Kinetic analysis of the reactions of 4-hydroperoxycyclophosphamide and acrolein with glutathione, mesna, and WR-1065

The kinetics of the reactions of glutathione (GSH) with 4-hydroperoxycyclophosphamide (400H-CP) and acrolein, a metabolite of 400H-CP, were investigated in a cell-free medium (pH ∼7.5) and peripheral blood mononuclear cells. The ability of the thiol drugs, sodium 2-mercaptoethane sulfonate (mesna) and S-2-(3-aminopropylamino)ethanethiol (WR-1065), to affect the reactions of cellular GSH with the alkyalting agents was also studied. The amount of unreacted thiols in the various reactions was determined by derivatization with monobromobimane, followed by separation of fluorescent-labeled thioether adducts using high-pressure liquid chromatography. The second-order rate constants (k₂) for reactions of GSH, mesna, and WR-1065 with 4OOH-CP in solution were 38 ± 5, 25 ± 5, and 880 ± 50 M^-1s^-1, respectively. The corresponding k₂ for reactions of GSH, mesna, and WR-1065 with acrolein were 490 ± 100, 700 ± 150, and >2000 M^-1s^-1, respectively. The apparent rate constants for reactions of cellular GSH with acrolein and 4OOH-CP were smaller than those obtained in solution. Assuming that the k₂ is the same inside and outside cells, we estimate the first-order rate constant (k₁) for transfer of 4OOH-CP and acrolein across the cell membrane as ∼0.01 and ∼0.04 s^-1, respectively. WR-1065 was more effective than mesna in blocking depletion of cellular GSH (because it passes into the cell more quickly and has higher reaction rates with the alkylators than the latter compound). When WR-1065 and mesna were used together, the protection against cellular depletion of GSH was additive. Our results are relevant to the administration of thiol drugs with highdose alkylating agents.