Anticancer dinuclear Ir(III) complex activates Nrf2 and interferes with NAD(H) in cancer cells

Dinuclear complex [Ir₂(μ-L1)(η⁵-Cp*)₂Cl₂](PF₆)₂ (1) exhibits low micromolar cytotoxic activity in vitro in various human cancer cells (GI₅₀ = 1.7–3.0 μM) and outperformed its mononuclear analogue [Ir(η⁵-Cp*)Cl(L2)]PF₆ (2; GI₅₀ > 40.0 μM); Cp* = pentamethylcyclopentadienyl, L1 = 4-chloro-2,6-bis[5-(pyridin-2-yl)-1,3,4-thiadiazol-2-yl]pyridine, L2 = 5-(pyridin-2-yl)-1,3,4-thiadiazol-2-amine. Compound 1 upregulated the Keap1/Nrf2 oxidative stress-protective pathway in the treated MV4‐11 acute myeloid leukemia cells. In connection with the redox-mediated mode of action of 1, its NADH-oxidizing activity was detected in solution (¹H NMR), while NAD⁺ remained intact (with formate as a hydride source). Surprisingly, only negligible NADH oxidation was detected in the presence of the reduced glutathione and ascorbate. Following the results of in-solution experiments, NAD(H) concentration was assessed in 1-treated MV4‐11 cancer cells. Besides the intracellular NADH oxidation in the presence of 1, the induced oxidative stress also led to a decrease of NAD⁺, resulting in depletion of both NAD⁺/NADH coenzymes. The discussed findings provide new insight into the biochemical effects of catalytic anticancer compounds that induce cell death via a redox-mediated mode of action.