The metabolism of beta-chloroprene: preliminary in-vitro studies using liver microsomes

Based on analogy with butadiene and isoprene, the metabolism of P-chloropre ne (2-chloro-1,3-butadiene, CD) to reactive intermediates is likely to be a key determinant of tumor development in laboratory rodents exposed to CD b y inhalation. The purpose of this study is to identify species differences in toxic metabolite (epoxide) formation and detoxification in rodents and h umans. The in-vitro metabolism of CD was studied in liver microsomes of B6C 3F1 mice, Fischer/344 and Wistar rats, Syrian hamsters, and humans. Microso mal oxidation of CD in the presence of NADP(+), extraction with diethyl eth er, and analysis by GC-mass selective detection (MSD) indicated that (1-chl oroethenyl)oxirane (CEO) was an important metabolite of CD in the liver mic rosomal suspensions of all species studied. Other potential water-soluble o xidative metabolites may have been present. The oxidation of CID was inhibi ted by 4-methyl pyrazole, an inhibitor of CYP 2E1. CEO was sufficiently vol atile at 37 degreesC for vial headspace analysis using GC-MSD single ion mo nitoring (m/z = 39). CEO was synthesized and used to conduct partition meas urements along with CD and further explore CEO metabolism in liver microsom es and cytosol. The liquid-to-air partition coefficients for CD and CEO in the microsomal suspensions were 0.7 and 58, respectively. Apparent species differences in the uptake of CEO by microsomal hydrolysis were hamster simi lar to human>rats> mice. Hydrolysis was inhibited by 1,1,1-trichloropropene oxide, a competitive inhibitor of epoxide hydrolase. A preliminary experim ent indicated that the uptake of CEO in liver cytosol by GSH conjugation wa s hamster>rats similar to mice (human cytosol not yet tested). In general, the results suggest that metabolism may help explain species differences sh owing a greater sensitivity for CD-induced tumorigenicity in mice, for exam ple, compared with hamsters. Additional experiments are in progress to quan tify the kinetic parameters of CD oxidation and CEO metabolism by enzymatic hydrolysis and conjugation by glutathione S-transferase for in cytosol. A future goal is to use the kinetic rates to parameterize a physiologically b ased toxicokinetic model and relate the burden of toxic metabolite to the c ancer dose-response observed in experimental animals. (C) 2001 Elsevier Sci ence Ireland Ltd. All rights reserved.