Purpose: Hydroxytyrosol (3,4-DHPEA) derivatives (disulfide, thioacetate and thiohydroxytyrosol) were synthesized in order to test in vitro if the combination of catechol moiety of 3,4-DHPEA and sulfur containing functions results in an improvement of the pro-apoptotic and anti-proliferative activities shown by 3,4-DHPEA. The involvement of H2O2 production in the cell culture medium has been studied.

Methods: The effects of thiohydroxytyrosol derivatives and 3,4-DHPEA on cell proliferation, apotosis and cell cycle of HL60 and its MDR variant HL60R were assessed by the Trypan Blue exclusion test, by fluorescence microscopy or by flow cytometry respectively. H2O2 concentrations in the culture medium was measured by the ferrous ion oxidation-xylenol orange method.

Results: We found that: i) all synthesized compounds were able to inhibit the proliferation inducing apoptosis on both cell lines HL60 and HL60R; ii) all thiohydroxytyrosol derivatives were more effective than 3,4-DHPEA in inducing apoptosis on HL60R; iii) differently from 3,4-DHPEA, the proapoptotic activities of thiohydroxytyrosol derivatives were not dependent upon the release of H2O2 in the culture medium; iv) the hydroxytyrosol disulfide was the most active pro-apoptotic and anti-proliferative compound on both HL60 and HL60R cells.

Conclusions: The combination of cathecol moiety and sulfur functions resulted in an improvement of 3,4-DHPEA proapoptotic activity which was particularly evident on HL60R cells suggesting that these compounds could be potentially used in cancer therapy and could be able to reverse the resistance toward the most common anticancer drugs.