Received for publication January 13, 2006.
Revised March 21, 2006.
Accepted for publication March 22, 2006.

In vitro characterization of lamotrigine N2-glucuronidation and the lamotrigine - valproic acid interaction

Abstract

Studies were performed to investigate the UDP-glucuronosyltransferase enzyme(s) responsible for the human liver microsomal (HLM) N2-glucuronidation of the anticonvulsant drug lamotrigine (LTG) and the mechanistic basis for the LTG-valproic acid (VPA) interaction in vivo. LTG N2-glucuronidation by microsomes from five livers exhibited atypical kinetics, best described by a model comprising the expressions for the Hill (1869 ± 1286µM, n 0.65 ± 0.16) and Michaelis-Menten (K_m 2234 ± 774µM) equations. The UGT1A4 inhibitor hecogenin abolished the Michaelis-Menten component, without affecting the Hill component. LTG N2-glucuronidation by recombinant UGT1A4 exhibited Michaelis-Menten kinetics, with a K_m of 1558µM. Although recombinant UGT2B7 exhibited only low activity towards LTG, inhibition by zidovudine and fluconazole and activation by BSA (2%) strongly suggested that this enzyme was responsible for the Hill component of microsomal LTG N2-glucuronidation. VPA (10mM) abolished the Hill component of microsomal LTG N2-glucuronidation, without affecting the Michaelis-Menten component or UGT1A4-catalyzed LTG metabolism. K_i values for inhibition of the Hill component of LTG N2-glucuronidation by VPA were 2465 ± 370µM and 387 ± 12µM in the absence and presence of BSA (2%), respectively. Consistent with published data for the effect of fluconazole on zidovudine glucuronidation by human liver microsomal UGT2B7, the K_i value generated in the presence of BSA predicted the magnitude of the LTG - VPA interaction reported in vivo. These data indicate that UGT2B7 and UGT1A4 are responsible for the Hill and Michaelis-Menten components of microsomal LTG N2-glucuronidation, respectively, and the LTG - VPA interaction in vivo arises from inhibition of UGT2B7.

Key words: drug-drug interactions, glucuronidation, in vitro-in vivo prediction, liver microsomes, phase II drug metabolism, UDP glucuronyltransferases

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