The use of transgenic cell lines for evaluating toxic metabolites of carbamazepine

Human lymphoblastoid cell lines transgenic for human CYP450s were evaluated for the identification of toxic metabolites of the anticonvulsant drug carbamazepine (CBZ). Human CYP450 isoforms expressed by these cell lines included 1A1, 1A2, 2E1, 2A6, and 3A4. A dose-dependent inhibition of population growth from 50-200 micrograms/ml CBZ was detected by measuring cell number and respiration. The inhibition increased with the growth rate of the various lines, which correlated inversely with the presence of CYP450s, and may have been caused by CBZ itself. Cytotoxicity was observed only at the highest dose and in the line lacking transfected CYP450s. Microsomal preparations from hCYP3A4/OR cells converted CBZ into its principal oxidative metabolite, carbamazepine-10,11-epoxide (CBZ-E), at a rate of 630 pmol/min per mg protein, confirming a major role of CYP3A4 in this reaction. However, no CBZ-E (or any metabolite) was recovered from any whole-cell incubation even though hCYP3A4 cells readily converted testosterone to 6 beta-hydroxytestosterone. This suggests that differences exist between whole-cell and microsomal preparations of lymphoblastoid cells in their ability to metabolize CBZ.