The invention describes compounds that inhibit both HDAC and GSK3β (i.e., HDAC/GSK3β dual inhibitors). The invention further describes compositions containing these HDAC/GSK3β dual inhibitors, as well as methods and kits using these HDAC/GSK3β dual inhibitors to treat various medical conditions. The invention also provides methods and kits using a HDAC inhibitor and a GSK3β to treat various medical conditions, and compositions containing a HDAC inhibitor and a GSK3β. Medical conditions treatable with various embodiments of the invention include but are not limited to cancers and tumors.