Anti-proliferative effects of clioquinol (CQ) + copper and pyrrolidine dithiocarbamate (PDTC) + copper. MDA-MB-231 cells were treated for 24 h with the following: copper (Cu; at 30 μM); CQ (CQ; at 30 μM); PDTC (P; 10 μM); tetrathiomolybdate (TM; 30 μM); CQ + copper (CC; 10, 20, 30 μM); PDTC + copper (PC; 1, 5, 10 μM); TM + copper (TC; 10, 20, 30 μM); or dimethylsulfoxide (DMSO) as a control. After 24 h the media was removed and cells were treated with 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazolium bromide (MTT) solution. Measurement of MTT conversion by absorbance at 560 nm showed that MDA-MB-231 cells responded in a dose-dependent manner to CQ-copper and PDTC-copper complexes. This suggests that these organic-copper complexes act as anti-proliferative agents.