PT-195 NMR KINETIC AND MECHANISTIC STUDIES OF CIS-DIAMMINEDICHLOROPLATINUM AND TRANS-DIAMMINEDICHLOROPLATINUM(II) BINDING TO DNA

The kinetics and mechanism of binding of the anticancer drug cis-diamminedichloroplatinum(II), or cis-DDP, and its inactive trans isomer to chicken erythrocyte DNA at 37 °C have been investigated by 195Pt NMR spectroscopy. Both cis and trans-DDP bind to DNA by two successive pseudo-first-order processes, forming monofunctional adducts (195Pt NMR shifts near-2300 ppm) that subsequently close to bifunctional lesions (chemical shifts near-2450 ppm). The pseudo-first-order rate constants at pH 6.5 and 37 °C for the first DNA binding step are k = 10.2 ± 0.7 ⨯ 10-5 s-1 (t1/2-1.9 ± 0.1 h) for cis-DDP and k = 9.6 X 10-5 s-1 (t1/2 = 2.0 ± 0.1 h) for trans-DDP. These rate constants are the same as for the rate of hydrolysis of the first chloride ion in this solution. The monofunctional adducts are bound predominantly at the N7 position of guanosine and retain a chloride ligand. The pseudo-first-order rate constants at pH 6.5 and 37 °C for closure of mono-to bifunctional adducts are 9.2 ± 1.4 X 10-5 s-1 (t1/2 = 2.1 ± 0.3 h) and 6.3 ± 0.1 X 10-5 s-1 (t1/2 = 3.1 ±0.1 h) for the cis and trans isomers, respectively, and are the same for closure on single-stranded DNA. These results indicate that the different biological activities of cis- and trans-DDP cannot, as others have suggested, be due to large differences in monofunctional adduct lifetimes. The similarity of the rate constants for monofunctional adduct closure to those for the second hydrolysis of cis-DDP and the absence of significant concentrations of equated species in the reaction mixture indicate that loss of chloride is the rate-limiting step in monofunctional adduct closure. The ΔS⋆ and ΔH⋆ values for monofunctional adduct closure are comparable to those for substitution reactions of square-planar Pt(II) complexes. The closure reactions are proposed to proceed through solvent-associated intermediates. Both cis- and trans-DDP monofunctional, but not bifunctional, adducts react rapidly with glutathione, forming sulfur-bound species with 195Pt NMR chemical shifts near-2900 ppm that are trapped and cannot close to form bifunctional lesions. Preliminary experiments indicate that trans-DDP monofunctional adducts react more rapidly with glutathione than those of the cis isomer, suggesting that selective trapping of trans-DDP monofunctional adducts in vivo could contribute to the biological inactivity of this isomer. © 1990, American Chemical Society. All rights reserved.