N-Glucuronidation of the antiepileptic drug retigabine: results from studies with human volunteers, heterologously expressed human UGTs, human liver, kidney, and liver microsomal membranes of Crigler-Najjar type II.

Retigabine (D-23129), an N-2-amino-4-(4-fluorobenzylamino)phenylcarbamine acid ethyl ester, is a novel antiepileptic drug which is currently in phase II clinical development. This drug undergoes N-glucuronidation. We aimed to identify the principal enzymes involved in the N-glucuronidation pathway of retigabine and compared our findings with those obtained from human liver (a pool of 30 donors) and kidney microsomes (a pool of 3 donors) and with results from a human absorption, distribution, metabolism, and excretion study upon administration of 200 μCi of [14C]-D-23129. Essentially, microsomal assays with UGT1A1 produced only one of the 2 N-glucuronides, whereas UGT1A9 is capable of forming both N-glucuronides. The rates of metabolism for UGT1A9, human liver microsomes, and UGT1A1 were 200, 100, and 100 pmol N-glucuronide per minute per milligram of protein, respectively. At the 50 μmol/L uridine diphosphate glucoronic acid (UDPGA) concentration, UGT1A4 also catalyzed the N-glucuronidation of retigabine, the rates being approximately 5 and 6 pmol/(min·mg protein). With UGT1A9, the production of metabolites 1 and 2 proceeded at a Km of 38 ± 25 and 45 ± 15 μmol/L, whereas the Km for retigabine N-glucuronidation by human liver microsomal fractions was 145  ± 39 μmol/L. Furthermore, a Vmax of 1.2 ± 0.3 (nmol/[min·mg protein]) was estimated for human liver microsomes (4 individual donors).