Current Topics in Medicinal Chemistry - Volume 14, Issue 21, 2014

The vitamin D nuclear receptor (VDR) and its ligand, 1α, 25-dihydroxyvitamin D3 (1,25(OH)2D3, or calcitriol) regulate numerous biological functions. Therefore, VDR represents an important therapeutic target in the treatment of various diseases such as cancers, psoriasis, rickets, renal osteodystrophy, and autoimmune dysfunctions. Despite the number of newly synthesized 1,25(OH)2D3 analogues, the need for highly potential modulators of VDR with precise cell-, gene- or coregulator-selectivity still exists. The information coming from the analysis of crystal structures of VDR-ligand complexes remains one of the most powerful tools to explain and validate the properties of the compounds and, furthermore, to gain new rationales for their modification. The number of reports on VDR-ligand recognition is constantly rising, and herein we review the recently published structural data. With the emphasis on the most promising compounds, such as secosteroidal compounds and 1,25(OH)2D3 mimics, we also highlight other natural ligands for VDR, evidence for the existence of an alternative ligand binding site within LBP, and identification of novel VDR modulators.

The first determination of the X-ray crystal structure of the ligand binding domain (LBD) of the vitamin D receptor (VDR) complexed with 1α,25-dihydroxyvitamin D3 was reported in 2000. Since then several dozen crystal structures of VDR accommodating various ligands have been presented. Almost all of these complexes display the canonical active conformation observed in the VDR-LBD/1α,25- dihydroxyvitamin D3 complex, and all have quite similar ligand binding pocket (LBP) architectures. To develop new VDR ligands as therapeutic agents, it is important to separate the various biological activities of 1α,25- dihydroxyvitamin D3, such as calcium regulation, cell differentiation and anti-proliferation, and immune modulation. We focused on the structure of the LBP and discovered that vitamin D analogs with a branched side chain induce structural rearrangement of the amino acid residues lining the LBP. These analogs formed an additional cavity in the LBP for accommodation of the side chain and thus altered the structure of the LBP. Interestingly, the ligands showed agonistic, partial agonistic, or antagonistic activity depending upon the structure of the side chain. These results indicate that ligands which alter the pocket structure open a new perspective for the development of VDR ligands exhibiting a specific biological activity.

The Gemini analogs are the last significant contribution to the family of vitamin D derivatives in medicine, for the treatment of cancer. The first Gemini analog was characterized by two symmetric side chains at C-20. Following numerous modifications, the most active analog bears a C-23-triple bond, C-26, 27- hexafluoro substituents on one side chain and a terminal trideuteromethylhydroxy group on the other side chain. This progression was possible due to improvements in the synthetic methods for the preparation of these derivatives, which allowed for increasing molecular complexity and complete diastereoselective control at C-20 and the substituted sidechains.

The development of non-microbial methods for the selective functionalization of non-activated C-H bonds has constituted a challenge, with important economical and environmental implications, for chemists for over a century. The present review provides a comprehensive and current compendium that illustrates the power of C-H functionalization and, namely, of remote functionalization strategies, to expeditiously access vitamin D analogs with intricate structures.

The active metabolite of vitamin D, 1α, 25 dihydroxyvitamin D3 (calcitriol) is classically known to regulate calcium and phosphate homeostasis and bone mineralization. In addition, calcitriol has also been documented to act as a potent anticancer agent in multiple cell culture and animal models of cancer. However, major side effects, such as hypercalcemia, hinder broad-spectrum therapeutic uses of calcitriol in cancer chemotherapy. Synthesis of calcitriol analogues with the same or increased antiproliferative and pro-differentiating activities, and with reduced undesired effects on calcium and bone metabolism, is getting significant attention towards rational therapeutics to treat cancer. In this regard, phosphonate analogues have been shown to display a certain degree of dissociation between the vitamin D activity in vitro and undesired hypercalcemia in vivo. However, few phosphonates have been described in the literature and fewer of them tested for antitumoral effects. Our group has synthesized a novel vitamin D analogue (EM1) bearing an alkynylphosphonate moiety that combines the low calcemic properties of phosphonates with the decreased metabolic inactivation due to the presence of a triple bond between C-23 and C-24. Biological assays demonstrated that this analogue has potent antiproliferative effects in a wide panel of tumour cell lines, even in those resistant to calcitriol treatment. Importantly, EM1 does not show toxic effects in animals, even administered at high doses and for extended periods of time. In the current review we discuss the effects and the potential application in cancer of vitamin D and its derivatives, with an emphasis on phosphonate analogues.

Vitamin D, through its hormonally active form 1α,25-dihydroxyvitamin D3 [1α,25- (OH)2-D3], exhibits a much broader spectrum of bio logical activities than expected in the endocrine system. However, 1α,25-(OH)2-D3 causes hypercalcemia a t pharmacologically r elevant doses wh ich forms a major obstacle in the clinical development of this compound. As a result, considerable effort has been made toward the synthesis of potential chemotherapeutic structurally related congeners with dissociation of beneficial effects from their toxic effects. Most of the analogues prepared have modifications on the upper side chain, more accessible from a synthetic point of view. Modifications of the A-ring are less extensive, although A-ring analogues exhibit a unique biological profile. This seco steroid can undergo a rotation around the 6,7 carbon-carbon single bond generating a wide array of molecular shapes, extending from the 6-s-cis to the more stable extended 6-s-trans conformation, which plays an important role in modulating the different biological activities of vitamin D. We review here, the synthetic strategies for the preparation of Vitamin D analogues with modific ations on the A-ring, including 6-s-cis locked derivatives that became of interest to further probe the less well investigate membrane actions of 1α,25-(OH)2-D3 for structure-activity relationship studies.

The hormonally active form of vitamin D3, 1α,25-dihydroxyvitamin D3 (1a), has a wide variety of biological activities and its major molecular target is considered to be the vitamin D receptor (VDR). The A-ring stereoisomers of 1a as well as its C2-modified analogues, which have different stereochemistry at the C1 and/or C3 hydroxy groups, are of interest since recent metabolic studies have shown that catabolism could occur through A-ring modification. In this review, a practical and versatile synthesis of the A-ring enyne precursors by the convergent method of Trost and coworkers, which is needed to construct all possible A-ring stereoisomers of 1,25-dihydroxyvitamin D3 (1a-d), and the C2-modified analogues (4a-d, 5a-d, 6a-d and 7a-d) is described. A strategy for the synthesis and evaluation of all possible A-ring stereoisomers of 1a and their A-ring modified analogues is important, and this will stimulate synthesis and biological studies into vitamin D.

As the first stable tachysterol analogs, 14-epi-19-nortachysterol and its 2-substituted derivatives were synthesized using the Stille coupling reaction between the A-ring precursor (three vinylstannanes) and the CD-ring vinyl trifrate. Among them, the 2-methylidene group was hydrogenated with Wilkinson’s catalyst regioselectively to obtain 2α- and 2β-methyl analogs after separation; therefore, five new 14-epi-19- nortachysterols were constructed. All 14-epi-19-nortachysterols showed moderate to strong human vitamin D receptor (hVDR) binding affinity except the 2α-(3-hydroxypropoxy) substituted analog. X-ray cocrystallographic analysis of the [truncated hVDR]-[2-methyl-14-epi-19-nortachysterol] complex exhibited an unusual binding structure that has not been observed previously.