WR-2721 (amifostine) infusion in patients with Ewing's sarcoma receiving ifosfamide and cyclophosphamide with mesna: drug and thiol levels in plasma and blood cells, a Pediatric Oncology Group study

Purpose: Previous WR-2721 human pharmacokinetic studies were limited to pla sma levels in patients receiving platinum-based compounds, and none include s the effects of WR-2721 on endogenous thiols. In the present study (Pediat ric Oncology Group study no. 9457), we measured the levels of WR-2721, its active metabolites, as well as cysteine and glutathione in whale blood, pla sma, and blood cells in patients receiving high-dose alkylating agents with mesna. Methods: WR-2721 was administered (15 min intravenous infusion of 8 25 mg/m(2) per dose x2) to five patients with metastatic Ewing's sarcoma re ceiving ifosfamide and cyclophosphamide with mesna. Intracellular and extra cellular blood thiols were labeled with monobromobimane (mBBr) at the time of collection, and the low molecular weight (LMW) thiols were subsequently separated by HPLC and detected by fluorescence. Results: The active metabol ite of the drug, WR-1065, peaked at 100 mu M in plasma and blood cells at t he end of WR-2721 infusion and decayed with a rapid initial half-life. Dete ctable levels of WR-1065 and its LMW disulfides were present in plasma and blood cells at similar to 1h after the WR-3721 infusion. By the end of the first WR-2721 infusion (prior to mesna infusion), the mean cysteine level m ore than doubled and the mean Cys-SS-LMW (cystine and the mixed LMW disulfi des) level decreased by similar to 50% in both plasma and blood cells. In f our of five patients, reduced glutathione levels in blood cells increased b y the end of the first WR-2721 infusions, the average increment being simil ar to 36%. Conclusions: (1) WR-1065 is rapidly formed from WR-2721 and equi librates between plasma and blood cells; (2) WR-1065 decays in plasma and b lood cells with similar rapid initial half-lives of similar to 16 min; (3) WR-2721 treatment increases cysteine in plasma and blood cells, an effect s imilar to that of mesna; (4) WR-2721 treatment appears to increase glutathi one levels in blood cells; (5) Mesna does not have a substantial effect on the fate of WR-3721 in patients.