Histone deacetylase (HDAC) inhibitors offer a promising strategy for cancer therapy, and the first generation HDAC inhibitors are currently in the clinic. Entirely novel ketone HDAC inhibitors have been developed from the cyclic tetrapeptide apicidin. These compounds show class I subtype selectivity and levels of cellular activity comparable to clinical candidates. A representative example has demonstrated tumor growth inhibition in a human colon HCT-116 carcinoma xenograft model comparable to known inhibitors.