Chlorzoxazone: a probe drug whose metabolism can be used to monitor toluene exposure in rats.

In this study we investigated cytochrome P450 (CYP) 2E1 expression using a probe drug, chlorzoxazone (CZX), whose metabolism can be used to monitor toluene exposure in rats. The animals received an i.p. injection of toluene (0.25, 0.5 and 1 ml/kg) once a day for 3 days. The total CYP and CYP2E1 content and the aniline and CZX hydroxylase activity (Vmax and CL(int)) increased depending on the dose of toluene administered. At the highest concentration (128 mM) of diethyldithiocarbamate, a specific inhibitor of CYP2E1, the production of 6-hydroxychlorzoxazone (HCZX) in microsomes from toluene-treated rats was reduced by about 80%. The IC50 values in microsomes from toluene-treated rats were between 3 and 5 microM. The production of HCZX and the activity of aniline hydroxylase in toluene-treated rats were correlated with the amount of rat CYP2E1 protein (r = 0.88 and r = 0.88, respectively). The elimination of CZX by toluene-treated rats was increased and the HCXZ production in the toluene-treated group was greater than that in the olive oil control group. The correlations between intrinsic clearance (CL(int): Vmax/Km) in vitro and total body clearance (CL(tot)) of CZX hydroxylation and the elimination half-life (t1/2) of CZX in vivo in toluene-treated rats were high (r = 0.784, P < 0.001; r = -0.678, P < 0.001, respectively). In addition, the metabolic plasma HCZX/CZX ratio did not require multiple blood sampling and 2 h after CZX administration in vivo there was also a high correlation with CL(int) (Vmax/Km) in vitro (r = -0.729, P < 0.001). In conclusion, these results demonstrate that CZX is a very good probe for monitoring induction in toluene-treated rats.