Kinetic Study on the Reactions of Platinum Drugs with Glutathione

The binding of platinum (Pt) drugs (oxaliplatin, carboplatin, and cisplatin) to glutathione (GSH, 6.75 mM) was investigated at 37°C in Hepes (100 mM, pH ∼7.4) or Tris-NO₃ (60 mM, pH 7.4) buffer and NaCl (4.62, 6.63, or 7.82 mM). The conditions were chosen to mimic passage of clinical concentrations of the drugs (135 μM) through the cytosol. The reactions were monitored by UV-absorption spectroscopy, high-performance liquid chromatography (HPLC), and atomic absorption spectroscopy. The initial rates, detected by UV absorbance, were similar for oxaliplatin and cisplatin reacting with GSH and were more than 5-fold faster than for carboplatin reacting with GSH. The Pt contents in HPLC eluates corresponding to unbound drug decreased exponentially with time, confirming that the reactions were first order in [Pt drug] and allowing determination of the pseudo first-order rate constants (k₁). The second-order rate constants (k₂) were calculated as k₁ divided by [GSH]. The k₂ value for oxaliplatin reacting with GSH was ∼3.8 × 10^-2 M ^-1 s^-1, for cisplatin reacting with GSH ∼2.7 × 10^-2 M^-1 s^-1, and for carboplatin reacting with GSH ∼1.2 × 10^-3 M^-1 s^-1 (∼32-fold slower than that of oxaliplatin and ∼23-fold slower than that of cisplatin). These results demonstrate an influence of ligands surrounding the Pt coordination sphere on the reactivity of Pt²⁺ with GSH.