Highly Potent Vitamin D Receptor Antagonists: Design, Synthesis, and Biological Evaluation

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.

Introduction of fluorine atoms to vitamin D3 side-chain and synthesis of 24,24-difluoro-25-hydroxyvitamin D3

During our ongoing studies of vitamin D, we focused on the vitamin D₃ side-chain 24-position, which is the major metabolic site of human CYP24A1. In order to inhibit the metabolism of vitamin D₃, 24,24-difluorovitamin D₃analogues are important candidates. In this paper, we report the practical introduction of the difluoro-unit to the 24-position to synthesize 24,24-difluoro-CD ring (1) and 24,24-difluoro-25-hydroxyvitamin D₃ (2).

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.

Update on pharmacologically-relevant vitamin D analogues

Pharmacologists have been interested in vitamin D since its metabolism was elucidated in the early 1970s. Despite the synthesis of thousands of vitamin D analogues in the hope of separating its calcemic and anti-proliferative properties, few molecules have reached the market for use in the treatment of clinical conditions from psoriasis to chronic kidney disease. This review discusses vitamin D drugs, recently developed or still under development, for use in various diseases, but in particular bone disease. In the process we explore the mechanisms postulated to explain the action of these vitamin D analogues including action through the vitamin D receptor, action through other receptors e.g. FAM57B2 and dual action on transcriptional processes.

Vitamin D Analogs with Nitrogen Atom at C2 Substitution and Effect on Bone Formation

The Arg274 residue of the ligand binding domain of human vitamin D receptor (hVDR) is important for 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3) binding as a specific ligand through forming a hydrogen bond with the 1α-OH group of the active vitamin D3, 1α,25(OH)2D3. An additional pincer-type hydrogen bond formation with Arg274 from a 2α-substituent of a synthetic 1α,25(OH)2D3 analog would enhance the binding affinity and biological activity. A series of 2α-[2-(heteroaryl)ethyl]-, 2α-(4-cyanobutyl)-, 2α-(ω-cyanoalkoxy)-, and 2β-(3-cyanopropoxy)-1α,25(OH)2D3 were designed and synthesized based on our original hVDR super agonists of 2α-(3-hydroxypropyl)- and 2α-(3-hydroxypropoxy)-1α,25(OH)2D3. Their potential biological activities, i.e., hVDR binding affinity, transactivation activity in HOS cells, and therapeutic effect on enhancing the bone mineral density of OVX rats, were studied.

Inhibitors for the Vitamin D Receptor-Coregulator Interaction

The vitamin D receptor (VDR) belongs to the superfamily of nuclear receptors and is activated by the endogenous ligand 1,25-dihydroxyvitamin D3. The genomic effects mediated by VDR consist of the activation and repression of gene transcription, which includes the formation of multiprotein complexes with coregulator proteins. Coregulators bind many nuclear receptors and can be categorized according to their role as coactivators (gene activation) or corepressors (gene repression). Herein, different approaches to develop compounds that modulate the interaction between VDR and coregulators are summarized. This includes coregulator peptides that were identified by creating phage display libraries. Subsequent modification of these peptides including the introduction of a tether or nonhydrolyzable bonds resulted in the first direct VDR-coregulator inhibitors. Later, small molecules that inhibit VDR-coregulator inhibitors were identified using rational drug design and high-throughput screening. Early on, allosteric inhibition of VDR-coregulator interactions was achieved with VDR antagonists that change the conformation of VDR and modulate the interactions with coregulators. A detailed discussion of their dual agonist/antagonist effects is given as well as a summary of their biological effects in cell-based assays and in vivo studies.

Synthesis of 14-epi-2α-hydroxypropoxy-1α,25-dihydroxy-19-nortachysterol and its hVDR binding

Recently, we evaluated a novel skeleton in the vitamin D family, 14-epi-1α,25(OH)2-19-nortachysterol, and discovered its unique binding configuration in the human vitamin D receptor (VDR) with the C5,6- and C7,8-s-trans triene configuration. Because of its unprecedented form, this skeleton has a promising characteristic profile for clinical use, and also the synthesis of its derivatives should be versatile. Therefore, we synthesized the novel analog, 2α-hydroxypropoxy substituted 14-epi-1α,25(OH)2-19-nortachysterol, and evaluated its human VDR binding affinity. Although this substitution is one of the promising modification of vitamin D3 such as eldecalcitol (ED-71), it had negative effects on the binding affinity, and the compound showed lower affinity than 1α,25(OH)2D3 and its parent compound, 14-epi-1α,25(OH)2-19-nortachysterol. It was thought that the unprecedented binding configuration of this skeleton should not allow the terminal hydroxyl group of the 2α-substituent to construct effective hydrogen bond networks around the amino acid residues in the binding pocket. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Peroxisome proliferation-activated receptor δ agonist GW0742 interacts weakly with multiple nuclear receptors, including the vitamin D receptor

A high-throughput screening campaign was conducted to identify small molecules with the ability to inhibit the interaction between the vitamin D receptor (VDR) and steroid receptor coactivator 2. These inhibitors represent novel molecular probes for modulating gene regulation mediated by VDR. Peroxisome proliferator-activated receptor (PPAR) δ agonist GW0742 was among the identified VDR-coactivator inhibitors and has been characterized herein as a pan nuclear receptor antagonist at concentrations of > 12.1 μM. The highest antagonist activity for GW0742 was found for VDR and the androgen receptor. Surprisingly, GW0742 behaved as a PPAR agonist and antagonist, activating transcription at lower concentrations and inhibiting this effect at higher concentrations. A unique spectroscopic property of GW0742 was identified as well. In the presence of rhodamine-derived molecules, GW0742 increased the fluorescence intensity and level of fluorescence polarization at an excitation wavelength of 595 nm and an emission wavelength of 615 nm in a dose-dependent manner. The GW0742-inhibited NR-coactivator binding resulted in a reduced level of expression of five different NR target genes in LNCaP cells in the presence of agonist. Especially VDR target genes CYP24A1, IGFBP-3, and TRPV6 were negatively regulated by GW0742. GW0742 is the first VDR ligand inhibitor lacking the secosteroid structure of VDR ligand antagonists. Nevertheless, the VDR-meditated downstream process of cell differentiation was antagonized by GW0742 in HL-60 cells that were pretreated with the endogenous VDR agonist 1,25-dihydroxyvitamin D3.

Discovery of the first irreversible small molecule inhibitors of the interaction between the vitamin D receptor and coactivators

The vitamin D receptor (VDR) is a nuclear hormone receptor that regulates cell proliferation, cell differentiation, and calcium homeostasis. The receptor is activated by vitamin D analogues that induce the disruption of VDR-corepressor binding and promote VDR-coactivator interactions. The interactions between VDR and coregulators are essential for VDR-mediated transcription. Small molecule inhibition of VDR-coregulator binding represents an alternative method to the traditional ligand-based approach in order to modulate the expression of VDR target genes. A high throughput fluorescence polarization screen that quantifies the inhibition of binding between VDR and a fluorescently labeled steroid receptor coactivator 2 peptide was applied to discover the new small molecule VDR-coactivator inhibitors, 3-indolylmethanamines. Structure-activity relationship studies with 3-indolylmethanamine analogues were used to determine their mode of VDR-binding and to produce the first VDR-selective and irreversible VDR-coactivator inhibitors with the ability to regulate the transcription of the human VDR target gene TRPV6.

Application of olefin metathesis in the synthesis of steroids

Over the past decade, ruthenium-mediated metathesis transformations, including cross-metathesis, ring-closing metathesis, enyne metathesis, ring-opening metathesis polymerization, and also tandem processes, belong to the most intensively studied reactions. Many applications of olefin metathesis in the synthesis of natural products have been recently described. Also in the field of steroid chemistry new methods of total synthesis and hemisynthesis based on metathesis reactions have been elaborated. Various biologically active compounds, e.g. vitamin D and hormone analogues, steroid dimers and macrocycles, etc. have been prepared using a variety of olefin-metathesis protocols.

An o-aminoanilide analogue of 1α,25-dihydroxyvitamin D(3) functions as a strong vitamin D receptor antagonist

Vitamin D receptor (VDR) antagonists have therapeutic potential in treatment of allergic conditions and hypercalcemia driven by granulomatous diseases. We have identified an o-aminoanilide analogue of the hormonal form of vitamin D with a dienyl side chain that functions as a strong VDR antagonist. Modeling studies indicate that antagonism arises from side chain rigidity, when compared to a more flexible saturated analogue, which interferes with H12 folding/alignment.

Synthesis of 2beta-substituted-14-epi-previtamin D3 and testing of its genomic activity

2beta-substituted analogs of 14-epi-previtamin D(3) were synthesized for the first time by the thermal isomerization of the corresponding 14-epi-vitamin D3 that were available using coupling reaction between the A-ring phosphine oxide derived from a chiral epoxide and CD-ring cis-hydrindanone. The VDR binding affinity and transactivation activity of osteocalcin promoter in HOS cells were evaluated, and the new analogs were found to be less active, 0.01-0.18% of VDR binding affinity compared with the natural hormone and EC50 1.0-9.1 nM for transactivation activity, than 14-epi-previtamin D3 with 0.5% (VDR) and EC50 0.46 nM, respectively.

Vitamin D analogs

Vitamin D has gone through a renaissance with the association of vitamin D deficiency with a wide array of common diseases including breast, colorectal and prostate cancers, cardio-vascular disease, autoimmune conditions and infections. Vitamin D analogs constitute a valuable group of compounds which can be used to regulate gene expression in functions as varied as calcium and phosphate homeostasis, as well as cell growth regulation and cell differentiation of a wide spectrum of cell types. This review will discuss the full range of vitamin D compounds currently available, some of their possible uses, and potential mechanisms of action.

Structural refinement of seco-steroidal skeleton and the biological activity through nuclear receptors

1alpha,25-Dihydroxyvitamin D(3) (1) regulates a variety of biological actions through vitamin D receptor (VDR), including calcium and phosphorus homeostasis, bone remodeling, cellular proliferation and differentiation and many other functions. To enhance its potency and to study the structure/function relationship, we synthesized a series of analogs of 1 with a modification at the C-2alpha position. Introducing 2alpha-methyl, 2alpha-(3-hydroxypropyl), or 2alpha-(3-hydroxypropoxy) group increased its binding affinity for the VDR 2- to 4-fold compared to 1. The crystal structures of the VDR bound to these analogs provide a molecular explanation for the interaction between the 2alpha-substituents and water molecules exist in the VDR-ligand binding domain. Based on the accumulated knowledge in VDR agonists, we synthesized 2-substituted analogs of 'double side chain' (gemini), 19-norvitamin D(3) (MART-10), TEI-9647 (VDR antagonist), 1-alkylated vitamin D(3), 14-epi-previtamin D(3) etc. Gemini analogs showed potent HL-60 cell differentiation activity (13-38 times compared to 1), and MART-10 exhibited remarkable antiproliferative activity on PZ-HPV-7 cells even at 10(-10) M. (24S)-2alpha-(3-Hydroxypropoxy)-24-propyl-TEI-9647 showed potent VDR antagonism, and its IC(50) value was 7.4 pM against 10 nM of 1. 1alpha-Methyl-2alpha-(3-hydroxypropyl)-25-hydroxyvitamin D(3) improved the binding affinity for the mutant VDR(Arg274Leu), which causes hereditary vitamin D resistant rickets. 1alpha,25-Dihydroxy-2alpha-methyl-14-epi-previtamin D(3) showed moderate osteocalcin transcriptional activity on HOS cells. We theorize that modification at A-ring alone and in combination with functionalization of the other parts of the vitamin D molecule would provide important new information on the mechanism of vitamin D actions that could lead to the development of new therapeutic regimes for the treatment of various diseases.

Synthesis and biological properties of 2-methylene-19-nor-25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactones--weak agonists

In a continuing effort to explore the 2-methylene-1alpha-hydroxy-19-norvitamin D(3) class of pharmacologically important vitamin D compounds, two novel 2-methylene-19-nor-25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactones, GC-3 and HLV, were synthesized and biologically tested. Based on reports of similarly structured molecules, it was hypothesized that these compounds might act as antagonists, at least in vitro. The pathway designed to synthesize these compounds was based on two key steps: first, the Lythgoe-type Wittig-Horner coupling of Windaus-Grundmann-type ketone 18, with phosphine oxide 15, followed, later in the synthesis, by the Zn-mediated Reformatsky-type allylation of aldehyde 20 with methylbromomethylacrylate 8. Our biological data show that neither compound has antagonistic activity but acts as weak agonists in vitro and in vivo.

[Creation of highly potent vitamin D receptor antagonists]

Vitamin D receptor antagonist has attracted significant level of interests because of its potential utility in the treatment of Paget's disease, which is known as the most flagrant example of disordered bone remodeling and the second most common bone disease after osteoporosis in Anglo-Saxons. Recent studies on Paget's disease suggested a specific increase in osteoclasts sensitivity to the differentiation activity of active vitamin D(3) as the principal mechanism for abnormal bone formation. We set out to conduct a structure-activity relationship study on the first VDR antagonists of TEI-9647 and TEI-9648 (25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone) toward improved VDR antagonistic activity. Given that both potent agonists and antagonists must have high affinity for the VDR, we hoped that our accumulated knowledge in VDR agonists would help us identify potent antagonists. First, 2alpha-modified TEI-9647 analogs were synthesized, and then, 24-substitution was next investigated to stabilize its lactone structure under the physiological conditions. Finally, 2alpha-modified 24-methyl-, 24,24-dimethyl-25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone analogs were synthesized. It was found that 2alpha,24,24-trimethyl-TEI-9647 was found to possess approximately 90-fold improved antagonistic activity (IC(50) 0.093 nM) over the original TEI-9647 (IC(50) 8.3 nM).

Synthesis and biological evaluation of new 6-s-cis locked 1,2,25-trihydroxyprevitamin D3 analogues

An efficient synthesis of several diastereomers of 2-hydroxy substituted 1alpha,25-dihydroxyprevitamin D3 derivatives was accomplished utilizing a practical route to the A-ring synthon. The biological activity of the analogues was evaluated in vitro. All the synthesized derivatives demonstrated low affinity for the vitamin D receptor and vitamin D-binding protein compared with 1alpha,25-dihydroxyvitamin D3, the natural hormone. 1alpha,2beta,25-trihydroxy-19-nor-pre-D3 was the most potent of the analogues in inhibiting proliferation of MCF-7 cells but requires higher EC50 concentrations than 1alpha,25-dihydroxyvitamin D3.