Effect of a cysteine prodrug (L-2-oxothiazolidine-4-carboxylic acid) on the metabolism and toxicity of bromobenzene: a repeated exposure study.

The relationship between dose, toxicity, and metabolism of bromobenzene and the use of urinary metabolite excretion as an index of internal exposure to the reactive electrophilic intermediate bromobenzene 3,4-epoxide after repeated treatments with bromobenzene in presence or absence of L-2-oxothiazolidine-4-carboxylic acid (OTCA) were evaluated in mice. Repeated treatments with bromobenzene doses of 0.5, 0.75, and 1.0 mmol/kg ip twice a day for 18 d produced a marked reduction in 24-h urinary excretion of bromophenylmercapturic acid and p-bromophenol; this was accompanied by increases in plasma transaminases. Treatment with OTCA (1.0-3.0 mmol/kg) prevented toxicity and enhanced the 24-h urinary excretion of various bromobenzene metabolites by approximately 30-75, 104-145, and 164-269% for bromobenzene doses of 0.5, 0.75, and 1.0 mmol/kg, respectively. The effect of OTCA was further characterized by investigating the metabolism of bromobenzene given as a challenge dose of 4.0 mmol/kg to mice pretreated with bromobenzene and OTCA for 18 d. Pretreatment with bromobenzene reduced the 0- to 6-h urinary excretion of all metabolites after the challenge dose; this effect was virtually reversed by OTCA. It is concluded that repeated bromobenzene administration reduces its own detoxification to mercapturic acid and phenolic metabolites and elicits toxicity. This phenomenon is reversed after OTCA administration. This study further provides evidence that internal exposure to the reactive electrophilic intermediate bromobenzene 3,4-epoxide could be assessed more accurately by summing the urinary excretion of bromophenylmercapturic acid and p-bromophenol after OTCA treatment.