Targeting RNA Polymerase I Transcription Activity in Osteosarcoma: Pre-Clinical Molecular and Animal Treatment Studies

The survival rate of patients with osteosarcoma (OS) has not improved over the last 30 years. Mutations in the genes TP53, RB1 and c-Myc frequently occur in OS and enhance RNA Polymerase I (Pol I) activity, thus supporting uncontrolled cancer cell proliferation. We therefore hypothesised that Pol I inhibition may be an effective therapeutic strategy for this aggressive cancer. The Pol I inhibitor CX-5461 has demonstrated therapeutic efficacy in different cancers in pre-clinical and phase I clinical trials; thus, the effects were determined on ten human OS cell lines. Following characterisation using genome profiling and Western blotting, RNA Pol I activity, cell proliferation and cell cycle progression were evaluated in vitro, and the growth of TP53 wild-type and mutant tumours was measured in a murine allograft model and in two human xenograft OS models. CX-5461 treatment resulted in reduced ribosomal DNA (rDNA) transcription and Growth 2 (G2)-phase cell cycle arrest in all OS cell lines. Additionally, tumour growth in all allograft and xenograft OS models was effectively suppressed without apparent toxicity. Our study demonstrates the efficacy of Pol I inhibition against OS with varying genetic alterations. This study provides pre-clinical evidence to support this novel therapeutic approach in OS.

Abstract 2155: PMR-116, a novel inhibitor of ribosome biogenesis with antitumor activity in preclinical models of prostate cancer

Advanced prostate cancer is characterised by mutations and amplifications of genes involved in regulating protein synthesis. PTEN-loss stimulates activity of the mTOR pathway, while amplification of MYC leads to increased ribosome biogenesis and elevated mRNA translation rate. Our previous work has demonstrated the efficacy of co-targeting ribosome biogenesis, via inhibition of RNA Pol I activity, and 4E-BP1 phosphorylation to suppress prostate cancer growth in vivo in GEMM of PCa and in patient-derived xenografts (Rebello et al., Clinical Cancer Research, 2016; Lawrence et al., European Urology, 2018).

In a collaboration with Pimera Inc., we investigated the efficacy of their new lead RNA Pol I inhibitor PMR-116 in models of prostate cancer. PMR-116 is well tolerated in vivo in mice and can be given at 300mg/kg weekly. Using the Hi-MYC mouse model of PCa we show that dosing 6 month old mice once weekly for 4 weeks can decrease the incidence of invasive lesions by up to 85% compared to vehicle control while reverting glands to patterns of low grade intraepithelial neoplasia. PMR-116 rapidly inhibits proliferation in the Hi-MYC model with a 50% decrease in Ki67 observed 12 hours after oral administration. Conversely, PMR-116 showed minimal anti-tumour efficacy in the PTEN-null model of PCa suggesting that elevated MYC signalling may be required for optimal response.

To further validate our promising GEMM results in more clinically relevant human-derived models, we used patient-derived xenografts lines we established from multidrug-resistant, metastatic PCa (Lawrence et al, European Urology, 2018). PMR-116 treatment decreased tumour volume in all PDX tested including complete response in a line in which tumour volume decreased by ~90% compared to baseline.

We believe this new RNA Pol I inhibitor shows promising results in a wide range of preclinical models and may exert higher efficacy in tumours expressing high levels of MYC. PMR-116 is currently in Phase I dose escalation trial in patient with solid tumours (ACTRN12620001146987).

Citation Format: Alisee Huglo, Richard Rebello, Mitchell Lawrence, Gail Risbridger, Denis Drygin, Mustapha Haddach, Katherine Hannan, Ross Hannan, Luc Furic. PMR-116, a novel inhibitor of ribosome biogenesis with antitumor activity in preclinical models of prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2155.

Cancer-cell-secreted extracellular vesicles suppress insulin secretion through miR-122 to impair systemic glucose homeostasis and contribute to tumour growth

Epidemiological studies demonstrate an association between breast cancer (BC) and systemic dysregulation of glucose metabolism. However, how BC influences glucose homeostasis remains unknown. We show that BC-derived extracellular vesicles (EVs) suppress pancreatic insulin secretion to impair glucose homeostasis. EV-encapsulated miR-122 targets PKM in β-cells to suppress glycolysis and ATP-dependent insulin exocytosis. Mice receiving high-miR-122 EVs or bearing BC tumours exhibit suppressed insulin secretion, enhanced endogenous glucose production, impaired glucose tolerance and fasting hyperglycaemia. These effects contribute to tumour growth and are abolished by inhibiting EV secretion or miR-122, restoring PKM in β-cells or supplementing insulin. Compared with non-cancer controls, patients with BC have higher levels of circulating EV-encapsulated miR-122 and fasting glucose concentrations but lower fasting insulin; miR-122 levels are positively associated with glucose and negatively associated with insulin. Therefore, EV-mediated impairment of whole-body glycaemic control may contribute to tumour progression and incidence of type 2 diabetes in some patients with BC.

The therapeutic potential of RNA Polymerase I transcription inhibitor, CX-5461, in uterine leiomyosarcoma

Background. Uterine leiomyosarcoma is a rare aggressive smooth muscle cancer with poor survival rates. RNA Polymerase I (Pol I) activity is elevated in many cancers supporting tumour growth and prior studies in uterine leiomyosarcoma revealed enlarged nucleoli and upregulated Pol I activity-related genes. This study aimed to investigate the anti-tumour potential of CX-5461, a Pol I transcription inhibitor currently being evaluated in clinical trials for several cancers, against the human uterine leiomyosarcoma cell line, SK-UT-1. Methods. SK-UT-1 was characterised using genome profiling and western blotting. The anti-tumour effects of CX-5461 were investigated using cell proliferation assays, expression analysis using qRT-PCR, and BrdU/PI based cell cycle analysis. Results. Genetic analysis of SK-UT-1 revealed mutations in TP53, RB1, PTEN, APC and TSC1 & 2, all potentially associated with increased Pol I activity. Protein expression analysis showed dysregulated p53, RB1 and c-Myc. CX-5461 treatment resulted in an anti-proliferation response, G2 phase cell-cycle arrest and on-target activity demonstrated by reduced ribosomal DNA transcription. Conclusions. SK-UT-1 was confirmed as a representative model of uterine leiomyosarcoma and CX-5461 has significant potential as a novel adjuvant for this rare cancer.

Ribosome biogenesis during cell cycle arrest fuels EMT in development and disease

Ribosome biogenesis is a canonical hallmark of cell growth and proliferation. Here we show that execution of Epithelial-to-Mesenchymal Transition (EMT), a migratory cellular program associated with development and tumor metastasis, is fueled by upregulation of ribosome biogenesis during G1/S arrest. This unexpected EMT feature is independent of species and initiating signal, and is accompanied by release of the repressive nucleolar chromatin remodeling complex (NoRC) from rDNA, together with recruitment of the EMT-driving transcription factor Snai1 (Snail1), RNA Polymerase I (Pol I) and the Upstream Binding Factor (UBF). EMT-associated ribosome biogenesis is also coincident with increased nucleolar recruitment of Rictor, an essential component of the EMT-promoting mammalian target of rapamycin complex 2 (mTORC2). Inhibition of rRNA synthesis in vivo differentiates primary tumors to a benign, Estrogen Receptor-alpha (ERα) positive, Rictor-negative phenotype and reduces metastasis. These findings implicate the EMT-associated ribosome biogenesis program with cellular plasticity, de-differentiation, cancer progression and metastatic disease.

Inhibition of RNA polymerase I transcription initiation by CX-5461 activates non-canonical ATM/ATR signaling

RNA polymerase I (Pol I)-mediated transcription of the ribosomal RNA genes (rDNA) is confined to the nucleolus and is a rate-limiting step for cell growth and proliferation. Inhibition of Pol I by CX-5461 can selectively induce p53-mediated apoptosis of tumour cells in vivo. Currently, CX-5461 is in clinical trial for patients with advanced haematological malignancies (Peter Mac, Melbourne). Here we demonstrate that CX-5461 also induces p53-independent cell cycle checkpoints mediated by ATM/ATR signaling in the absence of DNA damage. Further, our data demonstrate that the combination of drugs targeting ATM/ATR signaling and CX-5461 leads to enhanced therapeutic benefit in treating p53-null tumours in vivo, which are normally refractory to each drug alone. Mechanistically, we show that CX-5461 induces an unusual chromatin structure in which transcriptionally competent relaxed rDNA repeats are devoid of transcribing Pol I leading to activation of ATM signaling within the nucleoli. Thus, we propose that acute inhibition of Pol transcription initiation by CX-5461 induces a novel nucleolar stress response that can be targeted to improve therapeutic efficacy.

Abstract IA10: Drugging the ribosome at the level of synthesis and translation to treat solid and hematologic cancers

Recent findings by our group have been instrumental in the development of the novel selective inhibitors of RNA Polymerase I (Pol I) (Drygin et al., Cancer Research, 2011; Bywater et al. Cancer Cell, 2012). This work has led to the fundamental discovery that ribosomal gene transcription by Pol I is not simply a “housekeeping” process in cancer cells but is highly regulated to maintain their viability (Bywater et al. Nature Reviews Cancer, 2013). Strikingly, inhibition of Pol I transcription shows a profound selectivity for malignant over normal cells in preclinical studies.

As with the majority of targeted therapies, despite initial favorable responses to approaches that target ribosome synthesis and/or function in MYC-driven lymphoma models, resistant disease emerges. It is increasingly clear that maximizing the inhibition of key signaling networks as a whole improves anti-tumor response. The well-established reliance of MYC-driven malignancies on elevated rates of ribosome biogenesis, mTORC1/eIF4E-driven protein synthesis, and cell growth makes them vulnerable to therapeutic strategies that target the ribosome. Thus we hypothesized that the simultaneous targeting of the ribosome at multiple points would antagonize the development of acquired resistance and consequently prolong survival in MYC-driven cancer models. We will present data to demonstrate that targeting both ribosome synthesis and function through the combination of novel inhibitors of RNA polymerase I transcription, and PI3K/AKT/mTOR signaling inhibitors or PIM Kinase inhibitors provides a significant increase in survival compared to treatment with single agents (Devlin et al., Cancer Discovery 2016; Rebello et al., Clinical Cancer Res. 2016). We will also discuss the molecular mechanism by which multipoint targeting of the ribosome synergizes to increase survival. Finally we will discuss our collaboration with Pimera, Inc. to develop highly selective second generation RNA Pol I inhibitors. The lead compound PMR-116 is showing exceptional activity in transgenic models of malignancy, including MLL-ENL AML and Vk*MYC driven multiple myeloma. We anticipate this compound will enter the clinic in 2017.

Citation Format: Ross D. Hannan, Nadine Hein, Katherine M. Hannan, Gretchen Poortinga, Elaine Sanij, Jirawas Sornkom, Kylee MacLachlan, Andrew Cuddihy, Carleen Cullinane, Luc Furic, Denis Drygin, Mustapha Haddach, Simon Harrison, Grant McArthur, Richard B. Pearson. Drugging the ribosome at the level of synthesis and translation to treat solid and hematologic cancers. [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr IA10.

Abstract B23: Inhibition of ribosomal RNA synthesis as a new therapeutic approach to treat advanced prostate cancer

Background: Prostate epithelium is exquisitely sensitive to the overexpression of the proto-oncogene MYC which causes neoplastic transformation. Indeed, MYC protein is almost universally overexpressed in metastatic castration-resistant prostate cancer (CRPC) making targeting MYC an attractive option for treating advanced stage disease. Unfortunately, the development of therapeutic agents directly targeting MYC has been largely unsuccessful, thus emphasizing the need to indirectly target MYC activity through inhibition of downstream cellular processes it regulates. One of the main effects of MYC in cancer cells is to accelerate proliferative growth via stimulation of high levels of ribosome biogenesis. Accordingly, the control of protein synthesis rate has emerged as the “Achilles' heel” of a wide array of tumors. MYC also regulates and cooperates with PIM kinases to increase the activity of the eIF4F translation initiation complex and MYC-driven tumors are addicted to eIF4E. Here, we investigate the efficacy of a single and dual approach targeting ribosome biogenesis and function to treat prostate cancer (PC).

Experimental design: We employed numerous models of PC, including a novel CRPC patient derived xenograft system, which showed the pre-clinical efficacy of therapies that combine to target MYC directed signaling to the ribosome. The inhibition of ribosomal RNA (rRNA) synthesis with CX-5461, a potent, selective and orally bioavailable inhibitor of RNA polymerase I (Pol I) transcription has been successfully exploited therapeutically, but only in models of hematological malignancy. CX-5461 and CX-6258, a pan-PIM kinase inhibitor, were tested alone and in combination in PC cell lines, in Hi-MYC and PTEN-deficient mouse models and in patient derived xenografts (PDX) of metastatic tissue obtained from a castration-resistant PC patient.

Results: CX-5461 inhibited anchorage-independent growth and induced cell cycle arrest in PC cell lines at nanomolar concentrations. Oral administration of 50 mg/kg CX-5461 induced p53 expression and activity and reduced proliferation (Ki-67) and invasion (loss of ductal actin) in Hi-MYC tumors, but not in PTEN null (low MYC) tumors. While 100 mg/kg CX-6258 showed limited effect alone, its combination with CX-5461 further suppressed proliferation and dramatically reduced large invasive lesions in both models. This rational combination strategy significantly inhibited proliferation and induced cell death in PDX of PC.

Conclusion: Our results demonstrate preclinical efficacy of targeting the ribosome at multiple levels and provide a new approach for the treatment of PC. In addition, a key conclusion of our study is that the androgen receptor (AR) presence or activity has no significant impact on the therapeutic activity of our novel combination therapy. Therefore, we believe our new exciting combination therapy could be used in the clinic in combination with current anti-androgens or as salvage therapy in multi-drug resistant CRPC.

Citation Format: Richard J. Rebello, Eric Kusnadi, Don P. Cameron, Helen B. Pearson, Analia Lesmana, Jennifer R. Devlin, Denis Drygin, Ashlee K. Clark, Laura Porter, John Pedersen, Shahneen Sandhu, Gail P. Risbridger, Richard B. Pearson, Ross D. Hannan, Luc Furic. Inhibition of ribosomal RNA synthesis as a new therapeutic approach to treat advanced prostate cancer. [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr B23.

The Dual Inhibition of RNA Pol I Transcription and PIM Kinase as a New Therapeutic Approach to Treat Advanced Prostate Cancer

PURPOSE

The MYC oncogene is frequently overexpressed in prostate cancer. Upregulation of ribosome biogenesis and function is characteristic of MYC-driven tumors. In addition, PIM kinases activate MYC signaling and mRNA translation in prostate cancer and cooperate with MYC to accelerate tumorigenesis. Here, we investigate the efficacy of a single and dual approach targeting ribosome biogenesis and function to treat prostate cancer.

EXPERIMENTAL DESIGN

The inhibition of ribosomal RNA (rRNA) synthesis with CX-5461, a potent, selective, and orally bioavailable inhibitor of RNA polymerase I (Pol I) transcription, has been successfully exploited therapeutically but only in models of hematologic malignancy. CX-5461 and CX-6258, a pan-PIM kinase inhibitor, were tested alone and in combination in prostate cancer cell lines, in Hi-MYC- and PTEN-deficient mouse models and in patient-derived xenografts (PDX) of metastatic tissue obtained from a patient with castration-resistant prostate cancer.

RESULTS

CX-5461 inhibited anchorage-independent growth and induced cell-cycle arrest in prostate cancer cell lines at nanomolar concentrations. Oral administration of 50 mg/kg CX-5461 induced TP53 expression and activity and reduced proliferation (MKI67) and invasion (loss of ductal actin) in Hi-MYC tumors, but not in PTEN-null (low MYC) tumors. While 100 mg/kg CX-6258 showed limited effect alone, its combination with CX-5461 further suppressed proliferation and dramatically reduced large invasive lesions in both models. This rational combination strategy significantly inhibited proliferation and induced cell death in PDX of prostate cancer.

CONCLUSIONS

Our results demonstrate preclinical efficacy of targeting the ribosome at multiple levels and provide a new approach for the treatment of prostate cancer. Clin Cancer Res; 22(22); 5539-52. ©2016 AACR.

The Dual Inhibition of RNA Pol I Transcription and PIM Kinase as a New Therapeutic Approach to Treat Advanced Prostate Cancer

PURPOSE

The MYC oncogene is frequently overexpressed in prostate cancer. Upregulation of ribosome biogenesis and function is characteristic of MYC-driven tumors. In addition, PIM kinases activate MYC signaling and mRNA translation in prostate cancer and cooperate with MYC to accelerate tumorigenesis. Here, we investigate the efficacy of a single and dual approach targeting ribosome biogenesis and function to treat prostate cancer.

EXPERIMENTAL DESIGN

The inhibition of ribosomal RNA (rRNA) synthesis with CX-5461, a potent, selective, and orally bioavailable inhibitor of RNA polymerase I (Pol I) transcription, has been successfully exploited therapeutically but only in models of hematologic malignancy. CX-5461 and CX-6258, a pan-PIM kinase inhibitor, were tested alone and in combination in prostate cancer cell lines, in Hi-MYC- and PTEN-deficient mouse models and in patient-derived xenografts (PDX) of metastatic tissue obtained from a patient with castration-resistant prostate cancer.

RESULTS

CX-5461 inhibited anchorage-independent growth and induced cell-cycle arrest in prostate cancer cell lines at nanomolar concentrations. Oral administration of 50 mg/kg CX-5461 induced TP53 expression and activity and reduced proliferation (MKI67) and invasion (loss of ductal actin) in Hi-MYC tumors, but not in PTEN-null (low MYC) tumors. While 100 mg/kg CX-6258 showed limited effect alone, its combination with CX-5461 further suppressed proliferation and dramatically reduced large invasive lesions in both models. This rational combination strategy significantly inhibited proliferation and induced cell death in PDX of prostate cancer.

CONCLUSIONS

Our results demonstrate preclinical efficacy of targeting the ribosome at multiple levels and provide a new approach for the treatment of prostate cancer. Clin Cancer Res; 22(22); 5539-52. ©2016 AACR.

Abstract 5181: Dual inhibition of RNA Pol I transcription and PIM kinase as a new therapy to treat prostate cancer

Background: The MYC oncogene is commonly over-expressed in prostate cancer (PC). Upregulation of ribosome biogenesis and function is a characteristic feature of MYC-driven tumors. Accordingly, inhibition of ribosomal RNA (rRNA) synthesis with CX-5461, a potent, selective and orally bioavailable inhibitor of RNA polymerase I (Pol I) transcription has been successfully exploited therapeutically in models of hematological malignancy characterized by elevated MYC expression. Additionally, PIM kinases activate MYC signaling and mRNA translation in PC and cooperate with MYC to accelerate tumorigenesis. Here we investigate the efficacy of a dual approach targeting ribosome biogenesis and function to treat PC by combining CX-5461 with the pan-PIM kinase inhibitor CX-6258 in murine and human models

Methods: The efficacy of CX-5461 and CX-6258, alone and in combination, was tested in PC cell lines, in the Hi-MYC mouse model of PC (n = 8-11 per group) and in PC metastatic tissues. Primary cell lines derived from Hi-MYC mice were used to analyze signaling events underpinning therapeutic efficacy. Triplicate experiments were analyzed with ANOVA followed by Dunnett's post hoc test. All statistical tests were two-sided.

Results: CX-5461 reduced anchorage independent growth and induced cell cycle arrest in human PC cell lines and in primary prostatic epithelial cells from Hi-MYC mice (P<0.001). CX-5461 treatment of Hi-MYC mice induced p53 expression and activity and significantly reduced prostate epithelial cell proliferation (P = 0.02) and invasion. While CX-6258 showed little effect alone, its combination with CX-5461 further suppressed proliferation (P = 0.01), dramatically reduced the incidence of large invasive lesions from 64% to 9% and preserved prostate ductal architecture.

This promising combination strategy prevented the growth of PDX tissue characterized by elevated MYC and resistance to conventional therapy (P = 0.04).

Conclusions: Our results demonstrate preclinical efficacy of combination therapies targeting the ribosome at multiple levels and provide a new approach for treatment of PC with high MYC activity.

Citation Format: Richard J. Rebello, Eric Kusnadi, Don Cameron, Analia Lesmana, Denis Drygin, Ashlee K. Clark, Laura Porter, Shahneen Sandhu, Gail P. Risbridger, Richard B. Pearson, Ross D. Hannan, Luc Furic. Dual inhibition of RNA Pol I transcription and PIM kinase as a new therapy to treat prostate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 5181.

Integrated transcriptomic and proteomic analysis identifies protein kinase CK2 as a key signaling node in an inflammatory cytokine network in ovarian cancer cells

We previously showed how key pathways in cancer-related inflammation and Notch signaling are part of an autocrine malignant cell network in ovarian cancer. This network, which we named the "TNF network", has paracrine actions within the tumor microenvironment, influencing angiogenesis and the immune cell infiltrate.The aim of this study was to identify critical regulators in the signaling pathways of the TNF network in ovarian cancer cells that might be therapeutic targets. To achieve our aim, we used a systems biology approach, combining data from phospho-proteomic mass spectrometry and gene expression array analysis. Among the potential therapeutic kinase targets identified was the protein kinase Casein kinase II (CK2).Knockdown of CK2 expression in malignant cells by siRNA or treatment with the specific CK2 inhibitor CX-4945 significantly decreased Notch signaling and reduced constitutive cytokine release in ovarian cancer cell lines that expressed the TNF network as well as malignant cells isolated from high grade serous ovarian cancer ascites. The expression of the same cytokines was also inhibited after treatment with CX-4945 in a 3D organotypic model. CK2 inhibition was associated with concomitant inhibition of proliferative activity, reduced angiogenesis and experimental peritoneal ovarian tumor growth.In conclusion, we have identified kinases, particularly CK2, associated with the TNF network that may play a central role in sustaining the cytokine network and/or mediating its effects in ovarian cancer.

Combination Therapy Targeting Ribosome Biogenesis and mRNA Translation Synergistically Extends Survival in MYC-Driven Lymphoma

Ribosome biogenesis and protein synthesis are dysregulated in many cancers, with those driven by the proto-oncogene c-MYC characterized by elevated Pol I-mediated ribosomal rDNA transcription and mTORC1/eIF4E-driven mRNA translation. Here, we demonstrate that coordinated targeting of rDNA transcription and PI3K-AKT-mTORC1-dependent ribosome biogenesis and protein synthesis provides a remarkable improvement in survival in MYC-driven B lymphoma. Combining an inhibitor of rDNA transcription (CX-5461) with the mTORC1 inhibitor everolimus more than doubled survival of Eμ-Myc lymphoma-bearing mice. The ability of each agent to trigger tumor cell death via independent pathways was central to their synergistic efficacy. CX-5461 induced nucleolar stress and p53 pathway activation, whereas everolimus induced expression of the proapoptotic protein BMF that was independent of p53 and reduced expression of RPL11 and RPL5. Thus, targeting the network controlling the synthesis and function of ribosomes at multiple points provides a potential new strategy to treat MYC-driven malignancies.

SIGNIFICANCE

Treatment options for the high proportion of cancers driven by MYC are limited. We demonstrate that combining pharmacologic targeting of ribosome biogenesis and mTORC1-dependent translation provides a remarkable therapeutic benefit to Eμ-Myc lymphoma-bearing mice. These results establish a rationale for targeting ribosome biogenesis and function to treat MYC-driven cancer.

Abstract A31: Elucidating mechanisms that cooperate with the therapeutic inhibition of RNA polymerase I to treat MYC-driven hematological malignancies

The MYC oncoprotein and transcription factor has a well-described role in harnessing and driving ribosome biogenesis (RiBi) via its regulation of all three RNA polymerases, and it is proposed that this is a crucial output of MYC's malignant function. Thus, the potential for therapeutic intervention in a broad range of MYC-driven cancers may converge on targeting the RiBi program, potentially as monotherapy or, more likely, as a basis for rational combination therapies. Indeed, small molecule (CX-5461) inhibition of RNA Polymerase I (Pol I), a major MYC target, selectively kills MYC-driven lymphoma in vivo via activation of a p53-dependent nucleolar stress response.

While CX-5461 potently inhibits ribosome biogenesis in a MYC-driven B-cell lymphoma via p53-dependent apoptosis, it is unclear whether all malignancies will respond similarly. Furthermore, as we predict resistance to CX-5461 and tumor relapse from our in vivo mouse studies, the need to identify pathways responsible for mediating resistance to Pol I inhibition in the background of oncogenic MYC is necessary for future development of rational combination therapies. To address these key questions, we have undertaken multiple approaches. Firstly, we are utilizing model systems of acute myeloid leukemia (AML) to take both an unbiased and a direct approach to understanding the role of MYC in mediating CX-5461 sensitivity. Secondly, to identify factors conferring acquired resistance to CX-5461 we have used the well-characterized Eµ-Myc mouse model of MYC-mediated lymphoma where, following a period of disease remission, prolonged dosing of CX-5461 in mice bearing lymphomas results in eventual relapse due to acquired drug resistance. Thus, we compared whole-exome sequencing data between isogenic sensitive and CX-5461 resistant B-cell lymphomas to identify additional mutations conferring drug resistance.

Finally, we are using the Eµ-Myc model to investigate the regulation of rDNA transcription itself during cancer progression. Compared to wild type B-cells, malignant MYC over-expressing B-cells have hyper-activated Pol I and increased rDNA transcription. This increase in transcription is associated with a dramatic increase in the number of transcriptionally active rDNA repeats, indicating changes in chromatin structure during transformation. In order to assess epigenetic changes at the rDNA loci and to identify long distance rDNA interactions occurring during malignant progression, we have used ChIP-seq and circularized chromosome conformation capture sequencing (4C-seq) at all cell stages of malignancy in vivo (wild type, pre-malignant and malignant Eµ-Myc B-cells). We demonstrate that transition from premalignancy to malignancy is associated with robust changes in rDNA chromatin as well as reorganization of rDNA-genome interactions with a significant increase in these interactions being detected in malignant cells. Our data provide evidence for MYC-driven activation of rDNA in cancer progression beyond ribosome biogenesis and offer novel insights into the spatial and transcriptional dynamics of the rDNA-associated genome during malignant transformation.

We will present our latest data on all fronts aimed at understanding the critical role that MYC plays in therapeutic inhibition of Pol I transcription.

Citation Format: Gretchen Poortinga, Jeannine Diesch, Elaine Sanij, Nadine Hein, Jirawas Sornkum, Donald Cameron, Megan J. Bywater, Ricky Johnstone, Denis Drygin, Sean O'Brien, Richard B. Pearson, Grant A. McArthur, Ross D. Hannan. Elucidating mechanisms that cooperate with the therapeutic inhibition of RNA polymerase I to treat MYC-driven hematological malignancies. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr A31.

Abstract 2735: Multi-point targeting of the synthetic lethal interactions between Myc, ribosome biogenesis and ribosome function cooperates to treat B-cell lymphoma

We recently demonstrated that transcription of the ribosomal genes (rRNA) by RNA Polymerase I (Pol I) can be therapeutically targeted with a novel small molecule, CX-5461, to selectively kill B-lymphoma cells in vivo while maintaining a viable wild-type B-cell population (Bywater et al Cancer Cell 2012; Bywater et al Nature Reviews Cancer 2013). The therapeutic effect was a consequence of nucleolar disruption, activation of ribosomal protein (Rp)-MDM2-P53 nucleolar stress response and apoptosis. We have recently launched a first-in-human clinical trial of CX-5461 in patients with hematological malignancies and although our pre-clinical data indicate immense potential of Pol I targeting for cancer therapy, some cancers still develop resistance. We hypothesized that simultaneously targeting the ribosome at multiple steps will extend survival. Thus we tested pharmacological inhibitors of PI3K/AKT/mTOR signalling in combination with CX-5461 as the former pathway is known to potently regulate both translational activity (Jefferies et al EMBO J, 1997; Pourdehnad M et al PNAS 2013) and ribosome biogenesis (Chan et al Science Signaling 2011; Devlin et al FEBS J 2013; Wall et al Cancer Discovery 2013).

Using the Eμ-Myc model of B-cell lymphoma we demonstrate that multiple pharmacological inhibitors of the PI3K/AKT/mTOR pathway suppress transcription of the rRNA genes and induce cell death similar to CX-5461. Unexpectedly however, PI3K/AKT/mTOR pathway blockade is not associated with nucleolar disruption, or activation of the Rp/MDM2/p53 nucleolar stress pathway. This is because inhibition of PI3K/AKT/mTOR signalling suppresses both rRNA synthesis and ribosomal protein synthesis equally and therefore does not increase the pool of free Rps necessary to suppress MDM2 E3 ligase that regulates p53 stability. Furthermore, we demonstrate that combined treatment of Eμ-Myc tumor-bearing mice with CX-5461 and Everolimus delayed relapse compare to single agent and significantly extended survival of tumor bearing mice.

These data demonstrate that dual targeting of the ribosome by selectively inhibiting Pol I transcription and inhibition of key signaling molecules regulating ribosome synthesis and function combine to potently treat MYC driven tumors. It therefore provides a rationale to combine such drugs in the clinic for the treatment of MYC driven cancer.

Citation Format: Richard B. Pearson, Jennifer R. Devlin, Katherine M. Hannan, Nadine Hein, Megan J. Bywater, Gretchen Poortinga, Donald Cameron, Denis Drygin, Sean O'Brien, Carleen Cullinane, Grant A. McArthur, Ross D. Hannan. Multi-point targeting of the synthetic lethal interactions between Myc, ribosome biogenesis and ribosome function cooperates to treat B-cell lymphoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2735. doi:10.1158/1538-7445.AM2014-2735

Targeting RNA polymerase I transcription and the nucleolus for cancer therapy

The nucleoli are the site of the production of ribosomes, the protein synthetic apparatus of the cell. The presence of enlarged nucleoli, reflecting increased ribosomal gene transcription, has long been used by pathologists as an indicator of aggressive tumors. However, over the last 10 years a growing body of evidence has revealed that the nucleolus contains a dynamic cohort of over 4500 proteins, the majority of which have no function in ribosome production. The activity of some of these proteins is modulated by their regulated sequestration and release from the nucleolus. In particular, the nucleolus plays a central role in sensing cellular stress to modulate the abundance of the critical tumor suppressor protein p53. The finding that p53 activity is dysregulated in up to 50% of all human cancers highlights the importance of the nucleolar stress response in limiting malignant transformation. The development of drugs to selectively inhibit transcription of the ribosomal RNA genes in the nucleolus has paved the way for a new therapeutic approach to hijack nucleolar stress to selectively and non-genotoxically activate p53 in tumor cells. Here, we describe the potential application of this exciting new class of drugs for the treatment of human cancer.

Abstract B16: Inhibition of RNA Polymerase I transcription by CX-5461 as a therapeutic strategy for the cancer-specific activation of p53 in highly refractory haematological malignancies

Malignant transformation is commonly associated with dysregulated ribosome biogenesis and for more than 100 years, pathologist have utilized the increase in size and number of nucleoli, the site of ribosome biogenesis as a marker for aggressive malignancies. Recently we demonstrated that hyperactivated RNA Polymerase I (Pol I) transcription, a rate limiting step in ribosome biogenesis, can be specifically targeted by the small molecule inhibitor, CX-5461. When evaluated for its anti-proliferative activity against a panel of genetically diverse human cancer cell lines, those derived from p53 wild type haemotological malignancies were the most sensitive (Drygin et al., Cancer Research, 2011). Evaluation of whether Pol I transcription inhibition could serve as therapeutic strategy in vivo revealed that CX-5461 administration significantly prolongs the overall survival of Eμ-Myc lymphoma bearing mice (Bywater et al., Cancer Cell, 2012). This survival advantage was associated with rapid activation of the ribosomal protein (Rp)-MDM2-p53 nucleolar stress response (Deisenroth et al., Oncogene, 2010) and subsequent induction of tumour-specific p53-dependent apoptosis (Bywater et al. Cancer Cell, 2012).

New data and future directions: To further explore the therapeutic potential of Pol I transcription inhibition in haemotological cancers that are refractory to standard therapy, we employed mouse models of MLL-driven Acute Myeloid Leukemia (AML) (MLL/AF9 + Nras and MLL/ENL + Nras) (Zuber et al., Genes Dev, 2009) and V*κ-Myc-driven Multiple Myeloma (MM) (Chesi et al. Cancer Cell, 2009). CX-5461 administration significantly increased the overall survival time of AML-bearing mice (MLL/ENL Nras 17 days for vehicle vs 36 days for drug, P<0.0001; MLL/AF9 Nras 15 days for vehicle vs 23 days for drug, P 0.0009))without severe adverse effects. The extended survival of MLL-driven AML bearing mice was associated with acute activation of p53, an aberrant G2/M cell cycle progression and consequently a delay in tumour progression as monitored by bioluminescence-imaging. In contrast to CX-5461, treatment of AML -bearing mice with Cytarabine/Doxorubicin, a regime used as standard-therapy in the clinic failed to induce p53 in MLL-driven AML resulting in a significantly less survival time than for CX-5461. In ongoing experiments with MM-bearing mice, chronic CX-5461 administration robustly reduced the secretion of serum monoclonal Ig, as detected by serum protein electrophoresis (SPEP), and significantly prolonged survival time demonstrating that CX-5461 possess potent anti-tumour activity in this highly refractory MM model. Analysis of the molecular mechanism responsible for the therapeutic benefit of CX-5461 in MM is ongoing but appears to involve both p53 mediated apoptosis and aberrant cell cycle progression.

Conclusion: These data demonstrate that hyperactivated Pol I transcription can be successfully targeted through small molecule inhibitors to therapeutically treat models of human AML and MM that are refractory to standard therapy.. Based on these and published results (Bywater et al., Cancer Cell 2012) and a favorable toxicology profile, we have launched a first-in-human clinical trial of this first in class drug, CX-5461 in patients with hematological malignancies. The outcomes of this trial will offer valuable insights as to whether inhibition of ribosome synthesis might offer a new non-genotoxic therapy approach in the fight against cancers, which are highly refractory to standard-therapies.

Citation Format: Nadine Hein, Denis Drygin, Sean O'Brien, Carleen Cullinane, Geoff Matthews, Marta Chesi, Leif Bergsagel, Johannes Zuber, Scott Lowe, Ricky Johnstone, Rick Pearson, Grant McArthur, Ross Hannan. Inhibition of RNA Polymerase I transcription by CX-5461 as a therapeutic strategy for the cancer-specific activation of p53 in highly refractory haematological malignancies. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr B16.

Abstract PR16: Combined inhibition of ribosome function and ribosomal RNA gene transcription cooperate to delay relapse and extend survival in MYC-driven tumors

Introduction: We recently demonstrated that transcription of the ribosomal genes (rDNA) by RNA Polymerase I (Pol I) can be therapeutically targeted with a novel small molecule, CX-5461, to selectively kill B-lymphoma cells in vivo while maintaining a viable wild-type B cell population (Bywater et al Cancer Cell 2012; Bywater et al Nature Reviews Cancer 2013). The therapeutic effect was a consequence of nucleolar disruption and activation of ribosomal protein (Rp)-MDM2-P53 nucleolar stress response (3) leading to apoptosis. Human leukemia and lymphoma cell lines also show high sensitivity to inhibition of rDNA transcription that is dependent on p53 mutational status. As our pre-clinical data indicate immense potential of Pol I targeting as a cancer therapy, we have launched a first-in-human clinical trial of CX-5461 in patients with hematological malignancies. Despite impressive initial responses with CX-5461 as a single agent, we decided to explore its combination potential. We hypothesised that simultaneously targeting the ribosome at multiple steps would reduce the instance of acquired resistance to CX-5461 and extend survival. Thus we tested pharmacological inhibitors of the PI3K/AKT/mTOR pathway in combination with CX-5461 as the former signaling molecules are known to be potent regulators of both ribosome translational activity (Jefferies et al EMBO J, 1997; Pourdehnad M et al PNAS 2013) and ribosome biogenesis (Chan et al Science Signaling 2011; Devlin et al FEBS J 2013).

Experimental procedures and new data: Using the Eμ-Myc model of B-cell lymphoma we demonstrate that multiple pharmacological inhibitors of the PI3K/AKT/mTOR pathway also suppress transcription of the rRNA genes and induce cell death to a similar extent as CX-5461. Unexpectedly however, PI3K/AKT/mTOR pathway blockade is not associated with nucleolar disruption, nor activation of the Rp/MDM2/p53 nucleolar stress pathway. We demonstrate this is because inhibition of PI3K/AKT/mTOR suppresses both rRNA synthesis and ribosomal protein synthesis equally and therefore does not result in a pool of free Rps that are necessary to suppress MDM2 E3 ligase that regulates p53 stability. Instead apoptosis induced by PI3K/AKT/mTOR was associated with up-regulated expression of the pro-apoptotic BH3-only protein BMF. Furthermore, we demonstrate that combined treatment of Eμ-Myc tumour-bearing mice with CX-5461 and Everolimus delayed relapse compare to single agent and significantly extended survival of tumor bearing mice.

Conclusions: These data demonstrate that dual targeting of the ribosome by selectively inhibiting Pol I transcription and by inhibition of key signaling molecules regulating ribosome synthesis and function combine to potently treat MYC driven tumours and provides a rationale to combine such drugs in the clinic for the treatment of MYC driven cancer. Moreover these data also demonstrate that MYCs control of Pol I transcription and nucleolar integrity is required for its oncogenic potential, independent of its well described function in function in controlling ribosome levels and protein translation.

This abstract is also presented as poster C64.

Citation Format: Ross Hannan, Jennifer Devlin, Katherine Hannan, Nadine Hein, Megan Bywater, Gretchen Poortinga, Don Cameron, Denis Drygin, Carleen Cullinane, Sean O'Brien, Grant McArthur, Richard Pearson. Combined inhibition of ribosome function and ribosomal RNA gene transcription cooperate to delay relapse and extend survival in MYC-driven tumors. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr PR16.

Targeting protein kinase CK2 suppresses prosurvival signaling pathways and growth of glioblastoma

PURPOSE

Gliomas are the most frequently occurring primary malignancies in the brain, and glioblastoma is the most aggressive of these tumors. Protein kinase CK2 is composed of two catalytic subunits (α and/or α') and two β regulatory subunits. CK2 suppresses apoptosis, promotes neoangiogenesis, and enhances activation of the JAK/STAT, NF-κB, PI3K/AKT, Hsp90, Wnt, and Hedgehog pathways. Aberrant activation of the NF-κB, PI3K/AKT, and JAK/STAT-3 pathways is implicated in glioblastoma progression. As CK2 is involved in their activation, the expression and function of CK2 in glioblastoma was evaluated.

EXPERIMENTAL DESIGN AND RESULTS

Analysis of 537 glioblastomas from The Cancer Genome Atlas Project demonstrates the CSNK2A1 gene, encoding CK2α, has gene dosage gains in glioblastoma (33.7%), and is significantly associated with the classical glioblastoma subtype. Inhibition of CK2 activity by CX-4945, a selective CK2 inhibitor, or CK2 knockdown by siRNA suppresses activation of the JAK/STAT, NF-κB, and AKT pathways and downstream gene expression in human glioblastoma xenografts. On a functional level, CX-4945 treatment decreases the adhesion and migration of glioblastoma cells, in part through inhibition of integrin β1 and α4 expression. In vivo, CX-4945 inhibits activation of STAT-3, NF-κB p65, and AKT, and promotes survival of mice with intracranial human glioblastoma xenografts.

CONCLUSIONS

CK2 inhibitors may be considered for treatment of patients with glioblastoma.

Abstract 4355: Inhibition of RNA Polymerase I as a strategy to treat cancer

Abnormalities of the nucleolus, the site of transcription of the ribosomal genes (rDNA) by RNA Polymerase I (Pol I), have been recognized as diagnostic for cancer for more then a century. However, a critical, unresolved question has been whether the accelerated ribosome biogenesis responsible for the nucleolar changes is required for malignancy.

Here we show that the PI3K/AKT pathway, deregulated in a high proportion of human tumours, is a critical regulator of ribosome biogenesis. Active AKT is sufficient to drive rRNA synthesis and ribosome biogenesis. Furthermore, AKT cooperates with c-MYC to drive these processes identifying the AKT//MYC network as a master controller of cell growth. Consistent with this concept, AKT activity is required for maximal activation of rRNA synthesis and tumour formation in the Eμ-Myc mouse model of Burkitt's lymphoma (1). Our findings raise the possibility that cancers characterized by unrestrained cellular growth may be vulnerable to therapeutic strategies that target ribosome biogenesis.

To directly test this hypothesis, we used a novel selective small molecule inhibitor of Pol I transcription (CX-5461) (2), to show that Pol I can be targeted in vivo to treat tumors in mouse models of lymphoma and leukemia through the activation of p53-dependent apoptosis, while sparing normal hematologic cells. Thus, selective inhibition of Pol I transcription can serve as a novel therapeutic strategy for the treatment of cancer (3). A Phase 1 trial of this first-in-class molecule begins in 2013 at the Peter MacCallum Cancer Centre for patients with haematologic malignancies. Strikingly, allosteric inhibitors of AKT suppress rRNA synthesis independent of p53 and cooperate with CX-5461 in killing Eμ-Myc lymphomas providing a clear rationale for combining these agents in future trials.

Citation Format: Megan J. Bywater, Katherine M. Hannan, Gretchen Poortinga, Jennifer R. Devlin, Carleen Cullinane, Denis Drygin, William G. Rice, Daniel Von Hoff, Ricky W. Johnstone, Grant A. McArthur, Ross D. Hannan, Richard B. Pearson. Inhibition of RNA Polymerase I as a strategy to treat cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4355. doi:10.1158/1538-7445.AM2013-4355

Discovery of CX-5461, the First Direct and Selective Inhibitor of RNA Polymerase I, for Cancer Therapeutics

Accelerated proliferation of solid tumor and hematologic cancer cells is linked to accelerated transcription of rDNA by the RNA polymerase I (Pol I) enzyme to produce elevated levels of rRNA (rRNA). Indeed, upregulation of Pol I, frequently caused by mutational alterations among tumor suppressors and oncogenes, is required for maintenance of the cancer phenotype and forms the basis for seeking selective inhibitors of Pol I as anticancer therapeutics. 2-(4-Methyl-[1,4]diazepan-1-yl)-5-oxo-5H-7-thia-1,11b-diaza-benzo[c]fluorene-6-carboxylic acid (5-methyl-pyrazin-2-ylmethyl)-amide (CX-5461, 7c) has been identified as the first potent, selective, and orally bioavailable inhibitor of RNA Pol I transcription with in vivo activity in tumor growth efficacy models. The preclinical data support the development of CX-5461 as an anticancer drug with potential for activity in several types of cancer.

Inhibition of RNA polymerase I as a therapeutic strategy to promote cancer-specific activation of p53

Increased transcription of ribosomal RNA genes (rDNA) by RNA Polymerase I is a common feature of human cancer, but whether it is required for the malignant phenotype remains unclear. We show that rDNA transcription can be therapeutically targeted with the small molecule CX-5461 to selectively kill B-lymphoma cells in vivo while maintaining a viable wild-type B cell population. The therapeutic effect is a consequence of nucleolar disruption and activation of p53-dependent apoptotic signaling. Human leukemia and lymphoma cell lines also show high sensitivity to inhibition of rDNA transcription that is dependent on p53 mutational status. These results identify selective inhibition of rDNA transcription as a therapeutic strategy for the cancer specific activation of p53 and treatment of hematologic malignancies.

Abstract 3765: Discovery and biological characterization of CX-8184, a potent inhibitor of protein kinase CK2

Protein kinase CK2 regulates multiple pathways that play key roles in carcinogenesis, including cell cycle progression, suppression of apoptosis, hypoxia, angiogenesis, inflammation and DNA damage repair. Overexpression of CK2 in various cancer tissues has been frequently linked to poor prognosis. All this makes targeting of CK2 highly attractive for both single agent and combination therapies. To date therapeutic targeting of CK2 proved to be challenging, with only one compound, CX-4945, entering clinical development. In phase I clinical trials CX-4945 was well tolerated, inhibited CK2 activity in PBMC, modulated the PI3K-AKT-mTOR oncogenic pathway, reduced tumor-related serum IL-6 and IL-8 biomarkers, resulted in stable disease for at least 16 weeks in 20% of treated patients, and caused single agent tumor shrinkage. Building on these successes with the first generation CK2 inhibitor, we sought to design a second generation CK2 inhibitor with distinct pharmacologic properties. The application of a focused screening cascade and iterative medicinal chemistry efforts generated CX-8184, a selective single digit nanomolar inhibitor of the CK2 enzyme that demonstrated desirable biologic properties. When evaluated against a panel of 32 cancer cell lines, CX-8184 demonstrated broad antiproliferative activity with median IC50 = 145 nM. CX-8184 suppressed NF-κB, Hif-1α and β-catenin driven transcription at concentrations of 100-300 nM, and inhibited IL-6 expression with IC50 = 30 nM. Since CK2 is known to potentiate AKT signaling, treatment of cancer cells with CX-8184 decreased phosphorylation of AKT, as well as its substrates. The Hsp90/Cdc37 machinery is regulated by CK2 and plays an important role in activation of various kinases, including members of the ErbB RTK family. CX-8184 rapidly suppressed phosphorylation of Cdc37 by CK2, leading to the deactivation of ErbB family members. In combination with erlotinib or lapatinib, CX-8184 produced synergistic killing of lung cancer and breast cancer cells, respectively. CX-8184 also inhibited the phosphorylation of XRCC1, a scaffold protein involved in DNA strand break repair that plays an important role in sensitivity of cancer cells to DNA damaging agents. Consequently, the treatment of ovarian cancer cells pre-challenged with gemcitabine and/or cisplatin with 300 nM CX-8184 led to synergistic induction of cell death. In vivo, CX-8184 demonstrated favorable PK properties with oral bioavailability and a half-life amenable to once daily dosing in mice. CX-8184 exhibited potent anti-tumor activity and was well tolerated in A431 xenograft model. The compound has also showed an acceptable pharmacological profile as evidenced by CYP, hERG and Ames testing. The pharmacokinetic and pharmacological properties and the biological activity of CX-8184, both as single agent or in combination, support its further development as a preclinical candidate.9

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3765. doi:1538-7445.AM2012-3765

Combined inhibition of EGFR and CK2 augments the attenuation of PI3K-Akt-mTOR signaling and the killing of cancer cells

Ser/Thr protein kinase CK2 regulates multiple processes that play important roles in the sensitivity of cancer to epidermal growth factor receptor targeting therapeutics, including PI3K-Akt-mTOR signaling, Hsp90 activity, and inhibition of apoptosis. We hypothesized that top-down inhibition of EGFR, combined with lateral suppression of multiple oncogenic pathways by targeting CK2, would create a pharmacologic synthetic lethal event and result in an improved cancer therapy compared to EGFR inhibition alone. This hypothesis was tested by combining CX-4945, a first-in-class clinical stage inhibitor of CK2, with the EGFR tyrosine kinase inhibitor, erlotinib, in vitro and in vivo in models of non-small cell lung carcinoma, NCI-H2170, and squamous cell carcinoma, A431. Our results demonstrate that combination of CX-4945 with erlotinib results in enhanced attenuation of the PI3K-Akt-mTOR pathway. We also observed an increase in apoptosis, synergistic killing of cancer cells in vitro, as well as improved antitumor efficacy in vivo. Taken together, these data position CK2 as a valid pharmacologic target for drug combinations and support further evaluation of CX-4945 in combination with EGFR targeting agents.

Discovery of CX-6258. A Potent, Selective, and Orally Efficacious pan-Pim Kinases Inhibitor

Structure-activity relationship analysis in a series of 3-(5-((2-oxoindolin-3-ylidene)methyl)furan-2-yl)amides identified compound 13, a pan-Pim kinases inhibitor with excellent biochemical potency and kinase selectivity. Compound 13 exhibited in vitro synergy with chemotherapeutics and robust in vivo efficacy in two Pim kinases driven tumor models.

CK2 inhibitor CX-4945 suppresses DNA repair response triggered by DNA-targeted anticancer drugs and augments efficacy: mechanistic rationale for drug combination therapy

Drug combination therapies are commonly used for the treatment of cancers to increase therapeutic efficacy, reduce toxicity, and decrease the incidence of drug resistance. Although drug combination therapies were originally devised primarily by empirical methods, the increased understanding of drug mechanisms and the pathways they modulate provides a unique opportunity to design combinations that are based on mechanistic rationale. We have identified protein kinase CK2 as a promising therapeutic target for combination therapy, because CK2 regulates not just one but many oncogenic pathways and processes that play important roles in drug resistance, including DNA repair, epidermal growth factor receptor signaling, PI3K/AKT/mTOR signaling, Hsp90 machinery activity, hypoxia, and interleukin-6 expression. In this article, we show that CX-4945, a clinical stage selective small molecule inhibitor of CK2, blocks the DNA repair response induced by gemcitabine and cisplatin and synergizes with these agents in models of ovarian cancer. Mechanistic studies show that the enhanced activity is a result of inactivation of XRCC1 and MDC1, two mediator/adaptor proteins that are essential for DNA repair and that require phosphorylation by CK2 for their function. These data position CK2 as a valid pharmacologic target for intelligent drug combinations and support the evaluation of CX-4945 in combination with gemcitabine and platinum-based chemotherapeutics in the clinical setting.

Abstract PR15: Inhibition of RNA Polymerase I as a therapeutic strategy for cancer-specific activation of p53

Increased transcription of the ribosomal genes (rDNA) by RNA Polymerase I (Pol I) is a common feature of human cancer[1]. However until now no studies have directly examined the requirement for dysregulated rDNA transcription in the maintenance of the malignant phenotype. Our studies show that increased rDNA transcription is necessary for MYC oncogenic activity and can be therapeutically targeted to treat tumors. We demonstrate that restoration of hyperactivated rDNA transcription rates in Eμ-MYC lymphoma cells to the levels observed in normal B cells by knock down of Pol I transcription factors UBF and Rrn3, is rapidly selected against in vitro as determined by loss from competitive culture with parental cells. This disadvantage is due to the induction of apoptosis and can be rescued by over expression of the anti-apoptotic protein BCL2. Furthermore, treatment of Eμ-MYC lymphoma cells with a small molecule inhibitor of Pol I (CX-5461) we have recently developed[2] is able to specifically inhibit Pol I transcription (IC50=45.64nM, metabolic labeling) and rapidly induce apoptosis and subsequent cell death by 16hrs (IC50=8.4nM, PI exclusion). This apoptotic response is not an indirect consequence of ribosome insufficiency but is due to induction of the ribosome biogenesis surveillance pathway[3] characterized by rapid nucleolar disruption, as determined by immunofluorescence of Fibrillarin relocalization, and the subsequent activation of p53-dependent apoptotic signaling, as determined by increased protein levels of p53 and increased expression of p53 target genes p21, Mdm2 and Puma at the mRNA and protein level within 1hr of treatment. Using CX-5461 we show that malignant B cells have a heightened dependence on elevated rDNA transcription that can be exploited in vivo as a therapeutic target for treatment of lymphoma. Treatment of mice transplanted Eμ-MYC lymphoma with 40mg/kg CX-5461 orally every 3 days is able to delay the onset of disease (median survival of 15 days for vehicle, 31 days for drug, P<0.0001), with this delay accompanied by a period of disease remission, as determined by peripheral white blood cell counts (58×10⁁3cellsμl±4.5 for vehicle, 4.2×10⁁3cellsμl±0.29 for drug, P<0.001) and FACS analysis of the peripheral blood (62%tumor cells±2.3 for vehicle, 0.7%tumor cells±0.05 for drug, P<0.001), and maintenance of a wild type B-cell population (20%B220+cells±1.0 for vehicle, 43%B220+cells±1.5 for drug, P<0.001). Similarly in vivo, elimination of tumor cells with the inhibition of Pol I transcription results from subsequent activation of p53-dependent apoptotic signaling that specifically occurs in malignant but not normal B-lymphocytes in the spleen and bone marrow of healthy mice treated with the same dose of CX-5461. Human leukemia and lymphoma cell lines also show high sensitivity to the inhibition of Pol I transcription that is dependent on p53 mutational status. Furthermore, CX-5461 was able to delay the growth of subcutaneous MV 4;11 (human acute myeloid leukemia) xenografts in mice dosed at 125mg/kg IP weekly (tumor growth inhibition=93% compared to vehicle). Our work reveals a previously unproven paradigm that links hyperactivated rDNA transcription and nucleolar integrity to maintenance of aggressive tumors independent of ribosome levels. Critically, these results also demonstrate how activation of a ribosome biogenesis surveillance pathway by selective inhibition of rDNA transcription can be used as a novel therapeutic target for the treatment of cancer. Furthermore, patients with hematological malignancies have been identified as the appropriate first cohort for trials of CX-5461 in man commencing next year.

1. R. J. White, Nat Rev Mol Cell Biol 6, 69 (Jan, 2005).

2. D. Drygin et al., Cancer Res, (Dec 15, 2010).

3. C. Deisenroth, Y. Zhang, Oncogene 29, 4253 (Jul 29, 2010).

This abstract is also presented as Poster A35.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr PR15.

Abstract 4511: CX-5461, a non-genotoxic activator of p53 through selective inhibition of RNA polymerase I for the treatment of hematological cancers

Cancer is characterized by hyperactivation of ribosome biogenesis which depends on increased RNA Polymerase I (Pol I) transcription. Inhibition of Pol I transcription causes nucleolar stress that leads to the release of ribosomal proteins from the nucleolus into the nucleoplasm where they can sequester the p53 inhibitory protein MDM2, causing activation of p53 and induction of apoptosis. The inhibition of Pol I transcription as a therapeutic approach is significant because it impacts two critically balanced processes, proliferation and apoptosis, that regulate cancer cell survival. CX-5461 is a potent and selective inhibitor of Pol I transcription. CX-5461 acts at the initiation stage of Pol I transcription through the disruption of the SL1/rDNA complex. CX-5461 is non-genotoxic and does not inhibit DNA replication, protein translation or RNA Polymerase II transcription. We have previously demonstrated that CX-5461 triggers autophagic cell death in solid tumor cell lines and exhibits antitumor activity in xenograft models, highlighting the importance of Pol I transcription in cancer (Drygin et al. Cancer Res. in press). In preparation for clinical testing we sought to identify the most sensitive indications by evaluating CX-5461 against a panel of genetically diverse cancer cell lines. CX-5461 exhibited a broad range of antiproliferative activity with wild-type (wt) p53 cells derived from hematological malignancies being the most sensitive (median IC50 = 5 nM). Other cell types, i.e. p53 mutated hematological, p53wt and p53 mutated solid tumors were less sensitive to CX-5461 (median IC50s = 94, 164 and 265 nM respectively). In contrast, the median IC50 against normal cells was 5 uM indicating that CX-5461 selectively kills cancer cells. Further molecular characterization revealed that CX-5461 treatment of p53wt hematologic cancer cells inhibited rRNA synthesis, stabilized p53, activated p21, caused cell cycle arrest and induced apoptosis. Chemical inhibition of p53 prevented the induction of apoptosis indicating that CX-5461 acts through activation of p53. Interestingly, while p53 mutations impacted the activity of CX-5461, other genetic alterations known to silence p53 response, i.e. deletion of ARF, did not affect sensitivity to CX-5461 (Bywater et al. 2010 AACR Ann Met Proceedings). Activation of p53 has long been considered an attractive approach for treating cancers because of the surveillance function of p53 to remove abnormal cells via induction of apoptotic cell death. The fact that mutations or deletions of the p53 gene are relatively rare in hematological malignancies, coupled with our data that p53wt hematologic cancer cells are particularly sensitive to CX-5461 provides compelling rationale for evaluating CX-5461 in this patient population.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4511. doi:10.1158/1538-7445.AM2011-4511

Abstract 5494: CX-4945, an inhibitor of protein kinase CK2, potentiates the antitumor activity of platinum chemotherapy in models of ovarian cancer by preventing phosphorylation of XRCC1 and MDC1 and disrupting DNA damage repair

Platinum-based chemotherapeutics are commonly used to treat solid tumors but may be restricted in their application by dose limiting toxicity and inherent or acquired resistance. Because efficient DNA damage repair mechanisms contribute to resistance, targeting components of the repair machinery has emerged as a strategy to increase the effectiveness of these and other DNA-damaging anti-cancer drugs. Protein kinase CK2 is a serine/threonine kinase that has emerged as an attractive molecular target due to its overexpression in cancer and regulatory role in key cellular processes including the cell cycle, apoptosis and DNA damage repair. Multiple lines of evidence suggest an increasingly important role for CK2 in the DNA damage response, including the phosphorylation and activation of the mediator/adaptor proteins XRCC1 and MDC1. XRCC1 is an essential component for nucleotide excision repair which is the major repair pathway responsible for removing platinum adducts. MDC1 is the predominant γ-H2AX recognition factor in mammalian cells and is essential for homologous recombination repair. CX-4945 is a first-in-class, selective, oral inhibitor of CK2 currently in Phase 1 clinical trials. We sought to determine if inhibiting CK2 activity with CX-4945 would prevent phosphorylation of XRCC1 and MDC1 and potentiate the activity of platinum-based drugs by preventing DNA damage repair. Treatment of the ovarian cancer cell lines A2780 and SKOV3 with CX-4945 led to decreased phosphorylation of XRCC1 at the CK2 specific phosphorylation sites and reduced total XRCC1 protein levels. Likewise, immunoprecipitation of MDC1 from SKOV3 cells treated with CX-4945 revealed significant reductions in phosphorylation at the CK2 specific sites, while in A2780 cells MDC1 protein levels were decreased. The reduction of MDC1 protein levels was reproduced by CK2 siRNA, confirming that CK2 activity supports MDC1 expression levels. Combined treatment of A2780 cells with CX-4945 and cycloheximide revealed a faster rate of MDC1 degradation than with cycloheximide alone but did not affect MDC1 mRNA levels, indicating that CK2 regulates MDC1 protein stability. When combined with cisplatin, CX-4945 enhanced the activation of CHK2 and increased levels of γ-H2AX and cleaved PARP. In antiproliferative experiments, CX-4945 exhibited synergy with cisplatin in A2780 and SKOV3 cell lines. The combination of CX-4945 with cisplatin or carboplatin significantly enhanced antitumor activity in ovarian xenograft models and was well tolerated. Thus, the inhibition of CK2 by CX-4945 enhanced the antitumor activity of platinum agents by preventing DNA damage repair and inducing apoptosis. These data provide compelling preclinical support for pursuing CX-4945 in combination with platinum chemotherapy in the clinic.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5494. doi:10.1158/1538-7445.AM2011-5494

Abstract 2560: The CK2 inhibitor CX-4945 enhances the antitumor activity of EGFR-targeted agents by attenuating signaling in the PI3K/AKT/mTOR pathway

The epidermal growth factor receptor (EGFR) regulates several oncogenic signaling pathways including PI3K/AKT/mTOR, MEK/ERK, and STAT. A number of EGFR antagonists have been approved for treatment of late-stage tumors of epithelial origin. Despite these advances, there are several limitations to these therapies, including primary and acquired resistance, prompting the need to combine EGFR antagonists with agents that target pathways downstream of EGFR. CK2 modulates multiple pro-proliferative and pro-survival signals through many of these same signaling pathways, and co-overexpression of EGFR and CK2 has been frequently observed in solid tumors. Here we present the combination of EGFR-targeted agents with CX-4945, a first-in-class selective CK2 inhibitor currently under evaluation in a Phase I clinical trial. CK2 phosphorylates several key proteins at multiple levels within the PI3K/AKT/mTOR signaling pathway including AKT, PTEN, and p70S6K1. CK2 also regulates the Hsp90/Cdc37 machinery, whose clients include EGFR, AKT, and Src. We proposed that top-down inhibition of EGFR, combined with lateral inhibition of the PI3K/ATK/mTOR pathway by CX-4945, would result in an improved cancer therapy compared to EGFR antagonism alone. This hypothesis was tested in vitro and in vivo in A431 squamous cell carcinoma (SCC) and non-small cell lung carcinoma (NSCLC) models of various genetic backgrounds. Signaling pathways and induction of apoptosis were analyzed by western blot and cell proliferation was measured in a 96-hour cell viability assay. The combination of CX-4945 and erlotinib resulted in enhanced reduction in phosphorylation of AKT (T308), AKT (S473), PRAS40 (S246), mTOR (S2481), mTOR (S2448), p70S6K1 (T389), S6 (S235/6), S6 (240/4), and 4E-BP1 (T37/46), and decreased Mcl-1 levels. These effects were accompanied by decreased cell proliferation and synergistic induction of apoptosis. CX-4945 plus erlotinib exhibited enhanced antitumor activity in A431 SCC and NCI-H2170 NSCLC xenograft models; moreover, in the erlotinib-resistant NCI-H1975 NSCLC xenograft model, CX-4945 plus cetuximab resulted in enhanced efficacy. These data suggest that CX-4945 in combination with EGFR-targeted agents may improve clinical outcomes in patients with EGFR and CK2-driven cancers by inhibiting multiple nodes of the EGFR signaling pathway.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2560. doi:10.1158/1538-7445.AM2011-2560

Discovery and SAR of 5-(3-chlorophenylamino)benzo[c][2,6]naphthyridine-8-carboxylic acid (CX-4945), the first clinical stage inhibitor of protein kinase CK2 for the treatment of cancer

Herein we chronicle the discovery of CX-4945 (25n), a first-in-class, orally bioavailable ATP-competitive inhibitor of protein kinase CK2 in clinical trials for cancer. CK2 has long been considered a prime cancer drug target because of the roles of deregulated and overexpressed CK2 in cancer-promoting prosurvival and antiapoptotic pathways. These biological properties as well as the suitability of CK2's small ATP binding site for the design of selective inhibitors, led us to fashion novel therapeutic agents for cancer. The optimization leading to 25n (K(i) = 0.38 nM) was guided by molecular modeling, suggesting a strong binding of 25n resulting from a combination of hydrophobic interactions, an ionic bridge with Lys68, and hydrogen bonding with the hinge region. 25n was found to be highly selective, orally bioavailable across species (20-51%) and efficacious in xenograft models. The discovery of 25n will allow the therapeutic targeting of CK2 in humans for the first time.

Targeting RNA polymerase I with an oral small molecule CX-5461 inhibits ribosomal RNA synthesis and solid tumor growth

Deregulated ribosomal RNA synthesis is associated with uncontrolled cancer cell proliferation. RNA polymerase (Pol) I, the multiprotein complex that synthesizes rRNA, is activated widely in cancer. Thus, selective inhibitors of Pol I may offer a general therapeutic strategy to block cancer cell proliferation. Coupling medicinal chemistry efforts to tandem cell- and molecular-based screening led to the design of CX-5461, a potent small-molecule inhibitor of rRNA synthesis in cancer cells. CX-5461 selectively inhibits Pol I-driven transcription relative to Pol II-driven transcription, DNA replication, and protein translation. Molecular studies demonstrate that CX-5461 inhibits the initiation stage of rRNA synthesis and induces both senescence and autophagy, but not apoptosis, through a p53-independent process in solid tumor cell lines. CX-5461 is orally bioavailable and demonstrates in vivo antitumor activity against human solid tumors in murine xenograft models. Our findings position CX-5461 for investigational clinical trials as a potent, selective, and orally administered agent for cancer treatment.

Abstract 3110: RNA Polymerase I: A novel target in the treatment of Myc-driven malignancy

c-MYC plays a prominent role in cancer. Intriguingly, many of the genes regulated by this oncoprotein are associated with ribosome biogenesis and we have previously demonstrated that Myc regulates a major rate limiting step in this pathway, transcription of the 45S rRNA genes by RNA Polymerase I (Pol I). These observations lead us to hypothesize that cMyc's regulation of rRNA synthesis may contribute to its oncogenic properties. We have tested this hypothesis in a mouse model of Myc-driven lymphoma, the EμMyc transgenic mouse.

B-cells purified from EμMyc mice display an increased growth rate in comparison to their wild-type littermates, with increased cell volume, total RNA and protein per cell. This phenotype is characterized by higher rates of 45S rRNA transcription and increased expression of factors specific for Pol I transcription. Knockdown of one of these factors, UBF, by RNAi in EμMyc lymphoma cell lines results in a selective proliferative disadvantage of cells in vitro, in a competition assay, and in vivo, in a transplant model. This phenotype is driven by an increased rate of apoptosis associated with a reduction in 45S rRNA transcription.

Based on these findings we explored the potential therapeutic effectiveness in this model of a novel specific small molecule inhibitor of Pol I, CX-5461, currently in preclinical development. Transplanted EμMyc tumors showed marked sensitivity to CX-5461 in vivo, with a dramatic reduction in tumor burden in the peripheral blood (97.54%±0.56) and lymph nodes (94.96%±0.90) due to induction of apoptosis 24hrs following a single oral dose at 75mg/kg. Importantly a normal B-cell population was preferentially maintained in treated mice (13%±1.39 wt B220+ cells versus 1.04%±0.24 tumor B220+ cells, as a percentage of total WBC) indicating specificity of the compound for tumor cells. Four doses of CX-5461, 75mg/kg orally every third day, significantly delayed time to endpoint by 9.5 days (P<0.0001) compared to untreated animals. This delay was accompanied by a period of complete remission with normal white blood cell counts (6.76±0.48 ×10^9cells/L) and no identifiable tumor cells in the peripheral blood. Interestingly, in vitro dose curves indicate a dependence of CX-5461 sensitivity on wild-type p53 function (p53 wt and ARF−/− cell line IC50=9.28nM±1.53 in comparison to p53 mutant and p53−/− IC50=1.70uM±0.03), which can be reduced with over expression of Bcl2 (Bcl2 IC50=2.33uM±1.3). Notably even in the more resistant p53 mutant and p53−/− cell lines, cell death also occurred via apoptosis, suggesting p53-dependent and independent mechanisms to be involved in CX-5461 mediated cell death.

In summary, this work with UBF RNAi and CX-5461 identifies inhibition of RNA Pol I transcription as a novel and effective target in the treatment of cMyc-driven malignancies, and for the first time establishes that dysregulation of rDNA transcription can directly contribute to malignant transformation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3110.

The RNA polymerase I transcription machinery: an emerging target for the treatment of cancer

The RNA polymerase I (Pol I) transcription machinery in the nucleolus is the key convergence point that collects and integrates a vast array of information from cellular signaling cascades to regulate ribosome production that in turn guides cell growth and proliferation. Cancer cells commonly harbor mutations that inactivate tumor suppressors, hyperactivate oncogenes, and upregulate protein kinases, all of which promote Pol I transcription and drive cell proliferation. The intimate balance between Pol I transcription and growth-factor signaling is perturbed in cancer cells, indicating that upregulation of rRNA synthesis is mandatory for all tumors. Though the emerging picture of transcriptional regulation reveals an unexpected level of complexity, we are beginning to understand the multiple links between rRNA biogenesis and cancer. In this review, we discuss experimental data and potential strategies to downregulate rRNA synthesis and induce an antiproliferative response in cancer cells.

Abstract A246: Discovery of selective small molecule Pan-Pim kinase inhibitors with potent oral efficacy in murine xenograft models

The PIM family of serine/threonine kinases are pro-proliferative kinases activated by multiple cytokines and growth factor signaling. The Pim kinases are unusual in that they are regulated primarily by transcription and not by membrane recruitment or phosphorylation like other serine/threonine kinases. Activated cytokine receptors recruit JAKs to induce STAT-dependent transcription of the Pim genes. They are proto-oncogenes and have been implicated in the process of lymphomagenesis and malignant transformation. Pim overexpression has been reported in diffuse B cell lymphoma, chronic lymphocytic leukemia, FLT3-mediated acute myelogenous leukemia and prostate cancer. Pim-2 is over expressed in leukemias and lymphomas, whereas Pim-3 overexpression has been observed in melanoma, pancreatic and gastric tumors. The recent reports of elevated levels of Pim-1 expression in human prostate tumor biopsies implicate the Pim family of protein kinases in the progression of human prostate tumors. Further, in transgenic animal models, Pim-1 expression has been shown to be elevated in prostate tumors that are caused by overexpression of the c-myc oncogene. Recent evidence reveals the overlapping and compensatory nature of Pim-1 and Pim-2 phosphorylation and highlights the importance of inhibiting all isoforms. The emerging role of the PIM kinase family in hematological malignancies and solid tumors and the druggable nature of their ATP binding pocket make them attractive targets for anticancer drug development.

Utilizing a highly distinct molecular scaffold, CX-6258 was developed as a selective and potent small molecule pan-PIM kinase inhibitor. CX-6258 inhibits Pims 1, 2 and 3 with IC50 values in the low nanomolar range and high selectivity as evidenced in a screening panel of over 100 kinases. CX-6258 demonstrates potent in vitro antiproliferative activity, particularly in leukemia derived cell lines expressing the FLT3-ITD. Moreover, CX-6258 inhibits the phosphorylation of BAD and 4EBP1, known substrates for PIMs 1, 2 and 3. When delivered orally, this pan-Pim inhibitor is well tolerated and demonstrates potent antitumor activity in murine xenograft models of PIM driven cancer. Using CX-6258 as our “path finder” molecule, we have created four additional unique chemical scaffolds as pan-Pim inhibitors, and certain molecules from these scaffolds can inhibit Pims 1, 2 and 3 in the picomolar range while exhibiting no inhibitory activity of the Flt3 protein kinase. The in vivo and in vitro profiles of these chemically diverse series are indicative of an effective and potent anti-cancer mechanism mediated through the selective inhibition of PIM kinase activity. Together, these findings exemplify that we have created multiple proprietary chemical series of pan-Pim inhibitors exhibiting picomolar potency and discerning selectivity.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A246.

Abstract C198: CX-4945, a novel small molecule inhibitor of CK2 protein kinase, reduces hyperactivated Akt signaling and synergizes with Akt inhibitors in breast cancer cells

Akt, a critical protein kinase in the PI3K signaling pathway regulates multiple biological processes that are important in tumorigenesis. This “Master Regulator Kinase” is often hyperactivated in cancer through various mechanisms, including mutations or deletions in Akt, PI3K or PTEN tumor suppressor. The last decade witnessed the emergence of another “Master Regulator Kinase” - CK2. Like Akt, CK2 controls the growth, proliferation and survival of cancer cells. Ironically, CK2 regulates Akt through phosphorylation and down regulation of PTEN and via direct and specific phosphorylation of Akt at Ser129. This phosphorylation event by CK2 further stimulates the activity of Akt, thereby enhancing the “driver effect” of Akt in promoting oncogenic signaling. Unlike Akt, CK2 does not require phosphorylation for activation but rather its activity appears to be regulated through expression levels. Due to the unique shape of the ATP-binding site and an incomplete understanding of the regulation of CK2 expression, pharmacological targeting of CK2 has proven to be very challenging. To date only one small molecule inhibitor of CK2, CX-4945, has advanced into clinical development. Herein, we describe the pharmacological characterization of CX-4945, its impact on Akt signaling and implications for combination therapies.

The high incidence of abnormalities found in the PI3K pathway in breast cancers and the distinct roles that CK2 and Akt play in this disease have made it an attractive tumor type to study the effects of CX-4945. A cell viability screen of 16 diverse but genetically well-characterized breast cancer lines revealed that cells carrying mutations causing Akt-activation were significantly more sensitive to CX-4945 than those that did not. Western blot analyses of these cell lines demonstrated good correlation between the phosphorylation of Akt at Ser129 and expression of catalytic subunits of CK2. Treatment with CX-4945 resulted in dramatic reductions of phosphorylation of Akt at Ser129 and reductions in phosphorylation of the downstream targets of Akt, e.g. p21. Upon combination of CX-4945 with inhibitors that targeted the PI3K/Akt pathway, we observed synergistic antiproliferative activity in breast cancer cells. Thus, evaluation of the effects of CX-4945 on the Akt pathway in breast cancer cell lines may allow for the identification of patient populations more sensitive to CX-4945 and guide the selection of more effective combination therapies for cancer patients.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C198.

Anticancer activity of CX-3543: a direct inhibitor of rRNA biogenesis

Hallmark deregulated signaling in cancer cells drives excessive ribosome biogenesis within the nucleolus, which elicits unbridled cell growth and proliferation. The rate-limiting step of ribosome biogenesis is synthesis of rRNA (building blocks of ribosomes) by RNA Polymerase I (Pol I). Numerous kinase pathways and products of proto-oncogenes can up-regulate Pol I, whereas tumor suppressor proteins can inhibit rRNA synthesis. In tumorigenesis, activating mutations in certain cancer-associated kinases and loss-of-function mutations in tumor suppressors lead to deregulated signaling that stimulates Pol I transcription with resultant increases in ribosome biogenesis, protein synthesis, cell growth, and proliferation. Certain anticancer therapeutics, such as cisplatin and 5-fluorouracil, reportedly exert, at least partially, their activity through disruption of ribosome biogenesis, yet many prime targets for anticancer drugs within the ribosome synthetic machinery of the nucleolus remain largely unexploited. Herein, we describe CX-3543, a small molecule nucleolus-targeting agent that selectively disrupts nucleolin/rDNA G-quadruplex complexes in the nucleolus, thereby inhibiting Pol I transcription and inducing apoptosis in cancer cells. CX-3543 is the first G-quadruplex interactive agent to enter human clinical trials, and it is currently under evaluation against carcinoid/neuroendocrine tumors in a phase II clinical trial.