Evaluation of quantitative assays for the identification of direct signal transducer and activator of transcription 3 (STAT3) inhibitors

Challenging the "Undruggable"─Targeting STAT3 but Identifying Potent TrkA-Targeted Inhibitors

Signal transducer and activator of transcription 3 (STAT3) is a promising yet challenging anticancer drug target due to its complex signaling and limited "druggability". To this end, we herein highlight a target engagement-focused screening and optimization pipeline pursuing the discovery of novel STAT3 inhibitors. From a STAT3 differential scanning fluorimetry high-throughput screen, we identified compounds that appeared to stabilize STAT3 toward thermal aggregation and moderately inhibited cellular STAT3 activity. Subsequent evaluation using complementary and orthogonal assays revealed their high affinity for tropomyosin receptor kinase A (TrkA). Applying a similar target engagement-inspired approach, we refined inhibitor binding and selectivity toward TrkA, showing efficacy in cellular TrkA cancer models. Top compound, PI-15, demonstrated successful target engagement in a cellular thermal shift assay and potently inhibited TrkA activity in cancer cells. These approaches highlight the importance of prioritizing rigorous target engagement validation early in the drug discovery pipeline, resulting in promising new inhibitors.

Small Molecule Induces Time-Dependent Inhibition of Stat3 Dimerization and DNA-Binding Activity and Regresses Human Breast Tumor Xenografts

Aberrantly-active signal transducer and activator of transcription (Stat)3 has a causal role in many human cancers and represents a validated anticancer drug target, though it has posed significant challenge to drug development. A new small molecule, JKB887, was identified through library screening and is predicted to interact with Lys591, Arg609 and Pro63 in the phospho-tyrosine (pTyr)-binding pocket of the Stat3 SH2 domain. JKB887 inhibited Stat3 DNA-binding activity in vitro in a time-dependent manner, with IC50 of 2.2-4.5 μM at 30-60-min incubation. It directly disrupted both the Stat3 binding to the cognate, high-affinity pTyr (pY) peptide, GpYLPQTV-NH2 in fluorescent polarization assay with IC50 of 3.5-5.5 μM at 60-90-min incubation, and to the IL-6 receptor/gp130 or Src in treated malignant cells. Treatment with JKB887 selectively blocked constitutive Stat3 phosphorylation, nuclear translocation and transcriptional activity, and Stat3-regulated gene expression, and decreased viable cell numbers, cell growth, colony formation, migration, and survival in human or mouse tumor cells. By contrast, JKB887 had minimal effects on Stat1, pErk1/2MAPK, pShc, pJAK2, or pSrc induction, or on cells that do not harbor aberrantly-active Stat3. Additionally, JKB887 inhibited growth of human breast cancer xenografts in mice. JKB887 is a Stat3-selective inhibitor with demonstrable antitumor effects against Stat3-dependent human cancers.

Novel inhibitors of the STAT3 signaling pathway: an updated patent review (2014-present)

INTRODUCTION

STAT3 is a critical transcription factor that transmits signals from the cell surface to the nucleus, thus influencing the transcriptional regulation of some oncogenes. The inhibition of the activation of STAT3 is considered a promising strategy for cancer therapy. Numerous STAT3 inhibitors bearing different scaffolds have been reported to date, with a few of them having been considered in clinical trials.

AREAS COVERED

This review summarizes the advances on STAT3 inhibitors with different structural skeletons, focusing on the structure-activity relationships in the related patent literature published from 2014 to date.

EXPERT OPINION

Since the X-ray crystal structure of STAT3β homo dimer bound to DNA was solved in 1998, the development of STAT3 inhibitors has gone through a boom in recent years. However, none of them have been approved for marketing, probably due to the complex biological functions of the STAT3 signaling pathway, including its character and the poor drug-like physicochemical properties of its inhibitors. Nonetheless, targeting STAT3 continues to be an exciting field for the development of anti-tumor agents along with the emergence of new STAT3 inhibitors with unique mechanisms of action.

The STAT3-MYC axis promotes survival of leukemia stem cells by regulating SLC1A5 and oxidative phosphorylation

Acute myeloid leukemia (AML) is characterized by the presence of leukemia stem cells (LSCs), and failure to fully eradicate this population contributes to disease persistence/relapse. Prior studies have characterized metabolic vulnerabilities of LSCs, which demonstrate preferential reliance on oxidative phosphorylation (OXPHOS) for energy metabolism and survival. In the present study, using both genetic and pharmacologic strategies in primary human AML specimens, we show that signal transducer and activator of transcription 3 (STAT3) mediates OXPHOS in LSCs. STAT3 regulates AML-specific expression of MYC, which in turn controls transcription of the neutral amino acid transporter gene SLC1A5. We show that genetic inhibition of MYC or SLC1A5 acts to phenocopy the impairment of OXPHOS observed with STAT3 inhibition, thereby establishing this axis as a regulatory mechanism linking STAT3 to energy metabolism. Inhibition of SLC1A5 reduces intracellular levels of glutamine, glutathione, and multiple tricarboxylic acid (TCA) cycle metabolites, leading to reduced TCA cycle activity and inhibition of OXPHOS. Based on these findings, we used a novel small molecule STAT3 inhibitor, which binds STAT3 and disrupts STAT3-DNA, to evaluate the biological role of STAT3. We show that STAT3 inhibition selectively leads to cell death in AML stem and progenitor cells derived from newly diagnosed patients and patients who have experienced relapse while sparing normal hematopoietic cells. Together, these findings establish a STAT3-mediated mechanism that controls energy metabolism and survival in primitive AML cells.

A novel small molecule LLL12B inhibits STAT3 signaling and sensitizes ovarian cancer cell to paclitaxel and cisplatin

Ovarian cancer is the fifth most common cause of cancer deaths among American women. Platinum and taxane combination chemotherapy represents the first-line approach for ovarian cancer, but treatment success is often limited by chemoresistance. Therefore, it is necessary to find new drugs to sensitize ovarian cancer cells to chemotherapy. Persistent activation of Signal Transducer and Activator of Transcription 3 (STAT3) signaling plays an important role in oncogenesis. Using a novel approach called advanced multiple ligand simultaneous docking (AMLSD), we developed a novel nonpeptide small molecule, LLL12B, which targets the STAT3 pathway. In this study, LLL12B inhibited STAT3 phosphorylation (tyrosine 705) and the expression of its downstream targets, which are associated with cancer cell proliferation and survival. We showed that LLL12B also inhibits cell viability, migration, and proliferation in human ovarian cancer cells. LLL12B combined with either paclitaxel or with cisplatin demonstrated synergistic inhibitory effects relative to monotherapy in inhibiting cell viability and LLL12B-paclitaxel or LLL12B-cisplatin combination exhibited greater inhibitory effects than cisplatin-paclitaxel combination in ovarian cancer cells. Furthermore, LLL12B-paclitaxel or LLL12B-cisplatin combination showed more significant in inhibiting cell migration and growth than monotherapy in ovarian cancer cells. In summary, our results support the novel small molecule LLL12B as a potent STAT3 inhibitor in human ovarian cancer cells and suggest that LLL12B in combination with the current front-line chemotherapeutic drugs cisplatin and paclitaxel may represent a promising approach for ovarian cancer therapy.

Comprehensive live-cell imaging analysis of cryptotanshinone and synergistic drug-screening effects in various human and canine cancer cell lines

Background

Several studies have highlighted both the extreme anticancer effects of Cryptotanshinone (CT), a Stat3 crippling component from Salvia miltiorrhiza, as well as other STAT3 inhibitors to fight cancer.

Methods

Data presented in this experiment incorporates 2 years of in vitro studies applying a comprehensive live-cell drug-screening analysis of human and canine cancer cells exposed to CT at 20 μM concentration, as well as to other drug combinations. As previously observed in other studies, dogs are natural cancer models, given to their similarity in cancer genetics, epidemiology and disease progression compared to humans.

Results

Results obtained from several types of human and canine cancer cells exposed to CT and varied drug combinations, verified CT efficacy at combating cancer by achieving an extremely high percentage of apoptosis within 24 hours of drug exposure.

Conclusions

CT anticancer efficacy in various human and canine cancer cell lines denotes its ability to interact across different biological processes and cancer regulatory cell networks, driving inhibition of cancer cell survival.

Role of STAT3 signaling transduction pathways in cancer stem cell-associated chemoresistance

Chemoresistance resulting from cancer stem cells (CSCs) and epithelial-mesenchymal transition (EMT) results in inconsistent chemotherapeutic efficacy. The co-existence of CSCs and the EMT allows cancer cells to interconvert between differentiated and stem-like states, a phenomenon known as cellular plasticity. Phosphorylated signal transducer and activator of transcription 3 (pSTAT3) has been increasingly identified as a major contributor to CSCs and the EMT, as evidenced from preclinical studies that reversed chemoresistance through STAT3 pathway inhibition. In this review, we discuss mechanisms that center on STAT3 and its target genes responsible for regulating the EMT. We also highlight the current status of clinical trials using STAT3 pathway inhibitors.

Validating Signal Transducer and Activator of Transcription (STAT) Protein-Inhibitor Interactions Using Biochemical and Cellular Thermal Shift Assays

Signal transducer and activator of transcription (STAT) proteins have important biological functions; however, deregulation of STAT signaling is a driving force behind the onset and progression of inflammatory diseases and cancer. While their biological roles suggest that STAT proteins would be valuable targets for developing therapeutic agents, STAT proteins are notoriously difficult to inhibit using small drug-like molecules, as they do not have a distinct inhibitor binding site. Despite this, a multitude of small-molecule STAT inhibitors have been proposed, primarily focusing on inhibiting STAT3 protein to generate novel cancer therapies. Demonstrating that inhibitors bind to their targets in cells has historically been a very challenging task. With the advent of modern target engagement techniques, such as the cellular thermal shift assay (CETSA), interactions between experimental compounds and their biological targets can be detected with relative ease. To investigate interactions between STAT proteins and inhibitors, we herein developed STAT CETSAs and evaluated known STAT3 inhibitors for their ability to engage STAT proteins in biological settings. While potent binding was detected between STAT proteins and peptidic STAT inhibitors, small-molecule inhibitors elicited variable responses, most of which failed to stabilize STAT3 proteins in cells and cell lysates. The described STAT thermal stability assays represent valuable tools for evaluating proposed STAT inhibitors.

Irreversible TrxR1 inhibitors block STAT3 activity and induce cancer cell death

Because of its key role in cancer development and progression, STAT3 has become an attractive target for developing new cancer therapeutics. While several STAT3 inhibitors have progressed to advanced stages of development, their underlying biology and mechanisms of action are often more complex than would be expected from specific binding to STAT3. Here, we have identified and optimized a series of compounds that block STAT3-dependent luciferase expression with nanomolar potency. Unexpectedly, our lead compounds did not bind to cellular STAT3 but to another prominent anticancer drug target, TrxR1. We further identified that TrxR1 inhibition induced Prx2 and STAT3 oxidation, which subsequently blocked STAT3-dependent transcription. Moreover, previously identified inhibitors of STAT3 were also found to inhibit TrxR1, and likewise, established TrxR1 inhibitors block STAT3-dependent transcriptional activity. These results provide new insights into the complexities of STAT3 redox regulation while highlighting a novel mechanism to block aberrant STAT3 signaling in cancer cells.

Optimization of a high-throughput fluorescence polarization assay for STAT5B DNA binding domain-targeting inhibitors

Signal transducer and activator of transcription 5B (STAT5B) is constitutively activated in multiple cancers as a result of hyperactivating mutations or dysregulation of upstream effectors. Therapeutic strategies have predominantly targeted the Src homology 2 (SH2) domain to inhibit STAT phosphorylation, a prerequisite for STAT5B transcriptional activation. An alternative approach for STAT5B pharmacologic inhibition involves targeting the DNA-binding domain (DBD). However, this strategy remains relatively unexplored and is further hindered by the lack of a high-throughput in vitro engagement assay. Herein, we present the development and optimization of a STAT5B DBD fluorescence polarization (FP) assay, which facilitates rapid screening of small molecules targeting the STAT5B DBD though displacement of a fluorescently labelled oligonucleotide. The assay can generate a complete DNA-binding profile in 10 min, with signal stability up to 2 h, and minimal changes under a range of conditions including 10 % (v/v) glycerol, 15 % (v/v) DMSO, 1 mM NaCl, 0.02 % (w/v) BSA, and 1 mM EDTA. This assay is compatible with both unphosphorylated and phosphorylated STAT5B and demonstrates suitability for high-throughput screening with a Z' factor of 0.68 ± 0.07 and a signal to noise ratio of 6.7 ± 0.84.

Niclosamide inhibits the cell proliferation and enhances the responsiveness of esophageal cancer cells to chemotherapeutic agents

Niclosamide is an FDA-approved anthelmintic drug, and may elicit antineoplastic effects through direct STAT3 inhibition, which has been revealed in numerous human cancer cells. Chemotherapy is the standard treatment for advanced esophageal cancers, but also causes severe systemic side effects. The present study represents the first study evaluating the anticancer efficacy of niclosamide in esophageal cancers. Through western blot assay, it was demonstrated that niclosamide suppressed the STAT3 signaling pathway in esophageal adenocarcinoma cells (BE3) and esophageal squamous cell carcinoma cells (CE48T and CE81T). In addition, niclosamide inhibited cell proliferation as determined by 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay and soft agar colony forming assay, and induced cell apoptosis as determined by Annexin V and PI staining. The induction of p21 and G1 arrest of the cell cycle also was revealed in niclosamide-treated CE81T cells by qPCR and flow cytometric assays, respectively. Furthermore, in the combination analysis of niclosamide and chemotherapeutic agents by MTS assay, low IC₅₀ values were detected in cells co-treated with niclosamide, with the exception of cisplatin-treated CE81T cells. To confirm the results using an apoptosis assay, the apoptotic enhancement of niclosamide was only demonstrated in CE48T cells co-treated with 5-FU, cisplatin, or paclitaxel, and in BE3 cells co-treated with paclitaxel, but not in CE81T cells. These findings indicate a future clinical application of niclosamide in esophageal cancers.

Revisiting signal transducer and activator of transcription 3 (STAT3) as an anticancer target and its inhibitor discovery: Where are we and where should we go?

As a transcription factor, STAT3 protein transduces extracellular signals to the nucleus and then activates transcription of target genes. STAT3 has been well validated as an attractive anticancer target due to its important roles in cancer initiation and progression. Identification of specific and potent STAT3 inhibitors has attracted much attention, while there has been no STAT3 targeted drug approved for clinical application. In this review, we will briefly introduce STAT3 protein and review its role in multiple aspects of cancer, and systematically summarize the recent advances in discovery of STAT3 inhibitors, especially the ones discovered in the past five years. In the last part of the review, we will discuss the possible new strategies to overcome the difficulties of developing potent and specific STAT3 inhibitors and hope to shed light on future drug design and inhibitor optimization.

Signal Transducer and Activator of Transcription Protein 3 (STAT3): An Update on its Direct Inhibitors as Promising Anticancer Agents

BACKGROUND

Since Signal Transducer and Activator of Transcription 3 (STAT3) is a transcription factor which plays an important role in multiple aspects of cancer, including progression and migration, and it is constitutively activated in various human tumors, STAT3 inhibition has emerged as a validated strategy for the treatment of several malignancies. The aim of this review is to provide an update on the identification of new promising direct inhibitors targeting STAT3 domains, as potential anticancer agents.

METHODS

A thorough literature search focused on recently reported STAT3 direct inhibitors was undertaken. We considered the relevant developments regarding the STAT3 domains, which have been identified as potential drug targets.

RESULTS

In detail, 135 peer-reviewed papers and 7 patents were cited; the inhibitors we took into account targeted the DNA binding domain (compounds were grouped into natural derivatives, small molecules, peptides, aptamers and oligonucleotides), the SH2 binding domain (natural, semi-synthetic and synthetic compounds) and specific residues, like cysteines (natural, semi-synthetic, synthetic compounds and dual inhibitors) and tyrosine 705.

CONCLUSION

The huge number of direct STAT3 inhibitors recently identified demonstrates a strong interest in the investigation of this target, although it represents a challenging task considering that no drug targeting this enzyme is currently available for anticancer therapy. Notably, many studies on the available inhibitors evidenced that some of them possess a dual mechanism of action.

Niclosamide repositioning for treating cancer: Challenges and nano-based drug delivery opportunities

Drug repositioning may be defined as a process when new biological effects for known drugs are identified, leading to recommendations for new therapeutic applications. Niclosamide, present in the Model List of Essential Medicines, from the World Health Organization, has been used since the 1960s for tapeworm infection. Several preclinical studies have been shown its impressive anticancer effects, which led to clinical trials for colon and prostate cancer. Despite high expectations, proof of efficacy and safety are still required, which are associated with diverse biopharmaceutical challenges, such as the physicochemical properties of the drug and its oral absorption, and their relationship with clinical outcomes. Nanostructured systems are innovative drug delivery strategies, which may provide interesting pharmaceutical advantages for this candidate. The aim of this review is to discuss challenges involving niclosamide repositioning for cancer diseases, and the opportunities of therapeutic benefits from nanosctrutured system formulations containing this compound.

Genome-Wide Inhibition of Pro-atherogenic Gene Expression by Multi-STAT Targeting Compounds as a Novel Treatment Strategy of CVDs

Cardiovascular diseases (CVDs), including atherosclerosis, are globally the leading cause of death. Key factors contributing to onset and progression of atherosclerosis include the pro-inflammatory cytokines Interferon (IFN)α and IFNγ and the Pattern Recognition Receptor (PRR) Toll-like receptor 4 (TLR4). Together, they trigger activation of Signal Transducer and Activator of Transcription (STAT)s. Searches for compounds targeting the pTyr-SH2 interaction area of STAT3, yielded many small molecules, including STATTIC and STX-0119. However, many of these inhibitors do not seem STAT3-specific. We hypothesized that multi-STAT-inhibitors that simultaneously block STAT1, STAT2, and STAT3 activity and pro-inflammatory target gene expression may be a promising strategy to treat CVDs. Using comparative in silico docking of multiple STAT-SH2 models on multi-million compound libraries, we identified the novel multi-STAT inhibitor, C01L_F03. This compound targets the SH2 domain of STAT1, STAT2, and STAT3 with the same affinity and simultaneously blocks their activity and expression of multiple STAT-target genes in HMECs in response to IFNα. The same in silico and in vitro multi-STAT inhibiting capacity was shown for STATTIC and STX-0119. Moreover, C01L_F03, STATTIC and STX-0119 were also able to affect genome-wide interactions between IFNγ and TLR4 by commonly inhibiting pro-inflammatory and pro-atherogenic gene expression directed by cooperative involvement of STATs with IRFs and/or NF-κB. Moreover, we observed that multi-STAT inhibitors could be used to inhibit IFNγ+LPS-induced HMECs migration, leukocyte adhesion to ECs as well as impairment of mesenteric artery contractility. Together, this implicates that application of a multi-STAT inhibitory strategy could provide great promise for the treatment of CVDs.

A high-throughput fluorescence polarization assay for discovering inhibitors targeting the DNA-binding domain of signal transducer and activator of transcription 3 (STAT3)

Anti-cancer drug discovery efforts to directly inhibit the signal transducer and activator of transcription 3 (STAT3) have been active for over a decade following the discovery that 70% of cancers exhibit elevated STAT3 activity. The majority of research has focused on attenuating STAT3 activity through preventing homo-dimerization by targeting the SH2 or transcriptional activation domains. Such dimerization inhibitors have not yet reached the market. However, an alternative strategy focussed on preventing STAT3 DNA-binding through targeting the DNA-binding domain (DBD) offers new drug design opportunities. Currently, only EMSA and ELISA-based methods have been implemented with suitable reliability to characterize STAT3 DBD inhibitors. Herein, we present a new orthogonal, fluorescence polarization (FP) assay suitable for high-throughput screening of molecules. This assay, using a STAT3^127-688 construct, was developed and optimized to screen molecules that attenuate the STAT3:DNA association with good reliability (Z’ value > 0.6) and a significant contrast (signal-to-noise ratio > 15.0) at equilibrium. The assay system was stable over a 48 hour period. Significantly, the assay is homogeneous and simple to implement for high-throughput screening compared to EMSA and ELISA. Overall, this FP assay offers a new way to identify and characterize novel molecules that inhibit STAT3:DNA association.

Silibinin is a direct inhibitor of STAT3

We herein combined experimental and computational efforts to delineate the mechanism of action through which the flavonolignan silibinin targets STAT3. Silibinin reduced IL-6 inducible, constitutive, and acquired feedback activation of STAT3 at tyrosine 705 (Y705). Silibinin attenuated the inducible phospho-activation of Y705 in GFP-STAT3 genetic fusions without drastically altering the kinase activity of the STAT3 upstream kinases JAK1 and JAK2. A comparative computational study based on docking and molecular dynamics simulation over 14 different STAT3 inhibitors (STAT3i) predicted that silibinin could directly bind with high affinity to both the Src homology-2 (SH2) domain and the DNA-binding domain (DBD) of STAT3. Silibinin partially overlapped with the cavity occupied by other STAT3i in the SH2 domain to indirectly prevent Y705 phosphorylation, yet showing a unique binding mode. Moreover, silibinin was the only STAT3i predicted to establish direct interactions with DNA in its targeting to the STAT3 DBD. The prevention of STAT3 nuclear translocation, the blockade of the binding of activated STAT3 to its consensus DNA sequence, and the suppression of STAT3-directed transcriptional activity confirmed silibinin as a direct STAT3i. The unique characteristics of silibinin as a bimodal SH2- and DBD-targeting STAT3i make silibinin a promising lead for designing new, more effective STAT3i.