Vol. 30, Issue 12, 1378-1384, December 2002

Kinetic Study of the Reaction of Cisplatin with Thiols

James C. Dabrowiak, Jerry Goodisman, and Abdul-Kader Souid

Department of Chemistry, Syracuse University, Syracuse, New York (J.C.D., J.G.); and Department of Pediatrics, Upstate Medical University, State University of New York, Syracuse, New York (A.-K.S.)

The reactions of cisplatin [cis-diamminedichloroplatinum(II), CDDP] with glutathione (GSH) and drug thiols were investigated at 37°C in 100 mM Tris-NO₃, pH ~7.4, using a clinically relevant concentration of CDDP (33 µM), 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 reactions are first order in [CDDP] and allowing determination of the pseudo first order rate constants (k₁). For reaction of the dichloro form of CDDP with GSH, the k₁ value was ~2.2 × 10⁴ s¹ (t_1/2 of ~53 min), giving the second order rate constant value (k₂) of ~1.3 × 10² M1 s¹. Reaction of a mixture of the aquated forms of CDDP with GSH gave a lower k₁ value (~0.9 × 10⁴ s¹). Reaction of CDDP with sodium 2-mercaptoethanesulfonate (mesna) gave a k₁ value of ~1.8 × 10⁴ s¹ (t_1/2 of ~65 min and k₂ of ~1.1 × 10² M¹ s¹). Reaction of CDDP with S-2-(3-aminopropylamino)ethanethiol (WR-1065) gave a k₁ value of ~12.0 × 10⁴ s¹ (t_1/2 of ~10 min and k₂ of ~7.3 × 10² M1 s¹). 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.