Abstract
An LC-MS method was set up and validated for determining low concentrations of phenethyl isothiocyanate (PEITC) in rat plasma. Following administration of dietary doses to rats, PEITC was rapidly absorbed, reaching peak plasma levels within an hour, and achieved high bioavailability, which increased substantially after repeated administration. However, PEITC displayed dose-dependent pharmacokinetic behaviour with bioavailability decreasing with increasing dose. Dietary doses of PEITC administered to rats enhanced hepatic CYP2B activity and expression whereas a modest inhibition of CYP2E1 was observed. Increase in hepatic CYP1A1 and 1A2 apoprotein levels was not accompanied by similar rises in activity. Further studies revealed that PEITC inhibited CYP1A1 activity in a mechanism-based fashion. In the lung, CYP2B activity was competitively inhibited. A marked rise in glutathione S-transferase (GST) and NADPH:quinone oxidoreductase (NQO1) activities in liver was noted after the same treatment, while in kidney only NQO1 was elevated, and in the lung no change was evident. Incubation of rat liver slices with PEITC decreased CYP1A2 activity and suppressed apoprotein levels, whereas a moderate rise in GST and, to a greater extent, in NQO1 activity was observed. When human liver slices were similarly incubated with PEITC, CYP1A2 activity was impaired as in rat. PEITC was a mechanism-based inhibitor of CYP1A1 in hepatic microsomes. When GST and NQO1 were monitored in the four donors, interindividual variability was observed. Long-term exposure of PEITC to rats prior to a single dose of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) suppressed urinary excretion of indirect-acting mutagens indicating an increase in the overall metabolism of IQ, which was not related to changes in CYP1A2 and GST activities implying that other enzymes are modulated. Finally, PEITC antagonised the benzo(a)pyrene-mediated induction of CYP1A1 in rat liver slices, whereas inconsistent results were obtained in liver slices from four human donors.