Therapeutic strategies to enhance the anticancer efficacy of histone deacetylase inhibitors.

Histone acetylation is a posttranslational modification that plays a role in regulating gene expression. More recently, other nonhistone proteins have been identified to be acetylated which can regulate their function, stability, localization, or interaction with other molecules. Modulating acetylation with histone deacetylase inhibitors (HDACi) has been validated to have anticancer effects in preclinical and clinical cancer models. This has led to development and approval of the first HDACi, vorinostat, for the treatment of cutaneous T cell lymphoma. However, to date, targeting acetylation with HDACi as a monotherapy has shown modest activity against other cancers. To improve their efficacy, HDACi have been paired with other antitumor agents. Here, we discuss several combination therapies, highlighting various epigenetic drugs, ROS-generating agents, proteasome inhibitors, and DNA-damaging compounds that together may provide a therapeutic advantage over single-agent strategies.

Combinatorial effects of histone deacetylase inhibitors (HDACi), vorinostat and entinostat, and adaphostin are characterized by distinct redox alterations.

PURPOSE: Amongst the epigenetically targeted therapies, targeting of the histone deacetylases (HDACs) has yielded numerous drugs for clinical use in hematological malignancies, but none as yet for acute lymphocytic leukemia (ALL). Single agent activity of HDAC inhibitors (HDACi) has been elusive in ALL, and has prompted study of combinatorial strategies. Because several HDACi raise levels of intracellular oxidative stress, we evaluated combinations of two structurally distinct HDACi with the redox active compound adaphostin in ALL. METHODS: The HDACi vorinostat and entinostat were tested in combination with adaphostin in human ALL cell lines. DNA fragmentation, caspase activation, mitochondrial disruption and levels of  intracellular peroxides, superoxide and glutathione were measured in cells treated with the HDACi/adaphostin combinations. Antioxidant blockade of cell death induction and gene expression profiling of cells treated with vorinostat/adaphostin versus entinostat/adaphostin combinations were evaluated. RESULTS: Both combinations synergistically induced apoptotic DNA fragmentation, which was preceded by an increase in superoxide levels, a reduction in mitochondrial membrane potential, and an increase in caspase-9 activation. The antioxidant N-acetylcysteine (NAC) blocked superoxide generation and prevented reduction of mitochondrial membrane potential. NAC decreased DNA fragmentation and caspase activity in cells treated with adaphostin and vorinostat, but not in those treated with adaphostin and entinostat. Gene expression arrays revealed differential regulation of several redox genes prior to cell death induction. CONCLUSIONS: A redox modulatory agent, adaphostin, enhances efficacy of two HDACi, vorinostat or entinostat, but via different mechanisms indicating a point of divergence in the mechanisms of synergy between the two distinct HDACi and adaphostin.

Redox control of leukemia: from molecular mechanisms to therapeutic opportunities.

Reactive oxygen species (ROS) play both positive and negative roles in the proliferation and survival of a cell. This dual nature has been exploited by leukemia cells to promote growth, survival, and genomic instability-some of the hallmarks of the cancer phenotype. In addition to altered ROS levels, many antioxidants are dysregulated in leukemia cells. Together, the production of ROS and the expression and activity of antioxidant enzymes make up the primary redox control of leukemia cells. By manipulating this system, leukemia cells gain proliferative and survival advantages, even in the face of therapeutic insults. Standard treatment options have improved leukemia patient survival rates in recent years, although relapse and the development of resistance are persistent challenges. Therapies targeting the redox environment show promise for these cases. This review highlights the molecular mechanisms that control the redox milieu of leukemia cells. In particular, ROS production by the mitochondrial electron transport chain, NADPH oxidase, xanthine oxidoreductase, and cytochrome P450 will be addressed. Expression and activation of antioxidant enzymes such as superoxide dismutase, catalase, heme oxygenase, glutathione, thioredoxin, and peroxiredoxin are perturbed in leukemia cells, and the functional consequences of these molecular alterations will be described. Lastly, we delve into how these pathways can be potentially exploited therapeutically to improve treatment regimens and promote better outcomes for leukemia patients.

PCI-24781, a Novel Hydroxamic Acid HDAC Inhibitor, Exerts Cytotoxicity and Histone Alterations via Caspase-8 and FADD in Leukemia Cells.

Histone deacetylase inhibitors (HDACi) have become a promising new avenue for cancer therapy, and many are currently in Phase I/II clinical trials for various tumor types. In the present study, we show that apoptosis induction and histone alterations by PCI-24781, a novel hydroxamic acid-based HDAC inhibitor, require caspase-8 and the adaptor molecule, Fas-associated death domain (FADD), in acute leukemia cells. PCI-24781 treatment also causes an increase in superoxide levels, which has been reported for other HDACi. However, an antioxidant does not reverse histone alterations caused by PCI-24781, indicating that ROS generation is likely downstream of the effects that PCI-24781 exerts on histone H3. Taken together, these results provide insight into the mechanism of apoptosis induction by PCI-24781 in leukemia by highlighting the roles of caspase-8, FADD and increased superoxide levels.