Kinetic study of the reaction of cisplatin with thiols

The reactions of cisplatin [cis-diamminedichloroplatinum(II), CDDP] with glutathione (GSH) and drug thiols were investigated at 37degreesC in 100 mM Tris-NO3, pH similar to7.4, using a clinically relevant concentration of CDDP (33 muM), a large excess of GSH (16.5 mM), and [NaCl] of 4.62 mM. The conditions were designed to mimic passage of CDDP through the cytosol. The reactions were studied by UV-absorption spectroscopy, high-pressure liquid chromatography (HPLC), and atomic absorption spectroscopy. The initial rates, detected by UV absorbance, confirmed that the reactions are first order in [CDDP]. The HPLC peak corresponding to CDDP was analyzed for platinum content by atomic absorption spectroscopy, which decreased exponentially with time, confirming that the the pseudo first order rate constants (k(1)). For reaction of the dichloro form of CDDP with GSH, the k(1) value was similar to2.2 x 10(-4) s(-1) (t(1/2) of similar to53 min), giving the second order rate constant value (k(2)) of similar to1.3 x 10(-2) M-1 s(-1). Reaction of a mixture of the aquated forms of CDDP with GSH gave a lower k(1) value (similar to0.9 x 10(-4) s(-1)). Reaction of CDDP with sodium 2-mercaptoethanesulfonate (mesna) gave a k(1) value of similar to1.8 x 10(-4) s(-1) (t(1/ 2) of similar to65 min and k(2) of similar to1.1 x 10(-2) M-1 s(-1)). Reaction of CDDP with S-2-(3-aminopropylamino) ethanethiol (WR-1065) gave a k(1) value of similar to12.0 x 10(-4) s(-1) (t(1/2) of similar to10 min and k(2) of similar to7.3 x 10(-2) M-1 s(-1)). The relatively slow reaction rate of CDDP with GSH is consistent with the efficient DNA platination by CDDP in the presence of millimolar concentration of GSH in the cytosol.