Direct evaluation of polarity of the ligand binding pocket in retinoid X receptor using a fluorescent solvatochromic agonist.

High selectivity of small-molecule drug candidates for their target molecule is important to minimize potential side effects. One factor that contributes to the selectivity is the internal polarity of the ligand-binding pocket (LBP) in the target molecule, but this is difficult to measure. Here, we first confirmed that the retinoid X receptor (RXR) agonist 6-(ethyl(1-isobutyl-2-oxo-4-(trifluoromethyl)-1,2-dihydroquinolin-7-yl)amino)nicotinic acid (NEt-iFQ, 1) exhibits fluorescence solvatochromism, i.e., its Stokes shift depends on the polarity of the solvent, and then we utilized this property to directly measure the internal polarity of the RXRα-LBP. The Stokes shift of 1 when bound to the RXRα-LBP corresponded to that of 1 in chloroform solution. This finding is expected to be helpful for designing RXR-selective ligands. A similar approach should be appliable to evaluate the internal polarity of the LBPs of other receptors.

Characterization and Interconversion of Two Crystal Forms of NEt-3IB, a Retinoid X Receptor Agonist.

Retinoid X receptor (RXR) agonist NEt-3IB (1) is a candidate for the treatment of inflammatory bowel disease (IBD), and we have established a process synthesis of 1 in which the final product is obtained by recrystallization from 70% EtOH. However, we found that there are two crystal forms of 1. Here, to characterize and clarify the relationship between them, we conducted thermogravimetry, powder X-ray diffraction, and single crystal X-ray diffraction. The crystal forms were identified as the monohydrate form I and anhydrate form II. The crystal form I, obtained as a stable form by our established synthesis, was easily dehydrated simply by drying to afford the form II', which was similar to the crystal form II obtained by recrystallization from anhydrous EtOH. Storage of the form II' in air regenerated the form I. The molecular conformations of 1 in the crystals of the two forms are similar, and they can be reversibly interconverted. The solubility of the monohydrate form I and anhydrate form II was examined and the former was found to be less soluble than the latter. Thus, form I may be superior to form II for targeting IBD, because of higher delivery to the lower gastrointestinal tract and reduction of systemic side effects associated with lower absorption due to lower water solubility.

Development of Scaled-Up Synthetic Method for Retinoid X Receptor Agonist NEt-3IB Contributing to Sustainable Development Goals.

Small-molecular drugs, which are generally inexpensive compared with biopharmaceuticals and can often be taken orally, may contribute to the Sustainable Development Goals (SDGs) adopted by the United Nations. We previously reported the retinoid X receptor (RXR) agonist 4-(ethyl(3-isobutoxy-4-isopropylphenyl)amino)benzoic acid (NEt-3IB, 1) as a small-molecular drug candidate to replace biopharmaceuticals for the treatment of inflammatory bowel disease. The previous synthetic method to 1 required a large amount of organic solvent and extensive purification. In line with the SDGs, we aimed to develop an environmentally friendly, inexpensive method for the large-scale synthesis of 1. The developed method requires only a hydrophobic ether and EtOH as reaction and extraction solvents. The product was purified by recrystallization twice to afford 99% pure 1 at 100 mmol scale in about 30% yield. The optimized process showed a 35-fold improvement of the E-factor (an index of environmental impact) compared to the original method. This work, which changes the solvent used to environmentally preferable ones based on the existing synthetic method for 1, illustrates how synthetic methods for small-molecular drugs can be adapted and improved to contribute to the SDGs.

Fluorescence properties of retinoid X receptor antagonist NEt-SB.

Ligands of retinoid X receptors (RXRs) are effective against various diseases, so there is a need for efficient screening methods to discover new ligands. Existing screening methods are complex and time-consuming, and a simple fluorescence assay would be highly desirable. Here, we focused on NEt-SB (4), which has a stilbene structure, as a candidate for this purpose, and examined its fluorescence properties in detail. The fluorescence intensity of 4 was remarkably increased in highly viscous solvents and upon binding to hRXRα-LBD, due to suppression of free rotation of the stilbene moiety. Although the relatively low fluorescence intensity and the short fluorescence wavelength of 4 make this compound itself unsuitable for use in RXR binding assay, our findings provide a basis for further structural evolution, which may lead to a derivative that would be suitable for fluorescence assay of RXR binders.

A partial agonist for retinoid X receptor mitigates experimental colitis.

Inflammatory bowel disease (IBD), including ulcerative colitis and Crohn's disease, is an intractable disease of the gastrointestinal tract. Multiple environmental factors, including food ingredients, have been implicated in the development of these diseases. For example, animal fat-rich diets are predisposing factors for ulcerative colitis, whereas n-3 unsaturated fatty acids such as docosahexaenoic acid (DHA) show protective effects in experimental colitis and are negatively correlated with the incidence of ulcerative colitis and Crohn's disease. Given that DHA exhibits agonistic activity on retinoid X receptor (RXR), activation of RXR could be a therapeutic strategy for IBD. However, conventional full RXR agonists are known to show considerable adverse effects. We therefore took advantage of a partial RXR agonist, CBt-PMN, to minimize the adverse effects, and evaluated its efficacy in dextran sodium sulfate-induced colitis. Administration of CBt-PMN efficiently ameliorated the symptoms of colitis. This effect was attributed to the down-regulation of pro-inflammatory cytokines such as Tnf and Il6 in colon-infiltrating monocytes. Down-regulation of pro-inflammatory cytokines by CBt-PMN was also evident in lipopolysaccharide-stimulated bone marrow-derived macrophages (BMDMs). Among many RXR-associated nuclear receptors, activation of peroxisome proliferator-activated receptor δ (PPARδ) and nuclear hormone receptor 77 (Nur77) suppressed cytokine production by BMDMs. These observations suggest that the activation of PPARδ/RXR and Nur77/RXR heterodimers by CBt-PMN through the permissive mechanism is responsible for diminishing the monocyte-mediated inflammatory response in the gut. Our data highlight the importance of RXR activation in the regulation of colitis.

A retinoid X receptor partial agonist attenuates pulmonary emphysema and airway inflammation.

BACKGROUND: Retinoid X receptors (RXRs) are members of the nuclear receptor (NR) superfamily that mediate signalling by 9-cis retinoic acid, a vitamin A derivative. RXRs play key roles not only as homodimers but also as heterodimeric partners, e.g., for retinoic acid receptors, vitamin D receptors, and peroxisome proliferator-activated receptors. The NR family may also play important roles in the development of emphysema. However, the role of RXRs in the pathogenesis of emphysema is not well defined. METHODS: We developed a novel RXR partial agonist (NEt-4IB) and investigated its effect and mechanism compared to a full agonist (bexarotene) in a murine model of emphysema. For emphysema induction, BALB/c mice received intraperitoneal cigarette smoke extract (CSE) or intratracheal porcine pancreas elastase (PPE). Treatment with RXR agonists was initiated before or after emphysema induction. RESULTS: Treatment with NEt-4IB significantly suppressed the increase in static lung compliance and emphysematous changes in CSE-induced emphysema and PPE-induced established and progressive emphysema. NEt-4IB significantly suppressed PPE-induced neutrophilic airway inflammation and the levels of keratinocyte chemoattractant (KC), C-X-C motif ligand5 (CXCL5), interferon (IFN)-γ and IL-17. NEt-4IB also improved the matrix metalloproteinase-9 (MMP-9)/tissue inhibitor of metalloproteinase-1 (TIMP-1) imbalance and the reduced anti-oxidant activity in bronchoalveolar lavage (BAL) fluid. NEt-4IB suppressed PPE-induced vascular endothelial growth factor (VEGF) expression in the airway. Treatment with NEt-4IB and bexarotene significantly suppressed the increase in static lung compliance and emphysematous changes. However, adverse effects of RXR agonists, including hypertriglyceridemia and hepatomegaly, were observed in bexarotene-treated mice but not in NEt-4IB-treated mice. CONCLUSION: These data suggest that RXRs play crucial roles in emphysema and airway inflammation, and novel partial RXR agonists could be potential therapeutic strategies for the treatment of PPE- and CSE-induced emphysema.

3H-Imidazo[4,5-b]pyridine-6-carboxylic acid derivatives as rexinoids with reduced teratogenicity.

Several retinoid X receptor (RXR) ligands (rexinoids), such as bexarotene (1), exhibit teratogenicity, which is a serious impediment to their clinical application. We considered that rexinoids with a lower level of maximal RXR transcription activation (i.e., partial agonists) and lower lipid solubility might show weaker adverse side effects. Based on this idea, we modified our previously reported pentamethyltetralin-type RXR partial agonists 5 and 6 to reduce their lipophilicity. Here, we report a new RXR partial agonist, 3-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydronaphthalen-2-yl)-2-(trifluoromethyl)-3H-imidazo[4,5-b]pyridine-6-carboxylic acid (8, CATF-PMN), which showed greatly reduced teratogenicity in zebrafish embryos.

Fluorine-18 (18F)-labeled retinoid x receptor (RXR) partial agonist whose tissue transferability is affected by other RXR ligands.

Bexarotene (1), a retinoid X receptor (RXR) agonist approved for the treatment of cutaneous T cell lymphoma (CTCL), was reported to migrate into baboon brain based on findings obtained by positron emission tomography (PET) with a 11C-labeled tracer. However, co-administration of non-radioactive 1 had no effect on the distribution of [11C]1, probably due to non-specific binding of 1 as a result of its high lipophilicity. Here, we report a fluorine-18 (18F)-labeled PET tracer [18F]6 derived from RXR partial agonist CBt-PMN (2), which has lower lipophilicity and weaker RXR-binding ability than [11C]1. The concomitant administration of 1 or 2 with [18F]6 with resulted in decreased accumulation of [18F]6 in liver, together with increased brain uptake and increased accumulation in kidney and muscle, as visualized by PET. A plausible explanation of these findings is the inhibition of [18F]6 uptake into the liver by concomitantly administered 1 or 2, leading to an increase in blood concentration of [18F]6 followed by increased accumulation in other tissues.

Retinoid X Receptor Antagonists.

Retinoid X receptor (RXR) antagonists are not only useful as chemical tools for biological research, but are also candidate drugs for the treatment of various diseases, including diabetes and allergies, although no RXR antagonist has yet been approved for clinical use. In this review, we present a brief overview of RXR structure, function, and target genes, and describe currently available RXR antagonists, their structural classification, and their evaluation, focusing on the latest research.

Effect of a retinoid X receptor partial agonist on airway inflammation and hyperresponsiveness in a murine model of asthma.

BACKGROUND: Retinoid X receptors (RXRs) are members of the nuclear receptor (NR) superfamily that mediate signaling by 9-cis retinoic acid, a vitamin A (retinol) derivative. RXRs play key roles not only as homodimers but also as heterodimeric partners-e.g., retinoic acid receptors (RARs), vitamin D receptors (VDRs), liver X receptors (LXRs), and peroxisome proliferator-activated receptors (PPARs). The NR family was recently associated with allergic diseases, but the role of RXRs in allergen-induced airway responses is not well defined. The goal of this study is to elucidate the role of RXRs in asthma pathogenesis and the potency of RXR partial agonist in the treatment of allergic airway inflammation and airway hyperresponsiveness using a murine model of asthma. METHODS: We investigated the effect of a novel RXR partial agonist (NEt-4IB) on the development of allergic airway inflammation and airway hyperresponsiveness (AHR) in a murine model of asthma. Balb/c mice were sensitized (days 0 and 14) and challenged (days 28-30) with ovalbumin (OVA), and airway inflammation and airway responses were monitored 48 h after the last OVA challenge. NEt-4IB was administered orally on days 25 to 32. RESULTS: Oral administration of NEt-4IB significantly suppressed AHR and inflammatory cell accumulation in the airways and attenuated the levels of TNF-α in the lung and IL-5, IL-13 and NO levels in bronchoalveolar lavage (BAL) fluid and the number of periodic acid Schiff (PAS)-positive goblet cells in lung tissue. Treatment with NEt-4IB also significantly suppressed NF-κB expression. CONCLUSION: These data suggest that RXRs may be of crucial importance in the mechanism of allergic asthma and that the novel RXR partial agonist NEt-4IB may be a promising candidate for the treatment of allergic airway inflammation and airway hyperresponsiveness in a model of allergic asthma.

Interspecies comparison of hepatic metabolism of six newly synthesized retinoid X receptor agonistic compounds possessing a 6-[N-ethyl-N-(alkoxyisopropylphenyl)amino]nicotinic acid skeleton in rat and human liver microsomes.

OBJECTIVE: The hepatic metabolism of six compounds newly synthesized as retinoid X receptor agonists was characterized in rat and human liver microsomes to examine the relationship between their hepatic metabolism profiles and side chain structures, considering the interspecies difference. MATERIALS AND METHODS: The compounds used have a 6-[N-ethyl-N-(3-alkoxy-4-isopropylphenyl)amino]nicotinic or 6-[N-ethyl-N-(4-alkoxy-3-isopropylphenyl)amino]nicotinic acid skeleton, in which the isopropoxy, isobutoxy or cyclopropylmethoxy group is employed for the alkoxy group. These compounds were incubated with the microsomes, and their Michaelis--Menten parameters were determined. The incubation study was also performed with various cytochrome P450 (CYP) inhibitors to examine their susceptibilities to the inhibitors. In addition, a molecular docking simulation was conducted to assess the compound's spatial configuration with the CYP isoform when necessary. RESULTS: The Michaelis--Menten parameters determined are comparable between rats and humans for the compounds having 3-isobutoxy, 4-isobutoxy, 4-isopropoxy and 4-cyclopropylmethoxy groups. However, it was indicated that all compounds except that having the 3-isobutoxy group are metabolized in a different manner between rats and humans. That is, the extent of the contribution of each CYP isozyme is different between those two species. A molecular docking simulation showed that the spatial configuration of the compound to be associated with CYP2D6 markedly changes depending on whether the isobutoxy group is situated at the 3- or 4-position. CONCLUSION: A slight difference in the side chain structures markedly alters the compound's metabolic profile, which amplifies the interspecies difference regarding the profile, increasing the difficulty in characterizing the profile in humans with the structural-property relationship and interspecies extrapolation.

Simple Fluorescence Labeling Method Enables Detection of Intracellular Distribution and Expression Level of Retinoid X Receptors.

There is no straightforward method to visualize the intracellular distribution of nuclear receptors, such as retinoid X receptors (RXRs), which are trafficked between the cytosol and nucleus. Here, in order to develop a simple fluorescence labeling method for RXRs, we designed and synthesized compound 4, consisting of an RXR-selective antagonist, CBTF-EE (2), linked via an ether bond to the fluorophore nitrobenzoxadiazole (NBD). Compound 4 is nonfluorescent, but the ether bond (-O-NBD) reacts with biothiols such as cysteine and homocysteine to generate a thioether (-S-NBD), followed by intramolecular Smiles rearrangement with an amino group such as that of lysine to form a fluorescent secondary amine (-NH-NBD) adjacent to the binding site. Fluorescence microscopy of intact or RXR-overexpressing MCF-7 cells after incubation with 4 enabled us to visualize RXR expression as well as nuclear transfer of RXR induced by the agonist bexarotene (1).

Structural characterization of 1,3-bis-tert-butyl monocyclic benzene derivatives with agonistic activity towards retinoid X receptor alpha.

Retinoid X receptor alpha (RXRα) plays pivotal roles in multiple biological processes, but limited information is available on the structural features of chemicals that show low affinity for RXRα, but nevertheless cause significant activation, though these may represent a human health hazard. We recently discovered that several industrial chemicals having 1,3-bis-tert-butylbenzene as a common chemical structure exhibit agonistic activity towards rat RXRα. In this study, we explored the structure-activity relationship of 1,3-bis-tert-butyl monocyclic benzene derivatives for RXRα activation by means of in vitro and in silico analyses. The results indicate that a bulky substituent at the 5-position is favorable for agonistic activity towards human RXRα. Since 1,3-bis-tert-butyl monocyclic benzene derivatives with bulky hydrophobic moieties differ structurally from known RXRα ligands such as 9-cis-retinoic acid and bexarotene, our findings may be helpful for the development of structural alerts in the safety evaluation of industrial chemicals for RXRα-based toxicity to living organisms.

Ligand Screening System for the RXRα Heterodimer Using the Fluorescence RXR Agonist CU-6PMN.

Retinoid X receptor (RXR), a nuclear receptor (NR) that regulates transcription of target genes in a ligand binding-dependent manner, is of interest as a drug target. RXR agonists have been developed as therapeutic agents for cutaneous invasive T-cell lymphoma (e.g., bexarotene (1)) and investigated as potential anti-inflammatory agents. Screening systems for the binding of RXR alone have been reported. However, although RXRs function as RXR heterodimers, information on systems to evaluate the differential binding of RXR agonists as RXR heterodimers has not been available until recently. Here we show that the fluorescent RXR agonist CU-6PMN (3), designed by our group, can be useful for assessing RXR binding to PPARγ/RXRα, and that the binding data differ from those of RXRα alone. This screening method opens a new avenue for binding assays for RXR heterodimers.

Feasibility of structural modification of retinoid X receptor agonists to separate blood glucose-lowering action from adverse effects: studies in KKA(y) type 2 diabetes model mice.

Retinoid X receptor (RXR) agonists are reported to exhibit blood glucose-lowering action owing to peroxisome proliferator-activated receptor (PPAR)/RXR or liver X receptor (LXR)/RXR activation, but may also cause adverse effects such as blood triglyceride elevation. In order to examine the feasibility of separating the glucose-lowering action from the adverse effects, we examined the effects of RXR agonists (NEt-TMN), NEt-3IB, and NEt-3IP, which have different heterodimer-activating patterns, in KKA(y) type 2 diabetes model mice. We found that NEt-3IB induced lower degrees of hepatomegaly and blood triglyceride (TG) elevation than the other RXR agonists, even though all of them showed similar blood glucose-lowering action on repeated administration. These findings indicate that structural modification of RXR agonists is a potentially effective strategy to reduce adverse effects while retaining desired activities.

Increased Molecular Flexibility Widens the Gap between K i and K d values in Screening for Retinoid X Receptor Modulators.

Screening for small-molecule modulators targeting a particular receptor is frequently based on measurement of K d, i.e., the binding constant between the receptor and the compound of interest. However, K d values also reflect binding at receptor protein sites other than the modulatory site. We designed derivatives of retinoid X receptor (RXR) antagonist CBTF-EE (1) with modifications that altered their conformational flexibility. Compounds 6a,b and 7a,b showed quite similar K d values, but 7a,b exhibited 10-fold higher K i values than those of 6a,b. Further, 6a,b showed potent RXR-antagonistic activity, while 7a,b were inactive. These results suggest that increased conformational flexibility promotes binding at nontarget receptor sites. In this situation, conventional determination of K d is less effective for screening purposes than the determination of K i using a ligand that binds specifically to the site regulating transcriptional activity. Thus, the use of K i values for orthosteric ligands may increase the hit rate in screening active regulatory molecules.

Teratogenicity and Fetal-Transfer Assessment of the Retinoid X Receptor Agonist Bexarotene.

Bexarotene, a retinoid X receptor (RXR) agonist, is used to treat cutaneous T-cell lymphoma, and drug repositioning research has also been reported, despite warnings of teratogenicity. However, fetal transfer of bexarotene and its effect on rat fetal bone formation have not been examined. In this study, we conducted a detailed teratogenicity and fetal transferability assessment of bexarotene in rats. Repeated administration of bexarotene during pregnancy caused marked fetal atrophy and bone dysplasia. Although fetal transfer was not detectable by dynamic imaging of [11C]bexarotene by means of positron emission tomography, transfer to the fetus was confirmed by using a gamma counter. Similar levels were found in mother and fetus. In addition, we found that bexarotene was accumulated in the placenta. These findings will be useful for the toxicity assessment of bexarotene as well as for drug discovery research targeting RXR agonists, which are expected to have therapeutic effects in various diseases.

Analysis of absorption-enhancing mechanisms for combinatorial use of spermine with sodium taurocholate in Caco-2 cells.

Previously, we reported that the combined use of spermine (SPM) and sodium taurocholate (STC) (SPM-STC) significantly improves the oral absorption of rebamipide (BCS class IV) and pulmonary absorption of interferon-α without any harmful histopathological changes in the gastrointestinal tract and lungs, respectively. In the present study, we examined the effect of SPM-STC on the transport of fluorescein isothiocyanate-labeled dextrans (FDs) across Caco-2 cell monolayers and attempted to clarify the mechanisms underlying the transport enhancement caused by SPM-STC. SPM-STC were found to significantly enhance the transport of FDs, while the treatment with SPM-STC was not harmful, and the decrease in transepithelial electrical resistance was transient and reversible. The voltage-clamp study clearly indicated that the opening of the paracellular route could be mainly responsible for the enhanced transport of FD-4. As for the mechanisms, it was found that SPM-STC caused a significant increase in membrane fluidity, which would lead to the enhanced transport of small-molecule drugs such as rebamipide. Since SPM-STC increased intracellular Ca2+ via Ca2+ uptake through Ca2+ channels and Ca2+ release from the endoplasmic reticulum stimulated by the IP3 pathway, the subsequent possible activation of the MLCK signaling pathway would have led to the contraction of the actin-myosin ring. The rearrangement of tight junction-constituting proteins induced through the MAPK pathway has also been suggested as a possible mechanism for opening tight junctions. Claudin-4, a key protein constituting the tight junction, merged with F-actin along with the plasma membrane, was significantly decreased, which would be at least partial structural evidence for the tight-junction opening.

Identification of a Vitamin-D Receptor Antagonist, MeTC7, which Inhibits the Growth of Xenograft and Transgenic Tumors In Vivo.

Vitamin-D receptor (VDR) mRNA is overexpressed in neuroblastoma and carcinomas of lung, pancreas, and ovaries and predicts poor prognoses. VDR antagonists may be able to inhibit tumors that overexpress VDR. However, the current antagonists are arduous to synthesize and are only partial antagonists, limiting their use. Here, we show that the VDR antagonist MeTC7 (5), which can be synthesized from 7-dehydrocholesterol (6) in two steps, inhibits VDR selectively, suppresses the viability of cancer cell-lines, and reduces the growth of the spontaneous transgenic TH-MYCN neuroblastoma and xenografts in vivo. The VDR selectivity of 5 against RXRα and PPAR-γ was confirmed, and docking studies using VDR-LBD indicated that 5 induces major changes in the binding motifs, which potentially result in VDR antagonistic effects. These data highlight the therapeutic benefits of targeting VDR for the treatment of malignancies and demonstrate the creation of selective VDR antagonists that are easy to synthesize.

Replacement of the hydrophobic part of 9-cis-retinoic acid with cyclic terpenoid moiety results in RXR-selective agonistic activity.

Retinoid X receptor (RXR) agonists are interesting candidates for the treatment of metabolic syndrome. 9-Cis-retinoic acid (9cRA: 1) is a natural RXR agonist, that also works as a retinoic acid receptor (RAR) agonist. This fact prompted us to study the structure-activity relationship (SAR) of RXR agonists derived from 1. Though 3 and 4, in which the cyclohexene part of 1 is replaced with bulkier hydrophobic moieties, show RXR-selective agonistic activity, some analogs containing other ring structures show RAR agonistic activity. Thus, we were interested in establishing what kind of ring skeleton is required for RXR-selective agonistic activity. In this study, we systematically prepared 5 and 6, in which the cyclohexene ring of 1 is replaced with various cyclic terpenoid moieties, and evaluated their RXR and RAR agonistic activities. Our previously reported CsF-promoted Stille coupling reaction was employed as a key step for the comprehensive synthesis of 5 and 6. The results of transcriptional assay showed that compounds 5b-f, which possess a menthane skeleton, exhibit RXR-selective agonistic activity. These results should be helpful for the design of superior RXR-selective agonists based on the structure of 1.