The structures, spectroscopy, and cytotoxicity of four novel nominally square-planar gold(III) chelates 1-4 with the general formula cis-AuCl2(X), where the ligand X is an anionic bidentate pyridyl- or isoquinolylamido chelating agent, are described. The Au-N-amido, Au-N-pyridyl, and Au-N-isoquinolyl distances are 2.002(9)-2.016(3), 2.01(1)-2.037(3), and 2.037(3) angstrom, respectively. Density functional theory simulations afforded accurate gold(III) coordination geometries for 1-4 (bond distances and angles to within 5% of the X-ray values), while accurate transition energies were limited to those calculated in the UV spectral region. The complexes had variable stability in dimethyl sulfoxide: compound 3 (relatively rigid) was indefinitely stable, compounds 1 and 2 (conformationally flexible) slowly demetalated over 30 days, and 4 (extensively aromatic) formed an insoluble precipitate after 10 days (72 h in an aqueous buffer). The isoquinolylamido derivative 4 was sufficiently cytotoxic in the NCI-60 screen to undergo full five-dose testing. Notably low GI(50) (1.8, 2.3, and 3.2 mu M) and IC50 (4.0, 9.8, and 15 mu M) values were recorded for the OVCAR-3, IGROV1, and SW-620 cell lines, respectively. Hierarchical cluster analysis employing the National Cancer Institute (NCI) data for known anticancer drugs and 4 revealed that compound 4 is mechanistically identical with the topoisomerase II alpha (Top2) poison zorubicin and statistically similar to the topoisomerase IB (Top 1) poisons camptothecin and 9-methoxycamptothecin. The Top2-catalyzed decatenation reaction of kinetoplast DNA was studied as a function of the concentration of 4: the compound acts as an interfacial poison of Top2 at low concentrations (<1 mu M) and a catalytic inhibitor of the enzyme above 5 mu M. Gel mobility shift assays (plasmid DNA substrate) showed that the catalytic inhibition of Top2 likely correlates with DNA binding by 4 at concentrations >5 mu M. Compound 4 is also a catalytic inhibitor of Top1 at higher concentrations, consistent with DNA binding by the complex.