GLUTATHIONE AND N-ACETYLCYSTEINE CONJUGATES OF 2-CHLOROETHYL ISOCYANATE - IDENTIFICATION AS METABOLITES OF N,N'-BIS(2-CHLOROETHYL)-N-NITROSOUREA IN THE RAT AND INHIBITORY PROPERTIES TOWARD GLUTATHIONE-REDUCTASE INVITRO

The antitumor agent N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU) is kn own to be unstable in aqueous solution, and to degrade spontaneously t o reactive alkylating and carbamoylating intermediates. Whereas the al kylating component is believed to be responsible for the antitumor eff ects of this drug, it has been speculated that the carbamoylating spec ies 2-chloroethyl isocyanate (CEIC) may mediate some of the serious ad verse effects of BCNU therapy. In order to determine whether CEIC is r eleased from BCNU in vivo, rats were administered an ip injection of t he drug and a targeted search was made by ionspray LC-MS/MS techniques for the glutathione (GSH) conjugate of CEIC in bile and for the corre sponding N-acetylcysteine (NAC) adduct in urine. Both of these S-linke d conjugates were identified on the basis of their HPLC and MS/MS char acteristics, which were identical to those of the respective reference compounds prepared by synthesis. Quantitative studies indicated that, following an ip dose of BCNU (24 mg kg-1), excretion of the GSH conju gate in bile over 4 h accounted for 3.90 +/- 0.64 % of the administere d dose, while excretion of the mercapturic acid derivative in urine ov er 24 h accounted for a further 18.1 +/- 3.3 % (n = 4). Experiments co nducted in vitro demonstrated that the S-linked conjugates of CEIC wer e of limited stability under simulated physiological conditions, decom posing to generate free GSH and NAC. In addition, both adducts inhibit ed rat liver glutathione reductase in vitro, when they were essentiall y equipotent to BCNU. It is concluded that CEIC is liberated from BCNU in vivo and that the resulting S-linked conjugates of this carbamoyla ting intermediate may contribute to the toxic effects of the parent ni trosourea.