Transferrin Receptor-Mediated Cellular Uptake of Fluorinated Chlorido[N,N'-bis(salicylidene)-1,2-phenylenediamine]iron(III) Complexes.

Fluorinated chlorido[salophene]iron(III) complexes (salophene = N,N'-bis(salicylidene)-1,2-phenylenediamine) are promising anticancer agents. Apoptosis and necrosis induction have already been described as part of their mode of action. However, the involvement of ferroptosis in cell death induction, as confirmed for other chlorido[salophene]iron(III) complexes, has not yet been investigated. Furthermore, the mechanism of cellular uptake of these compounds is unknown. Therefore, the biological activity of the fluorescent chlorido[salophene]iron(III) complexes with a fluorine substituent at positions 3, 4, 5, or 6 at the salicylidene moieties (C1-C4) was evaluated in malignant and nonmalignant cell lines with focus on the involvement of the transferrin receptor-1 (TfR-1) in cellular uptake, the influence of the complexes on mitochondrial function, and the analysis of the molecular mechanism of cell death. All complexes significantly decreased the metabolic activity in the tested ovarian cancer (A2780, A2780cis), breast cancer (MDA-MB 231), and leukemia (HL-60) cell lines, while the nonmalignant human stroma cell line HS-5 at a concentration of 0.5 µM, which represents the IC50 of the complexes in most of the used tumorigenic cell lines, was not affected. The mitochondrial function was impaired, as evidenced by a reduced mitochondrial membrane potential ΔΨm and decreased mitochondrial activity. Besides apoptosis and necroptosis, ferroptosis was identified as part of the mode of action. It was further demonstrated for the first time that fluorinated chlorido[salophene]iron(III) complexes downregulate TfR-1 expression, comparable to ferristatin II, an iron transport inhibitor that acts via TfR-1 degradation. FerroOrange staining further indicated that the complexes strongly increased the intracellular iron(II) level as a driving force to induce ferroptosis. In conclusion, these fluorinated chlorido[salophene]iron(III) complexes are potent, tumor cell-specific chemotherapeutic agents, with the potential to treat various types of cancers.

Design, Synthesis, Electrochemical, and Biological Evaluation of Fluorescent Chlorido[N,N'-bis(methoxy/hydroxy)salicylidene-1,2-bis(4-methoxyphenyl)ethylenediamine]iron(III) Complexes as Anticancer Agents.

The impact of methoxy and hydroxyl groups at the salicylidene moiety of chlorido[N,N'-bis(methoxy/hydroxy)salicylidene-1,2-bis(4-methoxyphenyl)ethylenediamine]iron(III) complexes was evaluated on human MDA-MB 231 breast cancer and HL-60 leukemia cells. Methoxylated complexes (C1-C3) inhibited proliferation, migration, and metabolic activity in a concentration-dependent manner following the rank order: C2 > C3 > C1. In particular, C2 was highly cytotoxic with an IC50 of 4.2 µM which was 6.6-fold lower than that of cisplatin (IC50 of 27.9 µM). In contrast, hydroxylated complexes C4-C6 were almost inactive up to the highest concentration tested due to lack of cellular uptake. C2 caused a dual mode of cell death, ferroptosis, and necroptosis, whereby at higher concentrations, ferroptosis was the preferred form. Ferroptotic morphology and the presence of ferrous iron and lipid reactive oxygen species proved the involvement of ferroptosis. C2 was identified as a promising lead compound for the design of drug candidates inducing ferroptosis.