Aza-retinoids as novel retinoid X receptor-specific agonists

Potencies of organotin compounds in scallop RXRa responsive activity with a GAL4-based reconstituted yeast assay in vitro

Retinoid X receptor (RXR) has been found to be a major target in various processes of endocrine disruption from the exposure to organotin compounds (OTCs), including imposex in gastropod mollusks. It was also reported in bivalves that OTCs caused intersex and skewed sex ratio. In order to evaluate the effect of these ligand-like OTCs, we constructed a reconstituted yeast system (CfRE system) based on GAL4 yeast two-hybrid principle using scallop Chlamys farreri retinoid X receptor (CfRXRa) and retinoid X response element (RXRE) to investigate the ligand-induced transactivation of CfRXRa. Responses of CfRXRa to 9-cis retinoic acid (9cRA) and tested four OTCs showed concentration-dependent response which is comparable with reported RXRa in vitro assay of human and gastropods. The detective limits of the CfRE system were found to be 100 nM for 9cRA and 10-1000 nM for the tested OTCs. While the tested non-Sn endocrine disrupting chemicals, including Benzo[a]pyrene, 2,4-Dichlorophenol, Nonylphenol, and Tetrabromobisphenol A, showed no effect on CfRXRa response. The present assay system may provide a valuable tool for screening assessments of unidentified environmental ligand chemicals on bivalve mollusks. It is also useful for comparison of sensitivity differences among species exposed to EDCs.

Deacetylative Amination of Acetyl Arenes and Alkanes with C-C Bond Cleavage

The Brønsted acid-catalyzed synthesis of primary amines from acetyl arenes and alkanes with C-C bond cleavage is described. Although the conversion from an acetyl group to amine has traditionally required multiple steps, the method described herein, which uses an oxime reagent as an amino group source, achieves the transformation directly via domino transoximation/Beckmann rearrangement/Pinner reaction. The method was also applied to the synthesis of γ-aminobutyric acids, such as baclophen and rolipram.

A review of the molecular design and biological activities of RXR agonists

An attractive approach to combat disease is to target theregulation of cell function. At the heart of this task are nuclear receptors (NRs); which control functions such as gene transcription. Arguably, the key player in this regulatory machinery is the retinoid X receptor (RXR). This NR associates with a third of the NRs found in humans. Scientists have hypothesized that controlling the activity of RXR is an attractive approach to control cellular functions that modulate diseases such as cancer, diabetes, Alzheimer's disease and Parkinson's disease. In this review, we will describe the key features of the RXR, present a historic perspective of the first RXR agonists, and discuss various templates that have been reported to activate RXR with a focus on their molecular structure, biological activity, and limitations. Finally, we will present an outlook of the field and future directions and considerations to synthesize or modulate RXR agonists to make these compounds a clinical reality.

Synthesis of medium-sized aryl-fused nitrogenous heterocycles via sequential aryne aza-Claisen rearrangement/ring-closing metathesis

The reaction of arynes and secondary allylamines furnished ortho-allyl-substituted N-arylanilines via an aza-Claisen rearrangement. In this transformation, the sequential formation of C–C and C–N bonds occurred by involving two aryne molecules under metal-free reaction conditions to provide moderate to good yields of the products.

Selective ligand activity at Nur/retinoid X receptor complexes revealed by dimer-specific bioluminescence resonance energy transfer-based sensors

Retinoid X receptors (RXRs) play a role as master regulators because of their capacity to form heterodimers with other nuclear receptors (NRs). Accordingly, retinoid signaling is involved in multiple biologic processes, including development, cell differentiation, metabolism, and cell death. However, the role and function of RXRs in different heterodimer complexes remain unidentified, mainly because most RXR drugs (called rexinoids) are not selective of specific heterodimer complexes. The lack of selectivity strongly limits the use of rexinoids for specific therapeutic approaches. To better characterize rexinoids at specific NR complexes, we have developed and optimized luciferase (Luc) protein complementation(PCA)-based bioluminescence resonance energy transfer (BRET) assays that can directly measure recruitment of a coactivator (CoA) motif fused to yellow fluorescent protein (YFP) by specific NR dimers. To validate the assays, we compared rexinoid modulation of CoA recruitment by the RXR homodimer and by the heterodimers Nur77/RXR and Nurr1/RXR. Results revealed that some rexinoids display selective CoA recruitment activities with homo- or heterodimer complexes. In particular, SR11237 (BMS649) has stronger potency for recruitment of CoA motif and transcriptional activity with the heterodimer Nur77/RXR than other complexes. This technology should be useful in identifying new compounds with specificity for individual dimeric species formed by NRs.

Modeling, synthesis, and biological evaluation of potential retinoid X receptor (RXR) selective agonists: novel analogues of 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethynyl]benzoic acid (bexarotene) and (E)-3-(3-(1,2,3,4-tetrahydro-1,1,4,4,6-pentamethylnaphthalen-7-yl)-4-hydroxyphenyl)acrylic acid (CD3254)

Three unreported analogues of 4-[1-(3,5,5,8,8-pentamethyl-5-6-7-8-tetrahydro-2-naphthyl)ethynyl]benzoic acid (1), otherwise known as bexarotene, as well as four novel analogues of (E)-3-(3-(1,2,3,4-tetrahydro-1,1,4,4,6-pentamethylnaphthalen-7-yl)-4-hydroxyphenyl)acrylic acid (CD3254), are described and evaluated for their retinoid X receptor (RXR) selective agonism. Compound 1 has FDA approval as a treatment for cutaneous T-cell lymphoma (CTCL), although treatment with 1 can elicit side-effects by disrupting other RXR-heterodimer receptor pathways. Of the seven modeled novel compounds, all analogues stimulate RXR-regulated transcription in mammalian 2 hybrid and RXRE-mediated assays, possess comparable or elevated biological activity based on EC50 profiles, and retain similar or improved apoptotic activity in CTCL assays compared to 1. All novel compounds demonstrate selectivity for RXR and minimal crossover onto the retinoic acid receptor (RAR) compared to all-trans-retinoic acid, with select analogues also reducing inhibition of other RXR-dependent pathways (e.g., VDR-RXR). Our results demonstrate that further improvements in biological potency and selectivity of bexarotene can be achieved through rational drug design.

Modeling, synthesis and biological evaluation of potential retinoid X receptor-selective agonists: novel halogenated analogues of 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethynyl]benzoic acid (bexarotene)

The synthesis of halogenated analogues of 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethynyl]benzoic acid (1), known commonly as bexarotene, and their evaluation for retinoid X receptor (RXR)-specific agonist performance is described. Compound 1 is FDA approved to treat cutaneous T-cell lymphoma (CTCL); however, bexarotene treatment can induce hypothyroidism and elevated triglyceride levels, presumably by disrupting RXR heterodimer pathways for other nuclear receptors. The novel halogenated analogues in this study were modeled and assessed for their ability to bind to RXR and stimulate RXR homodimerization in an RXRE-mediated transcriptional assay as well as an RXR mammalian-2-hybrid assay. In an array of eight novel compounds, four analogues were discovered to promote RXR-mediated transcription with EC(50) values similar to that of 1 and are selective RXR agonists. Our approach also uncovered a periodic trend of increased binding and homodimerization of RXR when substituting a halogen atom for a proton ortho to the carboxylic acid on 1.

The retinoid X receptors and their ligands

This chapter presents an overview of the current status of studies on the structural and molecular biology of the retinoid X receptor subtypes α, β, and γ (RXRs, NR2B1-3), their nuclear and cytoplasmic functions, post-transcriptional processing, and recently reported ligands. Points of interest are the different changes in the ligand-binding pocket induced by variously shaped agonists, the communication of the ligand-bound pocket with the coactivator binding surface and the heterodimerization interface, and recently identified ligands that are natural products, those that function as environmental toxins or drugs that had been originally designed to interact with other targets, as well as those that were deliberately designed as RXR-selective transcriptional agonists, synergists, or antagonists. Of these synthetic ligands, the general trend in design appears to be away from fully aromatic rigid structures to those containing partial elements of the flexible tetraene side chain of 9-cis-retinoic acid. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Modeling, synthesis and biological evaluation of potential retinoid X receptor (RXR) selective agonists: novel analogues of 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethynyl]benzoic acid (bexarotene)

This report describes the synthesis of analogues of 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethynyl]benzoic acid (1), commonly known as bexarotene, and their analysis in acting as retinoid X receptor (RXR)-specific agonists. Compound 1 has FDA approval to treat cutaneous T-cell lymphoma (CTCL); however, its use can cause side effects such as hypothyroidism and increased triglyceride concentrations, presumably by disruption of RXR heterodimerization with other nuclear receptors. The novel analogues in the present study have been evaluated for RXR activation in an RXR mammalian-2-hybrid assay as well as an RXRE-mediated transcriptional assay and for their ability to induce apoptosis as well as for their mutagenicity and cytotoxicity. Analysis of 11 novel compounds revealed the discovery of three analogues that best induce RXR-mediated transcriptional activity, stimulate apoptosis, have comparable K(i) and EC(50) values to 1, and are selective RXR agonists. Our experimental approach suggests that rational drug design can develop new rexinoids with improved biological properties.

3D-pharmacophore model for RXR(gamma) agonists

Three-dimensional pharmacophore models were generated for retinoid X receptor (RXR(gamma)) agonists using quantitative approach (CATALYST HypoRefine). One optimal pharmacophore model for selective RXR(gamma) agonists was determined through careful validation processes. The best quantitative model (Hypo-1) had five features and five excluded volumes: three hydrophobic aliphatic groups (HAL1, HAL2, and HAL3), one hydrophobic aromatic ring (HAR), and one hydrogen bond acceptor (HBA). The model was validated using a wide range of test molecules. It could predict agonist activity and identify highly potent molecules. The present results are valuable to discover and develop specific RXR(gamma) agonists with desired biological activities.