Conformationally restricted analogs of 1 alpha, 25-dihydroxyvitamin D3 and its 20-epimer: compounds for study of the three-dimensional structure of vitamin D responsible for binding to the receptor

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.

Synthesis and Biological Activity of 2,22-Dimethylene Analogues of 19-Norcalcitriol and Related Compounds

Continuing our search for vitamin D analogues, we explored the modification of the steroidal side chain and inserted a methylene moiety in position C-22 together with either lengthening the side chain or introducing a ring at the terminal end. Our conformational studies confirmed that the presence of a methylene group attached to C-22 restricts the conformational flexibility of the side chain, which can result in changes in biological characteristics of a molecule. All synthesized 1α,25-dihydroxy-2,22-dimethylene-19-norvitamin D₃ analogues proved equal to calcitriol in their ability to bind to the vitamin D receptor, and most of them exert significantly higher differentiation and transcriptional activity than calcitriol. The most active compounds were characterized by the presence of an elongated side chain or 26,27-dimethylene bridge. The synthetic strategy was based on the Wittig-Horner coupling of the known A-ring phosphine oxide with the corresponding Grundmann ketones prepared from a 20-epi-Inhoffen-Lythgoe diol derived from vitamin D₂.

SNP rs11185644 of RXRA gene is identified for dose-response variability to vitamin D3 supplementation: a randomized clinical trial

The level of serum 25-Hydroxyvitamin D [25(OH)D] has high heritability, suggesting that genes may contribute to variations in serum 25(OH)D level and vitamin D dose-response. As vitamin D deficiency has been linked to numerous diseases, understanding how genetic variation contributes to vitamin D dose-response is important for personalized vitamin D treatment and cost-effective disease prevention. To identify genetic variants responsible for vitamin D status and dose-response, we performed two vitamin D3 and calcium clinical supplementation trials in 2,207 postmenopausal Caucasian women. We examined the association of 291 SNPs with baseline serum 25(OH)D levels and 25(OH)D dose-response. Five SNPs, rs10500804 (P = 4.93 × 10⁻⁷), rs2060793 (P = 6.63 × 10⁻⁷), rs10741657 (P = 1.49 × 10⁻⁶), rs10766197 (P = 1.05 × 10⁻⁵) and rs11023380 (P = 7.67 × 10⁻⁵) in the CYP2R1 gene, as well as 6 SNPs, rs4588 (P = 7.86 × 10⁻⁷), rs2298850 (P = 1.94 × 10⁻⁶), rs1155563 (P = 6.39 × 10⁻⁶), rs705119 (P = 2.80 × 10⁻⁵), rs705120 (P = 1.08 × 10⁻⁴) and rs222040 (P = 1.59 × 10⁻⁴) in the GC gene were associated with baseline serum 25(OH)D levels. SNP rs11185644 near the RXRA was significantly associated with 25(OH)D dose-response (P = 1.01 × 10⁻⁴). Our data suggest that polymorphisms in the CYP2R1 and GC gene may contribute to variation in baseline serum 25(OH)D concentration, and that polymorphism rs11185644 may contribute to variation in 25(OH)D dose-response in healthy postmenopausal Caucasian women.

The serum 25-hydroxyvitamin D response to vitamin D supplementation is related to genetic factors, BMI, and baseline levels

OBJECTIVE

The serum 25-hydroxyvitamin D (25(OH)D) level is not only dependent on vitamin D intake and production in the skin but also dependent on genetic factors. Thus, in large genome-wide association studies, it has been shown that single nucleotide polymorphisms (SNPs) in the vitamin D binding protein (DBP), as well as in enzymes related to activation or degradation of vitamin D and its metabolites, are as important for the serum 25(OH)D level as the effect of season. How these SNPs affect the serum 25(OH)D response to vitamin D supplementation is uncertain.

DESIGN AND METHODS

Data were pooled from three randomized controlled trials where 40, 000 IU vitamin D/week was given for 6 months. Serum 25(OH)D was measured before and at the end of the intervention, and the subjects were genotyped for SNPs related to the serum 25(OH)D level.

RESULTS

Baseline 25(OH)D levels were significantly related to SNPs in the DBP and CYP2R1 genes. Those with SNPs associated with the lowest baseline 25(OH)D levels also had the smallest increase (delta) after supplementation. Those with the lowest baseline serum 25(OH)D (without regard to genotypes) had the highest increase (delta) after supplementation. Subjects with high BMI had lowest baseline 25(OH)D levels and also the smallest increase (delta) after supplementation.

CONCLUSIONS

The serum 25(OH)D response to supplementation depends on genes, baseline level, and BMI. However, whether this is clinically important or not depends on the therapeutic window of vitamin D, an issue that is still not settled.

Butyl pocket formation in the vitamin D receptor strongly affects the agonistic or antagonistic behavior of ligands

Previously, we reported that 22S-butyl-25,26,27-trinor-1α,24-dihydroxyvitamin D(3)2 represents a new class of antagonist for the vitamin D receptor (VDR). The crystal structure of the ligand-binding domain (LBD) of VDR complexed with 2 showed the formation of a butyl pocket to accommodate the 22-butyl group and insufficient interactions between ligand 2 and the C-terminus of VDR. Here, we designed and synthesized new analogues 5a-c and evaluated their biological activities to probe whether agonistic activity is recovered when the analogue restores interactions with the C-terminus of VDR. Analogues 5a-c exhibited full agonistic activity in transactivation. Interestingly, 5c, which bears a 24-diethyl group, completely recovered agonistic activity, although 3c and 4c act as an antagonist and a weak agonist, respectively. The crystal structures of VDR-LBD complexed with 3a, 4a, 5a, and 5c were solved, and the results confirmed that butyl pocket formation in VDR strongly affects the agonistic or antagonistic behaviors of ligands.

A 20S combined with a 22R configuration markedly increases both in vivo and in vitro biological activity of 1α,25-dihydroxy-22-methyl-2-methylene-19-norvitamin D3

Six new analogues of 1α,25-dihydroxy-19-norvitamin D(3) (3a-4b, 5, and 6) were prepared by a convergent synthesis applying the Wittig-Horner reaction as a key step. The influence of methyl groups at C-22 on their biological activity was examined. It was established that both in vitro and in vivo activity is strongly dependent on the configuration of the stereogenic centers at C-20 and C-22. Introduction of the second methyl group at C-22 (analogues 5 and 6) generates the compounds that are slightly more potent than 1α,25-(OH)(2)D(3) in the in vitro tests but much less potent in vivo. The greatest in vitro and in vivo biological activity was achieved when the C-20 is in the S configuration and the C-22 is in the R configuration. The building blocks for the synthesis, the respective (20R,22R)-, (20R,22S)-, (20S,22R)-, and (20S,22S)-diols, were obtained by fractional crystallization of mixtures of the corresponding diastereomers. Structures and absolute configurations of the diols 21a, 21b, and 22a as well as analogues 3a, 5, and 6 were confirmed by the X-ray crystallography.

Potent antagonist for the vitamin D receptor: vitamin D analogues with simple side chain structure

We previously reported that 22S-butyl-25,26,27-trinor-1alpha,24-dihydroxyvitamin D(3) 2 was a potent VDR antagonist. The X-ray crystal structure of the ligand binding domain of VDR complexed with 2 indicated that this ligand induces an extra cavity within the ligand-binding pocket. The structure also showed that the ligand forms only poor hydrophobic interactions with helix 12 of the protein. Here, to study the effects of the induction of the extra cavity and of insufficient interactions with helix 12 on antagonism, we designed and synthesized a series of vitamin D(3) analogues with or without a 22-alkyl substituent and evaluated their biological potency. We found that the 22-butyl analogues 3c and 5c act as full antagonists, the 22-ethyl analogues 3b, 4b, 5b, and 22-butyl analogue 4c act as partial agonists, and the others (3a, 4a, 5a, 6a, 6b, and 6c) act as full agonists for VDR. It is intriguing that 6c is a potent agonist for VDR, whereas its 26,27-dinor analogue 5c is a potent antagonist. Analogue 6c recruited coactivator SRC-1 well, but 5c did not. These results indicate that a combination of induction of the extra cavity and insufficient hydrophobic interactions with helix 12 is important for VDR antagonism in this class of ligands.

Peroxisome proliferator activated receptor gamma and oxidized docosahexaenoic acids as new class of ligand

PPARgamma regulates the expression of numerous genes. In addition to their anti-diabetic activity, PPARgamma agonists have been reported to have beneficial effects for cancer, inflammation including inflammatory bowel disease, atherosclerosis and brain inflammation, as well as bone turnover. To investigate a potential new class of ligands for PPARgamma, we designed with reference to the crystal structure of the ligand-binding domain of PPARgamma oxidized docosahexaenoic acid (DHA) derivatives, which have a hydrophilic substituent at the C(4)-position and are putative metabolites of DHA. We synthesized 14 compounds and evaluated their activities in vitro. We found that these DHA derivatives show PPARgamma transactivation higher than, or comparable to, that of pioglitazone, which is a thiazolidinedione derivative used as an antidiabetic agent. Furthermore, one of them showed anti-diabetic activity in animal models. In this paper, we review the potential of PPARgamma as a drug target and oxidized DHA as a new class of ligand for PPARgamma.

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.

Vitamin D3 derivatives with adamantane or lactone ring side chains are cell type-selective vitamin D receptor modulators

The vitamin D receptor (VDR) mediates the biological actions of 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], the active form of vitamin D, which regulates calcium homeostasis, immunity, cellular differentiation, and other physiological processes. We investigated the effects of three 1,25(OH)(2)D(3) derivatives on VDR function. AD47 has an adamantane ring and LAC67a and LAC67b have lactone ring substituents at the side chain position. These vitamin D derivatives bind to VDR but do not stabilize an active cofactor conformation. In a VDR transfection assay, AD47 and LAC67b act as partial agonists and all three compounds inhibit VDR activation by 1,25(OH)(2)D(3). The derivatives enhanced the heterodimerization of VDR with the retinoid X receptor, an effect unrelated to agonist/antagonist activity. AD47 and LAC67b weakly induced recruitment of the SRC-1 cofactor to VDR, and all three derivatives inhibited the recruitment of p160 family cofactors to VDR induced by 1,25(OH)(2)D(3). It is noteworthy that AD47 induced DRIP205 recruitment as effectively as 1,25(OH)(2)D(3), whereas LAC67a and LAC67b were not effective. We examined the expression of endogenous VDR target genes and the nuclear protein levels of VDR and cofactors in several cell lines, including cells derived from intestine, bone, and monocytes, and found that the vitamin D(3) derivatives act as cell type-selective VDR modulators. The data indicate that side chain modification is useful in the development of VDR antagonists and tissue-selective modulators. Further elucidation of the molecular mechanisms of action of selective VDR modulators will be essential for their clinical application.

Synthesis and biological evaluation of a des-C,D-analog of 2-methylene-19-nor-1alpha,25-(OH)2D3

Structure-activity studies directed at novel Vitamin D compounds of potential therapeutic value has long attracted an attention of numerous research groups. To date more than 3000 analogs of the natural hormone 1alpha,25-dihydroxyvitamin D(3) (1) have been synthesized and tested. As a continuation of our studies on 19-norvitamin D compounds we have obtained (20S)-des-C,D-2-methylene-1alpha,25-dihydroxy-19-norvitamin D(3) (6) and examined its biological activity. This Vitamin D compound was efficiently prepared in a convergent synthesis. The "upper" fragment was synthesized starting with methyl (R)-(-)-3-hydroxy-2-methylpropionate (9). The key synthetic step involved Lythgoe type Wittig-Horner coupling of the protected hydroxy aldehyde 19 with the phosphine oxide 20. The prepared Vitamin D analog 6 exhibited limited binding ability to the rat intestinal Vitamin D receptor being ca. 80 times less potent when compared to the natural hormone 1. Although the analog 6 did retain some cell differentiating and transcriptional activities, preliminary in vivo tests do not show any activity of this analog in animals.

22-Alkyl-20-epi-1α,25-dihydroxyvitamin D3 Compounds of Superagonistic Activity: Syntheses, Biological Activities and Interaction with the Receptor

We previously reported that 22R-methyl-20-epi-1,25-(OH)2D3 (3) possesses strong binding affinity for the vitamin D receptor (VDR) and shows superagonistic biological activities. To examine the effect of the length of an alkyl substituent at C(22) and to extend our compound library, we successfully synthesized 22R-ethyl-20-epi-1,25-(OH)2D3 (4) and 22R-butyl-20-epi-1,25-(OH)2D3 (5). Surprisingly, 22-ethyl analogue 4 showed stronger VDR binding affinity and transactivation potency than the superagonist of methyl analogue 3, but its calcemic activity in vivo was weaker than that of both the methyl analogue 3 and the natural hormone (1), while 22-butyl analogue 5 showed activities comparable to those of the hormone (1). A study of the docking of these new analogues to the VDR-LBD and alanine scanning mutational analysis demonstrated that 22-methyl and 22-ethyl substituents enhance the favorable hydrophobic interactions with residues lining the ligand binding pocket of the VDR, and that 22-butyl analogue 5 binds to the VDR by an induced fit mechanism.

New 19-nor-(20S)-1alpha,25-dihydroxyvitamin D3 analogs strongly stimulate osteoclast formation both in vivo and in vitro

2-Methylene-19-nor-(20S)-1alpha,25-dihydroxyvitamin D3 (2MD), an analog of 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3], has been shown to strongly induce bone formation both in vitro and in vivo. We have synthesized four substituents at carbon 2 of 2MD (2MD analogs), four stereoisomers at carbon 20 of the respective 2MD analogs (2MD analog-C20 isomers) and four 2MD analogs with an oxygen atom at carbon 22 (2MD-22-oxa analogs) and examined their ability to stimulate osteoclastogenesis and induce hypercalcemia. 2MD analogs were 100 times as potent as 1alpha,25(OH)2D3 in stimulating the formation of osteoclasts in vitro and in inducing the expression of receptor activator of NF-kappaB ligand (RANKL) and 25-hydroxyvitamin D3-24 hydroxylase mRNAs in osteoblasts. The osteoclast-inducing activities of 2MD analog-C20 isomers and 2MD 22-oxa analogs were much weaker than those of 2MD analogs. In addition, the activity of a 2MD analog in inducing dentine resorption was much stronger than that of 1alpha,25(OH)2D3 in the pit formation assay. Affinities to the vitamin D receptor and transcriptional activities of these compounds did not always correlate with their osteoclastogenic activities. Osteoprotegerin-deficient (OPG-/-) mice provide a suitable model for investigating in vivo effects of 2MD analogs because they exhibit extremely high concentrations of serum RANKL. The same amounts of 2MD analogs and 1alpha,25(OH)2D3 were administered daily to OPG-/- mice for 2 days. The elevation in serum concentrations of RANKL and calcium was much greater in 2MD analog-treated OPG-/- mice than in 1alpha,25(OH)2D3-treated ones. A 2MD analog was much more potent than 1alpha,25(OH)2D3 in causing hypercalcemia and in increasing soluble RANKL with enhanced osteoclastogenesis even in wild-type mice. In contrast, the administration of the 2MD analog to c-fos-deficient mice failed to induce osteoclastogenesis and hypercalcemia. These results suggest that new substituents at carbon 2 of 2MD strongly stimulate osteoclast formation in vitro and in vivo, and that osteoclastic bone resorption is indispensable for their hypercalcemic action of 2MD analogs in vivo.

Synthesis and biological activities of new 1α,25-dihydroxy-19-norvitamin D3 analogs with modifications in both the A-ring and the side chain

In a series of studies on structure-activity relationships of 2-substituted 19-norvitamin D analogs, we found that 1alpha,25-dihydroxy-19-norvitamin D3 analogs with 2beta-hydroxyethoxy or 2E-hydroxyethylidene moieties show strong binding affinity for the vitamin D receptor (VDR) as well as marked transcriptional activity. To further examine the effects of side chain structure on the activity of 2-substituted 19-norvitamin D analogs, we have synthesized new 19-norvitamin D3 analogs with modifications in both the A-ring at the C(2) position and the side chain. The side chains of these analogs contained a double bond between C(22) and C(23) or an oxygen atom at C(22). The biological activity of the analogs was evaluated in vitro. All the side chain-modified analogs were less active than 1alpha,25-dihydroxyvitamin D31e and the parent compounds 3-6e possessing a natural 20R-configuration in binding to the VDR, but, except for the (20R)-22-oxa analogs 3-6d, were significantly more potent in transcriptional activity. Of the side-chain-modified analogs 4 and 5, the 2beta-hydroxyethoxy- and 2E-hydroxyethylidene-22,24-diene-24a,26a,27a-trihomo analogs showed markedly higher transcriptional activity (25- and 17.5-fold, respectively) compared with 1e. Elongation of the side chain at the C-24, C-26, and C-27 positions and introduction of a 22,24-diene moiety strongly increased transcriptional activity, as seen in the 20-epi analogs 3-6f.

Synthesis of 26,27-bisnorcastasterone analogs and analysis of conformation–activity relationship for brassinolide-like activity

Three castasterone (CS) derivatives with varied side-chain moieties, 26,27-bisnorcastasterone (20S-bisnorCS), 20-epi-26,27-bisnorcastasterone (20R-bisnorCS), and 21,26,27-trisnorcastasterone (trisnorCS), were synthesized stereoselectively from either stigmasterol or dehydroisoandrosterone. The 50% effective doses (ED50, nmol/plant) in the concentration-response curve for brassinolide-like activity in the rice lamina inclination assay were determined to be 0.020 nmol (pED50 = 10.7) for 20S-bisnorCS, 3.2 nmol (pED50 = 8.5) for 20R-bisnorCS, and 2.0 nmol (pED50 = 8.7) for trisnorCS. An analog containing an ester linkage between the steroid and the side-chain moiety of 20S-bisnorCS was also synthesized and its activity was evaluated to be 3.2 nmol (pED50 = 8.5), being equipotent to 20R-bisnorCS and trisnorCS. The activity of 20S-bisnorCS was 1/40 that of CS. The conformation analysis was conducted using a systematic search, showing that the activity decreases with an increase in the degree of freedom of the side chain of the steroidal skeleton.

Vitamin D Receptor: Ligand Recognition and Allosteric Network

To investigate the allosteric effects of ligands in the function of nuclear receptors, we performed exhaustive alanine scanning mutational analysis (ASMA) of the residues lining the ligand-binding pocket (LBP) of the human vitamin D receptor. The effects of ligands were examined in this system (termed two-dimensional (2D) ASMA) using 10 structurally and biologically characteristic ligands that included agonists, partial agonists, and a full antagonist. The results clearly revealed the role and importance of all the amino acid residues lining the LBP and the relationships between ligand binding and transcriptional potency. 2D ASMA indicated ligand-specific ligand-protein interactions, which have key importance in determining the transactivation potency of the ligand. Taking the results as a whole, we suggest a ligand-mediated allosteric network through which information from ligands is transmitted to the interfaces with protein cofactors and which was shown to be linked to part of the network found by statistical coupling analysis.

Identification of putative metabolites of docosahexaenoic acid as potent PPARgamma agonists and antidiabetic agents

We found that putative metabolites of docosahexaenoic acid (DHA) are strong PPARgamma activators and potential antidiabetic agents. We designed DHA derivatives based on the crystal structure of PPARgamma, synthesized them and evaluated their activities in vitro and in vivo. The efficacy of 5E-4-hydroxy-DHA 2a as a PPARgamma activator was about fourfold stronger than that of pioglitazone. Furthermore, the 4-keto derivative (10b) showed antidiabetic activity in animal models without producing undesirable effects such as obesity and hepatotoxicity.

Biological activity and conformational analysis of C20 and C14 epimers of CD-ring modified trans-decalin 1alpha,25-dihydroxyvitamin D analogs

In the context of our ongoing study of vitamin D structure-function relationships and in an attempt to obtain a better dissociation of their prodifferentiating (HL-60) and/or antiproliferative (MCF-7) activities and their calcemic activity, further 20-epi and 14-epi modifications were made to three trans-decalin CD-ring analogs of 1,25-dihydroxyvitamin D(3), the hormonally active metabolite of vitamin D(3), possessing a natural 20R side chain and featuring additional structural modifications in the seco-B-ring and in the A-ring. Following a previously observed trend and in agreement with the conformational analysis results, all three 20-epi derivatives show substantially lower biological activities, opposite to what is usually observed for analogs having the natural CD-ring. The 14-epi modification (cis-decalins) has little effect on the biological activity of the ynediene type and the saturated derivative, but results in an approximate 10-fold reduction in activity of the previtamin derivative. No better dissociation of the prodifferentiating and/or antiproliferative activities and the calcemic activity was achieved.

Two-dimensional alanine scanning mutational analysis of the interaction between the vitamin D receptor and its ligands: studies of A-ring modified 19-norvitamin D analogs

To clarify the structure-function relationship (SFR) of vitamin D analogs in terms of their interaction with the vitamin D receptor (VDR), we have proposed a new approach, two-dimensional alanine scanning mutational analysis (2D-ASMA). In this paper, attention was focused on the interactions around the A-ring of vitamin D. For this purpose, we synthesized four new 2-substituted 19-norvitamin D derivatives (3-6). The VDR affinity (3-6: 1, 5, 2 and 1/140, respectively) and transcriptional activity (3-6: 10, 30, 2 and 0.3, respectively) of the four compounds were evaluated relative to 1,25-(OH)(2)D(3) (5) (normalized to 1). Then, the transcriptional activities of wild-type and 18 mutant VDRs induced by the four compounds (3-6) were investigated. The results of this 18 x 4 2D-ASMA were presented as a patch table, and the effects of the mutations were analyzed in comparison with the natural hormone (1) and 2-methylene-19-nor-20-epi-1,25-(OH)(2)D(3) (2MD, 2). Of the four A-ring analogs, the 2alpha-hydroxyethoxy derivative (3) showed striking differences in the pattern on the patch table. From the results, we suggest a docking mode of this compound (3) in which the A-ring adopts the alpha conformation.

Studies on intramolecular Diels-Alder reactions of furo[3,4-c]pyridines in the synthesis of conformationally restricted analogues of nicotine and anabasine

En route to conformationally restricted analogues of nicotine and anabasine, a novel synthetic route to bridged anabasines is described that hinges on a domino intramolecular [4 + 2]-cycloaddition/ring opening-elimination sequence of 3-amino-substituted furo[3,4-c]pyridines. Extension of this route to bridged nicotines, however, proved abortive, even when the dienophile tether is activated by a p-tolylsulfonyl group or when the diene moiety is activated by an electron-releasing methoxy substituent. A detailed density functional theoretical study (B3LYP/6-31+G) was undertaken to provide insight into the factors that facilitate an intramolecular Diels-Alder reaction in the former case.

Interaction between vitamin D receptor and vitamin D ligands: two-dimensional alanine scanning mutational analysis

We present a new method to investigate the details of interaction between vitamin D nuclear receptor (VDR) and various ligands, namely a two-dimensional alanine scanning mutational analysis. In this method, the transactivation of various ligands is studied in conjunction with a series of alanine scanning mutations of the residues lining the ligand binding pocket (LBP) of VDR, and the complete set of results is profiled in a patch table. We investigated examples from four structurally diverse groups of known VDR ligands: the native vitamin D hormone and two compounds with the same side chain configuration; four 20-epi compounds; three 19-nor compounds; and two nonsecosteroids. The patch table of the results indicates characteristics of each group in terms of its interaction with 18 LBP residues. We demonstrate the validity of this approach by application to docking studies of the two nonsecosteroids.

Structure-function relationships of vitamin D including ligand recognition by the vitamin D receptor

First, the general structure and function of nuclear receptors (NRs) are described briefly to help our understanding of the mechanism of action of vitamin D mediated by the vitamin D receptor (VDR), a member of the NRs. Then we discuss the structure-function relationship (SFR) of vitamin D on the basis of ligand structures and the interaction of the ligand with the VDR. The SFR of vitamin D side chain analogs is discussed extensively in terms of our active space group concept, which was derived from conformational analyses of the side chains of vitamin D analogs and from studies with conformationally restricted 22-methyl-1,25-(OH)(2)D(3) isomers. The mobile area of the side chain of vitamin D can be grouped into five regions (E, G, EA, EG, and F), and the SFR has been analyzed in terms of these spatial regions. The SFR of ligand/VDR interaction is discussed on the basis of the crystal structure of VDR-LBD(delta 165-215), docking of various vitamin D ligands into the ligand binding pocket (LBP) of the VDR, and functional analysis of amino acids lining the LBP. Finally, we discuss total SFR, combining the results of the two approaches, and future aspects of structure-based design of vitamin D analogs.

Rational design, synthesis, and biological activity of novel conformationally restricted vitamin D analogues, (22R)- and (22S)-22-ethyl-1,25-dihydroxy-23,24-didehydro-24a,24b-dihomo-20-epivitamin D(3)

Two new vitamin D analogues, (22R)- and (22S)-22-ethyl-1,25-dihydroxy-23,24-didehydro-24a,24b-dihomo-20-epivitamin D(3) (3 and 4), were rationally designed on the basis of the active space group concept previously proposed by us. The 22R ethyl group of 3 restricts the mobility of the side chain to active space regions, whereas the 22S ethyl group of 4 confines the side chain to an inactive region. The double bond at C(23) further restricts the side chain flexibility. These compounds (3 and 4) were synthesized using ortho ester Claisen rearrangement as the key step. As expected, the 22R isomer 3 has nearly 100 times higher efficacy than 1,25-dihydroxyvitamin D(3) (1) in cell differentiation, although its affinity for the vitamin D receptor (VDR) was one-seventh of that of 1. The 22S isomer 4 has significantly lower efficacy than 3. A docking study in combination with site-directed mutation analysis revealed that two carbon elongated side chain analogue 3 could be fitted in the ligand binding pocket of the VDR by adopting a stable conformation.

Fluorinated vitamin D analogs to probe the conformation of vitamin D in its receptor complex: 19F-NMR studies and biological activity

To investigate vitamin D conformation, specifically the A-ring and seco-B ring parts, in its complex with the vitamin D receptor (VDR), we have previously suggested the use of 19F-NMR spectroscopy and have synthesized three fluorovitamin D derivatives to be used for the study, 4,4-difluoro-1alpha,25-dihydroxyvitamin D3 [4,4-F2-1,25-(OH)2D3, 2a], and (10Z)- and (10E)-19-fluoro-1alpha,25-dihydroxyvitamin D3 [19-F-1,25-(OH)2D3, 3a, 4a]. In this paper, we examined the 19F-NMR spectra of these and related fluorovitamin D compounds in detail. In the low temperature 19F-NMR spectra, we observed two well-separated rigid conformations of 2a (51:49) and 4a (84:16), while only one conformation was detected with 3a. The two observed conformers of 2a and 4a were respectively assigned to the known alpha- and beta-conformers formed by the flipping of the A-ring where the C(19) exocyclic methylene points to the alpha- and beta-faces. The single conformation observed for 3a was assigned to the alpha-conformer. We detected no other conformation in the 19F-NMR of all vitamin D compounds examined. The effect of solvents on the 19F chemical shifts of fluorovitamin D compounds was found to be small (less than 6.3 ppm). This was much smaller than the difference between the two A-ring conformers (13-30 ppm). Using the dynamic 1H-NMR studies of fluorovitamin D compounds, we determined the free energy of activation for the interconversion between the two conformations of 2a (9.9 kcal/mol) and 4a (10.8, 11.5 kcal/mol). Introduction of the 1alpha-hydroxyl group raised the activation energy about 1 kcal/mol. The affinity for VDR was evaluated, and the relative potency of 2a, 3a and 4a was found to be 1%, 8% and 9%, respectively, of that of 1,25-(OH)2D3 (1). Though the affinity for VDR was considerably reduced in these compounds, the ability to activate gene transcription was similar and remained in the range 30-50% of the effect of 1. This biological information in combination with the NMR properties indicates that 2a and 4a are promising probes for studying the VDR-bound A-ring conformation of vitamin D.

Ligand recognition by the vitamin D receptor

Three-dimensional structure of the ligand binding domain (LBD) of the vitamin D receptor (VDR) docked with the natural ligand 1 alpha,25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] has been mostly solved by the X-ray crystallographic analysis of the deletion mutant (VDR-LBD Delta 165-215). The important focus, from now on, is how the VDR recognizes and interacts with potent synthetic ligands. We now report the docking models of the VDR with three functionally and structurally interesting ligands, 22-oxa-1,25-(OH)(2)D(3) (OCT), 20-epi-1,25-(OH)(2)D(3) and 20-epi-22-oxa-24,26,27-trihomo-1,25-(OH)(2)D(3). In parallel with the computational docking studies, we prepared twelve one-point mutants of amino acid residues lining the ligand binding pocket of the VDR and examined their transactivation potency induced by 1,25-(OH)(2)D(3) and these synthetic ligands. The results indicate that L233, R274, W286, H397 and Y401 are essential for holding the all ligands tested, S278 and Q400 are not important at all, and the importance of S237, V234, S275, C288 and H305 is variable depending on the side-chain structure of the ligands. Based on these studies, we suggested key structural factors to bestow the selective action on OCT and the augmented activities on 20-epi-ligands. Furthermore, the docking models coincided well with our proposed active space-region theory of vitamin D based on the conformational analyses of ligands.

Three-dimensional structure-function relationship of vitamin D and vitamin D receptor model

On the basis of conformational analysis of the vitamin D side chain and studies using conformationally restricted synthetic vitamin D analogs, we have suggested the active space region concept of vitamin D: The vitamin D side-chain region was grouped into four regions (A, G, EA and EG) and the A and EA regions were suggested to be important for vitamin D actions. We extended our theory to known highly potent vitamin D analogs and found a new region F. The analogs which occupy the F region have such modifications as 22-oxa, 22-ene, 16-ene and 18-nor. Altogether, the following relationship between the space region and activity was found: Affinity for vitamin D receptor (VDR), EA > A> F > G > EG; Affinity for vitamin D binding protein (DBP), A >> G,EA,EG; Target gene transactivation, EA > F > A > EG > or = G; Cell differentiation, EA > F > A > EG > or = G; Bone calcium mobilization, EA > GA > F > or = EG; Intestinal calcium absorption, EA = A > or = G >> EG. We modeled the 3D structure of VDR-LBD (ligand binding domain) using hRARgamma as a template, to develop our structure-function theory into a theory involving VDR. 1alpha,25(OH)(2)D(3) was docked into the ligand binding pocket of the VDR with the side chain heading the wide cavity at the H-11 site, the A-ring toward the narrow beta-turn site, and the beta-face of the CD ring facing H3. Amino acid residues forming hydrogen bonds with the 1alpha- and 25-OH groups were specified: S237 and R274 forming a pincer type hydrogen-bond for the 1alpha-OH and H397 for the 25-OH. Mutants of several amino acid residues that are hydrogen-bond candidates were prepared and their biologic properties were evaluated. All of our mutation results together with known mutation data support our VDR model docked with the natural ligand.

Why do we need a three-dimensional architecture of the ligand-binding domain of the nuclear 1alpha,25-dihydroxyvitamin D(3) receptor?

Highly specific binding of 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) by vitamin D receptor (VDR), a nuclear transcriptional factor, activates a genomic mechanism that is manifested in the multiple biologic properties of 1alpha,25(OH)(2)D(3). Numerous synthetic analogs of 1alpha,25(OH)(2)D(3) have been employed to study the interaction between 1alpha,25(OH)(2)D(3) and VDR, and to identify structural markers in 1alpha,25(OH)(2)D(3) that are important for VDR-binding. On the other hand the three-dimensional structure of VDR remained elusive till very recently. In the present study we employed affinity labeling (by 1alpha,25-dihydroxyvitamin D(3)-3-bromoacetate, 1alpha,25(OH)(2)D(3)-3-BE) of VDR to identify C(288) as the anchoring residue for the 3-hydroxyl group of 1alpha,25(OH)(2)D(3) inside the ligand-binding domain of VDR (VDR-LBD). In addition we carried out mutation/hormone-binding analyses to determine the importance of M(284) and W(286) toward hormone binding. We incorporated this information with the three-dimensional structure of the LBD of progesterone receptor to develop a homology-extension model of VDR-LBD. This model identified several amino acid residues as ligand-contact points inside the LBD. Mutational and hormone-binding analyses of these residues verified the structure-functional authenticity of this model, in comparison with the crystal structure of VDR, bound to 1alpha,25(OH)(2)D(3).

History of the development of new vitamin D analogs: studies on 22-oxacalcitriol (OCT) and 2beta-(3-hydroxypropoxy)calcitriol (ED-71)

In 1981 Suda and his colleagues first reported the new activity of calcitriol namely its ability to differentiate the myeloid leukemia cells into normal monocytes-macrophages. However, the possibility of using calcitriol as an antileukemic drug was not feasible because of its potent calcemic effects. Based on these observations, several pharmaceutical companies initiated the synthesis of vitamin D analogs with the aim to separate the calcemic actions of calcitriol from its actions on regulating the cell growth and differentiation. As a result, numerous noncalcemic analogs with a potential for the treatment of leukemia and other cancers were synthesized. The group at Chugai introduced two characteristic analogs of opposite type namely, 22-oxacalcitriol (OCT) and 2beta-(3-hydroxypropoxy)calcitriol (ED-71) which have been shown to have therapeutic value and are already being used clinically. The work on OCT and ED-71 together with the work on calcipotriol and KH-1060 by Leo Laboratories, and 1alpha,25(OH)(2)-16-ene-23-yne-D(3) by Hoffmann-La Roche, vigorously stimulated research world-wide in the development of vitamin D analogs into pharmaceutical products. More recently new impressive vitamin D analogs such as 3-epi analogs, 19-nor analogs, 18-nor analogs, 2-methyl-20-epi-calcitriol, non-steroidal vitamin D analogs are being developed. The authors are convinced that various vitamin D analogs will become highly effective therapeutic agents at the clinical level in the new century, and also that a new theory on the mechanism of vitamin D action will be generated.

(10Z)- and (10E)-19-fluoro-1alpha,25-dihydroxyvitamin D3: an improved synthesis via 19-nor-10-oxo-vitamin D

An efficient synthetic route to (10Z)- and (10E)-19-fluoro-1alpha,25-dihydroxyvitamin D3 was developed. The key feature of this pathway is the introduction of a 19-fluoromethylene group to a (5E)-19-nor-10-oxo-vitamin D derivative. The 10-oxo-compound was obtained via a 1,3-dipolar cycloaddition reaction of (5E)-1alpha,25-dihydroxyvitamin D with in situ generated nitrile oxide followed by ring cleavage of the formed isoxazoline moiety with molybdenum hexacarbonyl. Conversion of the keto group of (5E)-19-nor-10-oxo-vitamin D to the E and Z fluoromethylene group was achieved through a two-step sequence involving a reaction of lithiofluoromethyl phenyl sulfone followed by the reductive desulfonylation of the alpha-fluoro-beta-hydroxy sulfone. The dye-sensitized photoisomerization of the (5E)-19-fluorovitamin D afforded the desired (5Z)-19-fluorovitamin D derivatives, (10Z)- and (10E)-19-fluoro-1alpha,25-dihydroxyvitamin D3.

Highly potent cell differentiation-inducing analogues of 1alpha,25-dihydroxyvitamin D3: synthesis and biological activity of 2-methyl-1,25-dihydroxyvitamin D3 with side-chain modifications

Eight 2-methyl substituted analogues of 20-epi-22R-methyl-1alpha,25-dihydroxyvitamin D3 (5) and 20-epi-24,26,27-trihomo-22-oxa-1alpha,25-dihydroxyvitamin D3 (6: KH-1060) were convergently synthesized. Preparation of the CD-ring portions with modified side chains of 5 and 6, followed by palladium-catalyzed cross-coupling with the A-ring enyne synthons (20a-d), (3S,4S,5R)-, (3S,4R,5R)-, (3S,4S,5S)- and (3R,4R,5S)-3,5-bis[(tert-butyldimethylsilyl)oxy]-4-methyloct-1-en-7-yne, afforded two sets of four A-ring stereoisomers of 20-epi-2,22-dimethyl-1,25-dihydroxyvitamin D3 (7a-d) and 20-epi-24,26,27-trihomo-2-methyl-22-oxa-1,25-dihydroxyvitamin D3 (8a-d). The biological profiles of the hybrid analogues were assessed in terms of affinity for vitamin D receptor (VDR) and HL-60 cell differentiation-inducing activity in comparison with the natural hormone. The combined modifications of the A-ring at the 2-position and the side chain yielded analogues with high potency.

Development of analogues of 1alpha,25-dihydroxyvitamin D3 with biased side chain orientation: methylated des-C,D-homo analogues

The discovery that 1alpha,25-dihydroxyvitamin D3 is effective in the inhibition of cellular proliferation and in the induction of cellular differentiation has led to a search for analogues in which these activities and the classical calcemic activity of this hormone are separated. In this context, the synthesis and biological evaluation are reported of the three stereoisomeric CD-ring modified structural analogues in order to enforce a particular and different orientation of the 25-hydroxylated side chain. Comparison of the results of the biological evaluation and conformational analysis of the side chain suggests one defined and "active" geometry.

1alpha,25-dihydroxyvitamin D(3) and its potent synthetic analogs downregulate tissue factor and upregulate thrombomodulin expression in monocytic cells, counteracting the effects of tumor necrosis factor and oxidized LDL

BACKGROUND

We have recently found that a hormonally active form of vitamin D, 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], exerts anticoagulant effects by upregulating the expression of an anticoagulant glycoprotein, thrombomodulin (TM), and downregulating the expression of a critical coagulation factor, tissue factor (TF), in monocytic cells including human peripheral monocytes. In this study, we investigated the counteracting effects of 1,25(OH)(2)D(3) and its potent analogs on TF induction and TM downregulation by tumor necrosis factor and oxidized LDL in monocytic cells and the modulatory effects of potent analogs on TF and TM expression.

METHODS AND RESULTS

Effects of 1,25(OH)(2)D(3) and its potent synthetic analogs (22R)-22-methyl-20-epi-1,25(OH)(2)D(3) (KY3) and 22-oxacalcitriol on TF and TM antigen levels, cell surface activities, and mRNA levels in monocytic cells were examined. 1, 25(OH)(2)D(3) and its potent analogs showed anticoagulant effects in monocytic cells by downregulating TF and upregulating TM expression, counteracting the effects of tumor necrosis factor and oxidized LDL. KY3 was most potent in its regulatory effect on TF and TM expression.

CONCLUSIONS

Because KY3 has the highest affinity for vitamin D receptor, our findings suggest that TF and TM regulation by 1, 25(OH)(2)D(3) analogs is also mediated by vitamin D receptor. The 1, 25(OH)(2)D(3) analogs KY3 and 22-oxacalcitriol may have the potential to serve as an agent for preventing and treating atherosclerotic and other cytokine-mediated thrombotic diseases and as a tool for studying the molecular mechanisms of TF and TM regulation.

Synthesis, biological evaluation, and conformational analysis of A-ring diastereomers of 2-methyl-1,25-dihydroxyvitamin D(3) and their 20-epimers: unique activity profiles depending on the stereochemistry of the A-ring and at C-20

All eight possible A-ring diastereomers of 2-methyl-1, 25-dihydroxyvitamin D(3) (2) and 2-methyl-20-epi-1, 25-dihydroxyvitamin D(3) (3) were convergently synthesized. The A-ring enyne synthons 19 were synthesized starting with methyl (S)-(+)- or (R)-(-)-3-hydroxy-2-methylpropionate (8). This was converted to the alcohol 14 as a 1:1 epimeric mixture in several steps. After having been separated by column chromatography, each isomer led to the requisite A-ring enyne synthons 19 again as 1:1 mixtures at C-1. Coupling of the resulting A-ring enynes 20a-h with the CD-ring portions 5a,b in the presence of a Pd catalyst afforded the 2-methyl analogues 2a-h and 3a-h in good yield. In this way, all possible A-ring diastereomers were synthesized. The synthesized analogues were biologically evaluated both in vitro and in vivo. The potency was highly dependent on the stereochemistry of each isomer. In particular, the alpha alpha beta-isomer 2g exhibited 4-fold higher potency than 1 alpha,25-dihydroxyvitamin D(3) (1) both in bovine thymus VDR binding and in elevation of rat serum calcium concentration and was twice as potent as the parent compound in HL-60 cell differentiation. Furthermore, its 20-epimer, that is, 20-epi-alpha alpha beta 3g, exhibited exceptionally high activities: 12-fold higher in VDR binding affinity, 7-fold higher in calcium mobilization, and 590-fold higher in HL-60 cell differentiation, as compared to 1 alpha,25-dihydroxyvitamin D(3) (1). Accordingly, the double modification of 2-methyl substitution and 20-epimerization resulted in unique activity profiles. Conformational analysis of the A-ring by (1)H NMR and an X-ray crystallographic analysis of the alpha alpha beta-isomer 2g are also described.

Characterization of a novel analogue of 1alpha,25(OH)(2)-vitamin D(3) with two side chains: interaction with its nuclear receptor and cellular actions

The hormone 1alpha,25(OH)(2)-vitamin D(3) (125D) binds to its nuclear receptor (VDR) to stimulate gene transcription activity. Inversion of configuration at C-20 of the side chain to generate 20-epi-1alpha,25(OH)(2)D(3) (20E-125D) increases transcription 200-5000-fold over 125D with its 20-normal (20N) side chain. This enhancement has been attributed to the VDR ligand-binding domain (LBD) having different contact sites for 20N and 20E side chains that generate different VDR conformations. We synthesized 1alpha, 25-dihydroxy-21-(3-hydroxy-3-methylbutyl)vitamin D(3) (Gemini) with two six-carbon side chains (both 20N and 20E orientations). Energy minimization calculations indicate the Gemini side chain possesses significantly more energy minima than either 125D or 20E-125D (2346, 207, and 127 minima, respectively). We compared activities of 125D, 20E-125D, and Gemini, respectively, in several assays: binding to wild-type (100%, 147%, and 38%) and C-terminal-truncated mutant VDR; transcriptional activity (of the transfected osteopontin promoter in ROS 17/2.8 cells: ED(50) 10, 0.005, and 1.0 nM); mediation of conformational changes in VDR assessed by protease clipping (major trypsin-resistant fragment of 34, 34, and 28 kDa). For inhibition of cellular clonal growth of human leukemia (HL-60) and breast cancer (MCF7) cell lines, the ED(50)(125D)/ED(50)(Gem) was respectively 380 and 316. We conclude that while Gemini readily binds to the VDR and generates unique conformational changes, none of them is able to permit a superior gene transcription activity despite the presence of a 20E side chain.

Three-dimensional modeling of and ligand docking to vitamin D receptor ligand binding domain

The ligand binding domain of the human vitamin D receptor (VDR) was modeled based on the crystal structure of the retinoic acid receptor. The ligand binding pocket of our VDR model is spacious at the helix 11 site and confined at the beta-turn site. The ligand 1alpha, 25-dihydroxyvitamin D(3) was assumed to be anchored in the ligand binding pocket with its side chain heading to helix 11 (site 2) and the A-ring toward the beta-turn (site 1). Three residues forming hydrogen bonds with the functionally important 1alpha- and 25-hydroxyl groups of 1alpha,25-dihydroxyvitamin D(3) were identified and confirmed by mutational analysis: the 1alpha-hydroxyl group is forming pincer-type hydrogen bonds with S237 and R274 and the 25-hydroxyl group is interacting with H397. Docking potential for various ligands to the VDR model was examined, and the results are in good agreement with our previous three-dimensional structure-function theory.

Synthesis and biological activity of 22-iodo- and (E)-20(22)-dehydro analogues of 1alpha,25-dihydroxyvitamin D3

Construction of 25-hydroxy-steroidal side chain substituted with iodine at C-22 was elaborated on a model PTAD-protected steroidal 5,7-diene and applied to a synthesis of (22R)- and (22S)-22-iodo-1alpha,25-dihydroxyvitamin D3. Configuration at C-22 in the iodinated vitamins, obtained by nucleophilic substitution of the corresponding 22S-tosylates with sodium iodide, was determined by comparison of their iodine-displacement processes and cyclizations leading to isomeric five-membered (22,25)-epoxy-1alpha-hydroxyvitamin D3 compounds. Also, 20(22)-dehydrosteroids have been obtained and their structures established by 1H NMR spectroscopy. When compared to the natural hormone, (E)-20(22)-dehydro-1alpha,25-dihydroxyvitamin D3 was found 4 times less potent in binding to the porcine intestinal vitamin D receptor (VDR) and 2 times less effective in differentiation of HL-60 cells. 22-Iodinated vitamin D analogues showed somewhat lower in vitro activity, whereas (22,25)-epoxy analogues were inactive. Interestingly, it was established that (22S)-22-iodo-1alpha,25-dihydroxyvitamin D3 was 3 times more potent than its (22R)-isomer in binding to VDR and four times more effective in HL-60 cell differentiation assay. The restricted mobility of the side chain of both 22-iodinated vitamin D compounds was analyzed by a systematic conformational search indicating different spatial regions occupied by their 25-oxygen atoms. Preliminary data on the in vivo calcemic activity of the synthesized vitamin D analogues indicate that (E)-20(22)-dehydro-1alpha,25-dihydroxyvitamin D3 and 22-iodo-1alpha,25-dihydroxyvitamin D3 isomers were ca. ten times less potent than the natural hormone 1alpha,25-(OH)2D3 both in intestinal calcium transport and bone calcium mobilization.

Synthesis, biological activity, and conformational analysis of four seco-D-15,19-bisnor-1alpha,25-dihydroxyvitamin D analogues, diastereomeric at C17 and C20

The synthesis of four CD-ring-modified 19-nor-1alpha, 25-dihydroxyvitamin D(3) derivatives lacking C15, referred to as 6C analogues, and diastereomeric at C17 and C20 is described. The synthesis involves an Ireland-Claisen rearrangement of a 3-methyl-substituted ester of (1R)-3-methyl-2-cyclohexen-1-ol as the key step, followed by elaboration of the side chain, transformation into a C8 cyclohexanone derivative, and final Wittig-Horner coupling with the 19-nor A-ring phosphine oxide. Despite possessing a more flexible side chain than the parent hormone, biological evaluation showed an unexpected superagonistic antiproliferative and prodifferentiating activity (10-50 times higher as compared to that of 1alpha,25(OH)(2)D(3)) for the diastereomer with the "natural" configuration at C17 and C20. The other diastereomers exhibit a 25-90% decrease in activity. All four analogues show a decreased binding affinity (45% or less), and their calcemic activity is 4-400 times less than that of 1alpha,25(OH)(2)D(3). The conformational behavior of their side chain was studied using molecular mechanics calculations, and the result is presented as volume maps. A relative activity volume was determined by subtraction of the volume map of the least active analogue from the volume map of the most active one. This shows three regions corresponding to preferred orientations in space of the side chain of the active analogue. One of these regions was found to overlap with the region that is preferentially occupied by the most active of the four diastereomeric 22-methyl-substituted 1alpha,25(OH)(2)D(3) analogues.

Three-dimensional structure-function relationship of vitamin D: side chain location and various activities

The various biological activities of side-chain mobility restricted analogs, four diastereomers at C(20) and C(22) of 22-methyl-1alpha,25-dihydroxyvitamin D3, were evaluated. The relationship between structure and the various activities of the analogs was discussed in terms of the active space region concept that we previously suggested.