Quantitative evaluation of bromodichloromethane metabolism, by recombinant rat and human cytochrome P450s

We report quantitative estimates of the parameters for metabolism of bromodichloromethane (BDCM) by recombinant preparations of hepatic cytochrome P450s (CYPs) from rat and human. Earlier work identified CYP2E1, CYP2B1/2 and CYP1A2 as activating enzymes necessary for hepatotoxicity in rat. In order to extend an existing PBPK model for rat to include a capability for extrapolation to humans, it is necessary to evaluate quantitatively the principal metabolic pathways in both species. We have conducted in vitro experiments using recombinant preparations of the three rat CYP isoenzymes mentioned above and for CYP2C11 and CYP3A1 as well. Similar experiments have been performed with human recombinant isoenzymes for CYP2E1, CYP1A2, CYP2A6, CYP2B6, CYP2D6 and CYP3A4. Results indicate that the principal metabolizing enzymes in rat are those identified previously, CYP2E1, CYP2B1/2 and CYP1A2. CYP3A1 may also have some activity. In human, CYP2E1, CYP1A2 and CYP3A4 show substantial activity, and CYP2A6 also measurably metabolizes BDCM. In both species, CYP2E1 is the low K-m isoenzyme, with each with a higher K-m for BDCM than CYP2E1, have been identified in both species. The results appear consistent with available in vivo data in rat and with the predictions of the existing PBPK model for BDCM in rat. The inclusion of multiple CYP isoenzymes in the PBPK model may improve the model's ability to predict the physiological distribution of BDCM. Evaluation of the metabolic parameters for rat and human are essential for the model to be extended to predict effects in humans.