Analogs of 1α,25-dihydroxyvitamin D3 with high potency in induction of osteoclastogenesis and prevention of dendritic cell differentiation: Synthesis and biological evaluation of 2-substituted 19-norvitamin D analogs

Strategies for the Synthesis of 19-nor-Vitamin D Analogs

1α,25-Dihydroxyvitamin D₃ [1α,25-(OH)₂-D₃], the hormonally active form of vitamin D₃, classically regulates bone formation, calcium, and phosphate homeostasis. In addition, this hormone also exerts non-classical effects in a wide variety of target tissues and cell types, such as inhibition of the proliferation and stimulation of the differentiation of normal and malignant cells. However, to produce these actions, supraphysiological doses are required resulting in calcemic effects that limit the use of this natural hormone. During the past 30 years, many structurally modified analogs of the 1α,25-(OH)₂-D₃ have been synthesized in order to find derivatives that can dissociate the beneficial antiproliferative effects from undesired calcemic effects. Among these candidates, 1α,25-(OH)₂-19-nor-D₃ analogs have shown promise as good derivatives since they show equal or better activity relative to the parent hormone but with reduced calcemic effects. In this review, we describe the synthetic strategies to obtain the 19-nor-D₃ derivatives and briefly describe their physiological activities.

Synthesis of 19-norcalcitriol analogs with alkylidene moieties at C-2 based on succinic acid and l-methionine

On the basis of the literature data, our previous research work and docking experiments, we designed novel 19-norvitamin D compounds having elongated 2-alkylidene substituents. These 19-norcalcitriol derivatives have attached 2-(3'-aminopropylidene) substituent in which the nitrogen atom bears acyl residue derived from succinic acid and l-methionine. Both compounds were obtained by the same synthetic strategy involving Julia coupling of the A-ring ketone with the known C/D-ring sulfone. In the obtained 1α,25-dihydroxy-19-norvitamin D₃ derivative, the alkylidene substituent at C-2 was further elaborated to the desired structures.

Recent developments of 19-nor-1,25-dihydroxyvitamin D3 analogues

The vitamin D hormone, 1α,25-dihydroxyvitamin D3 [1,25-(OH)2 D3 ], exerts its hormonal effects predominantly on intestine, bone, and kidney, where it plays a crucial role in calcium and phosphorus homeostasis and bone mineralization. In addition to its classical actions, 1,25(OH)2 D3 exerts pleiotropic effects in a wide variety of target tissues and cell types, often in an autocrine/paracrine fashion. These biological activities of 1,25(OH)2 D3 have suggested a multitude of potential therapeutic applications for the vitamin D hormone in the treatment of hyperproliferative disorders (e.g. cancer and psoriasis), immune dysfunction (autoimmune diseases), and endocrine disorders (e.g. hyperparathyroidism). However, the calcemic effects induced by 1,25(OH)2 D3--hypercalcemia, increased bone resorption, and soft tissue calcification--limit the use of the natural ligand in these clinical applications. Therefore, numerous 1,25(OH)2 D3 analogues have been synthesized with the intent of producing therapeutic agents devoid of hypercalcemic and hyperphosphatemic side effects. To this aim, much attention has been focused on the development of 19-nor-vitamin D3 derivatives that lack the ring-A exocyclic methylene group (C19). In this review, the 19-nor-1,25(OH)2 D3 analogues are classified according to modifications made at the A-ring, the side chain, or both the A-ring and side chain, as well as other positions. The biological activities of these 19-nor-1,25(OH)2 D3 analogues are summarized and their structure-activity relationships and binding features with the vitamin D receptor (VDR) are discussed.

A new role for vitamin D receptor activation in chronic kidney disease

Vitamin D has proven to be much more than a simple "calcium hormone." The fact that the vitamin D receptor has been found in cells not related to mineral metabolism supports that statement. The interest of nephrologists in vitamin D and its effects beyond mineral metabolism has increased over the last few years, evidencing the importance of this so-called "sunshine hormone." In the present review, we highlight the most recent developments in the traditional use of vitamin D in chronic kidney disease (CKD) patients, namely, the control of secondary hyperparathyroidism (sHPT). Furthermore, we also explore the data available regarding the new possible therapeutic uses of vitamin D for the treatment of other complications present in CKD patients, such as vascular calcification, left ventricular hypertrophy, or proteinuria. Finally, some still scarce but very promising data regarding a possible role of vitamin D in kidney transplant patients also are reviewed. The available data point to a potential beneficial effect of vitamin D in CKD patients beyond the control of mineral metabolism.

The skeleton as an intracrine organ for vitamin D metabolism

The endocrine hormone, 1alpha,25-dihydroxyvitamin D(3) (1,25D) is an important regulator of calcium and phosphorus homeostasis. In this context, 1,25D is generally recognized as necessary for the maintenance of a healthy skeleton through its actions on the small intestine. In this review, we highlight the direct effects of 1,25D on the constituent cells of the bone, actions that are independent of effects on the intestine and kidney. We also consider the evidence that 25D levels, not 1,25D levels, correlate best with parameters of bone health, and that the bone itself is a site of metabolic conversion of 25D into 1,25D, by virtue of its expression of the 25-hydroxyvitamin D 1alpha-hydroxylase, CYP27B1. We review the evidence that at least osteoblasts and chondrocytes, and possibly also bone resorbing osteoclasts, are capable of such metabolic conversion, and therefore that these cells likely participate in autocrine and paracrine loops of vitamin D metabolism. We conclude that the skeleton is an intracrine organ for vitamin D metabolism, challenging the long-held notion that 1,25D is solely an endocrine hormone.

2-Substituted-16-ene-22-thia-1alpha,25-dihydroxy-26,27-dimethyl-19-norvitamin D3 analogs: Synthesis, biological evaluation, and crystal structure

Recently, we have found that 16-ene-22-thia-26,27-dimethyl-19-norvitamin D(3) analogs 1a (n=2, 3) are 20 times more active than the natural hormone 1alpha,25-dihydroxyvitamin D(3) in terms of transcriptional activity. To further investigate the effects of the A-ring modification of 1a, b on the biological activity profile, novel 22-thia-19-norvitamin D analogs 2-11 bearing a hydroxyethoxy-, hydroxyethylidene- or methyl group at C-2 in combination with 20S- and 20R-isomers were prepared and tested for their in vitro biological activities. All of the synthesized analogs showed 0.5-140% of the activity of the natural hormone in binding to the vitamin D receptor (VDR). When compared with the transcriptional activity of C-2 or C-20 isomeric pairs of the 22-thia analogs, the 20S-isomers 2-11a were more potent than the 20R-isomers 2, 3, 8-11b, and the 2beta-hydroxyethoxy, 2E-hydroxyethylidene, and 2alpha-methyl-2beta-hydroxy-22-thia isomers showed higher potency than their corresponding counterparts. In particular, 3a exhibited an extremely higher level of potency (210-fold) than the natural hormone. To elucidate the action mode of superagonist 3a at the molecular level, we determined the crystal structures of the rat VDR-ligand-binding domain complexed with 3a or 3b in the presence of peptide containing a nuclear box motif (LxxLL) at 1.9-2.0A resolution. The crystal structures demonstrated that the 1alpha-OH, 3beta-OH, and 25-OH groups of the natural hormone and 3a were anchored by the same amino acid residues in the ligand-binding pocket, and the terminal OH moiety of the substituent at C-2 formed hydrogen bonds with Arg270 and a water molecule to create a tight water molecule network. Moreover, the methyl groups at C-26a and C-27a make additional contact with hydrophobic residues such as Leu223, Ala227, Val230, and Ala299. These hydrophilic and hydrophobic interactions in 3a may underlie the induction of superagonistic activity.

Identification of a highly potent vitamin D receptor antagonist: (25S)-26-Adamantyl-25-hydroxy-2-methylene-22,23-didehydro-19,27-dinor-20-epi-vitamin D3 (ADMI3)

We synthesized four new vitamin D derivatives, diastereomers at C20 and C25 of 26-adamantyl-1,25-dihydroxy-2-methylene-22,23-didehydro-19,27-dinorvitamin D3 (ADMI1-4), which have the bulky and rigid adamantane ring system at the side chain terminus. These compounds had significant VDR affinity (1/6-1/30 that of the natural hormone) but their efficacies of transactivation in transient transcription assay was low (approximately 1/10). All ADMI compounds antagonized the action of 1,25(OH)2D3 in transient transcription assay in COS-7 cells with ADMI3 (20S,25S-isomer) was the most potent (IC50, 3 nM). ADMI3 (1 microM) suppressed the endogenous CYP24A1 gene expression induced by 1,25(OH)2D3 (10 nM) in HEK293 cells to nearly control level. Thus we have identified 26-adamantyl vitamin D compound as a novel highly potent VDR antagonist/partial agonist. A docking model of ADMI3 reveals that a terminal part of the large adamantane ring crowds the H12 residues (Val318 and Phe422) and this would prevent the H12 adopting the active conformation.