Synthesis and Biological Activity of 2‑Chloro-8-methoxy-5-methyl‑5H‑indolo [2,3‑b] Quinoline for the Treatment of Colorectal Cancer by Modulating PI3K/AKT/mTOR Pathways

Developing novel drugs from natural products has proven to be a very effective strategy. Neocryptolepine was isolated from Cryptolepis sanguinolenta, a traditional endemic African herb, which exerts a wide range of biological activities such as antimalaria, antibacterial, and antitumor. 2-Chloro-8-methoxy-5-methyl-5H-indolo [2,3-b] quinoline (compound 49) was synthesized, and its cytotoxicity was assessed on pancreatic cancer PANC-1 cells, colorectal cancer HCT116 cells, liver cancer SMMC-7721 cells, and gastric cancer AGS cells in vitro. The results of the in vitro assay showed that compound 49 exerted remarkable cytotoxicity on colorectal cancer HCT116 and Caco-2 cells. The cytotoxicity of compound 49 to colorectal cancer HCT116 cells was 17 times higher than that of neocryptolepine and to human normal intestinal epithelial HIEC cells was significantly reduced. Compound 49 exhibited significant cytotoxicity against the colorectal cancer HCT116 and Caco-2 cells, with IC₅₀ of 0.35 and 0.54 μM, respectively. The mechanism of cytotoxicity of compound 49 to colorectal cancer HCT116 and Caco-2 cells was further investigated. The results showed that compound 49 could inhibit colony formation and cell migration. Moreover, compound 49 could arrest the cell cycle at the G2/M phase, promote the production of reactive oxygen species, reduce mitochondrial membrane potential, and induce apoptosis. The results of Western blot indicated that compound 49 showed cytotoxicity on HCT116 and Caco-2 cells by modulating the PI3K/AKT/mTOR signaling pathway. In conclusion, these results suggested that compound 49 may be a potentially promising lead compound for the treatment of colorectal cancer.