conformational changes in the protein subunits, opening a pore in the ion channel macromolecular structure. Currently, the P2X7R has been studied as a potential therapeutic target of anti-inflammatory drugs. Based on this, a series of eight boronic acids (NO) analogs were evaluated on the biologic effect of this pharmacophoric group on the human and murine P2X7R. The boronic acids derivatives NO-01 and NO-12 inhibited in vitro human and murine P2X7R function. These analogs compounds showed effect better than compound BBG and similar to inhibitor A740003 for inhibiting dye uptake, in vitro IL-1β release and ATP-induced paw edema in vivo. In both, in vitro and in vivo assays the compound NO-01 showed to be the hit compound in the present series of the arylboronic acids analogs. The molecular docking suggests that the NO derivatives bind into the upper body domain of the P2X7 pore and that the main intermolecular interaction with the two most active NO derivatives occur with the residues Phe 95, 103 and 293 by hydrophobic interactions and with Leu97, Gln98 and Ser101 by hydrogen bonds.. These results indicate that the boronic acid derivative NO-01 shows the lead compound characteristics to be used as a scaffold structure to the development of new P2X7R inhibitors with anti-inflammatory action.

De Novo Design of Boron-Based Peptidomimetics as Potent Inhibitors of Human ClpP in the Presence of Human ClpX

Boronic acids have attracted the attention of synthetic and medicinal chemists due to boron's ability to modulate enzyme function. Recently, we demonstrated that boron-containing amphoteric building blocks facilitate the discovery of bioactive aminoboronic acids. Herein, we have augmented this capability with a de novo library design and a virtual screening platform modified for covalent ligands. This technique has allowed us to rapidly design and identify a series of α-aminoboronic acids as the first inhibitors of human ClpXP, which is responsible for the degradation of misfolded proteins.

Benzoxaboroles as dynamic covalent receptors for bioconjugation and transport of nucleosides and related drugs: Proof of action in HeLa cells

In this work we describe not previously explored binding studies on the reversible interaction of benzoxaborole with ligands of medical and pharmaceutical interest such as nucleosidic drugs gemcitabine and capecitabine, as well as the hydrophobic chemotherapeutic doxorubicin. We include functional derivatives of benzoxaborole such as a near infrared fluorescent boronolectine, Cy-Bx, The dynamic covalent interaction in physiological conditions was assessed by spectroscopic techniques yielding moderate to high binding affinities. The cytotoxic activity of the drugs upon conjugation to the boronolectins was evaluated revealing significant influence of the bioconjugation status on the cellular viability. The availability of the conjugate for cellular uptake and localization in the model cancer cell line HeLa was assessed by fluorescence imaging. Benzoxaborole and the fluorescent boronolectin Cy-Bx, proved to be versatile conjugation tools for 1,2 and 1,3-diol containing pharmacophores as well as bioisosteric forms such as 1,2-hydroxyamino, envisioning these small boronolectins as components in systems for drug release with tracking capability.

Ligand-Controlled Regiodivergent Enantioselective Rhodium-Catalyzed Alkene Hydroboration

Regiocontrol in the rhodium-catalyzed boration of vinyl arenes is typically dominated by the presence of the conjugated aryl substituent. However, small differences in TADDOL-derived chiral monophosphite ligands can override this effect and direct rhodium-catalyzed hydroboration of β-aryl and β-heteroaryl methylidenes by pinacolborane to selectively produce either chiral primary or tertiary borated products. The regiodivergent behavior is coupled with enantiodivergent addition of the borane. The nature of the TADDOL backbone substituents and that of the phosphite moiety function synergistically to direct the sense and extent of regioselectivity and enantioinduction. Twenty substrates are shown to undergo each reaction mode with regioselectivity values reaching greater than 20:1 and enantiomer ratios reaching up to 98:2. A variety of subsequent transformations illustrate the potential utility of each product.

Gene expression alterations of human liver cancer cells following borax exposure

Borax is a boron compound that is becoming widely recognized for its biological effects, including lipid peroxidation, cytotoxicity, genotoxicity, antioxidant activity and potential therapeutic benefits. However, it remains unknown whether exposure of human liver cancer (HepG2) cells to borax affects the gene expression of these cells. HepG2 cells were treated with 4 mM borax for either 2 or 24 h. Gene expression analysis was performed using Affymetrix GeneChip Human Gene 2.0 ST Arrays, which was followed by gene ontology analysis and pathway analysis. The clustering result was validated using reverse transcription-quantitative polymerase chain reaction. A cell proliferation assay was performed using Celigo Image Cytometer Instrumentation. Following this, 2- or 24-h exposure to borax significantly altered the expression level of a number of genes in HepG2 cells, specifically 530 genes (384 upregulated and 146 downregulated) or 1,763 genes (1,044 upregulated and 719 downregulated) compared with the control group, respectively (≥2-fold; P<0.05). Twenty downregulated genes were abundantly expressed in HepG2 cells under normal conditions. Furthermore, the growth of HepG2 cells was inhibited through the downregulation of PRUNE1, NBPF1, PPcaspase-1, UPF2 and MBTPS1 (≥1.5-fold, P<0.05). The dysregulated genes potentially serve important roles in various biological processes, including the inflammation response, stress response, cellular growth, proliferation, apoptosis and tumorigenesis/oncolysis.

Effects of boron-containing compounds on immune responses: review and patenting trends

INTRODUCTION

Boron-containing compounds induce effects on immune responses. Such effects are interesting to the biomedical field for the development of therapeutic tools to modulate the immune system.

AREAS COVERED

The scope of BCC use to modify immune responses is expanding, mainly with regard to inflammatory diseases. The information was organized to demonstrate the breadth of reported effects. BCCs act as modulators of innate and adaptive immunity, with the former including regulation of cluster differentiation and cytokine production. In addition, BCCs exert effects on inflammation induced by infectious and noninfectious agents, and there are also reports regarding their effects on mechanisms involving hypersensitivity and transplants. Finally, the authors discuss the beneficial effects of BCCs on pathologies involving various targets and mechanisms.

EXPERT OPINION

Some BCCs are currently used as drugs in humans. The mechanisms by which these BCCs modulate immune responses, as well as the required structure-activity relationship for each observed mechanism of action, should be clarified. The former will allow for the development of improved immunomodulatory drugs with extensive applications in medicine. Patenting trends involve claims concerning the synthesis and actions of identified molecules with a defined profile regarding cytokines, cell differentiation, proliferation, and antibody production.

Boron's neurophysiological effects and tumoricidal activity on glioblastoma cells with implications for clinical treatment

Purpose: To define the actions of boron on normal neurophysiology and glioblastoma growth. Materials and Methods: PubMed and other relevant databases were searched. Results: Discovery of novel boron compounds in treatment of glioblastoma is being actively investigated, but the majority of such studies is focused on the synthesis of boron compounds as sensitizers to Boron Neutron Capture Therapy (BNCT). Nonetheless, the translational functionality of boron compounds is not limited to BNCT as many boron compounds possess direct tumoricidal activity and there is substantial evidence that certain boron compounds can cross the blood-brain barrier. Moreover, boron-containing compounds interfere with several tumorigenic pathways including intratumoral IGF-I levels, molybdenum Fe-S containing flavin hydroxylases, glycolysis, Transient Receptor Potential (TRP) and Store Operated Calcium Entry (SOCE) channels. Conclusions: Boron compounds deserve to be studied further in treatment of systemic cancers and glioblastoma due to their versatile antineoplastic functions.

Suppressive Role of Boron on Adipogenic Differentiation and Fat Deposition in Human Mesenchymal Stem Cells

Over the past years, adipose tissue has become an invaluable source of mesenchymal stem cells (MSCs) due to development of improved isolation methodologies. In a recent work, our group established a primary culture of human adipose-derived stem cells (hADSCs), which were characterized for their stem cell characteristics in detail and studied their myogenic differentiation potential in presence of boron. In the current study, we focused on the effects of a boron-containing compound, sodium pentaborate pentahydrate (NaB), on the adipogenic differentiation of hADSCs. Incorporation of boron in various chemical derivates has been a novel interest in drug-discovery attempts due to increasing number of reports on their anticancer, antibacterial, antiviral, and antifungal activities. In this report, a striking suppressive activity of boron on adipogenic differentiation of hADSCs is observed in a dose-dependent manner. Higher concentrations of NaB (20, 50, and 100 μg/mL (68, 170 and 340 μM)) resulted in a progressive decrease of lipid deposition, suppressed master regulators of adipogenesis transcriptional programming at the mRNA and protein levels, while having no evident cytotoxicity on the cells. The findings of this study are encouraging to undertake further investigations on potential beneficial effects boron in terms of its impact on normal and dysfunctional adipose biology. In that respect, these results pave the path to evaluate boron-based compounds in prevention and treatment of obesity which is a modern age pandemic that is predominant worldwide and found in strong association with comorbidities, including type 2 diabetes, hypertension, cardiovascular disease, cancers, and others."

Turning fear of boron toxicity into boron-containing drug design

BACKGROUND

Despite the historical employment of boron-containing compounds (BCCs) with medicinal purposes, the reported cases of BCC toxicity in humans during the twentieth century were driving us towards an "boron-withdrawal" period. Fortunately, the use of boric acid for specific purposes remains, and the discovery of natural BCCs with biological action attractive for therapeutic purposes as well as the introduction of some new BCCs for clinical use have reactivated the interest in studying the properties of these BCCs.

METHODS

We carried out a structured search of bibliographic databases for scientific peer-reviewed research literature regarding boron toxicity and linked that information to that about BCCs in drug design and development. A deductive qualitative content analysis methodology was applied to analyse the interventions and findings of the included studies using a theoretical outline.

RESULTS

This review recapitulates the following on a timeline: the boron uses in medicine, the data known about the toxicological profiles of some BCCs, the pharmacological properties of some BCCs that are employed in cancer and infectious disease therapies, and the known properties of BCCs recently introduced into clinical assays as well as the identification of their structure-activity relationships for toxicity and therapeutic use. Then, we discuss the use of new approaches taking advantage of some toxicological data to identify potent and efficient BCCs for prevention and therapy while limiting their toxic effects.

CONCLUSION

Data for boron toxicity can be strategically used for boron-containing drug design.