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Phytoalexins are substances with antimicrobial properties produced by plants after being attacked by microorganisms, especially phytopathogenic fungi and viruses. They are also currently being studied for their antitumor effect. We aimed to study the apoptosis-stimulating effect of homobrassinin and thiazino[6,5-b]indol in human ovarian adenocarcinoma A2780 and A2780cis cells via flow cytometric analysis of annexin V/PI, caspase 3 and 9 activity, cytochrome C release, and smac-diablo accumulation. Using the western blot technique, we also monitored the effect of both indoles on the response of heat shock proteins in these cells. Thiazino[6,5-b]indol showed more pronounced sensitizing and/or pro-apoptotic effect compared to homobrassinin accompanied by increased smac-diablo accumulation at earlier time intervals and pronounced externalization of phosphatidylserine at 72 h in A2780cis compared to A2780 cells. The apoptosis stimulating effect of thiazino[6,5-b]indol in A2780cis cells was associated with significant irreversible downregulation of HSP70 and HSP90 and partly with a decrease of HSP40. On the other hand, cisplatin-induced the apoptosis of sensitive A2780 cells with reversible downregulation of HSP40 and HSP57. In conclusion, the effect of thiazino[6,5-b]indol on resistant A2780cis cells could have a great utility in both the potential prevention and the treatment of other cisplatin-resistant tumor cells.

Acridine derivatives were first used as antibacterial and antiparasitic agents, later as antimalarial and anti-HIV drugs, and now as potentially anti-cancer agents due to their high cytotoxic activity. Due to their serious adverse effects, new synthetic derivatives were introduced and tested based on modified naturally occurring substances, such as acridone derivatives, which also exhibit potential anti-tumor activity. Most of them are DNA-damaging substances, causing relatively strong and selective destruction of tumor cells. We have tested in vitro anti-proliferative effects of newly-synthesized tetrahydroacrid derivatives, namely tacrine-coumarin hybrid molecules, on various human and mouse cancer cell lines. Our results showed that tacrine-coumarin hybrids with seven, eight, and nine methylene groups in spacer reduce proliferation of cancer cells. A hybrid with nine methylene groups had the most significant anti-cancer effect.