Structural and functional effects of benzimidazole/thioether–copper complexes with antitumor activity on cell membranes and molecular models

•Benzimidazole/thioether–Cu(II) complexes have significant cytotoxic activity.•N-methylation of benzimidazole results in enhanced cytotoxicity.•Cu-L2Me high lipophilicity may account for its higher antiproliferative effect.•Both Cu(II) complexes induce in vitro change of shapes in human erythrocytes.

Two cytotoxic copper(II) complexes with N–H and N-methylated benzimidazole-derived ligands (Cu-L2 and Cu-L2Me) were synthesized and made to interact with human erythrocytes and molecular models of their plasmatic membranes. The latter consisted in lipid bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE), lipids of the types present in the outer and inner monolayers of the human erythrocyte membrane, respectively. Initial assessment of the interaction of the complexes with DMPC and DMPE consisted of X-ray diffraction studies, which showed preferential interactions with the former. Scanning electron microscopy (SEM) of erythrocytes incubated with solutions of the Cu(II) complexes evidenced deformation of the cells to stomatocytes and knizocytes by Cu-L2 and Cu-L2Me due to interactions with the inner and outer leaflets of the cell membranes, respectively. This was further confirmed by real-time observation of the dose-dependent effects of the complexes on live erythrocytes by defocusing microscopy (DM). The combined observations, including the increased antiproliferative activity of the N-methylated complex Cu-L2Me over that of Cu-L2 is rationalized based on the higher lipophilicity of the former. This property would facilitate passive diffusion of Cu-L2Me through the cell membrane, particularly in the initial stages when the DMPC-rich outer leaflet is involved. In contrast, the benzimidazole N–H groups of Cu-L2 may participate in hydrogen bonding with DMPE polar groups; this result is consistent with the formation of stomatocyte induced by the latter complex.

Graphical abstractBenzidimidazole/thioether–Cu(II) complexes interact with phospholipid bilayers as models of cell membranes, and also with the membranes of live erythrocytes. N-methylation of the benzimidazole moiety confers a greater lipophilicity that results in enhanced cytotoxic activity, likely via passive diffusion through cell membranes.Figure optionsDownload full-size imageDownload as PowerPoint slide