ABSTRACT
This chapter focuses on naphthoquinone-contained terrestrial plants that had been employed for cancer treatment, the major cytotoxic naphthoquinones from their extracts, the antitumor activities, and the working mechanism of the naphthoquinones. The naturally occurring naphthoquinones are a class of phenolic compounds with a conjugated diketone moiety on their naphthalene skeletons. Naphthazarin is a hydroxylated naphthoquinone with the symmetric structural scaffold and also intramolecular hydrogen bonds. The enantiomeric natural naphthoquinone products shikonin and alkannin exhibited striking antitumor activity and also attracted considerable interest from both academia and industry. Despite excellent anticancer potential, juglone as a naphthoquinone was also reported to exert some toxic effects to normal tissues including acute irritant contact dermatitis. Mechanistic investigations indicated that one of the potential pathways through which juglone exerted its anticancer effects was the formation of the juglone semiquinone radical and subsequent production of superoxide anion radical in the mitochondria as well as in the cytoplasm.
