Effect of the geometry of the central coordination sphere in antitumor trinuclear platinum complexes on DNA binding.

Polynuclear platinum compounds comprise a unique class of anticancer agents with chemical and biological properties different from mononuclear platinum drugs. The lead compound of this class is bifunctional trinuclear platinum complex [[trans-PtCl(NH(3))(2)](2)mu-trans-Pt(NH(3))(2)[H(2)N(CH(2))(6)NH(2)](2)](4+) (1,0,1/t,t,t, BBR 3464). Interestingly, the geometry of the coordination spheres in this compound affects potency. For example, the central cis unit of [[trans-PtCl(NH(3))(2)](2)mu-cis-Pt(NH(3))(2)[H(2)N(CH(2))(6)NH(2)](2)](4+) (1,0,1/t,c,t, BBR 3499) results in substantially reduced cytotoxicity. It has been shown that the interactions of polynuclear platinum drugs with target DNA are distinct from the mononuclear-based cisplatin family. In the present work the DNA binding of 1,0,1/t,c,t in cell-free media was examined by the methods of molecular biophysics and compared to the binding of 1,0,1/t,t,t. The binding of 1,0,1/t,c,t is slower and less sequence specific. 1,0,1/t,c,t also forms on DNA long-range delocalized intrastrand and interstrand cross-links similarly as 1,0,1/t,t,t, although the frequency of interstrand adducts is markedly enhanced. Importantly, the adducts of 1,0,1/t,c,t distort DNA conformation and are repaired by cell-free extracts considerably more than the adducts of 1,0,1/t,t,t. It has been suggested that the unique properties of long-range interstrand cross-links of bifunctional trinuclear platinum complexes and resulting conformational alterations in DNA have critical consequences for their antitumor effects.