Synthesis and Biological Evaluation of Benzothiazole Derivatives Bearing the ortho-Hydroxy-N-acylhydrazone Moiety as Potent Antitumor Agents

A novel series of benzothiazole derivatives bearing the ortho-hydroxy-N-acylhydrazone moiety were designed, synthesized, and evaluated for their procaspase-3 kinase activation activities and antiproliferative activities against five cancer cell lines (NCI-H226, SK-N-SH, HT29, MKN-45, and MDA-MB-231). Most target compounds showed moderate to excellent cytotoxic activity against all five tested cancer lines. The most promising compound 18e (procaspase-3 EC50 = 0.31 mu M) with IC50 values ranging from 0.24 to 0.92 mu M against all tested cell lines was 4.24-12.2 times more active than PAC-1 (procaspase-3 EC50 = 0.41 mu M). Structure-activity relationship studies indicated that the phenyl group on the 2-hydroxyphenyl ring (moiety A) was critical for pharmacological activity in vitro. In addition, introduction of a benzyloxyl group on moiety A and a mono-electron-withdrawing group at the 4-position of the benzyloxyl group were more favorable for antitumor activity. Moreover, reduction of the electron density in the phenyl ring of the benzyloxy group led to a dramatic decrease in the procaspase-3 kinase activation activity.