The Checkpoint Kinase 1 Inhibitor Prexasertib Induces Regression of Preclinical Models of Human Neuroblastoma

Therapeutic Targeting of DNA Repair Pathways in Pediatric Extracranial Solid Tumors: Current State and Implications for Immunotherapy

DNA damage is fundamental to tumorigenesis, and the inability to repair DNA damage is a hallmark of many human cancers. DNA is repaired via the DNA damage repair (DDR) apparatus, which includes five major pathways. DDR deficiencies in cancers give rise to potential therapeutic targets, as cancers harboring DDR deficiencies become increasingly dependent on alternative DDR pathways for survival. In this review, we summarize the DDR apparatus, and examine the current state of research efforts focused on identifying vulnerabilities in DDR pathways that can be therapeutically exploited in pediatric extracranial solid tumors. We assess the potential for synergistic combinations of different DDR inhibitors as well as combinations of DDR inhibitors with chemotherapy. Lastly, we discuss the immunomodulatory implications of targeting DDR pathways and the potential for using DDR inhibitors to enhance tumor immunogenicity, with the goal of improving the response to immune checkpoint blockade in pediatric solid tumors. We review the ongoing and future research into DDR in pediatric tumors and the subsequent pediatric clinical trials that will be critical to further elucidate the efficacy of the approaches targeting DDR.

A new wave of innovations within the DNA damage response

Genome instability has been identified as one of the enabling hallmarks in cancer. DNA damage response (DDR) network is responsible for maintenance of genome integrity in cells. As cancer cells frequently carry DDR gene deficiencies or suffer from replicative stress, targeting DDR processes could induce excessive DNA damages (or unrepaired DNA) that eventually lead to cell death. Poly (ADP-ribose) polymerase (PARP) inhibitors have brought impressive benefit to patients with breast cancer gene (BRCA) mutation or homologous recombination deficiency (HRD), which proves the concept of synthetic lethality in cancer treatment. Moreover, the other two scenarios of DDR inhibitor application, replication stress and combination with chemo- or radio- therapy, are under active clinical exploration. In this review, we revisited the progress of DDR targeting therapy beyond the launched first-generation PARP inhibitors. Next generation PARP1 selective inhibitors, which could maintain the efficacy while mitigating side effects, may diversify the application scenarios of PARP inhibitor in clinic. Albeit with unavoidable on-mechanism toxicities, several small molecules targeting DNA damage checkpoints (gatekeepers) have shown great promise in preliminary clinical results, which may warrant further evaluations. In addition, inhibitors for other DNA repair pathways (caretakers) are also under active preclinical or clinical development. With these progresses and efforts, we envision that a new wave of innovations within DDR has come of age.

Identification of hub genes and their correlation with infiltration of immune cells in MYCN positive neuroblastoma based on WGCNA and LASSO algorithm

Background

The prognosis of MYCN positive NB is poor, and there is no targeted drug for N-myc at present. This study aims to screen out hub genes closely related to MYCN, analyze the relationship between hub genes and NB microenvironment, and provide basis for molecular targeted therapy of MYCN positive NB.

Methods

We combined the microarray data of GSE45547 (n=649) and GSE49710 (n=498), screened the DEGs between MYCN positive (n=185) and MYCN negative NB (n=951), performed WGCNA, Lasso regression and Roc analyses on the merged matrix, and obtained the hub genes related to MYCN in the training group. We performed ssGSEA on the experimental group to calculate the infiltration level of 28 kinds of immune cells in each sample, compared the differences of immune cell infiltration between MYCN positive and MYCN negative group. The influences of hub genes on the distribution of each immune cell were also analyzed by ssGSEA. The expression differences of the three hub genes were verified in the E-MTAB-8248 cohort (n=223), and the correlation between hub genes and prognosis of NB was calculated by Kaplan-Meier method in GSE62564 (n=498) and the validation group. We also verified the expression differences of hub genes by qRT-PCR in SK-N-BE(2), SKNDZ, Kelly and SH-SY5Y cell lines.

Results

Here were 880 DEGs including 420 upregulated and 460 downregulated genes in MYCN positive NB in the training group. Overlap of the DEGs and WGCNA networks identified four shared genes, namely, ZNF695, CHEK1, C15ORF42 and EXO1, as candidate hub genes in MYCN positive NB. Three core genes, ZNF695, CHEK1 and C15ORF42, were finally identified by Lasso regression and Roc analyses. ZNF695, CHEK1 and C15ORF42 were highly expressed in MYCN positive NB tissues and cell lines. These three genes were closely related to the prognosis of children with NB. Except that Activated CD4 T cell and Type2 T helper cell increased, the infiltration levels of the other 26 cells decreased significantly in MYCN positive NB tissues. The infiltration levels of Type2 T helper cell and Activated CD4 T cell were also significantly positively correlated with the expression levels of the three hub genes.

Conclusion

ZNF695, CHEK1 and C15ORF42 are highly expressed in MYCN positive NB, and their expression levels are negatively correlated with the prognosis of children with NB. The infiltration levels of Activated CD4 T cell and Type2 T helper cell increased in the microenvironment of MYCN positive NB and were significantly positively correlated with the expression levels of the three hub genes. The results of this study provide that ZNF695, CHEK1 and C15ORF42 may be potential prognostic markers and immunotherapy targets for MYCN positive NB.

DNA damage response revisited: the p53 family and its regulators provide endless cancer therapy opportunities

Antitumor therapeutic strategies that fundamentally rely on the induction of DNA damage to eradicate and inhibit the growth of cancer cells are integral approaches to cancer therapy. Although DNA-damaging therapies advance the battle with cancer, resistance, and recurrence following treatment are common. Thus, searching for vulnerabilities that facilitate the action of DNA-damaging agents by sensitizing cancer cells is an active research area. Therefore, it is crucial to decipher the detailed molecular events involved in DNA damage responses (DDRs) to DNA-damaging agents in cancer. The tumor suppressor p53 is active at the hub of the DDR. Researchers have identified an increasing number of genes regulated by p53 transcriptional functions that have been shown to be critical direct or indirect mediators of cell fate, cell cycle regulation, and DNA repair. Posttranslational modifications (PTMs) primarily orchestrate and direct the activity of p53 in response to DNA damage. Many molecules mediating PTMs on p53 have been identified. The anticancer potential realized by targeting these molecules has been shown through experiments and clinical trials to sensitize cancer cells to DNA-damaging agents. This review briefly acknowledges the complexity of DDR pathways/networks. We specifically focus on p53 regulators, protein kinases, and E3/E4 ubiquitin ligases and their anticancer potential.

Chromosome 11q loss and MYCN amplification demonstrate synthetic lethality with checkpoint kinase 1 inhibition in neuroblastoma

Neuroblastoma is the most common extracranial solid tumor found in children and despite intense multi-modal therapeutic approaches, low overall survival rates of high-risk patients persist. Tumors with heterozygous loss of chromosome 11q and MYCN amplification are two genetically distinct subsets of neuroblastoma that are associated with poor patient outcome. Using an isogenic 11q deleted model system and high-throughput drug screening, we identify checkpoint kinase 1 (CHK1) as a potential therapeutic target for 11q deleted neuroblastoma. Further investigation reveals MYCN amplification as a possible additional biomarker for CHK1 inhibition, independent of 11q loss. Overall, our study highlights the potential power of studying chromosomal aberrations to guide preclinical development of novel drug targets and combinations. Additionally, our study builds on the growing evidence that DNA damage repair and replication stress response pathways offer therapeutic vulnerabilities for the treatment of neuroblastoma.

Inhibition of Chk2 promotes neuroprotection, axon regeneration, and functional recovery after CNS injury

DNA double-strand breaks occur in many acute and long-term neurological conditions, including neurodegeneration, neurotrauma, and stroke. Nonrepaired breaks chronically activate the DNA damage response in neurons, leading to neural dysfunction and apoptosis. Here, we show that targeting of the central ATM-Chk2 pathway regulating the response to double-strand breaks slows neural decline in Drosophila models of chronic neurodegeneration. Inhibitors of ATM-Chk2, but not the parallel ATR-Chk1 pathway, also promote marked, functional recovery after acute central nervous system injury in rats, suggesting that inhibiting nonhomologous end-joining rather than homologous recombination is crucial for neuroprotection. We demonstrate that the Chk2 inhibitor, prexasertib, which has been evaluated in phase 2 clinical trials for cancer, has potent neuroprotective effects and represents a new treatment option to promote functional recovery after spinal cord or optic nerve injury.

RRM2 enhances MYCN-driven neuroblastoma formation and acts as a synergistic target with CHK1 inhibition

High-risk neuroblastoma, a pediatric tumor originating from the sympathetic nervous system, has a low mutation load but highly recurrent somatic DNA copy number variants. Previously, segmental gains and/or amplifications allowed identification of drivers for neuroblastoma development. Using this approach, combined with gene dosage impact on expression and survival, we identified ribonucleotide reductase subunit M2 (RRM2) as a candidate dependency factor further supported by growth inhibition upon in vitro knockdown and accelerated tumor formation in a neuroblastoma zebrafish model coexpressing human RRM2 with MYCN. Forced RRM2 induction alleviates excessive replicative stress induced by CHK1 inhibition, while high RRM2 expression in human neuroblastomas correlates with high CHK1 activity. MYCN-driven zebrafish tumors with RRM2 co-overexpression exhibit differentially expressed DNA repair genes in keeping with enhanced ATR-CHK1 signaling activity. In vitro, RRM2 inhibition enhances intrinsic replication stress checkpoint addiction. Last, combinatorial RRM2-CHK1 inhibition acts synergistic in high-risk neuroblastoma cell lines and patient-derived xenograft models, illustrating the therapeutic potential.

Increased Replication Stress Determines ATR Inhibitor Sensitivity in Neuroblastoma Cells

Despite intensive high-dose multimodal therapy, high-risk neuroblastoma (NB) confers a less than 50% survival rate. This study investigates the role of replication stress in sensitivity to inhibition of Ataxia telangiectasia and Rad3-related (ATR) in pre-clinical models of high-risk NB. Amplification of the oncogene MYCN always imparts high-risk disease and occurs in 25% of all NB. Here, we show that MYCN-induced replication stress directly increases sensitivity to the ATR inhibitors VE-821 and AZD6738. PARP inhibition with Olaparib also results in replication stress and ATR activation, and sensitises NB cells to ATR inhibition independently of MYCN status, with synergistic levels of cell death seen in MYCN expressing ATR- and PARP-inhibited cells. Mechanistically, we demonstrate that ATR inhibition increases the number of persistent stalled and collapsed replication forks, exacerbating replication stress. It also abrogates S and G2 cell cycle checkpoints leading to death during mitosis in cells treated with an ATR inhibitor combined with PARP inhibition. In summary, increased replication stress through high MYCN expression, PARP inhibition or chemotherapeutic agents results in sensitivity to ATR inhibition. Our findings provide a mechanistic rationale for the inclusion of ATR and PARP inhibitors as a potential treatment strategy for high-risk NB.

Synthetic Heterocyclic Derivatives as Kinase Inhibitors Tested for the Treatment of Neuroblastoma

In the last few years, small molecules endowed with different heterocyclic scaffolds have been developed as kinase inhibitors. Some of them are being tested at preclinical or clinical levels for the potential treatment of neuroblastoma (NB). This disease is the most common extracranial solid tumor in childhood and is responsible for 10% to 15% of pediatric cancer deaths. Despite the availability of some treatments, including the use of very toxic cytotoxic chemotherapeutic agents, high-risk (HR)-NB patients still have a poor prognosis and a survival rate below 50%. For these reasons, new pharmacological options are urgently needed. This review focuses on synthetic heterocyclic compounds published in the last five years, which showed at least some activity on this severe disease and act as kinase inhibitors. The specific mechanism of action, selectivity, and biological activity of these drug candidates are described, when established. Moreover, the most remarkable clinical trials are reported. Importantly, kinase inhibitors approved for other diseases have shown to be active and endowed with lower toxicity compared to conventional cytotoxic agents. The data collected in this article can be particularly useful for the researchers working in this area.

FGFR2 loss sensitizes MYCN-amplified neuroblastoma CHP134 cells to CHK1 inhibitor-induced apoptosis

Checkpoint kinase 1 (CHK1) plays a key role in genome surveillance and integrity throughout the cell cycle. Selective inhibitors of CHK1 (CHK1i) are undergoing clinical evaluation for various human malignancies, including neuroblastoma. In this study, one CHK1i‐sensitive neuroblastoma cell line, CHP134, was investigated, which characteristically carries MYCN amplification and a chromosome deletion within the 10q region. Among several cancer‐related genes in the chromosome 10q region, mRNA expression of fibroblast growth factor receptor 2 (FGFR2) was altered in CHP134 cells and associated with an unfavorable prognosis of patients with neuroblastoma. Induced expression of FGFR2 in CHP134 cells reactivated downstream MEK/ERK signaling and resulted in cells resistant to CHK1i‐mediated cell growth inhibition. Consistently, the MEK1/2 inhibitor, trametinib, potentiated CHK1 inhibitor–mediated cell death in these cells. These results suggested that FGFR2 loss might be prone to highly effective CHK1i treatment. In conclusion, extreme cellular dependency of ERK activation may imply a possible application for the MEK1/2 inhibitor, either as a single inhibitor or in combination with CHK1i in MYCN‐amplified neuroblastomas.

A combination of PARP and CHK1 inhibitors efficiently antagonizes MYCN-driven tumors

MYCN drives aggressive behavior and refractoriness to chemotherapy, in several tumors. Since MYCN inactivation in clinical settings is not achievable, alternative vulnerabilities of MYCN-driven tumors need to be explored to identify more effective and less toxic therapies. We previously demonstrated that PARP inhibitors enhance MYCN-induced replication stress and promote mitotic catastrophe, counteracted by CHK1. Here, we showed that PARP and CHK1 inhibitors synergized to induce death in neuroblastoma cells and in primary cultures of SHH-dependent medulloblastoma, their combination being more effective in MYCN amplified and MYCN overexpressing cells compared to MYCN non-amplified cells. Although the MYCN amplified IMR-32 cell line carrying the p.Val2716Ala ATM mutation showed the highest sensitivity to the drug combination, this was not related to ATM status, as indicated by CRISPR/Cas9-based correction of the mutation. Suboptimal doses of the CHK1 inhibitor MK-8776 plus the PARP inhibitor olaparib led to a MYCN-dependent accumulation of DNA damage and cell death in vitro and significantly reduced the growth of four in vivo models of MYCN-driven tumors, without major toxicities. Our data highlight the combination of PARP and CHK1 inhibitors as a new potential chemo-free strategy to treat MYCN-driven tumors, which might be promptly translated into clinical trials.

A phase 1 study of prexasertib (LY2606368), a CHK1/2 inhibitor, in pediatric patients with recurrent or refractory solid tumors, including CNS tumors: A report from the Children's Oncology Group Pediatric Early Phase Clinical Trials Network (ADVL1515)

BACKGROUND

Prexasertib (LY2606368) is a novel, second-generation, selective dual inhibitor of checkpoint kinase proteins 1 (CHK1) and 2 (CHK2). We conducted a phase 1 trial of prexasertib to estimate the maximum-tolerated dose (MTD) and/or recommended phase 2 dose (RP2D), to define and describe the toxicities, and to characterize the pharmacokinetics (PK) of prexasertib in pediatric patients with recurrent or refractory solid and central nervous system (CNS) tumors.

METHODS

Prexasertib was administered intravenously (i.v.) on days 1 and 15 of a 28-day cycle. Four dose levels, 80, 100, 125, and 150 mg/m² , were evaluated using a rolling-six design. PK analysis was performed during cycle 1. Tumor tissue was examined for biomarkers (CHK1 and TP53) of prexasertib activity.

RESULTS

Thirty patients were enrolled; 25 were evaluable. The median age was 9.5 years (range: 2-20) and 21 (70%) were male. Twelve patients (40%) had solid tumors and 18 patients (60%) had CNS tumors. There were no cycle 1 or later dose-limiting toxicities. Common cycle 1, drug-related grade 3/4 toxicities (> 10% of patients) included neutropenia (100%), leukopenia (68%), thrombocytopenia (24%), lymphopenia (24%), and anemia (12%). There were no objective responses; best overall response was stable disease in three patients for five cycles (hepatocellular carcinoma), three cycles (ependymoma), and five cycles (undifferentiated sarcoma). The PK appeared dose proportional across the 80-150 mg/m² dose range.

CONCLUSIONS

Although the MTD of prexasertib was not defined by this study, 150 mg/m² administered i.v. on days 1 and 15 of a 28-day cycle was determined to be the RP2D.

Small-molecule screen reveals synergy of cell cycle checkpoint kinase inhibitors with DNA-damaging chemotherapies in medulloblastoma

Medulloblastoma (MB) consists of four core molecular subgroups with distinct clinical features and prognoses. Treatment consists of surgery, followed by radiotherapy and cytotoxic chemotherapy. Despite this intensive approach, outcome remains dismal for patients with certain subtypes of MB, namely, MYC-amplified Group 3 and TP53-mutated SHH. Using high-throughput assays, six human MB cell lines were screened against a library of 3208 unique compounds. We identified 45 effective compounds from the screen and found that cell cycle checkpoint kinase (CHK1/2) inhibition synergistically enhanced the cytotoxic activity of clinically used chemotherapeutics cyclophosphamide, cisplatin, and gemcitabine. To identify the best-in-class inhibitor, multiple CHK1/2 inhibitors were assessed in mice bearing intracranial MB. When combined with DNA-damaging chemotherapeutics, CHK1/2 inhibition reduced tumor burden and increased survival of animals with high-risk MB, across multiple different models. In total, we tested 14 different models, representing distinct MB subgroups, and data were validated in three independent laboratories. Pharmacodynamics studies confirmed central nervous system penetration. In mice, combination treatment significantly increased DNA damage and apoptosis compared to chemotherapy alone, and studies with cultured cells showed that CHK inhibition disrupted chemotherapy-induced cell cycle arrest. Our findings indicated CHK1/2 inhibition, specifically with LY2606368 (prexasertib), has strong chemosensitizing activity in MB that warrants further clinical investigation. Moreover, these data demonstrated that we developed a robust and collaborative preclinical assessment platform that can be used to identify potentially effective new therapies for clinical evaluation for pediatric MB.

Therapeutic targeting of KSP in preclinical models of high-risk neuroblastoma

Neuroblastoma is a childhood malignancy with often dismal prognosis; relapse is common despite intense treatment. Here, we used human tumor organoids representing multiple MYCN-amplified high-risk neuroblastomas to perform a high-throughput drug screen with approved or emerging oncology drugs. Tumor-selective effects were calculated using drug sensitivity scores. Several drugs with previously unreported anti-neuroblastoma effects were identified by stringent selection criteria. ARRY-520, an inhibitor of kinesin spindle protein (KSP), was among those causing reduced viability. High expression of the KSP-encoding gene KIF11 was associated with poor outcome in neuroblastoma. Genome-scale loss-of-function screens in hundreds of human cancer cell lines across 22 tumor types revealed that KIF11 is particularly important for neuroblastoma cell viability. KSP inhibition in neuroblastoma patient-derived xenograft (PDX) cells resulted in the formation of abnormal monoastral spindles, mitotic arrest, up-regulation of mitosis-associated genes, and apoptosis. In vivo, KSP inhibition caused regression of MYCN-amplified neuroblastoma PDX tumors. Furthermore, treatment of mice harboring orthotopic neuroblastoma PDX tumors resulted in increased survival. Our results suggested that KSP inhibition could be a promising treatment strategy in children with high-risk neuroblastoma.

Acceleration or Brakes: Which Is Rational for Cell Cycle-Targeting Neuroblastoma Therapy?

Unrestrained proliferation is a common feature of malignant neoplasms. Targeting the cell cycle is a therapeutic strategy to prevent unlimited cell division. Recently developed rationales for these selective inhibitors can be subdivided into two categories with antithetical functionality. One applies a “brake” to the cell cycle to halt cell proliferation, such as with inhibitors of cell cycle kinases. The other “accelerates” the cell cycle to initiate replication/mitotic catastrophe, such as with inhibitors of cell cycle checkpoint kinases. The fate of cell cycle progression or arrest is tightly regulated by the presence of tolerable or excessive DNA damage, respectively. This suggests that there is compatibility between inhibitors of DNA repair kinases, such as PARP inhibitors, and inhibitors of cell cycle checkpoint kinases. In the present review, we explore alterations to the cell cycle that are concomitant with altered DNA damage repair machinery in unfavorable neuroblastomas, with respect to their unique genomic and molecular features. We highlight the vulnerabilities of these alterations that are attributable to the features of each. Based on the assessment, we offer possible therapeutic approaches for personalized medicine, which are seemingly antithetical, but both are promising strategies for targeting the altered cell cycle in unfavorable neuroblastomas.

Comparative Activity and Off-Target Effects in Cells of the CHK1 Inhibitors MK-8776, SRA737, and LY2606368

DNA damage activates the checkpoint protein CHK1 to arrest cell cycle progression, providing time for repair and recovery. Consequently, inhibitors of CHK1 (CHK1i) enhance damage-induced cell death. Additionally, CHK1i elicits single agent cytotoxicity in some cell lines. We compared three CHK1i that have undergone clinical trials and exhibited different toxicities. Each CHK1i inhibits other targets at higher concentrations, and whether these contribute to the toxicity is unknown. We compared their sensitivity in a panel of cell lines, their efficacy at inhibiting CHK1 and CHK2, and their ability to induce DNA damage and abrogate damage-induced S phase arrest. Published in vitro kinase analyses were a poor predictor of selectivity and potency in cells. LY2606368 was far more potent at inhibiting CHK1 and inducing growth arrest, while all three CHK1i inhibited CHK2 at concentrations 10- (MK-8776 and SRA737) to 100- (LY2606368) fold higher. MK-8776 and SRA737 exhibited similar off-target effects: higher concentrations demonstrated transient protection from growth inhibition, circumvented DNA damage, and prevented checkpoint abrogation, possibly due to inhibition of CDK2. Acquired resistance to LY2606368 resulted in limited cross-resistance to other CHK1i. LY2606368-resistant cells still abrogated DNA damage-induced S phase arrest, which requires low CDK2 activity, whereas inappropriately high CDK2 activity is responsible for sensitivity to CHK1i alone. All three CHK1i inhibited protein synthesis in a sensitive cell line correlating with cell death, whereas resistant cells failed to inhibit protein synthesis and underwent transient cytostasis. LY2606368 appears to be the most selective CHK1i, suggesting that further clinical development of this drug is warranted.

Effects of checkpoint kinase 1 inhibition by prexasertib on the tumor immune microenvironment of head and neck squamous cell carcinoma

Prognosis for patients with recurrent and/or metastatic head and neck squamous cell carcinoma (HNSCC) remains poor. Development of more effective and less toxic targeted therapies is necessary for HNSCC patients. Checkpoint kinase 1 (CHK1) plays a vital role in cell cycle regulation and is a promising therapeutic target in HNSCC. Prexasertib, a CHK1 inhibitor, induces DNA damage and cell death, however, its effect on the tumor immune microenvironment (TIME) is largely unknown. Therefore, we evaluated a short-term and long-term effects of prexasertib in HNSCC and its TIME. Prexasertib caused increased DNA damage and cell death in vitro and significant tumor regression and improved survival in vivo. The gene expression and multiplex immunohistochemistry (mIHC) analyses of the in vivo tumors demonstrated increased expression of genes that are related to T-cell activation and increased immune cell trafficking, and decreased expression of genes that related to immunosuppression. However, increased expression of genes related to immunosuppression emerged over time suggesting evasion of immune surveillances. These findings in gene expression analyses were confirmed using mIHC which showed differential modulation of TIME in the tumor margins and as well as cores over time. These results suggest that evasion of immune surveillance, at least in part, may contribute to the acquired resistance to prexasertib in HNSCC.

CDK2-Mediated Upregulation of TNFα as a Mechanism of Selective Cytotoxicity in Acute Leukemia

Although inhibitors of the kinases CHK1, ATR, and WEE1 are undergoing clinical testing, it remains unclear how these three classes of agents kill susceptible cells and whether they utilize the same cytotoxic mechanism. Here we observed that CHK1 inhibition induces apoptosis in a subset of acute leukemia cell lines in vitro, including TP53-null acute myeloid leukemia (AML) and BCR/ABL-positive acute lymphoid leukemia (ALL), and inhibits leukemic colony formation in clinical AML samples ex vivo. In further studies, downregulation or inhibition of CHK1 triggered signaling in sensitive human acute leukemia cell lines that involved CDK2 activation followed by AP1-dependent TNF transactivation, TNFα production, and engagement of a TNFR1- and BID-dependent apoptotic pathway. AML lines that were intrinsically resistant to CHK1 inhibition exhibited high CHK1 expression and were sensitized by CHK1 downregulation. Signaling through this same CDK2-AP1-TNF cytotoxic pathway was also initiated by ATR or WEE1 inhibitors in vitro and during CHK1 inhibitor treatment of AML xenografts in vivo. Collectively, these observations not only identify new contributors to the antileukemic cell action of CHK1, ATR, and WEE1 inhibitors, but also delineate a previously undescribed pathway leading from aberrant CDK2 activation to death ligand-induced killing that can potentially be exploited for acute leukemia treatment. SIGNIFICANCE: This study demonstrates that replication checkpoint inhibitors can kill AML cells through a pathway involving AP1-mediated TNF gene activation and subsequent TP53-independent, TNFα-induced apoptosis, which can potentially be exploited clinically.

The translational repressor 4E-BP1 regulates RRM2 levels and functions as a tumor suppressor in Ewing sarcoma tumors

Ribonucleotide reductase (RNR), which is a heterodimeric tetramer composed of RRM1 and RRM2 subunits, is the rate-limiting enzyme in the synthesis of deoxyribonucleoside triphosphates (dNTPs) and essential for both DNA replication and the repair of DNA damage. The activity of RNR is coordinated with the cell cycle and regulated by fluctuations in the level of the RRM2 subunit. Multiple cancer types, including Ewing sarcoma tumors, are sensitive to inhibitors of RNR or a reduction in the levels of either the RRM1 or RRM2 subunits of RNR. Here, we show that the expression of the RRM2 protein is dependent on active protein synthesis and that 4E-BP1, a repressor of cap-dependent protein translation, specifically regulates the level of the RRM2 protein. Furthermore, inhibition of mTORC1/2, but not mTORC1, activates 4E-BP1, inhibits protein synthesis, and reduces the level of the RRM2 protein in multiple sarcoma cell lines. This effect of mTORC1/2 inhibitors on protein synthesis and RRM2 levels was rescued in cell lines with the CRISPR/Cas9-mediated knockout of 4E-BP1. In addition, the inducible expression of a mutant 4E-BP1 protein that cannot be phosphorylated by mTOR blocked protein synthesis and inhibited the growth of Ewing sarcoma cells in vitro and in vivo in a xenograft. Overall, these results provide insight into the multifaceted regulation of RRM2 protein levels and identify a regulatory link between protein translation and DNA replication.

Bcl-2/Bcl-xL inhibitor navitoclax increases the antitumor effect of Chk1 inhibitor prexasertib by inducing apoptosis in pancreatic cancer cells via inhibition of Bcl-xL but not Bcl-2

In our previous study, we showed that prexasertib, a checkpoint kinase 1 (Chk1) inhibitor, enhances the effects of standard drugs for pancreatic cancer, including gemcitabine (GEM), S-1, and the combination of GEM and S-1 (GS). The combination of prexasertib and GS has a strong antitumor effect and induces apoptosis in pancreatic cancer cells by downregulating anti-apoptotic protein Bcl-2. In the present study, we investigated the combined effect of GEM, S-1, and prexasertib with a selective Bcl-2 inhibitor (venetoclax) and a non-selective Bcl-2 inhibitor (navitoclax) in SUIT-2 pancreatic cancer cells. An MTT assay revealed that the combination of prexasertib with navitoclax showed a synergistic effect but the combination with venetoclax did not. Investigation of the pancreatic cancer cell lines SUIT-2, MIA PaCa-2, and BxPC-3 revealed that BxPC-3 also showed a high synergistic effect when combined with prexasertib and navitoclax but not venetoclax. Mechanistic analysis of the combined effect showed that apoptosis was induced. Bcl-2 knockdown with siRNA and prexasertib treatment did not induce apoptosis, whereas Bcl-xL knockdown with siRNA and prexasertib treatment resulted in strong induction of apoptosis. In addition, among the three cell lines, the combined effect of prexasertib and navitoclax resulted in increased apoptotic cell death because the protein expression levels of Bcl-xL and Chk1 were higher. Our results demonstrate that the combination of prexasertib and navitoclax has a strong antitumor effect and induces apoptosis in pancreatic cancer cells by downregulating Bcl-xL. Simultaneous inhibition of Chk1 and Bcl-xL could be a new strategy for treating pancreatic cancer.

Prexasertib (LY2606368) reduces clonogenic survival by inducing apoptosis in primary patient-derived osteosarcoma cells and synergizes with cisplatin and talazoparib

Progress in the systemic control of osteosarcoma has been limited over the past decades thus indicating the urgent clinical need for the development of novel treatment strategies. Therefore, we have recently developed new preclinical models to study promising novel agents for the treatment of pediatric osteosarcoma. The checkpoint kinase (chk) inhibitor prexasertib (LY2606368) and its salt form (LSN2940930) have recently been shown to be active in adult and pediatric malignancies, including sarcoma. We have now tested the potency of prexasertib in clonogenic survival assays in two new lines of primary patient-derived osteosarcoma cells and in two established osteosarcoma cell lines as a single agent and in combination with cisplatin and the poly ADP-ribose polymerase (PARP) inhibitor talazoparib. Prexasertib alone results in strongly reduced clonogenic survival at low nanomolar concentrations and acts by affecting cell cycle progression, induction of apoptosis and induction of double-stranded DNA breakage at concentrations that are well below clinically tolerable and safe plasma concentrations. In combination with cisplatin and talazoparib, prexasertib acts in a synergistic fashion. Chk1 inhibition by prexasertib and its combination with the DNA damaging agent cisplatin and the PARP-inhibitor talazoparib thus emerges as a potential new treatment option for pediatric osteosarcoma which will now have to be tested in preclinical primary patient derived in vivo models and clinical studies.

Clinical outcomes of prexasertib monotherapy in recurrent BRCA wild-type high-grade serous ovarian cancer involve innate and adaptive immune responses

BACKGROUND

Preclinical data suggest cell cycle checkpoint blockade may induce an immunostimulatory tumor microenvironment. However, it remains elusive whether immunomodulation occurs in the clinical setting. To test this, we used blood and fresh tissue samples collected at baseline and post therapy from a phase II trial of the cell cycle checkpoint 1 inhibitor (CHK1i) prexasertib in recurrent ovarian cancer.

METHODS

Paired blood samples and fresh core biopsies, taken before treatment was started at baseline (cycle 1 day 1 (C1D1)) and post second dose on day 15 of cycle 1 (C1D15), were collected. To evaluate changes in the immune responses after treatment, multiparametric flow cytometry for DNA damage markers and immune cell subsets was performed on paired blood samples. RNA sequencing (RNAseq) of paired core biopsies was also analyzed. Archival tissue immune microenvironment was evaluated with immunohistochemistry. All correlative study statistical analyses used two-sided significance with a cut-off of p=0.05.

RESULTS

Flow cytometric analysis showed significantly increased γ-H2AX staining after CHK1i treatment, accompanied by increased monocyte populations, suggestive of an activated innate immune response (median 31.6% vs 45.6%, p=0.005). Increased expressions of immunocompetence marker HLA-DR (Human Leukocyte Antigen DR antigen) on monocytes and of TBK1, a marker of STING (stimulator of interferon genes) pathway activation, in biopsies were associated with improved progression-free survival (PFS) (9.25 vs 3.5 months, p=0.019; 9 vs 3 months, p=0.003, respectively). Computational analysis of RNAseq data indicated increased infiltration of tumor niches by naïve B-cells and resting memory T-cells, suggestive of a possibly activated adaptive immune response, and greater T-reg infiltration after treatment correlated with worse PFS (9.25 vs 3.5 months, p=0.007). An immunosuppressive adaptive immune response, perhaps compensatory, was also observed on flow cytometry, including lymphodepletion of total peripheral CD4+ and CD8+T cells after CHK1i and an increase in the proportion of T-regs among these T-cells. Additionally, there was a trend of improved PFS with greater tumor-infiltrating lymphocytes (TILs) in archival tissues (13.7 months >30% TILs vs 5.5 months ≤30% TILs, p=0.05).

CONCLUSION

Our study demonstrates that a favorable clinical response in high-grade serous ovarian carcinoma patients treated with CHK1i is possibly associated with enhanced innate and adaptive immunity, requiring further mechanistic studies. It is supportive of current efforts for a clinical development strategy for therapeutic combinations with immunotherapy in ovarian cancer.

DNA Repair and Ovarian Carcinogenesis: Impact on Risk, Prognosis and Therapy Outcome

There is ample evidence for the essential involvement of DNA repair and DNA damage response in the onset of solid malignancies, including ovarian cancer. Indeed, high-penetrance germline mutations in DNA repair genes are important players in familial cancers: BRCA1, BRCA2 mutations or mismatch repair, and polymerase deficiency in colorectal, breast, and ovarian cancers. Recently, some molecular hallmarks (e.g., TP53, KRAS, BRAF, RAD51C/D or PTEN mutations) of ovarian carcinomas were identified. The manuscript overviews the role of DNA repair machinery in ovarian cancer, its risk, prognosis, and therapy outcome. We have attempted to expose molecular hallmarks of ovarian cancer with a focus on DNA repair system and scrutinized genetic, epigenetic, functional, and protein alterations in individual DNA repair pathways (homologous recombination, non-homologous end-joining, DNA mismatch repair, base- and nucleotide-excision repair, and direct repair). We suggest that lack of knowledge particularly in non-homologous end joining repair pathway and the interplay between DNA repair pathways needs to be confronted. The most important genes of the DNA repair system are emphasized and their targeting in ovarian cancer will deserve further attention. The function of those genes, as well as the functional status of the entire DNA repair pathways, should be investigated in detail in the near future.

DNA damage checkpoint kinases in cancer

DNA damage response (DDR) pathway prevents high level endogenous and environmental DNA damage being replicated and passed on to the next generation of cells via an orchestrated and integrated network of cell cycle checkpoint signalling and DNA repair pathways. Depending on the type of damage, and where in the cell cycle it occurs different pathways are involved, with the ATM-CHK2-p53 pathway controlling the G1 checkpoint or ATR-CHK1-Wee1 pathway controlling the S and G2/M checkpoints. Loss of G1 checkpoint control is common in cancer through TP53, ATM mutations, Rb loss or cyclin E overexpression, providing a stronger rationale for targeting the S/G2 checkpoints. This review will focus on the ATM-CHK2-p53-p21 pathway and the ATR-CHK1-WEE1 pathway and ongoing efforts to target these pathways for patient benefit.

Limited antitumor activity of combined BET and MEK inhibition in neuroblastoma

BACKGROUND

The treatment of high-risk neuroblastoma continues to present a formidable challenge to pediatric oncology. Previous studies have shown that Bromodomain and extraterminal (BET) inhibitors can inhibit MYCN expression and suppress MYCN-amplified neuroblastoma in vivo. Furthermore, alterations within RAS-MAPK (mitogen-activated protein kinase) signaling play significant roles in neuroblastoma initiation, maintenance, and relapse, and mitogen-activated extracellular signal-regulated kinase (MEK) inhibitors demonstrate efficacy in subsets of neuroblastoma preclinical models. Finally, hyperactivation of RAS-MAPK signaling has been shown to promote resistance to BET inhibitors. Therefore, we examined the antitumor efficacy of combined BET/MEK inhibition utilizing I-BET726 or I-BET762 and trametinib in high-risk neuroblastoma.

PROCEDURE

Utilizing a panel of genomically annotated neuroblastoma cell line models, we investigated the in vitro effects of combined BET/MEK inhibition on cell proliferation and apoptosis. Furthermore, we evaluated the effects of combined inhibition in neuroblastoma xenograft models.

RESULTS

Combined BET and MEK inhibition demonstrated synergistic effects on the growth and survival of a large panel of neuroblastoma cell lines through augmentation of apoptosis. A combination therapy slowed tumor growth in a non-MYCN-amplified, NRAS-mutated neuroblastoma xenograft model, but had no efficacy in an MYCN-amplified model harboring a loss-of-function mutation in NF1.

CONCLUSIONS

Combinatorial BET and MEK inhibition was synergistic in the vast majority of neuroblastoma cell lines in the in vitro setting but showed limited antitumor activity in vivo. Collectively, these data do not support clinical development of this combination in high-risk neuroblastoma.

Identification of key candidate genes and pathways in anaplastic thyroid cancer by bioinformatics analysis

BACKGROUND

Anaplastic thyroid carcinoma (ATC) is a refractory and poor prognosis tumor Present study aimed to investigate the underlying biological functions and pathways involved in the development of ATC and to identify potential hub genes and candidate biomarkers of ATC.

MATERIALS AND METHODS

Bioinformatics analyses were performed to identify the differentially expressed genes (DEGs) between ATC tissue samples and adjacent normal tissue samples. Protein-protein interaction (PPI) networks of the DEGs were constructed using Search Tool for the Retrieval of Interacting Genes online tool and Cytoscape software and divided into sub-networks using the Molecular Complex Detection (MCODE) plug-in. DEGs in each module was analyzed by enrichment analysis of the KEGG Orthology Based Annotation System (KOBAS) web software version 3.0. Eventually, the hub genes from bioinformatics analysis were verified by qRT-PCR assay in different ATC cell lines.

RESULTS

Thirty hub genes were selected and three modules were built by the Cytoscape software from the PPI network. Seven genes (CDK1, CCNB2, BUB1B, CDC20, RRM2, CHEK1 and CDC45) were screened from thirty hub genes. Enrichment analysis showed that these hub genes were primarily accumulated in 'cell cycle', 'p53 signaling pathway', 'viral carcinogenesis', 'pyrimidine metabolism' and 'ubiquitin mediated proteolysis'. The results of qRT-PCR indicated that seven hub genes were unregulated in three ATC cell lines compared with normal thyroid gland cell.

CONCLUSIONS

These findings suggest that CDK1, CCNB2, BUB1B, CDC20, RRM2, CHEK1 and CDC45 may serve as novel diagnosis biomarkers and potential therapeutic target for ATC.

Targeting the DNA Damage Response for the Treatment of High Risk Neuroblastoma

Despite intensive multimodal therapy, the survival rate for high risk neuroblastoma (HR-NB) remains <50%. Most cases initially respond to treatment but almost half will subsequently relapse with aggressive treatment resistant disease. Novel treatments exploiting the molecular pathology of NB and/or overcoming resistance to current genotoxic therapies are needed before survival rates can significantly improve. DNA damage response (DDR) defects are frequently observed in HR-NB including allelic deletion and loss of function mutations in key DDR genes, oncogene induced replication stress and cell cycle checkpoint dysfunction. Exploiting defects in the DDR has been a successful treatment strategy in some adult cancers. Here we review the genetic features of HR-NB which lead to DDR defects and the emerging molecular targeting agents to exploit them.

Novel therapy for pediatric and adolescent kidney cancer

Pediatric and adolescent renal tumors account for approximately 7% of all new cancer diagnoses in the USA each year. The prognosis and treatment are varied based on factors including the underlying histology and tumor stage, with survival rates ranging from greater than 90% in favorable histology Wilms tumor to almost universally fatal in other disease types, including those patients with advanced stage malignant rhabdoid tumor and renal medullary carcinoma. In recent years, our understanding of the underlying genetic drivers of the different types of pediatric kidney cancer has dramatically increased, opening the door to utilization of new targeted biologic agents alone or in combination with conventional chemotherapy to improve outcomes. Several ongoing clinical trials are investigating the use of a variety of targeted agents in pediatric patients with underlying genetic aberrations. In this manuscript, the underlying biology and early phase clinical trials relevant to pediatric renal cancers are reviewed.

A pan-cancer transcriptome analysis identifies replication fork and innate immunity genes as modifiers of response to the CHK1 inhibitor prexasertib

The combined influence of oncogenic drivers, genomic instability, and/or DNA damage repair deficiencies increases replication stress in cancer. Cells with high replication stress rely on the upregulation of checkpoints like those governed by CHK1 for survival. Previous studies of the CHK1 inhibitor prexasertib demonstrated activity across multiple cancer types. Therefore, we sought to (1) identify markers of prexasertib sensitivity and (2) define the molecular mechanism(s) of intrinsic and acquired resistance using preclinical models representing multiple tumor types. Our findings indicate that while cyclin E dysregulation is a driving mechanism of prexasertib response, biomarkers associated with this aberration lack sufficient predictive power to render them clinically actionable for patient selection. Transcriptome analysis of a pan-cancer cell line panel and in vivo models revealed an association between expression of E2F target genes and prexasertib sensitivity and identified innate immunity genes associated with prexasertib resistance. Functional RNAi studies supported a causal role of replication fork components as modulators of prexasertib response. Mechanisms that protect cells from oncogene-induced replication stress may safeguard tumors from such stress induced by a CHK1 inhibitor, resulting in acquired drug resistance. Furthermore, resistance to prexasertib may be shaped by innate immunity.

Inhibition of the ATR-CHK1 Pathway in Ewing Sarcoma Cells Causes DNA Damage and Apoptosis via the CDK2-Mediated Degradation of RRM2

Inhibition of ribonucleotide reductase (RNR), the rate-limiting enzyme in the synthesis of deoxyribonucleotides, causes DNA replication stress and activates the ataxia telangiectasia and rad3-related protein (ATR)-checkpoint kinase 1 (CHK1) pathway. Notably, a number of different cancers, including Ewing sarcoma tumors, are sensitive to the combination of RNR and ATR-CHK1 inhibitors. However, multiple, overlapping mechanisms are reported to underlie the toxicity of ATR-CHK1 inhibitors, both as single agents and in combination with RNR inhibitors, toward cancer cells. Here, we identified a feedback loop in Ewing sarcoma cells in which inhibition of the ATR-CHK1 pathway depletes RRM2, the small subunit of RNR, and exacerbates the DNA replication stress and DNA damage caused by RNR inhibitors. Mechanistically, we identified that the inhibition of ATR-CHK1 activates CDK2, which targets RRM2 for degradation via the proteasome. Similarly, activation of CDK2 by inhibition or knockdown of the WEE1 kinase also depletes RRM2 and causes DNA damage and apoptosis. Moreover, we show that the concurrent inhibition of ATR and WEE1 has a synergistic effect in Ewing sarcoma cells. Overall, our results provide novel insight into the response to DNA replication stress, as well as a rationale for targeting the ATR, CHK1, and WEE1 pathways, in Ewing sarcoma tumors. IMPLICATIONS: Targeting the ATR, CHK1, and WEE1 kinases in Ewing sarcoma cells activates CDK2 and increases DNA replication stress by promoting the proteasome-mediated degradation of RRM2.

CNS penetration and pharmacodynamics of the CHK1 inhibitor prexasertib in a mouse Group 3 medulloblastoma model

Prexasertib (LY2606368) is a potent and selective small molecule inhibitor of cell-cycle checkpoint CHK1 and CHK2 protein kinases and is currently under clinical evaluation for treatment of pediatric malignancies. As a candidate therapy for pediatric Group 3 medulloblastoma (G3MB), prexasertib CNS penetration was evaluated in mice using cerebral microdialysis and pharmacokinetic modeling. A plasma pharmacokinetic study with a population-based design was performed in CD1 nude mice bearing G3MB orthotopically implanted in the brain and receiving a single dose of prexasertib (10 mg/kg, subcutaneously) to characterize prexasertib disposition and to establish a limited plasma sampling model for the microdialysis studies. The microdialysis studies were performed in both non-tumor bearing mice and in mice bearing G3MB receiving 10 mg/kg prexasertib subcutaneously, for up to 24 h post-dose. Plasma and extracellular fluid (ECF) concentrations were quantified using validated LC MS/MS methods, and analyzed using a population pharmacokinetic model. Model-derived prexasertib tumor/ECF to plasma partition coefficient K_p,uu (ratio of tumor/brain ECF to unbound plasma AUC_0-24 h) was significantly greater in G3MB tumor-bearing mice (0.17 ± 0.08) compared to non-tumor bearing mice (0.09 ± 0.04, p = 0.04). A pharmacodynamic study was then performed in mice bearing G3MB (20 mg/kg, IV) to evaluate prexasertib-induced target engagement after a single dose. Phosphorylated CHK1 serine 345 (pCHK1 S345), phosphorylated Histone 2A variant (γ-H2AX), and cleaved caspase-3 were quantified in mouse G3MB tumor tissues by immunohistochemistry at different time points up to 24 h post-dose. The induction of pCHK1 S345 and γ-H2AX peaked at 2 h after the dose and was elevated above baseline for at least 6 h, reflecting relevant CHK1 inhibition and DNA damage. Cleaved caspase-3 levels increased at 24 h suggesting initiation of cell apoptosis. Adequate unbound prexasertib exposure reached the brain tumor site relative to target engagement in G3MB tumor bearing mice at a clinically relevant dosage. These results support further preclinical and clinical development of prexasertib to treat children with medulloblastoma.

Challenges and Opportunities for Childhood Cancer Drug Development

Cancer in children is rare with approximately 15,700 new cases diagnosed in the United States annually. Through use of multimodality therapy (surgery, radiation therapy, and aggressive chemotherapy), 70% of patients will be "cured" of their disease, and 5-year event-free survival exceeds 80%. However, for patients surviving their malignancy, therapy-related long-term adverse effects are severe, with an estimated 50% having chronic life-threatening toxicities related to therapy in their fourth or fifth decade of life. While overall intensive therapy with cytotoxic agents continues to reduce cancer-related mortality, new understanding of the molecular etiology of many childhood cancers offers an opportunity to redirect efforts to develop effective, less genotoxic therapeutic options, including agents that target oncogenic drivers directly, and the potential for use of agents that target the tumor microenvironment and immune-directed therapies. However, for many high-risk cancers, significant challenges remain.

Anti-VEGFR2 therapy delays growth of preclinical pediatric tumor models and enhances anti-tumor activity of chemotherapy

Vascular endothelial growth factor receptor 2 (VEGFR2) is an attractive therapeutic target in solid malignancies due to its central role in tumor angiogenesis. Ramucirumab (Cyramza^®, LY3009806) is a human monoclonal antibody specific for VEGFR2 approved for several adult indications and currently in a phase 1 clinical trial for pediatric patients with solid tumors (NCT02564198). Here, we evaluated ramucirumab in vitro and the anti-murine VEGFR2 antibody DC101 in vivo with or without chemotherapy across a range of pediatric cancer models. Ramucirumab abrogated in vitro endothelial cord formation driven by cancer cell lines representing multiple pediatric histologies; this response was independent of the origin of the tumor cell-line. Several pediatric cancer mouse models responded to single agent DC101-mediated VEGFR2 inhibition with tumor growth delay. Preclinical stable disease and partial xenograft regressions were observed in mouse models of Ewing’s sarcoma, synovial sarcoma, neuroblastoma, and desmoplastic small round cell tumor treated with DC101 and cytotoxic chemotherapy. In contrast, DC101 treatment in osteosarcoma models had limited efficacy alone or in combination with chemotherapeutics. Our data indicate differential efficacy of targeting the VEGFR2 pathway in pediatric models and support the continued evaluation of VEGFR2 inhibition in combination with cytotoxic chemotherapy in multiple pediatric indications.

Prexasertib, a checkpoint kinase inhibitor: from preclinical data to clinical development

Checkpoint kinases 1 and 2 (CHK1 and CHK2) are important multifunctional proteins of the kinase family. Their main function is to regulate DNA replication and DNA damage response. If a cell is exposed to exogenous damage to its DNA, CHK1/CHK2 stops the cell cycle to give time to the cellular mechanisms to repair DNA breakage and apoptosis too, if the damage is not repairable to activate programmed cell death. CHK1/CHK2 plays a crucial role in the repair of recombination-mediated double-stranded DNA breaks. The other important functions performed by these proteins are the beginning of DNA replication, the stabilization of replication forks, the resolution of replication stress and the coordination of mitosis, even in the absence of exogenous DNA damage. Prexasertib (LY2606368) is a small ATP-competitive selective inhibitor of CHK1 and CHK2. In preclinical studies, prexasertib in monotherapy has shown to induce DNA damage and tumor cells apoptosis. The preclinical data and early clinical studies advocate the use of prexasertib in solid tumors both in monotherapy and in combination with other drugs (antimetabolites, PARP inhibitors and platinum-based chemotherapy). The safety and the efficacy of combination therapies with prexasertib need to be better evaluated in ongoing clinical trials.

Everything in Moderation: Lessons Learned by Exploiting Moderate Replication Stress in Cancer

The poor selectivity of standard cytotoxic chemotherapy regimens causes severe side-effects in patients and reduces the quality of life during treatment. Targeting cancer-specific vulnerabilities can improve response rates, increase overall survival and limit toxic side effects in patients. Oncogene-induced replication stress serves as a tumour specific vulnerability and rationale for the clinical development of inhibitors targeting the DNA damage response (DDR) kinases (CHK1, ATR, ATM and WEE1). CHK1 inhibitors (CHK1i) have served as the pilot compounds in this class and their efficacy in clinical trials as single agents has been disappointing. Initial attempts to combine CHK1i with chemotherapies agents that enhance replication stress (such as gemcitabine) were reported to be excessively toxic. More recently, it has emerged that combining CHK1i with subclinical doses of replication stress inducers is more effective, better tolerated and more compatible with immunotherapies. Here we focus on the lessons learned during the clinical development of CHK1i with the goal of improving the design of future clinical trials utilizing DDR inhibitors to target replication stress in cancer.

Novel CHK1 inhibitor MU380 exhibits significant single-agent activity in TP53-mutated chronic lymphocytic leukemia cells

Introduction of small-molecule inhibitors of B-cell receptor signaling and BCL2 protein significantly improves therapeutic options in chronic lymphocytic leukemia. However, some patients suffer from adverse effects mandating treatment discontinuation, and cases with TP53 defects more frequently experience early progression of the disease. Development of alternative therapeutic approaches is, therefore, of critical importance. Here we report details of the anti-chronic lymphocytic leukemia single-agent activity of MU380, our recently identified potent, selective, and metabolically robust inhibitor of checkpoint kinase 1. We also describe a newly developed enantioselective synthesis of MU380, which allows preparation of gram quantities of the substance. Checkpoint kinase 1 is a master regulator of replication operating primarily in intra-S and G₂/M cell cycle checkpoints. Initially tested in leukemia and lymphoma cell lines, MU380 significantly potentiated efficacy of gemcitabine, a clinically used inducer of replication stress. Moreover, MU380 manifested substantial single-agent activity in both TP53-wild type and TP53-mutated leukemia and lymphoma cell lines. In chronic lymphocytic leukemia-derived cell lines MEC-1, MEC-2 (both TP53-mut), and OSU-CLL (TP53-wt) the inhibitor impaired cell cycle progression and induced apoptosis. In primary clinical samples, MU380 used as a single-agent noticeably reduced the viability of unstimulated chronic lymphocytic leukemia cells as well as those induced to proliferate by anti-CD40/IL-4 stimuli. In both cases, effects were comparable in samples harboring p53 pathway dysfunction (TP53 mutations or ATM mutations) and TP53-wt/ATM-wt cells. Lastly, MU380 also exhibited significant in vivo activity in a xenotransplant mouse model (immunodeficient strain NOD-scid IL2Rγ^null) where it efficiently suppressed growth of subcutaneous tumors generated from MEC-1 cells.

Broad Spectrum Activity of the Checkpoint Kinase 1 Inhibitor Prexasertib as a Single Agent or Chemopotentiator Across a Range of Preclinical Pediatric Tumor Models

PURPOSE

Checkpoint kinase 1 (CHK1) inhibitors potentiate the DNA-damaging effects of cytotoxic therapies and/or promote elevated levels of replication stress, leading to tumor cell death. Prexasertib (LY2606368) is a CHK1 small-molecule inhibitor under clinical evaluation in multiple adult and pediatric cancers. In this study, prexasertib was tested in a large panel of preclinical models of pediatric solid malignancies alone or in combination with chemotherapy.

EXPERIMENTAL DESIGN

DNA damage and changes in cell signaling following in vitro prexasertib treatment in pediatric sarcoma cell lines were analyzed by Western blot and high content imaging. Antitumor activity of prexasertib as a single agent or in combination with different chemotherapies was explored in cell line-derived (CDX) and patient-derived xenograft (PDX) mouse models representing nine different pediatric cancer histologies.

RESULTS

Pediatric sarcoma cell lines were highly sensitive to prexasertib treatment in vitro, resulting in activation of the DNA damage response. Two PDX models of desmoplastic small round cell tumor and one malignant rhabdoid tumor CDX model responded to prexasertib with complete regression. Prexasertib monotherapy also elicited robust responses in mouse models of rhabdomyosarcoma. Concurrent administration with chemotherapy was sufficient to overcome innate resistance or prevent acquired resistance to prexasertib in preclinical models of neuroblastoma, osteosarcoma, and Ewing sarcoma, or alveolar rhabdomyosarcoma, respectively.

CONCLUSIONS

Prexasertib has significant antitumor effects as a monotherapy or in combination with chemotherapy in multiple preclinical models of pediatric cancer. These findings support further investigation of prexasertib in pediatric malignancies.

Dose-finding study of the checkpoint kinase 1 inhibitor, prexasertib, in Japanese patients with advanced solid tumors

Prexasertib is a novel inhibitor of checkpoint kinase 1. The primary objective of this study was to evaluate prexasertib tolerability in Japanese patients with advanced solid tumors. This nonrandomized single-arm open-label phase 1 study of prexasertib consisted of 2 dose levels, 80 mg/m² and the global-recommended dose based on a US study of 105 mg/m² , administered intravenously once every 14 days (n = 6 for each dose). Transition to the higher dose proceeded if the frequency of dose-limiting toxicity observed in cycle 1 was <33% at the lower dose. Safety measures, pharmacokinetics and antitumor activity were assessed. A total of 12 patients were treated. Two patients, one in each dose group, experienced dose-limiting toxicities of febrile neutropenia, one grade 4 and the other grade 3; both patients recovered and continued the study treatment. The grade 4 treatment-emergent adverse events related to study treatment were neutropenia (6 patients [50.0%]), leukopenia (4 patients [33.3%]), and 1 instance each (8.3%) of anemia, febrile neutropenia and thrombocytopenia. Neutropenia was generally transient and reversible; 11 patients (91.7%) required granulocyte colony-stimulating factor treatment during the study. There were no discontinuations due to adverse events or deaths. The prexasertib pharmacokinetics displayed dose-independent and time-independent behavior across both dose levels, similar to the profile observed in the US-based phase 1 study. Eight patients had a best overall response of stable disease. These data are consistent with the known safety profile for prexasertib and confirm its tolerability in Japanese patients with advanced solid tumors.

mTORC1/2 and Protein Translation Regulate Levels of CHK1 and the Sensitivity to CHK1 Inhibitors in Ewing Sarcoma Cells

The treatment of Ewing sarcoma has changed very little in the past two decades and novel treatment approaches are needed. We recently identified that Ewing sarcoma cells are uniquely vulnerable to inhibitors of ribonucleotide reductase (RNR), the rate-limiting enzyme in the synthesis of deoxyribonucleotides. We subsequently found that the inhibition of checkpoint kinase 1 (CHK1) increases the sensitivity of Ewing sarcoma cells to inhibitors of RNR, such as gemcitabine. However, Ewing sarcoma cells exhibit high levels of the CHK1 protein, which may represent an adaptive response to elevated levels of endogenous DNA replication stress. Consequently, we began this work with the aim of determining the impact of CHK1 levels on drug sensitivity, as well as identifying the mechanisms and pathways that regulate CHK1 levels in Ewing sarcoma cells. In this report, we show that the high levels of the CHK1 protein in Ewing sarcoma cells limit the efficacy of CHK1 inhibitors. However, inhibition of mTORC1/2 activates the translational repressor 4E-BP1, reduces protein synthesis, and decreases levels of the CHK1 protein in Ewing sarcoma cells. Similarly, we identified that the CHK1 inhibitor prexasertib also activates 4E-BP1, inhibits protein synthesis, and reduces CHK1 protein levels in Ewing sarcoma cells. Moreover, the combination of prexasertib and gemcitabine was synergistic in vitro, caused tumor regression in vivo, and significantly prolonged mouse survival in a Ewing sarcoma xenograft experiment. Overall, our results provide insight into Ewing sarcoma biology and support further investigation of the CHK1 pathway as a therapeutic target in Ewing sarcoma tumors.

MRE11 inhibition highlights a replication stress-dependent vulnerability of MYCN-driven tumors

MRE11 is a component of the MRE11/RAD50/NBS1 (MRN) complex, whose activity is essential to control faithful DNA replication and to prevent accumulation of deleterious DNA double-strand breaks. In humans, hypomorphic mutations in these genes lead to DNA damage response (DDR)-defective and cancer-prone syndromes. Moreover, MRN complex dysfunction dramatically affects the nervous system, where MRE11 is required to restrain MYCN-dependent replication stress, during the rapid expansion of progenitor cells. MYCN activation, often due to genetic amplification, represents the driving oncogenic event for a number of human tumors, conferring bad prognosis and predicting very poor responses even to the most aggressive therapeutic protocols. This is prototypically exemplified by neuroblastoma, where MYCN amplification occurs in about 25% of the cases. Intriguingly, MRE11 is highly expressed and predicts bad prognosis in MYCN-amplified neuroblastoma. Due to the lack of direct means to target MYCN, we explored the possibility to trigger intolerable levels of replication stress-dependent DNA damage, by inhibiting MRE11 in MYCN-amplified preclinical models. Indeed, either MRE11 knockdown or its pharmacological inhibitor mirin induce accumulation of replication stress and DNA damage biomarkers in MYCN-amplified cells. The consequent DDR recruits p53 and promotes a p53-dependent cell death, as indicated by p53 loss- and gain-of-function experiments. Encapsulation of mirin in nanoparticles allowed its use on MYCN-amplified neuroblastoma xenografts in vivo, which resulted in a sharp impairment of tumor growth, associated with DDR activation, p53 accumulation, and cell death. Therefore, we propose that MRE11 inhibition might be an effective strategy to treat MYCN-amplified and p53 wild-type neuroblastoma, and suggest that targeting replication stress with appropriate tools should be further exploited to tackle MYCN-driven tumors.

Abemaciclib Is Active in Preclinical Models of Ewing Sarcoma via Multipronged Regulation of Cell Cycle, DNA Methylation, and Interferon Pathway Signaling

PURPOSE

Ewing sarcoma (ES) is a rare and highly malignant cancer that occurs in the bone and surrounding tissue of children and adolescents. The EWS/ETS fusion transcription factor that drives ES pathobiology was previously demonstrated to modulate cyclin D1 expression. In this study, we evaluated abemaciclib, a small-molecule CDK4 and CDK6 (CDK4 and 6) inhibitor currently under clinical investigation in pediatric solid tumors, in preclinical models of ES.

EXPERIMENTAL DESIGN

Using Western blot, high-content imaging, flow cytometry, ELISA, RNA sequencing, and CpG methylation assays, we characterized the in vitro response of ES cell lines to abemaciclib. We then evaluated abemaciclib in vivo in cell line-derived xenograft (CDX) and patient-derived xenograft (PDX) mouse models of ES as either a monotherapy or in combination with chemotherapy.

RESULTS

Abemaciclib induced quiescence in ES cell lines via a G₁ cell-cycle block, characterized by decreased proliferation and reduction of Ki-67 and FOXM1 expression and retinoblastoma protein (RB) phosphorylation. In addition, abemaciclib reduced DNMT1 expression and promoted an inflammatory immune response as measured by cytokine secretion, antigen presentation, and interferon pathway upregulation. Single-agent abemaciclib reduced ES tumor volume in preclinical mouse models and, when given in combination with doxorubicin or temozolomide plus irinotecan, durable disease control was observed.

CONCLUSIONS

Collectively, our data demonstrate that the antitumor effects of abemaciclib in preclinical ES models are multifaceted and include cell-cycle inhibition, DNA demethylation, and immunogenic changes.

Too many targets, not enough patients: rethinking neuroblastoma clinical trials

Neuroblastoma is a rare solid tumour of infancy and early childhood with a disproportionate contribution to paediatric cancer mortality and morbidity. Combination chemotherapy, radiation therapy and immunotherapy remains the standard approach to treat high-risk disease, with few recurrent, actionable genetic aberrations identified at diagnosis. However, recent studies indicate that actionable aberrations are far more common in relapsed neuroblastoma, possibly as a result of clonal expansion. In addition, although the major validated disease driver, MYCN, is not currently directly targetable, multiple promising approaches to target MYCN indirectly are in development. We propose that clinical trial design needs to be rethought in order to meet the challenge of providing rigorous, evidence-based assessment of these new approaches within a fairly small patient population and that experimental therapies need to be assessed at diagnosis in very-high-risk patients rather than in relapsed and refractory patients.

Molecular mechanisms and therapeutic targets in neuroblastoma

Neuroblastoma is the most common extracranical tumor of childhood and the most deadly tumor of infancy. It is characterized by early age onset and high frequencies of metastatic disease but also the capacity to spontaneously regress. Despite intensive therapy, the survival for patients with high-risk neuroblastoma and those with recurrent or relapsed disease is low. Hence, there is an urgent need to develop new therapies for these patient groups. The molecular pathogenesis based on high-throughput omics technologies of neuroblastoma is beginning to be resolved which have given the opportunity to develop personalized therapies for high-risk patients. Here we discuss the potential of developing targeted therapies against aberrantly expressed molecules detected in sub-populations of neuroblastoma patients and how these selected targets can be drugged in order to overcome treatment resistance, improve survival and quality of life for these patients and also the possibilities to transfer preclinical research into clinical testing.

Prexasertib, a cell cycle checkpoint kinase 1 and 2 inhibitor, in BRCA wild-type recurrent high-grade serous ovarian cancer: a first-in-class proof-of-concept phase 2 study

Background

High-grade serous ovarian carcinoma (HGSOC) is characterized by TP53 mutations, DNA repair defects, and genomic instability. We hypothesized that prexasertib, a cell cycle checkpoint kinase 1 and 2 inhibitor, would be active in BRCA wild-type HGSOC.

Methods

In this open label, single centre, two-stage proof-of-concept phase 2 study, women aged 18 years or older with measurable, recurrent high-grade serous or high-grade endometrioid ovarian carcinoma were enrolled. All patients must have had either a negative family history of hereditary breast and ovarian cancer or known BRCA wild-type for BRCA wild-type cohort. Other key eligibility criteria were an Eastern Cooperative Oncology Group performance status of 0 or 1 or 2, and adequate haematological, renal, and hepatic function. Patients received intravenous prexasertib 105mg/m² once every 2 weeks until disease progression, unacceptable toxicity or patient withdrawal of consent. The primary endpoint was investigator-assessed tumour response per protocol based on Response Evaluation Criteria in Solid Tumors, version 1·1 in evaluable patients. The final analysis of this cohort is reported here. This ongoing trial is registered with ClinicalTrials.gov (NCT02203513) and enrolling the patients of BRCA mutation cohort.

Findings

Between January 2015 and November 2016, 28 women (median age 64-year-old [IQR 58–69·5], with median 5 prior systemic therapies [IQR 2·5–5]) were enrolled and received at least one dose of prexasertib. Eight of 24 evaluable patients had a partial response (PR; 33%, 95% CI: 16–55) and 50% had a GCIG CA125 response. The RR in the intention-to-treat population was 29% (8/28, 95% CI: 13–49). The common (>10%) grade 3 or 4 treatment-emergent adverse events were neutropenia (26 [93%] patients), thrombocytopenia (seven [25%] patients), and anaemia (three [11%] patients). Grade 4 neutropenia occurred in 22 (79%) patients after the first dose and was transient ≤ 7 days (median 6 days [IQR 4–8]) without growth factor support; the incidence of febrile neutropenia was 7% (2/28).

Interpretation

We demonstrate clinical activity of prexasertib in BRCA wild-type HGSOC, especially patients with platinum-resistant or refractory ovarian cancer. These results warrant further development for this unmet patient need.

Funding

Intramural Research Program of the National Institutes of Health, National Cancer Institute, Center for Cancer Research, USA.

Development of Chemotherapy with Cell-Cycle Inhibitors for Adult and Pediatric Cancer Therapy

Preclinical and clinical development of agents that inhibit cell-cycle progression have brought an understanding of the feasibility of targeting various cell-cycle regulators in patients with cancer. Small molecule inhibitors targeting key proteins that participate in cell-cycle progression including the cyclin-dependent kinases and checkpoint kinases induce cell-cycle arrest and apoptosis in neoplastic cells. Early phase I studies demonstrate targeted inhibitors can be administered safely in adult and pediatric cancer patients, but these agents generally show limited clinical benefits as single agents. In this review, we discuss biological mechanisms that support dual combination strategies of cell-cycle inhibition with chemotherapeutic agents that are anticipated to achieve rationally targeted therapies for cancer patients. The rationale for evaluating these combination strategies is that DNA damage renders tumors highly responsive to irreversible cell-cycle arrest therapy. This approach is predicted to generate less intensive therapies and to maximize the efficacy of individual agents against solid tumors and hematologic malignancies. Cancer Res; 78(2); 320-5. ©2018 AACR.

DNA damage response inhibitors: Mechanisms and potential applications in cancer therapy

Over the last decade the unravelling of the molecular mechanisms of the DNA damage response pathways and of the genomic landscape of human tumors have paved the road to new therapeutic approaches in oncology. It is now clear that tumors harbour defects in different DNA damage response steps, mainly signalling and repair, rendering them more dependent on the remaining pathways. We here focus on the proteins ATM, ATR, CHK1 and WEE1, reviewing their roles in the DNA damage response and as targets in cancer therapy. In the last decade specific inhibitors of these proteins have been designed, and their potential antineoplastic activity has been explored both in monotherapy strategies against tumors with specific defects (synthetic lethality approach) and in combination with radiotherapy or chemotherapeutic or molecular targeted agents. The preclinical and clinical evidence of antitumor activity of these inhibitors emanating from these research efforts will be critically reviewed. Lastly, the potential therapeutic feasibility of combining together such inhibitors with the aim to target particular subsets of tumors will be also discussed.