5. Camptothecin
Camptothecin (CPT), an anticancer drug produced from the roots ofOphiorrhiza prostrate (Fig. 10 ), is a member of the monoterpenoid family. Its synthesis involves some crucial steps, which are not yet fully elucidated. The main precursors for its synthesis are tryptamine and secologanin, which are synthesized from the shikimate pathway and MEP pathway, respectively. The intermediate strictosidine is produced from these two precursors in the presence of strictosidine synthase. After a series of enzymatic reactions involving the formation of multiple intermediates, camptothecin is formed from strictosamide in the plant (Cui et al., 2015; Yamazaki et al., 2004) (Fig. 8) . Camptothecin inhibits the action of DNA topoisomerase I by binding to it and block DNA re-ligation, which results in DNA damage. Three analogs of CPT, topotecan, irinotecan, and belotecan have received approval from the FDA for clinical treatment of several types of cancer (Liu et al., 2015b).