Methyl-substituted 1,10-phenanthroline derivatives in copper(II) complexes enhance antitumor activity and inhibit NHE1 in MDA-MB-231 breast cancer cells

Three copper(II) complexes containing 1,10-phenanthroline derivatives, [CuCl2(phen)].0.5H2O (1), [CuCl2(neo)].0.75H2O (2), and [CuCl2(tmp)].H2O (3), were evaluated for their antitumor activity in MDA-MB-231 breast cancer cells. All complexes markedly reduced cell viability, exhibiting significantly lower IC values and higher selectivity indices than the corresponding free ligands, CuCl2, and cisplatin, highlighting the therapeutic advantage of metal complexation. Among them, only 1 induced DNA damage at sub-IC concentrations. Cytotoxicity in all complexes was associated with intracellular ROS generation and apoptosis, although 2 and 3 produced a stronger oxidative response. Complex 3 additionally promoted necrosis. Cell proliferation was inhibited in a concentration-dependent manner, accompanied by increased intracellular copper accumulation. Moreover, 2 and 3 significantly inhibited Na/H exchanger 1 (NHE1) activity, reducing cell migration and MMP-9 activity. Western blot analysis further demonstrated that all three complexes modulated the expression of NHE1, G protein-coupled estrogen receptor (GPER), and apoptosis-related proteins. Overall, our findings show that ligand methylation enhances the antitumor activity of copper(II) complexes and shifts their mechanism of action from DNA damage-driven cytotoxicity toward ROS-mediated apoptosis, while also improving inhibition of NHE1-dependent migratory pathways. These results identify NHE1 as a key molecular target underlying the enhanced antitumor activity of methylated phenanthroline copper(II) complexes.