Table 1.

Inhibitory plant activities of clinically used, naturally occurring compounds or their precursors.

Anticancer Drug	Anticancer Activity	Plant Inhibition Activity
Paclitaxel	Paclitaxel stabilized microtubes in cancer cells and arrested the replication of cancer cells [16].	Paclitaxel arrested onion and maize root cells from dividing by stabilizing microtubules [72,73].
Vinblastine	Vinblastine destabilized microtubules in cancer cells and arrested replication [74].	Vinblastine bound to microtubules and created abnormal multipolar division in Allium cepa L. [73].
Podophyllotoxin, a precursor of etoposide and teniposide	Podophyllotoxin inhibited microtubule organization in cancer cells [75]. Etoposide killed cancer cells by inhibiting topoisomerase II (TopoII) [76].	Podophyllotoxin inhibited onion (Allium cepa L.) root growth by affecting the formation of mitotic microtubular organizing centers [77]. Etoposide inhibited the division of polyploid cells of grass pea (Lathyrus sativus L.) seedlings. The presumed binding regions of etoposide to TopoII were conserved in plants, Drosophila melanogaster, yeast, and humans [78].
Camptothecin, a precursor to irinotecan and topotecan	Camptothecin killed cancer cells by inhibiting topoisomerase 1 [16].	Camptothecin selectively caused the inhibition of young developing vascular tissues of the axillary buds of Nicotiana tabacum L. Camptothecin inhibited the sprouting of potatoes by interfering with cell division in the meristem [79]. Early reports showed this using a partially purified enzyme from barley seeds, and strong inhibition of the relaxation of supercoiled pBR322 DNA by the barley DNA enzyme was observed with camptothecin [79]. Later work showed that plants contained a conserved Topo1, and camptothecin-producing plants, including Camptotheca acuminata, Ophiorrhiza pumila, and Ophiorrhiza liukiuensis, had point mutations in Topo1 that conferred resistance to autotoxicity [80].
Homoharringtonine	Homoharringtonine was used for tyrosine kinase inhibitor-resistant chronic myelogenous leukemia (CML). It worked by binding to the A-site of the 80S ribosome and inhibiting translation [81].	Harringtonine alkaloids, which are related to homoharringtonine, had plant growth regulating activity [79]. The 80S ribosome was conserved across species [82].
Maytansine, a precursor to trastuzumab-emtansine.	Maytansine bound to β-tubulin and blocked the formation of longitudinal tubulin interactions in microtubules [83].	Maytansine inhibited growth in tobacco callus (Nicotiana tabacum L.) and rice seedling bioassays [79].
Ellipticine, a precursor to elliptinium	Elliptinium is approved in France for the treatment of metastatic breast cancer. Elliptinium and ellipticine inhibited topoisomerase II [84].	Ellipticine potently inhibited mungbean hypocotyls (Chen, Witham). Ellipticine has been postulated to bind to the same regions of TopoII as etoposide [85]. These regions were conserved in plants, Drosophila melanogaster, yeast, and humans [78].