Necrosis-Inducing High-Valent Oxo-Rhenium(V) Complexes with Potent Antitumor Activity: Synthesis, Aquation Chemistry, Cisplatin Cross-Resistance Profile, and Mechanism of Action

Chemotherapy with the cytotoxic platinum (Pt) drugs cisplatin, carboplatin, and oxaliplatin is the mainstay of Anticancer therapy in the clinic. The antitumor activity of Pt drugs originates from their ability to induce Apoptosis via covalent adduct formation with nuclear DNA. While the phenomenal clinical success is highly encouraging, resistance and adverse toxic side effects limit the wider applicability of Pt drugs. To circumvent these limitations, we embarked on an effort to explore the antitumor potential of a new class of oxo-rhenium(V) complexes of the type [(N^∧N)(EG)Re(O)Cl] (where EG = ethylene glycolate and N^∧N = bipyridine, Bpy (1); phenanthroline, Phen (2); 3,4,7,8-tetramethyl-phenanthroline, Me₄Phen (3)). Investigation of speciation chemistry in aqueous media revealed the formation of [(N^∧N)Re(O)(OH)₃] as the biologically active species. Complex 3 was found to be the most potent among the three, with IC₅₀ values ranging from 0.1 to 0.4 μM against a panel of Cancer cells, which is 5-70-fold lower when compared with cisplatin. The higher potency of 3 is attributed to its higher lipophilicity, which enhanced cellular uptake. Importantly, complex 3 efficiently overcomes cisplatin resistance in ovarian, lung, and prostate Cancer cells. In addition to reporting the aquation chemistry and identifying the active species in aqueous media, we performed in-depth in vitro mechanistic studies, which revealed that complex 3 preferentially accumulates in mitochondria, depletes mitochondrial membrane potential, and upregulates intracellular Reactive Oxygen Species (ROS), leading to ER stress-mediated necrosis-mediated Cancer cell death.