Synthesis, Structural Insights, and In Vitro Evaluation of Novel Silver Complexes Supported by Amantadine-Functionalized Bis(pyrazolyl)acetate Ligands as Anticancer Agents

Bis(pyrazol-1-yl)acetic acid (HC(pz)2COOH) and bis(3,5-dimethyl-pyrazol-1-yl)acetic acid (HC(pzMe2)2COOH) were conjugated with amantadine to yield the ligands LAd and L2Ad, respectively. These chelating ligands were used for the synthesis of silver complexes 1–5, whose electronic and molecular structures were investigated by X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. The coordination geometry of the noble metal ion was assessed by combining X-ray Absorption Spectroscopy (XAS) data analysis with Density Functional Theory (DFT) modeling. The structures of the ligand LAd and of the complex [Ag(L2Ad)(PPh3)]NO3·0.5CH3CN (3a) were determined by single-crystal X-ray diffraction analysis. The bis(pyrazolyl)acetamide ligand crystallizes in the I2/a space group and is similar to the 3,5-dimethyl analogue. The complex crystallizes in the P-1 space group and is the first reported example of a tetra-coordinated silver(I) complex incorporating a tridentate bis(pyrazolyl)acetate ligand. The cytotoxicity potential of the new phosphane Ag(I) complexes was tested on various human cancer cell lines, together with that of the two unfunctionalized analogs [Ag(L2OiPr)(PPh3)]NO3 (6) and [Ag(L2OiPr)(PTA)]NO3 (7), synthesized and studied for useful comparison. Notably, the novel complexes exhibited pronounced cytotoxic effects against the human-derived pancreatic cancer BxPC-3 cell line, both in 2D monolayers and in 3D spheroid models. Mechanistic studies revealed efficient intracellular accumulation of the complexes, likely favored by increased lipophilicity, and inhibition of thioredoxin reductase (TrxR), ultimately disrupting redox homeostasis and promoting oxidative stress, which are key contributors to their antiproliferative effects.