[Basic Organic and Inorganic Chemistry of Boron Clusters and Its Application to Drug Discovery].

In the past, drug discovery using low-molecular-weight compounds was dominated by structural design based on combinations of non-metallic elements such as carbon, nitrogen, oxygen, and halogens. Recent drug discovery efforts have shown extraordinary progress, an example of which is the adoption of non-universal elements. The approval of boron neutron capture therapy (BNCT) using a neutron accelerator in Japan ahead of other countries is still fresh in our memory. Other small-molecule drugs containing boron atoms have also been developed, and boron is becoming widely recognized as a constituent element for drug discovery. It is known that borane (BH3) is unstable because of its electron-deficient bonds; nevertheless, its stability has been improved by the formation of clusters through multimerization. Carborane (C2B10H12), one of the borane clusters, has an icosahedral structure with two carbon atoms and ten boron atoms and exhibits properties that vastly differ from conventional boron compounds. In this symposium review, we will introduce the basic chemistry of carboranes and their application to drug discovery. Boron is an essential element in plant cell wall formation and has extremely low toxicity to humans. I hope that this symposium review will be an opportunity for us to free ourselves from existing prejudices and constraints in drug discovery, and that new modalities that skillfully utilize the characteristics of boron and boron clusters will be developed one after another.

Challenging Approach to the Development of Novel Estrogen Receptor Modulators Based on the Chemical Properties of Guaiazulene.

Tamoxifen, a therapeutic agent for breast cancer, has been associated with genetic polymorphisms in the metabolism of N,N-dialkylaminoethyl substituent, which plays an important role in the expression of selective estrogen receptor modulator (SERM) activity. To solve this problem, we developed a novel estrogen receptor (ER) modulator, Az-01, on the basis of the aromaticity, dipole moment, and isopropyl group of guaiazulene. Az-01 showed four-fold lower binding affinity for ER than E2 but had similar ER-binding affinity to that of 4-hydroxytamoxifen (4-HOtam). Unlike tamoxifen, Az-01 acted as a partial agonist with very weak estrogenic activity at high concentrations when used alone, and it showed potent anti-estrogenic activity in the presence of E2. The cell proliferation and inhibition activities of Az-01 were specific to ER-expressing MCF-7 cells, and no effect of Az-01 on other cell proliferation signals was observed. These findings are important for the development of new types of SERMs without the N,N-dialkylaminoethyl substituent as a privileged functional group for SERMs.

Development of Force Field Parameters for p-Carborane to Investigate the Structural Influence of Carborane Derivatives on Drug Targets by Complex Formation.

Androgen receptor (AR) has a key role in the development and progression of prostate cancer, and AR antagonists are used for its remedy. Recently, carborane derivatives, which are carbon-containing boron clusters have attracted attention as new AR ligands. Here we determined the force field parameters of 10-vertex and 12-vertex p-carborane to facilitate in silico drug design of boron clusters. Then, molecular dynamics (MD) simulations of complexes of AR-carborane derivatives were performed to evaluate the parameters and investigate the influences of carborane derivatives on the three-dimensional structure of AR. Energy profiles were obtained using quantum chemical calculations, and the force-field parameters were determined by curve fitting of the energy profiles. The results of MD simulations indicated that binding of the antagonist-BA341 changed some hydrogen-bond formations involved in the structure and location of helix 12. Those results were consistent with previously reported data. The determined parameters are also useful for refining the structure of the carborane-receptor complex obtained by docking simulations and development of new ligands with carborane cages not only for AR but also for various receptors.

Antidepressant effect of BE360, a new selective estrogen receptor modulator, activated via CREB/BDNF, Bcl-2 signaling pathways in ovariectomized mice.

We have previously reported that the carborane compound BE360, a novel selective estrogen receptor modulator, has a therapeutic potential against dementia. This study aimed to explore the effects and underlying mechanisms of BE360 on depression-like behaviors in ovariectomized (OVX) mice subjected to subchronic stress, which are postmenopausal depression models. BE360 was subcutaneously administrated using a mini-osmotic pump, for 2 weeks. Depression-like behaviors were evaluated using the forced swimming test. Neurogenesis in the hippocampal dentate gyrus (DG) was measured by analyzing cells expressing doublecortin (DCX) following 5-bromo-2'-deoxyuridine (BrdU) uptake. The levels of phosphorylated cyclic-AMP response element-binding protein (p-CREB), brain-derived neurotrophic factor (BDNF), and Bcl-2 were measured using immunohistochemistry or immunoblotting. Depression-like behaviors in OVX + Stress-exposed mice improved after chronic treatment with BE360. BE360 treatment in OVX + Stress-exposed mice increased p-CREB, BDNF, and Bcl-2 expressions in the hippocampus. Immunohistochemistry showed that the number of BrdU/DCX double-positive cells in the DG of the hippocampus, which decreased significantly in OVX + Stress-exposed mice, increased after subchronic treatment with BE360. The present study demonstrates that BE360 exerts antidepressant effects via hippocampal neurogenesis, potentially activated through CREB/BDNF, Bcl-2 signaling pathways. These results indicate that BE360 may have therapeutic potential against postmenopausal depression.

Design and Synthesis of Novel Breast Cancer Therapeutic Drug Candidates Based upon the Hydrophobic Feedback Approach of Antiestrogens.

Based upon hydrophobic feedback approaches, we designed and synthesized novel sulfur-containing ERα modulators (4 and 5) as breast cancer therapeutic drug candidates. The tetrahydrothiepine derivative 5a showed the highest binding affinity toward ERα because of its high hydrophobicity, and it acted as an agonist toward MCF-7 cell proliferation. The corresponding alkylamino derivative 5d maintained high binding affinity to ERα and potently inhibited MCF-7 cell proliferation (IC50: 0.09 µM). Docking simulation studies of compound 5d with the ERα BD revealed that the large hydrophobic moiety of compound 5d fit well into the hydrophobic pocket of the ERα LBD and that the sulfur atom of compound 5d formed a sulfur-π interaction with the amino acid residue His524 of the ERα LBD. These interactions play important roles for the binding affinity of compound 5d to the ERα LBD.

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.

Anti-cancer activity of m-carborane-containing trimethoxyphenyl derivatives through tubulin polymerization inhibition.

m-Carborane-containing compound 1a was identified as a cell growth inhibitor from a random screening of a boron compound library. As 1a is a mixture of diastereomers due to the presence of two chiral carbons, we designed achiral derivatives 2-4 and studied the structure-activity relationships of the methoxy groups on the benzene ring. 3,4,5-Trimethoxybenzyl derivative 2a and 3,4,5-trimethoxybenzoyl derivative 3a showed more potent anti-cancer activity against the human breast cancer cell line MDA-MB-453 than lead compound 1a. Compound 3a inhibited tubulin polymerization in a dose-dependent manner.

An automated microliter-scale high-throughput screening system (MSHTS) for real-time monitoring of protein aggregation using quantum-dot nanoprobes.

Protein aggregation is the principal component of numerous protein misfolding pathologies termed proteinopathies, such as Alzheimer's disease, Parkinson's disease, prion disease, and AA amyloidosis with unmet treatment needs. Protein aggregation inhibitors have great potential for the prevention and treatment of proteinopathies. Here we report the development of an automated real-time microliter-scale high throughput screening (MSHTS) system for amyloid aggregation inhibitors using quantum-dot nanoprobes. Screening 504 crude extracts and 134 low molecular weight aromatic compounds revealed the relationship of amyloid-ß (Aß) aggregation inhibitory activities of plant extracts using a plant-based classification. Within the eudicots, rosids, Geraniales and Myrtales showed higher activity. Screening low molecular weight aromatic compounds demonstrated that the structure of tropolone endows it with potential Aß aggregation inhibitory activity. The activity of the most active tropolone derivative was higher than that of rosmarinic acid. MSHTS also identified three chaperone molecules as tau aggregation inhibitors. These results demonstrate that our automated MSHTS system is a novel and robust tool that can be adapted to a wide range of compounds and aggregation-prone polypeptides.

Novel androgen receptor full antagonists: Design, synthesis, and a docking study of glycerol and aminoglycerol derivatives that contain p-carborane cages.

Based on the co-crystal structure of bicalutamide with a T877A-mutated androgen receptor (AR), glycerol and aminoglycerol derivatives were designed and synthesized as a novel type of carborane-containing AR modulators. The (R)-isomer of 6c, whose chirality is derived from the glycerol group, showed 20 times more potent cell inhibitory activity against LNCaP cell lines expressing T877A-mutated AR than the corresponding (S)-isomer. Docking studies of both isomers with AR suggested that (R)-6c is in closer spatial proximity to helix-12 of the AR than (S)-6c, which is the most important common motif in the secondary structure of AR for the expression of antagonistic activity.

Synthesis, structure-activity relationships, and mechanistic studies of 5-arylazo-tropolone derivatives as novel xanthine oxidase (XO) inhibitors.

Xanthine oxidase (XO) is an enzyme that contains molybdenum at the active site and catalyzes the oxidation of purine bases to uric acid. Even though XO inhibitors are widely used for the treatment of hyperuricemia and gout, only very few such compounds are clinically used as drugs for the treatment of these diseases. Given the unique physicochemical properties of tropolone, i.e., its chelating effect and the pKa value that is similar to that of carboxylic acid, we have synthesized 22 5-arylazotropolone derivatives as potential XO inhibitors. In vitro enzyme-inhibitory assays for XO revealed that 3-nitro derivative 1j showed the most potent XO inhibitory activity, which is by one order of magnitude more potent than allopurinol. An enzyme-kinetic study revealed that 1j inhibited the production of uric acid by XO both competitively and non-competitively. A docking-simulation study of 1j with XO suggested that the carbonyl and hydroxyl groups of the tropolone ring interact with the hydroxy group that acts as a ligand for molybdenum and the amino acid residues around the active site of XO.

Design and synthesis of p-carborane-containing sulfamates as multitarget anti-breast cancer agents.

The development of multitarget anticancer agents is of high interest to medicinal chemists in terms of overcoming drug resistance and preventing cancer-cell migration. Based on the structure of the potent carborane-containing estrogen BE120, non-steroidal multitarget anti-breast cancer agent candidates 1a-1j were designed and synthesized. Compound 1f shows potent STS-inhibitory activity (IC50 = 1.8 µM), cell-growth-inhibitory (CGI) activity against 39 human cancer cell lines (MG-MID = 2.8 µM), and tubulin-polymerization-inhibitory (TPI) activity. An analysis of the DNA content for MDA-MB-453 breast cancer cells revealed that 1f arrests the cell cycle in the G2/M phase and induces apoptosis. Accordingly, 1f should be a promising therapeutic agent for hormone-dependent breast cancer.

Novel p-carborane-containing multitarget anticancer agents inspired by the metabolism of 17ß-estradiol.

The female hormone 17 ß-estradiol (E2) is synthesized from estrone by steroid sulfatase (STS), and metabolized into 2-methoxyestradiol (2-ME), whereby the biological activity of the latter is substantially different from that of E2. Based on the metabolic pathways of E2, a carborane-containing 2-ME mimic (1c) and its derivatives (1 and 2) were designed and synthesized as novel multitarget anticancer agents. Bissulfamate 1f exhibited potent STS-inhibitory activity and tubulin-polymerization-inhibitory activity. Moreover, the cell-growth-inhibitory (CGI) activity of 1f was similar to that of 2-ME in a panel screening against 39 human cancer cell lines. Accordingly, 1f should be a promising perspective therapeutic agent for hormone-dependent breast tissue.

Design and synthesis of iodocarborane-containing ligands with high affinity and selectivity toward ERß.

The selectivity and the binding affinity of previously reported carborane-containing ligands 2 and 3 toward ERß remains to be optimized. To improve their biological profiles, a series of iodinated carboranyl phenol derivatives (4-6) were designed and synthesized as prospective ERß-selective ligands with high affinity. Several iodinated carboranyl phenols showed high relative binding affinity (RBA) values for both ERs, and especially for ERß, due to suitable hydrophobic interactions of the iodine atoms with the hydrophobic amino acid residues of the ERß ligand-binding domains. Among these derivatives, 9,10-diiodo-m-carborane 5f exhibited a more than 100% increase of the RBA values toward ERß, a 14-fold increased selectivity for ERß over ERα, and ER-agonistic activity in MCF-7 cell proliferation assays.

Structure-activity relations of rosmarinic acid derivatives for the amyloid ß aggregation inhibition and antioxidant properties.

Amyloid-ß aggregation inhibitors are expected to be therapeutic or prophylactic agents for Alzheimer's disease. Rosmarinic acid, which is one of the main aggregation inhibitors derived from Lamiaceae, was employed as a lead compound and its 25 derivatives were synthesized. In this study, the structure-activity relations of rosmarinic acid derivatives for the amyloid-ß aggregation inhibitory effect (MSHTS assay), antioxidant properties, and xanthine oxidase inhibition were evaluated. Among the tested compounds, compounds 16d and 19 were found to the most potent amyloid aggregation inhibitors. The SAR revealed that the necessity of the presence of the phenolic hydroxyl on one side of the molecule as well as the lipophilicity of the entire molecule. The importance of these structural properties was also supported by docking simulations.

Targeting Cancer with PCPA-Drug Conjugates: LSD1 Inhibition-Triggered Release of 4-Hydroxytamoxifen.

Targeting cancer with small molecule prodrugs should help overcome problems associated with conventional cancer-targeting methods. Herein, we focused on lysine-specific demethylase 1 (LSD1) to trigger the controlled release of anticancer drugs in cancer cells, where LSD1 is highly expressed. Conjugates of the LSD1 inhibitor trans-2-phenylcyclopropylamine (PCPA) were used as novel prodrugs to selectively release anticancer drugs by LSD1 inhibition. As PCPA-drug conjugate (PDC) prototypes, we designed PCPA-tamoxifen conjugates 1 a and 1 b, which released 4-hydroxytamoxifen in the presence of LSD1 in vitro. Furthermore, 1 a and 1 b inhibited the growth of breast cancer cells by the simultaneous inhibition of LSD1 and the estrogen receptor without exhibiting cytotoxicity toward normal cells. These results demonstrate that PDCs provide a useful prodrug method that may facilitate the selective release of drugs in cancer cells.

Design, synthesis, and anti-proliferative activity of 1-(4-methoxyphenyl)-12-hydroxymethyl-p-carborane derivatives.

1-(4-Methoxyphenyl)-12-hydroxymethyl-p-carborane (2a), which is a precursor to the previously developed potent carborane-containing ER agonist BE120, exhibited weak cell growth inhibitory activity against four human cancer cell lines (MCF-7, MDA-MB-453, LNCaP, and PC-3). The biological evaluation of a series of derivatives of 2a revealed that an increased number of methoxy groups on the benzene ring of 2a enhanced the cell growth inhibitory activity. Trimethoxyphenyl derivative 2g afforded the most potent cell growth inhibitory activity (mean GI50 value: 5.8 µM) in a panel screening using 39 human cancer cell lines. Moreover, 2g induced for MDA-MB-453 breast cancer cell lines an arrest of the cell cycle in the G2/M phase and apoptosis mediated by caspase-3/7.

Symmetric 4,4'-(piperidin-4-ylidenemethylene)bisphenol derivatives as novel tunable estrogen receptor (ER) modulators.

We designed and synthesized 4,4'-(piperidin-4-ylidenemethylene)bisphenol derivatives as novel tunable estrogen receptor (ER) modulators. The introduction of hydrophobic substituents on the nitrogen atom of the piperidine ring enhanced ERα binding affinity. In addition, the introduction of four methyl groups adjacent to the piperidine ring nitrogen atom remarkably enhanced ERα binding affinity. N-Acetyl-2,2,6,6-tetramethylpiperidine derivative 3b showed high ERα binding affinity, high MCF-7 cell proliferation inducing activity, and high metabolic stability in rat liver S9 fractions.

BE360, a new selective estrogen receptor modulator, produces antidepressant and antidementia effects through the enhancement of hippocampal cell proliferation in olfactory bulbectomized mice.

We have reported that the carborane compound BE360 is a novel selective estrogen receptor modulator and new therapy option for osteoporosis. The aim of this study was to explore the effects and underlying mechanisms of BE360 on depressive-like behavior and memory impairment in the olfactory bulbectomized (OBX) mice, an experimental animal model of depression and dementia. BE360 was administered subcutaneously to mice using a mini-osmotic pump for 2 weeks. Depressive-like behavior was measured as the reduced intake of a sweet solution in the sucrose preference test. Short-term memory was assessed using the Y-maze test. Cell proliferation was assessed by the analysis of cells expressing 5-bromo-2'-deoxyuridine (BrdU) uptake. The expression of phosphorylated cyclic-AMP response element binding protein (pCREB) and brain-derived neurotrophic factor (BDNF) were measured by immunoblot. The depressive-like behavior and memory impairment in OBX mice were improved by the chronic treatment with BE360. Immunohistochemical analysis showed that the number of BrdU-positive cells in the dentate gyrus of the hippocampus significantly decreased in OBX mice whereas they increased after the chronic treatment with BE360. Immunoblotting studies revealed that pCREB and BDNF were significantly increased in the hippocampus of OBX mice treated with BE360. The present study has shown that BE360 has antidepressant and antidementia effects characterized by hippocampal cell proliferation potentially activated via CREB/BDNF signaling pathways. These results indicate that BE360 may have valuable therapeutic potential against depression and neurodegenerative diseases.