Interaction of bi-nuclear gallium(III) complexes with human serum albumin and impact on potency towards osteosarcoma...

Metastasised osteosarcoma has very poor survival rates, in part due to the presence of osteosarcoma stem cells (OSCs), a resistant subpopulation with self-renewal and differentiation capabilities. Recently, we reported the first metal complex (a bi-nuclear gallium(III) complex comprising tridentate Schiff base ligands and 8-hydroxyquinoline moieties) capable of eliciting an immunogenic response against OSCs. Here, we present an in-depth investigation of the interaction between this anti-OSC bi-nuclear gallium(III) complex and related complexes (4-7) with human serum albumin (HSA). Given that HSA is the predominant protein in human blood and that most osteosarcoma drugs are administered intravenously, the interaction of complexes 4-7 with HSA is of significant translational relevance. The binding of complexes 4-7 to HSA was examined using spectroscopic titration experiments. The resulting quenching (K_q) and binding (K_a) constants were in the 10⁴-10⁵ M^-1 range, indicating strong binding affinity. Molecular docking studies suggest that representative bi-nuclear gallium(III) complexes 4 and 5 interact with amino acid residues within subdomain IIA, primarily through hydrophobic and π-π interactions. Biophysical analyses indicate that, although the bi-nuclear gallium(III) complexes bind tightly to HSA, they do not induce significant structural perturbations. Notably, HSA binding improves the aqueous solubility of complexes 4-7. Cytotoxicity studies demonstrate that HSA binding enhances the potency of complexes 4-7 towards both bulk osteosarcoma cells (U2OS) and OSC-enriched cells (U2OS-MTX). Moreover, the HSA-bound bi-nuclear gallium(III) complexes (HSA-4-7) more effectively inhibit three-dimensional sarcosphere formation and viability compared to clinically used metallodrugs cisplatin and carboplatin, and the established anti-cancer stem cell agent salinomycin.