DNA-targeting copper(II) reagents have emerged as suitable drug candidates owing to the clinical success of the copper-activated, natural chemotherapeutic drug bleomycin. This agent and the synthetic chemical nuclease copper(II) bis-1,10-phenanthroline represent important templates for inorganic drug design owing to their ability to initiate free radical DNA scission. Herein, we report the synthesis and biological properties of 1:1:1 square-planar copper(II) complexes incorporating the dicarboxylate o-phthalate and 1,10-phenanthroline (1) or 2,2′-dipyridyl (2) ligands. Their broad-spectrum chemotherapeutic potential has been assessed at 24- and 96-h intervals, along with that of the clinical agent cisplatin, using breast (MCF-7), prostate (DU145), colon (HT29), and intrinsically cisplatin-resistant ovarian (SK-OV-3) human cancer cells. 1 represents a potent cytotoxic agent with IC50 values ranging from 5.6 to 3.4 μM across all cell lines, including SK-OV-3. The production of endogenous reactive oxygen species within SK-OV-3 cancer cells was monitored using the fluorophore 2′,7′-dichlorodihydrofluorescin diacetate, and results indicate a concentration-dependent propensity toward ROS generation by 1 and 2 that mirrors their antitumoral behavior. DNA interaction studies, using fluorescence and viscosity measurements, were conducted in tandem with the DNA-targeting drugs actinomycin D and pentamidine, using calf thymus DNA, poly[d(A-T)2], and poly[d(G-C)2], with intercalation of 1 and 2 at the minor groove appearing to be the likely interaction mode. DNA cleavage reactions using superhelical plasmid DNA, in the presence of exogenous reductant, l-ascorbic acid, revealed excellent agreement between double-stranded DNA scission capability and antitumoral IC50 concentration. The presence of double-strand DNA breaks (DSBs) was confirmed within SK-OV-3 cancer cells using immunodetection of γ-H2AX foci by confocal microscopy and flow cytometry, with complex 1 quantitatively producing superior numbers of DSBs compared with complex 2. Superoxide dismutase and catalase mimetic activity assays were conducted, and these activities are related to the ability of both complexes to cleave DNA through free radical generation. © 2012 Elsevier Inc. All rights reserved.