Cancer therapy using oligonucleotide-based STAT3 inhibitors: will they deliver?

B Cell Lymphoma Immunotherapy Using TLR9-Targeted Oligonucleotide STAT3 Inhibitors

Growing evidence links the aggressiveness of non-Hodgkin’s lymphoma, especially the activated B cell-like type diffuse large B cell lymphomas (ABC-DLBCLs) to Toll-like receptor 9 (TLR9)/MyD88 and STAT3 transcription factor signaling. Here, we describe a dual-function molecule consisting of a clinically relevant TLR9 agonist (CpG7909) and a STAT3 inhibitor in the form of a high-affinity decoy oligodeoxynucleotide (dODN). The CpG-STAT3dODN blocked STAT3 DNA binding and activity, thus reducing expression of downstream target genes, such as MYC and BCL2L1, in human and mouse lymphoma cells. We further demonstrated that injections (i.v.) of CpG-STAT3dODN inhibited growth of human OCI-Ly3 lymphoma in immunodeficient mice. Moreover, systemic CpG-STAT3dODN administration induced complete regression of the syngeneic A20 lymphoma, resulting in long-term survival of immunocompetent mice. Both TLR9 stimulation and concurrent STAT3 inhibition were critical for immune-mediated therapeutic effects, since neither CpG7909 alone nor CpG7909 co-injected with unconjugated STAT3dODN extended mouse survival. The CpG-STAT3dODN induced expression of genes critical to antigen-processing/presentation and Th1 cell activation while suppressing survival signaling. These effects resulted in the generation of lymphoma cell-specific CD8/CD4-dependent T cell immunity protecting mice from tumor rechallenge. Our results suggest that CpG-STAT3dODN as a systemic/local monotherapy or in combination with PD1 blockade can provide an opportunity for treating patients with B cell NHL.

Targeted inhibition of STATs and IRFs as a potential treatment strategy in cardiovascular disease

Key factors contributing to early stages of atherosclerosis and plaque development include the pro-inflammatory cytokines Interferon (IFN)α, IFNγ and Interleukin (IL)-6 and Toll-like receptor 4 (TLR4) stimuli. Together, they trigger activation of Signal Transducer and Activator of Transcription (STAT) and Interferon Regulatory Factor (IRF) families. In particular, STAT1, 2 and 3; IRF1 and 8 have recently been recognized as prominent modulators of inflammation, especially in immune and vascular cells during atherosclerosis. Moreover, inflammation-mediated activation of these STATs and IRFs coordinates a platform for synergistic amplification leading to pro-atherogenic responses.

Searches for STAT3-targeting compounds, exploring the pTyr-SH2 interaction area of STAT3, yielded many small molecules including natural products. Only a few inhibitors for other STATs, but none for IRFs, are described. Promising results for several STAT3 inhibitors in recent clinical trials predicts STAT3-inhibiting strategies may find their way to the clinic. However, many of these inhibitors do not seem STAT-specific, display toxicity and are not very potent. This illustrates the need for better models, and screening and validation tools for novel STAT and IRF inhibitors.

This review presents a summary of these findings. It postulates STAT1, STAT2 and STAT3 and IRF1 and IRF8 as interesting therapeutic targets and targeted inhibition could be a potential treatment strategy in CVDs. In addition, it proposes a pipeline approach that combines comparative in silico docking of STAT-SH2 and IRF-DBD models with in vitro STAT and IRF activation inhibition validation, as a novel tool to screen multi-million compound libraries and identify specific inhibitors for STATs and IRFs.

STAT3 Gene Silencing by Aptamer-siRNA Chimera as Selective Therapeutic for Glioblastoma

Glioblastoma (GBM) is the most frequent and aggressive primary brain tumor in adults, and despite advances in neuro-oncology, the prognosis for patients remains dismal. The signal transducer and activator of transcription-3 (STAT3) has been reported as a key regulator of the highly aggressive mesenchymal GBM subtype, and its direct silencing (by RNAi oligonucleotides) has revealed a great potential as an anti-cancer therapy. However, clinical use of oligonucleotide-based therapies is dependent on safer ways for tissue-specific targeting and increased membrane penetration. The objective of this study is to explore the use of nucleic acid aptamers as carriers to specifically drive a STAT3 siRNA to GBM cells in a receptor-dependent manner. Using an aptamer that binds to and antagonizes the oncogenic receptor tyrosine kinase PDGFRβ (Gint4.T), here we describe the design of a novel aptamer-siRNA chimera (Gint4.T-STAT3) to target STAT3. We demonstrate the efficient delivery and silencing of STAT3 in PDGFRβ⁺ GBM cells. Importantly, the conjugate reduces cell viability and migration in vitro and inhibits tumor growth and angiogenesis in vivo in a subcutaneous xenograft mouse model. Our data reveals Gint4.T-STAT3 conjugate as a novel molecule with great translational potential for GBM therapy.

Myeloid cells as a target for oligonucleotide therapeutics: turning obstacles into opportunities

Immunotherapies emerged as an alternative for cancer treatment, yet their clinical efficacies are still limited, especially in case of solid tumors. Myeloid immune cells, such as macrophages and myeloid-derived suppressor cells (MDSCs), are often hijacked by tumors and become pivotal inhibitors of antitumor immunity. Immunosuppressive functions of tumor-associated myeloid cells result from the activity of Signal Transducer and Activator of Transcription 3 (STAT3), a transcription factor with well-defined tumorigenic and tolerogenic roles in human cancers. To overcome challenges in the development of pharmacological STAT3 inhibitors, we recently developed oligonucleotide-based strategies for cell-selective, in vivo STAT3 targeting. Conjugation of a STAT3siRNA or decoy STAT3 inhibitors to synthetic Toll-like Receptor 9 (TLR9) agonists, CpG oligonucleotides, allowed for selective delivery into TLR9-positive cells. Cellular target for CpG-STAT3 inhibitors include non-malignant, tumor-associated myeloid cells, such as polymorphonuclear MDSCs, as well as cancer cells in acute myeloid leukemia, B cell lymphoma and in certain solid tumors. The chemically modified CpG-STAT3 inhibitors resist serum nucleases and thus can be administered intravenously. Their potency relies on the intracellular gain-of-function effect: release of the central immune checkpoint regulator (STAT3) to unleash proinflammatory signaling (CpG/TLR9) in the same antigen-presenting cell. At the cellular level, CpG-STAT3 inhibitors exert two-pronged effect by rescuing T cells from the immune checkpoint control while decreasing survival of cancer cells. In this article, we review the preclinical data on CpG-STAT3 inhibitors and discuss perspectives of using TLR9-targeted delivery of oligonucleotide therapeutics for the generation of novel, more effective and safer cancer immunotherapies.

CADM1 inhibits squamous cell carcinoma progression by reducing STAT3 activity

Although squamous cell carcinomas (SqCCs) of the lungs, head and neck, oesophagus, and cervix account for up to 30% of cancer deaths, the mechanisms that regulate disease progression remain incompletely understood. Here, we use gene transduction and human tumor xenograft assays to establish that the tumour suppressor Cell adhesion molecule 1 (CADM1) inhibits SqCC proliferation and invasion, processes fundamental to disease progression. We determine that the extracellular domain of CADM1 mediates these effects by forming a complex with HER2 and integrin α6β4 at the cell surface that disrupts downstream STAT3 activity. We subsequently show that treating CADM1 null tumours with the JAK/STAT inhibitor ruxolitinib mimics CADM1 gene restoration in preventing SqCC growth and metastases. Overall, this study identifies a novel mechanism by which CADM1 prevents SqCC progression and suggests that screening tumours for loss of CADM1 expression will help identify those patients most likely to benefit from JAK/STAT targeted chemotherapies.

Serum-resistant CpG-STAT3 decoy for targeting survival and immune checkpoint signaling in acute myeloid leukemia

Targeting oncogenic transcription factor signal transducer and activator of transcription 3 (STAT3) in acute myeloid leukemia (AML) can reduce blast survival and tumor immune evasion. Decoy oligodeoxynucleotides (dODNs), which comprise STAT3-specific DNA sequences are competitive inhibition of STAT3 transcriptional activity. To deliver STAT3dODN specifically to myeloid cells, we linked STAT3dODN to the Toll-like receptor 9 (TLR9) ligand, cytosine guanine dinucleotide (CpG). The CpG-STAT3dODN conjugates are quickly internalized by human and mouse TLR9(+)immune cells (dendritic cells, B cells) and the majority of patients' derived AML blasts, including leukemia stem/progenitor cells. Following uptake, CpG-STAT3dODNs are released from endosomes, and bind and sequester cytoplasmic STAT3, thereby inhibiting downstream gene expression in target cells. STAT3 inhibition in patients' AML cells limits their immunosuppressive potential by reduced arginase expression, thereby partly restoring T-cell proliferation. Partly chemically modified CpG-STAT3dODNs have >60 hours serum half-life which allows for IV administration to leukemia-bearing mice (50% effective dose ∼ 2.5 mg/kg). Repeated administration of CpG-STAT3dODN resulted in regression of human MV4-11 AML in mice. The antitumor efficacy of this strategy is further enhanced in immunocompetent mice by combining direct leukemia-specific cytotoxicity with immunogenic effects of STAT3 blocking/TLR9 triggering. CpG-STAT3dODN effectively reducedCbfb/MYH11/MplAML burden in various organs and eliminated leukemia stem/progenitor cells, mainly through CD8/CD4 T-cell-mediated immune responses. In contrast, small-molecule Janus kinase 2/STAT3 inhibitor failed to reproduce therapeutic effects of cell-selective CpG-STAT3dODN strategy. These results demonstrate therapeutic potential of CpG-STAT3dODN inhibitors with broad implications for treatment of AML and potentially other hematologic malignancies.