Small Molecule Inhibitor Of Chk1 Research Articles

Inhibition of Chk1 stimulates cytotoxic action of platinum-based drugs and TRAIL combination in human prostate cancer cells.

Checkpoint kinase 1 (Chk1) plays an important role in regulation of the cell cycle, DNA damage response and cell death, and represents an attractive target in anticancer therapy. Small-molecule inhibitors of Chk1 have been intensively investigated either as single agents or in combination with various chemotherapeutic drugs and they can enhance the chemosensitivity of numerous tumor types. Here we newly demonstrate that pharmacological inhibition of Chk1 using potent and selective inhibitor SCH900776, currently profiled in phase II clinical trials, significantly enhances cytotoxic effects of the combination of platinum-based drugs (cisplatin or LA-12) and TRAIL (tumor necrosis factor-related apoptosis inducing ligand) in human prostate cancer cells. The specific role of Chk1 in the drug combination-induced cytotoxicity was confirmed by siRNA-mediated silencing of this kinase. Using RNAi-based methods we also showed the importance of Bak-dependent mitochondrial apoptotic pathway in the combined anticancer action of SCH900776, cisplatin and TRAIL. The triple drug combination-induced cytotoxicity was partially enhanced by siRNA-mediated Mcl-1 silencing. Our findings suggest that targeting Chk1 may be used as an efficient strategy for sensitization of prostate cancer cells to killing action of platinum-based chemotherapeutic drugs and TRAIL.

Elucidating the structure and cytochrome P450-mediated mechanism for novel metabolites of GDC-0575 in rats

1. GDC-0575 is an ATP-competitive small-molecule inhibitor of ChK1 that is being developed by Genentech for the treatment of various human malignancies. 2. In a radiolabeled mass balance study of GDC-0575 in rats, two novel metabolites, named M12 (-71 Da,) and M17 (+288 Da), were detected as abundant circulating metabolites. 3. Subsequent mass spectrometry and nuclear magnetic resonance analysis showed that M12 was a cyclized metabolite of GDC-0575, whereas M17 was its heterodimer to the parent. We further determined that M12 was mainly generated by cytochrome P450 (Cyp) 2d2. 4. We proposed the potential mechanism was initiated by the oxidation on the pyrrole ring and subsequent cyclisation of the free primary amine onto C-3 of the pyrrole ring. This was followed by expulsion of cyclopropylcarboxamide and a loss of water to form intermediate I, which can be further oxidised to form M12, or dimerise with another molecule of GDC-0575 as nucleophile to form M17. 5. To verify this hypothesis, we attempted to trap the intermediate I with glutathione (GSH) trapping assay and the GSH conjugate on the pyrrole ring was identified. This suggests the oxidation on the pyrrole led to reactive metabolite formation and supported this proposed mechanism.

Enhanced Efficacy of Combined Therapy with Checkpoint Kinase 1 Inhibitor and Rucaparib via Regulation of Rad51 Expression in BRCA Wild-Type Epithelial Ovarian Cancer Cells.

PurposeThis study aimed to evaluate anticancer effects of combination treatment with poly(ADP-ribose) polymerase (PARP) and checkpoint kinase 1 (Chk1) inhibitors in BRCA wild-type ovarian cancer. PARP inhibitors can function as DNA-damaging agents in BRCA wild-type cancer, even if clinical activity is limited. Most epithelial ovarian cancers are characterized by a TP53 mutation causing dysfunction at the G1/S checkpoint, which makes tumor cells highly dependent on Chk1-mediated G/M phase cell-cycle arrest for DNA repair.Materials and MethodsWe investigated the anticancer effects of combination treatment with prexasertib (LY2606368), a selective ATP competitive small molecule inhibitor of Chk1 and Chk2, and rucaparib, a PARP inhibitor, in BRCA wild-type ovarian cancer cell lines (OVCAR3 and SKOV3).ResultsWe found that combined treatment significantly decreased cell viability in all cell lines and induced greater DNA damage and apoptosis than in the control and/or using monotherapies. Moreover, we found that prexasertib significantly inhibited homologous recombination–mediated DNA repair and thus showed a marked anticancer effect in combination treatment with rucaparib. The anticancer mechanism of prexasertib and rucaparib was considered to be caused by an impaired G2/M checkpoint due to prexasertib treatment, which forced mitotic catastrophe in the presence of rucaparib.ConclusionOur results suggest a novel effective therapeutic strategy for BRCA wild-type epithelial ovarian cancer using a combination of Chk1 and PARP inhibitors.

A Sampling of Highlights from the Literature

Prostaglandin E2 (from WikiMedia Commons)Neutrophils can be immunosuppressive when located in the microenvironment of tumors. GM-CSF in the tumor microenvironment stimulates STAT5-upregulation of fatty-acid transporter protein FATP2 primarily on tumor neutrophils. This increases arachidonic acid uptake and the generation of PGE2, with subsequent suppression of tumor-specific T-cell responses. By combining inhibition of FATP2 with blockade of CTLA-4, tumor growth can be effectively inhibited in two mouse tumor models.Veglia F, …, Gabrilovich DI. Nature 2019 Apr 17;569:73–8.Neo open reading frame peptides (from Koster et al., Fig. 2)Cancer cell genomes can contain frameshift mutations, from which long neopeptides can be translated. Examination of over 10,000 TGCA tumor samples finds 143,444 frame-shift mutations, and 70,000 unique peptides ≥ 10 amino acids. Tumor-drivers are commonly frame-shifted genes. The neo-open-reading-frame peptides (NOPs) converge on common peptide sequences, in that by including only 6 driver genes, 1244 new peptides could be found in 30% of the patients. NOPs offer a new source of commonly shared potential neoantigens that could inform production of an “off-the-shelf” library of vaccine reagents.Koster J and Plasterk RHA. Sci Rep 2019 Apr 29;9:6577.Antibody binding sites on PD-1 (from Fenwick et al., Fig. 2F)Clinically useful mAbs to PD-1 block the PD-L1 binding site and thereby block the inhibition signal. Fenwick and colleagues find two new mAbs that bind to PD-1, but structural analysis indicates that binding is on locations away from the PD-1 binding site. The nonblocking mAbs act through the CD28 coreceptor, restoring signaling through the AKT–NF-κB pathway, and helping further rescue exhausted T cells. Combining both types of antibodies in tumor models enhances tumor clearance over monotherapy.Fenwick C, …, Pantaleo G. J Exp Med 2019 May 23. DOI: 10.1084/jem.20182359.Effects of DDR and PD-L1 inhibition (from Sen et al., Fig. 7E)Immune checkpoint blockade is successful only in some cases of small cell lung cancer (SCLC). Sen et al. find that FDA-approved small molecule inhibitors of CHK1 and PARP, which participate in the DNA damage response, can enhance PD-L1 blockade of the CD8+ T cell antitumor response. This is mediated in part through activation of the STING pathway that leads to increased production of T cell chemoattractants like CXCL10 and CCL5. The results suggest dual therapy could improve immunotherapy in SCLC.Sen T, …, Byers LA. Cancer Discov 2019 May 1;9:646–61.Melanoma-associated lymph node (from Broggi et al., Fig. 6A)Two papers show the promise of using tumor-draining lymph node exudate as the substrate for tumor biomarkers and evaluation of antitumor immune responses. Garcia-Silva et al. make use of extracellular vesicles (EVs) to detect markers of melanoma progression and the status of BRAF mutations. Broggi et al. compare plasma to draining lymph node exudate and find the exudate and EVs to be enriched in metastatic tumor–derived factors with protein signatures that can differentiate early from advanced metastasis and provide insights into tumor interactions with the immune system.García-Silva S, …, Peinado H. J Exp Med 2019 MAY 6;216: 1061–70.Broggi MAS, …, Swartz MA. J Exp Med 2019 May 6;216:1091–107.Moderately differentiated gastric tumor (by Nephron via Wikimedia Commons)The ramifications of PD-1 blockade on regulatory T cells (Tregs) are not clear. Almost 10% of patients with gastric cancer treated with anti–PD-1 respond with rapid progression of their tumors. In these patients, treatment increases the proliferative rate and immune suppressive activity of their Tregs. Tregs from PD-1–deficient mice lack PD-1 and both proliferate strongly and have enhanced immunosuppressive activity of antitumor responses. This association of hyperprogressive disease with PD-1 treatment suggests that Tregs be routinely monitored during checkpoint blockade therapy to identify this patient subset.Kamada T, …, Nishikawa H. Proc Natl Acad Sci U S A 2019 May 14;116:9999–10008.

Inhibition of checkpoint kinase 1 following gemcitabine-mediated S phase arrest results in CDC7- and CDK2-dependent replication catastrophe

Combining DNA-damaging drugs with DNA checkpoint inhibitors is an emerging strategy to manage cancer. Checkpoint kinase 1 inhibitors (CHK1is) sensitize most cancer cell lines to DNA-damaging drugs and also elicit single-agent cytotoxicity in 15% of cell lines. Consequently, combination therapy may be effective in a broader patient population. Here, we characterized the molecular mechanism of sensitization to gemcitabine by the CHK1i MK8776. Brief gemcitabine incubation irreversibly inhibited ribonucleotide reductase, depleting dNTPs, resulting in durable S phase arrest. Addition of CHK1i 18 h after gemcitabine elicited cell division cycle 7 (CDC7)- and cyclin-dependent kinase 2 (CDK2)-dependent reactivation of the replicative helicase, but did not reinitiate DNA synthesis due to continued lack of dNTPs. Helicase reactivation generated extensive single-strand (ss)DNA that exceeded the protective capacity of the ssDNA-binding protein, replication protein A. The subsequent cleavage of unprotected ssDNA has been termed replication catastrophe. This mechanism did not occur with concurrent CHK1i plus gemcitabine treatment, providing support for delayed administration of CHK1i in patients. Alternative mechanisms of CHK1i-mediated sensitization to gemcitabine have been proposed, but their role was ruled out; these mechanisms include premature mitosis, inhibition of homologous recombination, and activation of double-strand break repair nuclease (MRE11). In contrast, single-agent activity of CHK1i was MRE11-dependent and was prevented by lower concentrations of a CDK2 inhibitor. Hence, both pathways require CDK2 but appear to depend on different CDK2 substrates. We conclude that a small-molecule inhibitor of CHK1 can elicit at least two distinct, context-dependent mechanisms of cytotoxicity in cancer cells.

Phase I study of the checkpoint kinase 1 inhibitor GDC-0575 in combination with gemcitabine in patients with refractory solid tumors

BackgroundCheckpoint kinase 1 (Chk1) inhibition following chemotherapy-elicited DNA damage overrides cell cycle arrest and induces mitotic catastrophe and cell death. GDC-0575 is a highly-selective oral small-molecule Chk1 inhibitor that results in tumor shrinkage and growth delay in xenograft models. We evaluated the safety, tolerability, and pharmacokinetic properties of GDC-0575 alone and in combination with gemcitabine. Antitumor activity and Chk1 pathway modulation were assessed. Patients and methodsIn this phase I open-label study, in the dose escalation stage, patients were enrolled in a GDC-0575 monotherapy Arm (1) or GDC-0575 combination with gemcitabine Arm (2) to determine the maximum tolerated dose. Patients in arm 2 received either i.v. gemcitabine 1000mg/m2 (arm 2a) or 500mg/m2 (arm 2b), followed by GDC-0575 (45 or 80mg, respectively, as RP2D). Stage II enrolled disease-specific cohorts. ResultsOf 102 patients treated, 70% were female, the median age was 59years (range 27–85), and 47% were Eastern Cooperative Oncology Group PS 0. The most common tumor type was breast (37%). The most frequent adverse events (all grades) related to GDC-0575 and/or gemcitabine were neutropenia (68%), anemia (48%), nausea (43%), fatigue (42%), and thrombocytopenia (35%). Maximum concentrations of GDC-0575 were achieved within 2hours of dosing, and half-life was ∼23hours. No pharmacokinetic drug–drug interaction was observed between GDC-0575 and gemcitabine. Among patients treated with GDC-0575 and gemcitabine, there were four confirmed partial responses, three occurring in patients with tumors harboring TP53 mutation. Pharmacodynamic data were consistent with GDC-0575 inhibition of gemcitabine-induced expression of pCDK1/2. ConclusionGDC-0575 can be safely administered as a monotherapy and in combination with gemcitabine; however, overall tolerability with gemcitabine was modest. Hematological toxicities were frequent but manageable. Preliminary antitumor activity was observed but limited to a small number of patients with a variety of refractory solid tumors treated with GDC-0575 and gemcitabine. Clinical trial numberNCT01564251.

Abstract 4090: Preclinical pharmacokinetics of CASC-578, a novel, selective, potent, and orally bioavailable small molecule checkpoint kinase 1 inhibitor

Abstract Introduction: Checkpoint kinase 1 (Chk1) is a serine/threonine protein kinase that regulates cell division in response to genotoxic stress by arresting cell cycle progression in the S & G2 phases. Pharmacological inhibition of Chk1 is proposed to selectively uncouple the completion of DNA replication from G2/M phase transition in tumor cells that have impaired DNA damage response networks, resulting in mitotic catastrophe and cell death. CASC-578 is a novel small molecule inhibitor of Chk1 that is selective, highly potent and orally bioavailable in multiple preclinical species. Methods: The in vitro ADME properties of CASC-578 were evaluated, including in vitro intrinsic microsomal clearance, Caco2 permeability, plasma protein binding and stability, blood to plasma partitioning, cytochrome P450 inhibition and induction, and transporter inhibition. Pharmacokinetic studies of CASC-578 were conducted in mice, rats, and cynomolgus monkeys as either single dose (IV and/or PO) or repeat dose (PO only). Multiple oral dose pharmacokinetic studies were conducted in mice, rats and cynomolgus monkeys for 5-7 days. Results: CASC-578 is highly bound in plasma protein across all species. The apparent permeability is high in Caco2 bi-directional transport study and correlated well with a rapid absorption profile observed in vivo. CASC-578 is not a substrate of P-glycoporotein (efflux =1). There was no direct nor time dependent inhibition on human CYP450 enzymes, and only a slight induction of CYP3A4 was seen at 10 uM drug concentration in a transporter cell-based induction assay. In all animal species, CASC-578 exhibited species-dependent systemic clearance resulting from both phase I and phase II metabolism, and a moderate to high volume of distribution. The elimination kinetics appeared to be monophasic. Oral bioavailability was high in all species studied (>60% F). There was no significant difference in the pharmacokinetics of the drug between genders. Maximal plasma concentration and total drug exposure (AUC) appeared to be proportional from repeat dose studies. Overall, CASC-578 has very desirable drug-like properties and ideal pharmacokinetics for an oral once daily drug, and represents a suitable candidate for clinical development as a novel potential therapeutic approach for the treatment of solid and hematological malignancies Citation Format: Dina Leviten, Teresa Sierra, Ashley Dozier, Richard Boyce, Bob Boyle, Scott Peterson, Alex C. Vo. Preclinical pharmacokinetics of CASC-578, a novel, selective, potent, and orally bioavailable small molecule checkpoint kinase 1 inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4090. doi:10.1158/1538-7445.AM2017-4090

Abstract 297: The novel orally available sub-nanomolar potent and selective checkpoint kinase 1 inhibitor CASC-578 is highly active in mantle cell lymphoma as a single agent and in combination with Wee1 inhibition

Abstract Background: Checkpoint kinase 1 (Chk1) is a serine/threonine protein kinase that regulates cell division by arresting progression in the S & G2 phases of the cell cycle in response to genotoxic stress. Pharmacological inhibition of Chk1 uncouples the completion of DNA replication from G2/M phase transition in tumor cells that have impaired DNA damage response networks, resulting in DNA damage induction, mitotic catastrophe and cell death. These properties make Chk1 inhibition an attractive therapeutic approach as a single agent, or to enhance the efficacy of other cancer drugs that target DNA damage response pathways. Methods and Results: This report highlights the activity of the orally bioavailable, selective, small molecule Chk1 inhibitor, CASC-578, in multiple mantle cell lymphoma (MCL) models in vitro and in tumor mouse xenograft studies. MCL have the common genetic biomarker t(11;14)(q13;q32), a chromosomal translocation leading to the constitutive expression of the oncogene CCND1 which encodes cyclin D1, a cell cycle-regulating protein. This genotypic characteristic has been observed in the clear majority of patients with MCL in the clinic and may provide a rationale for sensitivity to Chk1 inhibition. Consistent with this idea, CASC-578 demonstrated remarkable potency as a single agent in blocking the proliferation of MCL cell lines in vitro, with an average IC50 of 61 nM. Treatment with CASC-575 as a single agent resulted in the induction of DNA damage, as measured by phosphorylated histone H2AX and activation of apoptosis, as measured by cleaved caspase. In a JEKO-1 xenograft tumor study, CASC-578 as a single agent completely regressed subcutaneously implanted tumors in the majority of mice after oral drug administration using 20 mg/kg daily or when dosed on an intermittent schedule at 25 mg/kg, with minimal effects on body weight. In addition to the observed single agent activity of CASC-578, the combination of CASC-578, and the Wee1 inhibitor AZD-1775, was highly synergistic in MCL cell lines including JEKO-1, REC-1, Z-138 and MAVER-1 in vitro. CASC-578 was also highly potent as a single agent, and in combination with AZD-1775, in blocking the proliferation of a variety of leukemia derived cell lines in vitro. Taken together, these data support the advancement of CASC-578 into clinical development as a potential therapeutic agent for the treatment of MCL, as well as other hematological malignancies. Citation Format: Robert Rosler, Janelle Taylor, Dina Leviten, Teresa Sierra, Ashley Dozier, Kevin Klucher, Rich Boyce, Bob Boyle, Alex Vo, Scott Peterson. The novel orally available sub-nanomolar potent and selective checkpoint kinase 1 inhibitor CASC-578 is highly active in mantle cell lymphoma as a single agent and in combination with Wee1 inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 297. doi:10.1158/1538-7445.AM2017-297

Abstract 295: CASC-578, a novel Chk1 inhibitor, is active as a single agent in solid tumors and displays synergistic anti-tumor activity in combination with Wee1 inhibition

Abstract Background: Checkpoint kinase 1 (Chk1) is a serine/threonine protein kinase that regulates cell division in response to genotoxic stress by arresting cell cycle progression in the S & G2 phases. Pharmacological inhibition of Chk1 is proposed to target tumor cells with increased DNA replication stress, resulting in the uncoupling of DNA replication checkpoint function and the induction of DNA damage and cell death. These properties make Chk1 inhibition a novel therapeutic approach as a single agent in cancers with high replication stress that is driven by oncogenic signaling and loss of parallel DNA damage response pathway function. Methods and Results: This report highlights the activity of the orally bioavailable, selective small molecule Chk1 inhibitor, CASC-578, in solid tumor derived cell lines. CASC-578 is a sub-nanomolar enzymatic inhibitor of Chk1 with limited off-target activity against a panel of protein kinases. When evaluated in a large cell line panel in vitro, CASC-578 demonstrated a broad potency range as a single agent in solid tumor derived cells lines, with IC50s ranging from 30 nM to greater than 50 μM. Several solid tumor types demonstrated enriched sensitivity to CASC-578 in vitro, including gastric, non-small cell lung and ovarian cancers. Treatment of sensitive cell lines with CASC-578 resulted in the induction of DNA damage, as measured by phosphorylated histone H2AX, and the induction of cell death. CASC-578 was active as a single agent in SK-MES-1 and NCI-H727 NSCLC tumor xenograft models in vivo with minimal effects on body weight in treated mice. In addition to the potent single agent activity of CASC-578, combination with the Wee1 inhibitor AZD-1775 was highly synergistic in vitro in multiple solid tumor cell lines and the combination was more efficacious than either agent alone in NSCLC tumor xenograft models. These data support the advancement of CASC-578 into clinical development as a potential therapeutic agent for the treatment of solid tumor diseases. Experiments are ongoing to identify biomarkers associated with sensitivity to CASC-578 as a single agent in solid tumor cell lines to prospectively identify tumor genotypes that are more responsive to the drug. Citation Format: Alex Vo, Janelle Taylor, Robert Rosler, Julia Piasecki, Dina Leviten, Teresa Sierra, Ashley Dozier, Kevin Klucher, Bob Boyle, Rich Boyce, Scott Peterson. CASC-578, a novel Chk1 inhibitor, is active as a single agent in solid tumors and displays synergistic anti-tumor activity in combination with Wee1 inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 295. doi:10.1158/1538-7445.AM2017-295

Abstract NTOC-095: THE SYNERGISTIC EFFECTS OF CARBOPLATIN AND PF–477736, A SMALL MOLECULE CHECKPOINT KINASE 1 INHIBITOR ON A PLATINUM RESISTANT OVARIAN CANCER CELL LINE

Abstract PURPOSE: To investigate the effects of PF-477736, a small molecular checkpoint kinase 1 (Chk1) inhibitor with and without carboplatin on cell apoptosis and proliferation in a p53-mutated platinum resistant ovarian cancer cell line. METHODS: OVCAR3 cells were treated with PF-477736 alone and in combination with carboplatin. Western blot analyses were performed to assess the protein expression of Chk1, phosphorylated Chk1, H2AX, and phosphorylated H2AX. Cell proliferation was evaluated with colony formation assays 10 days following treatment and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) proliferation assays over the course of 5 days following treatment. Cell apoptosis was measured with flow cytometry after staining with FITC Annexin V and propidium iodide. Paired t-tests were performed for comparative analyses. RESULTS: PF-477736 decreased the expression of phosphorylated Chk1 on serine 249 within 24 hours following treatment as seen on Western blot assay. Total Chk1 expression was unchanged after treatment with PF-477736. The combination treatment of PF-477736 and carboplatin effectively decreased OVCAR3 cell proliferation. Colony formation assays revealed fewer colonies after dual treatment with PF-477736 and carboplatin compared to PF-477736 alone (average colony count 1.5 vs 13.6, p = 0.03) and compared to carboplatin alone (average colony count 1.5 vs 9.7, p < 0.001). MTT assays demonstrated a synergistic effect with PF-477736 and carboplatin: dual treated cells had lower mean dye absorbance values than PF-477736 alone (mean absorbance fold change 1.7 vs 3.2, p = 0.009) and carboplatin alone (mean absorbance fold change 1.7 vs 5.6, p < 0.001). OVCAR3 cells after treatment with PF-477736 and carboplatin had higher rates of apoptosis. On flow cytometry, a higher proportion of cells stained with the apoptotic markers Annexin V and propidium iodide after treatment with PF-477736 and carboplatin compared to PF-477736 alone (percentage of apoptotic cells 34.9% vs 13.0%, p = 0.004) and carboplatin alone (percentage of apoptotic cells 34.9% vs 16.0%, p = 0.008). In addition, western blot analysis of the cell lysates following dual treatment of PF-477736 and carboplatin showed increased expression of phosphorylated H2AX, a marker of double-stranded DNA damage while there was minimal expression with PF-477736 alone and carboplatin alone. CONCLUSIONS: Carboplatin and PF-477736, a small molecule Chk1 inhibitor, synergistically decreased cell proliferation and increased cell apoptosis in OVCAR3 cells, an in vitro model of platinum resistant ovarian cancer. PF-477736 may be a potential new therapeutic strategy for the resensitization of platinum resistant ovarian cancer. Citation Format: Jessica Lee MD, Stephanie V. Blank MD, Robert J. Schneider MD. THE SYNERGISTIC EFFECTS OF CARBOPLATIN AND PF–477736, A SMALL MOLECULE CHECKPOINT KINASE 1 INHIBITOR ON A PLATINUM RESISTANT OVARIAN CANCER CELL LINE [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr NTOC-095.

Abstract 1600: Keeping glioblastoma (GBM)in check by targeting the CHK1-STAT3-CIP2A axis

Abstract Checkpoint kinase1 (CHK1) is a DNA damage kinase, which has been observed to be constitutively active in human cancers. Unfortunately a number of new generation CHK1 inhibitors, currently in clinical trials, have shown deleterious effects on normal cells, thus limiting their clinical utility. Therefore, identification and targeting of cancer specific effectors of CHK1 signaling has emerged as an attractive therapeutic alternative. Analysis of the REMBRANDT and TCGA datasets revealed a strong positive correlation for CHK1 and CIP2A expression in 422/522 and 356/454 glioma patients respectively. By contrast, there was negligible correlation observed in normal samples. Additionally, high mRNA expression of both CHK1 and CIP2A was associated with reduced overall survival in glioma patients and marked a more aggressive form of the disease. Notably, CIP2A amplification found in 14.72% cases in the Rembrandt study was associated with worse overall survival in GBM patients. Mechanistically, we identify STAT3 as a transcriptional mediator for CHK-dependent CIP2A expression in GBM cells. We demonstrate that CHK1 positively regulates CIP2A transcription by promoting phosphorylation of STAT3 at Tyrosine 705 both in vitro and in vivo. Further, depletion of STAT3 by siRNA or chemical inhibitors resulted in decreased CIP2A expression in GBM cells. Functionally, both CHK1 and CIP2A promoted viability, clonogenicity and anchorage-independent growth of GBM cells. Importantly, inhibition of cancer cell viability and clonogenicity by Chk1 inhibition can be partially rescued by exogenous overexpression of CIP2A. Higher CIP2A, p-S345-CHK1 and p-Ty705-STAT3 levels were observed in de novo and patient-derived GBM, and U251MG cells relative to levels in normal human astrocytes (NHA). Analogously, CHK1 and CIP2A mRNA expression is increased 10 to 15 fold in genetically engineered mouse(GEM) models of human GBM (Pdgf-Cre-driven Ptenf/f and Pdgf-Cre-driven Ptenf/f;/p53f/f double mutant mice compared with wild-type mice). Finally, using xenograft mouse models we show that both PF477736 (a small molecule inhibitor of CHK1) and CIP2A depletion inhibits tumor growth. These results highlight CHK1 and CIP2A expression as potential diagnostic and prognostic markers in human GBMs. Further, these findings identify CIP2A as a cancer specific therapeutic target downstream of constitutively active CHK1 and STAT3 signaling in GBM. Together, these data serves as a platform for using CIP2A expression in GBM, to either directly target CIP2A or to stratify patients for CHK1 and/or STAT3 inhibition. Note: This abstract was not presented at the meeting. Citation Format: Anchit Khanna, Brett Stringer, Bryan Day, Kathleen Ensbey, Han Shen, Andrew Boyd, Kerrie McDonald, John E. Pimanda. Keeping glioblastoma (GBM)in check by targeting the CHK1-STAT3-CIP2A axis. [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 1600. doi:10.1158/1538-7445.AM2014-1600

Pharmacological targeting the ATR-CHK1-WEE1 axis involves balancing cell growth stimulation and apoptosis.

The ATR-CHK1-WEE1 kinase cascade's functions in the DNA damage checkpoints are well established. Moreover, its roles in the unperturbed cell cycle are also increasingly being recognized. In this connection, a number of small-molecule inhibitors of ATR, CHK1, and WEE1 are being evaluated in clinical trials. Understanding precisely how cells respond to different concentrations of inhibitors is therefore of paramount importance and has broad clinical implications. Here we present evidence that in the absence of DNA damage, pharmacological inactivation of ATR was less effective in inducing mitotic catastrophe than inhibition of WEE1 and CHK1. Small-molecule inhibitors of CHK1 (AZD7762) or WEE1 (MK-1775) induced mitotic catastrophe, as characterized by dephosphorylation of CDK1(Tyr15), phosphorylation of histone H39(Ser10), and apoptosis. Unexpectedly, partial inhibition of WEE1 and CHK1 had the opposite effect of accelerating the cell cycle without inducing apoptosis, thereby increasing the overall cell proliferation. This was also corroborated by the finding that cell proliferation was enhanced by kinase-inactive versions of WEE1. We demonstrated that these potential limitations of the inhibitors could be overcome by targeting more than one components of the ATR-CHK1-WEE1 simultaneously. These observations reveal insights into the complex responses to pharmacological inactivation of the ATR-CHK1-WEE1 axis.

Inhibition of the checkpoint kinase Chk1 induces DNA damage and cell death in human Leukemia and Lymphoma cells.

BackgroundChk1 forms a core component of the DNA damage response and small molecule inhibitors are currently being investigated in the clinic as cytotoxic chemotherapy potentiators. Recent evidence suggests that Chk1 inhibitors may demonstrate significant single agent activity in tumors with specific DNA repair defects, a constitutively activated DNA damage response or oncogene induced replicative stress.MethodsGrowth inhibition induced by the small molecule Chk1 inhibitor V158411 was assessed in a panel of human leukemia and lymphoma cell lines and compared to cancer cell lines derived from solid tumors. The effects on cell cycle and DNA damage response markers were further evaluated.ResultsLeukemia and lymphoma cell lines were identified as particularly sensitive to the Chk1 inhibitor V158411 (mean GI50 0.17 μM) compared to colon (2.8 μM) or lung (6.9 μM) cancer cell lines. Chk1 inhibition by V158411 in the leukemia and lymphoma cell lines induced DNA fragmentation and cell death that was both caspase dependent and independent, and prevented cells undergoing mitosis. An analysis of in vitro pharmacodynamic markers identified a dose dependent decrease in Chk1 and cyclin B1 protein levels and Cdc2 Thr15 phosphorylation along with a concomitant increase in H2AX phosphorylation at Ser139 following V158411 treatment.ConclusionsThese data support the further evaluation of Chk1 inhibitors in hematopoietic cancers as single agents as well as in combination with standard of care cytotoxic drugs.

Dovitinib induces mitotic defects and activates the G2DNAdamage checkpoint

Dovitinib (TKI258; formerly CHIR-258) is an orally bioavailable inhibitor of multiple receptor tyrosine kinases. Interestingly, Dovitinib triggered a G2/M arrest in cancer cell lines from diverse origins including HeLa, nasopharyngeal carcinoma, and hepatocellular carcinoma. Single-cell analysis revealed that Dovitinib promoted a delay in mitotic exit in a subset of cells, causing the cells to undergo mitotic slippage. Higher concentrations of Dovitinib induced a G2 arrest similar to the G2 DNA damage checkpoint. In support of this, DNA damage was triggered by Dovitinib as revealed by γ-H2AX and comet assays. The mitotic kinase CDK1 was found to be inactivated by phosphorylation in the presence of Dovitinib. Furthermore, the G2 arrest could be overcome by abrogation of the G2 DNA damage checkpoint using small molecule inhibitors of CHK1 and WEE1. Finally, Dovitinib-mediated G2 cell cycle arrest and subsequent cell death could be promoted after DNA damage repair was disrupted by inhibitors of poly(ADP-ribose) polymerases. These results are consistent with the recent finding that Dovitinib can also target topoisomerases. Collectively, these results suggest additional directions for use of Dovitinib, in particular with agents that target the DNA damage checkpoint.

CHK1 and WEE1 inhibition combine synergistically to enhance therapeutic efficacy in acute myeloid leukemia ex vivo

Novel combinations targeting new molecular vulnerabilities are needed to improve the outcome of patients with acute myeloid leukemia. We recently identified WEE1 kinase as a novel target in leukemias. To identify genes that are synthetically lethal with WEE1 inhibition, we performed a short interfering RNA screen directed against cell cycle and DNA repair genes during concurrent treatment with the WEE1 inhibitor MK1775. CHK1 and ATR, genes encoding two replication checkpoint kinases, were among the genes whose silencing enhanced the effects of WEE1 inhibition most, whereas CDK2 short interfering RNA antagonized MK1775 effects. Building on this observation, we examined the impact of combining MK1775 with selective small molecule inhibitors of CHK1, ATR and cyclin-dependent kinases. The CHK1 inhibitor MK8776 sensitized acute myeloid leukemia cell lines and primary leukemia specimens to MK1775 ex vivo, whereas smaller effects were observed with the MK1775/MK8776 combination in normal myeloid progenitors. The ATR inhibitor VE-821 likewise enhanced the antiproliferative effects of MK1775, whereas the cyclin-dependent kinase inhibitor roscovitine antagonized MK1775. Further studies showed that MK8776 enhanced MK1775-mediated activation of the ATR/CHK1 pathway in acute leukemia cell lines and ex vivo. These results indicate that combined cell cycle checkpoint interference with MK1775/MK8776 warrants further investigation as a potential treatment for acute myeloid leukemia.

Abstract 1459: Combined inhibition of Wee1 and PARP1 sensitizes pancreatic cancer cells to radiation

Abstract PARP1 [poly (ADP-ribose) polymerase-1] inhibitors are maximally effective in cells with HRR (homologous recombination repair) defects, a concept known as synthetic lethality. We have previously shown that inhibition of HRR and the G2 checkpoint by small molecule inhibitors of Chk1 (checkpoint kinase 1), can induce synthetic lethality by increasing PARP1 inhibitor-mediated radiosensitization selectively in tumor cells and preferentially in tumor cells with p53 mutations. In order to begin to determine whether other inhibitors of the DNA damage response might induce synthetic lethality in combination with PARP1 inhibition and radiation, we evaluated MK-1775, a Wee1 kinase inhibitor. Wee1 regulates the G2 checkpoint by phosphorylation of Cdk1, thus preventing entry into mitosis in the presence of DNA damage, while its influence on HRR is not clear. To determine whether inhibition of Wee1 might synergistically radiosensitize in combination with PARP1 inhibition, we treated pancreatic cancer cells with MK-1775 (100-200nM) and the PARP1 inhibitor, olaparib (1uM) and assessed radiosensitization by clonogenic survival. We found that the combination of MK-1775 with olaparib induced marked radiosensitization in MiaPaCa-2 (RER: 1.9) and AsPC-1 (RER: 1.7) pancreatic cancer cells, which was greater than the radiosensitization produced by either agent alone. In order to determine the mechanism(s) by which Wee1 and PARP1 inhibitors interact to produce radiosensitization, we investigated the G2 checkpoint as well as HRR. In response to radiation, MK-1775 abrogated the G2 checkpoint while olaparib treatment resulted in increased G2 accumulation which was associated with increased γH2AX, suggesting the presence of unrepaired DNA double strand breaks. Abrogation of the olaparib+radiation-mediated G2 checkpoint, by MK-1775 resulted in even greater DNA damage as assessed by γH2AX expression. Given the efficacy of PARP1 inhibitors in HRR defective cells, we also investigated the effect of MK-1775 on HRR. We found that MK-1775 inhibited homology-directed repair of an I-SceI endonuclease-induced DNA double strand break in MiaPaCa-2 cells (Control 5.2% versus MK-1775 3.3%, P<0.05) as well as impaired Rad51 focus formation in response to radiation. Taken together, these results suggest that inhibition of HRR and the G2 checkpoint by inhibition of Wee1 can induce synthetic lethality with PARP1 inhibitors in combination with radiation by increasing unrepaired DNA double strand breaks. These studies support the continued investigation of combined molecularly targeted agents such as Wee1 and PARP1 inhibitors with radiation, to induce synthetic lethality selectively in tumor cells. 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 1459. doi:1538-7445.AM2012-1459

Abstract A198: Identification and preclinical testing of novel targeted therapies for human triple-negative breast cancer.

Abstract Basal-like triple negative breast cancer (BTNBC) is characterized by the absence of estrogen receptor, progesterone receptor, and Her2 expression, and represents an estimated 15% of all human breast cancers. This subtype is highly prevalent in African American women (35%) and is associated with distant metastasis and poor prognosis. As there are currently no targeted therapies for this subtype of breast cancer, our research focuses on the development of improved therapies to increase patient survival. We have previously shown that primary mammary tumors from the C3(1)/SV40-TAg genetically engineered mouse (GEM) model express a critical proliferation/ DNA repair/ cell cycle gene signature (TAg signature) that is shared with human BTNBC. SV40-TAg functionally inactivates p53 and pRb, two critical tumor suppressor genes whose functions are lost in most human BTNBC. We hypothesize that targeting dysregulated genes and biological networks present in the TAg signature will lead to the identification of novel targets and potential combination therapies. To test this idea we developed a custom siRNA library screen to inhibit the expression of individual genes represented in the TAg signature using human MDA-MB-231 BTNBC cells. Additionally, we performed a synthetic lethal screen in which TAg signature genes were knocked down in MDA-MB-231 cells using pooled siRNAs in combination with either UCN-01 or gemcitabine, small molecule inhibitors of CHK1 and RRM1/2 respectively. It was found that the knockdown of BMI-1, EZH2, GeneA, GeneB, or GeneC significantly decreased cell proliferation with the addition of UCN-01 or gemcitabine. A second synthetic lethal screen was performed in which the above genes were knocked down using four deconvoluted siRNAs per gene with the addition of gemcitabine, UCN-01, or AZD7762 (a more specific CHK1 inhibitor). Synergistic activity was shown between the siRNA knockdown of these genes and UCN-01 and AZD7762 compared to siRNA alone. To determine whether the knockdown of these genes alters sensitivity to gemcitabine or CHK1 inhibitors, the EC50 values of these drugs are currently being evaluated in stable knockdown MDA-MB-231 cell lines expressing shRNA to the selected genes. Our results suggest that using GEM models to help identify new targets may be useful in the development of novel therapeutics for BTNBC in combination with available CHK1 inhibitors and/or gemcitabine. These studies will enhance our understanding of BTNBC and allow us to identify innovative combination therapies to treat this challenging form of breast cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A198.

Single-Agent Inhibition of Chk1 Is Antiproliferative in Human Cancer Cell Lines In Vitro and Inhibits Tumor Xenograft Growth In Vivo

Chk1 is a serine/threonine kinase that plays several important roles in the cellular response to genotoxic stress. Since many current standard-of-care therapies for human cancer directly damage DNA or inhibit DNA synthesis, there is interest in using small molecule inhibitors of Chk1 to potentiate their clinical activity. Additionally, Chk1 is known to be critically involved in cell cycle progression of unperturbed cells. Therefore, it is plausible that treatment with a Chkl inhibitor alone could also be an efficacious cancer therapy. Here we report that Chk1-A, a potent and highly selective small molecule inhibitor of Chk1, is antiproliferative as a single agent in a variety of human cancer cell lines in vitro. The inhibition of proliferation is associated with collapse of DNA replication and apoptosis. Rapid decreases in inhibitory phosphorylation of CDKs and a concomitant increase in CDK kinase activity and chromatin loading of Cdc45 suggest that the antiproliferative and proapoptotic activity of Chk1-A is at least in part due to deregulation of DNA synthesis. We extend these in vitro studies by demonstrating that Chk1-A inhibits the growth of tumor xenografts in vivo in a treatment regimen that is well tolerated. Together, these results suggest that single-agent inhibition of Chk1 may be an effective treatment strategy for selected human malignancies.

Abstract 4758: Inhibition of checkpoint kinase 1 (Chk1) as a potential therapeutic for pediatric neuroblastoma

Abstract Neuroblastoma is a cancer derived from cells of the sympathetic nervous system that manifests with significant clinical heterogeneity. Although children diagnosed with low-risk neuroblastoma are very likely to be cured, high-risk patients are frequently resistant to even the most intensive of multi-modal regimens, and at least half of these children suffer relapse that is almost always fatal. To address the unmet need for novel therapeutic targets, our lab conducted an unbiased siRNA screen and determined that the loss of cell cycle checkpoint kinase 1 (Chk1) produced the most robust cytotoxic effect across multiple neuroblastoma cell lines. We subsequently confirmed that many neuroblastoma cell lines express constitutively phosphorylated Chk1 – a discovery that is supported by identical findings in tumors obtained from high-risk patients. Here we present evidence that the combination of a small molecule inhibitor of Chk1 with either irinotecan (SN-38) or cisplatin was synergistic or additive in vitro in cell lines sensitive to single agent Chk1 inhibition. Inhibition of Chk1 signaling as potential therapy in neuroblastoma is further supported by complimentary experiments in which we found that induction of shRNA specific for Chk1 results in marked cytotoxicity. To further understand the chemosensitizing aspect of Chk1 inhibition in neuroblastoma cells, we assayed the ATR/Chk1 pathway for potential biomarkers predictive of response. Candidate signaling partners in this pathway were isolated based upon either pharmacological inhibition or shRNA-induced silencing of Chk1, followed by hybridization to antibody microarrays. This technique has allowed for identification of Chk1-related signaling proteins involved in the DNA damage response pathway. Lastly, we have assessed the efficacy of Chk1 inhibition in an in vivo xenograft mouse model of pediatric neuroblastoma. We found that daily administration of a small molecule Chk1 inhibitor, currently undergoing Phase I clinical testing, was able to significantly impede the growth of NB1643 neuroblastoma xenografts (p=0.001). Efforts are currently underway in our laboratory to investigate a potentially synergistic effect in vivo through co-administration of this inhibitor with irinotecan or cisplatin. Taken together, these results provide a strong rationale for further studies to assess the use of Chk1 inhibition as either a single-agent or combinatorial therapy in pediatric neuroblastoma. 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 4758. doi:10.1158/1538-7445.AM2011-4758

Targeting the Replication Checkpoint Using SCH 900776, a Potent and Functionally Selective CHK1 Inhibitor Identified via High Content Screening

Checkpoint kinase 1 (CHK1) is an essential serine/threonine kinase that responds to DNA damage and stalled DNA replication. CHK1 is essential for maintenance of replication fork viability during exposure to DNA antimetabolites. In human tumor cell lines, ablation of CHK1 function during antimetabolite exposure led to accumulation of double-strand DNA breaks and cell death. Here, we extend these observations and confirm ablation of CHK2 does not contribute to these phenotypes and may diminish them. Furthermore, concomitant suppression of cyclin-dependent kinase (CDK) activity is sufficient to completely antagonize the desired CHK1 ablation phenotypes. These mechanism-based observations prompted the development of a high-content, cell-based screen for γ-H2AX induction, a surrogate marker for double-strand DNA breaks. This mechanism-based functional approach was used to optimize small molecule inhibitors of CHK1. Specifically, the assay was used to mechanistically define the optimal in-cell profile with compounds exhibiting varying degrees of CHK1, CHK2, and CDK selectivity. Using this approach, SCH 900776 was identified as a highly potent and functionally optimal CHK1 inhibitor with minimal intrinsic antagonistic properties. SCH 900776 exposure phenocopies short interfering RNA-mediated CHK1 ablation and interacts synergistically with DNA antimetabolite agents in vitro and in vivo to selectively induce dsDNA breaks and cell death in tumor cell backgrounds.