Rhenium(I) tricarbonyl complexes with 1,10-phenanthroline derivatives: potential anticancer agents for hepatocellular...

Hepatocellular carcinoma (HCC) is a highly lethal malignancy with limited therapeutic options for advanced disease, motivating the development of metal-based agents that can bypass resistance and improve tumor selectivity.

This study aimed to evaluate rhenium(I) tricarbonyl complexes bearing 1,10-phenanthroline derivatives, with emphasis on fac-[Re(CO)₃(5-chloro-1,10-phenanthroline)Cl] (Re(3)), as potential anti-HCC candidates and to elucidate mechanistic determinants of activity in HepG2 cells.

Complexes fac-[Re(CO)₃(NH₂phen)Cl] (Re(1)), fac-[Re(CO)₃(NO₂phen)Cl] (Re(2)), and Re(3) were synthesized under argon and characterized by ¹H NMR; electronic structure was examined by DFT (ZORA-PW91/TZP) with COSMO (water), including global reactivity descriptors and frontier orbital composition. Antiproliferative effects were quantified in HepG2 and non-tumor Vero cells using a 72 h resazurin assay (1-150 μM), while apoptosis, caspase activity, oxidative stress, and signaling changes were assessed by Annexin V/7-AAD flow cytometry, Caspase-Glo 3/7, DCFH-DA ROS measurements, western blotting, and immunofluorescence microscopy; statistics used Kruskal-Wallis/Dunn or ANOVA with p < 0.05.

Re(3) showed pronounced, selective cytotoxicity (IC₅₀ 6 ± 1.1 μM at 72 h; selective index 5.7; Vero IC₅₀ > 30 μM), whereas Re(1) and Re(2) were largely inactive (IC₅₀ > 100 μM). Re(3) induced predominantly apoptotic death (>60% apoptotic cells at 48 h; minimal necrosis), increased caspase-3/7 activity (4-16 h), triggered an early ROS burst (1-20 μM; significant at 2 and 4 h, comparable to CuSO₄), upregulated cytochrome c, caspase-9 and caspase-3, downregulated Bcl-XL, and increased phosphorylated p53, with negligible extrinsic signaling via caspase-8. DFT indicated Re(3) has a stabilized LUMO (-3.34 eV) and electrophilicity (ω = 4.32 eV) with an "optimal" hardness (η = 2.37 eV) supporting sufficient redox reactivity yet kinetic stability; the LUMO localizes (>90%) on the chloro-phenanthroline, consistent with MLCT-driven redox stress.

Collectively, Re(3) functions as a tuned redox-active agent, triggering apoptosis through the intrinsic pathway. This process possibly involves mitochondrial modulation with tumor selectivity, supporting its advancement as a lead rhenium-based prototype for HCC and motivating in vivo pharmacology and broader HCC panel validation.