ABSTRACT:The reactions of cisplatin [cis-diamminedichloroplatinum(II), CDDP] with glutathione (GSH) and drug thiols were investigated at 37°C in 100 mM Tris-NO 3 , pH ϳ7.4, using a clinically relevant concentration of CDDP (33 ⌴), 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. CDDP 1 exerts its antitumor activity by binding to cellular DNA (Rosenberg, 1971;Gelasco and Lippard, 1999). When the drug enters the cell, it passes through the cytosol, enters the nuclear envelope, binds to nitrogen atoms on the bases of DNA, and promotes cell death by apoptosis (Bellon et al., 1991;Demarcq et al., 1994). The cytotoxic activity of CDDP thus seems to correlate with the amount of platinum (Pt) bound to DNA (Zwelling et al., 1979;Lindauer and Holler, 1996).Biological thiols such as GSH, metallothionine, and other protein thiols defend the cell against CDDP (Kraker et al., 1985;Pattanaik et al., 1992;Ishikawa and Ali-Osman, 1993;Reedijk and Teuben, 1999). Pt ions entering the cell may preferentially bind to GSH and metallothionine (Dedon and Borch, 1987), both present in millimolar concentrations in the cytoplasm (Souid et al., 1999(Souid et al., , 2001; A.-K. Souid personal observation). Because Pt-thiol adducts interact less well with DNA (Volckova et al., 2002), formation of these complexes limits the amount of drug available for binding to DNA. Furthermore, continued exposure to CDDP can increase the cytosolic sulfhydryl level (Schilder et al., 1990;Eastman 1991;Godwin et al., 1992), which may produce CDDP resistance.The sulfhydryl agents WR-1065 and mesna are often administered to mitigate toxicities of Pt-based compounds and alkylating agents, such as cyclophosphamide (Brock et al., 1982;Souid et al., 2001;Tacka et al., 2002). The protective mechanism of WR-1065 and mesna involves reaction of the thiolate ion with CDDP in a manner analogous to that of GSH (Leeuwenkamp et al., 1991;Treskes et al., 1991).Previously, we measured the rate of CDDP binding to DNA in peripheral blood mononuclear cells and ovarian cancer cells (Sadowitz et al., 2002). By blocking the cellular thiol groups and by adding known concentrations of WR-1065 or mesna, we showed how the amount of CDDP reaching the DNA is affected by thiol concentration. For example, blocking all cellular thiol groups with N-ethylmaleimide increased DNA platination by ϳ8-fold. These data were used to generate a simple kinetic model that reproduced the measured Pt bound to DNA as a function of incubation time and CDDP and thiol concentrations. The model included passage of CDDP through the cell membrane, binding of CDDP to cellular thiols, and platination of DNA. We also studied the reaction of GSH with 4-hydroperoxycyclophosphamide and acrolein ; the latter is a toxic metabolite of cyclophosphamide.Although CDDP reaction with thiols has been studied ex...