Multicenter phase II study of oral bexarotene for patients with metastatic breast cancer

IRX4204 Induces Senescence and Cell Death in HER2-positive Breast Cancer and Synergizes with Anti-HER2 Therapy

PURPOSE

Rexinoids, agonists of nuclear retinoid X receptor (RXR), have been used for the treatment of cancers and are well tolerated in both animals and humans. However, the usefulness of rexinoids in treatment of breast cancer remains unknown. This study examines the efficacy of IRX4204, a highly specific rexinoid, in breast cancer cell lines and preclinical models to identify a biomarker for response and potential mechanism of action.

EXPERIMENTAL DESIGN

IRX4204 effects on breast cancer cell growth and viability were determined using cell lines, syngeneic mouse models, and primary patient-derived xenograft (PDX) tumors. In vitro assays of cell cycle, apoptosis, senescence, and lipid metabolism were used to uncover a potential mechanism of action. Standard anti-HER2 therapies were screened in combination with IRX4204 on a panel of breast cancer cell lines to determine drug synergy.

RESULTS

IRX4204 significantly inhibits the growth of HER2-positive breast cancer cell lines, including trastuzumab and lapatinib-resistant JIMT-1 and HCC1954. Treatment with IRX4204 reduced tumor growth rate in the MMTV-ErbB2 mouse and HER2-positive PDX model by 49% and 44%, respectively. Mechanistic studies revealed IRX4204 modulates lipid metabolism and induces senescence of HER2-positive cells. In addition, IRX4204 demonstrates additivity and synergy with HER2-targeted mAbs, tyrosine kinase inhibitors, and antibody-drug conjugates.

CONCLUSIONS

These findings identify HER2 as a biomarker for IRX4204 treatment response and demonstrate a novel use of RXR agonists to synergize with current anti-HER2 therapies. Furthermore, our results suggest that RXR agonists can be useful for the treatment of anti-HER2 resistant and metastatic HER2-positive breast cancer.

Cancer drugs with high repositioning potential for Alzheimer's disease

INTRODUCTION

Despite the recent full FDA approval of lecanemab, there is currently no disease modifying therapy (DMT) that can efficiently slow down the progression of Alzheimer's disease (AD) in the general population. This statement emphasizes the need to identify novel DMTs in the shortest time possible to prevent a global epidemic of AD cases as the world population experiences an increase in lifespan.

AREAS COVERED

Here, we review several classes of anti-cancer drugs that have been or are being investigated in Phase II/III clinical trials for AD, including immunomodulatory drugs, RXR agonists, sex hormone therapies, tyrosine kinase inhibitors, and monoclonal antibodies.

EXPERT OPINION

Given the overall course of brain pathologies during the progression of AD, we express a great enthusiasm for the repositioning of anti-cancer drugs as possible AD DMTs. We anticipate an increasing number of combinatorial therapy strategies to tackle AD symptoms and their underlying pathologies. However, we strongly encourage improvements in clinical trial study designs to better assess target engagement and possible efficacy over sufficient periods of drug exposure.

Retinoid X Receptor agonists as selective modulators of the immune system for the treatment of cancer

Upon heterodimerizing with other nuclear receptors, retinoid X receptors (RXR) act as ligand-dependent transcription factors, regulating transcription of critical signaling pathways that impact numerous hallmarks of cancer. By controlling both inflammation and immune responses, ligands that activate RXR can modulate the tumor microenvironment. Several small molecule agonists of these essential receptors have been synthesized. Historically, RXR agonists were tested for inhibition of growth in cancer cells, but more recent drug discovery programs screen new molecules for inhibition of inflammation or activation of immune cells. Bexarotene is the first successful example of an effective therapeutic that molecularly targets RXR; this drug was approved to treat cutaneous T cell lymphoma and is still used as a standard of care treatment for this disease. No additional RXR agonists have yet achieved FDA approval, but several promising novel compounds are being developed. In this review, we provide an overview of the multiple mechanisms by which RXR signaling regulates inflammation and tumor immunity. We also discuss the potential of RXR-dependent immune cell modulation for the treatment or prevention of cancer and concomitant challenges and opportunities.

The novel function of bexarotene for neurological diseases

Bexarotene, a retinoid X receptor (RXR) agonist, is approved by FDA to treat cutaneous T-cell lymphoma. However, it has also demonstrated promising therapeutic potential for neurological diseases such as stroke, traumatic brain injury, Parkinson's disease, and particularly Alzheimer's disease(AD). In AD, bexarotene inhibits the production and aggregation of amyloid β (Aβ), activates Liver X Receptor/RXR heterodimers to increase lipidated apolipoprotein E to remove Aβ, mitigates the negative impact of Aβ, regulates neuroinflammation, and ultimately improves cognitive function. For other neurological diseases, its mechanisms of action include inhibiting inflammatory responses, up-regulating microglial phagocytosis, and reducing misfolded protein aggregation, all of which aid in alleviating neurological damage. Here, we briefly discuss the characteristics, applications, and adverse effects of bexarotene, summarize its pharmacological mechanisms and therapeutic results in various neurological diseases, and elaborate on the problems encountered in preclinical research, with the aim of providing help for the further application of bexarotene in central nervous system diseases.

FOXA1 Reprogramming Dictates Retinoid X Receptor Response in ESR1-Mutant Breast Cancer

Estrogen receptor alpha (ER/ESR1) mutations occur in 30% to 40% of endocrine resistant ER-positive (ER+) breast cancer. Forkhead box A1 (FOXA1) is a key pioneer factor mediating ER-chromatin interactions and endocrine response in ER+ breast cancer, but its role in ESR1-mutant breast cancer remains unclear. Our previous FOXA1 chromatin immunoprecipitation sequencing (ChIP-seq) identified a large portion of redistributed binding sites in T47D genome-edited Y537S and D538G ESR1-mutant cells. Here, we further integrated FOXA1 genomic binding profile with the isogenic ER cistrome, accessible genome, and transcriptome data of T47D cell model. FOXA1 redistribution was significantly associated with transcriptomic alterations caused by ESR1 mutations. Furthermore, in ESR1-mutant cells, FOXA1-binding sites less frequently overlapped with ER, and differential gene expression was less associated with the canonical FOXA1-ER axis. Motif analysis revealed a unique enrichment of retinoid X receptor (RXR) motifs in FOXA1-binding sites of ESR1-mutant cells. Consistently, ESR1-mutant cells were more sensitive to growth stimulation with the RXR agonist LG268. The mutant-specific response was dependent on two RXR isoforms, RXR-α and RXR-β, with a stronger dependency on the latter. In addition, T3, the agonist of thyroid receptor (TR) also showed a similar growth-promoting effect in ESR1-mutant cells. Importantly, RXR antagonist HX531 blocked growth of ESR1-mutant cells and a patient-derived xenograft (PDX)-derived organoid with an ESR1 D538G mutation. Collectively, our data support the evidence for a stronger RXR response associated with FOXA1 reprograming in ESR1-mutant cells, suggesting development of therapeutic strategies targeting RXR pathways in breast tumors with ESR1 mutation.

IMPLICATIONS

It provides comprehensive characterization of the role of FOXA1 in ESR1-mutant breast cancer and potential therapeutic strategy through blocking RXR activation.

Modulating retinoid-X-receptor alpha (RXRA) expression sensitizes chronic myeloid leukemia cells to imatinib in vitro and reduces disease burden in vivo

Introduction: The ligand-activated transcription factors, nuclear hormone receptors (NHRs), remain unexplored in hematological malignancies except for retinoic acid receptor alpha (RARA). Methods: Here we profiled the expression of various NHRs and their coregulators in Chronic myeloid leukemia (CML) cell lines and identified a significant differential expression pattern between inherently imatinib mesylate (IM)-sensitive and resistant cell lines. Results: Retinoid-X-receptor alpha (RXRA) was downregulated in CML cell lines inherently resistant to IM and in primary CML CD34⁺ cells. Pre-treatment with clinically relevant RXRA ligands improved sensitivity to IM in-vitro in both CML cell lines and primary CML cells. This combination effectively reduced the viability and colony-forming capacity of CML CD34⁺ cells in-vitro. In-vivo, this combination reduced leukemic burden and prolonged survival. Overexpression (OE) of RXRA inhibited proliferation and improved sensitivity to IM in-vitro. In-vivo, RXRA OE cells showed reduced engraftment of cells in the bone marrow, improved sensitivity to IM, and prolonged survival. Both RXRA OE and ligand treatment markedly reduced BCR::ABL1 downstream kinase activation, activating apoptotic cascades and improving sensitivity to IM. Importantly, RXRA OE also led to the disruption of the oxidative capacity of these cells. Conclusion: Combining IM with clinically available RXRA ligands could form an alternative treatment strategy in CML patients with suboptimal response to IM.

The Novel RXR Agonist MSU-42011 Differentially Regulates Gene Expression in Mammary Tumors of MMTV-Neu Mice

Retinoid X receptor (RXR) agonists, which activate the RXR nuclear receptor, are effective in multiple preclinical cancer models for both treatment and prevention. While RXR is the direct target of these compounds, the downstream changes in gene expression differ between compounds. RNA sequencing was used to elucidate the effects of the novel RXRα agonist MSU-42011 on the transcriptome in mammary tumors of HER2+ mouse mammary tumor virus (MMTV)-Neu mice. For comparison, mammary tumors treated with the FDA approved RXR agonist bexarotene were also analyzed. Each treatment differentially regulated cancer-relevant gene categories, including focal adhesion, extracellular matrix, and immune pathways. The most prominent genes altered by RXR agonists positively correlate with survival in breast cancer patients. While MSU-42011 and bexarotene act on many common pathways, these experiments highlight the differences in gene expression between these two RXR agonists. MSU-42011 targets immune regulatory and biosynthetic pathways, while bexarotene acts on several proteoglycan and matrix metalloproteinase pathways. Exploration of these differential effects on gene transcription may lead to an increased understanding of the complex biology behind RXR agonists and how the activities of this diverse class of compounds can be utilized to treat cancer.

Chronicle of a discovery: the retinoic acid receptor

The landmark 1987 discovery of the retinoic acid receptor (RAR) came as a surprise, uncovering a genomic kinship between the fields of vitamin A biology and steroid receptors. This stunning breakthrough triggered a cascade of studies to deconstruct the roles played by the RAR and its natural and synthetic ligands in embryonic development, skin, growth, physiology, vision, and disease as well as providing a template to elucidate the molecular mechanisms by which nuclear receptors regulate gene expression. In this review, written from historic and personal perspectives, we highlight the milestones that led to the discovery of the RAR and the subsequent studies that enriched our knowledge of the molecular mechanisms by which a low-abundant dietary compound could be so essential to the generation and maintenance of life itself.

Bexarotene-induced cell death in ovarian cancer cells through Caspase-4-gasdermin E mediated pyroptosis

Bexarotene selectively activates retinoid X receptor, which is a commonly used anticancer agent for cutaneous T-cell lymphoma. In this study, we aimed to investigate the anticancer effect of bexarotene and its underlying mechanism in ovarian cancer in vitro. The ES2 and NIH:OVACAR3 ovarian cancer cell lines were treated with 0, 5, 10, or 20 µM of bexarotene. After 24 h, cell number measurement and lactate dehydrogenase (LDH) cytotoxicity assay were performed. The effect of bexarotene on CDKN1A expression, cell cycle-related protein, cell cycle, pyroptosis, and apoptosis was evaluated. Bexarotene reduced cell proliferation in all concentrations in both the cells. At concentrations of > 10 µM, extracellular LDH activity increased with cell rupture. Treatment using 10 µM of bexarotene increased CDKN1A mRNA levels, decreased cell cycle-related protein expression, and increased the sub-G1 cell population in both cells. In ES2 cells, caspase-4 and GSDME were activated, whereas caspase-3 was not, indicating that bexarotene-induced cell death might be pyroptosis. A clinical setting concentration of bexarotene induced cell death through caspase-4-mediated pyroptosis in ovarian cancer cell lines. Thus, bexarotene may serve as a novel therapeutic agent for ovarian cancer.

The rexinoid V-125 reduces tumor growth in preclinical models of breast and lung cancer

Rexinoids are ligands which activate retinoid X receptors (RXRs), regulating transcription of genes involved in cancer-relevant processes. Rexinoids have anti-neoplastic activity in multiple preclinical studies. Bexarotene, used to treat cutaneous T cell lymphoma, is the only FDA-approved rexinoid. Bexarotene has also been evaluated in clinical trials for lung and metastatic breast cancer, wherein subsets of patients responded despite advanced disease. By modifying structures of known rexinoids, we can improve potency and toxicity. We previously screened a series of novel rexinoids and selected V-125 as the lead based on performance in optimized in vitro assays. To validate our screening paradigm, we tested V-125 in clinically relevant mouse models of breast and lung cancer. V-125 significantly (p < 0.001) increased time to tumor development in the MMTV-Neu breast cancer model. Treatment of established mammary tumors with V-125 significantly (p < 0.05) increased overall survival. In the A/J lung cancer model, V-125 significantly (p < 0.01) decreased number, size, and burden of lung tumors. Although bexarotene elevated triglycerides and cholesterol in these models, V-125 demonstrated an improved safety profile. These studies provide evidence that our screening paradigm predicts novel rexinoid efficacy and suggest that V-125 could be developed into a new cancer therapeutic.

Translational Advances in Cancer Prevention Agent Development (TACPAD) Virtual Workshop on Immunomodulatory Agents: Report

The National Cancer Institute (NCI) Division of Cancer Prevention (DCP) convened the "Translational Advances in Cancer Prevention Agent Development (TACPAD) Workshop on Immunomodulatory Agents" as a virtual 2-day workshop on September 13 to 14, 2021. The main goals of this workshop were to foster the exchange of ideas and potentially new collaborative interactions among leading cancer immunoprevention researchers from basic and clinical research and highlight new and emerging trends in immunoprevention. The workshop included an overview of the mechanistic classes of immunomodulatory agents and three sessions covering the gamut from preclinical to clinical studies. The workshop convened individuals working in immunology and cancer prevention to discuss trends in discovery and development of immunomodulatory agents individually and in combination with other chemopreventive agents or vaccines.

Insights into the SARS-CoV-2-Mediated Alteration in the Stress Granule Protein Regulatory Networks in Humans

The rapidly and constantly evolving coronavirus, SARS-CoV-2, imposes a great threat to human health causing severe lung disease and significant mortality. Cytoplasmic stress granules (SGs) exert anti-viral activities due to their involvement in translation inhibition and innate immune signaling. SARS-CoV-2 sequesters important SG nucleator proteins and impairs SG formation, thus evading the host response for efficient viral replication. However, the significance of SGs in COVID-19 infection remains elusive. In this study, we utilize a protein-protein interaction network approach to systematically dissect the crosstalk of human post-translational regulatory networks governed by SG proteins due to SARS-CoV-2 infection. We uncovered that 116 human SG proteins directly interact with SARS-CoV-2 proteins and are involved in 430 different brain disorders including COVID-19. Further, we performed gene set enrichment analysis to identify the drugs against three important key SG proteins (DYNC1H1, DCTN1, and LMNA) and also looked for potential microRNAs (miRNAs) targeting these proteins. We identified bexarotene as a potential drug molecule and miRNAs, hsa-miR-615-3p, hsa-miR-221-3p, and hsa-miR-124-3p as potential candidates for the treatment of COVID-19 and associated manifestations.

Fatty acid oxidation is a druggable gateway regulating cellular plasticity for driving metastasis in breast cancer

Cell state transitions control the functional behavior of cancer cells. Epithelial-to-mesenchymal transition (EMT) confers cancer stem cell-like properties, enhanced tumorigenicity and drug resistance to tumor cells, while mesenchymal-epithelial transition (MET) reverses these phenotypes. Using high-throughput chemical library screens, retinoids are found to be potent promoters of MET that inhibit tumorigenicity in basal-like breast cancer. Cell state transitions are defined by reprogramming of lipid metabolism. Retinoids bind cognate nuclear receptors, which target lipid metabolism genes, thereby redirecting fatty acids for β-oxidation in the mesenchymal cell state towards lipid storage in the epithelial cell state. Disruptions of key metabolic enzymes mediating this flux inhibit MET. Conversely, perturbations to fatty acid oxidation (FAO) rechannel fatty acid flux and promote a more epithelial cell phenotype, blocking EMT-driven breast cancer metastasis in animal models. FAO impinges on the epigenetic control of EMT through acetyl-CoA-dependent regulation of histone acetylation on EMT genes, thus determining cell states.

The RXR Agonist MSU42011 Is Effective for the Treatment of Preclinical HER2+ Breast Cancer and Kras-Driven Lung Cancer

(1) Background: Notwithstanding numerous therapeutic advances, 176,000 deaths from breast and lung cancers will occur in the United States in 2021 alone. The tumor microenvironment and its modulation by drugs have gained increasing attention and relevance, especially with the introduction of immunotherapy as a standard of care in clinical practice. Retinoid X receptors (RXRs) are members of the nuclear receptor superfamily and upon ligand binding, function as transcription factors to modulate multiple cell functions. Bexarotene, the only FDA-approved RXR agonist, is still used to treat cutaneous T-cell lymphoma. (2) Methods: To test the immunomodulatory and anti-tumor effects of MSU42011, a new RXR agonist, we used two different immunocompetent murine models (MMTV-Neu mice, a HER2 positive model of breast cancer and the A/J mouse model, in which vinyl carbamate is used to initiate lung tumorigenesis) and an immunodeficient xenograft lung cancer model. (3) Results: Treatment of established tumors in immunocompetent models of HER2-positive breast cancer and Kras-driven lung cancer with MSU42011 significantly decreased the tumor burden and increased the ratio of CD8/CD4, CD25 T cells, which correlates with enhanced anti-tumor efficacy. Moreover, the combination of MSU42011 and immunotherapy (anti-PDL1 and anti-PD1 antibodies) significantly (p < 0.05) reduced tumor size vs. individual treatments. However, MSU42011 was ineffective in an athymic human A549 lung cancer xenograft model, supporting an immunomodulatory mechanism of action. (4) Conclusions: Collectively, these data suggest that the RXR agonist MSU42011 can be used to modulate the tumor microenvironment in breast and lung cancer.

Inhibitory activity of FDA-approved drugs cetilistat, abiraterone, diiodohydroxyquinoline, bexarotene, remdesivir, and hydroxychloroquine on COVID-19 main protease and human ACE2 receptor: A comparative in silico approach

By September 1, 2021, SARS-CoV-2, a respiratory virus that prompted Coronavirus Disease in 2019, had infected approximately 218,567,442 patients and claimed 4,534,151 lives. There are currently no specific treatments available for this lethal virus, although several drugs, including remdesivir and hydroxychloroquine, have been tested. The purpose of this study is to assess the activity of FDA-approved drugs cetilistat, abiraterone, diiodohydroxyquinoline, bexarotene, remdesivir, and hydroxychloroquine as potential SARS-CoV-2 main protease inhibitors. Additionally, this study aims to provide insight into the development of potential inhibitors that may inhibit ACE2, thereby preventing SARS-CoV-2 entry into the host cell and infection. To this end, remdesivir and hydroxychloroquine were used as comparator drugs. The calculations revealed that cetilistat, abiraterone, diiodohydroxyquinoline, and bexarotene inhibit main protease and ACE2 receptors more effectively than the well-known drug hydroxychloroquine when used against COVID-19. Meanwhile, bexarotene and cetilistat bind more tightly to the SARS-CoV-2 main protease and the ACE2 receptor, respectively, than remdesivir, a potential treatment for COVID-19 that is the first FDA-approved drug against this virus. As a result, the molecular dynamic simulations of these two drugs in the presence of proteins were investigated. The MD simulation results demonstrated that these drugs interact to stabilize the systems, allowing them to be used as effective inhibitors of these proteins. Meanwhile, bexarotene, abiraterone, cetilistat, and diiodohydroxyquinoline's systemic effects should be further investigated in suitable ex vivo human organ culture or organoids, animal models, or clinical trials.

Photodegradation of Bexarotene and Its Implication for Cytotoxicity

A detailed understanding of the stability of an active pharmaceutical ingredient and a pharmaceutical dosage form is essential for the drug-development process and for safe and effective use of medicines. Photostability testing as an inherent part of stability studies provides valuable knowledge on degradation pathways and structures of products generated under UV irradiation. Photostability is particularly important for topically administered drugs, as they are more exposed to UV radiation. Bexarotene is a more recent third-generation retinoid approved by the U.S. Food and Drug Administration and the European Medicines Agency as a topically applied anticancer agent. The present study aimed to assess bexarotene photostability, including the presence of UV filters, which have been permitted to be used in cosmetic products in Europe and the USA. The bexarotene photostability testing was performed in ethanol solutions and in formulations applied on PMMA plates. The UPLC-MS/MS technique was used to determine the tested substance. The presence of photocatalysts such as TiO₂ or ZnO, as well as the organic UV filters avobenzone, benzophenone-3, meradimate, and homosalate, could contribute to degradation of bexarotene under UV irradiation. Four photocatalytic degradation products of bexarotene were identified for the first time. The antiproliferative properties of the degradation products of bexarotene were assessed by MTT assay on a panel of human adherent cancer cells, and concentration-dependent growth inhibition was evidenced on all tested cell lines. The cytotoxicity of the formed products after 4 h of UV irradiation was significantly higher than that of the parent compound (p < 0.05). Furthermore non-cancerous murine fibroblasts exhibited marked concentration-dependent inhibition by bexarotene, while the degradation products elicited more pronounced antiproliferative action only at the highest applied concentration.

Cytoplasmic Localization of RXRα Determines Outcome in Breast Cancer

Simple Summary

Considering the immense development of today’s therapeutic approaches in oncology towards customized therapy, this study aimed to assess the prognostic value of nuclear versus cytoplasmic retinoid X receptor α (RXRα) expression in breast cancer. Our results demonstrate that RXRα expression may have different roles in tumorigenesis according to its subcellular localization. This study strengthens the need for further research on the behavior of RXRα, depending on its intracellular localization.

Abstract

The aim of this retrospective study was to assess the prognostic value of cytoplasmic versus nuclear RXRα expression in breast cancer (BC) tissue samples and to correlate the results with clinicopathological parameters. In 319 BC patients, the expression of RXRα was evaluated via immunohistochemistry. Prognosis-determining aspects were calculated through uni- and multivariate analyses. Correlation analysis revealed a trend association with nuclear RXRα expression regarding an improved overall survival (OS) (p = 0.078), whereas cytoplasmic RXRα expression was significantly correlated with a poor outcomes in terms of both OS (p = 0.038) and disease-free survival (DFS) (p = 0.037). Strengthening these results, cytoplasmic RXRα was found to be an independent marker for DFS (p = 0.023), when adjusted to clinicopathological parameters, whereas nuclear RXRα expression was positively associated with lower TNM-staging, i.e., pT (p = 0.01), pN (p = 0.029) and pM (p = 0.001). Additionally, cytoplasmic RXRα expression was positively associated with a higher histopathological tumor grading (p = 0.02). Cytoplasmic RXRα was also found to be a negative prognosticator for Her-2neu-negative and triple-negative patients. Altogether, these findings support the hypothesis that the subcellular localization of RXRα plays an important role in carcinogenesis and the prognosis of BC. The expression of cytoplasmic RXRα is correlated with a more aggressive course of the disease, whereas nuclear RXRα expression appears to be a protective factor. These data may help to identify high-risk BC subgroups in order to find possible specific options in targeted tumor therapy.

Vitamin A Update: Forms, Sources, Kinetics, Detection, Function, Deficiency, Therapeutic Use and Toxicity

Vitamin A is a group of vital micronutrients widely present in the human diet. Animal-based products are a rich source of the retinyl ester form of the vitamin, while vegetables and fruits contain carotenoids, most of which are provitamin A. Vitamin A plays a key role in the correct functioning of multiple physiological functions. The human organism can metabolize natural forms of vitamin A and provitamin A into biologically active forms (retinol, retinal, retinoic acid), which interact with multiple molecular targets, including nuclear receptors, opsin in the retina and, according to the latest research, also some enzymes. In this review, we aim to provide a complex view on the present knowledge about vitamin A ranging from its sources through its physiological functions to consequences of its deficiency and metabolic fate up to possible pharmacological administration and potential toxicity. Current analytical methods used for its detection in real samples are included as well.

Molecular mechanism of liver X receptors in cancer therapeutics

Liver X receptors (LXRs) are receptors that belong to the nuclear receptor superfamily (NRs). It was originally called the "orphan receptor" when it was firstly discovered. Then it was found to be activated by oxysterol and it was officially named LXRs. LXRs are activated by ligands and bind to the retinol X receptor to form a heterodimer and regulate metabolism. Numerous studies have shown that LXRs are involved in regulating immune function and maintaining immune tolerance. Activating LXRs can also inhibit the tumorigenesis and promote apoptosis of tumor cells, which make LXRs as potential targets in cancer treatment. This review will discuss the recent progress of LXRs from the structure and function of LXRs, the signaling pathway of LXRs, the molecular mechanism of LXRs activation in cancers, and the potential targets of LXRs in cancer therapy.

Potential therapeutic uses of rexinoids

The discovery of nuclear receptors, particularly retinoid X receptors (RXR), and their involvement in numerous pathways related to development sparked interest in their immunomodulatory properties. Genetic models using deletion or overexpression of RXR and the subsequent development of several small molecules that are agonists or antagonists of this receptor support a promising therapeutic role for these receptors in immunology. Bexarotene was approved in 1999 for the treatment of cutaneous T cell lymphoma. Several other small molecule RXR agonists have since been synthesized with limited preclinical development, but none have yet achieved FDA approval. Cancer treatment has recently been revolutionized with the introduction of immune checkpoint inhibitors, but their success has been restricted to a minority of patients. This review showcases the emerging immunomodulatory effects of RXR and the potential of small molecules that target this receptor as therapies for cancer and other diseases. Here we describe the essential roles that RXR and partner receptors play in T cells, dendritic cells, macrophages and epithelial cells, especially within the tumor microenvironment. Most of these effects are site and cancer type dependent but skew immune cells toward an anti-inflammatory and anti-tumor effect. This beneficial effect on immune cells supports the promise of combining rexinoids with approved checkpoint blockade therapies in order to enhance efficacy of the latter and to delay or potentially eliminate drug resistance. The data compiled in this review strongly suggest that targeting RXR nuclear receptors is a promising new avenue in immunomodulation for cancer and other chronic inflammatory diseases.

Amyloid binding and beyond: a new approach for Alzheimer's disease drug discovery targeting Aβo-PrPC binding and downstream pathways

Amyloid β oligomers (Aβo) are the main toxic species in Alzheimer's disease, which have been targeted for single drug treatment with very little success. In this work we report a new approach for identifying functional Aβo binding compounds. A tailored library of 971 fluorine containing compounds was selected by a computational method, developed to generate molecular diversity. These compounds were screened for Aβo binding by a combined 19F and STD NMR technique. Six hits were evaluated in three parallel biochemical and functional assays. Two compounds disrupted Aβo binding to its receptor PrPC in HEK293 cells. They reduced the pFyn levels triggered by Aβo treatment in neuroprogenitor cells derived from human induced pluripotent stem cells (hiPSC). Inhibitory effects on pTau production in cortical neurons derived from hiPSC were also observed. These drug-like compounds connect three of the pillars in Alzheimer's disease pathology, i.e. prion, Aβ and Tau, affecting three different pathways through specific binding to Aβo and are, indeed, promising candidates for further development.

Development and validation of bexarote by bioanalytical methods using liquid chromatography-tandem mass spectroscopy (LC-MS/MS)

Background

The aim of this study was to develop and validate accurate and precise UPLC method with tandem mass spectrometry (Waters) for the determination of bexarotene in human plasma using bexarotene D4 as internal standard (IS).

Results

The retention time of bexarotene was 2.75 ± 0.30 min. The method was validated with respect to system suitability, linearity, accuracy, precision, matrix effect, auto sampler carryover test, and recovery. Linearity was found to be 1.04 to 351.93 μg/mL. LOQQC, LQC, INTQC, MQC, and HQC were found to be 1.0550, 2.7800, 25.2700, 131.61, and 263.23 respectively. The mean percentage recovery was found to be 95.72%

Conclusion

The bioanalytical method, a selective and sensitive liquid chromatography-mass spectrometry method to quantitate bexarotene in K2EDTA human plasma over the concentration range 1.0440 to 351.9320 ng/mL, was successfully validated. This method is suitable for sample analysis to support bioequivalence/bioavailability and/or pharmacokinetic studies involving formulations of bexarotene.

The Prognostic Impact of Retinoid X Receptor and Thyroid Hormone Receptor alpha in Unifocal vs. Multifocal/Multicentric Breast Cancer

The aim of this study was to assess the prognostic value of the steroid hormone receptor expression, counting the retinoid X receptor (RXR) and thyroid hormone receptors (THRs), on the two different breast cancer (BC) entities: multifocal/multicentric versus unifocal. The overall and disease-free survival were considered as the prognosis determining aspects and analyzed by uni- and multi-variate analysis. Furthermore, histopathological grading and TNM staging (T = tumor size, N = lymph node involvement, M = distant metastasis) were examined in relation to RXR and THRs expression. A retrospective statistical analysis was carried out on survival-related events in a series of 319 sporadic BC patients treated at the Department of Gynecology and Obstetrics at the Ludwig-Maximillian's University in Munich between 2000 and 2002. The expression of RXR and THRs, including its two major isoforms THRα1 and THRα2, was analyzed by immunohistochemistry and showed to have a significant correlation for both BC entities in regard to survival analysis. Patients with multifocal/multicentric BC were exposed to a significantly worse disease-free survival (DFS) when expressing RXR. Patients with unifocal BC showed a significantly worse DFS when expressing THRα1. In contrast, a statistically significant positive association between THRα2 expression and enhanced DFS in multifocal/multicentric BC was shown. Especially the RXR expression in multifocal/multicentric BC was found to play a remarkably contradictory role for BC prognosis. The findings imply the need for a critical review of possible molecular therapies targeting steroid hormone receptors in BC treatment. Our results strengthen the need to further investigate the behavior of the nuclear receptor family, especially in relation to BC focality.

Convenient Retinoid X Receptor Binding Assay Based on Fluorescence Change of the Antagonist NEt-C343

Retinoid X receptor (RXR) modulators (rexinoids) are considered to have therapeutic potential for multiple diseases, such as Alzheimer's disease and Parkinson's disease. To overcome various disadvantages of prior screening methods, we previously developed an RXR binding assay using a fluorescent RXR ligand, CU-6PMN (4). However, this ligand binds not only at the ligand-binding domain (LBD) but also at the dimer-dimer interface of hRXRα. Here, we present a new fluorescent RXR antagonist 6-[N-ethyl-N-(5-isobutoxy-4-isopropyl-2-(11-oxo-2,3,6,7-tetrahydro-1H,5H,11H-pyrano[2,3-f]pyrido[3,2,1-ij]quinoline-10-carboxamido)phenyl)amino]nicotinic acid (NEt-C343, 7), which emits strong fluorescence only when bound to the RXR-LBD. It allows us to perform a rapid, simple, and nonhazardous binding assay that does not require bound/free separation and uses a standard plate reader. The obtained Ki values of known compounds were correlated with the Ki values obtained using the standard [³H]9cis-retinoic acid assay. This assay should be useful for drug discovery as well as for research on endocrine disruptors, functional foods, and natural products.

Discovery of the FDA-approved drugs bexarotene, cetilistat, diiodohydroxyquinoline, and abiraterone as potential COVID-19 treatments with a robust two-tier screening system

Coronavirus Disease 2019 (COVID-19) caused by the emerging severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is associated with a crude case fatality rate of about 0.5-10 % depending on locality. A few clinically approved drugs, such as remdesivir, chloroquine, hydroxychloroquine, nafamostat, camostat, and ivermectin, exhibited anti-SARS-CoV-2 activity in vitro and/or in a small number of patients. However, their clinical use may be limited by anti-SARS-CoV-2 50 % maximal effective concentrations (EC₅₀) that exceeded their achievable peak serum concentrations (Cmax), side effects, and/or availability. To find more immediately available COVID-19 antivirals, we established a two-tier drug screening system that combines SARS-CoV-2 enzyme-linked immunosorbent assay and cell viability assay, and applied it to screen a library consisting 1528 FDA-approved drugs. Cetilistat (anti-pancreatic lipase), diiodohydroxyquinoline (anti-parasitic), abiraterone acetate (synthetic androstane steroid), and bexarotene (antineoplastic retinoid) exhibited potent in vitro anti-SARS-CoV-2 activity (EC₅₀ 1.13-2.01 μM). Bexarotene demonstrated the highest Cmax:EC₅₀ ratio (1.69) which was higher than those of chloroquine, hydroxychloroquine, and ivermectin. These results demonstrated the efficacy of the two-tier screening system and identified potential COVID-19 treatments which can achieve effective levels if given by inhalation or systemically depending on their pharmacokinetics.

Retinoid X receptor agonist LG100268 modulates the immune microenvironment in preclinical breast cancer models

Despite numerous therapeutic advances in the past decade, breast cancer is expected to cause over 42,000 deaths in the United States in 2019. Breast cancer had been considered an immunologically silent tumor; however recent findings suggest that immune cells play important roles in tumor growth even in the breast. Retinoid X receptors (RXRs) are a subclass of nuclear receptors that act as ligand-dependent transcription factors that regulate a variety of cellular processes including proliferation and differentiation; in addition, they are essential for macrophage biology. Rexinoids are synthetic molecules that bind and activate RXRs. Bexarotene is the only rexinoid approved by the FDA for the treatment of refractory cutaneous T-cell lymphoma. Other more-potent rexinoids have been synthesized, such as LG100268 (LG268). Here, we report that treatment with LG 268, but not bexarotene, decreased infiltration of myeloid-derived suppressor cells and CD206-expressing macrophages, increased the expression of PD-L1 by 50%, and increased the ratio of CD8/CD4, CD25 T cells, which correlates with increased cytotoxic activity of CD8 T cells in tumors of MMTV-Neu mice (a model of HER2-positive breast cancer). In the MMTV-PyMT murine model of triple negative breast cancer, LG268 treatment of established tumors prolonged survival, and in combination with anti-PD-L1 antibodies, significantly (p = 0.05) increased the infiltration of cytotoxic CD8 T cells and apoptosis. Collectively, these data suggest that the use of LG268, a RXR agonist, can improve response to immune checkpoint blockade in HER2+ or triple-negative breast cancer.

The success and the challenge of all-trans retinoic acid in the treatment of cancer

All-trans retinoic acid (ATRA), an active metabolite of vitamin A, plays important roles in cell proliferation, cell differentiation, apoptosis, and embryonic development. The effects of ATRA are mediated by nuclear retinoid receptors as well as non-genomic signal pathway, such as MAPK and PKA. The great success of differentiation therapy with ATRA in acute promyelocytic leukemia (APL) not only improved the prognosis of APL but also spurred the studies of ATRA in the treatment of other tumors. Since the genetic and physiopathological simplicity of APL is not common in human malignancies, the combination of ATRA with other agents (chemotherapy, epigenetic modifiers, and arsenic trioxide, etc) had been extensively investigated in a variety of tumors. In this review, we will discuss in details about ATRA and its role in cancer treatment.

Natural products as modulators of the nuclear receptors and metabolic sensors LXR, FXR and RXR

Nuclear receptors (NRs) represent attractive targets for the treatment of metabolic syndrome-related diseases. In addition, natural products are an interesting pool of potential ligands since they have been refined under evolutionary pressure to interact with proteins or other biological targets. This review aims to briefly summarize current basic knowledge regarding the liver X (LXR) and farnesoid X receptors (FXR) that form permissive heterodimers with retinoid X receptors (RXR). Natural product-based ligands for these receptors are summarized and the potential of LXR, FXR and RXR as targets in precision medicine is discussed.

Molecular Signaling Mechanisms of Natural and Synthetic Retinoids for Inhibition of Pathogenesis in Alzheimer's Disease

Retinoids, which are vitamin A derivatives, interact through retinoic acid receptors (RARs) and retinoid X receptors (RXRs) and have profound effects on several physiological and pathological processes in the brain. The presence of retinoic acid signaling is extensively detected in the adult central nervous system, including the amygdala, cortex, hypothalamus, hippocampus, and other brain areas. Retinoids are primarily involved in neural patterning, differentiation, and axon outgrowth. Retinoids also play a key role in the preservation of the differentiated state of adult neurons. Impairment in retinoic acid signaling can result in neurodegeneration and progression of Alzheimer's disease (AD). Recent studies demonstrated severe deficiencies in spatial learning and memory in mice during retinoic acid (vitamin A) deprivation indicating its significance in preserving memory function. Defective cholinergic neurotransmission plays an important role in cognitive deficits in AD. All-trans retinoic acid is known to enhance the expression and activity of choline acetyltransferase in neuronal cell lines. Activation of RAR and RXR is also known to impede the pathogenesis of AD in mice by inhibiting accumulation of amyloids. In addition, retinoids have been shown to inhibit the expression of chemokines and pro-inflammatory cytokines in microglia and astrocytes, which are activated in AD. In this review article, we have described the chemistry and molecular signaling mechanisms of natural and synthetic retinoids and current understandings of their therapeutic potentials in prevention of AD pathology.

Dual targeting of retinoid X receptor and histone deacetylase with DW22 as a novel antitumor approach

Retinoid X receptor (RXR) and Histone deacetylase (HDAC) are considered important targets for cancer therapy due to their crucial roles in genetic or epigenetic regulations of cancer development and progression. Here, we evaluated the potential of dual targeting of RXR and HDAC using DW22 as a novel therapeutic approach to cancer treatment. We found that the co-expression of RXR-α and HDAC1 was frequently appeared in lung cancer and breast cancer tissues and cell lines. RXR was activated by DW22 in RXRα and HDAC1 overexpressed A549 and MDA-MB-435 cell lines. Meanwhile, DW22 inhibited the activity of HDAC by decreasing its expression in A549 and MDA-MB-435 cell lines, but not in RXRα and HDAC1 deficient cell lines. Moreover, DW22 suppressed cell growth, induced cell differentiation, prompted cell apoptosis and arrested cell cycle in A549, MDA-MB-435 or HL60 cell lines. Treatment human umbilical vascular endothelial cells (HUVECs) with DW22 suppressed migration, invasion and tube formation through decreasing VEGF expression. The up-regulation of Ac-H3 and p21, and down-regulation of VEGF caused by DW22 was markedly attenuated by silencing of HDAC1. Furthermore, knockdown of RXRα by siRNA completely blocked DW22-induced cell differentiation, but partially attenuated DW22-caused inhibition of cell proliferation, induction of cell apoptosis, and suppression of cell migration, invasion and tube formation. Moreover, intravenous administration of DW22 significantly retarded tumor growth of A549 and MDA-MB-435 xenograft mice models, and induced no substantial weight loss and gross toxicity. In addition, DW22 also reduced cell proliferation, angiogenesis, and induced cell apoptosis in vivo. Collectively, our data demonstrates that dual targeting of RXR and HDAC using DW22 possesses pleiotropic antitumor activities both in vitro and in vivo, providing a novel therapeutic approach for cancer treatment.

Targeting Retinoid Receptors to Treat Schizophrenia: Rationale and Progress to Date

This review provides the rationale and reports on the progress to date regarding the targeting of retinoid receptors for the treatment of schizophrenia and schizoaffective disorder and the role of retinoic acid in functions of the normal brain, and in psychotic states. After a brief introduction, we describe the normal function of retinoic acid in the brain. We then examine the evidence regarding retinoid dysregulation in schizophrenia. Finally, findings from two add-on clinical trials with a retinoid (bexarotene) are discussed. The authors of this review suggest that targeting retinoid receptors may be a novel approach to treat schizophrenia and schizoaffective disorder. Further studies are warranted.

Investigating the Inhibitory Effect of Wortmannin in the Hotspot Mutation at Codon 1047 of PIK3CA Kinase Domain: A Molecular Docking and Molecular Dynamics Approach

Oncogenic mutations in phosphatidylinositol-4,5-bisphosphate 3-kinase, catalytic subunit alpha (PIK3CA) are the most frequently reported in association with various forms of cancer. Several studies have reported the significance of hotspot mutations in a catalytic subunit of PIK3CA in association with breast cancer. Mutations are frequently observed in the highly conserved region of the kinase domain (797-1068 amino acids) of PIK3CA are activating or gain-of-function mutations. Mutation in codon 1047 occurs in the C-terminal region of the kinase domain with histidine (H) replaced by arginine (R), lysine (L), and tyrosine (Y). Pathogenicity and protein stability predictors PhD-SNP, Align GVGD, HANSA, iStable, and MUpro classified H1047R as highly deleterious when compared to H1047L and H1047Y. To explore the inhibitory activity of Wortmannin toward PIK3CA, the three-dimensional structure of the mutant protein was determined using homology modeling followed by molecular docking and molecular dynamics analysis. Docking studies were performed for the three mutants and native with Wortmannin to measure the differences in their binding pattern. Comparative docking study revealed that H1047R-Wortmannin complex has a higher number of hydrogen bonds as well as the best binding affinity next to the native protein. Furthermore, 100 ns molecular dynamics simulation was initiated with the docked complexes to understand the various changes induced by the mutation. Though Wortmannin was found to nullify the effect of H1047R over the protein, further studies are required for designing a better compound. As SNPs are major genetic variations observed in disease condition, personalized medicine would provide enhanced drug therapy.

Retinoids and rexinoids in cancer prevention: from laboratory to clinic

Early in the age of modern medicine the consequences of vitamin A deficiency drew attention to the fundamental link between retinoid-dependent homeostatic regulation and malignant hyperproliferative diseases. The term "retinoid" includes a handful of endogenous and a large group of synthetic derivatives of vitamin A. These multifunctional lipid-soluble compounds directly regulate target genes of specific biological functions and critical signaling pathways to orchestrate complex functions from vision to development, metabolism, and inflammation. Many of the retinoid activities on the cellular level have been well characterized and translated to the regulation of processes like differentiation and cell death, which play critical roles in the outcome of malignant transformation of tissues. In fact, retinoid-based differentiation therapy of acute promyelocytic leukemia was one of the first successful examples of molecularly targeted treatment strategies. The selectivity, high receptor binding affinity and the ability of retinoids to directly modulate gene expression programs present a distinct pharmacological opportunity for cancer treatment and prevention. However, to fully exploit their potential, the adverse effects of retinoids must be averted. In this review we provide an overview of the biology of retinoid (activated by nuclear retinoic acid receptors [RARs]) and rexinoid (engaged by nuclear retinoid X receptors [RXRs]) action concluded from a long line of preclinical studies, in relation to normal and transformed states of cells. We will also discuss the past and current uses of retinoids in the treatment of malignancies, the potential of rexinoids in the cancer prevention setting, both as single agents and in combinations.

Targeting liver X receptors in cancer therapeutics

Members of the nuclear receptor superfamily of ligand-dependent transcription factors carry out vital cellular functions and are highly druggable therapeutic targets. Liver X receptors (LXRs) are nuclear receptor family members that function in cholesterol transport, glucose metabolism and the modulation of inflammatory responses. There is now accumulating evidence to support the involvement of LXRs in a variety of malignancies and the potential efficacy of their ligands in these diseases. This Review summarizes the discovery and characterization of LXRs and their ligands, their effects and mechanisms in preclinical cancer models, and the future directions of basic and translational LXR research in cancer therapeutics.

Analysis of differential secondary effects of novel rexinoids: select rexinoid X receptor ligands demonstrate differentiated side effect profiles

In order to determine the feasibility of utilizing novel rexinoids for chemotherapeutics and as potential treatments for neurological conditions, we undertook an assessment of the side effect profile of select rexinoid X receptor (RXR) analogs that we reported previously. We assessed pharmacokinetic profiles, lipid and thyroid-stimulating hormone (TSH) levels in rats, and cell culture activity of rexinoids in sterol regulatory element-binding protein (SREBP) induction and thyroid hormone inhibition assays. We also performed RNA sequencing of the brain tissues of rats that had been dosed with the compounds. We show here for the first time that potent rexinoid activity can be uncoupled from drastic lipid changes and thyroid axis variations, and we propose that rexinoids can be developed with improved side effect profiles than the parent compound, bexarotene (1).

Retinoic acid receptors: from molecular mechanisms to cancer therapy

Retinoic acid (RA), the major bioactive metabolite of retinol or vitamin A, induces a spectrum of pleiotropic effects in cell growth and differentiation that are relevant for embryonic development and adult physiology. The RA activity is mediated primarily by members of the retinoic acid receptor (RAR) subfamily, namely RARα, RARβ and RARγ, which belong to the nuclear receptor (NR) superfamily of transcription factors. RARs form heterodimers with members of the retinoid X receptor (RXR) subfamily and act as ligand-regulated transcription factors through binding specific RA response elements (RAREs) located in target genes promoters. RARs also have non-genomic effects and activate kinase signaling pathways, which fine-tune the transcription of the RA target genes. The disruption of RA signaling pathways is thought to underlie the etiology of a number of hematological and non-hematological malignancies, including leukemias, skin cancer, head/neck cancer, lung cancer, breast cancer, ovarian cancer, prostate cancer, renal cell carcinoma, pancreatic cancer, liver cancer, glioblastoma and neuroblastoma. Of note, RA and its derivatives (retinoids) are employed as potential chemotherapeutic or chemopreventive agents because of their differentiation, anti-proliferative, pro-apoptotic, and anti-oxidant effects. In humans, retinoids reverse premalignant epithelial lesions, induce the differentiation of myeloid normal and leukemic cells, and prevent lung, liver, and breast cancer. Here, we provide an overview of the biochemical and molecular mechanisms that regulate the RA and retinoid signaling pathways. Moreover, mechanisms through which deregulation of RA signaling pathways ultimately impact on cancer are examined. Finally, the therapeutic effects of retinoids are reported.

Advances in Preventive Therapy for Estrogen-Receptor-Negative Breast Cancer

Preventing breast cancer is an effective strategy for reducing breast cancer deaths. The purpose of chemoprevention (also termed preventive therapy) is to reduce cancer incidence by use of natural, synthetic, or biological agents. The efficacy of tamoxifen, raloxifene, and exemestane as preventive therapy against estrogen-receptor (ER)-positive breast cancer is well established for women at increased risk for breast cancer. However, because breast cancer is a heterogeneous disease, distinct preventive approaches may be required for effective prevention of each subtype. Current research is, therefore, focused on identifying alternative mechanisms by which biologically active compounds can reduce the risk of all breast cancer subtypes including ER-negative breast cancer. Promising agents are currently being developed for prevention of HER2-positive and triple-negative breast cancer (TNBC) and include inhibitors of the ErbB family receptors, COX-2 inhibitors, metformin, retinoids, statins, poly(ADP-ribose) polymerase inhibitors, and natural compounds. This review focuses on recent progress in research to develop more effective preventive agents, in particular for prevention of ER-negative breast cancer.

Retinoids and breast cancer: from basic studies to the clinic and back again

All-trans retinoic acid (ATRA) is the most important active metabolite of vitamin A controlling segmentation in the developing organism and the homeostasis of various tissues in the adult. ATRA as well as natural and synthetic derivatives, collectively known as retinoids, are also promising agents in the treatment and chemoprevention of different types of neoplasia including breast cancer. The major aim of the present article is to review the basic knowledge acquired on the anti-tumor activity of classic retinoids, like ATRA, in mammary tumors, focusing on the underlying cellular and molecular mechanisms and the determinants of retinoid sensitivity/resistance. In the first part, an analysis of the large number of pre-clinical studies available is provided, stressing the point that this has resulted in a limited number of clinical trials. This is followed by an overview of the knowledge acquired on the role played by the retinoid nuclear receptors in the anti-tumor responses triggered by retinoids. The body of the article emphasizes the potential of ATRA and derivatives in modulating and in being influenced by some of the most relevant cellular pathways involved in the growth and progression of breast cancer. We review the studies centering on the cross-talk between retinoids and some of the growth-factor pathways which control the homeostasis of the mammary tumor cell. In addition, we consider the cross-talk with relevant intra-cellular second messenger pathways. The information provided lays the foundation for the development of rational and retinoid-based therapeutic strategies to be used for the management of breast cancer.

Discovery of a small molecular compound simultaneously targeting RXR and HADC: design, synthesis, molecular docking and bioassay

Retinoid X receptor (RXR) and Histone deacetylase (HDAC) are considered important targets for anti-cancer therapy due to their crucial roles in genetic or epigenetic regulations of cancer development and progression. Here, we have designed and synthesized a novel compound which targets both RXR and HADC. This dual-targeting agent is derived from bexarotene and suberoylanilide hydroxamic acid (SAHA), prototypical RXR agonist and HDAC inhibitor, respectively. Molecular docking studies demonstrate that this agent has a relatively strong affinity to RXR and HADC. Importantly, it presents the potentials of activation of RXR and inhibition of HDAC in both cell-free and whole-cell assays, and displays anti-proliferative effect on representative cancer cell lines and drug-resistant cancer cell lines.

Design, synthesis, and biological evaluation of indenoisoquinoline rexinoids with chemopreventive potential

Nuclear receptors, such as the retinoid X receptor (RXR), are proteins that regulate a myriad of cellular processes. Molecules that function as RXR agonists are of special interest for the prevention and control of carcinogenesis. The majority of these ligands possess an acidic moiety that is believed to be key for RXR activation. This communication presents the design, synthesis, and biological evaluation of both acidic and nonacidic indenoisoquinolines as new RXR ligands. In addition, a comprehensive structure-activity relationship study is presented that identifies the important features of the indenoisoquinoline rexinoids. The ease of modification of the indenoisoquinoline core and the lack of the necessity of a carboxyl group for activity make them an attractive and unusual family of RXR agonists. This work establishes a structural foundation for the design of new and novel rexinoid cancer chemopreventive agents.

Bexarotene induces cellular senescence in MMTV-Neu mouse model of mammary carcinogenesis

Previous studies have shown that retinoids and rexinoids can prevent breast cancer in animal models and in women with increased risk of developing the disease. The cellular effects of these vitamin A analogues have been primarily associated with induction of differentiation and inhibition of proliferation. In this study, we tested the hypothesis that bexarotene (LGD1069, Targretin), a rexinoid, can not only inhibit cell proliferation but also induce cellular senescence in mammary epithelial cells, premalignant lesions, and tumors of the MMTV-Neu model of mammary carcinogenesis, which develops estrogen receptor-negative tumors. Mice with palpable mammary tumors were treated for 4 weeks with bexarotene at 80 or 40 mg/kg body weight, and senescent cells were determined by SA-β-Gal assay. Bexarotene decreased in a dose-dependent manner the multiplicity of premalignant lesions and tumors, and this was associated with inhibition of cell proliferation and induction of cellular senescence and apoptosis. By double labeling of senescent cells, first by SA-β-Gal and then by antibodies against genes related to cellular senescence, we found that p21, p16, and RARβ, but not p53, were upregulated by bexarotene in mammary tumors and in breast cancer cell lines, suggesting involvement of multiple signaling pathways in mediating the senescence program of rexinoids. These findings indicate that, in addition to cell proliferation and apoptosis, cellular senescence could be used as a potential biomarker of response in breast cancer prevention and therapy studies with rexinoids and possibly with other antitumor agents.

Synthesis and SAR study of modulators inhibiting tRXRα-dependent AKT activation

RXRα represents an intriguing and unique target for pharmacologic interventions. We recently showed that Sulindac and a designed analog could bind to RXRα and modulate its biological activity, including inhibition of the interaction of an N-terminally truncated RXRα (tRXRα) with the p85α regulatory subunit of phosphatidylinositol-3-OH kinase (PI3K). Here we report the synthesis, testing and SAR of a series of novel analogs of Sulindac as potential modulators for inhibiting tRXRα-dependent AKT activation. A new compound 30 was identified to have improved biological activity.

Molecular pathways: current role and future directions of the retinoic acid pathway in cancer prevention and treatment

Retinoids and their naturally metabolized and synthetic products (e.g., all-trans retinoic acid, 13-cis retinoic acid, bexarotene) induce differentiation in various cell types. Retinoids exert their actions mainly through binding to the nuclear retinoic acid receptors (α, β, γ), which are transcriptional and homeostatic regulators with functions that are often compromised early in neoplastic transformation. The retinoids have been investigated extensively for their use in cancer prevention and treatment. Success has been achieved with their use in the treatment of subtypes of leukemia harboring chromosomal translocations. Promising results have been observed in the breast cancer prevention setting, where fenretinide prevention trials have provided a strong rationale for further investigation in young women at high risk for breast cancer. Ongoing phase III randomized trials investigating retinoids in combination with chemotherapy in non-small cell lung cancer aim to definitively characterize the role of retinoids in this tumor type. The limited treatment success observed to date in the prevention and treatment of solid tumors may relate to the frequent epigenetic silencing of RARβ. Robust evaluation of RARβ and downstream genes may permit optimized use of retinoids in the solid tumor arena.

Phase I trial of exemestane in combination with metformin and rosiglitazone in nondiabetic obese postmenopausal women with hormone receptor-positive metastatic breast cancer

PURPOSE

Obese women with breast cancer have worse prognosis than women with normal body mass index. Endocrine therapy resistance is in part mediated by insulin resistance in obese women with breast cancer. We investigated the tolerability and pharmacokinetics of exemestane in combination with metformin and rosiglitazone in nondiabetic overweight and obese postmenopausal women with hormone receptor-positive metastatic breast cancer.

METHODS

Patients had previously received chemotherapy and endocrine therapy for breast cancer. Exemestane was given as 25 mg orally per day. Metformin (M) and rosiglitazone (R) were given twice daily. Dose level 1 consisted of M 1,500 mg/day and R 6 mg/day. Dose level 2 consisted of M 2,000 mg/day and R 8 mg/day. Plasma concentrations of exemestane were measured on days 1, 8, and 15.

RESULTS

Twenty patients were enrolled. Fourteen patients received exemestane, metformin, and rosiglitazone. Six patients received exemestane with metformin only (2,000 mg/day). Both regimens were well tolerated at the highest doses tested, and there were no notable changes in plasma exemestane levels. Six patients (30%) had stable disease for 6 months or longer.

CONCLUSIONS

Oral daily administration of exemestane (25 mg) and metformin (2,000 mg) with and without rosiglitazone (8 mg) daily was well tolerated. Exemestane pharmacokinetics were not altered by metformin and rosiglitazone.

Advances in drug design with RXR modulators

INTRODUCTION

Retinoid X receptors (subtypes RXRα or NR2B1, RXRβ or NR2B2 and RXRγ or NR2B3, which originate from three distinct genes) are promiscuous partners with heterodimeric associations to other members of the Nuclear Receptor (NR) superfamily. Some of the heterodimers are "permissive" and transcriptionally active in the presence of either an RXR ligand ("rexinoid") or a NR partner ligand, whereas others are "non-permissive" and unresponsive to rexinoids alone. In rodent models, rexinoids and partner agonists (mainly PPARγ, LXR, FXR) produce beneficial effects on insulin sensitization, diabetes and obesity, but secondary effects have also been noted, such as a raise in tryglyceride levels, supression of the thyroid hormone axis and induction of hepatomegaly.

AREAS COVERED

The authors review recent advances in rexinoid design, including further optimization of known scaffolds, and the discovery of novel RXR modulators by virtual ligand screening or from bioactive natural products. The understanding of rexinoid functions in permissive and non-permissive heterodimers is firmly based on structural knowledge. By strenghtening or disrupting the interaction surface with coregulators rexinoids exert agonist or (partial) antagonist activities. The activity state of the heterodimer can also be fine-tuned by the cellular context and the nature of coregulators.

EXPERT OPINION

The synthetic chemistry toolbox has provided a panel of agonists, partial (ant)agonists and/or heterodimer-selective rexinoids starting from existing, naturally occurring or serendipitously discovered scaffolds. These compounds have an unexplored therapeutic potential that might overcome some of the current limitations of rexinoids in therapy, such as hypertriglyceridemia.

The use of retinoids in ovarian cancer: a review of the literature

Ovarian cancer is the deadliest of all gynecologic malignancies. The search for novel treatment modalities to augment traditional chemotherapy and improve quality of life is ongoing. Retinoids, a class of compounds composed of vitamin A, its natural derivatives, and synthetic analogs, have been studied extensively in both the prevention and treatment of gynecologic malignancies. In this article, we reviewed preclinical studies and clinical trials conducted using retinoids in ovarian cancer.

Identification, synthesis, and biological evaluation of the metabolites of 3-amino-6-(3'-aminopropyl)-5H-indeno[1,2-c]isoquinoline-5,11-(6H)dione (AM6-36), a promising rexinoid lead compound for the development of cancer chemotherapeutic and chemopreventive agents

Activation of the retinoid X receptor (RXR), which is involved in cell proliferation, differentiation, and apoptosis, is a strategy for cancer chemotherapy and chemoprevention, and 3-amino-6-(3'-aminopropyl)-5H-indeno[1,2-c]isoquinoline-5,11-(6H)dione (AM6-36) (3) is among the few RXR ligands known. The presently reported studies of 3 include its binding to human plasma proteins, metabolic stability using human liver microsomes, metabolism by human liver microsomes and hepatocytes, and in vivo disposition in rat serum, liver, and mammary tissue. Compound 3 was 75% bound to human plasma proteins, and its metabolic stability was much greater than propranolol. One phase I metabolite was formed by human liver microsomes, seven phase I and II metabolites were formed by human hepatocytes, and five metabolites were detected in rat serum and liver after oral administration. The putative metabolites predicted using LC-MS-MS were synthesized to confirm their structures and to provide sufficient material for investigation of induction of RXRE transcriptional activity and inhibition of NFκB.