Design, Synthesis and Biological Evaluation of C(6)-Modified Celastrol Derivatives as Potential Antitumor Agents

New six C6-celastrol derivatives were designed, synthesized, and evaluated for their in vitro cytotoxic activities against nine human cancer cell lines (BGC-823, H4, Bel7402, H522, Colo 205, HepG2 and MDA-MB-468). The results showed that most of the compounds displayed potent inhibition against BGC823, H4, and Bel7402, with IC50s of 1.84–0.39 μM. The best compound NST001A was tested in an in vivo antitumor assay on nude mice bearing Colo 205 xenografts, and showed significant inhibition of tumor growth at low concentrations. Therefore, celastrol C-6 derivatives are potential drug candidates for treating cancer.