Antitumor Activity of Natural Phenanthroindolizines and Daphnane Diterpenes

Natural products have played major roles in drug discovery programs including development of anticancer agents. In our continuing efforts to identify and characterize antitumor agents from natural products, we found the extract of the root of Cynanchum paniculatum (Asclepiadaceae) exhibited potent anti-proliferative effects in cultured human cancer cells. Bioassay-guided fractionation further led to the isolation of the bioactive constituent and was elucidated as antofine, a phenanthroindolizidine alkaloid. Antofine markedly inhibited the growth of several human cancer cells with the IC₅₀ values in the nanomolar range. In view of the potential anti-proliferative effects against cancer cells we developed the procedure of total synthesis of antofine and further investigated the anti-proliferative mechanisms and antitumor effects. In HCT 116 human colon cancer cells, the plausible anti-proliferative mechanisms of antofine might be associated with the down-regulation of cyclin D1, cyclin E, and CDK4 expression, the inhibition of DNA synthesis, the modulation of Wnt signaling activation, and the induction of differentiation. Antofine also exhibited the potential growth inhibition of paclitaxel-resistant human lung cancer cells (A549-PA) along with the down-regulation of P-glycoprotein expression. Antofine also effectively suppressed tumor growth in the tumor xenograft model. In addition, daphnane-type diterpenoids were isolated from the flower of Daphne genkwa (Thymelaceae) with a potential anti-proliferative activity against human lung cancer cells. Yuanhuadine, one of the most potent isolates from D. genkwa, showed potent growth inhibition and a relatively strong selectivity against human lung cancer cells compared to a panel of several cancer cell lines. In A549 human lung cancer cells, yuanhuadine induced cell cycle arrest at either G0/G1 or G2/M phase depending on incubation time. The cell cycle arrest was well correlated with the expression of checkpoint proteins including the upregulation of cyclin-dependent kinase inhibitor p21, and down-regulation of cyclin B1, cyclin dependent kinases and c-myc. Yuanhuadine also inhibited the phosphorylation of Akt, the mammalian target of rapamycin (mTOR) and its downstream effector molecules, p70 S6 kinase 1 and eukaryotic initiation factor 4E binding protein 1, indicating that the induction of cell cycle arrest by yuanhuadine might be in part mediated by the suppression of Akt/mTOR signaling pathway. Taken together, these findings suggest that antofine and yuanhuadine might provide potential lead candidates for the development of cancer chemotherapeutic agents derived from natural products.