Structural Development of Silicon-Containing Retinoids: Structure-Activity Relationship Study of the Hydrophobic Pharmacophore of Retinobenzoic Acids Using Silyl Functionalities.

We designed and synthesized a series of retinobenzoic acids bearing various silyl functionalities in order to explore in detail the structure-activity relationship (SAR) at the hydrophobic moiety of retinoids. Among the synthesized compounds, 24 c bearing a t-butyldimethylsilyl (TBS) group at the hydrophobic site exhibited potent retinoid activity comparable to that of the lead compound Am555S (4). Compound 24 c exhibited transcription-promoting activity towards all three subtypes of retinoic acid receptor (RAR), but showed the highest activity towards RARγ, in contrast to the high RARα-selectivity of Am80 (3) and Am555S (4). The SARs presented here should be helpful in the development of subtype-selective retinoids, and in particular 24 c might be a promising lead compound for new RARγ ligands.

Lithocholic Acid Derivatives as Potent Vitamin D Receptor Agonists.

Lithocholic acid (2) was identified as a second endogenous ligand of vitamin D receptor (VDR), though its activity is very weak. In this study, we designed novel lithocholic acid derivatives based on the crystal structure of VDR-ligand-binding domain (LBD) bound to 2. Among the synthesized compounds, 6 bearing a 2-hydroxy-2-methylprop-1-yl group instead of the 3-hydroxy group at the 3α-position of 2 showed dramatically increased activity in HL-60 cell differentiation assay, being at least 10 000 times more potent than lithocholic acid (2) and 3 times more potent than 1α,25-dihydroxyvitamin D3 (1). Although the binding affinities of 6 and its epimer 7 were less than that of 1, their transactivation activities were greater than that of 1. X-ray structure analyses of VDR LBD bound to 6 or 7 showed that the binding positions of these compounds in the ligand-binding pocket are similar to that of 1.

Development of Androgen-Antagonistic Coumarinamides with a Unique Aromatic Folded Pharmacophore.

First-generation nonsteroidal androgen receptor (AR) antagonists, such as flutamide (2a) and bicalutamide (3), are effective for most prostate cancer patients, but resistance often appears after several years due to the mutation of AR. Second-generation AR antagonists are effective against some of these castration-resistant prostate cancers, but their structural variety is still limited. In this study, we designed and synthesized 4-methyl-7-(N-alkyl-arylcarboxamido)coumarins as AR antagonist candidates and evaluated their growth-inhibitory activity toward androgen-dependent SC-3 cells. Coumarinamides with a secondary amide bond did not show inhibitory activity, but their N-methylated derivatives exhibited AR-antagonistic activity. Especially, 19b and 31b were more potent than the lead compound 7b, which was comparable to hydroxyflutamide (2b). Conformational analysis showed that the inactive coumarinamides with a secondary amide bond have an extended structure with a trans-amide bond, while the active N-methylated coumarinamides have a folded structure with a cis-amide bond, in which the two aromatic rings are placed face-to-face. Docking study suggested that this folded structure is important for binding to AR. Selected coumarinamide derivatives showed AR-antagonistic activity toward LNCaP cells with T877A AR, and they had weak progesterone receptor (PR)-antagonistic activity. The folded coumarinamide structure appears to be a unique pharmacophore, different from those of conventional AR antagonists.

Development of novel lithocholic acid derivatives as vitamin D receptor agonists.

Lithocholic acid (2) was identified as the second endogenous ligand of vitamin D receptor (VDR), though its binding affinity to VDR and its vitamin D activity are very weak compared to those of the active metabolite of vitamin D3, 1α,25-dihydroxyvitamin D3 (1). 3-Acylated lithocholic acids were reported to be slightly more potent than lithocholic acid (2) as VDR agonists. Here, aiming to develop more potent lithocholic acid derivatives, we synthesized several derivatives bearing a 3-sulfonate/carbonate or 3-amino/amide substituent, and examined their differentiation-inducing activity toward human promyelocytic leukemia HL-60 cells. Introduction of a nitrogen atom at the 3-position of lithocholic acid (2) decreased the activity, but compound 6 bearing a 3-methylsulfonate group showed more potent activity than lithocholic acid (2) or its acylated derivatives. The binding of 6 to VDR was confirmed by competitive binding assay and X-ray crystallographic analysis of the complex of VDR ligand-binding domain (LBD) with 6.

Design and synthesis of 4-benzyl-1-(2H)-phthalazinone derivatives as novel androgen receptor antagonists.

The androgen receptor (AR) plays important roles in multiple physiological functions, including differentiation, growth, and maintenance of male reproductive organs, and also has effects on hair and skin. In this paper, we report the synthesis of nonsteroidal AR antagonists having a 4-benzyl-1-(2H)-phthalazinone skeleton. Among the synthesized compounds, 11c with two ortho-substituents on the phenyl group potently inhibited SC-3 cell proliferation (IC50: 0.18 µM) and showed high wt AR-binding affinity (IC50: 10.9 µM), comparable to that of hydroxyflutamide (3). Compound 11c also inhibited proliferation of LNCaP cells containing T877A-mutated AR. Docking study of 11c with the AR ligand-binding domain indicated that the benzyl group is important for the antagonism. These phthalazinone derivatives may be useful for investigating potential clinical applications of AR antagonists.

Structural development of p-carborane-based potent non-secosteroidal vitamin D analogs.

Non-secosteroidal vitamin D receptor (VDR) ligands are promising candidates for many clinical applications. We recently developed novel non-secosteroidal VDR agonists based on p-carborane (an icosahedral carbon-containing boron cluster) as a hydrophobic core structure. Here, we report the design, synthesis and biological evaluation of carborane-based vitamin D analogs bearing various substituents at the diol moiety. Among the synthesized compounds, methylene derivative 31 exhibited the most potent vitamin D activity, which was comparable to that of the natural hormone, 1α,25(OH)2D3. This compound is one of the most potent non-secosteroidal VDR agonists reported to date, and is a promising lead for development of novel drug candidates.

Design and synthesis of tetraol derivatives of 1,12-dicarba-closo-dodecaborane as non-secosteroidal vitamin D analogs.

Vitamin D receptor (VDR), a nuclear receptor for 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3, 1), is a promising target for multiple clinical applications. We recently developed non-secosteroidal VDR ligands based on a carbon-containing boron cluster, 1,12-dicarba-closo-dodecaborane (p-carborane), and examined the binding of one of them to VDR by means of crystallographic analysis. Here, we utilized that X-ray structure to design novel p-carborane-based tetraol-type vitamin D analogs, and we examined the biological activities of the synthesized compounds. Structure-activity relationship study revealed that introduction of an ω-hydroxyalkoxy functionality enhanced the biological activity, and the configuration of the substituent significantly influenced the potency. Among the synthesized compounds, 4-hydroxybutoxy derivative 9a exhibited the most potent activity, which was equal to that of the secosteroidal vitamin D analog, 19-nor-1α,25-dihydroxyvitamin D3 (2).

Synthesis and structure-activity relationship of p-carborane-based non-secosteroidal vitamin D analogs.

1α,25-Dihydroxyvitamin D3 [1α,25(OH)2D3: 1] is a specific modulator of nuclear vitamin D receptor (VDR), and novel vitamin D analogs are therapeutic candidates for multiple clinical applications. We recently developed non-secosteroidal VDR agonists bearing a p-carborane cage (a carbon-containing boron cluster) as a hydrophobic core structure. These carborane derivatives are structurally quite different from classical secosteroidal vitamin D analogs. Here, we report systematic synthesis and activity evaluation of carborane-based non-secosteroidal vitamin D analogs. The structure-activity relationships of carborane derivatives are different from those of secosteroidal vitamin D derivatives, and in particular, the length and the substituent position of the dihydroxylated side chain are rather flexible in carborane derivatives. The structure-activity relationships presented here should be helpful in development of non-secosteroidal vitamin D analogs for clinical applications.

Structure-activity relationship study on benzoic acid part of diphenylamine-based retinoids.

Based on structure-activity relationship studies of the benzoic acid part of diphenylamine-based retinoids, the potent RXR agonist 4 was derivatized to obtain retinoid agonists, synergists, and an antagonist. Cinnamic acid derivatives 5 and phenylpropionic acid derivatives 6 showed retinoid agonistic and synergistic activities, respectively. The difference of the activities is considered to be due to differences in the flexibility of the carboxylic acid-containing substituent on the diphenylamine skeleton. Compound 7, bearing a methyl group at the meta position to the carboxyl group, was an antagonist, dose-dependently inhibiting HL-60 cell differentiation induced by 3.3 × 10(-10)M Am80.

Novel vitamin D receptor ligands bearing a spherical hydrophobic core structure--comparison of bicyclic hydrocarbon derivatives with boron cluster derivatives.

Vitamin D receptor (VDR) is a nuclear receptor for 1α,25-dihydroxyvitamin D(3) (1α,25(OH)(2)D(3)), and is an attractive target for multiple clinical applications. We recently developed novel non-secosteroidal VDR ligands bearing a hydrophobic p-carborane cage, thereby establishing the utility of this spherical hydrophobic core structure for development of VDR ligands. Here, we synthesized two series of novel non-secosteroidal VDR ligands with different spherical hydrophobic cores, that is, bicyclo[2.2.2]octane derivatives and p-carborane derivatives, and compared their biological activities in order to examine the difference between the interactions of the C-H hydrocarbon surface and the B-H carborane surface with the receptor. Carborane derivatives exhibited more potent differentiation-inducing activity toward HL-60 cells than did the corresponding bicyclo[2.2.2]octane derivatives. These results suggest that the hydrophobic carborane cage may interact more efficiently than the hydrocarbons with the hydrophobic surface of VDR. This finding further supports the view that carborane structure is a promising option for drug development.

Boron cluster-based development of potent nonsecosteroidal vitamin D receptor ligands: direct observation of hydrophobic interaction between protein surface and carborane.

We report here the design and synthesis of a novel vitamin D receptor (VDR) agonist whose hydrophobic core structure is p-carborane (1,12-dicarba-closo-dodecaborane, an icosahedral carbon-containing boron cluster having remarkable thermal and chemical stability and a characteristically hydrophobic B-H surface). This carborane-based VDR ligand exhibited moderate vitamin D activity, comparable to that of the natural hormone, despite its simple and flexible structure. X-ray structure analysis provided direct evidence that the carborane cage binds to the hydrophobic surface of the ligand-binding pocket of the receptor, promoting transition to the active conformation. These results indicate that the spherical B-H surface of carborane can function efficiently as a hydrophobic anchor in binding to the receptor surface, thereby allowing induced fitting of the three essential hydroxyl groups on the alkyl chains to the appropriate positions for interaction with the VDR binding site, despite the entropic disadvantage of the flexible structure. We suggest that carborane structure is a promising option in the design of novel drug candidates.

Diphenylamine-based retinoid antagonists: regulation of RAR and RXR function depending on the N-substituent.

Based upon the structure-activity relationships of diphenylamine derivatives with retinoid synergistic activity (RXR agonists), novel diphenylamine derivatives with a long alkyl chain (9a and 9b) or a benzyl group (10a-f) as the N-substituent were designed and synthesized. All the synthesized compounds dose-dependently inhibited HL-60 cell differentiation induced by 3.3×10(-10)M Am80. Among them, compound 10f showed the most potent inhibitory activity, and the mechanism was shown, by means of transactivation assay for RARs and RXRs, to involve antagonism against RARs. The N-substituent of the diphenylamine skeleton plays an important role in determining the receptor selectivity for RARs or RXRs, as well as the agonist or antagonist nature of the activity.

Novel thyroid hormone receptor antagonists with an N-alkylated diphenylamine skeleton.

Thyroid hormones play important roles in growth, development and homeostasis, and disruption of their functions induces serious disease, so novel synthetic thyroid hormone analogues are candidates for clinical application. We designed and synthesized novel diphenylamine derivatives with a thiazolidinedione moiety as the terminal polar group as thyroid hormone receptor (TR) antagonists. Compounds bearing an appropriately sized N-alkyl group showed antagonistic activities towards both the hTRalpha1 and hTRbeta1 subtypes.

Novel retinoid X receptor (RXR) antagonists having a dicarba-closo-dodecaborane as a hydrophobic moiety.

We designed and synthesized novel retinoid X receptor (RXR)-selective antagonists bearing a carborane moiety. Compounds 8a-d or 9a-d themselves have no differentiation-inducing activity toward HL-60 cells and no inhibitory activity towards a retinoic acid receptor (RAR) agonist. However, they inhibit the synergistic activity of an RXR agonist, PA024, in the presence of Am80 on the cell differentiation of HL-60. Transactivation assay using RARs and RXRs suggested that the inhibitory activity of 9b resulted from the selective antagonism at the RXR site of RXR-RAR heterodimers.

Novel retinoid X receptor antagonists: specific inhibition of retinoid synergism in RXR-RAR heterodimer actions.

Several 2-(arylamino)pyrimidine-5-carboxylic acids were designed as novel retinoid X receptor (RXR) antagonists. Compound 6a or 6b alone did not exhibit differentiation-inducing activity toward HL-60 cells and did not affect the activity of a retinoic acid receptor (RAR) agonist, Am80, but did inhibit the synergistic activity of an RXR agonist, PA024 (3), in the presence of Am80. The activity of 6 was ascribed to selective antagonism at the RXR site of RXR-RAR heterodimers.