Triethylphosphinegold(I) Complexes with Secnidazole-Derived Thiosemicarbazones: Cytotoxic Activity against HCT-116 Colorectal Cancer Cells under Hypoxia Conditions

[Image: see text] Triethylphosphinegold(I) complexes [Au(HL1)P(CH(2)CH(3))(3)]PF(6) (1), [Au(HL2)P(CH(2)CH(3))(3)]PF(6) (2), and [Au(HL3)P(CH(2)CH(3))(3)]PF(6) (3) were obtained with (E)-2-(1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-ylidene)hydrazinecarbothioamide (HL1), (E)-N-methyl-2-(1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-ylidene)hydrazinecarbothioamide (HL2), and (E)-2-(1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-ylidene)-N-phenylhydrazinecarbothioamide (HL3). All compounds were assayed for their cytotoxic activities against HCT-116 colorectal carcinoma cells under normoxia and hypoxia conditions and against nonmalignant HEK-293 human embryonic kidney cells under normoxia conditions. The thiosemicarbazone ligands HL1-HL3 were inactive against HCT-116 cells under hypoxia but while HL3 was inactive, HL1 and HL2 proved to be cytotoxic to both cell lineages under normoxia conditions. Complexes (1–3) and the triethylphosphinegod(I) precursor proved to be active against both cell lineages in normoxia as well as in hypoxia. While 1 and 3 revealed to be active against HEK-293 and HCT-116 cells, being approximately as active against HCT-116 cells in normoxia as under hypoxia, complex (2) proved to be more active against HCT-116 cells under hypoxia than under normoxia conditions, and more active against HCT-116 cells than against the nonmalignant HEK-293 cells, with the selectivity index, calculated as SI = IC(50HEK-293)/IC(50HCT-116hypoxia), equal to 3.7, similar to the value obtained for the control drug tirapazamine (tirapazamine (TPZ), SI = 4). Although the compounds showed distinct cytotoxic activities, the electrochemical behaviors of HL1-HL3 were very similar, as were the behaviors of complexes (1–3). Complex (2) deserves special interest since it was significantly more active under hypoxia than under normoxia conditions. Hence, in this case, selective reduction of the nitro group in a low oxygen pressure environment, resulting in toxic reactive oxygen species (ROS) and damage to DNA or other biomolecules, might operate, while for the remaining compounds, other modes of action probably occur.