Boron Cluster-based Development of Potent Nonsecosteroidal Vitamin D Receptor Ligands: Direct Observation of Hydrophobic Interaction between Protein Surface and Carborane

Recent Advances in Carborane-Based Crystalline Porous Materials

The field of carborane research has witnessed continuous development, leading to the construction and development of a diverse range of crystalline porous materials for various applications. Moreover, innovative synthetic approaches are expanding in this field. Since the first report of carborane-based crystalline porous materials (CCPMs) in 2007, the synthesis of carborane ligands, particularly through innovative methods, has consistently posed a significant challenge in discovering new structures of CCPMs. This paper provides a comprehensive summary of recent advances in various synthetic approaches for CCPMs, along with their applications in different domains. The primary challenges and future opportunities are expected to stimulate further multidisciplinary development in the field of CCPMs.

Molecular engineering of AIE-active boron clustoluminogens for enhanced boron neutron capture therapy

The development of boron delivery agents bearing an imaging capability is crucial for boron neutron capture therapy (BNCT), yet it has been rarely explored. Here we present a new type of boron delivery agent that integrates aggregation-induced emission (AIE)-active imaging and a carborane cluster for the first time. In doing so, the new boron delivery agents have been rationally designed by incorporating a high boron content unit of a carborane cluster, an erlotinib targeting unit towards lung cancer cells, and a donor-acceptor type AIE unit bearing naphthalimide. The new boron delivery agents demonstrate both excellent AIE properties for imaging purposes and highly selective accumulation in tumors. For example, at a boron delivery agent dose of 15 mg kg-1, the boron amount reaches over 20 μg g-1, and both tumor/blood (T/B) and tumor/normal cell (T/N) ratios reach 20-30 times higher than those required by BNCT. The neutron irradiation experiments demonstrate highly efficient tumor growth suppression without any observable physical tissue damage and abnormal behavior in vivo. This study not only expands the application scopes of both AIE-active molecules and boron clusters, but also provides a new molecular engineering strategy for a deep-penetrating cancer therapeutic protocol based on BNCT.

A computational study to determine the role of σ-hole in Br/OH substituted nido-carborane and its binding capabilities

A detailed investigation of the σ-hole on the halogen atom present in the nido-heteroboranes is made by employing quantum mechanical methods. The bromide and the hydroxyl groups are incorporated in the exo-substituents of the nido-boranes. The potential of the bromide σ-hole was compared to that of electrostatic potential of hydroxyl group counterpart. The presence of a carbon atom vertex, in a different position of a system, influences the σ-hole and hence its binding abilities. Bromide substituted nido-carboranes have less potential and hence weaker binding ability compared to their closo-counterparts. Binding affinity with aliphatic is found to be more compared to that of aromatic system. The presence of solvent dampened the electrostatic interactions. Apart from the neutral system, the binding capabilities of charged nido-heteroboranes were also studied. The results of this study will be further useful for several applications viz., crystal engineering, drug designing (Pharmaceuticals), medicine, material science, energy storage devices, etc.

Hydrophobic tag-based protein degradation: Development, opportunity and challenge

Targeted protein degradation (TPD) has emerged as a promising approach for drug development, particularly for undruggable targets. TPD technology has also been instrumental in overcoming drug resistance. While some TPD molecules utilizing proteolysis-targeting chimera (PROTACs) or molecular glue strategies have been approved or evaluated in clinical trials, hydrophobic tag-based protein degradation (HyT-PD) has also gained significant attention as a tool for medicinal chemists. The increasing number of reported HyT-PD molecules possessing high efficiency in degrading protein and good pharmacokinetic (PK) properties, has further fueled interest in this approach. This review aims to present the design rationale, hydrophobic tags in use, and diverse mechanisms of action of HyT-PD. Additionally, the advantages and disadvantages of HyT-PD in protein degradation are discussed. This review may help inspire the development of more HyT-PDs with superior drug-like properties for clinical evaluation.

Design, synthesis and anti-hepatic fibrosis activity of novel diphenyl vitamin D receptor agonists

Hepatic fibrosis poses a significant threat to human health due to excessive extracellular matrix (ECM) deposition leading to liver function damage. Ligand-activated vitamin D receptor (VDR) has been identified as an effective target for hepatic fibrosis, reducing ECM by inhibiting hepatic stellate cell (HSC) activation. Here, a series of novel diphenyl VDR agonists have been rationally designed and synthesized. Among these, compounds 15b, 16i, and 28m showed better transcriptional activity compared to sw-22, which was previously reported to be a potent non-secosteroidal VDR modulator. Moreover, these compounds exhibited outstanding efficacy to inhibit collagen deposition in vitro. In models of CCl₄-induced and bile duct ligation-induced hepatic fibrosis, compound 16i showed the most significant therapeutic effect by ultrasound imaging and histological examination. Moreover, 16i was able to repair liver tissue by reducing the expression levels of fibrosis genes and serum liver function indexes without causing hypercalcemia in mice. In conclusion, compound 16i is a potent VDR agonist with significant anti-hepatic fibrosis action both in vitro and in vivo.

Preparation of Carborane-Tailored Covalent Organic Frameworks by a Postsynthetic Modification Strategy as a Barrier to Polysulfide in Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) have attracted much attention due to their high energy density and theoretical specific capacity. However, the "shuttle effect" of polysulfides limits their application. Herein, we propose a postsynthetic modification (PSM) strategy to synthesize a fibrous carborane-tailored covalent organic framework (PMCB-COF). Benefiting from its amphiphilicity, strong adsorption ability, high specific surface area, and accessible Li⁺ transport channels, PMCB-COF could serve as a barrier to polysulfide to inhibit the shuttle effect. The cell assembled with PMCB-COF exhibits a high initial capacity of 926 mAh g^-1 at 1 C. A Coulombic efficiency of 98% and a fading rate of only 0.039% per cycle are exhibited even after 1500 cycles. So far as we know, PMCB-COF is one of the best materials as a separator of LSBs. This work provides a safe and efficient avenue for tailoring COFs with carborane and might help promote the development of secure, low-cost, and durable rechargeable batteries.

The Centennial Collection of VDR Ligands: Metabolites, Analogs, Hybrids and Non-Secosteroidal Ligands

Since the discovery of vitamin D a century ago, a great number of metabolites, analogs, hybrids and nonsteroidal VDR ligands have been developed. An enormous effort has been made to synthesize compounds which present beneficial properties while attaining lower calcium serum levels than calcitriol. This structural review covers VDR ligands published to date.

Boron Clusters as Enhancers of RNase H Activity in the Smart Strategy of Gene Silencing by Antisense Oligonucleotides

Boron cluster-conjugated antisense oligonucleotides (B-ASOs) have already been developed as therapeutic agents with “two faces”, namely as potential antisense inhibitors of gene expression and as boron carriers for boron neutron capture therapy (BNCT). The previously observed high antisense activity of some B-ASOs targeting the epidermal growth factor receptor (EGFR) could not be rationally assigned to the positioning of the boron cluster unit: 1,2-dicarba-closo-dodecaborane (0), [(3,3′-Iron-1,2,1′,2′-dicarbollide) (1-), FESAN], and dodecaborate (2-) in the ASO chain and its structure or charge. For further understanding of this observation, we performed systematic studies on the efficiency of RNase H against a series of B-ASOs models. The results of kinetic analysis showed that pyrimidine-enriched B-ASO oligomers activated RNase H more efficiently than non-modified ASO. The presence of a single FESAN unit at a specific position of the B-ASO increased the kinetics of enzymatic hydrolysis of complementary RNA more than 30-fold compared with unmodified duplex ASO/RNA. Moreover, the rate of RNA hydrolysis enhanced with the increase in the negative charge of the boron cluster in the B-ASO chain. In conclusion, a “smart” strategy using ASOs conjugated with boron clusters is a milestone for the development of more efficient antisense therapeutic nucleic acids as inhibitors of gene expression.

Carboranes in drug discovery, chemical biology and molecular imaging

There exists a paucity of structural innovation and limited molecular diversity associated with molecular frameworks in drug discovery and biomolecular imaging/chemical probe design. The discovery and exploitation of new molecular entities for medical and biological applications will necessarily involve voyaging into previously unexplored regions of chemical space. Boron clusters, notably the carboranes, offer an alternative to conventional (poly)cyclic organic frameworks that may address some of the limitations associated with the use of novel molecular frameworks in chemical biology or medicine. The high thermal stability, unique 3D structure and aromaticity, kinetic inertness to metabolism and ability to engage in unusual types of intermolecular interactions, such as dihydrogen bonds, with biological receptors make carboranes exquisite frameworks in the design of probes for chemical biology, novel drug candidates and biomolecular imaging agents. This Review highlights the key developments of carborane derivatives made over the last decade as new design tools in medicinal chemistry and chemical biology, showcasing the versatility of this unique family of boron compounds.

The Boron Advantage: The Evolution and Diversification of Boron’s Applications in Medicinal Chemistry

In this review, the history of boron’s early use in drugs, and the history of the use of boron functional groups in medicinal chemistry applications are discussed. This includes diazaborines, boronic acids, benzoxaboroles, boron clusters, and carboranes. Furthermore, critical developments from these functional groups are highlighted along with recent developments, which exemplify potential prospects. Lastly, the application of boron in the form of a prodrug, softdrug, and as a nanocarrier are discussed to showcase boron’s emergence into new and exciting fields. Overall, we emphasize the evolution of organoboron therapeutic agents as privileged structures in medicinal chemistry and outline the impact that boron has had on drug discovery and development.

Development of Prodrugs for Treatment of Parkinson's Disease: New Inorganic Scaffolds for Blood-Brain Barrier Permeation

The treatment of Parkinson's disease (PD) has not been consistently modified for more than 60 years. L-DOPA, the blood-brain barrier permeable precursor prodrug of dopamine, is to date the only effective therapy on the market. However, it is well known that prolonged treatment with L-DOPA leads to several side effects, which may affect the patient's life expectancy (i.e., the wearing-off phenomenon, on-off fluctuations, and dyskinesia). For this reason, modifications, and supplements to L-DOPA treatment have been and are being studied, which, however, have not yet resulted in a valid alternative to the cornerstone drug. This review aims to summarize the main formulations currently in use for PD treatment, explaining advantages and disadvantages for each class. The attention will be focused on the promising prodrug concept, aimed at finding a suitable L-DOPA substitute with improved pharmacokinetic behavior. In this respect, new potential candidates which show interesting properties for the intended scope, the so-called dicarba-closo-dodecaboranes(12) (carboranes), will be discussed. Carboranes are inorganic molecular icosahedral boron-carbon clusters with 12 vertices and 20 deltahedral faces. They have been extensively studied for applications in medicine as potential pharmacophores, reagents in boron neutron capture therapy (BNCT) and radiotherapy. Here, we discuss them as inorganic scaffolds for dopamine delivery at the central nervous system (CNS) level.

Comprehensive exploration of chemical space using trisubstituted carboranes

A total of 42 trisubstituted carboranes categorised into five scaffolds were systematically designed and synthesized by exploiting the different reactivities of the twelve vertices of o-, m-, and p-carboranes to cover all directions in chemical space. Significant inhibitors of hypoxia inducible factor transcriptional activitay were mainly observed among scaffold V compounds (e.g., Vi–m, and Vo), whereas anti-rabies virus activity was observed among scaffold V (Va–h), scaffold II (IIb–g), and scaffold IV (IVb) compounds. The pharmacophore model predicted from compounds with scaffold V, which exhibited significant anti-rabies virus activity, agreed well with compounds IIb–g with scaffold II and compound IVb with scaffold IV. Normalized principal moment of inertia analysis indicated that carboranes with scaffolds I–V cover all regions in the chemical space. Furthermore, the first compounds shown to stimulate the proliferation of the rabies virus were found among scaffold V carboranes.

Carborane as an Alternative Efficient Hydrophobic Tag for Protein Degradation

Carboranes 1 and 2 were designed and synthesized for hydrophobic tag (HyT)-induced degradation of HaloTag fusion proteins. The levels of the hemagglutinin (HA)-HaloTag2-green fluorescent protein (EGFP) stably expressed in Flp-In 293 cells were significantly reduced by HyT13, HyT55, and carboranes 1 and 2, with expression levels of 49, 79, 43, and 65%, respectively, indicating that carborane is an alternative novel hydrophobic tag (HyT) for protein degradation under an intracellular environment. To clarify the mechanism of HyT-induced proteolysis, bovine serum albumin (BSA) was chosen as an extracellular protein and modified with maleimide-conjugated m-carborane (MIC). The measurement of the ζ-potentials and the lysine residue modification with fluorescein isothiocyanate (FITC) of BSA-MIC conjugates suggested that the conjugation of carborane induced the exposure of lysine residues on BSA, resulting in the degradation via ubiquitin E3 ligase-related proteasome pathways in the cell.

Discovery of a Vitamin D Receptor-Silent Vitamin D Derivative That Impairs Sterol Regulatory Element-Binding Protein In Vivo

Vitamin D₃ metabolites inhibit the expression of lipogenic genes by impairing sterol regulatory element-binding protein (SREBP), a master transcription factor of lipogenesis, independent of their canonical activity through a vitamin D receptor (VDR). Herein, we designed and synthesized a series of vitamin D derivatives to search for a drug-like small molecule that suppresses the SREBP-induced lipogenesis without affecting the VDR-controlled calcium homeostasis in vivo. Evaluation of the derivatives in cultured cells and mice led to the discovery of VDR-silent SREBP inhibitors and to the development of KK-052 (50), the first vitamin D-based SREBP inhibitor that has been demonstrated to mitigate hepatic lipid accumulation without calcemic action in mice. KK-052 maintained the ability of 25-hydroxyvitamin D₃ to induce the degradation of SREBP but lacked in the VDR-mediated activity. KK-052 serves as a valuable compound for interrogating SREBP/SCAP in vivo and may represent an unprecedented translational opportunity of synthetic vitamin D analogues.

Lithocholic Acid Derivatives as Potent Vitamin D Receptor Agonists

Lithocholic acid (2) was identified as a second endogenous ligand of vitamin D receptor (VDR), though its activity is very weak. In this study, we designed novel lithocholic acid derivatives based on the crystal structure of VDR-ligand-binding domain (LBD) bound to 2. Among the synthesized compounds, 6 bearing a 2-hydroxy-2-methylprop-1-yl group instead of the 3-hydroxy group at the 3α-position of 2 showed dramatically increased activity in HL-60 cell differentiation assay, being at least 10 000 times more potent than lithocholic acid (2) and 3 times more potent than 1α,25-dihydroxyvitamin D₃ (1). Although the binding affinities of 6 and its epimer 7 were less than that of 1, their transactivation activities were greater than that of 1. X-ray structure analyses of VDR LBD bound to 6 or 7 showed that the binding positions of these compounds in the ligand-binding pocket are similar to that of 1.

Vitamin D and Its Synthetic Analogs

For many individuals, in particular during winter, supplementation with the secosteroid vitamin D₃ is essential for the prevention of bone disorders, muscle weakness, autoimmune diseases, and possibly also different types of cancer. Vitamin D₃ acts via its metabolite 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] as potent agonist of the transcription factor vitamin D receptor (VDR). Thus, vitamin D directly affects chromatin structure and gene regulation at thousands of genomic loci, i.e., the epigenome and transcriptome of its target tissues. Modifications of 1,25(OH)₂D₃ at its side-chain, A-ring, triene system, or C-ring, alone and in combination, as well as nonsteroidal mimics provided numerous potent VDR agonists and some antagonists. The nearly 150 crystal structures of VDR’s ligand-binding domain with various vitamin D compounds allow a detailed molecular understanding of their action. This review discusses the most important vitamin D analogs presented during the past 10 years and molecular insight derived from new structural information on the VDR protein.

closo-Carboranes as dual CH⋯π and BH⋯π donors: theoretical study and biological significance

In this manuscript the ability ofcloso-carboranes to establish CH⋯π and BH⋯π interactions with several aromatic moieties exhibiting different electronic natures has been evaluated at the PBE0-D3/def2-TZVP level of theory.

Structural development of non-secosteroidal vitamin D receptor (VDR) ligands without any asymmetric carbon

Non-secosteroidal VDR ligands without any assymmetric carbon were designed and synthesized based on the structure of the previously reported non-secosteroidal VDR agonist LG190178. The VDR-agonistic activity of all synthesized compounds was evaluated, and 7b emerged as a potent agonist activity with an EC₅₀ value of 9.26 nM. Moreover, a docking simulation analysis was also performed to determine the binding mode of 7b with VDR-LBD.

Further Developments of the Phenyl-Pyrrolyl Pentane Series of Nonsteroidal Vitamin D Receptor Modulators as Anticancer Agents

The vitamin D₃ receptor (VDR), which belongs to the nuclear-receptor superfamily, is a potential molecular target for anticancer-drug discovery. In this study, a series of nonsteroidal vitamin D mimics with phenyl-pyrrolyl pentane skeletons with therapeutic potentials in cancer treatment were synthesized. Among them, 11b and 11g were identified as the most effective agents in reducing the viability of four cancer-cell lines, particularly those of breast-cancer cells, with IC₅₀ values in the submicromolar-concentration range. In addition, 11b and 11g possessed VDR-binding affinities and displayed significant partial VDR-agonistic activities determined by dual-luciferase-reporter assays and human-leukemia-cell-line (HL-60)-differentiation assays. Furthermore, 11b and 11g inhibited tumor growth in an orthotopic breast-tumor model via inhibition of cell proliferation and induction of cell apoptosis. More importantly, 11b and 11g exhibited favorable pharmacokinetic behavior in vivo and did not increase serum calcium levels or cause any other apparent side effects. In summary, 11b and 11g act as novel VDR modulators and may be promising candidates for cancer chemotherapy.

Novel VDR antagonists based on the GW0742 scaffold

The vitamin D receptor is a nuclear hormone receptor that regulates cell proliferation, cell differentiation and calcium homeostasis. The receptor is endogenously activated by 1,25-dihydroxyvitamin D₃, which induces transcription of VDR targets genes regulated by coactivator binding. VDR antagonists and partial agonists have been developed based on the secosteroid scaffold of vitamin D. Only a few non-secosteroid VDR antagonists are known. Herein, we report the rational design of non-secosteroid VDR antagonists using GW0742 as a scaffold. GW0742 is a PPARδ agonist previously identified by our group as a VDR antagonist. Several modifications including the replacement of the thiazole ring with an oxazole ring led to compound 7b, which inhibited VDR-mediated transcription (IC₅₀ = 660 nM) without activating PPARδ-mediated transcription. However, inhibition of transcription mediated by other nuclear receptors was observed.

Novel Non-steroidal Progesterone Receptor Ligands Based on m-Carborane Containing a Secondary Alcohol: Effect of Chirality on Ligand Activity

The progesterone receptor (PR) controls various physiological processes, including the female reproductive system, and nonsteroidal PR ligands are considered to be drug candidates for treatment of various diseases without significant adverse effects. Here, we designed and synthesized m-carborane-based secondary alcohols and investigated their PR-ligand activity. All the synthesized alcohols exhibited PR-antagonistic activity at subnanomolar concentration. Among them, alcohols having a small alkyl side chain and a 4-cyanophenyl group also exhibited PR-agonistic activity in a relatively high concentration range. Optical resolution of secondary alcohols having a methyl side chain was performed, and the PR-ligand activity and PR-binding affinity of the purified enantiomers were examined. The chirality of the secondary alcohol appears to have a more significant influence on PR-agonistic activity than on antagonistic activity.

Concise and Versatile Synthesis of Sulfoquinovosyl Acyl Glycerol Derivatives for Biological Applications

Sulfoquinovosyl acylpropanediol (SQAP), a chemically modified analogue of sulfoquinovosyl acylglycerol (SQAG) that occurs in sea algae, has been reported to show a variety of biological activities, including accumulation in tumor cells and the inhibition of tumor cell growth. We report herein on a new concise and versatile synthesis of SQAP itself and derivatives bearing iodoaryl groups and boronclusters. This method should be useful for the design and synthesis of SQAG/SQAP derivatives for diagnosis and the treatment of cancer and related diseases.

Transition from nematic to gyroid-type cubic soft self-assembly by side-chain engineering of π-conjugated sticky rods

A sequence of liquid crystalline phases, involving cybotactic nematics, a lamellar phase, bicontinuous cubics and triangular honeycombs, was observed for oligo(phenylene ethynylene) based X-shaped bolapolyphiles with two long lateral alkyl chains and sticky ends provided by glycerol groups. In the cubic phase with Ia3[combining macron]d lattice - which is tailored by alkyl chain engineering - the aromatic cores are organized on the gyroid minimal surface in 3D curved layers of almost parallel aligned π-conjugated rods. It is shown that this type of cubic phase is a general mode of soft self-assembly of X-shaped bolapolyphiles at the cross-over from the (long or short range) lamellar to the triangular honeycomb-like organization. Cubic phase formation is found only in a narrow range with respect to temperature and chain-length for the non-fluorinated compounds and in much wider ranges for related core-fluorinated molecules.

Boron's journey: advances in the study and application of pharmacokinetics

Boron-containing compounds (BCCs) are attractive chemical entities in drug development. Some of these compounds have been used in the treatment of human disease, and studies on their pharmacodynamics suggest that they employ multiple forms of activity. However, less is known about the pharmacokinetic profile of these molecules. Areas covered: The herein compiled reported data is presented in accordance with the classical 'ADME' system for identifying the scope of BCCs in the respective fields. Our analysis suggests that these compounds have several distinct ways to move within the human body, and that the specific structural features of each molecule account for its distinct pharmacokinetic profile. These insights should be useful for designing BCCs with a desired effect. Expert opinion: Increasing knowledge about the pharmacokinetics of BCCs is providing a broader understanding about the design of new release systems and potential drugs, as well as probable protein transporters that could be related to key roles in physiological processes. These transporters may be involved in sodium transport, hormone release and regulation of the cell cycle. The shared features among groups of BCCs are being identified in order to apply these insights to the design of advantageous compounds.

Vitamin D receptor 2016: novel ligands and structural insights

INTRODUCTION

Vitamin D₃ activates via its hormonal form 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃), the transcription factor vitamin D receptor (VDR). VDR is expressed in most human tissues and has more than 1,000 target genes. Thus, 1α,25(OH)₂D₃ and its synthetic analogs have a broad physiological impact. The crystal structures of the VDR ligand-binding domain (LBD), and its various ligands, allows further the understanding of the receptor's molecular actions. Areas covered: We discuss the most important novel VDR ligands and the further insight derived from new structural information on VDR. Expert opinion: There is an increasing appreciation of the impact of vitamin D and its receptor VDR not only in bone biology, but also for metabolic diseases, immunological disorders, and cancer. Detailed structural analysis of the interaction of additional novel ligands with VDR highlight helices 6 and 7 of the LBD as being most critical for stabilizing the receptor for an efficient interaction with co-activator proteins, i.e. for efficient agonistic action. This permits the design of even more effective VDR agonists. In addition, chemists took more liberty in replacing major parts of the 1α,25(OH)₂D₃ molecule, such as the A- and CD-rings or the side chain, with significantly different structures, such as carboranes, and still obtained functional VDR agonists.

Recent developments with boron as a platform for novel drug design

INTRODUCTION

After decades of development, the medicinal chemistry of compounds that contain a single boron atom has matured to the present status of having equal rights with other branches of drug discovery, although it remains a relative newcomer. In contrast, the medicinal chemistry of boron clusters is less advanced, but it is expanding and may soon become a productive area of drug discovery.

AREAS COVERED

The author reviews the current developments of medicinal chemistry of boron and its applications in drug design. First generation boron drugs that bear a single boron atom and second generation boron drugs that utilize boron clusters as pharmacophores or modulators of bioactive molecules are discussed. The advantages and gaps in our current understanding of boron medicinal chemistry, with a special focus on boron clusters, are highlighted.

EXPERT OPINION

Boron is not a panacea for every drug discovery problem, but there is a good chance that it will become a useful addition to the medicinal chemistry tool box. The present status of boron resembles the medicinal chemistry status of fluorine three decades ago; indeed, currently, approximately 20% of pharmaceuticals on the market contain fluorine. The fact that novel boron compounds, especially those based on abiotic polyhedral boron hydrides, are currently unfamiliar could be advantageous because organisms may be less prone to developing resistance against boron cluster-based drugs.

Design and synthesis of 3-arylbenzopyran based non-steroidal vitamin-D3mimics as osteogenic agents

27benhanced osteoblast differentiation at 1 pM in mouse calvarial osteoblast cells without inherent toxicity.

Expanding the chemical space of hydrophobic pharmacophores: the role of hydrophobic substructures in the development of novel transcription modulators

Interactions between biologically active compounds and their targets often involve hydrophobic interactions, and hydrophobicity also influences the pharmacokinetic profile.

Structural Studies of Vitamin D Nuclear Receptor Ligand-Binding Properties

The vitamin D nuclear receptor (VDR) and its natural ligand, 1α,25-dihydroxyvitamin D3 hormone (1,25(OH)2D3, or calcitriol), classically regulate mineral homeostasis and metabolism but also much broader range of biological functions, such as cell growth, differentiation, antiproliferation, apoptosis, adaptive/innate immune responses. Being widely expressed in various tissues, VDR represents an important therapeutic target in the treatment of diverse disorders. Since ligand binding is a key step in VDR-mediated signaling, numerous 1,25(OH)2D3 analogs have been synthesized in order to selectively modulate the receptor activity. Most of the synthetic analogs have been developed by modification of a parental compound and some of them mimic 1,25(OH)2D3 scaffold without being structurally related to it. The ability of ligands that have different size and conformation to bind to VDR and to demonstrate biological effects is intriguing, and therefore, ligand-binding properties of the receptor have been extensively investigated using a variety of biochemical, biophysical, and computational methods. In this chapter, we describe different aspects of the structure-function relationship of VDR in complex with natural and synthetic ligands coming from structural analysis. With the emphasis on the binding modes of the most promising compounds, such as secosteroidal agonists and 1,25(OH)2D3 mimics, we also highlight the action of VDR antagonists and the evidence for the existence of an alternative ligand-binding site within the receptor. Additionally, we describe the crystal structures of VDR mutants associated with hereditary vitamin D-resistant rickets that display impaired ligand-binding function.

Carborane-based design of a potent vitamin D receptor agonist

The development of a promising clinical antitumor vitamin D analog possessing a side-chain o-carborane cluster that efficiently binds to VDR by unconventional dihydrogen bonding (BH···HN) is described.

The vitamin D nuclear receptor (VDR) is a potential target for cancer therapy. It is expressed in many tumors and its ligand shows anticancer actions. To combine these properties with the application of boron neutron capture therapy (BNCT), we design and synthesize a potent VDR agonist based on the skeleton of the hormone 1α,25-dihydroxyvitamin D₃ (1,25D) and an o-carborane (dicarba-o-closo-1,2-dodecaborane) at the end of its side chain. The present ligand is the first secosteroidal analog with the carborane unit that efficiently binds to VDR and functions as an agonist with 1,25D-like potency in transcriptional assay on vitamin D target genes. Moreover it exhibits similar antiproliferative and pro-differentiating activities but is significantly less hypercalcemic than 1,25D. The crystal structure of its complex with VDR ligand binding domain reveals its binding mechanism involving boron-mediated dihydrogen bonds that mimic vitamin D hydroxyl interactions. In addition to the therapeutic interest, this study establishes the basis for the design of new unconventional vitamin D analogs containing carborane moieties for specific molecular recognition, and drug research and development.

The properties of substituted 3D-aromatic neutral carboranes: the potential for σ-hole bonding

The calculated properties of substituted carboranes such as dipole moment, polarisability, the magnitude of the σ-hole and the desolvation free energy are compared with these properties in comparable aromatic and cyclic aliphatic organic compounds. Dispersion and charge transfer energies are similar. However, the predicted strength of the halogen bonds with the same electron donor (based on the magnitude of the σ-hole) is larger for neutral C-vertex halogen-substituted carboranes than for their organic counterparts. Furthermore, the desolvation penalties of substituted carboranes are smaller than those of the corresponding organic compounds, which should further strengthen the halogen bonds of the former in the solvent. It is predicted that substituted carboranes have the potential to form stronger halogen bonds than comparable aromatic hydrocarbons, which will be even more pronounced in the medium. This theoretical study thus lays ground for the rational engineering of halogen bonding in inorganic crystals as well as in biomolecular complexes.