Effects of Geometric Isomerism and Anions on the Kinetics and Mechanism of the Stepwise Formation of Long-Range DNA Interstrand Cross-Links by Dinuclear Platinum Antitumor Complexes

Reported herein is a detailed study of the kinetics and mechanism of formation of a 1,4‐GG interstrand cross‐link by the dinuclear platinum anticancer compound [15N][{cis‐PtCl(NH3)2}2{μ‐NH2(CH2)6NH2}]2+ (1,1/c,c (1)). The reaction of [15N]1 with 5′‐{d(ATATGTACATAT)2} (I) has been studied by [1H,15N] HSQC NMR spectroscopy in the presence of different concentrations of phosphate. In contrast with the geometric trans isomer (1,1/t,t), there was no evidence for an electrostatic preassociation of 1,1/c,c with the polyanionic DNA surface, and the pseudo‐first‐order rate constant for the aquation of [15N]1 was actually slightly higher (rather than lower) than that in the absence of DNA. When phosphate is absent, the overall rate of formation of the cross‐link is quite similar for the two geometric isomers, occurring slightly faster for 1,1/t,t. A major difference in the DNA binding pathways is the observation of phosphate‐bound intermediates only in the case of 1,1/c,c. 15 mM phosphate causes a dramatic slowing in the overall rate of formation of DNA interstrand cross‐links due to both the slow formation and slow closure of the phosphate‐bound monofunctional adduct. A comparison of the molecular models of the bifunctional adducts of the two isomers shows that helical distortion is minimal and globally the structures of the 1,4 interstrand cross‐links are quite similar. The effect of carrier ligand was investigated by similar studies of the ethylenediamine derivative [15N]1‐en. A pKa value of 5.43 was determined for the [15N]1,1/c,c‐en diaquated species. The rate of reaction of [15N]1‐en with duplex I is similar to that of 1,1/c,c and the overall conformation of the final adduct appears to be similar. The significance of these results to the development of “second‐generation” polynuclear platinum clinical candidates based on the 1,1/c,c chelate (dach) series is discussed. Geometry affects the reaction pathway: [1H,15N] HSQC NMR studies on the cis (1,1/c,c) and trans (1,1/t,t) isomers of dinuclear platinum anticancer complexes reveal subtle differences in the individual steps that lead to the formation of DNA interstrand cross‐links (see figure). The presence of phosphate causes a dramatic slowing of cross‐link formation only for the cis isomer due to the formation of phosphate‐bound intermediates.