Ligand Design Controls Biomolecule Binding and Cytotoxicity in Platinum(II) Complexes with ONS-Type Tridentate Ligands

Abstract

Two types of ONS-tridentate ligands were obtained by condensation of salicylaldehyde derivatives with 2-(methylthio)-aniline (L1 – L6) or 2-(methylthio)ethylamine (L8 and L9) while N-phenyl-o-diaminobenzene was used to create NNO-tridentate ligand (L7) for systematic studies. These ligands were used to generate nine platinum complexes (Pt1 – Pt9). Single-crystal X-ray diffraction revealed that the O^N^S complexes adopt a conserved, slightly distorted square-planar geometry, largely unaffected by electronic variation of aryl substituents; N^N^O containing complexes have more distorted square planar structures. All the complexes exhibited good solution stability; P1–Pt6 behaved predominantly as neutral molecules in DMSO while P7-Pt9 showed higher tendencies for solvolysis. Lipophilicity (log D) was strongly ligand-dependent and increased with alkoxy substitution and halogenation. DNA-binding studies demonstrated strong, static interactions for the rigid, aromatic ONS-containing complexes, with intercalative or groove-assisted binding confirmed by viscosity measurements. Bromo- and ethoxy-substituted complexes showed the highest DNA affinity, whereas flexible ONS-analogues displayed weak binding. All complexes interacted favorably with bovine serum albumin through a static binding mechanism, suggesting possible interactions relevant to transport and distribution in biological media. Cytotoxicity evaluation against HepG2, HCT-116, and MCF-7 cancer cell lines revealed pronounced structure- and cell line-dependent cytotoxicity. Pt2 and Pt4 with their rigid ONS-containing ligands exhibited high potency and superior selectivity indices compared with cisplatin. Cell-cycle analysis showed that some complexes induce G2/M arrest, while the most active compounds (Pt2 and Pt4) promote apoptotic progression. Annexin V-FITC/PI assays confirmed apoptosis as the dominant mode of cell death, with Pt2 and Pt4 driving rapid late-stage apoptosis. The gene expression data highlights that the most effective complexes (Pt2 and Pt4) successfully combine proliferation arrest with a dominant pro-apoptotic signal. Overall, this work demonstrates that incorporation of a rigid, planar ONS-chelate represents a promising design strategy for developing platinum anticancer agents with enhanced selectivity and favorable biological performance.