Blood thiols following amifostine and mesna infusions, a Pediatric Oncology Group study

The Pediatric Oncology Group study for metastatic Ewing's sarcoma used amif ostine and mesna with the alkylating agents. To determine the fate of combi ned drug thiols, we measured thiol levels in plasma, red blood cells (RBC), and peripheral blood mononuclear cells (PBMC) of four patients. We also co nducted analogous measurements on two patients who received mesna alone and a volunteer's blood following in vitro treatment. Thiols were labeled with monobromobimane, separated on high-pressure liquid chromatography, and det ected by fluorescence. Incubation of a volunteer's blood with mesna, WR-106 5, or both revealed that cellular uptake of total reducible drug was simila r to 10% of plasma level for mesna but similar to 60% for WR-1065. Cellular drugs were mainly the thiol form, whereas half of the plasma drugs were di sulfides. Combined incubation with both thiols did not change the extent or form of uptake. WR-1065 and mesna prevented glutathione depletion by 4-hyd roperoxycyclophosphamide. Results from patients were similar. WR-1065 and m esna appeared in the cells by the end of the drug infusions, although WR-10 65 uptake was more efficient than mesna. The concentration-time profiles of mesna in RBC paralleled those in plasma. Amifostine administration during mesna infusion caused transient increase in mesna levels. Both agents incre ased blood cysteine and decreased total reducible cysteine. Mesna alone and mesna plus amifostine prevented cellular glutathione depletion. In conclus ion, mesna is imported by RBC and PBMC, but less efficiently than WR-1065. When present at equal levels, these thiols do not influence each other's up take. Adequate dosing of either drug is necessary for protecting the cells from toxic effects of alkylating agents.