Synthesis, characterisation and biological evaluation of monometallic Re(I) and heterobimetallic Re(I)/Fe(II) complexes with a 1,2,3-triazolyl pyridine chelating moiety.

Bioorganometallic complexes have attracted considerable interest and have shown promise for potential application in the treatment and diagnosis of cancer, as well as bioimaging agents, some acting as theranostic agents. The series of novel ferrocene, benzimidazo[1,2-a]quinoline and fluorescein derivatives with bidentate pyridyl-1,2,3-triazole and 2,2'-dipyridylamine and their tricarbonylrhenium(I) complexes was prepared and fully characterised by NMR, single-crystal X-ray diffraction, UV-Vis and fluorescence spectroscopy in biorelevant conditions. The fluorescein and benzimidazo[1,2-a]quinoline ligands and their complexes with Re(I) showed interactions with ds-DNA/RNA and HSA, characterised by thermal denaturation measurements, fluorimetric and circular dichroism titrations. The binding constants revealed that addition of Re(I) increases the affinity of fluorescein but decreases the affinity of benzimidazo[1,2-a]quinoline. The complexation of Re(I) had the opposite effect on fluorescein and benzimidazo[1,2-a]quinoline ligands' fluorimetric sensitivity upon biomacromolecule binding, Re(I) fluorescein complex emission being strongly quenched by DNA/RNA or HSA, while emission of Re(I) benzimidazo[1,2-a]quinolone complex was enhanced, particularly for HSA, making it a promising fluorescent probe. Some mono- and heterobimetallic complexes showed considerable antiproliferative activity on colon cancer cells (CT26 and HT29), with ferrocene dipyridylamine complexes exhibiting the best inhibitory activity, comparable to cisplatin. The correlation of the cytotoxicity data with the linker type between the ferrocene and the 1,2,3-triazole ring suggests that direct binding of the metallocene to the 1,2,3-triazole is favourable for antitumor activity. The Re(I) benzimidazo[1,2-a]quinolone complex showed moderate antiproliferative activity, in contrast to the Re(I) fluorescein complex, which exhibited weak activity on CT26 cells and no activity on HT29 cells. The accumulation of the Re(I) benzimidazo[1,2-a]quinolone complex in the lysosomes of CT26 cells indicates the site of its bioactivity, thus making this complex a potential theranostic agent.

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