A novel atypical retinoid endowed with proapoptotic and antitumor activity

Reduced Levels of Lagging Strand Polymerases Shape Stem Cell Chromatin

Stem cells display asymmetric histone inheritance while non-stem progenitor cells exhibit symmetric patterns in the Drosophila male germline lineage. Here, we report that components involved in lagging strand synthesis, such as DNA polymerase α and δ (Polα and Polδ), have significantly reduced levels in stem cells compared to progenitor cells. Compromising Polα genetically induces the replication-coupled histone incorporation pattern in progenitor cells to be indistinguishable from that in stem cells, which can be recapitulated using a Polα inhibitor in a concentration-dependent manner. Furthermore, stem cell-derived chromatin fibers display a higher degree of old histone recycling by the leading strand compared to progenitor cell-derived chromatin fibers. However, upon reducing Polα levels in progenitor cells, the chromatin fibers now display asymmetric old histone recycling just like GSC-derived fibers. The old versus new histone asymmetry is comparable between stem cells and progenitor cells at both S-phase and M-phase. Together, these results indicate that developmentally programmed expression of key DNA replication components is important to shape stem cell chromatin. Furthermore, manipulating one crucial DNA replication component can induce replication-coupled histone dynamics in non-stem cells in a manner similar to that in stem cells.

The CHK1 inhibitor prexasertib in BRCA wild-type platinum-resistant recurrent high-grade serous ovarian carcinoma: a phase 2 trial

The multi-cohort phase 2 trial NCT02203513 was designed to evaluate the clinical activity of the CHK1 inhibitor (CHK1i) prexasertib in patients with breast or ovarian cancer. Here we report the activity of CHK1i in platinum-resistant high-grade serous ovarian carcinoma (HGSOC) with measurable and biopsiable disease (cohort 5), or without biopsiable disease (cohort 6). The primary endpoint was objective response rate (ORR). Secondary outcomes were safety and progression-free survival (PFS). 49 heavily pretreated patients were enrolled (24 in cohort 5, 25 in cohort 6). Among the 39 RECISTv1.1-evaluable patients, ORR was 33.3% in cohort 5 and 28.6% in cohort 6. Primary endpoint was not evaluable due to early stop of the trial. The median PFS was 4 months in cohort 5 and 6 months in cohort 6. Toxicity was manageable. Translational research was an exploratory endpoint. Potential biomarkers were investigated using pre-treatment fresh biopsies and serial blood samples. Transcriptomic analysis revealed high levels of DNA replication-related genes (POLA1, POLE, GINS3) associated with lack of clinical benefit [defined post-hoc as PFS < 6 months]. Subsequent preclinical experiments demonstrated significant cytotoxicity of POLA1 silencing in combination with CHK1i in platinum-resistant HGSOC cell line models. Therefore, POLA1 expression may be predictive for CHK1i resistance, and the concurrent POLA1 inhibition may improve the efficacy of CHK1i monotherapy in this hard-to-treat population, deserving further investigation.

Targeting breast cancer stem cells through retinoids: A new hope for treatment

Breast cancer is a complex and diverse disease accounting for nearly 30% of all cancers diagnosed in females. But unfortunately, patients develop resistance to the existing chemotherapeutic regimen, resulting in approximately 90% treatment failure. With over half a million deaths annually, it is imperative to explore new therapeutic approaches to combat the disease. Within a breast tumor, a small sub-population of heterogeneous cells, with a unique ability of self-renew and differentiation and responsible for tumor formation, initiation, and recurrence are referred to as breast cancer stem cells (BCSCs). These BCSCs have been identified as one of the main contributors to chemoresistance in breast cancer, making them an attractive target for developing novel therapeutic strategies. These cells exhibit surface biomarkers such as CD44+, CD24-/LOW, ALDH, CD133, and CD49f phenotypes. Higher expression of CD44+ and ALDH activity has been associated with the formation of tumors in various cancers. Moreover, the abnormal regulation of signaling pathways, including Hedgehog, Notch, β-catenin, JAK/STAT, and P13K/AKT/mTOR, leads to the formation of cancer stem cells, resulting in the development of tumors. The growing drug resistance in BC is a significant challenge, highlighting the need for new therapeutic strategies to combat this dreadful disease. Retinoids, a large group of synthetic derivatives of vitamin A, have been studied as chemopreventive agents in clinical trials and have been shown to regulate various crucial biological functions including vision, development, inflammation, and metabolism. On a cellular level, the retinoid activity has been well characterized and translated and is known to induce differentiation and apoptosis, which play important roles in the outcome of the transformation of tissues into malignant. Retinoids have been investigated extensively for their use in the treatment and prevention of cancer due to their high receptor-binding affinity to directly modulate gene expression programs. Therefore, in this study, we aim to summarize the current understanding of BCSCs, their biomarkers, and the associated signaling pathways. Retinoids, such as Adapalene, a third-generation retinoid, have shown promising anti-cancer potential and may serve as therapeutic agents to target BCSCs.

The Antitumor Effect of the DNA Polymerase Alpha Inhibitor ST1926 in Glioblastoma: A Proteomics Approach

Glioblastoma Multiforme (GBM) is the most aggressive form of malignant brain tumor. The median survival rate does not exceed two years, indicating an imminent need to develop novel therapies. The atypical adamantyl retinoid ST1926 induces apoptosis and growth inhibition in different cancer types. We have shown that ST1926 is an inhibitor of the catalytic subunit of DNA polymerase alpha (POLA1), which is involved in initiating DNA synthesis in eukaryotic cells. POLA1 levels are elevated in GBM versus normal brain tissues. Therefore, we studied the antitumor effects of ST1926 in several human GBM cell lines. We further explored the global protein expression profiles in GBM cell lines using liquid chromatography coupled with tandem mass spectrometry to identify new targets of ST1926. Low sub-micromolar concentrations of ST1926 potently decreased cell viability, induced cell damage and apoptosis, and reduced POLA1 protein levels in GBM cells. The proteomics profiles revealed 197 proteins significantly differentially altered upon ST1926 treatment of GBM cells involved in various cellular processes. We explored the differential gene and protein expression of significantly altered proteins in GBM compared to normal brain tissues.

Augmented Therapeutic Potential of EC-Synthetic Retinoids in Caco-2 Cancer Cells Using an In Vitro Approach

Colorectal cancer therapies have produced promising clinical responses, but tumor cells rapidly develop resistance to these drugs. It has been previously shown that EC19 and EC23, two EC-synthetic retinoids, have single-agent preclinical anticancer activity in colorectal carcinoma. Here, isobologram analysis revealed that they have synergistic cytotoxicity with retinoic acid receptor (RAR) isoform-selective agonistic retinoids such as AC261066 (RARβ2-selective agonist) and CD437 (RARγ-selective agonist) in Caco-2 cells. This synergism was confirmed by calculating the combination index (lower than 1) and the dose reduction index (higher than 1). Flow cytometry of combinatorial IC₅₀ (the concentration causing 50% cell death) confirmed the cell cycle arrest at the SubG₀-G₁ phase with potentiated apoptotic and necrotic effects. The reported synergistic anticancer activity can be attributed to their ability to reduce the expression of ATP-binding cassette (ABC) transporters including P-glycoprotein (P-gp1), breast cancer resistance protein (BCRP) and multi-drug resistance-associated protein-1 (MRP1) and Heat Shock Protein 70 (Hsp70). This adds up to the apoptosis-promoting activity of EC19 and EC23, as shown by the increased Caspase-3/7 activities and DNA fragmentation leading to DNA double-strand breaks. This study sheds the light on the possible use of EC-synthetic retinoids in the rescue of multi-drug resistance in colorectal cancer using Caco-2 as a model and suggests new promising combinations between different synthetic retinoids. The current in vitro results pave the way for future studies on these compounds as possible cures for colorectal carcinoma.

The unfolding role of ceramide in coordinating retinoid-based cancer therapy

Sphingolipid-mediated regulation in cancer development and treatment is largely ceramide-centered with the complex sphingolipid metabolic pathways unfolding as attractive targets for anticancer drug discovery. The dynamic interconversion of sphingolipids is tightly controlled at the level of enzymes and cellular compartments in response to endogenous or exogenous stimuli, such as anticancer drugs, including retinoids. Over the past two decades, evidence emerged that retinoids owe part of their potency in cancer therapy to modulation of sphingolipid metabolism and ceramide generation. Ceramide has been proposed as a 'tumor-suppressor lipid' that orchestrates cell growth, cell cycle arrest, cell death, senescence, autophagy, and metastasis. There is accumulating evidence that cancer development is promoted by the dysregulation of tumor-promoting sphingolipids whereas cancer treatments can kill tumor cells by inducing the accumulation of endogenous ceramide levels. Resistance to cancer therapy may develop due to a disrupted equilibrium between the opposing roles of tumor-suppressor and tumor-promoter sphingolipids. Despite the undulating effect and complexity of sphingolipid pathways, there are emerging opportunities for a plethora of enzyme-targeted therapeutic interventions that overcome resistance resulting from perturbed sphingolipid pathways. Here, we have revisited the interconnectivity of sphingolipid metabolism and the instrumental role of ceramide-biosynthetic and degradative enzymes, including bioactive sphingolipid products, how they closely relate to cancer treatment and pathogenesis, and the interplay with retinoid signaling in cancer. We focused on retinoid targeting, alone or in combination, of sphingolipid metabolism nodes in cancer to enhance ceramide-based therapeutics. Retinoid and ceramide-based cancer therapy using novel strategies such as combination treatments, synthetic retinoids, ceramide modulators, and delivery formulations hold promise in the battle against cancer.

Restoration of ceramide de novo synthesis by the synthetic retinoid ST1926 as it induces adult T-cell leukemia cell death

Ceramide (Cer) is a bioactive cellular lipid with compartmentalized and tightly regulated levels. Distinct metabolic pathways lead to the generation of Cer species with distinguishable roles in oncogenesis. Deregulation of Cer pathways has emerged as an important mechanism for acquired chemotherapeutic resistance. Adult T-cell leukemia (ATL) cells are defective in Cer synthesis. ATL is an aggressive neoplasm that develops following infection with human T-cell lymphotropic virus-1 (HTLV-1) where the viral oncogene Tax contributes to the pathogenesis of the disease. ATL cells, resistant to all-trans-retinoic acid, are sensitive to pharmacologically achievable concentrations of the synthetic retinoid ST1926. We studied the effects of ST1926 on Cer pathways in ATL cells. ST1926 treatment resulted in early Tax oncoprotein degradation in HTLV-1-treated cells. ST1926 induced cell death and a dose- and time-dependent accumulation of Cer in malignant T cells. The kinetics and degree of Cer production showed an early response upon ST1926 treatment. ST1926 enhanced de novo Cer synthesis via activation of ceramide synthase CerS(s) without inhibiting dihydroceramide desaturase, thereby accumulating Cer rather than the less bioactive dihydroceramide. Using labeling experiments with the unnatural 17-carbon sphinganine and measuring the generated Cer species, we showed that ST1926 preferentially induces the activities of a distinct set of CerS(s). We detected a delay in cell death response and interruption of Cer generation in response to ST1926 in Molt-4 cells overexpressing Bcl-2. These results highlight the potential role of ST1926 in inducing Cer levels, thus lowering the threshold for cell death in ATL cells.

Natural and synthetic retinoids in preclinical colorectal cancer models

Colorectal cancer (CRC) remains a leading cause of cancer-related morbidity and mortality worldwide. Although targeted therapy in combination with chemotherapy in CRC prolongs the overall survival of patients with metastatic disease, acquired resistance and relapse hinder their clinical benefits. Moreover, patients with some specific genetic profile are unlikely to benefit from targeted therapy, suggesting the need for safe and effective treatment strategies. Retinoids, comprising of natural and synthetic analogs, are a class of chemical compounds that regulate cellular proliferation, differentiation, and cell death. Retinoids have been used in the clinic for several leukemias and solid tumors, either as single agents or in combination therapy. Furthermore, retinoids have shown potent chemotherapeutic and chemopreventive properties in different cancer models, including CRC. In this review, we summarize the major preclinical findings in CRC in which natural and synthetic retinoids showed promising antitumor activities and stress on the proposed mechanisms of action. Understanding of the retinoids' antitumor mechanisms would provide insights to support and warrant their development in the management of CRC.

Novel adamantyl retinoid-related molecules with POLA1 inhibitory activity

Atypical retinoids (AR) or retinoid-related molecules (RRMs) represent a promising class of antitumor compounds. Among AR, E-3-(3'-adamantan-1-yl-4'-hydroxybiphenyl-4-yl)acrylic acid (adarotene), has been extensively investigated. In the present work we report the results of our efforts to develop new adarotene-related atypical retinoids endowed also with POLA1 inhibitory activity. The effects of the synthesized compounds on cell growth were determined on a panel of human and hematological cancer cell lines. The most promising compounds showed antitumor activity against several tumor histotypes and increased cytotoxic activity against an adarotene-resistant cell line, compared to the parent molecule. The antitumor activity of a selected compound was evaluated on HT-29 human colon carcinoma and human mesothelioma (MM487) xenografts. Particularly significant was the in vivo activity of the compound as a single agent compared to adarotene and cisplatin, against pleural mesothelioma MM487. No reduction of mice body weight was observed, thus suggesting a higher tolerability with respect to the parent compound adarotene.

Structure-Activity Relationship and Anticancer Profile of Second-Generation Anti-MRSA Synthetic Retinoids

We previously reported the antibacterial activity of CD437, a known antitumor compound. It proved to be a potent antimicrobial agent effective against both growing and persister cells of methicillin-resistant Staphylococcus aureus (MRSA). Herein, we report the synthesis of a panel of analogs and their effect on both MRSA and cancer cells. The hydrophobic group of the parent compound was varied in steric bulk, and lipid-mimicking analogs were tested. Biological assessment confirmed that the adamantane moiety is the most effective substitution for antibacterial activity, and some preferential action in cancer over MRSA was achieved.

The synthetic retinoid ST1926 attenuates prostate cancer growth and potentially targets prostate cancer stem-like cells

Retinoids are vitamin A derivatives that regulate crucial biological processes such as cellular proliferation, apoptosis, and differentiation. The use of natural retinoids in cancer therapy is limited due to their toxicity and the acquired resistance by cancer cells. Therefore, synthetic retinoids were developed, such as the atypical adamantyl retinoid ST1926 that provides enhanced bioavailability and reduced toxicity. We have assessed the in vitro and in vivo antitumor properties and mechanism of action of ST1926 in targeting cancer stem-like cells population of human prostate cancer (PCa) cell lines, DU145 and PC3, and mouse PCa cell lines, PLum-AD and PLum-AI. We demonstrated that ST1926 substantially reduced proliferation of PCa cells and induced cell cycle arrest, p53-independent apoptosis, and early DNA damage. It also decreased migration and invasion of PCa cells and significantly reduced prostate spheres formation ability in vitro denoting sufficient eradication of the self-renewal ability of the highly androgen-resistant cancer stem cells. Importantly, ST1926 potently inhibited PCa tumor growth and progression in vivo. Our results highlight the potential of ST1926 in PCa therapy and warrant its clinical development.

Role of the HTLV-1 viral factors in the induction of apoptosis

Adult T-cell leukemia (ATL) and HTLV-1-associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP) are the two main diseases that are caused by the HTLV-1 virus. One of the features of HTLV-1 infection is its resistance against programmed cell death, which maintains the survival of cells to oncogenic transformation and underlies the viruses' therapeutic resistance. Two main genes by which the virus develops cancer are Tax and HBZ; playing an essential role in angiogenesis in regulating viral transcription and modulating multiple host factors as well as apoptosis pathways. Here we have reviewed by prior research how the apoptosis pathways are suppressed by the Tax and HBZ and new drugs which have been designed to deal with this suppression.

The antitumor toxin CD437 is a direct inhibitor of DNA polymerase α

CD437 is a retinoid-like small molecule that selectively induces apoptosis in cancer cells, but not in normal cells, through an unknown mechanism. We used a forward-genetic strategy to discover mutations in POLA1 that coincide with CD437 resistance (POLA1(R)). Introduction of one of these mutations into cancer cells by CRISPR-Cas9 genome editing conferred CD437 resistance, demonstrating causality. POLA1 encodes DNA polymerase α, the enzyme responsible for initiating DNA synthesis during the S phase of the cell cycle. CD437 inhibits DNA replication in cells and recombinant POLA1 activity in vitro. Both effects are abrogated by the identified POLA1 mutations, supporting POLA1 as the direct antitumor target of CD437. In addition, we detected an increase in the total fluorescence intensity and anisotropy of CD437 in the presence of increasing concentrations of POLA1 that is consistent with a direct binding interaction. The discovery of POLA1 as the direct anticancer target for CD437 has the potential to catalyze the development of CD437 into an anticancer therapeutic.

The synthetic retinoid ST1926 as a novel therapeutic agent in rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most frequent soft tissue sarcoma in children. Despite multiple attempts at intensifying chemotherapeutic approaches to treatment, only moderate improvements in survival have been made for patients with advanced disease. Retinoic acid is a differentiation agent that has shown some antitumor efficacy in RMS cells in vitro; however, the effects are of low magnitude. E-3-(4'-hydroxyl-3'-adamantylbiphenyl-4-yl) acrylic acid (ST1926) is a novel orally available synthetic atypical retinoid, shown to have more potent activity than retinoic acid in several types of cancer cells. We used in vitro and in vivo models of RMS to explore the efficacy of ST1926 as a possible therapeutic agent in this sarcoma. We found that ST1926 reduced RMS cell viability in all tested alveolar (ARMS) and embryonal (ERMS) RMS cell lines, at readily achievable micromolar concentrations in mice. ST1926 induced an early DNA damage response (DDR), which led to increase in apoptosis, in addition to S-phase cell cycle arrest and a reduction in protein levels of the cell cycle kinase CDK1. Effects were irrespective of TP53 mutational status. Interestingly, in ARMS cells, ST1926 treatment decreased PAX3-FOXO1 fusion oncoprotein levels, and this suppression occurred at a post-transcriptional level. In vivo, ST1926 was effective in inhibiting growth of ARMS and ERMS xenografts, and induced a prominent DDR. We conclude that ST1926 has preclinical efficacy against RMS, and should be further developed in this disease in clinical trials.

Resistance to the atypical retinoid ST1926 in SK-N-AS cells selected the subline rAS-ST with enhanced sensitivity to ATRA mediated by not conventional mechanisms: DNA damage, G2 accumulation and late telomerase inhibition

BACKGROUND AND PURPOSE

13-cis-Retinoic acid represents a well-established clinical strategy for the management of minimal residual disease of high risk neuroblastoma (NB) patients. However, the clinical efficacy on the overall survival of these patients remains limited, addressing the issue of better understanding the molecular mechanisms and intracellular pathways mediating Retinoic Acid (RA) clinical effects.

EXPERIMENTAL APPROACH

This work investigates the mechanism underlying the sensitivity/resistance to RA in NB by taking advantage of the paired SK-N-AS/rAS-ST cells showing different responsivity to ATRA. The subline rAS-ST was selected by inducing resistance to the novel retinoid ST1926 in the NB SK-N-AS cell line.

KEY RESULTS

Resistance to ST1926 was neither dependent on cellular uptake nor on multi-drug resistance phenotype. Rather, both delayed/lower DNA damage and apoptosis appeared involved in reduced sensitivity of rAS-ST cells to ST1926. This subline showed enhanced responsivity to ATRA compared to the wt counterpart, that was associated with enhanced RARα/β expression, DNA damage, G2 accumulation, PI3K/AKT pathway inhibition, cellular differentiation and delayed telomerase inhibition, without involvement of either p27/p53 or caspase-mediated apoptosis.

CONCLUSIONS AND IMPLICATIONS

The present data add important information to the understanding of RA sensitivity in NB, providing further insights towards a more efficacious clinical use of this drug.

Preclinical efficacy of the synthetic retinoid ST1926 for treating adult T-cell leukemia/lymphoma

Adult T-cell leukemia/lymphoma (ATL) is an aggressive neoplasm caused by human T-cell leukemia virus type 1 (HTLV-1). The HTLV-1 oncoprotein Tax plays an important role in ATL pathogenesis. ATL carries a poor prognosis due to chemotherapy resistance, stressing the need for alternative therapies. Here, we investigate the preclinical efficacy of the synthetic retinoid ST1926 in ATL and peripheral T-cell lymphomas. Clinically achievable concentrations of ST1926 induced a dramatic inhibition of cell proliferation in malignant T-cell lines and primary ATL cells with minimal effect on resting or activated normal lymphocytes. ST1926 induced apoptosis, DNA damage, and upregulation of p53 proteins in malignant T cells, whereas it caused an early downregulation of Tax proteins in HTLV-1-positive cells. In murine ATL, oral treatment with ST1926 prolonged survival and reduced leukemia cell infiltration, white blood cell counts, and spleen mass. In spleens of ST1926-treated animals, p53 and p21 proteins were upregulated, poly (ADP-ribose) polymerase was cleaved, and Tax transcripts were reduced. These results highlight the promising use of ST1926 as a targeted therapy for ATL.

Cellular and Pharmacological Bases of the Antitumor Activity of a Novel Adamantyl Retinoid, ST1926

ST1926 is a novel related adamantyl retinoid endowed with potent antiproliferative and apoptogenic activity. The drug induced an early G1/S cell cycle arrest which was associated with a typical DNA damage response including modulation of genes involved in cell cycle regulation and DNA repair. The evidence of the drug ability to induce a significant extent of DNA strand breaks after short-term exposure is consistent with the cellular response. ST1926 is active by oral administration both on hematological and on solid tumors. The more marked antitumor effect showed by ST1926 in immuno-competent mice rather than in tumor xenografts suggests a contribution of indirect host-mediated antitumor effects in addition to a direct antiproliferative activity against tumor cells.

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.

Ion-exchange-resin-catalyzed adamantylation of phenol derivatives with adamantanols: Developing a clean process for synthesis of 2-(1-adamantyl)-4-bromophenol, a key intermediate of adapalene

A clean process has been developed for the synthesis of 2-adamantylphenol derivatives through adamantylation of substituted phenols with adamantanols catalyzed by commercially available and recyclable ion-exchange sulfonic acid resin in acetic acid. The sole byproduct of the adamantylation reaction, namely water, could be converted into the solvent acetic acid by addition of a slight excess of acetic anhydride during the work-up procedure, making the process waste-free except for regeneration of the ion-exchange resin, and facilitating the recycling of the resin catalyst. The ion-exchange sulfonic acid resin catalyst could be readily recycled by filtration and directly reused at least ten times without a significant loss of activity. The key intermediate of adapalene, 2-(1-adamantyl)-4-bromophenol, could be produced by means of this waste-free process.

Identification of a novel non-retinoid pan inverse agonist of the retinoic acid receptors

Retinoids are potent forms of vitamin A and are involved in a broad range of physiological processes and the pharmacological effects of retinoids are primarily mediated by the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). Several natural and synthetic RAR modulators have proven to be clinically useful for a number of therapeutic indications including cancer, psoriasis, and diabetes. Unfortunately, these agents lead to a number of significant side effects. Most synthetic retinoid ligands are based on the retinoid scaffold and thus have similarities to the natural ligand with all previously disclosed RAR ligands having a carboxylic acid that makes a critical ionic bridge within the ligand binding domain of the receptors. The potential therapeutic value offered from RAR modulation provides the impetus to identify novel ligands based on unique scaffolds that may offer improved toxicity and pharmacokinetic profiles. Here we describe the identification of an atypical RAR inverse agonist that represents the first non-acid, non-retinoid direct modulator of RAR receptor subfamily. SR-0065 functions as a pan-RAR inverse agonist suppressing the basal activity of RARα, RARβ, and RARγ, as well as inhibiting agonist-induced RAR activity. SR-0065 treatment enhanced receptor interaction with a peptide representative of the corepressor SMRT, and in cells SR-0065 enhances recruitment of SMRT to the promoter of the RARγ dependent gene, Cyp26A1. The acid form of SR-0065, SR-1758, was inactive in all assays. Thus, SR-0065 represents a new class of non-acid, non-retinoid RAR modulator that may be used as a point to initiate development of improved RAR-targeted drugs.

Enhanced cell cycle perturbation and apoptosis mediate the synergistic effects of ST1926 and ATRA in neuroblastoma preclinical models

Retinoic acid therapy is nowadays an important component of treatment for residual disease of stage IV neuroblastoma after multimodal therapy. Nevertheless, arising resistance and treatment toxicity could represent relevant limiting factors. In the present study, we show that retinoic acid enhances the cytostatic and apoptogenic properties of the novel adamantyl retinoid ST1926 in a panel of neuroblastoma cells with different p53 status and caspase 8 expression, resulting in synergistic effects as assessed by Combination Index and Isobologram analysis. Under conditions where the two drugs alone produced no toxic effects, their combination resulted in enhanced G2-M arrest and sub-G1 population as shown by BrdU pulse-chase and labeling experiments. PARP cleavage, caspase 3, 8 and 9 activation and modulation of DR4 and FAS were indicative of enhanced apoptosis triggered by the co-incubation of the two drugs whereas neither ST1926-mediated genotoxic damage nor ATRA-differentiating effects were affected by the combined treatment. Caspase-3 and 8-mediated apoptosis appeared to play an important role in the drugs synergism. In fact, the addition of a pan-caspase inhibitor ZVAD-FMK reverted this effect in SK-N-DZ cells, and synergism was confined to limited drugs doses in HTLA cells not expressing caspase-8. Although not modulated, p53 appeared to enhance cells responsiveness to retinoid/ATRA combination. In vivo studies in the most sensitive neuroblastoma model SK-N-DZ, confirmed enhanced activity of the drugs combination vs single treatments. The study provides important lines of evidence that such a drugs combination could represent a less toxic and more effective approach for maintenance treatment in children with neuroblastoma.

Insights into the binding mode and mechanism of action of some atypical retinoids as ligands of the small heterodimer partner (SHP)

The Small Heterodimer Partner (SHP) is an orphan nuclear receptor and an atypical member of the nuclear receptor superfamily Since its discovery, a growing body of evidences have pointed out a pivotal role for SHP in the transcriptional regulation of a variety of target genes involved in diverse metabolic pathways. While we have previously developed a homology model of the structure of SHP that was instrumental to identify a putative ligand binding pocket and suggest the possibility of the development of synthetic modulators, others reported that some atypical retinoids may represent the first synthetic ligands for this receptor. In this work, we report a combined computational approach aimed at shedding further lights on the binding mode and mechanism of action of some atypical retinoids as ligands of SHP. The results have been instrumental to design mutagenesis experiments whose preliminary data suggest the presence of a functional site in SHP as defined by residues Phe96, Arg138 and Arg238. While further experimental studies are ongoing, these findings constitute the basis for the design and identification of novel synthetic modulators of SHP functions.

Development and validation of a liquid chromatography-tandem mass spectrometry method for the determination of ST1926, a novel oral antitumor agent, adamantyl retinoid derivative, in plasma of patients in a Phase I study

E-3-(4'-Hydroxy-3'-adamantylbiphenyl-4-yl) acrylic acid (ST1926) is a novel oral synthetic adamantyl retinoid derivative, now under early clinical investigation in patients with ovarian cancer. To investigate the pharmacokinetics of this new antitumor agent in human plasma, we developed and validated a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method based on the addition of ST2222 as internal standard and simple protein precipitation with methanol. The method requires a small volume of sample (100microL); it is rapid and selective, allowing a good resolution of peaks from the plasma matrix in 9min. The method offers high recovery (>90%), it is sensitive, precise and accurate with overall precision expressed as CV% less than 8.2%. We set the lower limit of quantitation at 20.0ng/mL and validated the assay up to the concentration of 1000.0ng/mL. The present method has been successfully applied to study ST1926 pharmacokinetics in patients with advanced ovarian cancer in a Phase I trial, administering the drug orally for five consecutive days. During analysis of the study samples, it became evident the presence of glucuroconjugates of the parent compound, established by LC-Orbitrap MS. Preliminary results show low and variable drug absorption in patients, with extensive glucuroconjugation influencing the bioavailability of ST1926.

Highly twisted adamantyl arotinoids: synthesis, antiproliferative effects and RXR transactivation profiles

Retinoid-related molecules with an adamantyl group (adamantyl arotinoids) have been described with selective activities towards the retinoid receptors as agonists for NR1B2 and NR1B3 (RARbeta,gamma) (CD437, MX3350-1) or RAR antagonists (MX781) that induce growth arrest and apoptosis in cancer cells. Since these molecules induce apoptosis independently of RAR transactivation, we set up to synthesize novel analogs with impaired RAR binding. Here we describe adamantyl arotinoids with 2,2'-disubstituted biaryl rings prepared using the Suzuki coupling of the corresponding fragments. Those with cinnamic and naphthoic acid end groups showed significant antiproliferative activity in several cancer cell lines, and this effect correlated with the induction of apoptosis as measured by caspase activity. Strikingly, some of these compounds, whereas devoid of RAR binding capacity, were able to activate RXR.

Involvement of thioredoxin-binding protein 2 in the antitumor activity of CD437

The present authors previously reported that a synthetic retinoid, CD437, induces endoplasmic reticulum stress-mediated apoptosis in ovarian adenocarcinoma cells in spite of no response to natural retinoids. However, the precise mechanism of its proapoptotic action has not been fully determined. The present study herein demonstrates that apoptosis induction of ovarian adenocarcinoma SKOV3 cells by CD437 involves the upregulation of thioredoxin-binding protein 2 (TBP2) by a mechanism that is dependent on the intracellular calcium concentration. TBP2 is known to bind to and suppress thioredoxin (TRX) activity whereas TRX has an anti-apoptotic effect by inhibiting apoptosis signal-regulating kinase 1 (ASK1). The activation of ASK1 and its downstream molecule, c-Jun N-terminal kinase, was observed after induction of TBP2 by CD437. Interestingly, CD437 induced the association of TBP2 with TRX and, in turn, facilitated the dissociation of ASK1 from TRX. Moreover, blockade of TBP2 induction by small interfering RNA (siRNA) significantly attenuated the cytotoxic effect of CD437. These results suggest that TBP2 plays a critical role in the mechanism by which CD437 exerts proapoptotic action against SKOV3 cells.

Adamantyl-substituted retinoid-derived molecules that interact with the orphan nuclear receptor small heterodimer partner: effects of replacing the 1-adamantyl or hydroxyl group on inhibition of cancer cell growth, induction of cancer cell apoptosis, and inhibition of SRC homology 2 domain-containing protein tyrosine phosphatase-2 activity

(E)-4-[3-(1-Adamantyl)-4'-hydroxyphenyl]-3-chlorocinnamic acid (3-Cl-AHPC) induces the cell-cycle arrest and apoptosis of leukemia and cancer cells. Studies demonstrated that 3-Cl-AHPC bound to the atypical orphan nuclear receptor small heterodimer partner (SHP). Although missing a DNA-binding domain, SHP heterodimerizes with the ligand-binding domains of other nuclear receptors to repress their abilities to induce or inhibit gene expression. 3-Cl-AHPC analogues having the 1-adamantyl and phenolic hydroxyl pharmacophoric elements replaced with isosteric groups were designed, synthesized, and evaluated for their inhibition of proliferation and induction of human cancer cell apoptosis. Structure-anticancer activity relationship studies indicated the importance of both groups to apoptotic activity. Docking of 3-Cl-AHPC and its analogues to an SHP computational model that was based on the crystal structure of ultraspiracle complexed with 1-stearoyl-2-palmitoylglycero-3-phosphoethanolamine suggested why these 3-Cl-AHPC groups could influence SHP activity. Inhibitory activity against Src homology 2 domain-containing protein tyrosine phosphatase 2 (Shp-2) was also assessed. The most active Shp-2 inhibitor was found to be the 3'-(3,3-dimethylbutynyl) analogue of 3-Cl-AHPC.

RAR and RXR modulation in cancer and metabolic disease

Retinoic acid receptors (RARs) are ligand-controlled transcription factors that function as heterodimers with retinoid X receptors (RXRs) to regulate cell growth and survival. The success of RAR modulation in the treatment of acute promyelocytic leukaemia (APL) has stimulated considerable interest in the development of RAR and RXR modulators. This has been aided by recent advances in the understanding of the biological role of RARs and RXRs and in the design of selective receptor modulators that might overcome the limitations of current drugs. Here, we discuss the challenges and opportunities for therapeutic strategies based on RXR and RAR modulators, with a focus on cancer and metabolic diseases such as diabetes and obesity.

Antitumor activity of the combination of synthetic retinoid ST1926 and cisplatin in ovarian carcinoma models

BACKGROUND

The novel adamantyl retinoid ST1926 is a potent inducer of apoptosis in ovarian carcinoma cells. Since the pro-apoptotic effect is associated with activation of p53, in this study we have investigated the efficacy of combination of ST1926 with cisplatin, a DNA-damaging agent that is known to induce p53-dependent apoptosis.

MATERIALS AND METHODS

The efficacy of ST1926 and its combination with cisplatin was evaluated in human ovarian carcinoma models, including resistant tumors.

RESULTS

Oral treatment with ST1926 alone caused a marginal tumor growth inhibition (<50%), but the combination with cisplatin resulted in an improved efficacy, most evident in terms of tumor growth delay without a substantial increase of toxicity. The combination therapy achieved the best effects against the HOC18 ovarian carcinoma tumor, resulting in an appreciable number of animals without evidence of disease at the end of the experiment. In contrast to the marginal effect of ST1926 alone against the subcutaneous-growing tumors, loco-regional (intraperitoneal) treatment achieved a marked increase of survival of animals with ascitic IGROV-1 tumor.

CONCLUSIONS

The present results document the efficacy of the combination of cisplatin with ST1926 and provide a rational basis for the design of novel, well-tolerated platinum-based treatment approaches in human ovarian carcinoma.

Synthesis and structure-activity relationships of new antiproliferative and proapoptotic retinoid-related biphenyl-4-yl-acrylic acids

Atypical retinoids, or retinoid-related molecules (RRMs), represent a class of proapoptotic agents with a promising potential in the treatment of neoplastic diseases. In the present work, the synthesis and structure-activity relationship of a series of 3'-adamantan-1-yl-biphenyl-4-yl-acrylic acids substituted in ring A were studied. The synthesized compounds were evaluated for their antiproliferative activity in a human promyelocitic leukemia cell line (NB4), and in an ovarian carcinoma cell system including IGROV-1, carrying a functional wild-type p53, and a cisplatin-resistant subline, IGROV-1/Pt-1. The presence of at least one oxygenated substituent in positions 4' or 5' appears determinant for the antiproliferative activity. With two substituents of this kind the activity increases, particularly in the case of alkylenedioxy compounds. The activation of DNA damage response as indicated by phosphorylation of H2AX histone, RPA-2 protein, and p53 at serine 15 by the most apoptotic compounds provides additional support to the hypothesis that the genotoxic stress is a critical event mediating apoptosis induction by compounds of this group.

New Adamantane-Based Spiro 1,2,4-Trioxanes Orally Effective against Rodent and Simian Malaria

New 6-arylvinyl- and 6-adamantylvinyl-substituted 1,2,4-trioxanes (13a-g and 14a,b) have been prepared and evaluated for antimalarial activity against multidrug resistant Plasmodium yoelii nigeriensis in mice by both oral and intramuscular routes. While all the 6-arylvinyl-substituted trioxanes, 13a-f, showed promising activity, none of the 6-adamantylvinyl-substituted trioxanes, 13g and 14a,b, exhibited significant activity. Trioxane, 13f, the most active compound of the series, provided 100% and 80% protection to malaria-infected mice at 48 mg/kg x 4 days and 24 mg/kg x 4 days, respectively, by oral route. In this model, beta-arteether (3) provided 100% protection at 48 mg/kg x 4 days and only 20% protection at 24 mg/kg x 4 days. Trioxane 13f also showed complete suppression of parasitaemia at 10 mg/kg x 4 days by oral route in rhesus monkeys infected with P. cynomolgi. None of these trioxanes, except 13f, showed significant activity by the intramuscular route.

The novel atypical retinoid ST1926 is active in ATRA resistant neuroblastoma cells acting by a different mechanism

E-3-(4'-Hydroxy-3'-adamantylbiphenyl-4-yl)acrylic acid (ST1926) is a novel orally available compound belonging to the class of synthetic atypical retinoids. These agents are attracting growing attention because of their unique mechanism of antitumor action that appears different from that of classical retinoic acid. This study aims at investigating the antitumor activity of ST1926 in neuroblastoma (NB) preclinical models. In vitro, ST1926 was more cytotoxic than both its prototype, CD437 and all-trans-retinoic acid (ATRA) and it was active in the SK-N-AS cell line, which is refractory to ATRA. We showed that unlike ATRA, ST1926 does not induce morphological differentiation in NB cells where it produces indirect DNA damage, cell cycle arrest in late S-G2 phases and p53-independent programmed cell death. DNA damage was not mediated by oxidative stress and was repaired by 24h after drug removal. The SK-N-DZ cell line appeared the most sensitive to the proapoptotic activity of ST1926, probably because both the extrinsic and intrinsic pathways appear involved in the process. Studies with Z-VAD-FMK, suggested that ST1926 might also mediate caspase-independent apoptosis in NB cells. In vivo, orally administered ST1926, appeared to inhibit tumor growth of NB xenografts with tolerable toxicity. Overall, our results support the view that ST1926 might represent a good drug candidate in this pediatric tumor.

Antitumor activity of the retinoid-related molecules (E)-3-(4'-hydroxy-3'-adamantylbiphenyl-4-yl)acrylic acid (ST1926) and 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437) in F9 teratocarcinoma: Role of retinoic acid receptor gamma and retinoid-independent pathways

The retinoid-related molecules (RRMs) ST1926 [(E)-3-(4'-hydroxy-3'-adamantylbiphenyl-4-yl)acrylic acid] and CD437 (6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid) are promising anticancer agents. We compared the retinoic acid receptor (RAR) trans-activating properties of the two RRMs and all-trans-retinoic acid (ATRA). ST1926 and CD437 are better RARgamma agonists than ATRA. We used three teratocarcinoma cell lines to evaluate the significance of RARgamma in the activity of RRMs: F9-wild type (WT); F9gamma-/-, lacking the RARgamma gene; F9gamma51, aF9gamma-/-derivative, complemented for the RARgamma deficit. Similar to ATRA, ST1926 and CD437 activate cytodifferentiation only in F9-WT cells. Unlike ATRA, ST1926 and CD437 arrest cells in the G2/M phase of the cell cycle and induce apoptosis in all F9 cell lines. Our data indicate that RARgamma and the classic retinoid pathway are not relevant for the antiproliferative and apoptotic activities of RRMs in vitro. Increases in cytosolic calcium are fundamental for apoptosis, in that intracellular calcium chelators abrogate the process. Comparison of the gene expression profiles associated with ST1926 and ATRA in F9-WT and F9gamma-/-indicates that the RRM activates a conspicuous nonretinoid response in addition to the classic and RAR-dependent pathway. The pattern of genes regulated by ST1926 selectively, in a RARgamma-independent manner, provides novel insights into the possible molecular determinants underlying the activity of RRMs in vitro. Furthermore, it suggests that RARgamma-dependent responses are relevant to the activity of RRMs in vivo. Indeed, the receptor hinders the antitumor activity in vivo, in that both syngeneic and immunosuppressed SCID mice bearing F9gamma-/- tumors have increased life spans after treatment with ST1926 and CD437 relative to their F9-WT counterparts.

Development of resistance to the atypical retinoid, ST1926, in the lung carcinoma cell line H460 is associated with reduced formation of DNA strand breaks and a defective DNA damage response

Atypical retinoids are potent inducers of apoptosis, but activation of the apoptotic pathway seems to be independent of retinoid receptors. Previous studies with a novel adamantyl retinoid, ST1926, have shown that apoptosis induction is associated with an early genotoxic stress. To better understand the relevance of these events, we have selected a subline of the H460 lung carcinoma cell line resistant to ST1926. Resistant cells exhibited cross-resistance to a related molecule, CD437, but not cross-resistance to agents with different mechanisms of action. In spite of a lack of defects in intracellular drug accumulation, induction of DNA strand breaks in resistant cells required exposure to a substantially higher concentration, which was consistent with the degree of resistance. At drug concentrations causing a similar antiproliferative effect (IC80) and a comparable extent of DNA lesions in sensitive and resistant cells, the apoptotic response was a delayed and less marked event in resistant cells, thus indicating a reduced susceptibility to apoptosis. In spite of recognition of DNA lesions in resistant cells, as supported by phosphorylation of p53 and histone H2AX, resistant cells exhibited no activation of the mitochondrial pathways of apoptosis. Following exposure to equitoxic drug concentrations, only sensitive cells exhibited a typical stress/DNA damage response, with activation of the S-phase checkpoint. The cellular resistance to ST1926 reflects alterations responsible for a reduced generation of DNA lesions and for an enhanced tolerance of the genotoxic stress, resulting in lack of activation of the intrinsic pathway of apoptosis. The defective DNA damage response, accompanied by a reduced susceptibility to apoptosis in resistant cells, provides further support to the involvement of genotoxic stress as a critical event in mediating apoptosis induction by ST1926.

Synthesis and Structure−Activity Relationships of a New Series of Retinoid-Related Biphenyl-4-ylacrylic Acids Endowed with Antiproliferative and Proapoptotic Activity

Atypical retinoids (AR) represent a class of proapoptotic agents with promising potential in the treatment of neoplastic diseases. In the present work 4'-hydroxybiphenyl-4-ylacrylic acids were studied as a novel series of AR. The synthesized compounds were evaluated for their antiproliferative activity in a human promyelocytic leukemia cell line (NB4) and in an ovarian carcinoma cell system including IGROV-1, carrying a functional wild-type p53, and a cisplatin-resistant subline, IGROV-1/Pt-1. The presence of a bulky lipophilic group at position 3' (adamantan-1-yl being the best) and the E configuration of the acrylic moiety appear essential for activity below 1 muM. No substitution on the rings or on the double bond improved the activity. A qualitative correlation between the log P and molecular volume of the 3'-substituent and the antiproliferative activity was found. From the study of a few selected compounds, it appears that the presence of the carboxylic group is an essential requirement for apoptogenic properties but not for antiproliferative activity, this being maintained in amide derivatives. On the other hand, compounds able to induce apoptosis produced a detectable level of genotoxic damage. This observation supports the hypothesis that the genotoxic stress is a critical event mediating apoptosis induction by compounds of this class. Among the compounds investigated, E-3-(3'-adamantan-1-yl-4'-hydroxybiphenyl-4-yl)acrylic acid (2) was chosen for further investigation.

Modulation of survival signaling pathways and persistence of the genotoxic stress as a basis for the synergistic interaction between the atypical retinoid ST1926 and the epidermal growth factor receptor inhibitor ZD1839

Strategies targeting apoptotic pathways may have relevance to improve the efficacy of antitumor therapy. Because synthetic atypical retinoids are potent inducers of apoptosis, there is an increasing interest in exploiting their potential in novel therapeutic approaches. In the present study, we have investigated the cellular effects of the combination of a novel atypical retinoid, ST1926, and the epidermal growth factor receptor inhibitor ZD1839. The results indicated a synergistic interaction between the two drugs associated with a dramatic enhancement of apoptotic response, up-regulation of the cell death receptor DR5, and caspase 8 activation. Other molecular events induced by the cotreatment included (a) a stabilization of the ST1926-induced genotoxic stress detected by formation of phosphorylated gamma-H2AX foci and (b) a complete inhibition of extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation associated with activation of the proapoptotic protein BAD (i.e., inhibition of phosphorylation on Ser112). In addition, ZD1839 itself inhibited survival pathways by causing a partial dephosphorylation of Akt and a marked down-regulation of survivin. The role of ERK-mediated survival pathways in the cellular response to the drug combination was further supported by the counteracting effect of stimulation of survival pathways by an alternative receptor tyrosine kinase and by the use of a specific inhibitor of the ERK pathway. In conclusion, the results support that the survival pathways activated by epidermal growth factor receptor are determinants of the cell susceptibility to ST1926-induced apoptosis and lowering survival signals may increase the cellular sensitivity to the atypical retinoid. The favorable pharmacologic profiles of both ST1926 and ZD1839 suggest that the combination of these well-tolerated agents may have therapeutic potential.

Discovery of 4-Aryl-4H-chromenes as a New Series of Apoptosis Inducers Using a Cell- and Caspase-based High-Throughput Screening Assay. 1. Structure−Activity Relationships of the 4-Aryl Group

By applying a novel cell- and caspase-based HTS assay, 2-amino-3-cyano-7-(dimethylamino)-4-(3-methoxy-4,5-methylenedioxyphenyl)-4H-chromene (1a) has been identified as a potent apoptosis inducer. Compound 1a was found to induce nuclear fragmentation and PARP cleavage, as well as to arrest cells at the G(2)/M stage and to induce apoptosis as determined by the flow cytometry analysis assay in multiple human cell lines (e.g. Jurkat, T47D). Through structure-activity relationship (SAR) studies of the 4-aryl group, a 4- and 7-fold increase in potency was obtained from the screening hit 1a to the lead compounds 2-amino-4-(3-bromo-4,5-dimethoxyphenyl)-3-cyano-7-(dimethylamino)-4H-chromene (1c) and 2-amino-3-cyano-7-(dimethylamino)-4-(5-methyl-3-pyridyl)-4H-chromene (4e), with an EC(50) of 19 and 11 nM in the caspase activation assay in T47D breast cancer cells, respectively. The 2-amino-4-aryl-3-cyano-7-(dimethylamino)-4H-chromenes also were found to be highly active in the growth inhibition MTT assay, with GI(50) values in the low nanomolar range for compound 1c. Significantly, compound 1c was found to have a GI(50) value of 2 nM in the paclitaxel resistant, p-glycoprotein overexpressed, MES-SA/DX5 tumor cells. Functionally, compound 1c was found to be a potent inhibitor of tubulin polymerization and to effectively inhibit the binding of colchicine to tubulin. These results confirm that the cell-based caspase activation assay is a powerful tool for the discovery of potent apoptosis inducers and suggest that the 4-aryl-4H-chromenes have the potential to be developed into future anticancer agents.

Antagonist Analogue of 6-[3‘-(1-Adamantyl)-4‘-hydroxyphenyl]-2-naphthalenecarboxylic Acid (AHPN) Family of Apoptosis Inducers That Effectively Blocks AHPN-Induced Apoptosis but Not Cell-Cycle Arrest

The retinoid 6-[3'-(1-adamantyl)-4'-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN) and its active analogues induce cell-cycle arrest and programmed cell death (apoptosis) in cancer cells independently of retinoic acid receptor (RAR) interaction. Its analogue, (E)-4-[3'-(1-adamantyl)-4'-hydroxyphenyl]-3-(3'-acetamidopropyloxy)cinnamic acid (3-A-AHPC) selectively antagonized cell apoptotic events (TR3/nur77/NGFI-B expression and nuclear-to-mitochondrial translocation) but not the proliferative events (cell-cycle arrest and p21(WAF1/CIP1) expression) induced by proapoptotic AHPN and its analogues. The syntheses of 3-A-AHPC and proapoptotic (E)-6-[3'-(1-adamantyl)-4'-hydroxyphenyl]-5-chloronaphthalenecarboxylic acid (5-Cl-AHPN) are described. Computational studies on AHPN, AHPC, and three substituted analogues (5-Cl-AHPN, 3-Cl-AHPC, and 3-A-AHPC) suggested reasons for their diametric effects on RAR activation. Density functional theory studies indicated that the 1-adamantyl (1-Ad) groups of the AHPN and AHPC configurations assumed positions that were nearly planar with the aromatic rings of their polar termini. In contrast, in the configurations of the substituted analogues having chloro and 3-acetamidopropyloxy groups, rather than a hydrogen, ortho to the diaryl bonds, the diaryl bond torsion angles increased so that the 1-Ad groups were oriented out of this plane. Docking and molecular dynamics of AHPN, AHPC, and these substituted analogues in the RARgamma ligand-binding domain illustrated how specific substituents on the AHPN and AHPC scaffolds modulated the positions and dynamics of the 1-Ad groups. As a result, the position of RARgamma helix H12 in forming the coactivator-binding site was impacted in a manner consistent with the experimental effect of each analogue on RARgamma transcriptional activation.