HDAC9 and breast cancer: In this study, compound 5a, of which the HDAC inhibitory activity (Fig. 2), but not cytotoxicity has been previously examined [59], showed strong cytotoxicity against human colon cancer SW620, breast cancer MCF7, prostate cancer PC3, pancreas cancer AsPC1 and lung cancer NCI-H460 cells (IC50 0.70, 1.80, 0.88, 2.71, 1.07 μM, respectively).