Synthesis, reactivity and biological studies of bioconjugated bis(pyrazolyl)acetate copper complexes

Among the well-known scorpionates bis(azol-1-yl)methane-based systems of general formula [(az)2CHX] (az = N-heterocyclic ring) bearing a neutral or an anionic functional donor group X with pendant donor arms (X = carboxylate, phenoxide, acetamidate, thioacetamidate, alkoxide, ether, thioether and related groups) have recently attracted considerable attention. In particular, bis(azol-1-yl)carboxylic acids are convenient starting materials for the synthesis of neutral heteroscorpionate systems based on bis(azol-1-yl)methane linked by acetate, acetamide or thioacetamide moieties. Recently, we have focused the research work on the development of two bis(pyrazol-1-yl)acetates [HC(COOH)(pz)2] (LH) and [HC(COOH)(pzMe2)2] (L2H) and the related analogues esterified with several alcohols such as methanol, ethanol, isopropanol and hexanol. All ligands have been used to obtain the copper(II) complexes starting from Cu(II) acceptors and to synthesize the related Cu(I) complexes by reaction of Cu(CH3CN)4PF6 and 1,3,5-triaza-7-phosphaadamantane, tris(hydroxymethyl)phosphine and triphenylphosphine in acetonitrile solution. In addition, based on the observation that copper nitroimidazole conjugates have recently shown additive or synergistic selectivity for tumor hypoxia compared to their individual components,1 Cu(I) and Cu(II) complexes of 5-nitroimidazole conjugated heteroscorpionate ligands have been prepared. The new ligands LHMN and LMeMN were synthesized by direct coupling of preformed side chain acids LH and L2H with 5-nitroimidazole and their coordination chemistry was investigated towards Cu(I) and Cu(II) acceptors.2,3 Finally, LH and L2H have been bioconjugated with the NMDA receptor antagonist (±)-(6,6-diphenyl-1,4-dioxan-2-yl)methanamine and the resulting ligands have been used to synthesize the related copper(II)4 and copper(I)5 derivatives, potentially acting through synergistic mechanisms of action due to the presence of the NMDA ligand and copper in the same chemical entity. Several new copper complexes as well as the corresponding uncoordinated ligands were evaluated for their cytotoxic activity against 2D monolayer cultures of multiple human cancer cell lines and the most promising derivatives against 3D-cultured HCT-15 colon cancer spheroids.