Mechanisms underlying the cytotoxic activity of syn/anti-isomers of dinuclear Au(I) NHC complexes

The syn- and anti-isomers of dinuclear Au(I) complexes of the type Au₂(^RL^OH)(PF₆)₂ (R = isopropyl or mesityl) bearing 2-hydroxyethane-1,1-diyl-bridged bisimidazolylidene ligands were separated by reversed phase high performance liquid chromatography (HPLC) and characterized by NMR spectroscopy, elemental analysis, ESI mass spectrometry as well as single crystal X-ray diffraction analysis. Evaluation of the antiproliferative activity of the isolated isomers has shown very small difference in their cytotoxic behavior in various cancer cell lines. Additional counter-anion exchange (hexafluorophosphate to chloride) allows to increase the water solubility of ^synAu₂(^MesL^OH)(PF₆)₂ and leads to higher antiproliferative activity when compared to the hexafluorophosphate-complex. Both isomers were treated with L-cysteine as nucleophilic thiol source and only the anti-isomer shows dissociation of one bisimidazolylidene ligand after 24 h. In the case of the syn-isomer, density functional theory calculations indicate a lower reactivity due to the higher steric hindrance of the N-substituents and additional hydrogen bond interaction, which prevents a nucleophilic attack. When the N-substituent is replaced by the bulkier mesityl group, both conformations remain unreactive and result to be the most cytotoxic complexes in the above-mentioned cancer cell lines. Interestingly, ^synAu₂(^MesL^OH)(PF₆)₂ exhibits a high selectivity in the MCF-7 cell line with a selectivity index (SI) of 19, which is superior to auranofin (SI < 1), making this compound an ideal candidate for further studies. Preliminary mechanistic studies reveal that the cytotoxic complexes possess mitochondrial-TrxR inhibition properties in the nanomolar range. Additionally, the cellular distribution studies by ICP-MS and nuclear microscopy have shown that the compound accumulates in the membranes. These results suggest that the mitochondrial membrane is the main target for this type of dinuclear complexes, causing oxidative stress by inhibiting mitochondrial thioredoxin reductase.