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 in vitro

Abstract

The antitumor agent N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU) is known to be unstable in aqueous solution, and to degrade spontaneously to reactive alkylating and carbamoylating intermediates. Whereas the alkylating component is believed to be responsible for the antitumor effects of this drug, it has been speculated that the carbamoylating species 2-chloroethyl isocyanate (CEIC) may mediate some of the serious adverse effects of BCNU therapy. In order to determine whether CEIC is released from BCNU in vivo, rats were administered an ip injection of the 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 corresponding N-acetylcysteine (NAC) adduct in urine. Both of these S-linked conjugates were identified on the basis of their HPLC and MS/MS characteristics, 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 conjugate in bile over 4 h accounted for 3.90 +/- 0.64% of the administered dose, while excretion of the mercapturic acid derivative in urine over 24 h accounted for a further 18.1 +/- 3.3% (n = 4). Experiments conducted in vitro demonstrated that the S-linked conjugates of CEIC were of limited stability under simulated physiological conditions, decomposing to generate free GSH and NAC. In addition, both adducts inhibited rat liver glutathione reductase in vitro, when they were essentially equipotent to BCNU.(ABSTRACT TRUNCATED AT 250 WORDS)