Design, synthesis and in vitro cell-based evaluation of the anti-cancer activities of hispolon analogs

Phytochemicals play an important role in cancer therapy. Hispolon and 26 of its analogs (9 known and 17 new) were synthesized and evaluated for their antiproliferative activities in a panel of six independent human cancer cell lines using the in vitro cell-based MTT assay. Among the hispolon analogs tested, compound VA-2, the most potent overall, produced its most significant effect in the colon cancer cell lines HCT-116 (IC₅₀ 1.4 ± 1.3 μM) and S1 (IC₅₀ 1.8 ± 0.9 μM) compared to its activity in the normal HEK293/pcDNA3.1 cell line (IC₅₀ 15.8 ± 3.7 μM; p <0.01 for each comparison). Based on our results, VA-2 was about 9- to 11-times more potent in colon cancer cells and 2- to 3-times more potent in prostate cancer cells compared to HEK293/pcDNA3.1 cells. Morphological analysis of VA-2 showed significant reduction of cell number, while the cells' sizes were also markedly increased and were obvious at 68 h of treatment with 1 μM in HCT-116 (colon) and PC-3 (prostate) cancer cells. A known analog, compound VA-4, prepared by simple modifications on the aromatic functional groups of hispolon, inhibited prostate and colon cancer cell lines with IC₅₀ values <10 μM. In addition, hispolon isoxazole and pyrazole analogs, VA-7 and VA-15 (known), respectively, have shown significant activity with the mean IC₅₀ values in the range 3.3-10.7 μM in all the cancer cell lines tested. Activity varied among the analogs in which aromatic functional groups and β-diketone functional groups are modified. But the activity of analogs VA-16 to VA-27 was completely lost when the side chain double-bond was hydrogenated indicating the crucial role of this functionality for anticancer activity. Furthermore, many of the compounds synthesized were not substrates for the ABCB1-transporter, the most common cause of multidrug resistance in anti-cancer drugs, suggesting they may be more effective anticancer agents.