Pro-Oxidant Activity of Amine-Pyridine-Based Iron Complexes Efficiently Kills Cancer and Cancer Stem-Like Cells

Abstract

Differential redox homeostasis in normal and malignant cells suggests that pro-oxidant-induced upregulation of cellular reactive oxygen species (ROS) should selectively target cancer cells without compromising the viability of untransformed cells. Consequently, a pro-oxidant deviation well-tolerated by nonmalignant cells might rapidly reach a cell-death threshold in malignant cells already at a high setpoint of constitutive oxidative stress. To test this hypothesis, we took advantage of a selected number of amine-pyridine-based Fe(II) complexes that operate as efficient and robust oxidation catalysts of organic substrates upon reaction with peroxides. Five of these Fe(II)-complexes and the corresponding aminopyridine ligands were selected to evaluate their anticancer properties. We found that the iron complexes failed to display any relevant activity, while the corresponding ligands exhibited significant antiproliferative activity. Among the ligands, none of which were hemolytic, compounds 1, 2 and 5 were cytotoxic in the low micromolar range against a panel of molecularly diverse human cancer cell lines. Importantly, the cytotoxic activity profile of some compounds remained unaltered in epithelial-to-mesenchymal (EMT)-induced stable populations of cancer stem-like cells, which acquired resistance to the well-known ROS inducer doxorubicin. Compounds 1, 2 and 5 inhibited the clonogenicity of cancer cells and induced apoptotic cell death accompanied by caspase 3/7 activation. Flow cytometry analyses indicated that ligands were strong inducers of oxidative stress, leading to a 7-fold increase in intracellular ROS levels. ROS induction was associated with their ability to bind intracellular iron and generate active coordination complexes inside of cells. In contrast, extracellular complexation of iron inhibited the activity of the ligands. Iron complexes showed a high proficiency to cleave DNA through oxidative-dependent mechanisms, suggesting a likely mechanism of cytotoxicity. In summary, we report that, upon chelation of intracellular iron, the pro-oxidant activity of amine-pyrimidine-based iron complexes efficiently kills cancer and cancer stem-like cells, thus providing functional evidence for an efficient family of redox-directed anti-cancer metallodrugs.

Delivering aminopyridine ligands into cancer cells through conjugation to the cell-penetrating peptide BP16

“…The analysis of these results revealed that the activity of the metal binding peptides was consistent with that observed for the ligands alone. 45 Thus, BP16 conjugates incorporating the (S,S′)-BPBP ligand were more active than the sequences bearing the Me2 PyTACN metal binding moiety. However, the conjugation of these ligands to BP16 decreased their activity.…”

“…Moreover, biological analysis of Me2 PyTACN and (S,S′)-BPBP ligands have shown their ability to chelate intracellular labile Fe(II) in cancer cells and that the resulting redox-active complexes induce apoptosis through pro-oxidant mechanisms. 45 In this work we envisioned that the conjugation of these ligands to a CPP could facilitate their uptake into cancer cells increasing their activity. Recently, we described the undecapeptide BP16 as a CPP able to favor the translocation of biologically active compounds across the cell membrane.…”

The Next Generation of Anticancer Metallopharmaceuticals: Cancer Stem Cell‐Active Inorganics

“…Pyrrolidine–pyridine ligands 6 and 7 (Scheme ), which have previously been shown to enhance ROS (peroxide) activation reactions upon binding to iron(II), display equipotency towards CSC‐enriched and CSC‐depleted breast cancer populations in vitro . In contrast, related aminopyridine ligand 8 (Scheme ), which is also capable of binding iron(II) and inducing ROS, displays preferential toxicity toward CSC‐depleted cells .…”

“…[19] In contrast, related aminopyridine ligand 8 (Scheme 3), which is also capable of binding iron(II) and inducing ROS,d isplays preferential toxicity toward CSC-depleted cells. [19] As the cytotoxicity of 6 and 7 is augmented upon pre-treatment with iron(II) chloride in the breast and pancreatic adenocarcinoma cell lines (MCF-7 and CAPAN-1, respectively), 6-a nd 7-induced cell death is proposed to result from their ability to bind intracellular iron(II) ions and generate active coordination iron(II) complexes inside cells. Furthers tudies in non-CSC lines showedt hat in the presence of an excess amount of iron(II) chloride, 6 and 7 induce high levels of intracellular ROS leading to caspase-dependenta poptosis.…”

A Cancer Stem Cell Potent Cobalt(III)–Cyclam Complex Bearing Two Tolfenamic Acid Moieties

“…The development of CSC-potent agents is in its infancy, and most of the drug candidates undergoing preclinical or clinical trials are completely organic in nature [7]. Our group and others have recently shown that metal complexes are capable of potently and selectively killing CSCs (over bulk cancer cells) [23][24][25][26][27][28].…”