Phase Ib Study of the Histone Deacetylase 6 Inhibitor Citarinostat in Combination With Paclitaxel in Patients With Advanced Solid Tumors

BackgroundCitarinostat (CC-96241; previously ACY-241), an oral inhibitor of histone deacetylases (HDACs) with selectivity for HDAC6, has demonstrated synergistic anticancer activity with paclitaxel in multiple solid tumor models. Combination therapy using citarinostat with paclitaxel was evaluated in this phase Ib 3 + 3 dose-escalation study in patients with advanced solid tumors.MethodsPatients with previously treated advanced solid tumors received citarinostat 180, 360, or 480 mg once daily on days 1 to 21 plus paclitaxel 80 mg/m2 on days 1, 8, and 15 of 28-day cycles until disease progression or unacceptable toxicity. The primary endpoint was determination of the maximum tolerated dose (MTD). Secondary endpoints included safety, antitumor activity, pharmacokinetics, and pharmacodynamics.ResultsTwenty patients were enrolled and received study treatment; 15 had received prior taxane therapy. No dose-limiting toxicities were reported at any dose; therefore, the MTD was not identified. Citarinostat 360 vs 480 mg was associated with reduced incidence and severity of neutropenia. Three patients experienced a confirmed partial response and 13 achieved stable disease. Pharmacokinetic parameters were linear up to citarinostat 360 mg, the dose at which the highest levels of histone and tubulin acetylation were observed in peripheral blood mononuclear cells.ConclusionsThe combination of citarinostat plus paclitaxel showed an acceptable safety profile, with no unexpected or dose-limiting toxicities and potential evidence of antitumor activity in patients with heavily pretreated advanced solid tumors. Citarinostat 360 mg once daily is considered the recommended phase II dose for use in combination with paclitaxel 80 mg/m2 every 3 of 4 weeks. This trial is registered on ClinicalTrials.gov (NCT02551185).

Transactive response DNA-binding Protein (TDP-43) regulates early HIV-1 entry and infection.

The transactive response DNA-binding protein (TDP-43) is an important regulator of mRNA, being reported to stabilize the anti-HIV factor, histone deacetylase 6 (HDAC6). However, little is known about the role of TDP-43 in HIV infection. In this work, we seek for the TDP-43 function on regulating CD4+ T cell permissibility to HIV infection. We observed that over-expression of wt-TDP-43 in CD4+ T cells stabilized HDAC6, increasing mRNA and the protein levels of this antiviral enzyme. Under this experimental condition, HIV-1 infection was impaired, independently of the viral envelope glycoprotein (Env) complex tropism. The results obtained by using an HIV-1 Env-mediated cell-to-cell fusion model, under the same experimental conditions, suggest that the increase in TDP-43 levels negatively affects the viral Env fusion capacity. Moreover, the specific siRNA silencing of endogenous TDP-43 in target cells lead to a significant decrease in the levels of HDAC6 which consistently induces an increase in the fusogenic and infection activities of the HIV-1 Env. These observations were confirmed by using primary viral Envs from HIV+ individuals with different clinical phenotypes. An increase in the level of expression of wt-TDP-43 strongly reduced the Envs infection activity of viremic non-progressors (VNP) and rapid progressors (RP) HIV+ individuals down to the levels of the inefficient HIV-1 Envs from long-term non-progressor elite controllers (LTNP-EC) individuals. On the contrary, low levels of endogenous TDP-43, obtained after specific siRNA-TDP-43 knocking-down, significantly favors the infection activity of primary HIV-1 Envs of VNP and RP individuals, leading to an increase in the infection ability of the primary HIV-1/LTNP-EC Envs. Based on this evidence, we interpret that TDP-43 conditions cell permissibility to HIV infection by affecting viral Env fusion and infection capacities, at least by altering the cellular levels of the antiviral enzyme HDAC6.

Arabidopsis SUMO E3 Ligase SIZ1 Interacts with HDA6 and Negatively Regulates HDA6 Function during Flowering

The changes in histone acetylation mediated by histone deacetylases (HDAC) play a crucial role in plant development and response to environmental changes. Mammalian HDACs are regulated by post-translational modifications (PTM), such as phosphorylation, acetylation, ubiquitination and small ubiquitin-like modifier (SUMO) modification (SUMOylation), which affect enzymatic activity and transcriptional repression. Whether PTMs of plant HDACs alter their functions are largely unknown. In this study, we demonstrated that the Arabidopsis SUMO E3 ligase SAP AND MIZ1 DOMAIN-CONTAINING LIGASE1 (SIZ1) interacts with HISTONE DEACETYLASE 6 (HDA6) both in vitro and in vivo. Biochemical analyses indicated that HDA6 is not modified by SUMO1. Overexpression of HDA6 in siz1-3 background results in a decreased level of histone H3 acetylation, indicating that the activity of HDA6 is increased in siz1-3 plants. Chromatin immunoprecipitation (ChIP) assays showed that SIZ1 represses HDA6 binding to its target genes FLOWERING LOCUS C (FLC) and MADS AFFECTING FLOWERING 4 (MAF4), resulting in the upregulation of FLC and MAF4 by increasing the level of histone H3 acetylation. Together, these findings indicate that the Arabidopsis SUMO E3 ligase SIZ1 interacts with HDA6 and negatively regulates HDA6 function.

Inhibition of Metastatic Uveal Melanoma Cell Proliferation by the Histone Deacetylase 6 Inhibitor, Ricolinostat, is Linked to Altered Microphthalmia-associated Transcription Factor and Phospho-ERK Expression

Metastatic uveal melanoma (MUM) is characterized by poor patient survival. Unfortunately, current treatment options demonstrate limited benefits. In this study, we evaluate the efficacy of ACY-1215, a histone deacetylase 6 inhibitor (HDAC6i), to attenuate MUM cell growth in vitro and in vivo, and elucidate the underlying molecular mechanisms. Treatment of OMM2.5 MUM cells with ACY-1215 resulted in a significant (p = 0.0001), dose-dependent reduction in cell survival and proliferation in vitro, and in vivo regression of primary OMM2.5 xenografts in zebrafish larvae. Furthermore, flow cytometry analysis revealed that ACY-1215 significantly arrested the OMM2.5 cell cycle in S phase (p = 0.0006) following 24 hours of treatment and significant apoptosis was triggered in a time- and dose-dependent manner (p = <0.0001). Additionally, ACY-1215 treatment resulted in a significant reduction in OMM2.5 p-ERK expression levels. Through proteome-profiling, attenuation of the microphthalmia-associated transcription factor (MITF) signaling pathway was linked to the observed anti-cancer effects of ACY-1215. In agreement, pharmacological inhibition of MITF signaling with ML329, significantly reduced OMM2.5 cell survival and viability in vitro (p = 0.0001) and in vivo (p = 0.0006). Our findings provide evidence that ACY-1215 and ML329 are efficacious against growth and survival of MUM cells and are potential therapeutic options for MUM.

The Role of HDAC6 in Autophagy and NLRP3 Inflammasome

Autophagy fights against harmful stimuli and degrades cytosolic macromolecules, organelles, and intracellular pathogens. Autophagy dysfunction is associated with many diseases, including infectious and inflammatory diseases. Recent studies have identified the critical role of the NACHT, LRR, and PYD domain-containing protein 3 (NLRP3) inflammasomes activation in the innate immune system, which mediates the secretion of proinflammatory cytokines IL-1β/IL-18 and cleaves Gasdermin D to induce pyroptosis in response to pathogenic and sterile stimuli. Accumulating evidence has highlighted the crosstalk between autophagy and NLRP3 inflammasome in multifaceted ways to influence host defense and inflammation. However, the underlying mechanisms require further clarification. Histone deacetylase 6 (HDAC6) is a class IIb deacetylase among the 18 mammalian HDACs, which mainly localizes in the cytoplasm. It is involved in two functional deacetylase domains and a ubiquitin-binding zinc finger domain (ZnF-BUZ). Due to its unique structure, HDAC6 regulates various physiological processes, including autophagy and NLRP3 inflammasome, and may play a role in the crosstalk between them. In this review, we provide insight into the mechanisms by which HDAC6 regulates autophagy and NLRP3 inflammasome and we explored the possibility and challenges of HDAC6 in the crosstalk between autophagy and NLRP3 inflammasome. Finally, we discuss HDAC6 inhibitors as a potential therapeutic approach targeting either autophagy or NLRP3 inflammasome as an anti-inflammatory strategy, although further clarification is required regarding their crosstalk.