Role of ligand exchange processes in the reaction kinetics of the antitumor drug cis-diamminedichloroplatinum(II) with its targets.

The kinetics of a model reaction between the antitumor drug cis-diamminedichloroplatinum(II)(cis-DDP) and the signal nucleotide diadenosine 5',5"'-P1,P4-tetraphosphate (Ap4A) has been investigated by spectrophotometry. The formation of the reactive platinum aquated species was first analyzed by potentiometry using a chloride-specific electrode. Both equilibrium and rate constants were measured. The rate constants for the release of the first and second chloride were found 1.1 +/- 0.5 (S.D.) X 10(-4) and 4.2 +/- 0.2 X 10(-5) sec-1, respectively, at 37 degrees. It was shown that anions such as acetate, phosphate, and pyrophosphate were able, in some conditions, to exchange with chloride to form acetato, phosphato, or pyrophosphato complexes. The reaction of cis-DDP with Ap4A or other targets involves at least four steps, which have been analyzed separately. The values of rate constants deduced from the analysis of the overall reaction are in agreement with those determined independently from the separated steps. The second-order rate constants for the reaction of acetato, phosphato, and pyrophosphato complexes with Ap4A (0.2 +/- 0.02, 0.20 +/- 0.02, and 0.16 +/- 0.03 M-1 sec-1, respectively) are close to that of monoaqua-monochloro (0.16 +/- 0.1 M-1 sec-1) and lower than that of the diaqua species (0.94 +/- 0.06 M-1 sec-1). At a high concentration of Ap4A, the reaction kinetics is slowed down. The formation of a complex of cis-platinum with the Ap4A phosphate groups is suggested. The intracellular concentration of phosphates, pyrophosphates, and carboxylates is large enough to displace chloride from cis-DDP. Inside cells, therefore, ligand exchange processes have to be taken into account to analyze the in vivo reactivity of cis-DDP with its potential targets.