The natural products trapoxin B and trichostatin A, as well as the novel marine natural product psammaplin A (PSMA) were found in a cell-based screen for compounds that induced the expression of the cyclin dependent kinase inhibitor p21waf1. The mechanism of p21waf1 induction for these compounds was via histone deacetylase (HDAC) inhibition. Of these compounds, PSMA was of interest because of its novel structure, but the physiological stability of it, and its analogs was poor. Thus, a directed medicinal chemistry effort was undertaken to prepare analogs of the simple HDAC inhibitor dimethylaminobenzamidylcaprylic hydroxamate (DBCH), which led to chromenone amide 9. This compound was efficacious in the HCT116 colon xenograft assay, but was difficult to formulate in pharmaceutically acceptable vehicles. In parallel with these efforts, a screen of the Novartis compound archive for novel HDAC inhibitors uncovered the cinnamyl hydroxamic acid NVP-LAK974. This compound had good enzyme and cellular potency, but poor efficacy in vivo. A systematic structural exploration of cinnamyl hydroxamates based on NVP-LAK974 was undertaken with the goal of finding a novel, well-tolerated and efficacious HDAC inhibitor. Several derivatives were found to be efficacious in the xenograft assay. Of those compounds, NVP-LAQ824 distinguished itself due to its tolerability, efficacy and potency. Based, in part, on these properties, NVP-LAQ824 is currently undergoing human clinical trials as a novel anti-cancer agent.