Cytotoxicity of Pyrazine-Based Cyclometalated (C∧NPZ∧C)Au(III) Carbene Complexes: Impact of the Nature of the Ancillary Ligand on the Biological Properties

The synthesis of a series of cyclometalated gold(III) complexes supported by pyrazitie-based ((CNC)-N-boolean AND-C-boolean AND)-type pincer ligands is reported, including the crystal structure of a cationic example. The compounds provide a new platform for the study of antiproliferative properties of gold(III) complexes. Seven complexes were tested: the neutral series ((CNC)-N-boolean AND-C-PZ boolean AND)AuX [X = Cl (1), 6-thioguanine (4), C equivalent to CPh (5), SPh (6)] and an ionic series that included the N-methyl complex [((CNC)-N-boolean AND-C-pzMe boolean AND)AUCl]BF4 (7) and the N-heterocyclic carbene complexes [((CNC)-N-boolean AND-C-pz boolean AND)AuL](+) with L = 1,3-dimethylbenzimidazol-2-ylidene (2) or 1,3,7,9-tetramethylxanthin.-8-ylidene (3). Tests against human leukemia, tells identified 1, 2, 3, and 4 as particularly promising, whereas protecting the nonCoordinated N atom on the pyrazine ring by methylation (as in 7) -reduced the cytotoxicity. Complex 2 proved to be the most effective of the entire series against the HL60, leukemia, MCF-7 breast cancer, and A549 lung cancer cell lines, with IC50 values down to submicromolar levels, associated, with a lower toxicity, toward healthy human lung fibroblast cells. The benzimidazolylidene complex 2 accumulated more effectively in human lung,,canter,cells than its caffeine-based analogue 3 and the gold(III) chloride 1. Compound 2 proved to be unaffected by ghitathione under physiological conditions for periods of up to 6 days and stabilizes the DNA G-quadruplex and 1 motif structures; the latter is the first such report for gold compounds. We also, show the first evidence of inhibition of MDM2 p53 protein-protein interactions by a gold-based compound and identified the binding mode of the compound with MDM2 using saturation transfer difference NMR spectroscopy combined with docking calculations.