Mechanisms responsible for the synergistic antileukemic interactions between ATR inhibition and cytarabine in acute myeloid leukemia cells

DNA damage response defects in hematologic malignancies: mechanistic insights and therapeutic strategies

ABSTRACT

The DNA damage response (DDR) encompasses the detection and repair of DNA lesions and is fundamental to the maintenance of genome integrity. Germ line DDR alterations underlie hereditary chromosome instability syndromes by promoting the acquisition of pathogenic structural variants in hematopoietic cells, resulting in increased predisposition to hematologic malignancies. Also frequent in hematologic malignancies are somatic mutations of DDR genes, typically arising from replication stress triggered by oncogene activation or deregulated tumor proliferation that provides a selective pressure for DDR loss. These defects impair homology-directed DNA repair or replication stress response, leading to an excessive reliance on error-prone DNA repair mechanisms that results in genomic instability and tumor progression. In hematologic malignancies, loss-of-function DDR alterations confer clonal growth advantage and adverse prognostic impact but may also provide therapeutic opportunities. Selective targeting of functional dependencies arising from these defects could achieve synthetic lethality, a therapeutic concept exemplified by inhibition of poly-(adenosine 5'-diphosphate ribose) polymerase or the ataxia telangiectasia and Rad 3 related-CHK1-WEE1 axis in malignancies harboring the BRCAness phenotype or genetic defects that increase replication stress. Furthermore, the role of DDR defects as a source of tumor immunogenicity, as well as their impact on the cross talk between DDR, inflammation, and tumor immunity are increasingly recognized, thus providing rationale for combining DDR modulation with immune modulation. The nature of the DDR-immune interface and the cellular vulnerabilities conferred by DDR defects may nonetheless be disease-specific and remain incompletely understood in many hematologic malignancies. Their comprehensive elucidation will be critical for optimizing therapeutic strategies to target DDR defects in these diseases.

Panobinostat sensitizes AraC-resistant AML cells to the combination of azacitidine and venetoclax

The majority of acute myeloid leukemia (AML) patients respond to intensive induction therapy, consisting of cytarabine (AraC) and an anthracycline, though more than half experience relapse. Relapsed/refractory (R/R) AML patients are difficult to treat, and their clinical outcomes remain dismal. Venetoclax (VEN) in combination with azacitidine (AZA) has provided a promising treatment option for R/R AML, though the overall survival (OS) could be improved (OS ranges from 4.3 to 9.1 months). Overexpression of c-Myc is associated with chemoresistance in AML. Histone deacetylase (HDAC) inhibitors have been shown to suppress c-Myc and enhance the antileukemic activity of VEN, as well as AZA, though combination of all three has not been fully explored. In this study, we investigated the HDAC inhibitor, panobinostat, in combination with VEN + AZA against AraC-resistant AML cells. Panobinostat treatment downregulated c-Myc and Bcl-xL and upregulated Bim, which enhanced the antileukemic activity of VEN + AZA against AraC-resistant AML cells. In addition, panobinostat alone and in combination with VEN + AZA suppressed oxidative phosphorylation and/or glycolysis in AraC-resistant AML cells. These findings support further development of panobinostat in combination with VEN + AZA for the treatment of AraC-resistant AML.

Targeting the DNA damage response in hematological malignancies

Deregulation of the DNA damage response (DDR) plays a critical role in the pathogenesis and progression of many cancers. The dependency of certain cancers on DDR pathways has enabled exploitation of such through synthetically lethal relationships e.g., Poly ADP-Ribose Polymerase (PARP) inhibitors for BRCA deficient ovarian cancers. Though lagging behind that of solid cancers, DDR inhibitors (DDRi) are being clinically developed for haematological cancers. Furthermore, a high proliferative index characterize many such cancers, suggesting a rationale for combinatorial strategies targeting DDR and replicative stress. In this review, we summarize pre-clinical and clinical data on DDR inhibition in haematological malignancies and highlight distinct haematological cancer subtypes with activity of DDR agents as single agents or in combination with chemotherapeutics and targeted agents. We aim to provide a framework to guide the design of future clinical trials involving haematological cancers for this important class of drugs.

Key Proteins of Replication Stress Response and Cell Cycle Control as Cancer Therapy Targets

Replication stress (RS) is a characteristic state of cancer cells as they tend to exchange precision of replication for fast proliferation and increased genomic instability. To overcome the consequences of improper replication control, malignant cells frequently inactivate parts of their DNA damage response (DDR) pathways (the ATM-CHK2-p53 pathway), while relying on other pathways which help to maintain replication fork stability (ATR-CHK1). This creates a dependency on the remaining DDR pathways, vulnerability to further destabilization of replication and synthetic lethality of DDR inhibitors with common oncogenic alterations such as mutations of TP53, RB1, ATM, amplifications of MYC, CCNE1 and others. The response to RS is normally limited by coordination of cell cycle, transcription and replication. Inhibition of WEE1 and PKMYT1 kinases, which prevent unscheduled mitosis entry, leads to fragility of under-replicated sites. Recent evidence also shows that inhibition of Cyclin-dependent kinases (CDKs), such as CDK4/6, CDK2, CDK8/19 and CDK12/13 can contribute to RS through disruption of DNA repair and replication control. Here, we review the main causes of RS in cancers as well as main therapeutic targets-ATR, CHK1, PARP and their inhibitors.

A Selective Nano Cell Cycle Checkpoint Inhibitor Overcomes Leukemia Chemoresistance

Cell cycle checkpoint activation promotes DNA damage repair, which is highly associated with the chemoresistance of various cancers including acute myeloid leukemia (AML). Selective cell cycle checkpoint inhibitors are strongly demanded to overcome chemoresistance, but remain unexplored. A selective nano cell cycle checkpoint inhibitor (NCCI: citric acid capped ultra-small iron oxide nanoparticles) that can catalytically inhibit the cell cycle checkpoint of AML to boost the chemotherapeutic efficacy of genotoxic agents is now reported. NCCI can selectively accumulate in AML cells and convert H2 O2 to • OH to cleave heat shock protein 90, leading to the degradation of ataxia telangiectasia and Rad3-related proteinand checkpoint kinase 1, and the subsequent dysfunction of the G2/M checkpoint. Consequently, NCCI revitalizes the anti-AML efficacy of cytarabine that is previously ineffective both in vitro and in vivo. This study offers new insights into designing selective cell cycle checkpoint inhibitors for biomedical applications.

Cytarabine-induced destabilization of MCL1 mRNA and protein triggers apoptosis in leukemia cells

Although cytarabine (Ara-C) is the mainstay of treatment for acute myeloid leukemia (AML), its cytotoxic mechanisms for inducing apoptosis are poorly understood. Therefore, we investigated the Ara-C-induced cell death pathway in human AML U937 cells. Ara-C-induced downregulation of MCL1 is associated with the induction of mitochondrial depolarization and apoptosis. Ara-C triggered NOX4-mediated ROS production, which in turn activated p38 MAPK but inactivated AKT. Ara-C-induced DNA damage modulates p38 MAPK activation without affecting AKT inactivation in U937 cells. Inactivated AKT promotes GSK3β-dependent CREB phosphorylation, which in turn increases NOXA transcription, thereby triggering the degradation of MCL1 protein. Activated p38 MAPK induces HuR downregulation, leading to accelerated MCL1 mRNA turnover. A similar pathway also explains the Ara-C-induced THP-1 cell death. Collectively, our data confirm that Ara-C-triggered apoptosis in the AML cell lines U937 and THP-1 is mediated through the destabilization of MCL1 mRNA and protein. Furthermore, Ara-C acts synergistically with the BCL2 inhibitor ABT-199 to induce cell death in ABT-199-resistant and parental U937 cells by inhibiting MCL1 expression.

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.

Inhibition of Mcl-1 Synergistically Enhances the Antileukemic Activity of Gilteritinib and MRX-2843 in Preclinical Models of FLT3-Mutated Acute Myeloid Leukemia

FMS-like tyrosine kinase 3 (FLT3)-internal tandem duplication (FLT3-ITD) mutations occur in about 25% of all acute myeloid leukemia (AML) patients and confer a poor prognosis. FLT3 inhibitors have been developed to treat patients with FLT3-mutated AML and have shown promise, though the acquisition of resistance occurs, highlighting the need for combination therapies to prolong the response to FLT3 inhibitors. In this study, we investigated the selective Mcl-1 inhibitor AZD5991 in combination with the FLT3 inhibitors gilteritinib and MRX-2843. The combinations synergistically induce apoptosis in AML cell lines and primary patient samples. The FLT3 inhibitors downregulate c-Myc transcripts through the suppression of the MEK/ERK and JAK2/STAT5 pathways, resulting in the decrease in c-Myc protein. This suppression of c-Myc plays an important role in the antileukemic activity of AZD5991. Interestingly, the suppression of c-Myc enhances AZD5991-inudced cytochrome c release and the subsequent induction of apoptosis. AZD5991 enhances the antileukemic activity of the FLT3 inhibitors gilteritinib and MRX-2843 against FLT3-mutated AML in vitro, warranting further development.

Molecular Threat of Splicing Factor Mutations to Myeloid Malignancies and Potential Therapeutic Modulations

Splicing factors are frequently mutated in myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). These mutations are presumed to contribute to oncogenic transformation, but the underlying mechanisms remain incompletely understood. While no specific treatment option is available for MDS/AML patients with spliceosome mutations, novel targeting strategies are actively explored, leading to clinical trials of small molecule inhibitors that target the spliceosome, DNA damage response pathway, and immune response pathway. Here, we review recent progress in mechanistic understanding of splicing factor mutations promoting disease progression and summarize potential therapeutic strategies, which, if successful, would provide clinical benefit to patients carrying splicing factor mutations.

Pharmacological inhibition of Carbonic Anhydrase IX and XII to enhance targeting of acute myeloid leukaemia cells under hypoxic conditions

Acute myeloid leukaemia (AML) is an aggressive form of blood cancer that carries a dismal prognosis. Several studies suggest that the poor outcome is due to a small fraction of leukaemic cells that elude treatment and survive in specialised, oxygen (O₂)‐deprived niches of the bone marrow. Although several AML drug targets such as FLT3, IDH1/2 and CD33 have been established in recent years, survival rates remain unsatisfactory, which indicates that other, yet unrecognized, mechanisms influence the ability of AML cells to escape cell death and to proliferate in hypoxic environments. Our data illustrates that Carbonic Anhydrases IX and XII (CA IX/XII) are critical for leukaemic cell survival in the O₂‐deprived milieu. CA IX and XII function as transmembrane proteins that mediate intracellular pH under low O₂ conditions. Because maintaining a neutral pH represents a key survival mechanism for tumour cells in O₂‐deprived settings, we sought to elucidate the role of dual CA IX/XII inhibition as a novel strategy to eliminate AML cells under hypoxic conditions. Our findings demonstrate that the dual CA IX/XII inhibitor FC531 may prove to be of value as an adjunct to chemotherapy for the treatment of AML.

Involvement of classic and alternative non-homologous end joining pathways in hematologic malignancies: targeting strategies for treatment

Chromosomal translocations are the main etiological factor of hematologic malignancies. These translocations are generally the consequence of aberrant DNA double-strand break (DSB) repair. DSBs arise either exogenously or endogenously in cells and are repaired by major pathways, including non-homologous end-joining (NHEJ), homologous recombination (HR), and other minor pathways such as alternative end-joining (A-EJ). Therefore, defective NHEJ, HR, or A-EJ pathways force hematopoietic cells toward tumorigenesis. As some components of these repair pathways are overactivated in various tumor entities, targeting these pathways in cancer cells can sensitize them, especially resistant clones, to radiation or chemotherapy agents. However, targeted therapy-based studies are currently underway in this area, and furtherly there are some biological pitfalls, clinical issues, and limitations related to these targeted therapies, which need to be considered. This review aimed to investigate the alteration of DNA repair elements of C-NHEJ and A-EJ in hematologic malignancies and evaluate the potential targeted therapies against these pathways.

The combination of CUDC-907 and gilteritinib shows promising in vitro and in vivo antileukemic activity against FLT3-ITD AML

About 25% of patients with acute myeloid leukemia (AML) harbor FMS-like tyrosine kinase 3 (FLT3) internal tandem duplication (ITD) mutations and their prognosis remains poor. Gilteritinib is a FLT3 inhibitor approved by the US FDA for use in adult FLT3-mutated relapsed or refractory AML patients. Monotherapy, while efficacious, shows short-lived responses, highlighting the need for combination therapies. Here we show that gilteritinib and CUDC-907, a dual inhibitor of PI3K and histone deacetylases, synergistically induce apoptosis in FLT3-ITD AML cell lines and primary patient samples and have striking in vivo efficacy. Upregulation of FLT3 and activation of ERK are mechanisms of resistance to gilteritinib, while activation of JAK2/STAT5 is a mechanism of resistance to CUDC-907. Gilteritinib and CUDC-907 reciprocally overcome these mechanisms of resistance. In addition, the combined treatment results in cooperative downregulation of cellular metabolites and persisting antileukemic effects. CUDC-907 plus gilteritinib shows synergistic antileukemic activity against FLT3-ITD AML in vitro and in vivo, demonstrating strong translational therapeutic potential.

The HDAC and PI3K dual inhibitor CUDC-907 synergistically enhances the antileukemic activity of venetoclax in preclinical models of acute myeloid leukemia

Venetoclax is a promising agent in the treatment of acute myeloid leukemia, though its antileukemic activity is limited to combination therapies. Mcl-1 downregulation, Bim upregulation, and DNA damage have been identified as potential ways to enhance venetoclax activity. In this study, we combine venetoclax with the dual PI3K and histone deacetylase inhibitor CUDC-907, which can downregulate Mcl-1, upregulate Bim, and induce DNA damage, as well as downregulate c-Myc. We establish that CUDC-907 and venetoclax synergistically induce apoptosis in acute myeloid leukemia cell lines and primary acute myeloid leukemia patient samples ex vivo. CUDC-907 downregulates CHK1, Wee1, RRM1, and c-Myc, which were found to play a role in venetoclax-induced apoptosis. Interestingly, we found that venetoclax treatment enhances CUDC-907-induced DNA damage potentially through inhibition of DNA repair. In vivo results show that CUDC-907 enhances venetoclax efficacy in an acute myeloid leukemia cell line derived xenograft mouse model, supporting the development of CUDC-907 in combination with venetoclax for the treatment of acute myeloid leukemia.

Targeting multiple signaling pathways: the new approach to acute myeloid leukemia therapy

Acute myeloid leukemia (AML) is the most common form of acute leukemia in adults and the second most common form of acute leukemia in children. Despite this, very little improvement in survival rates has been achieved over the past few decades. This is partially due to the heterogeneity of AML and the need for more targeted therapeutics than the traditional cytotoxic chemotherapies that have been a mainstay in therapy for the past 50 years. In the past 20 years, research has been diversifying the approach to treating AML by investigating molecular pathways uniquely relevant to AML cell proliferation and survival. Here we review the development of novel therapeutics in targeting apoptosis, receptor tyrosine kinase (RTK) signaling, hedgehog (HH) pathway, mitochondrial function, DNA repair, and c-Myc signaling. There has been an impressive effort into better understanding the diversity of AML cell characteristics and here we highlight important preclinical studies that have supported therapeutic development and continue to promote new ways to target AML cells. In addition, we describe clinical investigations that have led to FDA approval of new targeted AML therapies and ongoing clinical trials of novel therapies targeting AML survival pathways. We also describe the complexity of targeting leukemia stem cells (LSCs) as an approach to addressing relapse and remission in AML and targetable pathways that are unique to LSC survival. This comprehensive review details what we currently understand about the signaling pathways that support AML cell survival and the exceptional ways in which we disrupt them.

Antiproliferative Properties of a Few Auranofin-Related Gold(I) and Silver(I) Complexes in Leukemia Cells and their Interferences with the Ubiquitin Proteasome System

A group of triethylphosphine gold(I) and silver(I) complexes, structurally related to auranofin, were prepared and investigated as potential anticancer drug candidates. The antiproliferative properties of these metal compounds were assessed against two leukemia cell lines, i.e., CCRF-CEM and its multidrug-resistant counterpart, CEM/ADR5000. Interestingly, potent cytotoxic effects were disclosed for both series of compounds against leukemia cells, with IC₅₀ values generally falling in the low-micromolar range, the gold derivatives being on the whole more effective than the silver analogues. Some initial structure-function relationships were drawn. Subsequently, the ability of the study compounds to inhibit the three main catalytic activities of the proteasome was investigated. Different patterns of enzyme inhibition emerged for the various metal complexes. Notably, gold compounds were able to inhibit effectively both the trypsin-like and chymotrypsin-like proteasome activities, being less effective toward the caspase-like catalytic activity. In most cases, a significant selectivity of the study compounds toward the proteasome proteolytic activities was detected when compared to other proteases. The implications of the obtained results are discussed.

A WEE1 family business: regulation of mitosis, cancer progression, and therapeutic target

The inhibition of the DNA damage response (DDR) pathway in the treatment of cancer has recently gained interest, and different DDR inhibitors have been developed. Among them, the most promising ones target the WEE1 kinase family, which has a crucial role in cell cycle regulation and DNA damage identification and repair in both nonmalignant and cancer cells. This review recapitulates and discusses the most recent findings on the biological function of WEE1/PKMYT1 during the cell cycle and in the DNA damage repair, with a focus on their dual role as tumor suppressors in nonmalignant cells and pseudo-oncogenes in cancer cells. We here report the available data on the molecular and functional alterations of WEE1/PKMYT1 kinases in both hematological and solid tumors. Moreover, we summarize the preclinical information on 36 chemo/radiotherapy agents, and in particular their effect on cell cycle checkpoints and on the cellular WEE1/PKMYT1-dependent response. Finally, this review outlines the most important pre-clinical and clinical data available on the efficacy of WEE1/PKMYT1 inhibitors in monotherapy and in combination with chemo/radiotherapy agents or with other selective inhibitors currently used or under evaluation for the treatment of cancer patients.

Expression of Fibroblast Growth Factor 4 in a Rat Model of Polydactyly of the Thumb Induced by Cytarabine

BACKGROUND The aim of this study was to assess the expression and mechanisms of fibroblast growth factor 4 in polydactyly of the thumb induced by cytarabine. MATERIAL AND METHODS Rats were intraperitoneally injected with cytarabine at different gestation periods (12.5 days, 13.5 days, and 14.5 days) to establish a polydactyly of the thumb model. Then, the expression of FGF4 in polydactyly was studied by whole-mount in situ hybridization. We used hematoxylin & eosin stain and cartilage stain to investigate the development of the skeleton and tissues in the embryo. Pictures were taken to determine the general shape of the deformity, then X-rays were taken to detect bone distortion of the rats born with a congenital malformation. RESULTS In the experimental group (11.5 days, 12.5 days, 13.5 days, and 14.5 days), whole-mount in situ hybridization showed that the FGF4 expression at the tip of the embryonic limb bud was significantly increased compared with the control group and FGF4 was distributed in a wider range and lasted longer than in the control group (P<0.01). HE staining and cartilage staining showed that there was an extra metacarpal bone and a phalanx in the rats with polydactyly of the thumb (P<0.01). Images of the deformed limbs showed polydactyly and syndactyly of the thumb in the rats. Further X-ray examination revealed 1 extra metacarpal bone and 1 extra phalanx. CONCLUSIONS Cytarabine can induce polydactyly and syndactyly of the thumb in rats. In this process, cytarabine can induce the expression of FGF4 on the tip of the embryonic limb bud, which further leads to abnormal development of the embryonic limb bud and eventually causes a congenital deformity.

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.

Classic and targeted anti-leukaemic agents interfere with the cholesterol biogenesis metagene in acute myeloid leukaemia: Therapeutic implications

Despite significant advances in deciphering the molecular landscape of acute myeloid leukaemia (AML), therapeutic outcomes of this haematological malignancy have only modestly improved over the past decades. Drug resistance and disease recurrence almost invariably occur, highlighting the need for a deeper understanding of these processes. While low O₂ compartments, such as bone marrow (BM) niches, are well‐recognized hosts of drug‐resistant leukaemic cells, standard in vitro studies are routinely performed under supra‐physiologic (21% O₂, ambient air) conditions, which limits clinical translatability. We hereby identify molecular pathways enriched in AML cells that survive acute challenges with classic or targeted therapeutic agents. Experiments took into account variations in O₂ tension encountered by leukaemic cells in clinical settings. Integrated RNA and protein profiles revealed that lipid biosynthesis, and particularly the cholesterol biogenesis branch, is a particularly therapy‐induced vulnerability in AML cells under low O₂ states. We also demonstrate that the impact of the cytotoxic agent cytarabine is selectively enhanced by a high‐potency statin. The cholesterol biosynthesis programme is amenable to additional translational opportunities within the expanding AML therapeutic landscape. Our findings support the further investigation of higher‐potency statin (eg rosuvastatin)–based combination therapies to enhance targeting residual AML cells that reside in low O₂ environments.

The PI3K/AKT Pathway Inhibitor ISC-4 Induces Apoptosis and Inhibits Growth of Leukemia in Preclinical Models of Acute Myeloid Leukemia

Acute myeloid leukemia is a heterogeneous disease with a 5-year survival rate of 28.3%, and current treatment options constrained by dose-limiting toxicities. One of the key signaling pathways known to be frequently activated and dysregulated in AML is PI3K/AKT. Its dysregulation is associated with aggressive cell growth and drug resistance. We investigated the activity of Phenybutyl isoselenocyanate (ISC-4) in primary cells obtained from newly diagnosed AML patients, diverse AML cell lines, and normal cord blood cells. ISC-4 significantly inhibited survival and clonogenicity of primary human AML cells without affecting normal cells. We demonstrated that ISC-4-mediated p-Akt inhibition caused apoptosis in primary AML (CD34⁺) stem cells and enhanced efficacy of cytarabine. ISC-4 impeded leukemia progression with improved overall survival in a syngeneic C1498 mouse model with no obvious toxic effects on normal myelopoiesis. In U937 xenograft model, bone marrow cells exhibited significant reduction in human CD45⁺ cells in ISC-4 (~87%) or AraC (~89%) monotherapy groups compared to control. Notably, combination treatment suppressed the leukemic infiltration significantly higher than the single-drug treatments (~94%). Together, the present findings suggest that ISC-4 might be a promising agent for AML treatment.

In vitro activity of a G-quadruplex-stabilizing small molecule that synergizes with Navitoclax to induce cytotoxicity in acute myeloid leukemia cells

Background

Acute Myeloid Leukemia (AML) is a malignancy of myeloid precursor cells that arise from genomic alterations in the expression of key growth regulatory genes causing cells to assume an undifferentiated state and continue to proliferate. Recent efforts have focused on developing therapies that target specific protein products of aberrantly expressed genes. However, many of the identified proteins are difficult to target and thought to be “undrugable” because of structural challenges, protein overexpression, or mutations that confer resistance to therapy. A novel technology that circumvents some of these issues is the use of small molecules that stabilize secondary DNA structures present in the promoters of many potential oncogenes and modulate their transcription.

Methods

This study characterizes the in vitro activity of the G-quadruplex-stabilizing small molecule GQC-05 in AML cells. The effect of GQC-05 on three AML cell lines was analyzed using viability and apoptosis assays. GQC-05 has been shown to down-regulate MYC through G-quadruplex stabilization in Burkitt’s lymphoma cell lines. MYC expression was evaluated through qPCR and immunoblotting in the three AML cell lines following the treatment of GQC-05. In order to identify other therapeutic agents that potentiate the activity of GQC-05, combination drug screening was performed. The drug combinations were validated using in vitro cytotoxicity assays and compared to other commonly used chemotherapeutic agents.

Results

GQC-05 treatment of KG-1a, CMK and TF-1 cells decreased cell viability and resulted in increased DNA damage and apoptosis. Additionally, treatment of KG-1a, CMK and TF-1 with GQC-05 resulted in decreased expression of MYC mRNA and protein, with a more pronounced effect in KG-1a cells. Combination drug screening identified the Bcl-2/Bcl-X_L inhibitor Navitoclax as a compound that potentiated GQC-05 activity. Co-treatment with GQC-05 and Navitoclax showed a synergistic decrease in cell viability of AML cells as determined by Chou-Talalay analysis, and induced more DNA damage, apoptosis, and rapid cytotoxicity. The cytotoxicity induced by GQC-05 and Navitoclax was more potent than that of Navitoclax combined with either cytarabine or doxorubicin.

Conclusion

These results suggest that the G-quadruplex stabilizing small molecule GQC-05 induces down regulated MYC expression and DNA damage in AML cells. Treatment with both GQC-05 with a Bcl-2/Bcl-X_L inhibitor Navitoclax results in increased cytotoxic activity, which is more pronounced than Navitoclax or GQC-05 alone, and more significant than Navitoclax in combination with cytarabine and doxorubicin that are currently being used clinically.

Antileukemic activity of novel adenosine derivatives

The present study investigated the effect of cladribine (CLA) and six of its derivatives containing a formamidine group at position 6 (CLA-FDM, CLA-FPAZ, CLA-FPIR, CLA-FPIP, CLA-FHEX, and CLA-FMOR) on acute promyelocytic, lymphoblastic, and acute monocytic leukemia cells. The role of ATR kinase in deoxycytidine kinase (dCK) activation in response to DNA damage was assessed. The presence of DNA lesions was assessed by measurement phosphorylation of H2AX and by using the alkaline comet assay with proteinase K post-treatment following assessment of the cell cycle. Apoptotic events such as alterations in intracellular calcium concentration, caspase-3/7 activity and increased sub-G1 cell population were measured. CLA derivatives were highly effective against leukemic cells, showing high cytotoxicity, causing DNA fragmentation, and inducing DNA-protein cross-links in leukemic cells. CLA-FMOR showed the highest efficacy. CLA derivatives increased the levels of intracellular calcium ions, caspase-3/7 and the percentage of sub-G1 apoptotic cells and blocked cells in the S phase of the cell cycle to a greater extent than free CLA. The selective ATR inhibitor VE-821 significantly suppressed the increase in dCK activity and decreased basal dCK activity. The present results suggested that ATR kinase controls dCK activity in response to synthetic CLA derivatives.

Inhibition of MEK and ATR is effective in a B-cell acute lymphoblastic leukemia model driven by Mll-Af4 and activated Ras

Infant B-cell acute lymphoblastic leukemias (B-ALLs) that harbor MLL-AF4 rearrangements are associated with a poor prognosis. One important obstacle to progress for this patient population is the lack of immunocompetent models that faithfully recapitulate the short latency and aggressiveness of this disease. Recent whole-genome sequencing of MLL-AF4 B-ALL samples revealed a high frequency of activating RAS mutations; however, single-agent targeting of downstream effectors of the RAS pathway in these mutated MLL-r B-ALLs has demonstrated limited and nondurable antileukemic effects. Here, we demonstrate that the expression of activating mutant N-Ras ^G12D cooperates with Mll-Af4 to generate a highly aggressive serially transplantable B-ALL in mice. We used our novel mouse model to test the sensitivity of Mll-Af4/N-Ras ^G12D leukemia to small molecule inhibitors and found potent and synergistic preclinical efficacy of dual targeting of the Mek and Atr pathways in mouse- and patient-derived xenografts with both mutations in vivo, suggesting this combination as an attractive therapeutic opportunity that might be used to treat patients with these mutations. Our studies indicate that this mouse model of Mll-Af4/N-Ras B-ALL is a powerful tool to explore the molecular and genetic pathogenesis of this disease subtype, as well as a preclinical discovery platform for novel therapeutic strategies.

Inhibition of Bcl-2 Synergistically Enhances the Antileukemic Activity of Midostaurin and Gilteritinib in Preclinical Models of FLT3-Mutated Acute Myeloid Leukemia

PURPOSE

To investigate the efficacy of the combination of the FLT3 inhibitors midostaurin or gilteritinib with the Bcl-2 inhibitor venetoclax in FLT3-internal tandem duplication (ITD) acute myeloid leukemia (AML) and the underlying molecular mechanism.

EXPERIMENTAL DESIGN

Using both FLT3-ITD cell lines and primary patient samples, Annexin V-FITC/propidium iodide staining and flow cytometry analysis were used to quantify cell death induced by midostaurin or gilteritinib, alone or in combination with venetoclax. Western blot analysis was performed to assess changes in protein expression levels of members of the JAK/STAT, MAPK/ERK, and PI3K/AKT pathways, and members of the Bcl-2 family of proteins. The MV4-11-derived xenograft mouse model was used to assess in vivo efficacy of the combination of gilteritinib and venetoclax. Lentiviral overexpression of Mcl-1 was used to confirm its role in cell death induced by midostaurin or gilteritinib with venetoclax. Changes of Mcl-1 transcript levels were assessed by RT-PCR.

RESULTS

The combination of midostaurin or gilteritinib with venetoclax potently and synergistically induces apoptosis in FLT3-ITD AML cell lines and primary patient samples. The FLT3 inhibitors induced downregulation of Mcl-1, enhancing venetoclax activity. Phosphorylated-ERK expression is induced by venetoclax but abolished by the combination of venetoclax with midostaurin or gilteritinib. Simultaneous downregulation of Mcl-1 by midostaurin or gilteritinib and inhibition of Bcl-2 by venetoclax results in "free" Bim, leading to synergistic induction of apoptosis. In vivo results show that gilteritinib in combination with venetoclax has therapeutic potential.

CONCLUSIONS

Inhibition of Bcl-2 via venetoclax synergistically enhances the efficacy of midostaurin and gilteritinib in FLT3-mutated AML.See related commentary by Perl, p. 6567.

ATR inhibition sensitizes HPV- and HPV+ head and neck squamous cell carcinoma to cisplatin

OBJECTIVES

Cisplatin is commonly used in the treatment of head and neck squamous cell carcinoma (HNSCC), and the repair of cisplatin-induced DNA damage involves activation of the DNA damage response protein ataxia telangiectasia and Rad3-related (ATR). Resistance to cisplatin therapy exacerbates adverse toxicities and is associated with poor outcomes. Since repair of cisplatin-induced DNA damage contributes to resistance, we hypothesized that inhibition of ATR using AZD6738, a well-tolerated and orally-bioavailable inhibitor, would enhance the sensitivity of HNSCC cells and tumors to cisplatin.

MATERIALS AND METHODS

A panel of human papilloma virus-negative (HPV^-) and HPV⁺ HNSCC cell lines were treated with cisplatin in the absence or presence of AZD6738, and effects on cell viability, colony formation, apoptosis signaling, and DNA damage were assessed. The impact of co-treatment with cisplatin plus AZD6738 on the growth of HPV^- and HPV⁺ cell line- and patient-derived xenograft tumors was also examined.

RESULTS

Inhibition of ATR with AZD6738 enhanced cisplatin-induced growth inhibition of HNSCC cell lines and tumors, in association with increased apoptosis signaling and DNA damage. Both HPV^- and HPV⁺ models were sensitized to cisplatin by ATR inhibition.

CONCLUSION

Inhibition of ATR promotes sensitization to cisplatin in preclinical in vitro and in vivo models of HPV^- and HVP⁺ HNSCC, supporting clinical evaluation of this strategy in this disease.

Inhibition of Wee1 sensitizes AML cells to ATR inhibitor VE-822-induced DNA damage and apoptosis

Resistance to standard induction therapy and relapse remain the primary challenges for improving therapeutic effects in acute myeloid leukemia (AML); thus, novel therapeutic strategies are urgently required. Ataxia telangiectasia and Rad3-related protein (ATR) is a key regulator of different types of DNA damage, which is crucial for the maintenance of genomic integrity. The ATR-selective inhibitor VE-822 has proper solubility, potency, and pharmacokinetic properties. In this study, we investigated the anti-leukemic effects of VE-822 alone or combined with Wee1-selective inhibitor AZD1775 in AML cells. Our results showed that VE-822 inhibited AML cell proliferation and induced apoptosis in a dose-dependent manner. AZD1775 significantly promoted VE-822-induced inhibition of AML cell proliferation and led to a decreased number of cells in the G2/M phase. VE-822 and AZD1775 decreased the protein levels of ribonucleotide reductase M1 (RRM1) and M2 (RRM2) subunits, key enzymes in the synthesis of deoxyribonucleoside triphosphate, which increased DNA replication stress. VE-822 combined with AZD1775 synergistically induced AML cell apoptosis and led to replication stress and DNA damage in AML cell lines. Our study demonstrated that AZD1775 synergistically promotes VE-822-induced anti-leukemic activity in AML cell lines and provides support for clinical research on VE-822 in combination with AZD1775 for the treatment of AML patients.

Antileukemic activity and mechanism of action of the novel PI3K and histone deacetylase dual inhibitor CUDC-907 in acute myeloid leukemia

Induction therapy for patients with acute myeloid leukemia (AML) has remained largely unchanged for over 40 years, while overall survival rates remain unacceptably low, highlighting the need for new therapies. The PI3K/Akt pathway is constitutively active in the majority of patients with AML. Given that histone deacetylase inhibitors have been shown to synergize with PI3K inhibitors in preclinical AML models, we investigated the novel dual-acting PI3K and histone deacetylase inhibitor CUDC-907 in AML cells both in vitro and in vivo. We demonstrated that CUDC-907 induces apoptosis in AML cell lines and primary AML samples and shows in vivo efficacy in an AML cell line-derived xenograft mouse model. CUDC-907-induced apoptosis was partially dependent on Mcl-1, Bim, and c-Myc. CUDC-907 induced DNA damage in AML cells while sparing normal hematopoietic cells. Downregulation of CHK1, Wee1, and RRM1, and induction of DNA damage also contributed to CUDC-907-induced apoptosis of AML cells. In addition, CUDC-907 treatment decreased leukemia progenitor cells in primary AML samples ex vivo, while also sparing normal hematopoietic progenitor cells. These findings support the clinical development of CUDC-907 for the treatment of AML.

Targeting ATR in cancer

The chemical treatment of cancer started with the realization that DNA damaging agents such as mustard gas present notable antitumoural properties. Consequently, early drug development focused on genotoxic chemicals, some of which are still widely used in the clinic. However, the efficacy of such therapies is often limited by the side effects of these drugs on healthy cells. A refinement to this approach is to use compounds that can exploit the presence of DNA damage in cancer cells. Given that replication stress (RS) is a major source of genomic instability in cancer, targeting the RS-response kinase ataxia telangiectasia and Rad3-related protein (ATR) has emerged as a promising alternative. With ATR inhibitors now entering clinical trials, we here revisit the biology behind this strategy and discuss potential biomarkers that could be used for a better selection of patients who respond to therapy.

Inhibition of ATR acutely sensitizes acute myeloid leukemia cells to nucleoside analogs that target ribonucleotide reductase

The ataxia telangiectasia and Rad3-related (ATR) protein kinase promotes cancer cell survival by signaling stalled replication forks generated by replication stress, a common feature of many cancers including acute myeloid leukemia (AML). Here we show that the antileukemic activity of the chemotherapeutic nucleoside analogs hydroxyurea and gemcitabine was significantly potentiated by ATR inhibition via a mechanism involving ribonucleotide reductase (RNR) abrogation and inhibition of replication fork progression. When administered in combination with gemcitabine, an inhibitor of the M1 RNR subunit, the ATR inhibitor VX-970, eradicated disseminated leukemia in an orthotopic mouse model, eliciting long-term survival and effective cure. These data identify a synergistic interaction between ATR inhibition and RNR loss that will inform the deployment of small molecule inhibitors for the treatment of AML and other hematologic malignancies.

Inhibiting checkpoint kinase 1 protects bone from bone resorption by mammary tumor in a mouse model

DNA damage response plays a critical role in tumor growth, but little is known about its potential role in bone metabolism. We employed selective inhibitors of Chk1 and examined their effects on the proliferation and migration of mammary tumor cells as well as the development of osteoblasts and osteoclasts. Further, using a mouse model of bone metastasis we evaluated the effects of Chk1 inhibitors on bone quality. Chk1 inhibitors blocked the proliferation, survival, and migration of tumor cells in vitro and suppressed the development of bone-resorbing osteoclasts by downregulating NFATc1. In the mouse model, Chk1 inhibitor reduced osteolytic lesions and prevented mechanical weakening of the femur and tibia. Analysis of RNA-seq expression data indicated that the observed effects were mediated through the regulation of eukaryotic translation initiation factor 2 alpha, stress to the endoplasmic reticulum, S100 proteins, and bone remodeling-linked genes. Our findings suggest that targeting Chk1 signaling without adding DNA damaging agents may protect bone from degradation while suppressing tumor growth and migration.

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.

The DNA damage response pathway in normal hematopoiesis and malignancies

In mammalian cells, the DNA damage response (DDR) prevents the replication and propagation of DNA errors to the next generation, thus maintaining genomic stability. At the heart of the DDR are the related signaling kinases ATM, ATR, and DNA-PK, which regulate DNA repair and associated events such as cell cycle checkpoints, chromatin remodeling, transcription, and ultimately apoptosis. Several findings highlight the occurrence of DDR in hemopoietic stem cells (HSCs), and persistence of DNA lesions in these cells promotes their functional decline and accumulation of leukemogenic mutations. Besides favoring tumor formation and progression, molecular defects that directly or indirectly inactivate certain DDR pathways can provide a therapeutic opportunity, since a reduced ability to repair DNA lesions renders hemopoietic malignancies vulnerable to genotoxic drugs acting also through synthetic lethal interactions. Here, we discuss the essential role of DDR in HSC maintenance and protection against leukemogenesis, and how acquired DDR dysfunctions or pharmacological agents that block this pathway can be effectively exploited for the treatment of various hematopoietic malignancies.

Reevaluation of ATR signaling in primary resting chronic lymphocytic leukemia cells: evidence for pro-survival or pro-apoptotic function

ATM, primarily activated by DNA double-strand breaks, and ATR, activated by single-stranded DNA, are master regulators of the cellular response to DNA damage. In primary chronic lymphocytic leukemia (CLL) cells, ATR signaling is considered to be switched off due to ATR downregulation. Here, we hypothesized that ATR, though expressed at low protein level, could play a role in primary resting CLL cells after genotoxic stress. By investigating the response of CLL cells to UV-C irradiation, a prototypical activator of ATR, we could detect phosphorylation of ATR at Thr-1989, a marker for ATR activation, and also observed that selective ATR inhibitors markedly decreased UV-C-induced phosphorylation of ATR targets, including H2AX and p53. Similar results were obtained with the purine analogs fludarabine and cladribine that were also shown to activate ATR and induce ATR-dependent phosphorylation of H2AX and p53. In addition, ATR inhibition was found to sensitize primary CLL cells to UV-C by decreasing DNA repair synthesis. Conversely, ATR inhibition rescued CLL cells against purine analogs by reducing expression of the pro-apoptotic genes PUMA and BAX. Collectively, our study indicates that ATR signaling can be activated in resting CLL cells and play a pro-survival or pro-apoptotic role, depending on the genotoxic context.