Search

Despite the advances in treatment strategies, cancer is still the second leading cause of death in the USA. A majority of the currently used cancer drugs have limitations in their clinical use due to poor selectivity, toxic side effects and multiple drug resistance, warranting the development of new anticancer drugs of different mechanisms of action. Here we describe the design, synthesis and initial biological evaluation of a new class of antimitotic agents that modulate tubulin polymerization. Structurally, these compounds are chalcone mimics containing a 1-(1H-imidazol-2-yl)ethan-1-one moiety, which was initially introduced to act as a metal-binding group and inhibit histone deacetylase enzymes. Although several analogues selectively inhibited purified HDAC8 with IC(50) values in low micromolar range, tissue culture studies suggest that HDAC inhibition is not a major mechanism responsible for cytotoxicity. The compounds demonstrated cell growth inhibition with GI(50) values of upper nanomolar to low micromolar potency with significant selectively for cancer over normal cells. Interestingly, several compounds arrested HeLaM cells in mitosis and seem to target tubulin to cause mitotic arrest. For example, when combined with inhibitors of Aurora B kinase, they led to dramatic disassembly of the mitotic spindle. In-vitro tubulin polymerization studies showed that the compounds reduced the rate of polymerization of microtubules during the elongation phase and lowered the amount of polymerized tubulin during the plateau phase. Finally, in silico docking studies identified binding of IPE-7 to the colchicine site with similar affinity as the test compound D64131. These compounds represent a new antimitotic pharmacophore with limited HDAC inhibitory activity.

Histone deacetylase (HDAC) enzymes govern the post-translational acetylation state of lysine residues on protein substrates, leading to regulatory changes in cell function. Due to their role in cancers, HDAC proteins have emerged as promising targets for cancer treatment. Four HDAC inhibitors have been approved as anti-cancer therapeutics, including SAHA (Suberoylanilide hydroxamic acid, Vorinostat, Zolinza). SAHA is a nonselective HDAC inhibitor that targets most of the eleven HDAC isoforms. The nonselectivity of SAHA might account for its clinical side effects, but certainly limits its use as a chemical tool to study cancer-related HDAC cell biology. Herein, the nonselective HDAC inhibitor SAHA was modified at the C4 position of the linker to explore activity and selectivity. Several C4-modified SAHA analogs exhibited dual HDAC6/8 selectivity. Interestingly, (R)-C4-benzyl SAHA displayed 520- to 1300-fold selectivity for HDAC6 and HDAC8 over HDAC1, 2, and 3, with IC50 values of 48 and 27 nM with HDAC6 and 8, respectively. In cellulo testing of the inhibitors was consistent with the observed in vitro selectivity. Docking studies provided a structural rationale for selectivity. The C4-SAHA analogs represent useful chemical tools to understand the role of HDAC6 and HDAC8 in cancer biology and exciting lead compounds for targeting of both HDAC6 and HDAC8 in various cancers.

Epigenetic regulation of gene expression without changing DNA sequences is a promising strategy in developing therapeutic agents for human diseases, especially cancer. Histone deacetylases (HDACs) are a family of enzymes involved in epigenetic modulation of gene expression by chromatin remodeling. Deacetylation of the lysine side chains of histones by HDAC proteins renders DNA transcriptionally inactive, resulting in the inhibition of expression of tumor suppressor genes, leading to tumorigenesis and tumor progression. Therefore, inhibiting HDAC enzymes has become an attractive strategy to modulate gene expression as a strategy for developing anticancer drugs. There are currently four FDA-approved cancer drugs in clinical use, and many more are in different stages of clinical trials. However, their clinical utility is limited due to undesirable side effects, mainly attributed to their lack of selectivity and the presence of a hydroxamic acid moiety as the metal-binding group. There are eighteen different isoforms of HDACs belonging to four classes that have been identified in humans. A major challenge in HDAC inhibitor development is how to make them selective for these HDAC isoforms and classes. We report the design and synthesis of new classes of HDAC inhibitors and the evaluation of their anticancer activity and selectivity.