The breast cancer stem cell (CSC) potency of a series of copper(II)-phenanthroline complexes containing the nonsteroidal anti-inflammatory drug (NSAID), indomethacin, is reported. The most effective copper(II) complex in this series, 4, selectivity kills breast CSC-enriched HMLER-shEcad cells over breast CSC-depleted HMLER cells. Furthermore, 4 reduces the formation, size, and viability of mammospheres, to a greater extent than salinomycin, a potassium ionophore known to selectively inhibit CSCs. Mechanistic studies revealed that the CSC-specificity observed for 4 arises from its ability to generate intracellular reactive oxygen species (ROS) and inhibit cyclooxygenase-2 (COX-2), an enzyme that is overexpressed in breast CSCs. The former induces DNA damage, activates JNK and p38 pathways, and leads to apoptosis.

We report the cancer stem cell (CSC) potency of a novel series of copper(ii)-phenanthroline complexes bearing nonsteriodial anti-inflammatory drugs: naproxen, tolfenamic acid, and indomethacin (2a-3c). Two of the complexes, 2a and 3c, kill breast CSC-enriched HMLER-shEcad cells (grown in both monolayer and three-dimensional cell cultures) to a significantly better extent than salinomycin, a well-established CSC toxin. The most potent complex in the series, 3c induces its cytotoxic effect by generating intracellular reactive oxygen species (ROS) and inhibiting cyclooxgenase-2 (COX-2) activity. Encapsulation of 3c using biodegradable methoxy poly(ethylene glycol)-b-poly(d,l-lactic-co-glycolic) acid (PEG-PLGA) copolymers at the appropriate feed (5%, 3c NP) enhances breast CSC uptake and reduces overall toxicity. The nanoparticle formulation, 3c NP selectively kills breast CSCs over bulk breast cancer cells, and evokes a similar cellular response to the payload, 3c. To the best of our knowledge, this is the first study to demonstrate that polymeric nanoparticles can be used to effectively deliver CSC-potent metal complexes into CSCs.

The cytotoxic properties of a series of nickel(II)-dithiocarbamate phenanthroline complexes is reported. The complexes 1-6 kill bulk cancer cells and cancer stem cells (CSCs) with micromolar potency. Two of the complexes, 2 and 6, kill twice as many breast cancer stem cell (CSC)-enriched HMLER-shEcad cells as compared to breast CSC-depleted HMLER cells. Complex 2 inhibits mammosphere formation to a similar extent as salinomycin (a CSC-specific toxin). Detailed mechanistic studies suggest that 2 induces CSC death by necroptosis, a programmed form of necrosis. Specifically, 2 triggers MLKL phosphorylation, oligomerization, and translocation to the cell membrane. Additionally, 2 induces necrosome-mediated propidium iodide (PI) uptake and mitochondrial membrane depolarisation, as well as morphological changes consistent with necroptotosis. Strikingly, 2 does not evoke necroptosis by intracellular reactive oxygen species (ROS) production or poly(ADP) ribose polymerase (PARP-1) activation.

Four copper(II) complexes, 1-4 containing regioisomeric vanillin Schiff base derivatives and the nonsteroidal anti-inflammatory drug (NSAID), naproxen, were synthesised and characterised. All complexes effectively cleave DNA in cell-free systems, with 4 displaying the highest nuclease activity. DNA binding studies suggest that 4 binds to DNA via the grooves prior to inducing oxidative DNA cleavage. Three of the complexes (1, 3, and 4) indiscriminately kill cancer stem cell (CSC)-enriched cells (HMLER-shEcad) and bulk cancer cells (HMLER) at micromolar concentrations. The most effective complex, 4 also reduced the formation and size of mammospheres to a similar extent as salinomycin, a well-established CSC-potent agent. Mechanistic studies show that 4 is readily taken up by CSCs, elevates intracellular reactive oxygen species (ROS) levels, causes DNA damage, and induces caspase-dependent apoptosis. Furthermore, 4 inhibits cyclooxygenase-2 (COX-2) expression and causes COX-2-dependent CSC death. The advantage of 4 over bulk cancer cell- or CSC-selective agents is that it has the potential to remove whole tumor populations (bulk cancer cells and CSCs) with a single dose.

We report the potency against cancer stem cells (CSCs) of a new cobalt(III)-cyclam complex (1) that bears the nonsteroidal anti-inflammatory drug, naproxen. The complex displays selective potency for breast CSC-enriched HMLER-shEcad cells over breast CSC-depleted HMLER cells. Additionally, it inhibited the formation of three-dimensional tumour-like mammospheres, and reduced their viability to a greater extent than clinically used breast cancer drugs (vinorelbine, cisplatin and paclitaxel). The anti-mammosphere potency of 1 was enhanced under hypoxia-mimicking conditions. Detailed mechanistic studies revealed that DNA damage and cyclooxygenase-2 (COX-2) inhibition contribute to the cytotoxic mechanism of 1. To the best of our knowledge, 1 is the first cobalt-containing compound to show selective potency for CSCs over bulk cancer cells.