Epigenetic modulators as therapeutic targets in prostate cancer
Pre-clinical activity of HDACi in prostate cancer
Several HDACi demonstrated encouraging results in pre-clinical phase studies, showing promise as candidates for future clinical trials.
Concerning the aliphatic acids family, exposure to sodium butyrate induced growth inhibition and increased differentiation and apoptosis of PC-3 and DU145 cells [188, 189]. Remarkably, treatment with sodium butyrate also induced H2B acetylation, and methylation on multiples lysine residues, as well as phosphorylation of Thr19, in DU145 cells [190]. Recently, this compound was shown to stimulate the morphological and molecular differentiation of LNCaP cells via inhibition of T-type Ca2+ channels [191]. Valproic acid (VPA) also reduced cell viability and induced apoptosis in vitro and was able to reduce tumor growth in xenograft models [192]. Moreover, this compound inhibited epithelial-mesenchymal transition (EMT) and invasion abilities of PC-3 cells by decreasing SMAD4 protein expression and upregulating the metastasis suppressor gene N-myc downstream regulated gene-1 (NDRG1), respectively [193, 194]. In a TRAMP model of PCa treated with VPA, decreased tumor growth and invasiveness correlated with the re-expression of CCND2, a frequently silenced gene in PCa [195]. Remarkably, this compound also induced AR and E-cadherin expression in PCa cell lines [196].
Among hydroxamic acids, vorinostat/SAHA demonstrated the ability to decrease PCa cell lines proliferation and to reduce tumor growth in vivo [197, 198]. Panobinostat also induced cell cycle arrest and DNA damage and reduced PCa tumor growth in vivo [199]. Moreover, the exposure of PCa cells to this compound lead to a decrease in AR levels and reversed resistance to hormone therapy in castration-resistant PCa cell lines [200]. Belinostat showed pronounced anti-tumor effects in androgen-responsive PCa cell lines increasing p21, p27, and p53 protein expression and leading to G2/M cell cycle arrest [201]. It also reduced the migration of PCa cells, increasing the expression of tissue inhibitor of metalloproteinase-1 (TIMP-1). Moreover, it decreased the expression of oncogenic proteins, such as mutant P53 and ERG. Notably, the cytotoxic activity of this compound was preferentially directed against tumor cells [202].
Concerning the cyclic peptides family, mice inoculated with the 22Rv1 cell line exposed to romidepsin not only experienced reduced metastasis formation but also induces a 61 % survival increase [203]. Largazole and 2-epi-largazole are potent class I-selective HDACi, purified from marine cyanobacteria, that decrease LNCaP and PC-3 cell viability [204].
The benzamide derivative MS-275 increased H3 acetylation, p21 protein expression, and induced growth arrest in LNCaP and PC-3 cells and apoptosis in DU145 cells. Moreover, MS-275 reduced tumor growth in xenograft mice [205], particularly when acting synergistically with radiation therapy [206]. This drug also lead to H3K4 methylation upregulation, inducing re-expression of tumor suppressor and cell differentiation genes [207].
Sulforaphane, an isothiocyanate isolated form broccoli, suppressed PCa tumor cell growth in male nude mice and significantly correlated with decreased HDAC activity in prostate tissue and mononuclear blood cells. Moreover, in human subjects, the consumption of BroccoSprouts (68 g) also inhibited HDAC activity in peripheral blood mononuclear cells [208]. Importantly, another study demonstrated that sulforaphane effects are selective, since it more potently induced cell cycle arrest apoptosis and acetylation of H3 at P21 promoter and inhibited HDAC activity in benign hyperplasia (BPH1) and cancer (LNCaP and PC-3) PCa cells than in the normal cell line PrEC [209]. It was also reported that this compound destabilizes AR by hyperacetylating HSP90, via restraining HDAC6, leading to AR proteasomal degradation [210]. Recently, it was shown that sulforaphane was able to decrease MYC expression, the activity of aldehyde dehydrogenase 1 (ALDH1), CD49f?+?fraction enrichment and the efficiency of sphere forming, all characteristics of PCa stem cells [211]. Phenethyl isothiocyanate (PEITC), another isothiocyanate, suppressed PCa progression in transgenic adenocarcinoma of mouse prostate mice by induction of autophagic cell death and overexpression of E-cadherin [212]. Another study demonstrated that PEITC suppressed androgen-responsive tumor growth in vivo, possibly by downregulation of integrin family proteins (?1, ?2, and ?6) and tumor platelet/endothelial cell adhesion molecule (PECAM-1/CD31) [213]. This compound also promoted apoptosis and cell cycle arrest and inhibited invasion and on in vitro and in vivo models of PCa [214–216]. Like sulforaphane, PEITC repressed AR transcription and expression [217].
New specific HDAC1 inhibitors designed and synthetized using click chemistry revealed anti-proliferative activity in DU145 cells at micromolar concentrations [218]. A specific inhibitor of HDAC6, N-hydroxy-4-(2-[(2-hydroxyethyl)(phenyl)amino]-2-oxoethyl)benzamide (HPOB) decreased viability of LNCaP cells without affecting cell death or causing DNA damage. Furthermore, this compound inhibited HDAC6 deacetylase activity but not its ubiquitin-binding activity and incremented the cell death effect of SAHA, etoposide, and doxorubicin [219]. A novel compound, 3-hydroxypyridin-2-thione (a non-hydroxamate chemotype), was able to reduce expression of HDAC6 and 8 and suppress viability of LNCaP cells. This might be due, in part, to induced hyperacetylation of Hsp90 that subsequently attenuates interactions of key proteins essential for LNCaP cells survival, such as AR [220]. New class II-selective hydroxamate inhibitors, that target HDAC4 and HDAC6, were effective in decreasing cell proliferation and inducing cell cycle arrest at G1 phase and nuclear histone acetylation of PC-3 and LNCaP cells [221]. Benzothiazole-containing analogues of vorinostat/SAHA compounds displayed not only anti-proliferative effects in PC-3 cells but it also reduced tumor growth in a PC-3 mouse xenograft with efficacy equivalent to vorinostat/SAHA [222].
Development of hybrid compounds that could modulate multiple targets with superior efficacy and fewer side effects than current single-target drugs is underway [133]. A new set of HDACi were generated to selectively accumulate in PCa cells. A non-steroidal anti-androgen scaffold based on cyanonilutamide was incorporated into a prototypical HDACi (vorinostat/SAHA) pharmacophore, creating an AR-HDACi which will first engage AR, selectively accumulate, and then released to engage HDACs. These compounds demonstrated improved inhibition of all HDACs’ activity compared to vorinostat/SAHA alone and were able to simultaneously antagonize AR. Moreover, they displayed anti-proliferative activity in AR-expressing cell lines [223]. Another hybrid compound that resulted from the combination of methotrexate and hydroxamate (methotrexate-caproic hydroxamic acid) reduced HDAC activity and decreased viability of PC-3 cells [224]. Additionally, a new drug, VPA–GFLG-iRGD, which conjugates VPA with a cell penetrating peptide (iRGD) and a lysosomally degradable tetrapeptide (–GlyPheLeuGly–, –GFLG–), induced a significant decrease in the proportion of DU145 cells in G2 phase with increased cytotoxicity. This might be related with RGB induced blockage of ???3 and ???5 integrin on DU145 cell surface [225]. Likewise, the synthesis of dual-acting histone deacetylase (vorinostat/SAHA) and topoisomerase II inhibitors (anthracycline daunorubicin) resulted in decreased proliferation of DU145 cells [226]. Recently, WJ35435, a hybrid vorinostat/SAHA and DACA (topoisomerase inhibitor) molecule with anti-HDAC activity, showed a more potent anti-cancer effect, inducing more potent cell cycle arrest, DNA damage and apoptosis, than either agent alone, in PC-3 and DU-145 cells. Furthermore, this compound revealed anti-tumor activity in vivo and, importantly, it did not affect benign prostate cells [227]. Recently, CUDC-101, which resulted from the incorporation of HDAC inhibitory functionality into the pharmacophore of epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor 2 (HER2)/NEU inhibitors [228], was able to reduce AR and AR-v7 expression, PCa cell proliferation in vitro and in vivo [229]. This compound is currently in phase I trial in solid tumors (NCT01702285).