The Anti-Breast Cancer Stem Cell Potency of Copper(I)-Non-Steroidal Anti-Inflammatory Drug Complexes

Abstract

Cancer stem cells (CSCs) are thought to be partly responsible for metastasis and cancer relapse. Currently, there are no effective therapeutic options that can remove CSCs at clinically safe doses. Here, we report the synthesis, characterisation, and anti-breast CSC properties of a series of copper(I) complexes, comprising of non-steroidal anti-inflammatory drugs (NSAIDs) and triphenylphosphine ligands (1-3). The copper(I) complexes are able to reduce the viability of breast CSCs grown in two- and three-dimensional cultures at micromolar concentrations. The potency of the copper(I) complexes towards breast CSCs was similar to salinomycin (an established anti-breast CSC agent) and cisplatin (a clinically used metallopharmaceutical). Cell-based studies showed that the copper(I) complexes are readily, and similarly, internalised by breast CSCs. The copper(I) complexes significantly increase the intracellular reactive oxygen species (ROS) levels in breast CSCs, and their ROS generation profile with respect to time is dependent on the NSAID component present. The generation of intracellular ROS by the copper(I) complexes could be part of the underlying mechanism by which they evoke breast CSC death. As far as we are aware, this is the first study to explore the anti-breast CSC properties of copper(I) complexes.

Keywords: cancer stem cells; copper(I) complexes; metallopharmaceuticals; non-steroidal anti-inflammatory drugs; reactive oxygen species.

Scheme 1

Reaction scheme for the preparation of copper(I) complexes containing triphenylphosphine ligands and diclofenac, naproxen, or salicylate moieties (1–3).

Figure 1

X-ray structures of the copper(I) complexes (A) 2 and (B) 3, comprising triphenylphosphine and naproxen or salicylate, respectively. Ellipsoids are shown at 50% probability. C atoms are in grey, P in purple, O in red, and Cu in blue. H atoms and disorder components have been omitted for clarity.

Figure 2

Copper content (ng of Cu/ 10⁶ cells) in HMLER-shEcad cells treated with 1–3 (5 µM for 24 h).

Figure 3

(A) Quantification of mammosphere formation with HMLER-shEcad cells, untreated, and treated with 1–3, cisplatin, or salinomycin at 0.5 µM for 5 days. Error bars = SD. (B) Representative bright-field images (×10) of the mammospheres, in the absence and presence of 1–3, at 0.5 µM for 5 days.

Figure 4

Normalised ROS activity in untreated HMLER-shEcad cells (control) and HMLER-shEcad cells treated with 3 (2 × IC₅₀ value, 0.5–24 h).