Remarkable effect of 2α-modification on the VDR antagonistic activity of 1α-hydroxyvitamin D3-26,23-lactones

Differential gene regulation by a synthetic vitamin D receptor ligand and active vitamin D in human cells

Vitamin D (VD) exerts a wide variety of biological functions including calcemic activity. VD nutritional status is closely associated with the onset and development of chronic diseases. To develop a VD analog with the desired VD activity but without calcemic activity, we screened synthetic VDR antagonists. We identified 1α,25-dihydroxyvitamin D3-26-23-lactams (DLAM)-2a-d (DLAM-2s) as nuclear vitamin D receptor (VDR) ligands in a competitive VDR binding assay for 1α,25(OH)2 vitamin D3 (1α,25(OH)2D3), and DLAM-2s showed an antagonistic effect on 1α,25(OH)2 D3-induced cell differentiation in HL60 cells. In a luciferase reporter assay in which human VDR was exogenously expressed in cultured COS-1 cells, DLAM-2s acted as transcriptional antagonists. Consistently, DLAM-2s had an antagonistic effect on the 1α,25(OH)2D3-induced expression of a known VD target gene [Cytochrome P450 24A1 (CYP24A1)], and VDR bound DLAM-2s was recruited to an endogenous VD response element in chromatin in human keratinocytes (HaCaT cells) endogenously expressing VDR. In an ATAC-seq assay, the effects of 1α,25(OH)2 D3 and DLAM-2b on chromatin reorganization were undetectable in HaCaT cells, while the effect of an androgen receptor (AR) antagonist (bicalutamide) was confirmed in prostate cancer cells (LNCaP) expressing endogenous AR. However, whole genome analysis using RNA-seq and ATAC (Assay for Transposase Accessible Chromatin)-seq revealed differential gene expression profiles regulated by DLAM-2b versus 1α,25(OH)2D3. The upregulated and downregulated genes only partially overlapped between cells treated with 1α,25(OH)2D3 and those treated with DLAM-2b. Thus, the present findings illustrate a novel VDR ligand with gene regulatory activity differing from that of 1α,25(OH)2D3.

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.

Chemical Synthesis of Side-Chain-Hydroxylated Vitamin D3 Derivatives and Their Metabolism by CYP27B1

1α,25-Dihydroxyvitamin D₃ (abbreviated here as 1,25D₃ ) is a hormonally active form of vitamin D₃ (D₃ ), and is produced from D₃ by CYP27 A1-mediated hydroxylation at C25, followed by CYP27B1-mediated hydroxylation at C1. Further hydroxylation of 25D₃ and 1,25D₃ occurs at C23, C24 and C26 to generate corresponding metabolites, except for 1,25R,26D₃ . Since the capability of CYP27B1 to hydroxylate C1 of side-chain-hydroxylated metabolites other than 23S,25D₃ and 24R,25D₃ has not been examined, we have here explored the role of CYP27B1 in the C1 hydroxylation of a series of side-chain-hydroxylated D₃ derivatives. We found that CYP27B1 hydroxylates the R diastereomers of 24,25D₃ and 25,26D₃ more effectively than the S diastereomers, but shows almost no activity towards either diastereomer of 23,25D₃ . This is the first report to show that CYP27B1 metabolizes 25,26D₃ to the corresponding 1α-hydroxylated derivative, 1,25,26D₃ . It will be interesting to examine the physiological relevance of this finding.

Synthetic studies of (23S,25R)-1α,25-dihydroxyvitamin D3 26,23-lactone (calcitriol lactone) and its derivatives

(23S,25R)-1α,25-Dihydroxyvitamin D₃ 26,23-lactone (calcitriol lactone) is a major metabolite of 1α,25-dihydroxyvitamin D₃ that binds to vitamin D receptor (VDR) and exhibits various biological activities. This lactone and its derivatives are considered to have potential as drug candidates to treat VDR-related diseases, but their biological activities have not yet been fully characterized, mainly because of their limited availability by chemical synthesis. This review deals with synthetic studies of calcitriol lactone, and its derivatives, i.e., methylene lactones (TEI-9647 and its derivatives) and calcitriol lactams (DLAMs). We also discuss their biological activities, VDR-binding affinity and structure-activity relationships.

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.

3D-QSAR Studies on C24-Monoalkylated Vitamin D3 26,23-Lactones and their C2α-Modified Derivatives with Inhibitory Activity to Vitamin D Receptor

The ligand-based three-dimensional quantitative structure-activity relationship (3D-QSAR) for 82 inhibitors of 25-dehydro-1α-hydroxyvitamin D3 -26,23-lactone analogs has been studied by using comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) models. The established CoMFA model in training set gives a cross-validated q(2) value of 0.516 and a non-cross-validated rncv (2) value of 0.667, while the CoMSIA model results in q(2) =0.517 and rncv (2) =0.632. In general, the predictive ability of the CoMFA model is superior to that of the CoMSIA model, with rpred (2) =0.639 for the CoMFA and rpred (2) =0.619 for the CoMSIA model. Based on the CoMFA contour maps, some key structural characters of vitamin D3 analogs responsible for inhibitory activity are identified, and some new C2α-modified 24-alkylvitamin D3 lactone analogs with high predicted pIC50 values are designed. The ligand functional group mutations by FEP simulation and docking studies reveal the rationality of the molecular design.

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.

2-Methylene 19-nor-25-dehydro-1α-hydroxyvitamin D3 26,23-lactones: Synthesis, biological activities and molecular basis of passive antagonism

To investigate the molecular mechanism of vitamin D receptor (VDR) antagonists having no structurally bulky group interfering with helix 12 of the ligand-binding domain of the VDR, we have synthesized four diastereomers at C(20) and C(23) of 19-nor-1alpha-hydroxyvitamin D(3) 25-methylene-26,23-lactone bearing a 2MD-type A-ring. All four analogs showed significant VDR affinity. Transactivation was tested by using Cos7 cells and HEK293 cells. In both types of cells, LAC67a showed little transactivation potency and inhibited the activation induced by the natural hormone concentration-dependently, indicating that LAC67a works as an antagonist for the VDR in these cells. LAC67b, LAC82a and LAC82b similarly acted as VDR antagonists in Cos7 cells, but in HEK293 cells they behaved as potent VDR agonists. Docking of four lactones into the VDR-LBD, followed by structural analysis, demonstrated that each lactone lacks the hydrophobic interaction with helix12 necessary for maintaining the active conformation of the VDR, indicating that these lactones are passive-type antagonists. Furthermore, each docking structure explained the characteristic transactivation profiles of the four lactones. On the basis of our present findings, we suggest that the ligand acts as an agonist if there are appropriate coactivators in the cells to bind to the looser VDR-ligand complex, and as an antagonist if there are no such appropriate coactivators. The molecular basis of the passive antagonism is discussed in detail.

[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).

Creative synthesis of novel vitamin D analogs for health and disease

We report new analogs of 1alpha,25-dihydroxyvitamin D(3) (1) in three categories. First, design and synthesis of ligands for a mutant vitamin D receptor (VDR)(Arg274Leu), which possess proper functional groups at both C1alpha and C2alpha positions of 1 to study the biological activity of the mutant VDR. Among our synthetic analogs, 1alpha-methyl-2alpha-(3-hydroxypropyl)-25-hydroxyvitamin D(3) (8) showed 7.3-fold greater transcriptional activity for the VDR(Arg274Leu) than that of 1. Next, we examined the antiproliferative activity of 2-substituted 19-norvitamin D(3) analogs on an immortalized normal prostate cell line, PZ-HPV-7, and we found MART 10 (14) showed the activity even at very low concentration of 10(-10) to 10(-11)M. We also synthesized 25-hydroxy-19-norvitamin D(3) (13) using Julia-type olefination to connect between the C5 and C6 positions, effectively, to test it as a prohormone type agent for antiprostate diseases. Synthesized compound 13 showed potent antiproliferative activity in PZ-HPV-7, which has high 1alpha-hydroxylase activity. Finally, we describe design and synthesis of a new TEI-9647 analog, 2alpha-(3-hydroxypropoxy)-24-propyl-25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone (17), which showed the strongest VDR antagonism. Its IC(50) value is 7.4pM to inhibit differentiation of HL-60 cells induced by 10nM of 1.

New insights into Vitamin D sterol-VDR proteolysis, allostery, structure-function from the perspective of a conformational ensemble model

Recently, we have developed a Vitamin D sterol (VDS)-VDR conformational ensemble model. This model can be broken down into three individual, yet interlinked parts: (a) the conformationally flexible VDS, (b) the apo/holo-VDR helix-12 (H12) conformational ensemble, and (c) the presence of two VDR ligand binding pockets (LBPs); one thermodynamically favored (the genomic pocket, G-pocket) and the other kinetically favored by VDSs (the alternative pocket, A-pocket). One focus of this study is to use directed VDR mutagenesis to (1) demonstrate H12 is stabilized in the transcriptionally active closed conformation (hVDR-c1) by three salt-bridges that span the length of H12 (cationic residues R154, K264 and R402), (2) to elucidate the VDR trypsin sites [R173 (hVDR-c1), K413 (hVDR-c2) and R402 (hVDR-c3)] and (3) demonstrate the apo-VDR H12 equilibrium can be shifted. The other focus of this study is to apply the model to generate a mechanistic understanding to discrepancies observed in structure-function data obtained with a variety of 1alpha,25(OH)(2)-Vitamin D(3) (1,25D) A-ring and side-chain analogs, and side-chain metabolites. We will demonstrate that these structure-function conundrums can be rationalized, for the most part by focusing on alterations in the VDS conformational flexibility and the elementary interaction between the VDS and the VDR A- and G-pockets, relative to the control, 1,25D.

Further Synthetic and Biological Studies on Vitamin D Hormone Antagonists Based on C24-Alkylation and C2α-Functionalization of 25-Dehydro-1α-hydroxyvitamin D3-26,23-lactones

An efficient synthesis and the biological evaluation of 80 novel analogs of 25-dehydro-1alpha-hydroxyvitamin D3-26,23S-lactone 2 (TEI-9647) and its 23R epimer (3) in which the lactone ring was systematically functionalized by introduction of a C1 to C4 primary alkyl group at the C24 position (5 sets of 4 diastereomers), together with their C2alpha-methyl, 3-hydroxypropyl, and 3-hydroxypropoxy-substituted derivatives were described. The triene structure of the vitamin D3 was constructed using palladium-catalyzed alkenylative cyclization of the A-ring precursor enyne with the CD-ring counterpart bromoolefin having the C24-alkylated lactone moiety on the side chain. The CD-ring precursors having 23,24-cis lactones were prepared by using a chromium-mediated syn-selective allylation-lactonization process, and the 23,24-trans lactone derivatives were derived from these via inversion of the C23 stereochemistry. The biological evaluation revealed that both binding affinity for chick vitamin D hormone receptor and antagonistic activity (inhibition of vitamin D hormone induced HL-60 cell differentiation) were affected by the orientation and chain-length of the primary alkyl group on the lactone ring. Furthermore, the C2alpha-functionalization of the C24-alkylated vitamin D3 lactones dramatically enhanced their biological activities. The most potent compound to emerge, (23S,24S)-2alpha-(3-hydroxypropoxy)-24-propyl exhibited almost 1000-fold stronger antagonistic activity (IC50=7.4 pM) than 2 (IC50=6.3 nM).

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

Vitamin D receptor (VDR) antagonists have attracted significant levels of interest because of their 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, TEI-9647 and TEI-9648 (25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone), with the goal of 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 the field of VDR agonists would help us identify potent antagonists. First 2alpha-modified TEI-9647 analogues were synthesized, and then 24-substitution to stabilize the lactone structure under physiological conditions was investigated. Finally, 2alpha-modified 24-methyl-, 24,24-dimethyl-, and 24,24-ethano-25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone analogues were synthesized. The synthesis of the 24,24-ethano-TEI lactone was accomplished through Ru-catalyzed intermolecular enyne metathesis of the alkynone CD-ring side chain with ethylene to give a dienone, followed by regioselective cyclopropanation. It was found that 2alpha,24,24-trimethyl-TEI-9647 (39) possessed an antagonistic activity (IC(50)=0.093 nM) approximately 90 times that of the original TEI-9647 (IC(50)=8.3 nM).

In silico studies using Radial Distribution Function approach for predicting affinity of 1 alpha,25-dihydroxyvitamin D(3) analogues for Vitamin D receptor

The Radial Distribution Function (RDF) approach has been applied to the study of the chick intestinal VDR affinity of 49 Vitamin D analogues. A model able to describe more than 77.5% of the variance in the experimental activity was developed with the use of the mentioned approach. In contrast, none of four different approaches, including the use of Topological, BCUT, Randić molecular profiles and Geometrical descriptors were able to explain more than 55% of the variance in the mentioned property, with the same number of variables in the equation.

Steroids: reactions and partial synthesis

This article reviews the progress in the chemistry of the steroids that was published between January and December 2003. The reactions and partial synthesis of estrogens, androgens, pregnanes, cholic acid derivatives, cholestanes and vitamin D analogues are covered. There are 152 references.