Valproic acid synergistically enhances the cytotoxicity of clofarabine in pediatric acute myeloid leukemia cells

c-Myc plays a critical role in the antileukemic activity of the Mcl-1-selective inhibitor AZD5991 in acute myeloid leukemia

Acute myeloid leukemia (AML) is an aggressive disease with a low 5-year overall survival rate of 29.5%. Thus, more effective therapies are in need to prolong survival of AML patients. Mcl-1 is overexpressed in AML and is associated with poor prognosis, representing a promising therapeutic target. The oncoprotein c-Myc is also overexpressed in AML and is a significant prognostic factor. In addition, Mcl-1 is required for c-Myc induced AML, indicating that c-Myc-driven AML harbors a Mcl-1 dependency and co-targeting of Mcl-1 and c-Myc represents a promising strategy to eradicate AML. In this study, we investigated the role of c-Myc in the antileukemic activity of Mcl-1 selective inhibitor AZD5991 and the antileukemic activity of co-targeting of Mcl-1 and c-Myc in preclinical models of AML. We found that c-Myc protein levels negatively correlated with AZD5991 EC₅₀s in AML cell lines and primary patient samples. AZD5991 combined with inhibition of c-Myc synergistically induced apoptosis in AML cell lines and primary patient samples, and cooperatively targeted leukemia progenitor cells. AML cells with acquired resistance to AZD5991 were resensitized to AZD5991 when c-Myc was inhibited. The combination also showed promising and synergistic antileukemic activity in vitro against AML cell lines with acquired resistance to the main chemotherapeutic drug AraC and primary AML cells derived from a patient at relapse post chemotherapy. The oncoprotein c-Myc represents a potential biomarker of AZD5991 sensitivity and inhibition of c-Myc synergistically enhances the antileukemic activity of AZD5991 against AML.

Enhanced antitumor activity of combined methotrexate and histone deacetylase inhibitor valproic acid on mammary cancer in vitro and in vivo

Histone deacetylase inhibitors (HDACIs) act as antiproliferative agents by promoting differentiation and inducing apoptosis. Valproic acid (VPA) is an HDACI that shows promising chemotherapeutic effect in several tumor cells. The present study aimed to investigate the inhibitory effect of VPA on the viability of mammary cancer cells and its enhancing effect with methotrexate (MTX) in vitro and in vivo. Treatment with VPA or MTX alone induced concentration-dependent cytotoxic effects in two breast cancer cell lines. VPA significantly increased the cytotoxicity of MTX 3 times against MCF7. VPA addition to MTX, however, did not produce any significant changes on MTX cytotoxicity against MDA-MB231. VPA (150 and 200 mg/kg) significantly inhibited the growth of IP and SC Ehrlich ascites carcinoma tumor mouse models and improved results were achieved for tumor inhibition when VPA was combined with MTX (1 and 2 mg/kg) in vivo. The antitumor activity was not associated with a significant increase in toxicity or mice mortality rate. All these findings suggest that the combination of MTX and VPA may have clinical and/or adjuvant therapeutic application in the treatment of mammary cancer.

Therapeutic Use of Valproic Acid and All-Trans Retinoic Acid in Acute Myeloid Leukemia-Literature Review and Discussion of Possible Use in Relapse after Allogeneic Stem Cell Transplantation

Even though allogeneic stem cell transplantation is the most intensive treatment for acute myeloid leukemia (AML), chemo-resistant leukemia relapse is still one of the most common causes of death for these patients, as is transplant-related mortality, i.e., graft versus host disease, infections, and organ damage. These relapse patients are not always candidates for additional intensive therapy or re-transplantation, and many of them have decreased quality of life and shortened expected survival. The efficiency of azacitidine for treatment of posttransplant AML relapse has been documented in several clinical trials. Valproic acid is an antiepileptic fatty acid that exerts antileukemic activity through histone deacetylase inhibition. The combination of valproic acid and all-trans retinoic acid (ATRA) is well tolerated even by unfit or elderly AML patients, and low-toxicity chemotherapy (e.g., azacitidine) can be added to this combination. The triple combination of azacitidine, valproic acid, and ATRA may therefore represent a low-intensity and low-toxicity alternative for these patients. In the present review, we review and discuss the general experience with valproic acid/ATRA in AML therapy and we discuss its possible use in low-intensity/toxicity treatment of post-allotransplant AML relapse. Our discussion is further illustrated by four case reports where combined treatments with sequential azacitidine/hydroxyurea, valproic acid, and ATRA were used.

HDAC8: A Promising Therapeutic Target for Acute Myeloid Leukemia

Histone deacetylase 8 (HDAC8), a class I HDAC that modifies non-histone proteins such as p53, is highly expressed in different hematological neoplasms including a subtype of acute myeloid leukemia (AML) bearing inversion of chromosome 16 [inv(16)]. To investigate HDAC8 contribution to hematopoietic stem cell maintenance and myeloid leukemic transformation, we generated a zebrafish model with Hdac8 overexpression and observed an increase in hematopoietic stem/progenitor cells, a phenotype that could be reverted using a specific HDAC8 inhibitor, PCI-34051 (PCI). In addition, we demonstrated that AML cell lines respond differently to PCI treatment: HDAC8 inhibition elicits cytotoxic effect with cell cycle arrest followed by apoptosis in THP-1 cells, and cytostatic effect in HL60 cells that lack p53. A combination of cytarabine, a standard anti-AML chemotherapeutic, with PCI resulted in a synergistic effect in all the cell lines tested. We, then, searched for a mechanism behind cell cycle arrest caused by HDAC8 inhibition in the absence of functional p53 and demonstrated an involvement of the canonical WNT signaling in zebrafish and in cell lines. Together, we provide the evidence for the role of HDAC8 in hematopoietic stem cell differentiation in zebrafish and AML cell lines, suggesting HDAC8 inhibition as a therapeutic target in hematological malignancies. Accordingly, we demonstrated the utility of a highly specific HDAC8 inhibition as a therapeutic strategy in combination with standard chemotherapy.

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.

Megakaryocyte lineage development is controlled by modulation of protein acetylation

Treatment with lysine deacetylase inhibitors (KDACi) for haematological malignancies, is accompanied by haematological side effects including thrombocytopenia, suggesting that modulation of protein acetylation affects normal myeloid development, and specifically megakaryocyte development. In the current study, utilising ex-vivo differentiation of human CD34+ haematopoietic progenitor cells, we investigated the effects of two functionally distinct KDACi, valproic acid (VPA), and nicotinamide (NAM), on megakaryocyte differentiation, and lineage choice decisions. Treatment with VPA increased the number of megakaryocyte/erythroid progenitors (MEP), accompanied by inhibition of megakaryocyte differentiation, whereas treatment with NAM accelerated megakaryocyte development, and stimulated polyploidisation. Treatment with both KDACi resulted in no significant effects on erythrocyte differentiation, suggesting that the effects of KDACi primarily affect megakaryocyte lineage development. H3K27Ac ChIP-sequencing analysis revealed that genes involved in myeloid development, as well as megakaryocyte/erythroid (ME)-lineage differentiation are uniquely modulated by specific KDACi treatment. Taken together, our data reveal distinct effects of specific KDACi on megakaryocyte development, and ME-lineage decisions, which can be partially explained by direct effects on promoter acetylation of genes involved in myeloid differentiation.

Valproic Acid Induces Endocytosis-Mediated Doxorubicin Internalization and Shows Synergistic Cytotoxic Effects in Hepatocellular Carcinoma Cells

Valproic acid (VPA), a well-known histone deacetylase (HDAC) inhibitor, is used as an anti-cancer drug for various cancers, but the synergistic anti-cancer effect of VPA and doxorubicin (DOX) combination treatment and its potential underlying mechanism in hepatocellular carcinoma (HCC) remain to be elucidated. Here, we evaluate the mono- and combination-therapy effects of VPA and DOX in HCC and identify a specific and efficient, synergistic anti-proliferative effect of the VPA and DOX combination in HCC cells, especially HepG2 cells; this effect was not apparent in MIHA cells, a normal hepatocyte cell line. The calculation of the coefficient of drug interaction confirmed the significant synergistic effect of the combination treatment. Concurrently, the synergistic apoptotic cell death caused by the VPA and DOX combination treatment was confirmed by Hoechst nuclear staining and Western blot analysis of caspase-3 and poly (ADP-ribose) polymerase (PARP) activation. Co-treatment with VPA and DOX enhanced reactive oxygen species (ROS) generation and autophagy, which were clearly attenuated by ROS and autophagy inhibitors, respectively. Furthermore, as an indication of the mechanism underlying the synergistic effect, we observed that DOX internalization, which was induced in the VPA and DOX combination-treated group, occurred via by the caveolae-mediated endocytosis pathway. Taken together, our study uncovered the potential effect of the VPA and DOX combination treatment with regard to cell death, including induction of cellular ROS, autophagy, and the caveolae-mediated endocytosis pathway. Therefore, these results present novel implications in drug delivery research for the treatment of HCC.

Inhibition of Mcl-1 enhances cell death induced by the Bcl-2-selective inhibitor ABT-199 in acute myeloid leukemia cells

Acute myeloid leukemia (AML) is a serious disease. The 5-year survival rates remain frustratingly low (65% for children and 26% for adults). Resistance to frontline chemotherapy (usually cytarabine) often develops; therefore a new treatment modality is needed. Bcl-2 family proteins play an important role in balancing cell survival and apoptosis. The antiapoptotic Bcl-2 family proteins have been found to be dysregulated in AML. ABT-199, a BH3 mimetic, was developed to target antiapoptotic protein Bcl-2. Although ABT-199 has demonstrated promising results, resistance occurs. Previous studies in AML show that ABT-199 alone decreases the association of proapoptotic protein Bim with Bcl-2, but this is compensated by increased association of Bim with prosurvival protein Mcl-1, stabilizing Mcl-1, resulting in resistance to ABT-199. In this study, we investigated the antileukemic activity of the Mcl-1-selective inhibitor A-1210477 in combination with ABT-199 in AML cells. We found that A-1210477 synergistically induced apoptosis with ABT-199 in AML cell lines and primary patient samples. The synergistic induction of apoptosis was decreased upon Bak, Bax and Bim knockdown. While A-1210477 treatment alone also increased Mcl-1 protein levels, combination with ABT-199 reduced binding of Bim to Mcl-1. Our results demonstrate that sequestration of Bim by Mcl-1, a mechanism of ABT-199 resistance, can be abrogated by combined treatment with the Mcl-1 inhibitor A-1201477.

Binding of Released Bim to Mcl-1 is a Mechanism of Intrinsic Resistance to ABT-199 which can be Overcome by Combination with Daunorubicin or Cytarabine in AML Cells

PURPOSE

To investigate the molecular mechanism underlying intrinsic resistance to ABT-199.

EXPERIMENTAL DESIGN

Western blots and real-time RT-PCR were used to determine levels of Mcl-1 after ABT-199 treatment alone or in combination with cytarabine or daunorubicin. Immunoprecipitation of Bim and Mcl-1 were used to determine the effect of ABT-199 treatment on their interactions with Bcl-2 family members. Lentiviral short hairpin RNA knockdown of Bim and CRISPR knockdown of Mcl-1 were used to confirm their role in resistance to ABT-199. JC-1 assays and flow cytometry were used to determine drug-induced apoptosis.

RESULTS

Immunoprecipitation of Bim from ABT-199-treated cell lines and a primary patient sample demonstrated decreased association with Bcl-2, but increased association with Mcl-1 without corresponding change in mitochondrial outer membrane potential. ABT-199 treatment resulted in increased levels of Mcl-1 protein, unchanged or decreased Mcl-1 transcript levels, and increased Mcl-1 protein half-life, suggesting that the association with Bim plays a role in stabilizing Mcl-1 protein. Combining conventional chemotherapeutic agent cytarabine or daunorubicin with ABT-199 resulted in increased DNA damage along with decreased Mcl-1 protein levels, compared with ABT-199 alone, and synergistic induction of cell death in both AML cell lines and primary patient samples obtained from AML patients at diagnosis.

CONCLUSIONS

Our results demonstrate that sequestration of Bim by Mcl-1 is a mechanism of intrinsic ABT-199 resistance and supports the clinical development of ABT-199 in combination with cytarabine or daunorubicin for the treatment of AML. Clin Cancer Res; 22(17); 4440-51. ©2016 AACR.

Synergistic antitumor interactions between MK-1775 and panobinostat in preclinical models of pancreatic cancer

Pancreatic cancer remains a clinical challenge, thus new therapies are urgently needed. The selective Wee1 inhibitor MK-1775 has demonstrated promising results when combined with DNA damaging agents, and more recently with CHK1 inhibitors in various malignancies. We have previously demonstrated that treatment with the pan-histone deacetylase inhibitor panobinostat (LBH589) can cause down-regulation of CHK1. Accordingly, we investigated using panobinostat to down-regulate CHK1 in combination with MK-1775 to enhance cell death in preclinical pancreatic cancer models. We demonstrate that MK-1775 treatment results in increased H2AX phosphorylation, indicating increased DNA double-strand breaks, and activation of CHK1, which are both dependent on CDK activity. Combination of MK-1775 and panobinostat resulted in synergistic antitumor activity in six pancreatic cancer cell lines. Finally, our in vivo study using a pancreatic xenograft model reveals promising cooperative antitumor activity between MK-1775 and panobinostat. Our study provides compelling evidence that the combination of MK-1775 and panobinostat has antitumor activity in preclinical models of pancreatic cancer and supports the clinical development of panobinostat in combination with MK-1775 for the treatment of this deadly disease.

Obatoclax potentiates the cytotoxic effect of cytarabine on acute myeloid leukemia cells by enhancing DNA damage

Resistance to cytarabine and anthracycline-based chemotherapy is a major cause of treatment failure for acute myeloid leukemia (AML) patients. Overexpression of Bcl-2, Bcl-xL, and/or Mcl-1 has been associated with chemoresistance in AML cell lines and with poor clinical outcome of AML patients. Thus, inhibitors of anti-apoptotic Bcl-2 family proteins could be novel therapeutic agents. In this study, we investigated how clinically achievable concentrations of obatoclax, a pan-Bcl-2 inhibitor, potentiate the antileukemic activity of cytarabine in AML cells. MTT assays in AML cell lines and diagnostic blasts, as well as flow cytometry analyses in AML cell lines revealed synergistic antileukemic activity between cytarabine and obatoclax. Bax activation was detected in the combined, but not the individual, drug treatments. This was accompanied by significantly increased loss of mitochondrial membrane potential. Most importantly, in AML cells treated with the combination, enhanced early induction of DNA double-strand breaks (DSBs) preceded a decrease of Mcl-1 levels, nuclear translocation of Bcl-2, Bcl-xL, and Mcl-1, and apoptosis. These results indicate that obatoclax enhances cytarabine-induced apoptosis by enhancing DNA DSBs. This novel mechanism provides compelling evidence for the clinical use of BH3 mimetics in combination with DNA-damaging agents in AML and possibly a broader range of malignancies.

Panobinostat enhances cytarabine and daunorubicin sensitivities in AML cells through suppressing the expression of BRCA1, CHK1, and Rad51

Acute myeloid leukemia (AML) remains a challenging disease to treat and urgently requires new therapies to improve its treatment outcome. In this study, we investigated the molecular mechanisms underlying the cooperative antileukemic activities of panobinostat and cytarabine or daunorubicin (DNR) in AML cell lines and diagnostic blast samples in vitro and in vivo. Panobinostat suppressed expression of BRCA1, CHK1, and RAD51 in AML cells in a dose-dependent manner. Further, panobinostat significantly increased cytarabine- or DNR-induced DNA double-strand breaks and apoptosis, and abrogated S and/or G2/M cell cycle checkpoints. Analogous results were obtained by shRNA knockdown of BRCA1, CHK1, or RAD51. Cotreatment of NOD-SCID-IL2Rγ^null mice bearing AML xenografts with panobinostat and cytarabine significantly increased survival compared to either cytarabine or panobinostat treatment alone. Additional studies revealed that panobinostat suppressed the expression of BRCA1, CHK1, and RAD51 through downregulation of E2F1 transcription factor. Our results establish a novel mechanism underlying the cooperative antileukemic activities of these drug combinations in which panobinostat suppresses expression of BRCA1, CHK1, and RAD51 to enhance cytarabine and daunorubicin sensitivities in AML cells.

Novel therapies for children with acute myeloid leukaemia

Significant improvements in survival for children with acute myeloid leukaemia (AML) have been made over the past three decades, with overall survival rates now approximately 60-70%. However, these gains can be largely attributed to more intensive use of conventional cytotoxics made possible by advances in supportive care, and although over 90% of children achieve remission with frontline therapy, approximately one third in current protocols relapse. Furthermore, late effects of therapy cause significant morbidity for many survivors. Novel therapies are therefore desperately needed. Early-phase paediatric trials of several new agents such as clofarabine, sorafenib and gemtuzumab ozogamicin have shown encouraging results in recent years. Due to the relatively low incidence of AML in childhood, the success of paediatric early-phase clinical trials is largely dependent upon collaborative clinical trial design by international cooperative study groups. Successfully incorporating novel therapies into frontline therapy remains a challenge, but the potential for significant improvement in the duration and quality of survival for children with AML is high.

Synergistically killing activity of aspirin and histone deacetylase inhibitor valproic acid (VPA) on hepatocellular cancer cells

Aspirin and valproic acid (VPA) have been extensively studied for inducing various malignancies growth inhibition respectively, despite their severe side effects. Here, we developed a novel combination by aspirin and VPA on hepatocellular cancer cells (HCCs). The viability of HCC lines were analyzed by MTT assay, apoptotic analysis of HepG2 and SMMC-7721 cell was performed. Real time-PCR and Western blotting were performed to determine the expression of apoptosis related genes and proteins such as Survivin, Bcl-2/Bax, Cyclin D1 and p15. Moreover, orthotopic xenograft tumors were challenged in nude mice to establish murine model, and then therapeutic effect was analyzed after drug combination therapy. The viability of HCC lines' significantly decreased after drug combination treatment, and cancer cell apoptosis in combination group increasingly induced compared with single drug use. Therapeutic effect was significantly enhanced by combination therapy in tumor volume and tumor weight decrease. From the data shown here, aspirin and VPA combination have a synergistic killing effect on hepatocellular cancers cells proliferation and apoptosis.

Chidamide, a novel histone deacetylase inhibitor, synergistically enhances gemcitabine cytotoxicity in pancreatic cancer cells

Pancreatic cancer is a lethal human malignancy with an extremely poor prognosis and urgently requires new therapies. Histone deacetylase inhibitors (HDACIs) represent a new class of anticancer agents and have shown promising antitumor activities in preclinical models of pancreatic cancer. In this study, we sought to determine the antitumor effects of a novel HDACI, chidamide (CS055), in pancreatic cancer cells alone or in combination with gemcitabine. Treatments of BxPC-3 or PANC-1 pancreatic cancer cell lines with chidamide resulted in dose- and time-dependent growth arrest, accompanied by induction of p21 expression. When combined in a sequential schedule, chidamide synergistically enhanced gemcitabine-induced cell growth arrest and apoptosis, accompanied by cooperative downregulation of Mcl-1 and loss of mitochondrial membrane potential (ΔΨm). Chidamide enhanced gemcitabine-induced DNA double-strand breaks and S phase arrest, and abrogated the G2/M cell cycle checkpoint, potentially through suppression of CHK1 expression. Our results suggest that chidamide has a therapeutic potential for treating pancreatic cancer, especially in combination with gemcitabine.

Class I and class II histone deacetylases are potential therapeutic targets for treating pancreatic cancer

Background

Pancreatic cancer is a highly malignant disease with an extremely poor prognosis. Histone deacetylase inhibitors (HDACIs) have shown promising antitumor activities against preclinical models of pancreatic cancer, either alone or in combination with chemotherapeutic agents. In this study, we sought to identify clinically relevant histone deacetylases (HDACs) to guide the selection of HDAC inhibitors (HDACIs) tailored to the treatment of pancreatic cancer.

Methodology

HDAC expression in seven pancreatic cancer cell lines and normal human pancreatic ductal epithelial cells was determined by Western blotting. Antitumor interactions between class I- and class II-selective HDACIs were determined by MTT assays and standard isobologram/CompuSyn software analyses. The effects of HDACIs on cell death, apoptosis and cell cycle progression, and histone H4, alpha-tubulin, p21, and γH2AX levels were determined by colony formation assays, flow cytometry analysis, and Western blotting, respectively.

Results

The majority of classes I and II HDACs were detected in the pancreatic cancer cell lines, albeit at variable levels. Treatments with MGCD0103 (a class I-selective HDACI) resulted in dose-dependent growth arrest, cell death/apoptosis, and cell cycle arrest in G2/M phase, accompanied by induction of p21 and DNA double-strand breaks (DSBs). In contrast, MC1568 (a class IIa-selective HDACI) or Tubastatin A (a HDAC6-selective inhibitor) showed minimal effects. When combined simultaneously, MC1568 significantly enhanced MGCD0103-induced growth arrest, cell death/apoptosis, and G2/M cell cycle arrest, while Tubastatin A only synergistically enhanced MGCD0103-induced growth arrest. Although MC1568 or Tubastatin A alone had no obvious effects on DNA DSBs and p21 expression, their combination with MGCD0103 resulted in cooperative induction of p21 in the cells.

Conclusion

Our results suggest that classes I and II HDACs are potential therapeutic targets for treating pancreatic cancer. Accordingly, treating pancreatic cancer with pan-HDACIs may be more beneficial than class- or isoform-selective inhibitors.