DNA Interstrand Cross‐Links of an Antitumor Trinuclear Platinum(II) Complex: Thermodynamic Analysis and Chemical Probing

The trinuclear platinum compound {trans‐PtCl(NH₃)₂}₂(μ‐trans‐Pt(NH₃)₂{NH₂(CH₂)₆NH₂}₂)]⁴⁺ (BBR3464) belongs to the polynuclear class of platinum‐based anticancer agents. These agents form in DNA long‐range (Pt,Pt) interstrand cross‐links, whose role in the antitumor effects of BBR3464 predominates. Our results show for the first time that the interstrand cross‐links formed by BBR3464 between two guanine bases in opposite strands separated by two base pairs (1,4‐interstrand cross‐links) exist as two distinct conformers, which are not interconvertible, not only if these cross‐links are formed in the 5′‐5′, but also in the less‐usual 3′‐3’ direction. Analysis of the conformers by differential scanning calorimetry, chemical probes of DNA conformation, and minor groove binder Hoechst 33258 demonstrate that each of the four conformers affects DNA in a distinctly different way and adopts a different conformation. The results also support the thesis that the molecule of antitumor BBR3464 when forming DNA interstrand cross‐links may adopt different global structures, including different configurations of the linker chain of BBR3464 in the minor groove of DNA. Our findings suggest that the multiple DNA interstrand cross‐links available to BBR3464 may all contribute substantially to its cytotoxicity.