Modulation of enantioselective metabolism and inversion of ibuprofen by xenobiotics in isolated rat hepatocytes.

R-ibuprofen undergoes chiral inversion by the formation of a coenzyme A (CoA) thioester and subsequent epimerization and hydrolysis. Using isolated rat hepatocytes, the interaction of xenobiotics with the inversion and oxidation pathways of ibuprofen enantiomers was determined from the time course of R- and S-ibuprofen and ibuprofenyl-CoA during 4-hr incubations with R- or S-ibuprofen (25 microM). By fitting a first-order model, the rate constants of the formation of ibuprofenyl-CoA (K12), oxidation of R-ibuprofen (K10), hydrolysis of ibuprofenyl-CoA (K21) and oxidation of S-ibuprofen (K30) were 1.306, 0.284, 6.858 and 0.496 hr-1, respectively. The fractional inversion of R-ibuprofen was 0.75 and the area under the curve for ibuprofenyl-CoA was 203.8 microM min. Coincubation with 50 microM of the cytochrome P450 inhibitors metyrapone and proadifen resulted in significant reductions of K10 and K30; the fractional inversion of R-ibuprofen increased to 116% and 127% and the area under the curve of ibuprofenyl-CoA to 145% and 144% of controls, respectively. Valproic acid and pivalic acid at 50 microM significantly reduced the K12 and increased the K21; the fractional inversion was unchanged but the area under the curve of ibuprofenyl-CoA was significantly reduced to 57% and 28% of controls, respectively. Valproic acid also significantly reduced K10 and K30. p-nitrobenzoic acid at 50 microM significantly increased K21 and reduced the area under the curve of ibuprofenyl-CoA to 44% of control but did not influence the fractional inversion. Selective inhibitors of ibuprofen oxidation were found to enhance significantly hepatocellular exposure to the potentially reactive ibuprofenyl-CoA intermediate.