Interaction of cisplatin and carboplatin with sodium thiosulfate: reaction rates and protein binding.

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Abstract
Toxicity of cisplatin can be decreased by concomitant administration of sodium thiosulfate, which perhaps chemically inactivates this platinum compound. We studied the disappearance of cisplatin and carboplatin in aqueous solutions of thiosulfate at 37 degrees C by means of liquid chromatography. At initial concentrations that were similar to therapeutic concentrations in plasma, both drugs disappeared, with half-lives of 66 and 537 min for cisplatin and carboplatin, respectively. At higher thiosulfate concentrations, as found in urine, the respective half-lives were 3.7 and 33.8 min. These values suggest that direct chemical interaction in the plasma compartment has limited therapeutic consequences, whereas the anti-toxic effect of thiosulfate might be explained by the rapid inactivation of cisplatin in the kidneys. Reaction products of cisplatin and thiosulfate bound instantaneously and mainly reversibly to plasma proteins. Protein-bound cisplatin was not released by added thiosulfate--which may explain why thiosulfate, to be effective, must be given in advance of and during cisplatin administration.