DNA cross-linking patterns induced by an antitumor-active trinuclear platinum complex and comparison with its dinuclear analogue.

The DNA binding and cross-linking modes of a trinuclear platinum complex [Pt(3)Cl(3)(hptab)][ClO(4)](3) (1; hptab = N,N,N',N',N'',N''-hexakis(2-pyridylmethyl)-1,3,5-tris(aminomethyl)benzene) and its dinuclear analogue [Pt(2)Cl(2)(m-tpxa)]Cl(2) (2; m-tpxa = N,N,N',N'-tetra(2-pyridylmethyl)-m-xylylene diamine) are reported and compared. The adducts of 1 and 2 with 18-mer duplex N1, 5'-d(GAAGAAGTCACAAAATGT)-3'5'-d(ACATTTTGTGACTTCTTC)-3', have been characterized by means of denaturing polyacrylamide gels, Maxam-Gilbert sequencing, and MALDI-TOF mass spectrometry combined with enzymatic degradation to obtain insights into structural features responsible for the differences in their antitumor activities. The cytotoxic-active complex 1 readily forms various DNA adducts, such as through 1,3- and 1,4-intrastrand cross-links, and in particular, the unique and unprecedented interstrand cross-linked triadducts. In contrast, the cytotoxic-inactive complex 2 preferentially forms 1,4-intrastrand rather than 1,3-intra- and -interstrand cross-links. Digestion of the DNA adducts of 1 shows that the cleavage is completely blocked at one nucleotide before the cross-linked guanine residues on the opposite strand, a feature that appears to be unprecedented in antitumor platinum complexes. In the case of 2, the cleavage bypasses the first platinated guanine site and stops at one nucleotide prior to the second platinated site, confirming that very few 1,3-intrastrand cross-links are formed by 2. These results are supported by molecular-modeling studies of intra- and interstrand cross-links of duplex N1 with 1 and 2. The remarkable differences between 1 and 2 in DNA binding and cross-linking provide mechanistic insights into their different cytotoxicity against the tumor cell lines; these insights are useful for designing future antitumor agents.