Stronger Cytotoxicity for Cancer Cells Than for Fast Proliferating Human Stem Cells by Rationally Designed Dinuclear Complexes

Abstract

Cytostatic metallo-drugs mostly bind to the nucleobases of DNA. A new family of dinuclear transition metal complexes was rationally designed to selectively target the phosphate diesters of the DNA backbone by covalent bonding. The synthesis and characterization of the first dinuclear NiII 2 complex of this family are presented, and its DNA binding and interference with DNA synthesis in polymerase chain reaction (PCR) are investigated and compared to those of the analogous CuII 2 complex. The NiII 2 complex also binds to DNA but forms fewer intermolecular DNA cross-links, while it interferes with DNA synthesis in PCR at lower concentrations than CuII 2. To simulate possible competing phosphate-based ligands in vivo, these effects have been studied for both complexes with 100–200-fold excesses of phosphate and ATP, which provided no disturbance. The cytotoxicity of both complexes has been studied for human cancer cells and human stem cells with similar rates of proliferation. CuII 2 shows the lowest IC50 values and a remarkable preference for killing the cancer cells. Three different assays show that the CuII 2 complex induces apoptosis in cancer cells. These results are discussed to gain insight into the mechanisms of action and demonstrate the potential of this family of dinuclear complexes as anticancer drugs acting by a new binding target.

Evaluation of the binding mode of a cytotoxic dinuclear nickel complex to two neighboring phosphates of the DNA backbone

“…The interaction of the complexes [(Htom ){Cu(OAc)} 2 ] (OAc) and [(Htom ){Ni(OAc)} 2 ](OAc) with DNA was studied by agarose gel electrophoresis, PCR, DNA stretching experi-ments with magnetic and optical tweezers, and AFM experiments. 29,31,32 The careful analysis of these combined experimental data provided the following mechanism for the interaction with DNA: the coordinated acetates of the complexes [(Htom ){Cu(OAc)} 2 ](OAc) and [(Htom ){Ni(OAc)} 2 ] (OAc) are substituted by water upon dissolution in water and aqueous buffer resulting in the complexes [(Htom ){Cu (H 2 O)} 2 ] 3+ and [(Htom ){Ni(H 2 O) 2 } 2 ] 3+ , respectively. 32 The tricationic charge enhances the driving force for the electrostatic attraction by the polyanionic DNA and the two preorganized metal ions bind, as intended, to two neighboring phosphates of the DNA backbone.…”

“…29,31,32 The careful analysis of these combined experimental data provided the following mechanism for the interaction with DNA: the coordinated acetates of the complexes [(Htom ){Cu(OAc)} 2 ](OAc) and [(Htom ){Ni(OAc)} 2 ] (OAc) are substituted by water upon dissolution in water and aqueous buffer resulting in the complexes [(Htom ){Cu (H 2 O)} 2 ] 3+ and [(Htom ){Ni(H 2 O) 2 } 2 ] 3+ , respectively. 32 The tricationic charge enhances the driving force for the electrostatic attraction by the polyanionic DNA and the two preorganized metal ions bind, as intended, to two neighboring phosphates of the DNA backbone. The hydrophobic naphthalene units stick outward from the DNA into the aqueous solution, which results in π-π interactions between these naphthalene units belonging to complexes bound to the same DNA molecule (intramolecular cross-links) and to other DNA molecules (intermolecular cross-links) resulting in an oligomerization.…”

“…Therefore, in gel electrophoresis experiments, DNA treated with the complexes did not enter the gel by increasing the concentration of added complexes. 29,32 Cytotoxic tests using different cancer cells and human stem cell lines that have the same proliferation rates as the cancer cells provided the important result that the complexes are more efficient in killing cancer cells than human stem cells. 32 Although the cytotoxic properties of the two complexes [(Htom ){Cu(OAc)} 2 ](OAc) and [(Htom ){Ni(OAc)} 2 ](OAc) are very promising and all measurements performed support the intended binding mode to two neighboring phosphates of the DNA backbone, they cannot prove it.…”

“…29,32 Cytotoxic tests using different cancer cells and human stem cell lines that have the same proliferation rates as the cancer cells provided the important result that the complexes are more efficient in killing cancer cells than human stem cells. 32 Although the cytotoxic properties of the two complexes [(Htom ){Cu(OAc)} 2 ](OAc) and [(Htom ){Ni(OAc)} 2 ](OAc) are very promising and all measurements performed support the intended binding mode to two neighboring phosphates of the DNA backbone, they cannot prove it. To prove the principle binding ability of this family of dinuclear complexes to phosphate diesters, we thought to use simple phosphate diesters O 2 P(OR) 2 − as models for the phosphate diesters of the DNA backbone.…”

“…The combined experimental and computational results of this study provide strong evidence that two neighboring phosphates of the DNA backbone bind monodentately to the two Ni II ions of [(Htom 6-Me ){Ni(H 2 O) 2 } 2 ] 3+ supported hydrogen bonds between the coordinated water ligands and the phosphate diesters, which is in line with all biochemical and biophysical experiments described recently. 29,32 Experimental section…”