Pyrazole-Infused Metal Complexes: Anticancer Activity, DNA Cleavage, and Biophysical Insights into DNA/BSA Interactions

A pyrazole-based ligand bearing dimethyl substituents was used in the synthesis of Co(II), Cu(II), and Zn(II) complexes, specifically [CoCl₂(dmp)₂] (1), [CuCl₂(dmp)₂]₂ (2), and [ZnCl₂(dmp)₂] (3), where dmp = 3,5-dimethylpyrazole. The ligand dmp and complexes 1–3 were precisely characterized through various spectroscopic and analytical techniques, i.e., FTIR, UV–vis, and ESI-HRMS. The molecular structures of all complexes were confirmed by single-crystal X-ray diffraction, except for the ligand dmp, which was additionally characterized by ¹H NMR spectroscopy. UV–vis and fluorescence studies revealed that these complexes strongly bind with bovine serum albumin and calf thymus DNA. The DNA cleavage activity of complexes 1–3 and ligand dmp was assessed using gel electrophoresis. Concentration-dependent studies revealed that the compounds act as self-activating chemical nucleases, inducing hydrolytic DNA cleavage at varying concentrations, with complex 2 exhibiting a particularly promising DNA cleavage activity. It was observed that complex 2 significantly enhances DNA cleavage at a very low concentration of 15 μM in the presence of activators, namely ascorbate and H₂O₂. However, the contribution of the GSH activator toward the DNA cleavage was found to be extremely negligible. Further, the use of reactive oxygen species (ROS) scavengers, such as hydroxyl radical scavenger, singlet oxygen scavenger, and superoxide scavenger, resulted in a significant decrease in the DNA cleavage efficiency. These suggest that the diffusible species, including hydroxyl radical (HO^•), singlet oxygen (¹O₂), and superoxide (O₂^•–), play a major role in the oxidative cleavage of DNA. The cytotoxicities of dmp and its complexes were investigated using the MTT assay, with the evaluation of their IC₅₀ values for the MCF-7 and MDA-MB-231 breast cancer cell lines, as well as a normal epithelial HEK-293 cell line. Furthermore, flow cytometry analysis using Annexin V-FITC/PI double staining revealed that the apoptotic effects of these potent compounds on MDA-MB-231 cells are primarily mediated through the late apoptotic pathway.