New 1,2,4-Oxadiazole Nortopsentin Derivatives with Cytotoxic Activity

New analogs of nortopsentin, a natural 2,4-bis(3'-indolyl)imidazole alkaloid, in which the central imidazole ring of the natural lead was replaced by a 1,2,4-oxadiazole moiety, and in which a 7-azaindole portion substituted the original indole moiety, were efficiently synthesized. Among all derivatives, prescreened against the HCT-116 colon rectal carcinoma cell line, the two most active compounds were selected and further investigated in different human tumor cells showing IC₅₀ values in the micromolar and submicromolar range. Flow cytometric analysis of propidium iodide-stained MCF-7 cells demonstrated that both the active derivatives caused cell cycle arrest in the G0⁻G1 phase. The cell death mechanism induced by the compounds was considered to be apoptotic by measuring the exposure of phosphatidylserine to the outer membrane and observed morphological evaluation using acridine orange/ethidium bromide double staining. Moreover, further tested on intestinal normal-like differentiated Caco-2 cell line, they exhibited preferential toxicity towards cancer cells.