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 [Pt₃Cl₃(hptab)][ClO₄]₃ (1. hptab = N,N,N',N',N",N"-hexakis(2-pyr- idylmethyl)-1,3,5-tris(aminomethyl)- benzene) and its dinuclear analogue [Pt₂Cl₂(m-tpxa)] Cl₂ (2. m-tpxa = N,N,N',N'-tetra(2-pyridylmethyl)-m-xy- lylene diamine) are reported and compared. The adducts of 1 and 2 with 18- mer duplex N1,5′-d(GAAGAAGTCA- CAAAATGT)-3′ 5′-d(ACATTTTGT- GACTTCTTC)-3′, have been characterized by means of denaturing poly- acrylamide 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 crosslinks, and in particular, the unique and unprecedented interstrand cross-linked triadducts. In contrast, the cytotoxic-in- active 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 residuesontheopposite strand, a feature that appears to be unprecedented in antitumor platinum complexes. In the case of 2, the cleavage bypasses the first platinated gua- nine 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..