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 37degreesC in Hepes (100 mM, pH similar to7.4) 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 muM) 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 (k(1)). The second-order rate constants (k(2)) were calculated as k(1) divided by [GSH]. The k(2) value for oxaliplatin reacting with GSH was similar to3.8 X 10(-2) M-1 s(-1), for cisplatin reacting with GSH similar to2.7 X 10(-2) M-1 s(-1), and for carboplatin reacting with GSH similar to1.2 X 10(-3) M-1 s(-1) (similar to32-fold slower than that of oxaliplatin and similar to23-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.