Synthesis, conformational analysis, and biological evaluation of 19-nor-vitamin D3 analogues with A-ring modifications

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 of C2-Alkoxy-Substituted 19-Nor Vitamin D3 Derivatives: Stereoselectivity and Biological Activity

The active form of vitamin D₃ (D₃), 1a,25-dihydroxyvitamn D₃ (1,25D₃), plays a central role in calcium and bone metabolism. Many structure–activity relationship (SAR) studies of D₃ have been conducted, with the aim of separating the biological activities of 1,25D₃ or reducing its side effects, such as hypercalcemia, and SAR studies have shown that the hypercalcemic activity of C2-substituted derivatives and 19-nor type derivatives is significantly suppressed. In the present paper, we describe the synthesis of 19-nor type 1,25D₃ derivatives with alkoxy groups at C2, by means of the Julia–Kocienski type coupling reaction between a C2 symmetrical A ring ketone and a CD ring synthon. The effect of C2 substituents on the stereoselectivity of the coupling reaction was evaluated. The biological activities of the synthesized derivatives were evaluated in an HL-60 cell-based assay. The a-methoxy-substituted C2α-7a was found to show potent cell-differentiating activity, with an ED₅₀ value of 0.38 nM, being 26-fold more potent than 1,25D₃.

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, biological activity and molecular modelling studies of shikimic acid derivatives as inhibitors of the shikimate dehydrogenase enzyme of Escherichia coli

Shikimic acid (SA) pathway is the common route used by bacteria, plants, fungi, algae, and certain Apicomplexa parasites for the biosynthesis of aromatic amino acids and other secondary metabolites. As this essential pathway is absent in mammals designing inhibitors against implied enzymes may lead to the development of antimicrobial and herbicidal agents harmless to humans. Shikimate dehydrogenase (SDH) is the fourth enzyme of the SA pathway. In this contribution, a series of SA amide derivatives were synthesised and evaluated for in vitro SDH inhibition and antibacterial activity against Escherichia coli. All tested compounds showed to be mixed type inhibitors; diamide derivatives displayed more inhibitory activity than synthesised monoamides. Among the evaluated compounds, molecules called 4a and 4b were the most active derivatives with IC50 588 and 589 µM, respectively. Molecular modelling studies suggested two different binding modes of monoamide and diamide derivatives to the SDH enzyme of E. coli.

A-Ring Synthons of 19-Nor Type Vitamin D Derivatives

Various 19-nor vitamin D analogs, which lack the methylene group at C19, exhibit significant vitamin D (VD)-related biological activities, but generally show a reduced calcemic side-effect compared with VD itself. Among them, paricalcitol is already used clinically for treatment and prevention of secondary hyperparathyroidism associated with chronic renal failure. Therefore, considerable synthetic efforts have been directed towards 19-nor VD analogs, focusing especially on A-ring synthons suitable for use in convergent synthetic strategies based on coupling of CD-ring synthons and A-ring synthons. For example, we have recently developed a new synthetic route to A-ring synthons from linear dienes based upon a ring-closing olefin metathesis strategy. Here, we review recent synthetic approaches to A-ring synthons for 19-nor VD derivatives.

Preparation of ORMOSIL nanoparticles conjugated with vitamin D3analogues and their biological evaluation

Dye-doped multifunctional organically modified silica (ORMOSIL) nanoparticles were prepared within surfactant stabilized microemulsions and conjugated with several vitamin D₃derivatives.

19-Norvitamin D analogs for breast cancer therapy

The active form of vitamin D3, 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3 or calcitriol), is known to inhibit the proliferation and invasiveness of many types of cancer cells, including breast, colon, pancreatic, prostate, and liver cancer cells. These findings support the use of 1α,25(OH)2D3 for the treatment of these types of cancer. However, 1α,25(OH)2D3 can cause hypercalcemia, so analogs of 1α,25(OH)2D3 that are less calcemic but exhibit more potent anti-tumor activity would be good candidates as therapeutic agents. Therefore, a series of 19-norvitamin D analogs, in which the methylidene group on C19 is replaced with 2 hydrogen atoms, have been synthesized by several laboratories. In our laboratory, we have designed and synthesized a series of 2α-functional group substituted 19-norvitamin D3 analogs and examined their anti-proliferative activity. Among them, 2α- and 2β-(3-hydroxypropyl)-1α,25-dihydroxy-19-norvitamin D3 (MART-10 and MART-11) were found to be the most promising. Here, we review the rationale and approaches for the synthesis of different 19-norvitamin D analogs, and the pre-clinical studies using these analogs in breast cancer cells, in particular, we chose MART-10 for its potential application to the prevention and treatment of breast cancer.

Synthesis of vitamin D3 analogues with A-ring modifications to directly measure vitamin D levels in biological samples

C-3-substituted 25-hydroxyvitamin D3 analogues were synthesized as tools to directly measure levels of vitamin D in biological samples. The strategy involves vinyloxycarbonylation of the 3β-hydroxy group and formation of a carbamate bond with a hydroxyl or amino group at the end of the alkyl chain. Biotinylated conjugates of synthesized derivatives were generated to be linked with vitamin D binding protein (DBP). The spacer group present in the alkyl chain is important in the binding of antibodies to the analogue-DBP complex. When compared to 25-hydroxyvitamin D3-DBP, the binding of some antibodies to the analogue-DBP complex of the 25-hydroxyvitamin D3 derivative 10 that posses an 8-aminoctyl alkyl chain is significantly reduced, but this analogue displaced [26,27-(3)H]-25-hydroxyvitamin D3 from DBP. In contrast, the 8-hydroxyoctyl alkyl chain analogue 9 showed less displacement.

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.