The chemistry of boron analogues of biomolecules

Cytotoxic and apoptotic effects of 1,2-diborolanes with strong donor substitutes on human cancer cells

In recent years, boron compounds have become more common as chemotherapy agents against certain types of cancers. Along with the development of boron-based therapeutic agents have come investigations into the various cancers and biochemical and molecular mechanisms affected by boron compounds and the relationships between boron compounds and chemical protection against cancer. In this preliminary study, the effects of new 1,2-N-substituted-1,2-diborolane derivatives on types of breast and liver cancers were examined for the first time. Four were found to significantly affect the cell viabilities and mitochondrial membrane potential changes in MCF-7, HepG2 and Hep3B cancer cells. Each was prepared in n-hexane at various concentrations (5, 10, 25, 50, 75 and 100 µg/mL). Human peripheral blood lymphocytes were used as control cells. Compounds 1, 2, 3a, and 3b 1,2-diborolane derivatives selectively killed cancer cells, but compound 1 was cytotoxic in a concentration-dependent manner on HepG2 and Hep3B and only at concentrations of at least 75 µg/mL on MCF-7 cells. Compound 3a exhibited cytotoxic effect on lymphocytes at 75 and 100 µgmL^-1 concentrations, but compounds 1, 2 and 3b, 3c and 3d have not possessed significant cytotoxic effect on lymphocytes. Compounds 3c and 3d have not possessed significant cytotoxic effects. Mitochondrial membrane potential assay results supported these findings. Our results reveal that 1,2-diborolane derivates have high cytotoxic and apoptotic activities on human hepatocarcinoma cells and are therefore potential candidates in the development of new drugs against liver cancer.

Design, Synthesis, and Biological Evaluation of Boron-Containing Macrocyclic Polyamines and Their Zinc(II) Complexes for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a binary therapeutic method for cancer treatment based on the use of a combination of a cancer-specific drug containing boron-10 (¹⁰B) and thermal neutron irradiation. For successful BNCT, ¹⁰B-containing molecules need to accumulate specifically in cancer cells, because destructive effect of the generated heavy particles is limited basically to boron-containing cells. Herein, we report on the design and synthesis of boron compounds that are functionalized with 9-, 12-, and 15-membered macrocyclic polyamines and their Zn²⁺ complexes. Their cytotoxicity, intracellular uptake activity into cancer cells and normal cells, and BNCT effect are also reported. The experimental data suggest that mono- and/or diprotonated forms of metal-free [12]aneN₄- and [15]aneN₅-type ligands are uptaken into cancer cells, and their complexes with intracellular metals such as Zn²⁺ would induce cell death upon thermal neutron irradiation, possibly via interactions with DNA.

Towards carborane-functionalised structures for the treatment of brain cancer

Boron neutron capture therapy (BNCT) is a promising targeted chemoradiotherapeutic technique for the management of invasive brain tumors, such as glioblastoma multiforme (GBM). A prerequisite for effective BNCT is the selective targeting of tumour cells with ¹⁰B-rich therapeutic moieties. To this end, polyhedral boranes, especially carboranes, have received considerable attention because they combine a high boron content with relative low toxicity and metabolic inertness. Here, we review progress in the molecular design of recently investigated carborane derivatives in light of the widely accepted performance requirements for effective BNCT.

Palladium-catalyzed regioselective and stereo-invertive ring-opening borylation of 2-arylaziridines with bis(pinacolato)diboron: experimental and computational studies

A palladium catalyzed regioselective borylative ring opening reaction of 2-arylaziridines to give β-amino-β-arylethylborates was developed. The reaction reported herein represents the first example of ring-opening borylation of non-vinylic aziridines and direct borylative C(sp3)-N bond cleavage of neutral organic substrates. NMR studies and density functional theory (DFT) calculations suggested that the active intermediate for the reaction is a PdL2 complex [L = P(t-Bu)2Me]. The multi-component artificial force-induced reaction method (MC-AFIR) located the transition states for the regioselectivity-determining aziridine ring opening that proceeds in an SN2 fashion, and explained the selectivity of the reaction. The full catalytic cycle consists of a selectivity-determining aziridine ring opening (oxidative addition), a proton transfer, phosphine ligand dissociation from the catalyst, boron-boron bond cleavage, and reductive elimination. Water is important to the drive the transmetalation step. The calculated overall mechanism and selectivity are consistent with the experimental results.

Boronated carbohydrate derivatives as potential boron neutron capture therapy reagents

The treatment of cancer remains one of the most challenging problems for humanity. Boron neutron capture therapy is a binary approach for cancer treatment that is particularly attractive in treating high-grade gliomas and metastatic brain tumors. Among the types of boron-containing molecules used as boron neutron capture therapy agents, boronated carbohydrate derivatives have received significant attention because of their preferential uptake by growing tumor cells. This review provides a summary of the recent developments in the chemistry of carborane-containing carbohydrates.

A Green Approach to the Production of Hybrid Diindolylmethane-Phenylboronic Acids via a 3MCR: Promising Antineoplasic Molecules

The current role of the “Green Chemistry Protocol” in multicomponent reactions is first highlighted. Then, the green approach to the production of three novel hybrid diindolylmethanes-phenylboronic acids via a 3MCR is discussed, which features the following: solventless conditions, the use of microwave irradiation to activate the reactions, the absence of catalyst, and an efficient atom economy. The products were achieved with moderate yields (41–61%) within a short time frame (5 min) and appropriately characterized by elemental analysis and spectroscopic methods (NMR:1H,13C,11B; MS: EI, CI, FAB+, HRMS). During the FAB+MS determinations, various artifacts that are associated with the boron atom via an interaction with thioglycerol from the matrix were noted in the corresponding FAB+mass spectra; in addition, the accurate mass determination of these adducts unequivocally confirmed the presence of the target molecules. Moreover, the activity of these target molecules was evaluated in the presence of six cancer cell lines (U251 = glia of the central nerve system, PC-3 = prostate, K562 = leukemia, HCT-15 = colon, MCF-7 = breast, and SKLU-1 = lung), which resulted in themeta-regioisomer being the most active. Finally, the products were also analyzed using computational chemistry in order to determine their most stable geometries and reactivities by computing the respective molecular electrostatic potentials.

Design and synthesis of 3,5-disubstituted boron-containing 1,2,4-oxadiazoles as potential combretastatin A-4 (CA-4) analogs

We have designed and synthesized a small library of 3,5-disubstituted-1,2,4-oxadiazole containing combretastatin A-4 (CA-4) analogs. Our objective is to increase the efficacy of the CA-4 as an anti-tubulin and antimitotic agent by substituting the cis-alkene bond with one of its bioisosteres, the 1,2,4-oxadiazole ring. We also modified the substituents attached to both of the phenyl rings (ring A and B in Fig. 1) of CA-4 for the purpose of diversifying our analogs based on SAR. These compounds were synthesized via a coupling reaction between an amidoxime and a carboxylic acid in DMF solvent, with HOBt as a base, and utilizing EDCI as a coupling reagent. Using this protocol, we synthesized a small library of 10 compounds with moderate to good yields. A detailed biological study is currently undergoing in our laboratory to evaluate the activity of these compounds.

Design and synthesis of boron containing potential pan-RAR inverse agonists

We designed and successfully synthesized the compounds 5 and 8 as potential pan-RAR (retinoic acid receptor) agonists. These two compounds were designed based on an existing pan-RAR agonist (BMS493). We synthesized compound 5, in which the carboxylic acid group in BMS 493 was replaced by boronic ester; and compound 8, in which the double bond of BMS 493 was changed to an oxadiazole (as bioisosteres of double bond) ring. The two target molecules 5 and 8 were synthesized from the commercially available 7-bromo-4,4-dimethyl-3,4-dihydronaphthalen-1(2H)-one 1. Compound 1 was derivatized to intermediate 5,5-dimethyl-8-(phenylethynyl)-5,6-dihydronaphthalene-2 carbaldehyde 4 by using alkylation, dehydration, and metal exchange reactions. The intermediate 4 was further converted to 5 by using a Wittig reaction and to 8 by amide coupling and dehydration to give overall 18% and 33% yields, respectively, after 8 steps in each case.

Dicarba-closo-dodecarborane-containing half-sandwich complexes of ruthenium, osmium, rhodium and iridium: biological relevance and synthetic strategies

This review describes how the incorporation of dicarba-closo-dodecarboranes into half-sandwich complexes of ruthenium, osmium, rhodium and iridium might lead to the development of a new class of compounds with applications in medicine. Such a combination not only has unexplored potential in traditional areas such as Boron Neutron Capture Therapy agents, but also as pharmacophores for the targeting of biologically important proteins and the development of targeted drugs. The synthetic pathways used for the syntheses of dicarba-closo-dodecarboranes-containing half-sandwich complexes of ruthenium, osmium, rhodium and iridium are also reviewed. Complexes with a wide variety of geometries and characteristics can be prepared. Examples of addition reactions on the metal centre, B-H activation, transmetalation reactions and/or direct formation of metal-metal bonds are discussed (103 references).

11B NMR sensing of d-block metal ions in vitro and in cells based on the carbon-boron bond cleavage of phenylboronic acid-pendant cyclen (cyclen = 1,4,7,10-tetraazacyclododecane)

Noninvasive magnetic resonance imaging (MRI) including the "chemical shift imaging (CSI)" technique based on (1)H NMR signals is a powerful method for the in vivo imaging of intracellular molecules and for monitoring various biological events. However, it has the drawback of low resolution because of background signals from intrinsic water protons. On the other hand, it is assumed that the (11)B NMR signals which can be applied to a CSI technique have certain advantages, since boron is an ultratrace element in animal cells and tissues. In this manuscript, we report on the sensing of biologically indispensable d-block metal cations such as zinc, copper, iron, cobalt, manganese, and nickel based on (11)B NMR signals of simple phenylboronic acid-pendant cyclen (cyclen = 1,4,7,10-tetraazacyclododecane), L(6) and L(7), in aqueous solution at physiological pH. The results indicate that the carbon-boron bond of L(6) is cleaved upon the addition of Zn(2+) and the broad (11)B NMR signal of L(6) at 31 ppm is shifted upfield to 19 ppm, which corresponds to the signal of B(OH)(3). (1)H NMR, X-ray single crystal structure analysis, and UV absorption spectra also provide support for the carbon-boron bond cleavage of ZnL(6). Because the cellular uptake of L(6) was very small, a more cell-membrane permeable ligand containing the boronic acid ester L(7) was synthesized and investigated for the sensing of d-block metal ions using (11)B NMR. Data on (11)B NMR sensing of Zn(2+) in Jurkat T cells using L(7) is also presented.

Bioengineering functional copolymers. XV. Synthesis of organoboron amide-ester branched derivatives of oligo(maleic anhydride) and their interaction with HeLa and L929 fibroblast cells

Novel bioengineering functional organoboron oligomers were synthesized by (i) amidolysis of oligo(maleic anhydride) (OMA) with 2-aminoethyldiphenylborinate (2-AEPB), (ii) esterification of organoboron oligomer (OMA-B) with α-hydroxy-ω-methoxypoly(ethylene oxide) (PEO) as a compatibilizer and (iii) conjugation of organoboron PEO branches (OMA-B-PEO) with folic acid as a taggering agent. Structure and composition of the synthesized oligomers were characterized by FTIR-ART and 1H (13C) NMR spectroscopy, chemical and physical analysis methods. Interaction of functional oligomers and oligomer···FA complex (OMA-B-PEO-F) with HeLa and L929 fibroblast cells were investigated by using different biochemical methods such as cytotoxicity, statistical, apoptotic and necrotic cell indexes, double staining and caspase-3 immunostaining, light and fluorescence inverted microscope analyses. It was found that citotoxisity and apoptotic/necrotic effects of oligomers significantly depend on the structure and composition of studied oligomers, and increase the following raw: OMA << OMA-B < OMA-B-PEO < OMA-B-PEO-F. A folic acid complex (MA-PEG-B-F) at 400 μg ml–1 (2.36 μmol ml–1) concentration as a therapeutic drug exhibits minimal toxcisity toward the fibroblast cells, but influential for HeLa cells.

Design, synthesis and biological study of pinacolyl boronate-substituted stilbenes as novel lipogenic inhibitors

A pilot library of novel 4,4,5,5-tetramethyl-2-(4-substitutedstyrylphenyl)-1,3,2 dioxaborolane derivatives has been synthesized. 4-(4,4,5,5-Tetramethyl-1,3,2-dioxaboratophenyl)-methyl triphenylphosphonium bromide 3 was treated with various aldehydes in the presence of 3 equiv of (t)BuONa in DMF, and stirred at room temperature for 4-6h to yield the corresponding boron-containing stilbene derivatives in 71-94% yields. Several of them, including BF102 and BF175, have the lipogenesis inhibitory effect by suppressing lipogenic gene expression in mammalian hepatocytes. Further, BF102 also inhibits cholesterol biosynthesis by suppressing HMG-CoA reductase gene expression in hepatocytes. Interestingly, our preliminary in vivo data suggests that BF102 has no significant toxicity in mice at the highest possible dose we can administered. Thus, BF102 is a potential lead for the next generation of lipid-lowering drugs.

Design and Synthesis of 3,5-Disubstituted 1,2,4-Oxadiazole Containing Retinoids from a Retinoic Acid Receptor Agonist

We previously synthesized novel retinoid libraries, and after screening for bioactivity found one compound BT10 that functions as a specific agonist for retinoic acid receptors. This lead compound was further derivatized using SAR and LRD to obtain 3,5-disubstituted-1,2,4-oxadiazole-containing retinoids. The new oxadiazole (amide bioisosters)-containing retinoids (compounds 1, 2, 3, 4, 5, and 6) were synthesized in 42-65% yield by reacting with (E)-4-((3-ethyl,2-4,4,4-trimethylcyclohex-2-enylidene)methyl)benzoic acid and phenyl substituted amidoxime in DMF using CDI as the coupling reagent. The biological activities of the synthesized compounds are currently being evaluated.

PXPd-catalyzed borylation of aniline derivatives

Addition of pinacolborane (HBO2C2Me4) to 2-iodoaniline can be catalyzed using a number of palladium complexes, including [(t-Bu)2PCl]2PdCl2 (PXPd), to give the corresponding boronate ester 2-H2NC6H4(BO2C2Me4) in excellent yields. The PXPd system could also be used in the catalyzed borylation of substituted anilines to give the corresponding aminoboronate esters in moderate to high yields.

Design and synthesis of novel pinacolylboronate containing combretastatin 'antimitotic agent' analogues

We developed a procedure to synthesize pinacolyl boronate containing stilbene derivatives and used this procedure to synthesize boron-containing combretastatin analogues. The key step involves the Wittig reaction of the ylide 4-(4,4,5,5-tetramethyl-1,3,2-dioxaboratophenyl)-methyl triphenylphosphonium bromide 11 with 3,4,5-trimethoxy benzaldehyde in the presence of (t)BuONa in DMF, providing 88% yield. We are now in a position to evaluate the biological activity of these derivatives as modulators of TGF-beta signaling pathways.

Elemental Isomerism: A Boron-Nitrogen Surrogate for a Carbon-Carbon Double Bond Increases the Chemical Diversity of Estrogen Receptor Ligands

To increase the chemical diversity of bioactive molecules by incorporating unusual elements, we have examined the replacement of a C=C double bond with the isoelectronic, isostructural B-N bond in the context of nonsteroidal estrogen receptor (ER) ligands. While the B-N bond was hydrolytically labile in the unhindered cyclofenil system, the more hindered anilino dimesitylboranes, analogs of triarylethylene estrogens, were easily prepared, hydrolytically stable, and demonstrated substantial affinity for ERs. X-ray analysis of one ERalpha-ligand complex revealed steric clashes with the para methyl groups distorting the receptor; removal of these groups resulted in an increase in affinity, potency, and transcriptional efficacy. These studies define the structural determinants of stability and cellular bioactivity of a B-N for C=C substitution in nonsteroidal estrogens and provide a framework for further exploration of "elemental isomerism" for diversification of drug-like molecules.

In vitro apoptotic activity of 2,2-diphenyl-1,3,2-oxazaborolidin-5-ones in L5178Y cells

Compounds containing B-N bonds have shown interesting biological activity. One class of such molecules is the 2,2-diphenyl-1,3,2-oxazaborolidin-5-ones (3a-j), which contain a B-N bond, have an alpha-amino acid moiety in the heterocycle, and have an exocyclic moiety related to an amino acid. The purpose of this work was to determine the inhibitory effects of 3a-j on the proliferation of murine L5178Y lymphoma cells. A new five-membered heterocyclic nucleus with apoptotic activity was found. The target products showed potent cytotoxicity in the L5178Y cell line. Among them, 3a exhibited the highest antineoplastic activity in L5178Y cells with an IC(50) value of 22.5+/-0.2 microM.

Expanding Roles for Organoboron Compounds – Versatile and Valuable Molecules for Synthetic, Biological and Medicinal Chemistry

The present essay offers an overview of the latest developments in the chemistry of organoboron compounds. The unique structural characteristics and the versatile reactivity profile of organoboron compounds continue to expand their roles in several areas of chemistry. A growing number of boron-mediated reactions have become vital tools for synthetic chemistry, particularly in asymmetric synthesis, metal-catalyzed processes, acid catalysis, and multicomponent reactions. As a result, boronic acids and related molecules have now evolved as major players in synthetic and medicinal chemistry. Moreover, their remnant electrophilic reactivity, even under physiological conditions, has allowed their incorporation in a growing number of bioactive molecules, including bortezomib, a clinically approved anticancer agent. Finally, the sensitive and selective binding of boronic acids to diols and carbohydrates has led to the development of a growing number of novel chemosensors for the detection, quantification, and imaging of glucose and other carbohydrates. There is no doubt that the chemistry of organoboron compounds will continue to expand into new discoveries and new applications in several fields of science.

Synthesis and biological evaluation of boronic acid containing cis-stilbenes as apoptotic tubulin polymerization inhibitors

A series of boronic acid containing cis-stilbenes as potent inhibitors of tubulin polymerization was synthesized by the introduction of boronic acid as an acceptor-type functional group into the aromatic ring B of the combretastatin framework. High cell-growth inhibition was observed with boron compounds 13 c and 13 d, in which a hydroxy group on the aromatic ring B of combretastatin A-4 was replaced with boronic acid; IC50 values toward B-16 and 1-87 cell lines are 0.48-2.1 microM. Compounds 13 c and 13 d exhibited significant inhibitory activity toward tubulin polymerization (IC50=21-22 microM). The carboxylic acid derivative 17, which can be considered as a mimic of boronic acid 13 c, did not show significant inhibition of cell growth or tubulin polymerization. According to the FACScan analysis using Jurkat cells, apoptosis was induced after incubation for 8 h with 13 c at a concentration of >10(-8) M. Growth inhibitory experiments against a panel of 39 human cancer cell lines revealed 13 c to inhibit growth differently than combretastatin A-4; the correlation coefficient (r) between the two compounds was 0.553 in the COMPARE analysis.

Evaluation of oxazaborolidine activity on Streptococcus mutans biofilm formation

Dental diseases are among the most prevalent afflictions of humankind. These diseases are associated with the formation of biofilms harbouring pathogenic bacteria. Eight different derivatives of oxazaborolidines were synthesised and evaluated for their affect on Streptococcus mutans adhesion and biofilm formation. Structure-activity relationship was observed. The B-butyl moiety of the oxazaborolidines contributed an anti-adhesion effect for all derivatives, whilst its effect diminished when the boron atom was incorporated in a fused heterocyclic ring. The B-phenyl group induced bacterial adhesion in all tested compounds Oxazaborolidines may serve as novel agents for affecting oral biofilm formation. Moreover, the ability to alter the oxazaborolidine molecule and thus affect biofilms offers an excellent opportunity to study biofilm formation.

Boron neutron capture therapy of cancer: current status and future prospects

BACKGROUND

Boron neutron capture therapy (BNCT) is based on the nuclear reaction that occurs when boron-10 is irradiated with low-energy thermal neutrons to yield high linear energy transfer alpha particles and recoiling lithium-7 nuclei. Clinical interest in BNCT has focused primarily on the treatment of high-grade gliomas and either cutaneous primaries or cerebral metastases of melanoma, most recently, head and neck and liver cancer. Neutron sources for BNCT currently are limited to nuclear reactors and these are available in the United States, Japan, several European countries, and Argentina. Accelerators also can be used to produce epithermal neutrons and these are being developed in several countries, but none are currently being used for BNCT.

BORON DELIVERY AGENTS

Two boron drugs have been used clinically, sodium borocaptate (Na(2)B(12)H(11)SH) and a dihydroxyboryl derivative of phenylalanine called boronophenylalanine. The major challenge in the development of boron delivery agents has been the requirement for selective tumor targeting to achieve boron concentrations ( approximately 20 microg/g tumor) sufficient to deliver therapeutic doses of radiation to the tumor with minimal normal tissue toxicity. Over the past 20 years, other classes of boron-containing compounds have been designed and synthesized that include boron-containing amino acids, biochemical precursors of nucleic acids, DNA-binding molecules, and porphyrin derivatives. High molecular weight delivery agents include monoclonal antibodies and their fragments, which can recognize a tumor-associated epitope, such as epidermal growth factor, and liposomes. However, it is unlikely that any single agent will target all or even most of the tumor cells, and most likely, combinations of agents will be required and their delivery will have to be optimized.

CLINICAL TRIALS

Current or recently completed clinical trials have been carried out in Japan, Europe, and the United States. The vast majority of patients have had high-grade gliomas. Treatment has consisted first of "debulking" surgery to remove as much of the tumor as possible, followed by BNCT at varying times after surgery. Sodium borocaptate and boronophenylalanine administered i.v. have been used as the boron delivery agents. The best survival data from these studies are at least comparable with those obtained by current standard therapy for glioblastoma multiforme, and the safety of the procedure has been established.

CONCLUSIONS

Critical issues that must be addressed include the need for more selective and effective boron delivery agents, the development of methods to provide semiquantitative estimates of tumor boron content before treatment, improvements in clinical implementation of BNCT, and a need for randomized clinical trials with an unequivocal demonstration of therapeutic efficacy. If these issues are adequately addressed, then BNCT could move forward as a treatment modality.