Identification of Mithramycin Analogues with Improved Targeting of the EWS-FLI1 Transcription Factor

Mithramycin and its analogs: Molecular features and antitumor action

The antitumor antibiotic mithramycin A (MTA) binds to G/C-rich DNA sequences in the presence of dications. MTA inhibits transcription regulated by the Sp1 transcription factor, often enhanced during tumor development. It shows antitumor activity, but its clinical use was discontinued due to toxic side effects. However, recent observations have led to its use being reconsidered. The MTA biosynthetic pathways have been modified to produce mithramycin analogs (mithralogs) that encompass lower toxicity and improved pharmacological activity. Some mithralogs reduce gene expression in human ovarian and prostate tumors, among other types of cancer. They down-regulate gene expression in various cellular processes, including Sp1-responsive genes that control tumor development. Moreover, MTA and several mithralogs, such as EC-8042 (DIG-MSK) and EC-8105, effectively treat Ewing sarcoma by inhibiting transcription controlled by the oncogenic EWS-FLI1 transcription factor.

Enhancing Standard of Care Chemotherapy Efficacy Using DNA-Dependent Protein Kinase (DNA-PK) Inhibition in Preclinical Models of Ewing Sarcoma

Disruption of DNA damage repair via impaired homologous recombination is characteristic of Ewing sarcoma (EWS) cells. We hypothesize that this disruption results in increased reliance on nonhomologous end joining to repair DNA damage. In this study, we investigated if pharmacologic inhibition of the enzyme responsible for nonhomologous end joining, the DNA-PK holoenzyme, alters the response of EWS cells to genotoxic standard of care chemotherapy. We used analyses of cell viability and proliferation to investigate the effects of clinical DNA-PK inhibitors (DNA-PKi) in combination with six therapeutic or experimental agents for EWS. We performed calculations of synergy using the Loewe additivity model. Immunoblotting evaluated treatment effects on DNA-PK, DNA damage, and apoptosis. Flow cytometric analyses evaluated effects on cell cycle and fate. We used orthotopic xenograft models to interrogate tolerability, drug mechanism, and efficacy in vivo. DNA-PKi demonstrated on-target activity, reducing phosphorylated DNA-PK levels in EWS cells. DNA-PKi sensitized EWS cell lines to agents that function as topoisomerase 2 (TOP2) poisons and enhanced the DNA damage induced by TOP2 poisons. Nanomolar concentrations of single-agent TOP2 poisons induced G2M arrest and little apoptotic response while adding DNA-PKi-mediated apoptosis. In vivo, the combination of AZD7648 and etoposide had limited tolerability but resulted in enhanced DNA damage, apoptosis, and EWS tumor shrinkage. The combination of DNA-PKi with standard of care TOP2 poisons in EWS models is synergistic, enhances DNA damage and cell death, and may form the basis of a promising future therapeutic strategy for EWS.

AMPK-SP1-Guided Dynein Expression Represents a New Energy-Responsive Mechanism and Therapeutic Target for Diabetic Nephropathy

Key Points

AMP kinase senses diabetic stresses in podocytes, subsequently upregulates specificity protein 1–mediated dynein expression and promotes podocyte injury. Pharmaceutical restoration of dynein expression by targeting specificity protein 1 represents an innovative therapeutic strategy for diabetic nephropathy.

Background

Diabetic nephropathy (DN) is a major complication of diabetes. Injury to podocytes, epithelial cells that form the molecular sieve of a kidney, is a preclinical feature of DN. Protein trafficking mediated by dynein, a motor protein complex, is a newly recognized pathophysiology of diabetic podocytopathy and is believed to be derived from the hyperglycemia-induced expression of subunits crucial for the transportation activity of the dynein complex. However, the mechanism underlying this transcriptional signature remains unknown.

Methods

Through promoter analysis, we identified binding sites for transcription factor specificity protein 1 (SP1) as the most shared motif among hyperglycemia-responsive dynein genes. We demonstrated the essential role of AMP-activated protein kinase (AMPK)–regulated SP1 in the transcription of dynein subunits and dynein-mediated trafficking in diabetic podocytopathy using chromatin immunoprecipitation quantitative PCR and live cell imaging. SP1-dependent dynein-driven pathogenesis of diabetic podocytopathy was demonstrated by pharmaceutical intervention with SP1 in a mouse model of streptozotocin-induced diabetes.

Results

Hyperglycemic conditions enhance SP1 binding to dynein promoters, promoted dynein expression, and enhanced dynein-mediated mistrafficking in cultured podocytes. These changes can be rescued by chemical inhibition or genetic silencing of SP1. The direct repression of AMPK, an energy sensor, replicates hyperglycemia-induced dynein expression by activating SP1. Mithramycin inhibition of SP1-directed dynein expression in streptozotocin-induced diabetic mice protected them from developing podocytopathy and prevented DN progression.

Conclusions

Our work implicates AMPK-SP1–regulated dynein expression as an early mechanism that translates energy disturbances in diabetes into podocyte dysfunction. Pharmaceutical restoration of dynein expression by targeting SP1 offers a new therapeutic strategy to prevent DN.

Differential Expression of SRY-Related HMG-Box Transcription Factor 2, Oligodendrocyte Lineage Transcription Factor 2, and Zinc Finger E-Box Binding Homeobox 1 in Serum-Derived Extracellular Vesicles: Implications for Mithramycin Sensitivity and Targeted Therapy in High-Grade Glioma

Glioblastoma multiforme (GBM) is the most aggressive type of glioma and is often resistant to traditional therapies. Evidence suggests that glioma stem cells (GSCs) contribute to this resistance. Mithramycin (Mit-A) targets GSCs and exhibits antitumor activity in GBM by affecting transcriptional targets such as SRY-related HMG-box transcription factor 2 (SOX2), oligodendrocyte lineage transcription factor 2 (OLIG2), and zinc finger E-box binding homeobox 1 (ZEB1). However, its clinical use has been limited by toxicity. This study explored the diagnostic potential of serum extracellular vesicles (EVs) to identify Mit-A responders. Serum EVs were isolated from 70 glioma patients, and targeted gene expression was analyzed using qRT-PCR. Using chemosensitivity assay, we identified 8 Mit-A responders and 17 nonresponders among 25 glioma patients. The M-score showed a significant correlation (p = 0.045) with isocitrate dehydrogenase 1 mutation but not other clinical variables. The genes SOX2 (p = 0.005), OLIG2 (p = 0.003), and ZEB1 (p = 0.0281) were found to be upregulated in the responder EVs. SOX2 had the highest diagnostic potential (AUC = 0.875), followed by OLIG2 (AUC = 0.772) and ZEB1 (AUC = 0.632).The combined gene panel showed significant diagnostic efficacy (AUC = 0.956) through logistic regression analysis. The gene panel was further validated in the serum EVs of 45 glioma patients. These findings highlight the potential of Mit-A as a targeted therapy for high-grade glioma based on differential gene expression in serum EVs. The gene panel could serve as a diagnostic tool to predict Mit-A sensitivity, offering a promising approach for personalized treatment strategies and emphasizing the role of GSCs in therapeutic resistance.

ELK4 Promotes Colorectal Cancer Progression by Activating the Neoangiogenic Factor LRG1 in a Noncanonical SP1/3-Dependent Manner

Although the MAPK/MEK/ERK pathway is prevalently activated in colorectal cancer (CRC), MEK/ERK inhibitors show limited efficiency in clinic. As a downstream target of MAPK, ELK4 is thought to work primarily by forming a complex with SRF. Whether ELK4 can serve as a potential therapeutic target is unclear and the transcriptional regulatory mechanism has not been systemically analyzed. Here, it is shown that ELK4 promotes CRC tumorigenesis. Integrated genomics- and proteomics-based approaches identified SP1 and SP3, instead of SRF, as cooperative functional partners of ELK4 at genome-wide level in CRC. Serum-induced phosphorylation of ELK4 by MAPKs facilitated its interaction with SP1/SP3. The pathological neoangiogenic factor LRG1 is identified as a direct target of the ELK4-SP1/SP3 complex. Furthermore, targeting the ELK4-SP1/SP3 complex by combination treatment with MEK/ERK inhibitor and the relatively specific SP1 inhibitor mithramycin A (MMA) elicited a synergistic antitumor effect on CRC. Clinically, ELK4 is a marker of poor prognosis in CRC. A 9-gene prognostic model based on the ELK4-SP1/3 complex-regulated gene set showed robust prognostic accuracy. The results demonstrate that ELK4 cooperates with SP1 and SP3 to transcriptionally regulate LRG1 to promote CRC tumorigenesis in an SRF-independent manner, identifying the ELK4-SP1/SP3 complex as a potential target for rational combination therapy.

Cell cycle arrest and apoptosis are early events in radiosensitization of EWS::FLI1+ Ewing sarcoma cells by Mithramycin A

PURPOSE

The oncogenic fusion protein EWS::FLI1 is an attractive therapeutic target in Ewing sarcoma (ES). Mithramycin A (MithA) is a potent and specific inhibitor of EWS::FLI1 that can selectively radiosensitize ES cells through transcriptional inhibition of DNA double-strand break (DSB) repair. Here, we evaluate temporal changes in cell cycle progression and apoptosis in ES cells treated with MithA and/or ionizing radiation (RTx), testing the hypothesis that combining MithA with ionizing radiation would synergistically impair cell cycle progression and enhance apoptotic elimination to a greater extent than either agent alone.

MATERIALS AND METHODS

Four EWS::FLI1+ ES cell lines TC-71, RD-ES, SK-ES-1, and A673, and one EWS::ERG cell line (CHLA-25) were exposed to 10nM MithA or vehicle and followed 24 h later by exposure to 2 Gy x-radiation or sham irradiation. Reactive oxygen species (ROS) activity was evaluated by cytometric assay, and assay of antioxidant gene expression by RT-qPCR. Cell cycle changes were evaluated by flow cytometry of nuclei stained with propidium iodide. Apoptosis was assessed by cytometric assessment of Caspase-3/7 activity and by immunoblotting of PARP-1 cleavage. Radiosensitization was evaluated by clonogenic survival assay. Proliferation (EdU) and apoptosis (TUNEL) were evaluated in SK-ES-1 xenograft tumors following pretreatment with 1 mg/kg MithA, followed 24 h later by a single 4 Gy fraction of x-radiation.

RESULTS

MithA-treated cells showed reduced levels of ROS, and were associated with increased expression of antioxidant genes SOD1, SOD2, and CAT. It nonetheless induced persistent G0/G1 arrest and a progressive increase of the sub-G1 fraction, suggesting apoptotic degeneration. In vitro assays of Caspase-3/7 activity and immunoblotting of Caspase-3/7 dependent cleavage of PARP-1 indicated that apoptosis began as early as 24 h after MithA exposure, reducing clonogenic survival. Tumors from xenograft mice treated with either radiation alone, or in combination with MithA showed a significant reduction of tumor cell proliferation, while apoptosis was significantly increased in the group receiving the combination of MithA and RTx.

CONCLUSIONS

Taken together, our data show that the anti-proliferative and cytotoxic effects of MithA are the prominent components of radiosensitization of EWS::FLI1+ ES, rather than the result of acutely enhanced ROS levels.

Shifting from a Biological-Agnostic Approach to a Molecular-Driven Strategy in Rare Cancers: Ewing Sarcoma Archetype

Sarcomas of the thoracic cavity are rare entities that predominantly affect children and young adults. They can be very heterogeneous encompassing several different histological entities. Ewing Sarcoma (ES) can potentially arise from every bone, soft tissue, or visceral site in the body. However, it represents an extremely rare finding when it affects the thoracic cavity. It represents the second most frequent type of thoracic sarcoma, after chondrosarcoma. ES arises more frequently in sites that differ from the thoracic cavity, but it displays the same biological features and behavior of extra-thoracic ones. Current management of ES often requires a multidisciplinary treatment approach including surgery, radiotherapy, and systemic therapy, as it can guarantee local and distant disease control, at least transiently, although the long-term outcome remains poor. Unfortunately, due to the paucity of clinical trials purposely designed for this rare malignancy, there are no optimal strategies that can be used for disease recurrence. As a result of its complex biological features, ES might be suitable for emerging biology-based therapeutic strategies. However, a deeper understanding of the molecular mechanisms driving tumor growth and treatment resistance, including those related to oncogenic pathways, epigenetic landscape, and immune microenvironment, is necessary in order to develop new valid therapeutic opportunities. Here, we provide an overview of the most recent therapeutic advances for ES in both the preclinical and clinical settings. We performed a review of the current available literature and of the ongoing clinical trials focusing on new treatment strategies, after failure of conventional multimodal treatments.

Mithplatins: Mithramycin SA-Pt(II) Complex Conjugates for the Treatment of Platinum-Resistant Ovarian Cancers

DNA coordinating platinum (Pt) containing compounds cisplatin and carboplatin have been used for the treatment of ovarian cancer therapy for four decades. However, recurrent Pt-resistant cancers are a major cause of mortality. To combat Pt-resistant ovarian cancers, we designed and synthesized a conjugate of an anticancer drug mithramycin with a reactive Pt(II) bearing moiety, which we termed mithplatin. The conjugates displayed both the Mg2+ -dependent noncovalent DNA binding characteristic of mithramycin and the covalent crosslinking to DNA of the Pt. The conjugate was three times as potent as cisplatin against ovarian cancer cells. The DNA lesions caused by the conjugate led to the generation of DNA double-strand breaks, as also observed with cisplatin. Nevertheless, the conjugate was highly active against both Pt-sensitive and Pt-resistant ovarian cancer cells. This study paves the way to developing mithplatins to combat Pt-resistant ovarian cancers.

Management of Unresectable Localized Pelvic Bone Sarcomas: Current Practice and Future Perspectives

Simple Summary

Some locally advanced pelvic bone tumors are deemed unresectable and, as such, not suitable for curative surgery. In this setting, treatment options are generally limited and not unanimous, with decisions being made on an individual basis after multidisciplinary discussion. Ultimately, and notwithstanding the bright prospects raised by novel therapeutic approaches, treatment should be patient-tailored, weighing a panoply of patient- and tumor-related factors.

Abstract

Bone sarcomas (BS) are rare mesenchymal tumors usually located in the extremities and pelvis. While surgical resection is the cornerstone of curative treatment, some locally advanced tumors are deemed unresectable and hence not suitable for curative intent. This is often true for pelvic sarcoma due to anatomic complexity and proximity to vital structures, making treatment options for these tumors generally limited and not unanimous, with decisions being made on an individual basis after multidisciplinary discussion. Several studies have been published in recent years focusing on innovative treatment options for patients with locally advanced sarcoma not amenable to local surgery. The present article reviews the evidence regarding the treatment of patients with locally advanced and unresectable pelvic BS, with the goal of providing an overview of treatment options for the main BS histologic subtypes involving this anatomic area and exploring future therapeutic perspectives. The management of unresectable localized pelvic BS represents a major challenge and is hampered by the lack of comprehensive and standardized guidelines. As such, the optimal treatment needs to be individually tailored, weighing a panoply of patient- and tumor-related factors. Despite the bright prospects raised by novel therapeutic approaches, the role of each treatment option in the therapeutic armamentarium of these patients requires solid clinical evidence before becoming fully established.

Establishment and characterization of a novel patient-derived Ewing sarcoma cell line, NCC-ES2-C1

Ewing sarcoma (ES) is a small round cell sarcoma that is characterized by the unique gene translocation EWSR1-FLI1. It is the second most common primary bone and soft tissue malignancy in children and adolescents. It constitutes 10-15% of all bone sarcomas and is highly aggressive and rapidly recurring. Although intensive treatments have improved the clinical outcome of ES patients, 20-25% of them exhibit metastases during diagnosis. Thus, the prognoses of these patients remain poor. Cell lines are pivotal resources to investigate the molecular background of disease progression and to develop novel therapeutic modalities. In this study, we established and characterized a novel ES cell line, NCC-ES2-C1. The presence of the EWSR1-FLI1 fusion gene in these cells was confirmed in the NCC-ES2-C1 cells. Furthermore, these cells exhibited constant proliferation, and invasion, but did not form tumors in mice. We screened the anti-tumor effects of 214 anti-cancer drugs in NCC-ES2-C1 cells and found that the drugs which effectively reduced the proliferation of NCC-ES2-C1 cells. We concluded that NCC-ES2-C1 cells are a useful resource to study functions of the EWSR1-FLI1 fusion gene, investigate phenotypic changes caused by genes and proteins, and evaluate the anti-tumor effects of novel drugs.

One oncogene, several vulnerabilities: EWS/FLI targeted therapies for Ewing sarcoma

EWS/FLI is the defining mutation of Ewing sarcoma. This oncogene drives malignant transformation and progression and occurs in a genetic background characterized by few other recurrent cooperating mutations. In addition, the tumor is absolutely dependent on the continued expression of EWS/FLI to maintain the malignant phenotype. However, EWS/FLI is a transcription factor and therefore a challenging drug target. The difficulty of directly targeting EWS/FLI stems from unique features of this fusion protein as well as the network of interacting proteins required to execute the transcriptional program. This network includes interacting proteins as well as upstream and downstream effectors that together reprogram the epigenome and transcriptome. While the vast number of proteins involved in this process challenge the development of a highly specific inhibitors, they also yield numerous therapeutic opportunities. In this report, we will review how this vast EWS-FLI transcriptional network has been exploited over the last two decades to identify compounds that directly target EWS/FLI and/or associated vulnerabilities.

A targeted combinatorial therapy for Ewing's sarcoma

Ewing's sarcoma (EwS) is the second most common bone cancer in children and adolescents. Current chemotherapy regimens are mainly ineffective in patients with relapsed disease and cause long-term effects in survivors. Therefore, we have developed a combinatorial therapy based on a novel drug candidate named ML111 that exhibits selective activity against EwS cells and synergizes with vincristine. To increase the aqueous solubility of hydrophobic ML111, polymeric nanoparticles (ML111-NP) were developed. In vitro data revealed that ML111-NP compromise viability of EwS cells without affecting non-malignant cells. Furthermore, ML111-NP exhibit strong synergistic effects in a combination with vincristine on EwS cells, while this drug pair exhibits antagonistic effects towards normal cells. Finally, animal studies validated that ML111-NP efficiently accumulate in orthotopic EwS xenografts after intravenous injection and provide superior therapeutic outcomes in a combination with vincristine without evident toxicity. These results support the potential of the ML111-based combinatorial therapy for EwS.

Mithramycin delivery systems to develop effective therapies in sarcomas

BACKGROUND

Sarcomas comprise a group of aggressive malignancies with very little treatment options beyond standard chemotherapy. Reposition of approved drugs represents an attractive approach to identify effective therapeutic compounds. One example is mithramycin (MTM), a natural antibiotic which has demonstrated a strong antitumour activity in several tumour types, including sarcomas. However, its widespread use in the clinic was limited by its poor toxicity profile.

RESULTS

In order to improve the therapeutic index of MTM, we have loaded MTM into newly developed nanocarrier formulations. First, polylactide (PLA) polymeric nanoparticles (NPs) were generated by nanoprecipitation. Also, liposomes (LIP) were prepared by ethanol injection and evaporation solvent method. Finally, MTM-loaded hydrogels (HG) were obtained by passive loading using a urea derivative non-peptidic hydrogelator. MTM-loaded NPs and LIP display optimal hydrodynamic radii between 80 and 105 nm with a very low polydispersity index (PdI) and encapsulation efficiencies (EE) of 92 and 30%, respectively. All formulations show a high stability and different release rates ranging from a fast release in HG (100% after 30 min) to more sustained release from NPs (100% after 24 h) and LIP (40% after 48 h). In vitro assays confirmed that all assayed MTM formulations retain the cytotoxic, anti-invasive and anti-stemness potential of free MTM in models of myxoid liposarcoma, undifferentiated pleomorphic sarcoma and chondrosarcoma. In addition, whole genome transcriptomic analysis evidenced the ability of MTM, both free and encapsulated, to act as a multi-repressor of several tumour-promoting pathways at once. Importantly, the treatment of mice bearing sarcoma xenografts showed that encapsulated MTM exhibited enhanced therapeutic effects and was better tolerated than free MTM.

CONCLUSIONS

Overall, these novel formulations may represent an efficient and safer MTM-delivering alternative for sarcoma treatment.

Targeted and immuno-based therapies in sarcoma: mechanisms and advances in clinical trials

Sarcomas represent a distinct group of rare malignant tumors with high heterogeneity. Limited options with clinical efficacy for the metastatic or local advanced sarcoma existed despite standard therapy. Recently, targeted therapy according to the molecular and genetic phenotype of individual sarcoma is a promising option. Among these drugs, anti-angiogenesis therapy achieved favorable efficacy in sarcomas. Inhibitors targeting cyclin-dependent kinase 4/6, poly-ADP-ribose polymerase, insulin-like growth factor-1 receptor, mTOR, NTRK, metabolisms, and epigenetic drugs are under clinical evaluation for sarcomas bearing the corresponding signals. Immunotherapy represents a promising and favorable method in advanced solid tumors. However, most sarcomas are immune "cold" tumors, with only alveolar soft part sarcoma and undifferentiated pleomorphic sarcoma respond to immune checkpoint inhibitors. Cellular therapies with TCR-engineered T cells, chimeric antigen receptor T cells, tumor infiltrating lymphocytes, and nature killer cells transfer show therapeutic potential. Identifying tumor-specific antigens and exploring immune modulation factors arguing the efficacy of these immunotherapies are the current challenges. This review focuses on the mechanisms, advances, and potential strategies of targeted and immune-based therapies in sarcomas.

Therapeutic Potential of EWSR1-FLI1 Inactivation by CRISPR/Cas9 in Ewing Sarcoma

Simple Summary

Ewing sarcoma is an aggressive tumor with still unacceptable survival rates, particularly in patients with metastatic disease and for which it is necessary to develop new and innovative therapies. These tumors are characterized by the presence of chromosomal translocations that give rise to chimeric transcription factors (i.e., EWSR1–FLI1) that govern the oncogenic process. In this article, we describe an efficient strategy to permanently inactivate the EWSR1–FLI1 oncogene characteristic of Ewing sarcoma using CRISPR/Cas9 gene editing technology. Although the application of gene therapy in cancer still has many limitations, for example, the strategy for delivery, studies like ours show that gene therapy can be a promising alternative, particularly for those tumors that are highly dependent on a particular oncogene as is the case in Ewing sarcoma.

Abstract

Ewing sarcoma is an aggressive bone cancer affecting children and young adults. The main molecular hallmark of Ewing sarcoma are chromosomal translocations that produce chimeric oncogenic transcription factors, the most frequent of which is the aberrant transcription factor EWSR1–FLI1. Because this is the principal oncogenic driver of Ewing sarcoma, its inactivation should be the best therapeutic strategy to block tumor growth. In this study, we genetically inactivated EWSR1–FLI1 using CRISPR-Cas9 technology in order to cause permanent gene inactivation. We found that gene editing at the exon 9 of FLI1 was able to block cell proliferation drastically and induce senescence massively in the well-studied Ewing sarcoma cell line A673. In comparison with an extensively used cellular model of EWSR1–FLI1 knockdown (A673/TR/shEF), genetic inactivation was more effective, particularly in its capability to block cell proliferation. In summary, genetic inactivation of EWSR1–FLI1 in A673 Ewing sarcoma cells blocks cell proliferation and induces a senescence phenotype that could be exploited therapeutically. Although efficient and specific in vivo CRISPR-Cas9 editing still presents many challenges today, our data suggest that complete inactivation of EWSR1–FLI1 at the cell level should be considered a therapeutic approach to develop in the future.

Insight into mithramycin disruption of ETS transcription leads to improved understanding of more selective analogs

Fusion products with the ETS family of transcription factors play critical roles in the etiology of several cancers. In this issue of Structure, Hou et al. (2020) provide insight into allosteric mechanisms by which mithramycin and its analogs perturb protein-DNA interactions in higher-order complexes at a DNA enhancer site.

Nano-Encapsulation of Mithramycin in Transfersomes and Polymeric Micelles for the Treatment of Sarcomas

Sarcomas are aggressive tumors which often show a poor response to current treatments. As a promising therapeutic alternative, we focused on mithramycin (MTM), a natural antibiotic with a promising anti-tumor activity but also a relevant systemic toxicity. Therefore, the encapsulation of MTM in nano-delivery systems may represent a way to increase its therapeutic window. Here, we designed novel transfersomes and PLGA polymeric micelles by combining different membrane components (phosphatidylcholine, Span 60, Tween 20 and cholesterol) to optimize the nanoparticle size, polydispersity index (PDI) and encapsulation efficiency (EE). Using both thin film hydration and the ethanol injection methods we obtained MTM-loaded transferosomes displaying an optimal hydrodynamic diameter of 100–130 nm and EE values higher than 50%. Additionally, we used the emulsion/solvent evaporation method to synthesize polymeric micelles with a mean size of 228 nm and a narrow PDI, capable of encapsulating MTM with EE values up to 87%. These MTM nano-delivery systems mimicked the potent anti-tumor activity of free MTM, both in adherent and cancer stem cell-enriched tumorsphere cultures of myxoid liposarcoma and chondrosarcoma models. Similarly to free MTM, nanocarrier-delivered MTM efficiently inhibits the signaling mediated by the pro-oncogenic factor SP1. In summary, we provide new formulations for the efficient encapsulation of MTM which may constitute a safer delivering alternative to be explored in future clinical uses.

Mithramycin induces promoter reprogramming and differentiation of rhabdoid tumor

Rhabdoid tumor (RT) is a pediatric cancer characterized by the inactivation of SMARCB1, a subunit of the SWI/SNF chromatin remodeling complex. Although this deletion is the known oncogenic driver, there are limited effective therapeutic options for these patients. Here we use unbiased screening of cell line panels to identify a heightened sensitivity of rhabdoid tumor to mithramycin and the second-generation analogue EC8042. The sensitivity of MMA and EC8042 was superior to traditional DNA damaging agents and linked to the causative mutation of the tumor, SMARCB1 deletion. Mithramycin blocks SMARCB1-deficient SWI/SNF activity and displaces the complex from chromatin to cause an increase in H3K27me3. This triggers chromatin remodeling and enrichment of H3K27ac at chromHMM-defined promoters to restore cellular differentiation. These effects occurred at concentrations not associated with DNA damage and were not due to global chromatin remodeling or widespread gene expression changes. Importantly, a single 3-day infusion of EC8042 caused dramatic regressions of RT xenografts, recapitulated the increase in H3K27me3, and cellular differentiation described in vitro to completely cure three out of eight mice.

Mithramycin A Radiosensitizes EWS:Fli1+ Ewing Sarcoma Cells by Inhibiting Double Strand Break Repair

PURPOSE

The oncogenic EWS:Fli1 fusion protein is a key transcriptional mediator of Ewing sarcoma initiation, progression, and therapeutic resistance. Mithramycin A (MithA) is a potent and specific inhibitor of transcription mediated by the EWS:Fli1. We tested the hypothesis that pretreatment with MithA could selectively radiosensitize EWS:Fli1⁺ tumor cells by altering the transcriptional response to radiation injury.

METHODS AND MATERIALS

A panel of 4 EWS:Fli1⁺ and 3 EWS:Fli1^- Ewing sarcoma cell lines and 1 nontumor cell line were subjected to MithA dose-response viability assays to determine the relative potency of MithA in cells possessing or lacking the EWS:Fli1 fusion. Radiosensitization by MithA was evaluated by clonogenic survival assays in vitro and in a murine xenograft model. DNA damage was evaluated by comet assay and γ-H2Ax flow cytometry. Immunoblotting, flow cytometry, and reverse-transcription, polymerase chain reaction were used to evaluate DNA damage-induced signaling and repair processes and apoptosis.

RESULTS

We found that MithA alone could potently and selectively inhibit the growth of EWS:Fli1⁺ tumor cells, but not cells lacking this fusion. Pretreatment with MithA for 24 hours before irradiation significantly reduced clonogenic survival in vitro and delayed tumor regrowth in vivo, prolonging survival of EWS:Fli1⁺ tumor-bearing mice. Although MithA did not increase the level of DNA double-strand breaks, mechanistic studies revealed that MithA pretreatment selectively inhibited DNA double-strand break repair through downregulation of EWS:Fli1-mediated transcription, leading to tumor cell death by apoptosis.

CONCLUSIONS

Our data indicate that MithA is an effective radiosensitizer of EWS:Fli1⁺ tumors and may achieve better local control at lower doses of radiation.

Mithramycin 2'-Oximes with Improved Selectivity, Pharmacokinetics, and Ewing Sarcoma Antitumor Efficacy

Mithramycin A (MTM) inhibits the oncogenic transcription factor EWS-FLI1 in Ewing sarcoma, but poor pharmacokinetics (PK) and toxicity limit its clinical use. To address this limitation, we report an efficient MTM 2'-oxime (MTM_ox) conjugation strategy for rapid MTM diversification. Comparative cytotoxicity assays of 41 MTM_ox analogues using E-twenty-six (ETS) fusion-dependent and ETS fusion-independent cancer cell lines revealed improved ETS fusion-independent/dependent selectivity indices for select 2'-conjugated analogues as compared to MTM. Luciferase-based reporter assays demonstrated target engagement at low nM concentrations, and molecular assays revealed that analogues inhibit the transcriptional activity of EWS-FLI1. These in vitro screens identified MTM_ox32E (a Phe-Trp dipeptide-based 2'-conjugate) for in vivo testing. Relative to MTM, MTM_ox32E displayed an 11-fold increase in plasma exposure and improved efficacy in an Ewing sarcoma xenograft. Importantly, these studies are the first to point to simple C3 aliphatic side-chain modification of MTM as an effective strategy to improve PK.

Sarcoma treatment in the era of molecular medicine

Sarcomas are heterogeneous and clinically challenging soft tissue and bone cancers. Although constituting only 1% of all human malignancies, sarcomas represent the second most common type of solid tumors in children and adolescents and comprise an important group of secondary malignancies. More than 100 histological subtypes have been characterized to date, and many more are being discovered due to molecular profiling. Owing to their mostly aggressive biological behavior, relative rarity, and occurrence at virtually every anatomical site, many sarcoma subtypes are in particular difficult-to-treat categories. Current multimodal treatment concepts combine surgery, polychemotherapy (with/without local hyperthermia), irradiation, immunotherapy, and/or targeted therapeutics. Recent scientific advancements have enabled a more precise molecular characterization of sarcoma subtypes and revealed novel therapeutic targets and prognostic/predictive biomarkers. This review aims at providing a comprehensive overview of the latest advances in the molecular biology of sarcomas and their effects on clinical oncology; it is meant for a broad readership ranging from novices to experts in the field of sarcoma.

Creating and screening natural product libraries

Covering: up to 2020The National Cancer Institute of the United States (NCI) has initiated a Cancer Moonshot program entitled the NCI Program for Natural Product Discovery. As part of this effort, the NCI is producing a library of 1 000 000 partially purified natural product fractions which are being plated into 384-well plates and provided to the research community free of charge. As the first 326 000 of these fractions have now been made available, this review seeks to describe the general methods used to collect organisms, extract those organisms, and create a prefractionated library. Importantly, this review also details both cell-based and cell-free bioassay methods and the adaptations necessary to those methods to productively screen natural product libraries. Finally, this review briefly describes post-screen dereplication and compound purification and scale up procedures which can efficiently identify active compounds and produce sufficient quantities of natural products for further pre-clinical development.

Insights into DNA-drug interactions in the era of omics

Despite the rise of sophisticated new targeting strategies in cancer chemotherapy, many classic DNA-binding drugs remain on the front line of the therapy against cancer. Based on examples primarily from the author's laboratory, this article reviews the capabilities of several DNA-binding drugs to alter gene expression. Research is ongoing about the molecular bases of the inhibition of gene expression and how alteration of the cellular transcriptome can commit cancer cells to die. The development of a variety of omic techniques allows us to gain insights into the effect of antitumor drugs. Genome-wide approaches provide unbiased genomic data that can facilitate a deeper understanding of the cellular response to DNA-binding drugs. Moreover, the results of large-scale genomic studies are gathered in publicly available databases that can be used in developing precision medicine in cancer treatment.

Biosynthesis of aromatic polyketides in microorganisms using type II polyketide synthases

Aromatic polyketides have attractive biological activities and pharmacological properties. Different from other polyketides, aromatic polyketides are characterized by their polycyclic aromatic structure. The biosynthesis of aromatic polyketides is usually accomplished by the type II polyketide synthases (PKSs), which produce highly diverse polyketide chains by sequential condensation of the starter units with extender units, followed by reduction, cyclization, aromatization and tailoring reactions. Recently, significant progress has been made in characterization and engineering of type II PKSs to produce novel products and improve product titers. In this review, we briefly summarize the architectural organizations and genetic contributions of PKS genes to provide insight into the biosynthetic process. We then review the most recent progress in engineered biosynthesis of aromatic polyketides, with emphasis on generating novel molecular structures. We also discuss the current challenges and future perspectives in the rational engineering of type II PKSs for large scale production of aromatic polyketides.

Heterologous reconstitution of the biosynthesis pathway for 4-demethyl-premithramycinone, the aglycon of antitumor polyketide mithramycin

BACKGROUND

Mithramycin is an anti-tumor compound of the aureolic acid family produced by Streptomyces argillaceus. Its biosynthesis gene cluster has been cloned and characterized, and several new analogs with improved pharmacological properties have been generated through combinatorial biosynthesis. To further study these compounds as potential new anticancer drugs requires their production yields to be improved significantly. The biosynthesis of mithramycin proceeds through the formation of the key intermediate 4-demethyl-premithramycinone. Extensive studies have characterized the biosynthesis pathway from this intermediate to mithramycin. However, the biosynthesis pathway for 4-demethyl-premithramycinone remains unclear.

RESULTS

Expression of cosmid cosAR7, containing a set of mithramycin biosynthesis genes, in Streptomyces albus resulted in the production of 4-demethyl-premithramycinone, delimiting genes required for its biosynthesis. Inactivation of mtmL, encoding an ATP-dependent acyl-CoA ligase, led to the accumulation of the tricyclic intermediate 2-hydroxy-nogalonic acid, proving its essential role in the formation of the fourth ring of 4-demethyl-premithramycinone. Expression of different sets of mithramycin biosynthesis genes as cassettes in S. albus and analysis of the resulting metabolites, allowed the reconstitution of the biosynthesis pathway for 4-demethyl-premithramycinone, assigning gene functions and establishing the order of biosynthetic steps.

CONCLUSIONS

We established the biosynthesis pathway for 4-demethyl-premithramycinone, and identified the minimal set of genes required for its assembly. We propose that the biosynthesis starts with the formation of a linear decaketide by the minimal polyketide synthase MtmPKS. Then, the cyclase/aromatase MtmQ catalyzes the cyclization of the first ring (C7-C12), followed by formation of the second and third rings (C5-C14; C3-C16) catalyzed by the cyclase MtmY. Formation of the fourth ring (C1-C18) requires MtmL and MtmX. Finally, further oxygenation and reduction is catalyzed by MtmOII and MtmTI/MtmTII respectively, to generate the final stable tetracyclic intermediate 4-demethyl-premithramycinone. Understanding the biosynthesis of this compound affords enhanced possibilities to generate new mithramycin analogs and improve their production titers for bioactivity investigation.

Current Approaches for Personalized Therapy of Soft Tissue Sarcomas

Soft tissue sarcomas (STS) are a highly heterogeneous group of cancers of mesenchymal origin with diverse morphologies and clinical behaviors. While surgical resection is the standard treatment for primary STS, advanced and metastatic STS patients are not eligible for surgery. Systemic treatments, including standard chemotherapy and newer chemical agents, still play the most relevant role in the management of the disease. Discovery of specific genetic alterations in distinct STS subtypes allowed better understanding of mechanisms driving their pathogenesis and treatment optimization. This review focuses on the available targeted drugs or drug combinations based on genetic aberration involved in STS development including chromosomal translocations, oncogenic mutations, gene amplifications, and their perspectives in STS treatment. Furthermore, in this review, we discuss the possible use of chemotherapy sensitivity and resistance assays (CSRA) for the adjustment of treatment for individual patients. In summary, current trends in personalized management of advanced and metastatic STS are based on combination of both genetic testing and CSRA.

CDK9 Blockade Exploits Context-dependent Transcriptional Changes to Improve Activity and Limit Toxicity of Mithramycin for Ewing Sarcoma

There is a need to develop novel approaches to improve the balance between efficacy and toxicity for transcription factor-targeted therapies. In this study, we exploit context-dependent differences in RNA polymerase II processivity as an approach to improve the activity and limit the toxicity of the EWS-FLI1-targeted small molecule, mithramycin, for Ewing sarcoma. The clinical activity of mithramycin for Ewing sarcoma is limited by off-target liver toxicity that restricts the serum concentration to levels insufficient to inhibit EWS-FLI1. In this study, we perform an siRNA screen of the druggable genome followed by a matrix drug screen to identify mithramycin potentiators and a synergistic "class" effect with cyclin-dependent kinase 9 (CDK9) inhibitors. These CDK9 inhibitors enhanced the mithramycin-mediated suppression of the EWS-FLI1 transcriptional program leading to a shift in the IC₅₀ and striking regressions of Ewing sarcoma xenografts. To determine whether these compounds may also be liver protective, we performed a qPCR screen of all known liver toxicity genes in HepG2 cells to identify mithramycin-driven transcriptional changes that contribute to the liver toxicity. Mithramycin induces expression of the BTG2 gene in HepG2 but not Ewing sarcoma cells, which leads to a liver-specific accumulation of reactive oxygen species (ROS). siRNA silencing of BTG2 rescues the induction of ROS and the cytotoxicity of mithramycin in these cells. Furthermore, CDK9 inhibition blocked the induction of BTG2 to limit cytotoxicity in HepG2, but not Ewing sarcoma cells. These studies provide the basis for a synergistic and less toxic EWS-FLI1-targeted combination therapy for Ewing sarcoma.

Precision medicine in Ewing sarcoma: a translational point of view

Ewing sarcoma is a rare tumor that arises in bones of children and teenagers but, in 15% of the patients it is presented as a primary soft tissue tumor. Balanced reciprocal chimeric translocation t(11;22)(q24;q12), which encodes an oncogenic protein fusion (EWSR1/FLI1), is the most generalized and characteristic molecular event. Using conventional treatments, (chemotherapy, surgery and radiotherapy) long-term overall survival rate is 30% for patients with disseminated disease and 65-75% for patients with localized tumors. Urgent new effective drug development is a challenge. This review summarizes the preclinical and clinical investigational knowledge about prognostic and targetable biomarkers in Ewing sarcoma, finally suggesting a workflow for precision medicine committees.

The Mithralog EC-7072 Induces Chronic Lymphocytic Leukemia Cell Death by Targeting Tonic B-Cell Receptor Signaling

B-cell receptor (BCR)-dependent signaling is central for leukemia B-cell homeostasis, as underscored by the promising clinical results obtained in patients with chronic lymphocytic leukemia (CLL) treated with novel agents targeting components of this pathway. Herein, we demonstrate that the mithralog EC-7072 displays high ex vivo cytotoxic activity against leukemia cells from CLL patients independently from high-risk prognostic markers and IGHV mutational status. EC-7072 was significantly less toxic against T cells and NK cells and did not alter the production of the immune effector molecules IFN-γ and perforin. EC-7072 directly triggered caspase-3-dependent CLL cell apoptosis, which was not abrogated by microenvironment-derived factors that sustain leukemia cell survival. RNA-sequencing analyses revealed a dramatic EC-7072-driven reprograming of the transcriptome of CLL cells, including a wide downregulation of multiple components and targets of the BCR signaling pathway. Accordingly, we found decreased levels of phosphorylated signaling nodes downstream of the BCR. Crosslinking-mediated BCR activation antagonized CLL cell death triggered by EC-7072, increased the phosphorylation levels of the abovementioned signaling nodes and upregulated BCL2 expression, suggesting that the mithralog disrupts CLL cell viability by targeting the BCR signaling axis at multiple levels. EC-7072 exerted similar or higher antileukemic activity than that of several available CLL therapies and displayed additive or synergistic interaction with these drugs in killing CLL cells. Overall, our findings provide rationale for future investigation to test whether EC-7072 may be a potential therapeutic option for patients with CLL and other B-cell malignancies.

Emerging trends in immunotherapy for pediatric sarcomas

While promising, immunotherapy has yet to be fully unlocked for the preponderance of cancers where conventional chemoradiation reigns. This remains particularly evident in pediatric sarcomas where standard of care has not appreciably changed in decades. Importantly, pediatric bone sarcomas, like osteosarcoma and Ewing’s sarcoma, possess unique tumor microenvironments driven by distinct molecular features, as do rhabdomyosarcomas and soft tissue sarcomas. A better understanding of each malignancy’s biology, heterogeneity, and tumor microenvironment may lend new insights toward immunotherapeutic targets in novel platform technologies for cancer vaccines and adoptive cellular therapy. These advances may pave the way toward new treatments requisite for pediatric sarcomas and patients in need of new therapies.

Exploiting Signaling Pathways and Immune Targets Beyond the Standard of Care for Ewing Sarcoma

Ewing sarcoma (ES) family of tumors includes bone and soft tissue tumors that are often characterized by a specific translocation between chromosome 11 and 22, resulting in the EWS-FLI1 fusion gene. With the advent of multi-modality treatment including cytotoxic chemotherapy, surgery, and radiation therapy, the prognosis for patients with ES has substantially improved. However, a therapeutic plateau is now reached for both localized and metastatic disease over the last two decades. Burdened by the toxicity limits associated with the current frontline systemic therapy, there is an urgent need for novel targeted therapeutic strategies. In this review, we discuss the current treatment paradigm of ES, and explore preclinical evidence and emerging treatments directed at tumor signaling pathways and immune targets.

Direct and Indirect Targeting of HOXA9 Transcription Factor in Acute Myeloid Leukemia

HOXA9 (Homeobox A9) is a homeotic transcription factor known for more than two decades to be associated with leukemia. The expression of HOXA9 homeoprotein is associated with anterior-posterior patterning during embryonic development, and its expression is then abolished in most adult cells, with the exception of hematopoietic progenitor cells. The oncogenic function of HOXA9 was first assessed in human acute myeloid leukemia (AML), particularly in the mixed-phenotype associated lineage leukemia (MPAL) subtype. HOXA9 expression in AML is associated with aggressiveness and a poor prognosis. Since then, HOXA9 has been involved in other hematopoietic malignancies and an increasing number of solid tumors. Despite this, HOXA9 was for a long time not targeted to treat cancer, mainly since, as a transcription factor, it belongs to a class of protein long considered to be an "undruggable" target; however, things have now evolved. The aim of the present review is to focus on the different aspects of HOXA9 targeting that could be achieved through multiple ways: (1) indirectly, through the inhibition of its expression, a strategy acting principally at the epigenetic level; or (2) directly, through the inhibition of its transcription factor function by acting at either the protein/protein interaction or the protein/DNA interaction interfaces.

Bioanalytical method for quantitative determination of mithramycin analogs in mouse plasma by HPLC-QTOF

Mithramycin (MTM) has potent anticancer activity, but severe toxicities restrict its clinical use. Semi-synthetic approaches have yielded novel MTM analogs with potentially lower toxicity and similar efficacy. In an effort to transition these analogs into in vivo models, a bioanalytical method was developed for their quantification in mouse plasma. Here we present the validation of the method for the quantitation of mithramycin SA-tryptophan (MTMSA-Trp) as well as the applicability of the methodology for assaying additional analogs, including MTM, mithramycin SK (MTMSK) and mithramycin SA-phenylalanine (MTMSA-Phe) with run times of 6 min. Assay linearity ranged from 5 to 100 ng/mL. Accuracies of calibration standards and quality control samples were within 15% of nominal with precision variability of <20%. MTMSA-Trp was stable for 30 days at -80°C and for at least three freeze-thaw cycles. Methanol (-80°C) extraction afforded 92% of MTMSA-Trp from plasma. Calibration curves for MTM and analogs were also linear from ≤5 to 100 ng/mL. This versatile method was used to quantitate MTM analogs in plasma samples collected during preclinical pharmacokinetic studies.

How mithramycin stereochemistry dictates its structure and DNA binding function

An aureolic acid natural product mithramycin (MTM) has been known for its potent antineoplastic properties. MTM inhibits cell growth by binding in the minor groove of double-stranded DNA as a dimer, in which the two molecules of MTM are coordinated to each other through a divalent metal ion. A crystal structure of an MTM analogue, MTM SA-Phe, in the active metal ion-coordinated dimeric form demonstrates how the stereochemical features of MTM define the helicity of the dimeric scaffold for its binding to a right-handed DNA double helix. We also show crystallographically and biochemically that MTM, but not MTM SA-Phe, can be inactivated by boric acid through formation of a large macrocyclic species, in which two molecules of MTM are crosslinked to each other through 3-side chain-boron-sugar intermolecular bonds. We discuss these structural and biochemical properties in the context of MTM biosynthesis and the design of MTM analogues as anticancer therapeutics.

Chromomycin A2 potently inhibits glucose-stimulated insulin secretion from pancreatic β cells

Drugs that target insulin secretion are useful to understand β cell function and the pathogenesis of diabetes. Kalwat et al. investigate an aureolic acid that inhibits insulin secretion and reveal that it disrupts Wnt signaling, interferes with gene expression, and suppresses Ca²⁺ influx in β cells.

Modulators of insulin secretion could be used to treat diabetes and as tools to investigate β cell regulatory pathways in order to increase our understanding of pancreatic islet function. Toward this goal, we previously used an insulin-linked luciferase that is cosecreted with insulin in MIN6 β cells to perform a high-throughput screen of natural products for chronic effects on glucose-stimulated insulin secretion. In this study, using multiple phenotypic analyses, we found that one of the top natural product hits, chromomycin A2 (CMA2), potently inhibited insulin secretion by at least three potential mechanisms: disruption of Wnt signaling, interference of β cell gene expression, and partial suppression of Ca²⁺ influx. Chronic treatment with CMA2 largely ablated glucose-stimulated insulin secretion even after washout, but it did not inhibit glucose-stimulated generation of ATP or Ca²⁺ influx. However, by using the K_ATP channel opener diazoxide, we uncovered defects in depolarization-induced Ca²⁺ influx that may contribute to the suppressed secretory response. Glucose-responsive ERK1/2 and S6 phosphorylation were also disrupted by chronic CMA2 treatment. By querying the FUSION bioinformatic database, we revealed that the phenotypic effects of CMA2 cluster with a number of Wnt–GSK3 pathway-related genes. Furthermore, CMA2 consistently decreased GSK3β phosphorylation and suppressed activation of a β-catenin activity reporter. CMA2 and a related compound, mithramycin, are known to have DNA interaction properties, possibly abrogating transcription factor binding to critical β cell gene promoters. We observed that CMA2 but not mithramycin suppressed expression of PDX1 and UCN3. However, neither expression of INSI/II nor insulin content was affected by chronic CMA2. The mechanisms of CMA2-induced insulin secretion defects may involve components both proximal and distal to Ca²⁺ influx. Therefore, CMA2 is an example of a chemical that can simultaneously disrupt β cell function through both noncytotoxic and cytotoxic mechanisms. Future therapeutic applications of CMA2 and similar aureolic acid analogues should consider their potential effects on pancreatic islet function.

Transcriptional Reprogramming and Inhibition of Tumor-propagating Stem-like Cells by EC-8042 in ERG-positive Prostate Cancer

BACKGROUND

The TMPRSS2-ERG gene fusion is the most frequent genetic rearrangement in prostate cancers and results in broad transcriptional reprogramming and major phenotypic changes. Interaction and cooperation of ERG and SP1 may be instrumental in sustaining the tumorigenic and metastatic phenotype and could represent a potential vulnerability in ERG fusion-positive tumors.

OBJECTIVE

To test the activity of EC-8042, a compound able to block SP1, in cellular and mouse models of ERG-positive prostate cancer.

DESIGN, SETTING, AND PARTICIPANTS

We evaluated the activity of EC-8042 in cell cultures and ERG/PTEN transgenic/knockout mice that provide reliable models for testing novel therapeutics in this specific disease context. Using a new protocol to generate tumor spheroids from ERG/PTEN mice, we also examined the effects of EC-8042 on tumor-propagating stem-like cancer cells with high self-renewal and tumorigenic capabilities.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

The efficacy of EC-8042 was determined by measuring the proliferative capacity and target gene expression in cell cultures, invasive and metastatic capabilities in chick chorioallantoic membrane assays, and tumor development in mice. Significance was determined using statistical test.

RESULTS AND LIMITATIONS

EC-8042 blocked transcription of ERG-regulated genes and reverted the invasive and metastatic phenotype of VCaP cells. EC-8042 blocked the expansion of stem-like tumor cells in tumor spheroids from VCaP cells and mouse-derived tumors. In ERG/PTEN mice, systemic treatment with EC-8042 inhibited ERG-regulated gene transcription, tumor progression, and tumor-propagating stem-like tumor cells.

CONCLUSIONS

Our data support clinical testing of EC-8042 for the treatment of ERG-positive prostate cancer in precision medicine approaches.

PATIENT SUMMARY

In this study, EC-8042, a novel compound with a favorable pharmacological and toxicological profile, exhibited relevant activity in cell cultures and in vivo in a genetically engineered mouse model that closely recapitulates the features of clinically aggressive ERG-positive prostate cancer. Our data indicate that further evaluation of EC-8042 in clinical trials is warranted.

Development of Mithramycin Analogues with Increased Selectivity toward ETS Transcription Factor Expressing Cancers

Mithramycin A (1) was identified as the top potential inhibitor of the aberrant ETS transcription factor EWS-FLI1, which causes Ewing sarcoma. Unfortunately, 1 has a narrow therapeutic window, compelling us to seek less toxic and more selective analogues. Here, we used MTMSA (2) to generate analogues via peptide coupling and fragment-based drug development strategies. Cytotoxicity assays in ETS and non-ETS dependent cell lines identified two dipeptide analogues, 60 and 61, with 19.1- and 15.6-fold selectivity, respectively, compared to 1.5-fold for 1. Importantly, the cytotoxicity of 60 and 61 is <100 nM in ETS cells. Molecular assays demonstrated the inhibitory capacity of these analogues against EWS-FLI1 mediated transcription in Ewing sarcoma. Structural analysis shows that positioning the tryptophan residue in a distal position improves selectivity, presumably via interaction with the ETS transcription factor. Thus, these analogues may present new ways to target transcription factors for clinical use.

Challenging transcription by DNA-binding antitumor drugs

Cancer has been associated with altered gene expression. Therefore, transcription and its regulation by transcription factors are considered key points to be explored in the pursuit of more efficient antitumor agents. This paper reviews the effects of DNA-binding drugs on the interaction between transcription factors and DNA, and it discusses recent advances in the understanding of the mechanisms by which small compounds interfere with the activity of transcription factors and gene expression. Many DNA-binding drugs, some of them in clinical use, can compete with a variety of transcription factors for their preferred binding sites in gene promoters, or they can covalently modify DNA, thus preventing transcription factors from recognizing their binding sites. On the other hand, transcription factor activity can be impaired through modification of the protein factors or their complexes. Several "omic" tools have been developed to explore the genome-wide changes in gene expression induced by DNA-binding drugs, which reveal details of the mechanisms of action. Transcriptomic profiles obtained from drug-treated cells and of samples collected from patients upon treatment provide insights into the in vivo mechanisms of drug action related to the inhibition of gene transcription. The information available about the molecular structure and mechanisms of action of both transcription factors and DNA-binding drugs, together with the new opportunities provided by functional genomics, should encourage the development of new more-selective DNA-binding antitumor drugs to target a single gene with little effect on others.

Small animal models for the study of bone sarcoma pathogenesis:characteristics, therapeutic interests and limitations

Osteosarcoma, Ewing sarcoma and chondrosarcoma are the three main entities of bone sarcoma which collectively encompass more than 50 heterogeneous entities of rare malignancies. In contrast to osteosarcoma and Ewing sarcoma which mainly affect adolescents and young adults and exhibit a high propensity to metastasise to the lungs, chondrosarcoma is more frequently observed after 40 years of age and is characterised by a high frequency of local recurrence. The combination of chemotherapy, surgical resection and radiotherapy has contributed to an improved outcome for these patients. However, a large number of patients still suffer significant therapy related toxicities or die of refractory and metastatic disease. To better delineate the pathogenesis of bone sarcomas and to identify and test new therapeutic options, major efforts have been invested over the past decades in the development of relevant pre-clinical animal models. Nowadays, in vivo models aspire to mimic all the steps and the clinical features of the human disease as accurately as possible and should ideally be manipulable. Considering these features and given their small size, their conduciveness to experiments, their affordability as well as their human-like bone-microenvironment and immunity, murine pre-clinical models are interesting in the context of these pathologies. This chapter will provide an overview of the murine models of bone sarcomas, paying specific attention for the models induced by inoculation of tumour cells. The genetically-engineered mouse models of bone sarcoma will also be summarized.

Potential approaches to the treatment of Ewing's sarcoma

Ewing’s sarcoma (ES) is a highly aggressive and metastatic tumor in children and young adults caused by a chromosomal fusion between the Ewing sarcoma breakpoint region 1 (EWSR1) gene and the transcription factor FLI1 gene. ES is managed with standard treatments, including chemotherapy, surgery and radiation. Although the 5-year survival rate for primary ES has improved, the survival rate for ES patients with metastases or recurrence remains low. Several novel molecular targets in ES have recently been identified and investigated in preclinical and clinical settings, and targeting the function of receptor tyrosine kinases (RTKs), the fusion protein EWS-FLI1 and mTOR has shown promise. There has also been increasing interest in the immune responses of ES patients. Immunotherapies using T cells, NK cells, cancer vaccines and monoclonal antibodies have been considered for ES, especially for recurrent patients. Because understanding the pathogenesis of ES is extremely important for the development of novel treatments, this review focuses on the mechanisms and functions of targeted therapies and immunotherapies in ES. It is anticipated that integrating the knowledge obtained from basic research and translational and clinical studies will lead to the development of novel therapeutic strategies for the treatment of ES.

Fusion genes: A promising tool combating against cancer

The driving roles of fusion genes during tumorigenesis have been recognized for decades, with efficacies demonstrated in clinical diagnosis and targeted therapy. With advances in sequencing technologies and computational biology, a surge in the identification of fusion genes has been witnessed during the past decade. The discovery and presence of splicing based fusions in normal tissues have challenged our canonical conceptions on fusion genes and offered us novel medical opportunities. The specificity of fusion genes to neoplastic tissues and their diverse functionalities during carcinogenesis foster them as promising tools in the battle against cancer. It is time to re-visit and comb through our cutting-edge knowledge on fusion genes to accelerate clinical translation of these internal markers. Urged as such, we are encouraged to categorize fusion events according to mechanisms leading to their generation, oncological consequences and clinical implications, offer insights on fusion occurrence across tumors from the system level, highlight feasible practices in fusion-related pharmaceutical development, and identify understudied yet important niches that may lead future research trend in this field.

BET bromodomain inhibitors suppress EWS-FLI1-dependent transcription and the IGF1 autocrine mechanism in Ewing sarcoma

Ewing sarcoma is driven by characteristic chromosomal translocations between the EWSR1 gene with genes encoding ETS family transcription factors (EWS-ETS), most commonly FLI1. However, direct pharmacological inhibition of transcription factors like EWS-FLI1 remains largely unsuccessful. Active gene transcription requires orchestrated actions of many epigenetic regulators, such as the bromodomain and extra-terminal domain (BET) family proteins. Emerging BET bromodomain inhibitors have exhibited promising antineoplastic activities via suppression of oncogenic transcription factors in various cancers. We reasoned that EWS-FLI1-mediated transcription activation might be susceptible to BET inhibition. In this study, we demonstrated that small molecule BET bromodomain inhibitors repressed EWS-FLI1-driven gene signatures and downregulated important target genes. However, expression of EWS-FLI1 was not significantly affected. Repression of autocrine IGF1 by BET inhibitors led to significant inhibition of the IGF1R/AKT pathway critical to Ewing sarcoma cell proliferation and survival. Consistently, BET inhibitors impaired viability and clonogenic survival of Ewing sarcoma cell lines and blocked EWS-FLI1-induced transformation of mouse NIH3T3 fibroblast cells. Selective depletion of individual BET genes partially phenocopied the actions of BET inhibitors. Finally, the prototypical BET inhibitor, JQ1, significantly repressed Ewing sarcoma xenograft tumor growth. These findings suggest therapeutic potential of BET inhibitors in Ewing sarcoma and highlight an emerging paradigm of using epigenetic agents to treat cancers driven by fusion transcription factors.

Oncogenes Activate an Autonomous Transcriptional Regulatory Circuit That Drives Glioblastoma

Efforts to identify and target glioblastoma (GBM) drivers have primarily focused on receptor tyrosine kinases (RTKs). Clinical benefits, however, have been elusive. Here, we identify an SRY-related box 2 (SOX2) transcriptional regulatory network that is independent of upstream RTKs and capable of driving glioma-initiating cells. We identified oligodendrocyte lineage transcription factor 2 (OLIG2) and zinc-finger E-box binding homeobox 1 (ZEB1), which are frequently co-expressed irrespective of driver mutations, as potential SOX2 targets. In murine glioma models, we show that different combinations of tumor suppressor and oncogene mutations can activate Sox2, Olig2, and Zeb1 expression. We demonstrate that ectopic co-expression of the three transcription factors can transform tumor-suppressor-deficient astrocytes into glioma-initiating cells in the absence of an upstream RTK oncogene. Finally, we demonstrate that the transcriptional inhibitor mithramycin downregulates SOX2 and its target genes, resulting in markedly reduced proliferation of GBM cells in vivo.

Mithramycin A Enhances Tumor Sensitivity to Mitotic Catastrophe Resulting From DNA Damage

PURPOSE

Specificity protein 1 (SP1) is involved in the transcription of several genes implicated in tumor maintenance. We investigated the effects of mithramycin A (MTA), an inhibitor of SP1 DNA binding, on radiation response.

METHODS AND MATERIALS

Clonogenic survival after irradiation was assessed in 2 tumor cell lines (A549, UM-UC-3) and 1 human fibroblast line (BJ) after SP1 knockdown or MTA treatment. DNA damage repair was evaluated using γH2AX foci formation, and mitotic catastrophe was assessed using nuclear morphology. Gene expression was evaluated using polymerase chain reaction arrays. In vivo tumor growth delay was used to evaluate the effects of MTA on radiosensitivity.

RESULTS

Targeting of SP1 with small interfering RNA or MTA sensitized A549 and UM-UC-3 to irradiation, with no effect on the BJ radiation response. MTA did not alter γH2AX foci formation after irradiation in tumor cells but did enhance mitotic catastrophe. Treatment with MTA suppressed transcription of genes involved in cell death. MTA administration to mice bearing A549 and UM-UC-3 xenografts enhanced radiation-induced tumor growth delay.

CONCLUSIONS

These results support SP1 as a target for radiation sensitization and confirm MTA as a radiation sensitizer in human tumor models.

A phase I/II trial and pharmacokinetic study of mithramycin in children and adults with refractory Ewing sarcoma and EWS-FLI1 fusion transcript

PURPOSE

In a preclinical drug screen, mithramycin was identified as a potent inhibitor of the Ewing sarcoma EWS-FLI1 transcription factor. We conducted a phase I/II trial to determine the dose-limiting toxicities (DLT), maximum tolerated dose (MTD), and pharmacokinetics (PK) of mithramycin in children with refractory solid tumors, and the activity in children and adults with refractory Ewing sarcoma.

PATIENTS AND METHODS

Mithramycin was administered intravenously over 6 h once daily for 7 days for 28 day cycles. Adult patients (phase II) initially received mithramycin at the previously determined recommended dose of 25 µg/kg/dose. The planned starting dose for children (phase I) was 17.5 µg/kg/dose. Plasma samples were obtained for mithramycin PK analysis.

RESULTS

The first two adult patients experienced reversible grade 4 alanine aminotransferase (ALT)/aspartate aminotransferase (AST) elevation exceeding the MTD. Subsequent adult patients received mithramycin at 17.5 µg/kg/dose, and children at 13 µg/kg/dose with dexamethasone pretreatment. None of the four subsequent adult and two pediatric patients experienced cycle 1 DLT. No clinical responses were observed. The average maximal mithramycin plasma concentration in four patients was 17.8 ± 4.6 ng/mL. This is substantially below the sustained mithramycin concentrations ≥50 nmol/L required to suppress EWS-FLI1 transcriptional activity in preclinical studies. Due to inability to safely achieve the desired mithramycin exposure, the trial was closed to enrollment.

CONCLUSIONS

Hepatotoxicity precluded the administration of a mithramycin at a dose required to inhibit EWS-FLI1. Evaluation of mithramycin in patients selected for decreased susceptibility to elevated transaminases may allow for improved drug exposure.

Advances in the Treatment of Pediatric Bone Sarcomas

Bone tumors make up a significant portion of noncentral nervous system solid tumor diagnoses in pediatric oncology patients. Ewing sarcoma and osteosarcoma, both with distinct clinical and pathologic features, are the two most commonly encountered bone cancers in pediatrics. Although mutations in the germline have classically been more associated with osteosarcoma, there is recent evidence germline alterations in patients with Ewing sarcoma also play a significant role in pathogenesis. Treatment advances in this patient population have lagged behind that of other pediatric malignancies, particularly targeted interventions directed at the biologic underpinnings of disease. Recent advances in biologic and genomic understanding of these two cancers has expanded the potential for therapeutic advancement and prevention. In Ewing sarcoma, directed focus on inhibition of EWSR1-FLI1 and its effectors has produced promising results. In osteosarcoma, instead of a concentrated focus on one particular change, largely due to tumor heterogeneity, a more diversified approach has been adopted including investigations of growth factors inhibitors, signaling pathway inhibitors, and immune modulation. Continuing recently made treatment advances relies on clinical trial design and enrollment. Clinical trials should include incorporation of biological findings; specifically, for Ewing sarcoma, assessment of alternative fusions and, for osteosarcoma, stratification utilizing biomarkers. Expanded cancer genomics knowledge, particularly with solid tumors, as it relates to heritability and incorporation of family history has led to early identification of patients with cancer predisposition. In these patients through application of cost-effective evidence-based screening techniques the ultimate goal of cancer prevention is becoming a realization.