Cancers classified as multidrug-resistant (MDR) are a family of diseases with poor prognosis despite access to increasingly sophisticated treatments. Several mechanisms explain these resistances involving both tumor cells and their microenvironment. It is now recognized that a multi-targeting approach offers a promising strategy to treat these MDR tumors. Inhibition of thioredoxin reductase (TrxR), a key enzyme in maintaining redox balance in cells, is a well-identified target for this approach. Auranofin was the first inorganic gold complex to be described as a powerful inhibitor of TrxR. In this review, we will first recall the main results obtained with this metallodrug. Then, we will focus on organometallic complexes reported as TrxR inhibitors. These include gold(I), gold(III) complexes and metallocifens, i.e., organometallic complexes of Fe and Os derived from tamoxifen. In these families of complexes, similarities and differences in the molecular mechanisms of TrxR inhibition will be highlighted. Finally, the possible relationship between TrxR inhibition and cytotoxicity will be discussed and put into perspective with their mode of action.

The [ferrocene-ene-phenol] motif has been identified as the pharmacophore responsible for the anticancer activity of the family of ferrocene-based molecules coined ferrocifens, owing to its unique redox properties. The addition of imide entities to the historical ferrociphenol scaffold tremendously enhanced the cytotoxic activity of a large panel of cancer cell cultures and preliminary studies showed that the reduction of one of the carbonyl groups of the imide groups to the corresponding α-hydroxylactams only slightly affected the antiproliferative activity. As a continuation to these studies, we took advantage of the facile conversion of α-hydroxylactams to highly electrophilic N-acyliminium ions to graft various substituents to the imide motif of phthalimido ferrocidiphenol. Cell viability studies showed that the newly synthesized compounds showed diverse cytotoxic activities on two breast cancer cell lines, while only one compound was significantly less active on the non-tumorigenic cell line hTERT-RPE1.

The family of ferrocifens initially built up from the anti‐oestrogen tamoxifen shows a broad antitumor activity bothin vitroandin vivo. Their mechanism of action relies on the presence of the redox motif [ferrocene‐ene‐phenol] that, under oxidative conditions, generates reactive oxygen species (ROS) and affords electrophilic quinone methides (QMs) having the ability to alkylate biological nucleophiles and in turn elicit a strong antiproliferative activity. In this context, the cysteine protease cathepsin B was initially presumed to be a target for ferrocenyl QMs.In vitroenzymatic assays ruled out this hypothesis but unexpectedly revealed that other ferrocifen metabolites, i. e. ferrocenyl indenes, acted as moderate inhibitors of cathepsin B. These experimental results were nicely confirmed by molecular docking calculations, that showed that the monophenol ferrocenyl indene and to a lower extent the diphenol interacted with the active site of cathepsin B, making it an unanticipated target of ferrocifens.