Several mixed-ligand copper(I) complexes of cyclodiphosphazanes, [(BuNP)-Bu-t(NC4H8X)](2) (1, X = O; 2, X = NMe), were synthesized by reacting the octanuclear copper(I) complexes [Cu-8(mu(2)-l)(8){[(BuNP)-Bu-t(NC4H8X)](2)}(4)] (3, X = O; 4, X = NMe) with various pyridyl ligands. Interaction of the metallomacrocyclic complex 3 or 4 with pyridine, 2,2'-bipyridine, and 1,10-phenanthroline afforded the neutral dinuclear complexes [(C5H5N)(4)Cu(2)l(2){[(BuNP)-Bu-t(NC4H8X)](2)}] (5, X = O; 6, X = NMe), [(2,2'-bpy)2Cu(2)l(2){[(BuNP)-Bu-t(NC4H8X)](2)}] (7, X = O; 8, X = NMe), and [(1,10-phen)(2)Cu(2)l(2){[(BuNP)-Bu-t-(NC4H8X)](2)}] (9, X =0; 10, X = NMe), respectively, in good yield. The new dinuclear complexes 3, 5, and 7-9 were tested for their cytotoxic propenies against human cervical cancer (HeLa) cells. The results indicated that all of the copper complexes have in vitro antitumor activity either similar to or better than that of cisplatin, a widely used anticancer drug. Among the compounds tested, complex 9 showed the most potent inhibitory activity in HeLa cells. In addition, complex 9 was found to potently inhibit proliferation of human breast cancer cells (MCF-7), highly metastatic breast cancer cells (MDA-MB 231), and nontransformed Chinese hamster ovary (CHO) cells. Complex 9 inhibited proliferation of these cells in culture more potently than cisplatin; for example, complex 9 was found to inhibit proliferation of HeLa and MCF-7 cells 3 and 5 times more efficiently than cisplatin. Complex 9 treatment damaged the DNA integrity, blocked the cells in the G1 phase of the cell cycle, and induced apoptosis via a p53-dependent pathway. The molecular structures of conplexes 9 and 10 were confirmed by single-crystal X-ray diffraction studies.