Abstract PL06-01: Targeted inhibition of chromatin-modifying enzymes as cancer therapeutics: The HDAC inhibitor vorinostat and beyond
Authors: Richon Victoria M. 
Published: May 2019

Abstract
Histone deacetylase (HDAC) inhibitors represent a promising new class of anti-tumor agents. HDACs are enzymes that catalyze the removal of acetyl groups from the lysine residues of proteins, most notably the core nucleosomal histones. Four classes of HDACs have been identified, and include Class I human HDACs (HDAC1, HDAC2, HDAC3, and HDAC8). Class II HDACs are subdivided into Class IIa enzymes (HDAC4, HDAC5, HDAC7 and HDAC9) and Class IIb enzymes (HDAC6 and HDAC10). The third class of human HDACs or sirtiuins consists of homologues of yeast Sir2 (SIRT1–7). The catalytic site of this third HDAC class is not similar to the catalytic site of the Class I and Class II enzymes. HDAC11 is the sole member of the fourth HDAC class and shares homology with both the class I and class II HDACs. Defects in the enzymes regulating histone acetylation have been found in a variety of cancers. These defects can cause an imbalance in histone acetylation that may lead to changes in chromatin structure and transcriptional dysregulation of genes involved in the control of cell cycle progression, differentiation, and apoptosis. Histone acetyltransferases have been found to be mutated in a variety of cancer while HDACs have been shown to be overexpressed, aberrantly recruited to oncogenic transcription factors and mutated in cancer. HDACs thus represent potential targets for small molecule inhibitors. The substrates for HDACs
HDAC inhibitors are currently undergoing evaluation for the treatment of cancer and can be divided into four classes based on their structure. The classes include the short chain fatty acids (e.g., valproate, phenylbutyrate); hydroxamic acid derivatives (e.g., vorinostat (SAHA), belinostat (PXD-101), panobinostat (LBH-589), JNJ-26481585, PCI-24781); benzamides (e.g., entinostat (SNDX-275, MS-275); mocetinostat (MGCD0103)); and cyclic tetrapeptides (e.g., romidepsin (FK-228)). The HDAC inhibitors under clinical development for cancer do not inhibit the class III enzymes rather they inhibit, in general, either both class I and II enzymes (hydroxamate derivatives) or selectively inhibit class I enzymes (benzamides).
Vorinostat inhibits the enzymatic activity of Class I HDACs, (HDAC1, HDAC2, and HDAC3) and the Class II HDAC, HDAC6, at low nanomolar concentrations (IC50 &lt; 86 nM) (1–3). Vorinostat was originally identified as an inducer of differentiation of murine erythroleukemia (MEL) cells and has also been found to inhibit proliferation, induce apoptosis or autophagy of cultured transformed cell lines derived from lymphomas, leukemias, non small cell lung carcinomas, colon carcinomas, central nervous system tumors, melanomas, ovarian carcinomas, renal cell carcinomas, prostate cancers, and breast cancers rat prostate cancer cells.
The molecular mechanisms of vorinostat-induced growth arrest, differentiation, apoptosis and autophagy have yet to be fully clarified. The acetylation of nucleosomal histones plays an important role in the transcriptional regulation of gene expression. Hypoacetylation of histones is associated with a condensed chromatin structure and repression of gene transcription. Conversely, acetylated histones are associated with a more open chromatin structure and activation of transcription. Vorinostat may exert its antitumor effects through by transcriptional and non-transcriptional mechanisms. Expression profiling studies have revealed that vorinostat treatment can also lead to both activation and repression of gene expression. The vorinostat-mediated transcriptional repression may involve both direct and indirect effects resulting from histone acetylation, or alternately, may involve the hyperacetylation of non-histone proteins. Many nonhistone proteins, (e.g., tubulin, Hsp90 and p53) are known to be reversibly acetylated on lysine residues and undergo hyperacetylation following exposure to vorinostat. Acetylation of these proteins may also contribute to the antitumor activity of vorinostat. A more complete understanding of the spectrum of substrates for the HDAC enzymes and the role of acetylation of these substrates in cancer will aid in our understanding of how to best utilize HDAC inhibitors.
Based on the antitumor mechanism of action of vorinostat, studies evaluating the activity of vorinostat with other cancer therapies have been performed. These studies include the evaluation of vorinostat with radiation, kinase inhibitors, cytotoxic agents, and proteasome inhibitors. The studies have demonstrated synergistic or additive activity in a variety of cultured human transformed cell lines. Vorinostat has demonstrated anti-tumor activity in a variety of in vivo rodent tumor models whether administered by the intraperitoneal, intravenous, or oral route in both carcinogen induced tumor formation and established tumor models. Tumor growth inhibition is observed at doses of vorinostat that do not produce toxic side effects.
Vorinostat is under evaluation in clinical trials for the treatment of cancer and was the first HDAC inhibitor approved by the US FDA for the treatment of cutaneous manifestations in patients with cutaneous T-cell lymphoma (CTCL) who have progressive, persistent or recurrent disease on or following two systemic therapies in 2006. In the initial oral phase I vorinostat clinical trial, accumulation of acetylated histone H3 was observed in patient peripheral blood mononuclear cells at all dose levels and was detected in tumor biopsies following administration of vorinostat. Additionally, in a Phase II trial for recurrent glioblastoma patients, an increase in acetylation of histones was detected in surgical samples post treatment with vorinostat. These studies demonstrate that vorinostat is able to inhibit HDAC activity at tolerable doses in patients. The most common adverse experiences were diarrhea, fatigue, nausea, and anorexia and were mostly mild to moderate in severity. Ongoing biomarker studies are required to further define optimal dose and schedule and aid in identifying responding patient populations. Clinical activity has thus far been observed in patients with acute myeloid leukemia, multiple myeloma, non-Hodgkin's lymphoma, glioblastoma multiforme, and metastatic mesothelioma, as well as breast, laryngeal, nasopharyngeal, papillary thyroid, ovarian, and non-small cell lung cancer. Further investigation of vorinostat in phase II and III trials for patients with other hematologic malignancies and solid tumors as a single agent and in combination therapy is ongoing.
Citation Information: Mol Cancer Ther 2009;8(12 Suppl):PL06-01.