Class I histone deacetylase-selective novel synthetic inhibitors potently inhibit human tumor proliferation

We have developed previously a class of synthetic hybrid histone deacetylase (HDAC) inhibitors, which were built from hydroxamic acid of trichostatin A and pyridyl ring of MS-275. In this study we evaluated the antitumor effects of these novel hybrid synthetic HDAC inhibitors, SK-7041 and SK-7068, on human cancer cells. Both SK-7041 and SK-7068 effectively inhibited cellular HDAC activity at nanomolar concentrations and induced the time-dependent hyperacetylation of histones H3 and H4. These HDAC inhibitors preferentially inhibited the enzymatic activities of HDAC1 and HDAC2, as compared with the other HDAC isotypes, indicating that class I HDAC is the major target of SK-7041 and SK-7068. We found that these compounds exhibited potent antiproliferative activity against various human cancer cells in vitro. Growth inhibition effect of SK-7041 and SK-7068 was related with the induction of aberrant mitosis and apoptosis in human gastric cancer cells. Both compounds induced the accumulation of cells at mitosis after 6 h of treatment, which was demonstrated by accumulation of tetraploid cells, lack of G₂ cyclin/cyclin-dependent kinase inactivation, and higher mitotic index. After 12 h of treatment, apoptotic cells were increased through mitochondrial and caspase-mediated pathway. Finally, in vivo experiment showed that SK-7041 or SK-7068 was found to reduce the growth of implanted human tumors in nude mice. Therefore, based on isotype specificity and antitumor activity, SK-7041 and SK-7068 HDAC inhibitors are expected to be promising anticancer therapeutic agents and need additional clinical development.