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).