Gavinostat is an orally bioavailable hydroxymate inhibitor of histone deacetylase (HDAC) with potential anti-inflammatory, anti-angiogenic, and antineoplastic activities. Gavinostat inhibits class I and class II HDACs, resulting in an accumulation of highly acetylated histones, followed by the induction of chromatin remodeling and an altered pattern of gene expression. At low, nonapoptotic concentrations, this agent inhibits the production of pro-inflammatory cytokines such as tumor necrosis factor- (TNF-), interleukin-1 (IL-1), IL-6 and interferon-gamma. It is currently in phase 2 trials for Myeloproliferative disorders, Polycythaemia vera and Phase III for Duchenne muscular dystrophy announced. In clinical trials of givinostat as a salvage therapy for advanced Hodgkin's lymphoma, the most common adverse reactions were fatigue, mild diarrhea or abdominal pain, moderate thrombocytopenia, and mild leukopenia.

Vorinostat (rINN) or suberoylanilide hydroxamic acid (SAHA), is a drug currently under investigation for the treatment of cutaneous T cell lymphoma (CTCL). Vorinostat inhibits the enzymatic activity of histone deacetylases HDAC1, HDAC2 and HDAC3 (Class I) and HDAC6 (Class II) at nanomolar concentrations (IC50< 86 nM). These enzymes catalyze the removal of acetyl groups from the lysine residues of histones proteins. In some cancer cells, there is an overexpression of HDACs, or an aberrant recruitment of HDACs to oncogenic transcription factors causing hypoacetylation of core nucleosomal histones. By inhibiting histone deacetylase, vorinostat causes the accumulation of acetylated histones and induces cell cycle arrest and/or apoptosis of some transformed cells. The mechanism of the antineoplastic effect of vorinostat has not been fully characterized. Vorinostat is used for the treatment of cutaneous manifestations in patients with cutaneous T-cell lymphoma who have progressive, persistent or recurrent disease on or following two systemic therapies. Vorinostat is marketed under the name Zolinza by Merck for the treatment of cutaneous manifestations in patients with cutaneous T cell lymphoma (CTCL) when the disease persists, gets worse, or comes back during or after two systemic therapies.

Dacinostat (also known as LAQ824), is a hydroxamate histone deacetylase inhibitor with potential anticancer activity. Dacinostat inhibits histone deacetylase enzymatic activities in vitro and transcriptionally activated the p21 promoter in reporter gene assays. Tumor cells treated with Dacinostat caused acetylation of HSP90 and degradation of its cargo oncoproteins. Flow cytometry studies revealed that both tumor cell lines and normal diploid fibroblasts arrested in the G2/M phase of the cell cycle after Dacinostat treatment. However, an increased sub-G1 population at 48 h (reminiscent of apoptotic cells) was only observed in the cancer cell lines treated with Dacinostat. Dacinostat exhibited antitumor effects in a xenograft animal models. In phase I trials, Dacinostat was well tolerated at doses that induced accumulation of histone acetylation, with higher doses inducing changes consistent with HSP90 inhibition. In another phase 1 in patients with advanced solid tumors, grade 3 or 4 toxicities were observed. Dacinostat had been in phase II clinical trials by Novartis for the treatment of solid tumors but further studies were discontinued.

Indoximod is an orally available Indoleamine 2,3-dioxigenase inhibitor. It shows higher potency in reversing IDO-mediated T cell suppression. Indoximod improves the efficacy of multiple chemotherapeutics agents and some immunological checkpoints mediators in Phase I/II clinical studies for metastatic breast cancer, metastatic melanoma, non-small cell lung cancer, primary malignant brain tumors, metastatic pancreatic cancer, as well as metastatic prostate cancer.

Ricolinostat is a selective inhibitor of HDAC6 with IC50 value of 4.7 nM. Ricolinostat demonstrated good anti-proliferative activity on different cell lines and clinical models of cancer. The drug is being tested in phase I/II for the treatment of multiple myeloma and lymphoid malignancies and in phase I in patients with breast cancer, gynecological cancer, cholangiocarcinoma, recurrent chronic lymphoid leukemia.

Trichostatin A (TSA) was originally isolated as an antifungal antibiotic along with its fermentation congeners trichostatin B ((TSA)3-Fe) and the D-glucopyranosides trichostatin C and D. TSA inhibits HDAC in the low nanomolar range and is an inducer of histone hyperacetylation, both in vitro and in vivo. It inhibits all class I and II deacetylases to a similar extent in both tumor and non-tumor cells, although HDAC4 is slightly resistant when compared with HDAC1 and HDAC6. Class III HDAC is not affected by TSA. It has been shown that TSA dosedependently inhibits growth and induces apoptosis in a plethora of carcinoma cell lines in vitro. Recently, it was also found that TSA inhibits angiogenesis, which is important for the growth and metastasis of solid tumors, both in vivo and in vitro. In HT-29 colon carcinoma cells, a single dose of TSA induced transient hyperacetylation of histone H4 resulting in the induction of p21WAF1/Cip1 and inhibition of cellular proliferation at both the G1 and G2 phases of the cell cycle. Growth inhibition was associated with decreased cyclin D1 mRNA and cdk6 protein levels and increased cyclin D3 protein and p21WAF1/Cip1 mRNA levels. Cyclin D1 protein, cyclin D3 mRNA, cdk2 and cdk4 remained unaffected. In addition, TSA induced apoptosis by upregulating the expression of the pro-apoptotic genes ID1, ID2 and ID3, whereas the expression of the anti-apoptotic genes BclxL and Hsp27 was decreased In vivo, TSA induces differentiation and shows chemotherapeutic activity against N-methylnitrosureainduced rat mammary cancer without toxic side effects. TSA may also have therapeutic potential for the treatment of a variety of genetic and infectious diseases since silenced, transduced genes are reactivated probably due to structural changes of the chromatin on integrated viral sequences.

HDAC-IN-2 (also known as citarinostat or ACY-241) was developed as a selective histone deacetylase (HDAC) 6 inhibitor with potential antineoplastic activity. Inhibition of HDAC leads to the inhibition of tumor oncogene transcription, and the selective transcription of tumor suppressor genes, which inhibit tumor cell division and induce tumor cell apoptosis. HDAC-IN-2 participates in phase 1b clinical trial in patients with multiple myeloma to determine the maximum tolerated dose (MTD) and evaluate the safety and preliminary antitumor activity. Besides, HDAC-IN-2 in combination with paclitaxel participates in phase Ib in patients with advanced solid tumors. In addition, HDAC-IN-2 in combination with nivolumab participates in phase I in patients with unresectable non-small cell lung cancer to determine the safety, tolerability, dose-limiting toxicities (DLTs), and maximum tolerated dose (MTD) of the drug.

Abexinostat (PCI-24781) is a novel, second-generation phenyl hydroxamic acid–based, orally bioavailable HDAC inhibitor that has previously been shown to have activity in vitro and in vivo against a broad array of cancers, including hematopoietic malignancies and bone and soft-tissue sarcomas. Abexinostat is a pan-HDAC inhibitor mostly targeting HDAC1 with Ki of 7 nM, modest potent to HDACs 2, 3, 6, and 10 and greater than 40-fold selectivity against HDAC8. Abexinostat exhibits potent antitumor activity against a variety of tumor cell lines with GI50 ranging from 0.15 uM to 3.09 uM. PCI-24781 also has an antiproliferative effect on HUVEC endothelial cells with GI50 of 0.43 uM. Abexinostat treatment causes dose-dependent accumulation of both acetylated histones and acetylated tubulin in HCT116 or DLD-1 cells, induces expression of p21, and leads to PARP cleavage and accumulation of the γH2AX. It has also shown good tolerability and activity in Phase I and II clinical trials against lymphoma, as well as against solid tumors in Phase-I trials. Additionally, it acts as a potent radiosensitizing agent and is synergistic with cytotoxic chemotherapy, such as doxorubicin in preclinical models.

Pyroxamide is a potent histone deacetylase (HDAC) inhibitor. Pyroxamide induced terminal differentiation in murine erythroleukemia (MEL) cells and inhibited the growth by cell cycle arrest or apoptosis in a variety of tumor cells. An accumulation of acetylated histones and increased levels of p21/WAF1 expression were detected in cancer cells and in prostate xenografts treated with Pyroxamide.