TAK-676: A Novel Stimulator of Interferon Genes (STING) Agonist Promoting Durable IFN-dependent Antitumor Immunity in Preclinical Studies.

Oncology therapies targeting the immune system have improved patient outcomes across a wide range of tumor types, but resistance due to an inadequate T-cell response in a suppressive tumor microenvironment (TME) remains a significant problem. New therapies that activate an innate immune response and relieve this suppression may be beneficial to overcome this hurdle. TAK-676 is a synthetic novel stimulator of interferon genes (STING) agonist designed for intravenous administration. Here we demonstrate that TAK-676 dose-dependently triggers activation of the STING signaling pathway and activation of type I interferons. Furthermore, we show that TAK-676 is a highly potent modulator of both the innate and adaptive immune system and that it promotes the activation of dendritic cells, natural killer cells, and T cells in preclinical models. In syngeneic murine tumor models in vivo, TAK-676 induces dose-dependent cytokine responses and increases the activation and proliferation of immune cells within the TME and tumor-associated lymphoid tissue. We also demonstrate that TAK-676 dosing results in significant STING-dependent antitumor activity, including complete regressions and durable memory T-cell immunity. We show that TAK-676 is well tolerated, exhibits dose-proportional pharmacokinetics in plasma, and exhibits higher exposure in tumor. The intravenous administration of TAK-676 provides potential treatment benefit in a broad range of tumor types. Further study of TAK-676 in first-in-human phase I trials is ongoing. Significance: TAK-676 is a novel systemic STING agonist demonstrating robust activation of innate and adaptive immune activity resulting in durable antitumor responses within multiple syngeneic tumor models. Clinical investigation of TAK-676 is ongoing.

Identification of Novel Carbocyclic Pyrimidine Cyclic Dinucleotide STING Agonists for Antitumor Immunotherapy Using Systemic Intravenous Route.

Stimulator of Interferon Genes (STING) plays an important role in innate immunity by inducing type I interferon production upon infection with intracellular pathogens. STING activation can promote increased T-cell activation and inflammation in the tumor microenvironment, resulting in antitumor immunity. Natural and synthetic cyclic dinucleotides (CDNs) are known to activate STING, and several synthetic CDN molecules are being investigated in the clinic using an intratumoral administration route. Here, we describe the identification of STING agonist 15a, a cyclic dinucleotide structurally diversified from natural ligands with optimized properties for systemic intravenous (iv) administration. Our studies have shown that STING activation by 15a leads to an acute innate immune response as measured by cytokine secretion and adaptive immune response via activation of CD8+ cytotoxic T-cells, which ultimately provides robust antitumor efficacy.

FAK activity sustains intrinsic and acquired ovarian cancer resistance to platinum chemotherapy.

Gene copy number alterations, tumor cell stemness, and the development of platinum chemotherapy resistance contribute to high-grade serous ovarian cancer (HGSOC) recurrence. Stem phenotypes involving Wnt-ß-catenin, aldehyde dehydrogenase activities, intrinsic platinum resistance, and tumorsphere formation are here associated with spontaneous gains in Kras, Myc and FAK (KMF) genes in a new aggressive murine model of ovarian cancer. Adhesion-independent FAK signaling sustained KMF and human tumorsphere proliferation as well as resistance to cisplatin cytotoxicity. Platinum-resistant tumorspheres can acquire a dependence on FAK for growth. Accordingly, increased FAK tyrosine phosphorylation was observed within HGSOC patient tumors surviving neo-adjuvant chemotherapy. Combining a FAK inhibitor with platinum overcame chemoresistance and triggered cell apoptosis. FAK transcriptomic analyses across knockout and reconstituted cells identified 135 targets, elevated in HGSOC, that were regulated by FAK activity and ß-catenin including Myc, pluripotency and DNA repair genes. These studies reveal an oncogenic FAK signaling role supporting chemoresistance.

Inhibition of FAK kinase activity preferentially targets cancer stem cells.

Because cancer stem cells (CSCs) have been implicated in chemo-resistance, metastasis and tumor recurrence, therapeutic targeting of CSCs holds promise to address these clinical challenges to cancer treatment. VS-4718 and VS-6063 are potent inhibitors of focal adhesion kinase (FAK), a non-receptor tyrosine kinase that mediates cell signals transmitted by integrins and growth factor receptors. We report here that inhibition of FAK kinase activity by VS-4718 or VS-6063 preferentially targets CSCs, as demonstrated by a panel of orthogonal CSC assays in cell line models and surgically resected primary breast tumor specimens cultured ex vivo. Oral administration of VS-4718 or VS-6063 to mice bearing xenograft models of triple-negative breast cancer (TNBC) significantly reduced the proportion of CSCs in the tumors, as evidenced by a reduced tumor-initiating capability upon re-implantation in limiting dilutions of cells prepared from these tumors. In contrast, the cytotoxic chemotherapeutic agents, paclitaxel and carboplatin, enriched for CSCs, consistent with previous reports that these cytotoxic agents preferentially target non-CSCs. Importantly, VS-4718 and VS-6063 attenuated the chemotherapy-induced enrichment of CSCs in vitro and delayed tumor regrowth following cessation of chemotherapy. An intriguing crosstalk between FAK and the Wnt/ß-catenin pathway was revealed wherein FAK inhibition blocks ß-catenin activation by reducing tyrosine 654 phosphorylation of ß-catenin. Furthermore, a constitutively active mutant form of ß-catenin reversed the preferential targeting of CSCs by FAK inhibition, suggesting that this targeting is mediated, at least in part, through attenuating ß-catenin activation. The preferential targeting of cancer stem cells by FAK inhibitors provides a rationale for the clinical development of FAK inhibitors aimed to increase durable responses for cancer patients.

Small-Molecule Reactivation of Mutant p53 to Wild-Type-like p53 through the p53-Hsp40 Regulatory Axis.

TP53 is the most frequently mutated gene in human cancer, and small-molecule reactivation of mutant p53 function represents an important anticancer strategy. A cell-based, high-throughput small-molecule screen identified chetomin (CTM) as a mutant p53 R175H reactivator. CTM enabled p53 to transactivate target genes, restored MDM2 negative regulation, and selectively inhibited the growth of cancer cells harboring mutant p53 R175H in vitro and in vivo. We found that CTM binds to Hsp40 and increases the binding capacity of Hsp40 to the p53 R175H mutant protein, causing a potential conformational change to a wild-type-like p53. Thus, CTM acts as a specific reactivator of the p53 R175H mutant form through Hsp40. These results provide new insights into the mechanism of reactivation of this specific p53 mutant.

PI3K/mTOR dual inhibitor VS-5584 preferentially targets cancer stem cells.

Cancer stem cells (CSC) have been implicated in disease recurrence, metastasis, and therapeutic resistance, but effective targeting strategies for these cells are still wanting. VS-5584 is a potent and selective dual inhibitor of mTORC1/2 and class I PI 3-kinases. Here, we report that VS-5584 is up to 30-fold more potent in inhibiting the proliferation and survival of CSC compared with non-CSC in solid tumor cell populations. VS-5584 preferentially diminished CSC levels in multiple mouse xenograft models of human cancer, as evidenced by marked reduction of tumor-initiating capacity in limiting dilution assays. Likewise, VS-5584 treatment ex vivo preferentially reduced CSC in surgically resected breast and ovarian patient tumors. In contrast, chemotherapeutics such as paclitaxel and cisplatin were less effective in targeting CSC than bulk tumor cells. Mechanistic investigations revealed that preferential targeting of CSC required inhibition of multiple components of the PI3K-mTOR pathway: coordinate RNAi-mediated silencing of PI3Kα, PI3Kß, and mTOR phenocopied the effect of VS-5584, exhibiting the strongest preferential targeting of CSC, while silencing of individual PI3K isoforms or mTOR failed to replicate the effect of VS-5584. Consistent with CSC ablation, VS-5584 delayed tumor regrowth following chemotherapy in xenograft models of small-cell lung cancer. Taken together, the preferential targeting of CSC prompts a new paradigm for clinical testing of VS-5584: clinical trials designed with CSC-directed endpoints may facilitate demonstration of the therapeutic benefit of VS-5584. We suggest that combining VS-5584 with classic chemotherapy that debulks tumors may engender a more effective strategy to achieve durable remissions in patients with cancer.

Merlin deficiency predicts FAK inhibitor sensitivity: a synthetic lethal relationship.

The goal of targeted therapy is to match a selective drug with a genetic lesion that predicts for drug sensitivity. In a diverse panel of cancer cell lines, we found that the cells most sensitive to focal adhesion kinase (FAK) inhibition lack expression of the neurofibromatosis type 2 (NF2) tumor suppressor gene product, Merlin. Merlin expression is often lost in malignant pleural mesothelioma (MPM), an asbestos-induced aggressive cancer with limited treatment options. Our data demonstrate that low Merlin expression predicts for increased sensitivity of MPM cells to a FAK inhibitor, VS-4718, in vitro and in tumor xenograft models. Disruption of MPM cell-cell or cell-extracellular matrix (ECM) contacts with blocking antibodies suggests that weak cell-cell adhesions in Merlin-negative MPM cells underlie their greater dependence on cell-ECM-induced FAK signaling. This provides one explanation of why Merlin-negative cells are vulnerable to FAK inhibitor treatment. Furthermore, we validated aldehyde dehydrogenase as a marker of cancer stem cells (CSCs) in MPM, a cell population thought to mediate tumor relapse after chemotherapy. Whereas pemetrexed and cisplatin, standard-of-care agents for MPM, enrich for CSCs, FAK inhibitor treatment preferentially eliminates these cells. These preclinical results provide the rationale for a clinical trial in MPM patients using a FAK inhibitor as a single agent after first-line chemotherapy. With this design, the FAK inhibitor could potentially induce a more durable clinical response through reduction of CSCs along with a strong antitumor effect. Furthermore, our data suggest that patients with Merlin-negative tumors may especially benefit from FAK inhibitor treatment.

Small GTPase RhoE/Rnd3 is a critical regulator of Notch1 signaling.

Aberrations of Notch signaling have been implicated in a variety of human cancers. Oncogenic mutations in NOTCH1 are common in human T-cell leukemia and lymphomas. However, loss-of-function somatic mutations in NOTCH1 arising in solid tumors imply a tumor suppressor function, which highlights the need to understand Notch signaling more completely. Here, we describe the small GTPase RhoE/Rnd3 as a downstream mediator of Notch signaling in squamous cell carcinomas (SCC) that arise in skin epithelia. RhoE is a transcriptional target of activated Notch1, which is attenuated broadly in SCC cells. RhoE depletion suppresses Notch1-mediated signaling in vitro, rendering primary keratinocytes resistant to Notch1-mediated differentiation and thereby favoring a proliferative cell fate. Mechanistic investigations indicated that RhoE controls a key step in Notch1 signaling by mediating nuclear translocation of the activated portion of Notch1 (N1IC) through interaction with importins. Our results define RhoE as a Notch1 target that is essential for recruitment of N1IC to the promoters of Notch1 target genes, establishing a regulatory feedback loop in Notch1 signaling. This molecular circuitry may inform distinct cell fate decisions to Notch1 in epithelial tissues, where carcinomas such as SCC arise.

High-throughput, high-content screening for novel pigmentation regulators using a keratinocyte/melanocyte co-culture system.

Skin pigmentation is a complex process including melanogenesis within melanocytes and melanin transfer to the keratinocytes. To develop a comprehensive screening method for novel pigmentation regulators, we used immortalized melanocytes and keratinocytes in co-culture to screen large numbers of compounds. High-throughput screening plates were subjected to digital automated microscopy to quantify the pigmentation via brightfield microscopy. Compounds with pigment suppression were secondarily tested for their effects on expression of microphthalmia transcription factor (MITF) and several pigment regulatory genes, and further validated in terms of non-toxicity to keratinocytes/melanocytes and dose-dependent activity. The results demonstrate a high-throughput, high-content screening approach, which is applicable to the analysis of large chemical libraries using a co-culture system. We identified candidate pigmentation inhibitors from 4000 screened compounds including zoxazolamine, 3-methoxycatechol and alpha-mangostin, which were also shown to modulate expression of MITF and several key pigmentation factors and are worthy of further evaluation for potential translation to clinical use.

Tumor suppressor alterations cooperate to drive aggressive mesotheliomas with enriched cancer stem cells via a p53-miR-34a-c-Met axis.

Malignant mesothelioma is a highly aggressive, asbestos-related cancer frequently marked by mutations of both NF2 and CDKN2A. We demonstrate that germline knockout of one allele of each of these genes causes accelerated onset and progression of asbestos-induced malignant mesothelioma compared with asbestos-exposed Nf2(+/-) or wild-type mice. Ascites from some Nf2(+/-);Cdkn2a(+/-) mice exhibited large tumor spheroids, and tail vein injections of malignant mesothelioma cells established from these mice, but not from Nf2(+/-) or wild-type mice, produced numerous tumors in the lung, suggesting increased metastatic potential of tumor cells from Nf2(+/-);Cdkn2a(+/-) mice. Intraperitoneal injections of malignant mesothelioma cells derived from Nf2(+/-);Cdkn2a(+/-) mice into severe combined immunodeficient mice produced tumors that penetrated the diaphragm and pleural cavity and harbored increased cancer stem cells (CSC). Malignant mesothelioma cells from Nf2(+/-);Cdkn2a(+/-) mice stained positively for CSC markers and formed CSC spheroids in vitro more efficiently than counterparts from wild-type mice. Moreover, tumor cells from Nf2(+/-);Cdkn2a(+/-) mice showed elevated c-Met expression/activation, which was partly dependent on p53-mediated regulation of miR-34a and required for tumor migration/invasiveness and maintenance of the CSC population. Collectively, these studies demonstrate in vivo that inactivation of Nf2 and Cdkn2a cooperate to drive the development of highly aggressive malignant mesotheliomas characterized by enhanced tumor spreading capability and the presence of a CSC population associated with p53/miR-34a-dependent activation of c-Met. These findings suggest that cooperativity between losses of Nf2 and Cdkn2a plays a fundamental role in driving the highly aggressive tumorigenic phenotype considered to be a hallmark of malignant mesothelioma.

A positive FGFR3/FOXN1 feedback loop underlies benign skin keratosis versus squamous cell carcinoma formation in humans.

Seborrheic keratoses (SKs) are common, benign epithelial tumors of the skin that do not, or very rarely, progress into malignancy, for reasons that are not understood. We investigated this by gene expression profiling of human SKs and cutaneous squamous cell carcinomas (SCCs) and found that several genes previously connected with keratinocyte tumor development were similarly modulated in SKs and SCCs, whereas the expression of others differed by only a few fold. In contrast, the tyrosine kinase receptor FGF receptor-3 (FGFR3) and the transcription factor forkhead box N1 (FOXN1) were highly expressed in SKs, and close to undetectable in SCCs. We also showed that increased FGFR3 activity was sufficient to induce FOXN1 expression, counteract the inhibitory effect of EGFR signaling on FOXN1 expression and differentiation, and induce differentiation in a FOXN1-dependent manner. Knockdown of FOXN1 expression in primary human keratinocytes cooperated with oncogenic RAS in the induction of SCC-like tumors, whereas increased FOXN1 expression triggered the SCC cells to shift to a benign SK-like tumor phenotype, which included increased FGFR3 expression. Thus,we have uncovered a positive regulatory loop between FGFR3 and FOXN1 that underlies a benign versus malignant skin tumor phenotype.

Novel cross-talk between three cardiovascular regulators: thrombin cleavage fragment of Jagged1 induces fibroblast growth factor 1 expression and release.

Angiogenesis is controlled by several regulatory mechanisms, including the Notch and fibroblast growth factor (FGF) signaling pathways. FGF1, a prototype member of FGF family, lacks a signal peptide and is released through an endoplasmic reticulum-Golgi-independent mechanism. A soluble extracellular domain of the Notch ligand Jagged1 (sJ1) inhibits Notch signaling and induces FGF1 release. Thrombin, a key protease of the blood coagulation cascade and a potent inducer of angiogenesis, stimulates rapid FGF1 release through a mechanism dependent on the major thrombin receptor protease-activated receptor (PAR) 1. This study demonstrates that thrombin cleaves Jagged1 in its extracellular domain. The sJ1 form produced as a result of thrombin cleavage inhibits Notch-mediated CBF1/Suppressor of Hairless [(Su(H)]/Lag-1-dependent transcription and induces FGF1 expression and release. The overexpression of Jagged1 in PAR1 null cells results in a rapid thrombin-induced export of FGF1. These data demonstrate the existence of novel cross-talk between thrombin, FGF, and Notch signaling pathways, which play important roles in vascular formation and remodeling.

EGFR signalling as a negative regulator of Notch1 gene transcription and function in proliferating keratinocytes and cancer.

The Notch1 gene has an important role in mammalian cell-fate decision and tumorigenesis. Upstream control mechanisms for transcription of this gene are still poorly understood. In a chemical genetics screen for small molecule activators of Notch signalling, we identified epidermal growth factor receptor (EGFR) as a key negative regulator of Notch1 gene expression in primary human keratinocytes, intact epidermis and skin squamous cell carcinomas (SCCs). The underlying mechanism for negative control of the Notch1 gene in human cells, as well as in a mouse model of EGFR-dependent skin carcinogenesis, involves transcriptional suppression of p53 by the EGFR effector c-Jun. Suppression of Notch signalling in cancer cells counteracts the differentiation-inducing effects of EGFR inhibitors while, at the same time, synergizing with these compounds in induction of apoptosis. Thus, our data reveal a key role of EGFR signalling in the negative regulation of Notch1 gene transcription, of potential relevance for combinatory approaches for cancer therapy.

Sphingosine kinase 1 is a critical component of the copper-dependent FGF1 export pathway.

Sphingosine kinase 1 catalyzes the formation of sphingosine-1-phosphate, a lipid mediator involved in the regulation of angiogenesis. Sphingosine kinase 1 is constitutively released from cells, even though it lacks a classical signal peptide sequence. Because copper-dependent non-classical stress-induced release of FGF1 also regulates angiogenesis, we questioned whether sphingosine kinase 1 is involved in the FGF1 release pathway. We report that (i) the coexpression of sphingosine kinase 1 with FGF1 inhibited the release of sphingosine kinase 1 at 37 degrees C; (ii) sphingosine kinase 1 was released at 42 degrees C in complex with FGF1; (iii) sphingosine kinase 1 null cells failed to release FGF1 at stress; (iv) sphingosine kinase 1 is a high affinity copper-binding protein which formed a complex with FGF1 in a cell-free system, and (v) sphingosine kinase 1 over expression rescued the release of FGF1 from inhibition by the copper chelator, tetrathiomolybdate. We propose that sphingosine kinase 1 is a component of the copper-dependent FGF1 release pathway.

Notch1 is a p53 target gene involved in human keratinocyte tumor suppression through negative regulation of ROCK1/2 and MRCKalpha kinases.

Little is known about the regulation and function of the Notch1 gene in negative control of human tumors. Here we show that Notch1 gene expression and activity are substantially down-modulated in keratinocyte cancer cell lines and tumors, with expression of this gene being under p53 control in these cells. Genetic suppression of Notch signaling in primary human keratinocytes is sufficient, together with activated ras, to cause aggressive squamous cell carcinoma formation. Similar tumor-promoting effects are also caused by in vivo treatment of mice, grafted with keratinocytes expressing oncogenic ras alone, with a pharmacological inhibitor of endogenous Notch signaling. These effects are linked with a lesser commitment of keratinocytes to differentiation, an expansion of stem cell populations, and a mechanism involving up-regulation of ROCK1/2 and MRCKalpha kinases, two key effectors of small Rho GTPases previously implicated in neoplastic progression. Thus, the Notch1 gene is a p53 target with a role in human tumor suppression through negative regulation of Rho effectors.

Thrombin induces rapid PAR1-mediated non-classical FGF1 release.

Thrombin induces cell proliferation and migration during vascular injury. We report that thrombin rapidly stimulated expression and release of the pro-angiogenic polypeptide fibroblast growth factor 1 (FGF1). Thrombin failed to induce FGF1 release from protease-activated receptor 1 (PAR1) null fibroblasts, indicating that this effect was dependent on PAR1. Similarly to thrombin, FGF1 expression and release were induced by TRAP, a specific oligopeptide agonist of PAR1. These results identify a novel aspect of the crosstalk between FGF and thrombin signaling pathways which both play important roles in tissue repair and angiogenesis.

Release of FGF1 and p40 synaptotagmin 1 correlates with their membrane destabilizing ability.

Fibroblast growth factor (FGF)1 is released from cells as a constituent of a complex that contains the small calcium binding protein S100A13, and the p40 kDa form of synaptotagmin (Syt)1, through an ER-Golgi-independent stress-induced pathway. FGF1 and the other components of its secretory complex are signal peptide-less proteins. We examined their capability to interact with lipid bilayers by studying protein-induced carboxyfluorescein release from liposomes of different phospholipid (pL) compositions. FGF1, p40 Syt1, and S100A13 induced destabilization of liposomes composed of acidic but not of zwitterionic pL. We produced mutants of FGF1 and p40 Syt1, in which specific basic amino acid residues in the regions that bind acidic pL were substituted. The ability of these mutants to induce liposomes destabilization was strongly attenuated, and they exhibited drastically diminished spontaneous and stress-induced release. Apparently, the non-classical release of FGF1 and p40 Syt1 involves destabilization of membranes containing acidic pL.

The intracellular domain of Notch ligand Delta1 induces cell growth arrest.

Notch signaling involves proteolytic cleavage of the transmembrane Notch receptor after binding to its transmembrane ligands, Delta or Jagged; and the resultant soluble intracellular domain of Notch stimulates a cascade of transcriptional events. The Delta1 ligand also undergoes proteolytic cleavage upon Notch binding, resulting in the production of a free intracellular domain. We demonstrate that the expression of the intracellular domain of Delta1 results in a non-proliferating senescent-like cell phenotype which is dependent on the expression of the cell cycle inhibitor, p21, and is abolished by co-expression of constitutively active Notch1. These data suggest a new intracellular role for Delta1.

The non-transmembrane form of Delta1, but not of Jagged1, induces normal migratory behavior accompanied by fibroblast growth factor receptor 1-dependent transformation.

The interactions between Notch (N) receptors and their transmembrane ligands, Jagged1 (JI) and Delta1 (Dl1), mediate signaling events between neighboring cells that are crucial during embryonal development and in adults. Since the non-transmembrane extracellular form of J1 acts as an antagonist of N activation in NIH 3T3 mouse fibroblast cells and induces fibroblast growth factor 1 (FGF1)-dependent transformation (Small, D., Kovalenko, D., Soldi, R., Mandinova, A., Kolev, V., Trifonova, R., Bagala, C., Kacer, D., Battelli, C., Liaw, L., Prudovsky, I., and Maciag, T. (2003) J. Biol. Chem. 278, 16405-16413), we examined the potential redundant functions of the two subfamilies of Notch ligands and report that while the soluble (s) forms of both Dl1 and J1 act as N signaling antagonists in NIH 3T3 cells, they do display disparate functions. While sJ1 induced an attenuation of cell motility which is accompanied by a decrease in actin stress fibers and an increase in adherence junctions, sDl1 does not. However, sJ1, like sDl1, induces a NIH 3T3 cell tranformed phenotype mediated by FGF signaling. Because the inhibition of N signaling by sJ1 and sDl1 is rescued by dominant-negative Src expression, we suggest that there may be cooperation between the Notch and Src signaling pathways.

Copper chelation represses the vascular response to injury.

The induction of an acute inflammatory response followed by the release of polypeptide cytokines and growth factors from peripheral blood monocytes has been implicated in mediating the response to vascular injury. Because the Cu2+-binding proteins IL-1alpha and fibroblast growth factor 1 are exported into the extracellular compartment in a stress-dependent manner by using intracellular Cu2+ to facilitate the formation of S100A13 heterotetrameric complexes and these signal peptideless polypeptides have been implicated as regulators of vascular injury in vivo, we examined the ability of Cu2+ chelation to repress neointimal thickening in response to injury. We observed that the oral administration of the Cu2+ chelator tetrathiomolybdate was able to reduce neointimal thickening after balloon injury in the rat. Interestingly, although immunohistochemical analysis of control neointimal sections exhibited prominent staining for MAC1, IL-1alpha, S100A13, and the acidic phospholipid phosphatidylserine, similar sections obtained from tetrathiomolybdate-treated animals did not. Further, adenoviral gene transfer of the IL-1 receptor antagonist during vascular injury also significantly reduced the area of neointimal thickening. Our data suggest that intracellular copper may be involved in mediating the response to injury in vivo by its ability to regulate the stress-induced release of IL-1alpha by using the nonclassical export mechanism employed by human peripheral blood mononuclear cells in vitro.