Identification of potent anticancer copper() complexes containing tripodal bis[2-ethyl-di(3,5-dialkyl-1-pyrazol-1-yl)]amine moiety

A series of heteroleptic copper( ii ) complexes of the composition [Cu(L 1-5 )Cl]X, where X = ClO 4 and/or PF 6 and [bis(2-ethyl-di(3,5-dimethyl-1 H -pyrazol-1-yl))-(6-methyl-(2-pyridylmethyl))]amine ( L 1 ), [bis(2-ethyl-di(3,5-dimethyl-1 H -pyrazol-1-yl))-(3,4-dimethoxy-(2-pyridylmethyl))]amine ( L 2 ), [bis(2-ethyl-di(3,5-dimethyl-1 H -pyrazol-1-yl)-(2-quinolymethyl)]amine ( L 3 ), [bis(2-ethyl-di(3,5-dimethyl-1 H -pyrazolyl)-(di(3,5-dimethyl-1 H -pyrazol-1-yl-methyl))]amine ( L 4 ) and [bis(2-ethyl-di(3,5-dimethyl-1 H -pyrazol-1-yl)-(5-methyl-3-phenyl-1 H -pyrazol-1-yl-methyl)]amine ( L 5 ), were prepared and thoroughly characterized including single-crystal X-ray diffraction technique. The in vitro cytotoxicity of complexes against A2780, A2780R, HOS and MCF-7 human cancer cell lines was evaluated using the MTT test. The results revealed that complexes [Cu(L 1 )Cl]PF 6 ( 1-PF 6 ), [Cu(L 2 )Cl]ClO 4 ( 2-ClO 4 ) and [Cu(L 3 )Cl]PF 6 ( 3-PF 6 ) are the most effective, with IC 50 values ranging from 1.4 to 6.3 μM, thus exceeding the cytotoxic potential of metallodrug cisplatin (IC 50 values ranging from 29.9 to 82.0 μM). The complexes [Cu(L 4 )Cl]PF 6 ( 4-PF 6 ) and [Cu(L 5 )Cl]PF 6 ( 5-PF 6 ) showed only moderate cytotoxicity against A2780, with IC 50 = 53.6 μM, and 33.8 μM, respectively. The cell cycle profile, time-resolved cellular uptake, interactions with small sulfur-containing biomolecules (cysteine and glutathione), intracellular ROS production, induction of apoptosis and activation of caspases 3/7 were also evaluated in the case of the selected complexes. It has been found that the best performing complexes 1 and 2 cause cell arrest in the G2/M phase and induce apoptosis via the increase in production of ROS, dominantly due to the overproduction of superoxide. Five Cu( ii ) complexes with tripodal pyrazolyl-amines have been synthesized and structurally characterized. Three of the complexes revealed significantly higher in vitro cytotoxicity than cisplatin against five human cancer cell lines.