Cytotoxic (salen)ruthenium(iii) anticancer complexes exhibit different modes of cell death directed by axial ligands

Abstract

Two novel series of (salen)ruthenium(iii) complexes bearing guanidine and amidine axial ligands were synthesized, characterized, and evaluated for anticancer activity. In vitro cytotoxicity tests demonstrate that these complexes are cytotoxic against various cancer cell lines and the leading complexes have remarkable cancer-cell selectivity. A detailed study of the guanidine complex 7 and the amidine complex 13 reveals two distinguished modes of action. Complex 7 weakly binds to DNA and induces DNA damage, cell cycle arrest, and typical apoptosis pathways in MCF-7 cells. In contrast, complex 13 induces paraptosis-like cell death hallmarked by massive vacuole formation, mitochondrial swelling, and ER stress, resulting in significant cytotoxicity against human breast cancer cells. Our results provide an extraordinary example of tuning the mechanism of action of (salen)ruthenium(iii) anticancer complexes by modifying the structure of the axial ligands.

Scheme 1. Preparation of (salen)ruthenium(iii) cyanamide and nitrile complexes.

Fig. 1. Synthesis of (A) Ru(iii) guanidine complexes 3–8 and (B) Ru(iii) amidine complexes 10–15. (C) ORTEP structures of the cations of complexes 3 and 15. Hydrogen atoms have been omitted for clarity.

Fig. 2. (A) Confocal images of Hoechst stained MCF-7 cells upon treatment with cisplatin, complex 7, or complex 13 at their IC₇₀ values for 24 h. (B) Western blot analysis of DNA damage marker γ-H2AX in MCF-7 cells upon treatment with complex 13, complex 7, or cisplatin at their IC₇₀ values for different periods of time.

Fig. 3. (A) Confocal images of MCF-7 cells treated with 3 μM complex 13 for 4 h with or without the pretreatment of inhibitors. (B) Morphology of mitochondria in MCF-7 cells upon treatment with 0.5 μM complex 7, or 0.1 μM complex 13. (C) Western blot analysis of ER stress marker GRp78 in MCF-7 cells upon treatment with complex 13, complex 7, or cisplatin at their IC₇₀ values for different periods of time.

Fig. 4. Proposed modes of action of complexes 7 and 13. Complex 7 leads to an apoptosis pathway, whereas complex 13 induces paraptosis-like cell death.