Initial clinical trial of a selective retinoid X receptor ligand, LGD1069

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.

Drug screening identifies tazarotene and bexarotene as therapeutic agents in multiple sulfatase deficiency

Multiple sulfatase deficiency (MSD, MIM #272200) results from pathogenic variants in the SUMF1 gene that impair proper function of the formylglycine-generating enzyme (FGE). FGE is essential for the posttranslational activation of cellular sulfatases. MSD patients display reduced or absent sulfatase activities and, as a result, clinical signs of single sulfatase disorders in a unique combination. Up to date therapeutic options for MSD are limited and mostly palliative. We performed a screen of FDA-approved drugs using immortalized MSD patient fibroblasts. Recovery of arylsulfatase A activity served as the primary readout. Subsequent analysis confirmed that treatment of primary MSD fibroblasts with tazarotene and bexarotene, two retinoids, led to a correction of MSD pathophysiology. Upon treatment, sulfatase activities increased in a dose- and time-dependent manner, reduced glycosaminoglycan content decreased and lysosomal position and size normalized. Treatment of MSD patient derived induced pluripotent stem cells (iPSC) differentiated into neuronal progenitor cells (NPC) resulted in a positive treatment response. Tazarotene and bexarotene act to ultimately increase the stability of FGE variants. The results lay the basis for future research on the development of a first therapeutic option for MSD patients.

Targeting the Retinoid X Receptor Pathway Prevents and Ameliorates Murine Chronic Graft-Versus-Host Disease

Most allogeneic hematopoietic stem cell transplant (allo-HSCT) recipients receive peripheral blood stem cell grafts resulting in a 30%-70% incidence of chronic graft-versus-host disease (cGVHD), a major cause of mortality and morbidity in long-term survivors. While systemic steroids remain the standard of care for first-line therapy, patients may require long-term administration, and those with steroid-resistant or refractory cGVHD have a worse prognosis. Although durable and deep responses with second-line therapies can be achieved in some patients, there remains an urgent need for new therapies. In this study, we evaluated the efficacy of IRX4204, a novel agonist that activates RXRs and is in clinical trials for cancer treatment to prevent and treat cGVHD in two complementary murine models. In a major histocompatibility complex mismatched, non-sclerodermatous multiorgan system model with bronchiolitis obliterans, IRX4204 prevented and reversed cGVHD including associated pulmonary dysfunction with restoration of germinal center T-follicular helper: T-follicular regulatory cell balance. In a minor histocompatibility antigen disparate sclerodermatous model, IRX4204 treatment significantly prevented and ameliorated skin cGVHD by reducing Th1 and Th17 differentiation due to anti-inflammatory properties. Together, these results indicate that IRX4204 is a promising therapeutic option to treat cGVHD with bronchiolitis obliterans or sclerodermatous manifestations.

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.

A highly sensitive and rapid LC-MS/MS method for quantification of bexarotene in mouse plasma and brain tissue: Application to mice pharmacokinetic study

Emerging evidence has suggested that bexarotene, a nearly 20-year-old skin cancer drug, may be a potential drug candidate to treat Alzheimer's disease (AD) and other neurodegenerative disorders. As described in this study, a highly sensitive and rapid method, using liquid chromatography-tandem mass spectrometry (LC-MS/MS) to determine bexarotene in mouse plasma and brain tissue, was established and validated for the first time. Single-step protein precipitation utilizing methanol solution (containing 0.05 % acetic acid) as precipitation agent was employed to prepare the samples of plasma and brain tissue. Chromatographic separation in gradient elution mode was conducted via an Agilent ZORBAX SB-C18 column (50 mm × 4.6 mm, 5 µm) employing methanol-ammonium acetate buffer (5 mM, pH adjusted to 4.6 with acetic acid) as mobile phase which flowed at 0.45 mL/min. The total run time was 6 min for each sample. Detection through mass spectrometric technique was operated by selected reaction monitoring (SRM) in negative electrospray ionization mode. The method was linear within the range of 10.0-15000 ng/mL for plasma and 10.0-600 ng/mL for brain tissue homogenate with the lower limit of quantification of 10.0 ng/ml. The plasma or tissue homogenate was only required 20 μL. The intra- and inter-day precision were less than 13.8 %, and the RE was between -7.4 % and 3.4 %. The method was applied to investigate the plasma pharmacokinetics and brain distribution of bexarotene in mice after being intragastrically administered with bexarotene at the dosage of 100 mg/kg. The results showed that both brain and plasma concentrations of bexarotene peaked at 1.0 h. Bexarotene was rapidly eliminated with a half-life of 2.0 h.

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.

Cancer Chemoprevention: Preclinical In Vivo Alternate Dosing Strategies to Reduce Drug Toxicities

Cancer chemopreventive agents inhibit the formation of precursor lesions and/or the progression of these lesions to late stage disease. This approach to disease control has the potential to reduce the physical and financial costs of cancer in society. Several drugs that have been approved by the FDA for other diseases and have been extensively evaluated for their safety and pharmacokinetic/pharmacodynamic characteristics have the potential to be repurposed for use as cancer chemopreventive agents. These agents often mechanistically inhibit signaling molecules that play key roles in the carcinogenic process. The safety profile of agents is a primary concern when considering the administration of drugs for chemoprevention, as the drugs will be given chronically to high-risk, asymptomatic individuals. To decrease drug toxicity while retaining efficacy, several approaches are currently being explored. In this short review, we describe studies that use preclinical in vivo models to assess efficacy of alternative drug dosing strategies and routes of drug administration on chemopreventive drug efficacy. In vivo drug dosing strategies that reduce toxicity while retaining efficacy will pave the way for future cancer prevention clinical trials.

The potential of retinoids for combination therapy of lung cancer: Updates and future directions

Lung cancer is the most common cancer-related death worldwide. Natural compounds have shown high biological and pharmaceutical relevance as anticancer agents. Retinoids are natural derivatives of vitamin A having many regulatory functions in the human body, including vision, cellular proliferation and differentiation, and activation of tumour suppressor genes. Retinoic acid (RA) is a highly active retinoid isoform with promising anti-lung cancer activity. The abnormal expression of retinoid receptors is associated with loss of anticancer activities and acquired resistance to RA in lung cancer. The preclinical promise has not translated to the general clinical utility of retinoids for lung cancer patients, especially those with a history of smoking. Newer retinoid nano-formulations and the combinatorial use of retinoids has been associated with lower toxicity and more favorably efficacy in both the preclinical and clinical settings. Here, we highlight epidemiological and clinical therapeutic studies involving retinoids and lung cancer. We also discuss the biological actions of retinoids in lung cancer, which include effects on cancer stem cell differentiation, angiogenesis, metastasis, and proliferative. We suggest that the use of retinoids in combination with conventional and targeted anticancer agents will broaden the utility of these potent anticancer compounds in the lung cancer clinic.

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.

Testing Novel Pyrimidinyl Rexinoids: A New Paradigm for Evaluating Rexinoids for Cancer Prevention

Rexinoids, selective ligands for retinoid X receptors (RXR), have shown promise in preventing many types of cancer. However, the limited efficacy and undesirable lipidemic side-effects of the only clinically approved rexinoid, bexarotene, drive the search for new and better rexinoids. Here we report the evaluation of novel pyrimidinyl (Py) analogues of two known chemopreventive rexinoids, bexarotene (Bex) and LG100268 (LG268) in a new paradigm. We show that these novel derivatives were more effective agents than bexarotene for preventing lung carcinogenesis induced by a carcinogen. In addition, these new analogues have an improved safety profile. PyBex caused less elevation of plasma triglyceride levels than bexarotene, while PyLG268 reduced plasma cholesterol levels and hepatomegaly compared with LG100268. Notably, this new paradigm mechanistically emphasizes the immunomodulatory and anti-inflammatory activities of rexinoids. We reveal new immunomodulatory actions of the above rexinoids, especially their ability to diminish the percentage of macrophages and myeloid-derived suppressor cells in the lung and to redirect activation of M2 macrophages. The rexinoids also potently inhibit critical inflammatory mediators including IL6, IL1β, CCL9, and nitric oxide synthase (iNOS) induced by lipopolysaccharide. Moreover, in vitro iNOS and SREBP (sterol regulatory element-binding protein) induction assays correlate with in vivo efficacy and toxicity, respectively. Our results not only report novel pyrimidine derivatives of existing rexinoids, but also describe a series of biological screening assays that will guide the synthesis of additional rexinoids. Further progress in rexinoid synthesis, potency, and safety should eventually lead to a clinically acceptable and useful new drug for patients with cancer.

Clinical Efficacy of a Novel Therapeutic Principle, Anakoinosis

Classic tumor therapy, consisting of cytotoxic agents and/or targeted therapy, has not overcome therapeutic limitations like poor risk genetic parameters, genetic heterogeneity at different metastatic sites or the problem of undruggable targets. Here we summarize data and trials principally following a completely different treatment concept tackling systems biologic processes: the principle of communicative reprogramming of tumor tissues, i.e., anakoinosis (ancient greek for communication), aims at establishing novel communicative behavior of tumor tissue, the hosting organ and organism via re-modeling gene expression, thus recovering differentiation, and apoptosis competence leading to cancer control - in contrast to an immediate, "poisoning" with maximal tolerable doses of targeted or cytotoxic therapies. Therefore, we introduce the term "Master modulators" for drugs or drug combinations promoting evolutionary processes or regulating homeostatic pathways. These "master modulators" comprise a broad diversity of drugs, characterized by the capacity for reprogramming tumor tissues, i.e., transcriptional modulators, metronomic low-dose chemotherapy, epigenetically modifying agents, protein binding pro-anakoinotic drugs, such as COX-2 inhibitors, IMiDs etc., or for example differentiation inducing therapies. Data on 97 anakoinosis inducing schedules indicate a favorable toxicity profile: The combined administration of master modulators, frequently (with poor or no monoactivity) may even induce continuous complete remission in refractory metastatic neoplasia, irrespectively of the tumor type. That means recessive components of the tumor, successively developing during tumor ontogenesis, are accessible by regulatory active drug combinations in a therapeutically meaningful way. Drug selection is now dependent on situative systems characteristics, to less extent histology dependent. To sum up, anakoinosis represents a new substantive therapy principle besides novel targeted therapies.

A phase I/II study of bexarotene with carboplatin and weekly paclitaxel for the treatment of patients with advanced non-small cell lung cancer

BACKGROUND

Rexinoids demonstrate anti-proliferative differentiation-inducing activity in multiple cancer types, including NSCLC. Prior studies have shown promising results when combining rexinoids with chemotherapy. This phase I/II study evaluates the tolerability and activity of a rexinoid, bexarotene, combined with weekly paclitaxel and monthly carboplatin.

METHODS

Patients with confirmed advanced stage IIIB or IV NSCLC and adequate organ function were enrolled. They were scheduled to receive carboplatin (AUC =6) and 3 doses of weekly paclitaxel (100 mg/m2) every 4 weeks. Oral bexarotene was administered daily at two doses: 300 and 400 mg/m2/day.

RESULTS

Thirty-three patients were enrolled. Fourteen received 300 mg/m2/day and 19 received 400 mg/m2/day of bexarotene. Hematologic toxicity included grade 3 neutropenia in 7 patients. Hyperlipidemia was a major non-hematologic toxicity which was medically managed. The recommended phase II dose of bexarotene was 400 mg/m2/day. Response rate was 35%. Median overall survival (OS) for all patients was 8.3 months with 1-year survival of 43%. Median OS for the 300 mg/m2 dose of bexarotene was 6.6 versus 9.8 months for the 400 mg/m2 dose (HR, 0.73; Log rank P=0.37). Patients who experienced hypertriglyceridemia had a median OS of 9.8 months compared to 4.9 months for those who did not (HR, 0.69; Log rank P=0.33).

CONCLUSIONS

The 43% 1-year survival for patients receiving bexarotene with weekly paclitaxel and monthly carboplatin is encouraging. With the availability of new classes of agents for lung cancer, further evaluation of this regimen in unselected patients is not warranted. Our study confirms prior subgroup analyses showing a significant correlation between bexarotene-induced hypertriglyceridemia and survival. Further research is needed to identify molecular biomarkers to identify this subset of patients and to explore rexinoids in other combinations, especially with immunotherapy.

Bexarotene inhibits the viability of non-small cell lung cancer cells via slc10a2/PPARγ/PTEN/mTOR signaling pathway

Background

Thirty to 40 % of non-small cell lung cancer (NSCLC) patients developed higher hypertriglyceridemia in the process of treatment with bexarotene. And bioinformatics studies discovered that the expression of slc10a2 was increased in high-grade hypertriglyceridemia patients. So, we will explore the mechanism which may involve in this process.

Methods

We constructed slc10a2 overexpressed A549 cells and H1299 cells as cell models, normal A549 cells and H1299 cells as control. Then we explored the effects of slc10a2 on A549 cells and H1299 cells behaviors, including proliferation, invasion and apoptosis. The expression of apoptotic related genes and anti-cancer genes also been detected.

Results

We found that the proliferation and migration were inhibited and the apoptosis of NSCLC cells was accelerated by bexarotene. In addition, overexpressed slc10a2 in NSCLC cells can further suppress the proliferation and migration, and promote apoptosis under the treatment of bexarotene. On the contrary, the opposite results were obtained after slc10a2 gene was silenced in NSCLC cells treated with bexarotene. Moreover, the expression of caspase 3, caspase 7, PTEN, P21, P53, LKB1, TSC2 were increased and the expression of Bcl-2, cyclin D1, c-FLIP were declined in NSCLC cells and slc10a2 overexpressed NSCLC cells with the treatment of bexarotene, and the opposite situations were seen after slc10a2 gene was silenced in NSCLC cells. The further studies revealed the increased expression of slc10a2 activated the expression of peroxisome proliferator-activated receptor γ (PPARγ), then up-regulated PTEN expression and down-regulated mTOR expression.

Conclusion

These results suggest that bexarotene inhibits the viability of lung cancer cells via slc10a2/PPARγ/PTEN/mTOR signaling pathway.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-4224-x) contains supplementary material, which is available to authorized users.

Cancer Prevention: Lessons Learned and Future Directions

In this review, we address selected areas that are central to the state-of-the-art of cancer prevention science. The emphasis on prevention as a viable and critical approach to decreasing cancer mortality has gained traction in recent years, evidenced by its inclusion in the US Vice President's Cancer Initiative (also termed 'Moonshot'). Cancer prevention occurs by arresting, slowing down, or reversing the carcinogenic process before invasion into surrounding tissue or by avoiding or blocking causative exposure. An important challenge is to identify individuals who will benefit most from preventive interventions with the least possible harm. Preventive interventions range from avoiding known carcinogens (e.g., tobacco or asbestos) to intervening with anticarcinogenic strategies (behavioral modifications , such as diet and exercise; medications; nutritional agents; and vaccination against causative agents). Here, we focus on active intervention with measures involving pharmaceutical and immunological agents.

Inventory of oral anticancer agents: Pharmaceutical formulation aspects with focus on the solid dispersion technique

Dissolution from the pharmaceutical formulation is a prerequisite for complete and consistent absorption of any orally administered drug, including anticancer agents (oncolytics). Poor dissolution of an oncolytic can result in low oral bioavailability, high variability in blood concentrations and with that suboptimal or even failing therapy. This review discusses pharmaceutical formulation aspects and absorption pharmacokinetics of currently licensed orally administered oncolytics. In nearly half of orally dosed oncolytics poor dissolution is likely to play a major role in low and unpredictable absorption. Dissolution-limited drug absorption can be improved with a solid dispersion which is a formulation method that induces super-saturated drug dissolution and with that it enhances in vivo absorption. This review discusses formulation principles with focus on the solid dispersion technology and how it works to enhance drug absorption. There are currently three licensed orally dosed oncolytics formulated as a solid dispersion (everolimus, vemurafenib and regorafenib) and these formulations result in remarkably improved dissolution and absorption compared to what can be achieved with conventional formulations of the respective oncolytics. Because of the successful implementation of these three solid dispersion formulations, we encourage the application of this formulation method for poorly soluble oral oncolytics.

Retinoid X Receptor Agonists Upregulate Genes Responsible for the Biosynthesis of All-Trans-Retinoic Acid in Human Epidermis

UAB30 is an RXR selective agonist that has been shown to have potential cancer chemopreventive properties. Due to high efficacy and low toxicity, it is currently being evaluated in human Phase I clinical trials by the National Cancer Institute. While UAB30 shows promise as a low toxicity chemopreventive drug, the mechanism of its action is not well understood. In this study, we investigated the effects of UAB30 on gene expression in human organotypic skin raft cultures and mouse epidermis. The results of this study indicate that treatment with UAB30 results in upregulation of genes responsible for the uptake and metabolism of all-trans-retinol to all-trans-retinoic acid (ATRA), the natural agonist of RAR nuclear receptors. Consistent with the increased expression of these genes, the steady-state levels of ATRA are elevated in human skin rafts. In ultraviolet B (UVB) irradiated mouse skin, the expression of ATRA target genes is found to be reduced. A reduced expression of ATRA sensitive genes is also observed in epidermis of mouse models of UVB-induced squamous cell carcinoma and basal cell carcinomas. However, treatment of mouse skin with UAB30 prior to UVB irradiation prevents the UVB-induced decrease in expression of some of the ATRA-responsive genes. Considering its positive effects on ATRA signaling in the epidermis and its low toxicity, UAB30 could be used as a chemoprophylactic agent in the treatment of non-melanoma skin cancer, particularly in organ transplant recipients and other high risk populations.

Effects of RXR Agonists on Cell Proliferation/Apoptosis and ACTH Secretion/Pomc Expression

Various retinoid X receptor (RXR) agonists have recently been developed, and some of them have shown anti-tumor effects both in vivo and in vitro. However, there has been no report showing the effects of RXR agonists on Cushing’s disease, which is caused by excessive ACTH secretion in a corticotroph tumor of the pituitary gland. Therefore, we examined the effects of synthetic RXR pan-agonists HX630 and PA024 on the proliferation, apoptosis, ACTH secretion, and pro-opiomelanocortin (Pomc) gene expression of murine pituitary corticotroph tumor AtT20 cells. We demonstrated that both RXR agonists induced apoptosis dose-dependently in AtT20 cells, and inhibited their proliferation at their higher doses. Microarray analysis identified a significant gene network associated with caspase 3 induced by high dose HX630. On the other hand, HX630, but not PA024, inhibited Pomc transcription, Pomc mRNA expression, and ACTH secretion dose-dependently. Furthermore, we provide new evidence that HX630 negatively regulates the Pomc promoter activity at the transcriptional level due to the suppression of the transcription factor Nur77 and Nurr1 mRNA expression and the reduction of Nur77/Nurr1 heterodimer recruiting to the Pomc promoter region. We also demonstrated that the HX630-mediated suppression of the Pomc gene expression was exerted via RXRα. Furthermore, HX630 inhibited tumor growth and decreased Pomc mRNA expression in corticotroph tumor cells in female nude mice in vivo. Thus, these results indicate that RXR agonists, especially HX630, could be a new therapeutic candidate for Cushing’s disease.

Conformationally Defined Rexinoids and Their Efficacy in the Prevention of Mammary Cancers

(2E,4E,6Z,8Z)-8-(3',4'-Dihydro-1'(2H)-naphthalen-1'-ylidene)-3,7-dimethyl-2,3,6-octatrienoinic acid (UAB30) is currently undergoing clinical evaluation as a novel cancer prevention agent. In efforts to develop even more highly potent rexinoids that prevent breast cancer without toxicity, we further explore here the structure-activity relationship of two separate classes of rexinoids. UAB30 belongs to the class II rexinoids and possesses a 9Z-tetraenoic acid chain bonded to a tetralone ring, whereas the class I rexinoids contain the same 9Z-tetraenoic acid chain bonded to a disubstituted cyclohexenyl ring. Among the 12 class I and class II rexinoids evaluated, the class I rexinoid 11 is most effective in preventing breast cancers in an in vivo rat model alone or in combination with tamoxifen. Rexinoid 11 also reduces the size of established tumors and exhibits a therapeutic effect. However, 11 induces hypertriglyceridemia at its effective dose. On the other hand rexinoid 10 does not increase triglyceride levels while being effective in the in vivo chemoprevention assay. X-ray studies of four rexinoids bound to the ligand binding domain of the retinoid X receptor reveal key structural aspects that enhance potency as well as those that enhance the synthesis of lipids.

Retinoids Bias Integrin Expression and Function in Cutaneous T-Cell Lymphoma

Cutaneous T-cell lymphoma (CTCL) is a heterogeneous group of malignancies characterized by accumulation of malignant T-cells within the skin. Retinoids, metabolic derivatives, and synthetic analogs of vitamin A embody an effective CTCL therapy with over three decades of clinical use. The established mechanism of action is induction of growth arrest and apoptosis. However, the natural role of retinoids in T-cell biology is imprinting gut-homing properties by inducing integrin α4β7 expression. How the natural role of retinoids relates to therapeutic effectiveness in CTCL has not been addressed and merits investigation. Here we provide evidence that retinoids, including Bexarotene, selectively induce CTCL lineages to increase integrin β7 expression and function prior to growth arrest and apoptosis. Interestingly, augmented CTCL cell adhesion obtained with retinoid exposure was potently attenuated by 1,25-dihydroxyvitamin D3, a metabolic vitamin derivative involved in prompting immune cell skin homing. The integrin-dependent adhesion changes in CTCL cells occurred through synergistic activation of RAR and RXR nuclear receptors. These data explore the early cellular changes induced by retinoids that may be pivotal to sensitizing CTCL cells to growth arrest and apoptosis.

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.

Methyl substitution of a rexinoid agonist improves potency and reveals site of lipid toxicity

(2E,4E,6Z,8E)-8-(3′,4′-Dihydro-1′(2′H)-naphthalen-1′-ylidene)-3,7-dimethyl-2,4,6-octatrienoic acid, 9cUAB30, is a selective rexinoid that displays substantial chemopreventive capacity with little toxicity. 4-Methyl-UAB30, an analogue of 9cUAB30, is a potent RXR agonist but caused increased lipid biosynthesis unlike 9cUAB30. To evaluate how methyl substitution influenced potency and lipid biosynthesis, we synthesized four 9cUAB30 homologues with methyl substitutions at the 5-, 6-, 7-, or 8-position of the tetralone ring. The syntheses and biological evaluations of these new analogues are reported here along with the X-ray crystal structures of each homologue bound to the ligand binding domain of hRXRα. We demonstrate that each homologue of 9cUAB30 is a more potent agonist, but only the 7-methyl-9cUAB30 caused severe hyperlipidemia in rats. On the basis of the X-ray crystal structures of these new rexinoids and bexarotene (Targretin) bound to hRXRα-LBD, we reveal that each rexinoid, which induced hyperlipidemia, had methyl groups that interacted with helix 7 residues of the LBD.

CNX-013-B2, a unique pan tissue acting rexinoid, modulates several nuclear receptors and controls multiple risk factors of the metabolic syndrome without risk of hypertriglyceridemia, hepatomegaly and body weight gain in animal models

BACKGROUND

In addition to their role in growth, cellular differentiation and homeostasis Retinoid X Receptors (RXR) regulate multiple physiological and metabolic pathways in various organs that have beneficial glucose and lipid (cholesterol) lowering, insulin sensitizing and anti-obesity effects. Rexinoids, compounds that specifically binds and activate RXR, are therefore considered as potential therapeutics for treating metabolic syndrome. Apparently many of the rexinoids developed in the past increased triglycerides, caused hepatomegaly and also suppressed the thyroid hormone axis. The aim of this study is to evaluate CNX-013-B2, a potent and highly selective rexinoid, for its potential to treat multiple risk factors of the metabolic syndrome.

METHODS

CNX-013-B2 was selected in a screening system designed to identify compounds that selectively activated only a chosen sub-set of heterodimer partners of RXR of importance to treat insulin resistance. Male C57BL/6j mice (n = 10) on high fat diet (HFD) and 16 week old ob/ob mice (n = 8) were treated orally with CNX-013-B2 (10 mg/kg twice daily) or vehicle for 10 weeks and 4 weeks respectively. Measurement of plasma glucose, triglyceride, cholesterol including LDL-C, glycerol, free fatty acids, feed intake, body weight, oral glucose tolerance and non-shivering thermogenesis were performed at selected time points. After study termination such measurements as organ weight, triglyceride content, mRNA levels, protein phosphorylation along with histological analysis were performed.

RESULTS

CNX-013-B2 selectively activates PPARs- α, β/δ and γ and modulates activity of LXR, THR and FXR. In ob/ob mice a significant reduction of 25% in fed glucose (p < 0.001 ), a 14% (p < 0.05) reduction in serum total cholesterol and 18% decrease (p < 0.01) in LDL-C and in DIO mice a reduction of 12% (p < 0.01 ) in fasting glucose, 20% in fed triglyceride (p < 0.01) and total cholesterol (p < 0.001) levels, coupled with enhanced insulin sensitivity, cold induced thermogenesis and 7% reduction in body weight were observed.

CONCLUSION

CNX-013-B2 is an orally bio available selective rexinoid that can be used as a novel therapeutic agent for management of multiple risk factors of the metabolic syndrome without the risk of side effects reported to be associated with rexinoids.

Methyl-substituted conformationally constrained rexinoid agonists for the retinoid X receptors demonstrate improved efficacy for cancer therapy and prevention

(2E,4E,6Z,8Z)-8-(3',4'-Dihydro-1'(2H)-naphthalen-1'-ylidene)-3,7-dimethyl-2,3,6-octatrienoinic acid, 9cUAB30, is a selective rexinoid for the retinoid X nuclear receptors (RXR). 9cUAB30 displays substantial chemopreventive capacity with little toxicity and is being translated to the clinic as a novel cancer prevention agent. To improve on the potency of 9cUAB30, we synthesized 4-methyl analogs of 9cUAB30, which introduced chirality at the 4-position of the tetralone ring. The syntheses and biological evaluations of the racemic homolog and enantiomers are reported. We demonstrate that the S-enantiomer is the most potent and least toxic even though these enantiomers bind in a similar conformation in the ligand binding domain of RXR.

Final results of phase II trial of doxorubicin HCl liposome injection followed by bexarotene in advanced cutaneous T-cell lymphoma

BACKGROUND

High response rates for doxorubicin HCl liposome injection (DLI) in cutaneous T-cell lymphoma (CTCL) have been reported with vague criteria until recently. Approximately 50% of CTCL patients respond to bexarotene (Bex).

PATIENTS AND METHODS

A phase II trial was carried out to clarify the true overall response rate (ORR) for DLI and to assess the role of sequential Bex. Patients were treated with DLI 20 mg/m(2) i.v. every 2 weeks for 16 weeks (8 doses) followed by 16 weeks with Bex 300 mg/m(2) orally. Response assessments were carried out after 16 (DLI) and 32 weeks (Bex). Skin responses were measured by the modified Severity-Weighted Assessment Tool (mSWAT) and the Composite Assessment of Index Lesion Severity (CA).

RESULTS

Thirty-seven patients were treated: stage IV (22, 8 with Sézary syndrome), IIB (10), earlier stage refractory to skin-directed therapies or radiation therapy (5). For 34 assessable patients: ORR 14/34 [41%: partial response (PR) 12, clinical complete response (CCR) 2]. Maximum responses were all seen after 16 weeks DLI. Median progression-free survival (PFS) was 5 months. There were 22 deaths: 21 of disease and 1 of heart failure. Twenty-seven grade 3 and 5 grade 4 toxic events were observed.

CONCLUSION(S)

With strict criteria, DLI ORR is among the highest reported for single agents in CTCL. Sequential Bex did not increase the response rate or duration.

Low dose bexarotene treatment rescues dopamine neurons and restores behavioral function in models of Parkinson's disease

Nurr1 is a nuclear hormone receptor (NucHR) strongly implicated in the growth, maintenance, and survival of dopaminergic neurons. Nurr1 may be unable to bind ligands directly, but it forms heterodimers with other NucHRs that do. Using bioluminescence resonance energy transfer (BRET) assays to directly monitor interactions of Nurr1 with other NucHRs, we found the cancer drug bexarotene (Targretin, also LGD1069) displayed biased interactions with Nurr1-RXR heterodimers compared with RXR-RXR homodimers. Remarkably, at doses up to 100-fold lower than those effective in rodent cancer models, bexarotene rescued dopamine neurons and reversed behavioral deficits in 6-hydroxydopamine (6-OHDA) lesioned rats. Compared to the high doses used in cancer therapy, low doses of bexarotene have significantly milder side effects including a reduced increase in plasma triglycerides and less suppression of thyroid function. On the basis of extrapolations from rat to human doses, we hypothesize that low oral doses of bexarotene may provide an effective and tolerated therapy for Parkinson's disease (PD).

Defining the communication between agonist and coactivator binding in the retinoid X receptor α ligand binding domain

Retinoid X receptors (RXRs) are obligate partners for several other nuclear receptors, and they play a key role in several signaling processes. Despite being a promiscuous heterodimer partner, this nuclear receptor is a target of therapeutic intervention through activation using selective RXR agonists (rexinoids). Agonist binding to RXR initiates a large conformational change in the receptor that allows for coactivator recruitment to its surface and enhanced transcription. Here we reveal the structural and dynamical changes produced when a coactivator peptide binds to the human RXRα ligand binding domain containing two clinically relevant rexinoids, Targretin and 9-cis-UAB30. Our results show that the structural changes are very similar for each rexinoid and similar to those for the pan-agonist 9-cis-retinoic acid. The four structural changes involve key residues on helix 3, helix 4, and helix 11 that move from a solvent-exposed environment to one that interacts extensively with helix 12. Hydrogen-deuterium exchange mass spectrometry reveals that the dynamics of helices 3, 11, and 12 are significantly decreased when the two rexinoids are bound to the receptor. When the pan-agonist 9-cis-retinoic acid is bound to the receptor, only the dynamics of helices 3 and 11 are reduced. The four structural changes are conserved in all x-ray structures of the RXR ligand-binding domain in the presence of agonist and coactivator peptide. They serve as hallmarks for how RXR changes conformation and dynamics in the presence of agonist and coactivator to initiate signaling.

Comment on "ApoE-directed therapeutics rapidly clear β-amyloid and reverse deficits in AD mouse models"

Cramer et al. (Reports, 23 March 2012, p. 1503; published online 9 February 2012) demonstrates short-term bexarotene treatment clearing preexisting β-amyloid deposits from the brains of APP/PS1ΔE9 mice with low amyloid burden, providing a rationale for repurposing this anticancer agent as an Alzheimer's disease (AD) therapeutic. Using a nearly identical treatment regimen, we were unable to detect any evidence of drug efficacy despite demonstration of target engagement.

The combination of tamoxifen and the rexinoid LG100268 prevents ER-positive and ER-negative mammary tumors in p53-null mammary gland mice

In pursuit of effective therapeutic agents for the estrogen receptor (ER)-negative breast cancer, we previously showed that bexarotene reduced mammary tumor development by 75% in ErbB2 mice. To further improve the effectiveness of breast cancer prevention, we have now investigated the effects of a combinatorial therapy consisting of two cancer preventive drugs. On the basis of the hypothesis, rexinoid LG100268 plus tamoxifen would more effectively prevent the development of both ER-positive and ER-negative breast cancer. We treated p53-null mammary gland mice with tamoxifen and LG100268, individually and in combination. By 60 weeks of age, vehicle-treated mice developed tumors in 52% of transplanted mammary glands, whereas mice treated with tamoxifen and LG100268 developed tumors in only 13% of transplanted mammary glands. To further define the mechanistic effects of this combinatorial treatment, we investigated the effects of tamoxifen and LG100268 on mammary tissue biomarkers. In mammary tissue harvested before tumor development, the proliferation markers Ki67 and cyclin D1 were significantly reduced in mice treated with the combination therapy. In addition, the rexinoid target genes ABCA1 and ABCG1 were induced in both the rexinoid and combination treatment groups, whereas expression remained constant in tamoxifen group. These results show that tamoxifen-LG100268 combinatorial treatment is more effective in preventing mammary tumors than either agent alone. In addition, these studies have identified relevant tissue biomarkers that can be used to show the effect of these agents on mammary tissue. These results support the development of clinical trials of antiestrogen and rexinoid combinatorial therapy for the prevention of patients with high-risk breast cancer.

Successful Treatment of a Panniculitis-Like Primary Cutaneous T-Cell Lymphoma of the α/β Type with Bexarotene

Subcutaneous panniculitis-like T-cell lymphoma (SPTL) of the α/β type is a rare subtype of non-Hodgkin's lymphoma of the skin. Although these tumors usually run an indolent course, disease-related morbidity is often severe. Clinical findings include subcutaneous tumors located on the extremities or trunk, often accompanied by systemic symptoms like fever or fatigue. Due to the low incidence of SPTL, no standardized therapy has been defined so far and there is currently no curative therapy available for this type of non-Hodgkin's lymphoma. By sharing our experience with bexarotene therapy, we present a safe and potentially improved treatment for patients with SPTL. In the case presented, bexarotene was able to induce remission even after recurrence of disease.

Genome-wide profiling of liver X receptor, retinoid X receptor, and peroxisome proliferator-activated receptor α in mouse liver reveals extensive sharing of binding sites

The liver X receptors (LXRs) are nuclear receptors that form permissive heterodimers with retinoid X receptor (RXR) and are important regulators of lipid metabolism in the liver. We have recently shown that RXR agonist-induced hypertriglyceridemia and hepatic steatosis in mice are dependent on LXRs and correlate with an LXR-dependent hepatic induction of lipogenic genes. To further investigate the roles of RXR and LXR in the regulation of hepatic gene expression, we have mapped the ligand-regulated genome-wide binding of these factors in mouse liver. We find that the RXR agonist bexarotene primarily increases the genomic binding of RXR, whereas the LXR agonist T0901317 greatly increases both LXR and RXR binding. Functional annotation of putative direct LXR target genes revealed a significant association with classical LXR-regulated pathways as well as peroxisome proliferator-activated receptor (PPAR) signaling pathways, and subsequent chromatin immunoprecipitation-sequencing (ChIP-seq) mapping of PPARα binding demonstrated binding of PPARα to 71 to 88% of the identified LXR-RXR binding sites. The combination of sequence analysis of shared binding regions and sequential ChIP on selected sites indicate that LXR-RXR and PPARα-RXR bind to degenerate response elements in a mutually exclusive manner. Together, our findings suggest extensive and unexpected cross talk between hepatic LXR and PPARα at the level of binding to shared genomic sites.

The effect of bexarotene on atorvastatin pharmacokinetics: results from a phase I trial of bexarotene plus chemotherapy in patients with advanced non-small cell lung cancer

PURPOSE

Bexarotene (Targretin(®) capsules) is a retinoid-X-receptor agonist and an inducer of CYP3A4-mediated metabolism. This phase I trial evaluated the pharmacokinetic (PK) and drug-drug interactions of bexarotene with chemotherapy and a lipid-lowering agent (atorvastatin or fenofibrate). This trial was run in parallel with phase III trials of the combinations to determine whether repeated doses of bexarotene capsules affect the pharmacokinetics (PK) of the chemotherapeutic or the lipid-lowering agents.

METHODS

Patients (n = 48) with advanced non-small cell lung cancer were treated with repetitive cycles of either paclitaxel/carboplatin or cisplatin/vinorelbine chemotherapy, bexarotene (400 mg/m(2)/day) administered continuously starting on day 4 of chemotherapy, and a lipid-lowering drug, either atorvastatin or fenofibrate, starting at least 5 days before chemotherapy due to hypertriglyceridemia induced by bexarotene. Extensive plasma sampling to characterize the PK profiles of the lipid-lowering drugs, relevant chemotherapy agents was performed on day 1 (without bexarotene) and during chemotherapy cycles 2 or 3 (with bexarotene).

RESULTS

Here, we report the drug-drug interactions between the lipid-lowering agents and bexarotene. Mean atorvastatin clearance and dose-corrected AUC values were reduced by nearly 50% with the addition of concomitant bexarotene. As fenofibrate was less effective at controlling hypertriglyceridemia, too few patients received this agent to make any meaningful conclusions about drug-drug interactions.

CONCLUSIONS

A drug-drug interaction was seen in this trial with bexarotene co-administration leading to a significant reduction in the AUC of atorvastatin. The likely mechanism for this interaction is through induction of CYP3A4 by bexarotene given the role of this enzyme in the metabolism of atorvastatin. Knowledge of this interaction is important for optimizing lipid management with atorvastatin for patients receiving bexarotene.

A phase I pharmacokinetic study of bexarotene with paclitaxel and carboplatin in patients with advanced non-small cell lung cancer (NSCLC)

PURPOSE

Preclinical data suggest that the synthetic retinoid bexarotene may be an effective chemopreventive agent and that it may act synergistically in combination with platinum-based chemotherapy. The primary objective of this study was to determine whether repeated doses of bexarotene capsules affect pharmacokinetic parameters of paclitaxel or carboplatin in patients with advanced non-small cell lung cancer.

METHODS

Patients received treatment with paclitaxel (200 mg/m(2)) and carboplatin to provide a target AUC of 6 mg min/mL (day 1) every 3 weeks. Continuous oral bexarotene therapy (400 mg/m(2)/day) was initiated on Day 4, and patients started lipid-lowering therapy prior to beginning chemotherapy. Blood sampling to characterize the pharmacokinetic profiles of the chemotherapeutic agents with or without bexarotene was performed during cycle 1 (without concomitant bexarotene) and during cycle 2 (with concomitant bexarotene).

RESULTS

An analysis of drug concentration data from 16 patients indicated that bexarotene did not affect the pharmacokinetics of paclitaxel, free carboplatin, or total carboplatin concentrations. However, both maximal plasma concentrations and total exposure of bexarotene increased by 80% in the presence of paclitaxel-carboplatin by an, as of yet, unexplained mechanism. The toxicities observed resembled those of either the chemotherapy regimen or bexarotene alone, and there was no evidence for an enhancement of any drug-related toxicity with the combined treatment.

CONCLUSIONS

The administration of bexarotene, paclitaxel, and carboplatin is feasible and safe; however, the increased bexarotene plasma concentrations and exposure warrant further investigation if this combination is to be utilized clinically.

Cancer-preventive rexinoid modulates neutral lipid contents of mammary epithelial cells through a peroxisome proliferator-activated receptor γ-dependent mechanism

Synthetic rexinoids effectively suppress both estrogen receptor-positive and estrogen receptor-negative mammary tumors in animal models, which makes them prime candidates for a novel class of cancer-preventive agents. When used in combination with chemotherapy for non-small-cell lung cancer, the rexinoid bexarotene was most effective for patients who developed hypertriglyceridemia as a side effect. Although serum triglycerides originate from the liver, the effect of bexarotene on lipogenesis in breast epithelial cells is not known. Gene expression studies with normal mammary epithelial cells indicated that rexinoids modulate lipid metabolism, particularly enzymes involved in triglyceride synthesis. High-content analysis revealed dose-dependent accumulation of neutral lipids within adipocyte differentiation-related protein-associated cytoplasmic lipid droplets after long-term bexarotene treatment. Bexarotene also induced mRNA and protein levels for peroxisome proliferator-activated receptor (PPAR) γ, whereas selective knockdown of PPARγ attenuated the induction of both lipid droplets and adipocyte differentiation-related protein. Pharmacological activation of PPARγ, but not PPARα or retinoic acid receptors, effectively induced lipid accumulation. Furthermore, the combination of the PPARγ agonist rosiglitazone with bexarotene synergistically suppressed the growth of human mammary epithelial cells and revealed a strong, nonlinear, inverse correlation of cell growth with lipid droplet accumulation in the cell population. These findings indicate that rexinoids activate a lipogenic program in mammary epithelial cells through a retinoid X receptor/PPARγ-mediated mechanism. It is noteworthy that combining low doses of bexarotene with the PPARγ agonist rosiglitazone provides effective growth suppression of mammary epithelial cells, potentially dissociating systemic adverse effects associated with standard bexarotene treatment from the antiproliferative effects on mammary epithelium.

Modulation of RXR function through ligand design

As the promiscuous partner of heterodimeric associations, retinoid X receptors (RXRs) play a key role within the Nuclear Receptor (NR) superfamily. Some of the heterodimers (PPAR/RXR, LXR/RXR, FXR/RXR) are "permissive" as they become transcriptionally active in the sole presence of either an RXR-selective ligand ("rexinoid") or a NR partner ligand. In contrast, "non-permissive" heterodimers (including RAR/RXR, VDR/RXR and TR/RXR) are unresponsive to rexinoids alone but these agonists superactivate transcription by synergizing with partner agonists. Despite their promiscuity in heterodimer formation and activation of multiple pathways, RXR is a target for drug discovery. Indeed, a rexinoid is used in the clinic for the treatment of cutaneous T-cell lymphoma. In addition to cancer RXR modulators hold therapeutical potential for the treatment of metabolic diseases. The modulation potential of the rexinoid (as agonist or antagonist ligand) is dictated by the precise conformation of the ligand-receptor complexes and the nature and extent of their interaction with co-regulators, which determine the specific physiological responses through transcription modulation of cognate gene networks. Notwithstanding the advances in this field, it is not yet possible to predict the correlation between ligand structure and physiological response. We will focus on this review on the modulation of PPARγ/RXR and LXR/RXR heterodimer activities by rexinoids. The genetic and pharmacological data from animal models of insulin resistance, diabetes and obesity demonstrate that RXR agonists and antagonists have promise as anti-obesity agents. However, the treatment with rexinoids raises triglycerides levels, suppresses the thyroid hormone axis, and induces hepatomegaly, which has complicated the development of these compounds as therapeutic agents for the treatment of type 2 diabetes and insulin resistance. The discovery of PPARγ/RXR and LXR/RXR heterodimer-selective rexinoids, which act differently than PPARγ or LXR agonists, might overcome some of these limitations.

Absence of modulation of CD4+CD25 regulatory T cells in CTCL patients treated with bexarotene

Cutaneous T-cell lymphoma (CTCL) are a heterogeneous group of lymphoproliferative disorders, characterized by the infiltration of the epidermis by mature and activated malignant CD4+ T-lymphocytes. Retinoids such as retinoic acid and synthetic analogues have long been used alone or in combination with other therapies for CTCL. Bexarotene, the first synthetic highly selective RXR retinoid, was approved for the treatment of all stages of CTCL in patients refractory to at least one systemic therapy. Recently, six cases in which the initiation of bexarotene therapy for CTCL was associated with the progression of internal disease despite improvement of cutaneous signs and symptoms were reported. Moreover, it has been established that retinoids promote the generation of CD4+ Foxp3+ regulatory T cells, raising the question of an induction of regulatory T-cells by bexarotene. The aim of this work was to determine if bexarotene induces an increase of functional regulatory T cells which could play a role in the development of secondary extra-cutaneous lymphomas. Regulatory T cells were studied both in cutaneous biopsy specimens using an immunohistochemical analysis of CD4, CD25 and Foxp3 and in blood where proportion and functionality of circulating CD4+CD25(high) T-cells were determined. The study was performed in 10 patients [five patients with Sézary syndrome (SS) and five mycosis fungoïdes (MF)], treated for 6 months with bexarotene. Four healthy donors were used as controls for phenotypic and functional analysis on PBL. We found that the frequency of CD4+CD25(high) Treg cells was not significantly different before starting bexarotene and after 6 months of treatment in CTCL patients. However, we observed that the frequency of CD4+CD25(high) Treg cells before the beginning of the treatment was significantly increased compared to healthy donors. In addition, functional assays demonstrated that Foxp3 expressing CD4+CD25(high) T-cells were capable of suppressing autologous CD4 + CD25- T-cell proliferation. In the present work, we detected the presence of functional circulating CD4+CD25(high) Foxp3+ regulatory T-cells in CTCL patients, with an increased frequency compared to healthy donors. The treatment with bexarotene does not seem to affect the regulatory T-cell compartment.

About the cutaneous targets of bexarotene in CTCL patients

There are several approved therapies for cutaneous T-cell lymphoma (CTCL). The retinoids are one of the major biologic response modifiers used in CTCL, producing good response rates but few complete responses. Bexarotene has been demonstrated to act on malignant T-cells by inducing their apoptosis, but nothing is known about its role on keratinocytes and Langerhans cells. Immunohistochemical analysis using CD1a, HLA-DR, ICAM-1 (activation markers), CD95 and CD40 (apoptosis markers) was conducted on frozen sections of bexarotene-exposed cutaneous explants and skin biopsy specimens from patients treated with bexarotene. None of the studied markers was significantly modulated both on cutaneous explants and on skin biopsy specimens after treatment with bexarotene, compared to controls. Langerhans cells and keratinocytes do not appear to play a central role in the therapeutic control of CTCL by bexarotene therapy. The main bexarotene's target thus remains T-cells by inducing their apoptosis, a mechanism that is different from the other retinoids used in CTCL.

Aerosolized bexarotene inhibits lung tumorigenesis without increasing plasma triglyceride and cholesterol levels in mice

Prior studies have shown the retinoid X receptor (RXR) agonist bexarotene has preventive efficacy in rodent models of mammary and lung tumorigenesis albeit causing hypertriglyceridemia and hypercholesterolemia. We reasoned that bexarotene delivered by inhalation may provide sufficient dose directly to the respiratory tract to achieve efficacy while avoiding these side effects. In this study, the chemopreventive activity of aerosolized bexarotene was investigated in the benzo(a)pyrene [B(a)P]-induced mouse lung tumor model as assessed by tumor multiplicity and tumor load. Aerosolized bexarotene significantly decreased tumor multiplicity and tumor load by 43% and 74%, respectively. Our data showed that bexarotene can both inhibit proliferation and promote apoptosis in vivo. Our data also show that aerosolized bexarotene did not increase plasma total cholesterol and triglyceride level compared with diet group. These results indicate that aerosolization may be a safe and effective route of administering bexarotene for chemoprevention of lung cancer.

Identification of retinoic acid in a high content screen for agents that overcome the anti-myogenic effect of TGF-beta-1

BACKGROUND

Transforming growth factor beta 1 (TGF-β1) is an inhibitor of muscle cell differentiation that is associated with fibrosis, poor regeneration and poor function in some diseases of muscle. When neutralizing antibodies to TGF-β1 or the angiotensin II inhibitor losartan were used to reduce TGF-β1 signaling, muscle morphology and function were restored in mouse models of Marfan Syndrome and muscular dystrophy. The goal of our studies was to identify additional agents that overcome the anti-myogenic effect of TGF-β1.

METHODOLOGY/PRINCIPAL FINDINGS

A high-content cell-based assay was developed in a 96-well plate format that detects the expression of myosin heavy chain (MHC) in C2C12 cells. The assay was used to quantify the dose-dependent responses of C2C12 cell differentiation to TGF-β1 and to the TGF-β1 Type 1 receptor kinase inhibitor, SB431542. Thirteen agents previously described as promoting C2C12 differentiation in the absence of TGF-β1 were screened in the presence of TGF-β1. Only all-trans retinoic acid and 9-cis retinoic acid allowed a maximal level of C2C12 cell differentiation in the presence of TGF-β1; the angiotensin-converting enzyme inhibitor captopril and 10 nM estrogen provided partial rescue. Vitamin D was a potent inhibitor of retinoic acid-induced myogenesis in the presence of TGF-β1. TGF-β1 inhibits myoblast differentiation through activation of Smad3; however, retinoic acid did not inhibit TGF-β1-induced activation of a Smad3-dependent reporter gene in C2C12 cells.

CONCLUSIONS/SIGNIFICANCE

Retinoic acid alleviated the anti-myogenic effect of TGF-β1 by a Smad3-independent mechanism. With regard to the goal of improving muscle regeneration and function in individuals with muscle disease, the identification of retinoic acid is intriguing in that some retinoids are already approved for human therapy. However, retinoids also have well-described adverse effects. The quantitative, high-content assay will be useful to screen for less-toxic retinoids or combinations of agents that promote myoblast differentiation in the presence of TGF-β1.

Effect of retinoic acid and delta-like 1 homologue (DLK1) on differentiation in neuroblastoma

The principal objective of this study was to evaluate the chemopreventive and therapeutic effects of a combination of all-trans-retinoic acid (RA) and knockdown of delta-like 1 homologue (Drosophila) (DLK1) on neuroblastoma, the most common malignant disease in children. As unfavorable neuroblastoma is poorly differentiated, neuroblastoma cell was induced differentiation by RA or DLK1 knockdown. Neuroblastoma cells showed elongated neurite growth, a hallmark of neuronal differentiation at various doses of RA, as well as by DLK1 knockdown. In order to determine whether or not a combination of RA and DLK1 knockdown exerts a greater chemotherapeutic effect on neuroblastoma, cells were incubated at 10 nM RA after being transfected with SiRNA-DLK1. Neuronal differentiation was increased more by a combination of RA and DLK1 knockdown than by single treatment. Additionally, in order to assess the signal pathway of neuroblastoma differentiation induced by RA and DLK1 knockdown, treatment with the specific MEK/ERK inhibitors, U0126 and PD 98059, was applied to differentiated neuroblastoma cells. Differentiation induced by RA and DLK1 knockdown increased ERK phosphorylation. The MEK/ERK inhibitor U0126 completely inhibited neuronal differentiation induced by both RA and DLK1 knockdown, whereas PD98059 partially blocked neuronal differentiation. After the withdrawal of inhibitors, cellular differentiation was fully recovered. This study is, to the best of our knowledge, the first to demonstrate that the specific inhibitors of the MEK/ERK pathway, U0126 and PD98059, exert differential effects on the ERK phosphorylation induced by RA or DLK1 knockdown. Based on the observations of this study, it can be concluded that a combination of RA and DLK1 knockdown increases neuronal differentiation for the control of the malignant growth of human neuroblastomas, and also that both MEK1 and MEK2 are required for the differentiation induced by RA and DLK1 knockdown.

Evaluation of retinoic acid therapy for OTX2-positive medulloblastomas

The homeobox transcription factor OTX2 plays an essential role during embryonic brain development. It is normally silenced in the adult brain, but is overexpressed by genomic amplification or other mechanisms in the majority of medulloblastomas (MBs). Retinoic acids (RAs) can suppress OTX2 expression and inhibit MB growth. In this study, 9-cis RA most potently inhibited MB cell growth. 9-cis RA functions through the downregulation of OTX2 expression, which subsequently induces neuronal differentiation of OTX2-expressing cells. Treatment with 9-cis RA reduced the growth of D425 flank xenograft tumors in mice. In an intracranial model, however, MB tumors showed resistance to 9-cis RA treatment, and we implicated fibroblast growth factor (FGF) as a potential mediator of resistance to RA therapy. These findings suggest a mechanism for RA-mediated anti-tumor effect on OTX2-positive MB cells and indicate that therapeutic targeting of OTX2 might be effective if FGF pathway-mediated resistance can be overcome.

Preventive effects of bexarotene and budesonide in a genetically engineered mouse model of small cell lung cancer

In the present study, we examined the effect of bexarotene (Targretin) and budesonide in the chemoprevention of small cell lung carcinoma using a lung-specific knockout model of Rb1 and p53. Upon treatment with bexarotene, tumor incidence, number, and load were significantly reduced (P < 0.05). Budesonide treatment trended to inhibition, but the effect was not statistically significant (P > 0.05). Immunohistochemical staining indicated that bexarotene treatment decreased cell proliferation and increased apoptosis in tumors. The Rb1/p53 gene-targeted mouse seems to be a valuable model for chemopreventive studies on human small cell lung cancer. Our results indicate that the retinoid X receptor agonist bexarotene may be a potent chemopreventive agent in this cancer type.

Rexinoid bexarotene modulates triglyceride but not cholesterol metabolism via gene-specific permissivity of the RXR/LXR heterodimer in the liver

OBJECTIVE

Bexarotene (Targretin) is a clinically used antitumoral agent which exerts its action through binding to and activation of the retinoid-X-receptor (RXR). The most frequent side-effect of bexarotene administration is an increase in plasma triglycerides, an independent risk factor of cardiovascular disease. The molecular mechanism behind this hypertriglyceridemia remains poorly understood.

METHODS AND RESULTS

Using wild-type and LXR alpha/beta-deficient mice, we show here that bexarotene induces hypertriglyceridemia and activates hepatic LXR-target genes of lipogenesis in an LXR-dependent manner, hence exerting a permissive effect on RXR/LXR heterodimers. Interestingly, RNA analysis and Chromatin Immunoprecipitation assays performed in the liver reveal that the in vivo permissive effect of bexarotene on the RXR/LXR heterodimer is restricted to lipogenic genes without modulation of genes controlling cholesterol homeostasis.

CONCLUSIONS

These findings demonstrate that the hypertriglyceridemic action of bexarotene occurs via the RXR/LXR heterodimer and show that RXR heterodimers can act with a selective permissivity on target genes of specific metabolic pathways in the liver.

Potential of retinoic acid derivatives for the treatment of corticotroph pituitary adenomas

Cushing's disease is a severe clinical condition caused by hypersecretion of corticosteroids due to excessive ACTH secretion from a pituitary adenoma. This complex endocrine disorder still represents a major challenge for the physician in terms of efficient treatment. In the last years there was only little progress in elucidating the molecular mechanisms responsible for the constitutive and autonomous ACTH secretion of pituitary corticotrophinomas. As a consequence, no effective drug therapy is currently available, particularly if surgical excision is not successful. In the present article we examine recent studies that have investigated the therapeutic potential of retinoic acid receptors as nuclear receptor targets for the treatment of Cushing's disease. Retinoic acid is an efficient drug used for the treatment of different types of cancers and it proved to act in animal models of Cushing's disease. The efficiency of this treatment in patients with this disorder still needs to be tested in clinical trials.

Combination chemoprevention of HER2/neu-induced breast cancer using a cyclooxygenase-2 inhibitor and a retinoid X receptor-selective retinoid

The inducible prostaglandin synthase isoform cyclooxygenase-2 (COX-2) is overexpressed in approximately 40% of human breast carcinomas and in precancerous breast lesions, particularly in association with overexpression of human epidermal growth factor receptor 2 (HER2/neu). Experimental breast cancer can be suppressed by pharmacologic inhibition or genetic ablation of Cox-2, suggesting potential clinical utility of COX-2 inhibitors with respect to breast cancer. Importantly, several clinical trials have found reduced colorectal adenoma formation in individuals administered selective COX-2 inhibitors. However, such trials also identified increased cardiovascular risk associated with COX-2 inhibitor use. The goal of this research was to test whether improved chemopreventive efficacy could be achieved by combining submaximal doses of a selective COX-2 inhibitor and a retinoid X receptor-selective retinoid (rexinoid). The rate of HER2/neu-induced mammary tumor formation was substantially delayed by coadministration of the COX-2 inhibitor celecoxib (500 ppm in diet) and the rexinoid LGD1069 (10 mg/kg body weight; oral gavage) to MMTV/neu mice. Median time to tumor formation was increased from 304 to >600 days (P < 0.0001). The combination was substantially more effective than either drug individually. Similarly, potent suppression of aromatase activity was observed in mammary tissues from the combination cohort (44% of control; P < 0.001). Regulation of aromatase expression and activity by COX-derived prostaglandins is well established. Interestingly however, single agent LGD1069 significantly reduced mammary aromatase activity (71% of control; P < 0.001) without modulating eicosanoid levels. Our data show that simultaneous blockade of COX/prostaglandin signaling and retinoid X receptor-dependent transcription confers potent anticancer efficacy, suggesting a novel avenue for clinical evaluation.

Randomized phase III trial comparing bexarotene (L1069-49)/cisplatin/vinorelbine with cisplatin/vinorelbine in chemotherapy-naive patients with advanced or metastatic non-small-cell lung cancer: SPIRIT I

PURPOSE

This study evaluated whether the combination of the synthetic rexinoid bexarotene with first-line cisplatin/vinorelbine therapy provides additional survival benefit in patients with advanced non-small-cell lung cancer (NSCLC).

PATIENTS AND METHODS

Patients with stage IIIB with pleural effusion or stage IV NSCLC and Eastern Cooperative Oncology Group performance status 0 to 1 were randomly assigned to open-label bexarotene 400 mg/m(2)/d with cisplatin/vinorelbine or to cisplatin/vinorelbine alone. Antilipid agents were initiated on or before day 1 in the bexarotene arm. Primary efficacy end point was overall survival. Primary, secondary and supportive efficacy analyses were conducted.

RESULTS

A total of 623 patients (312 control, 311 bexarotene) were enrolled. Overall, no significant difference in survival occurred between the two treatment groups. However, an unplanned retrospective analysis showed that a subpopulation of bexarotene patients (n = 98 of 306) who experienced National Cancer Institute grade 3/4 hypertriglyceridemia had longer median survival compared with control patients (12.3 v 9.9 months; log-rank P = .08). Within that subgroup, those who benefited the most included males, smokers, those with stage IV disease, and those with a 6-month prior weight loss of 5% or more. Incidence, type and severity of grade 3/4 adverse events were comparable between arms, except for leukopenia (higher in chemotherapy arm) and hyperlipemia, hypothyroidism, dyspnea, and headache (higher in chemotherapy/bexarotene arm).

CONCLUSION

The addition of bexarotene to first-line chemotherapy did not increase survival in patients with advanced NSCLC. However, a subgroup (32%) of bexarotene-treated patients developing high-grade hypertriglyceridemia appeared to have better survival (12.3 months) than controls; thus triglyceride response may be a biomarker of survival benefit with bexarotene.