In vitro N-glucuronidation of a novel antiepileptic drug, lamotrigine, by human liver microsomes.

The metabolism of the antiepileptic drug lamotrigine was characterized in human liver microsomes. For that purpose a high performance liquid chromatography method allowing the separation of lamotrigine glucuronide from the parent compound, and the quantitation of the glucuronide, was developed. The drug undergoes glucuronidation on the 2-nitrogen atom of the triazine ring, leading to a quaternary ammonium-linked glucuronide. This metabolite was positively identified from its hydrolysis by beta-glucuronidase and its associated radioactivity when UDP-[U-14C] glucuronic acid was used as the cosubstrate. Structural confirmation of the glucuronide was finally obtained by high performance liquid chromatography-mass spectrometry, by using a thermospray interface. The reaction proceeded with an apparent Vmax of 0.65 nmol/min/mg and Km of 2.56 mM. The average value of lamotrigine glucuronidation in four human samples of transplantable liver was 0.43 +/- 0.14 nmol/min/mg, thus indicating a large interindividual variation. An interspecies comparison of hepatic lamotrigine glucuronidation (human, rabbit, rat, monkey) revealed that the rate of glucuronidation was low. Of all the species considered, humans glucuronidated the drug to the greatest extent, with a specific activity 2-fold higher than that observed in rabbit liver microsomes. In contrast, the activity was greater than 20 times lower in monkey (0.019 nmol/min/mg) and at the limit of detection in rat liver microsomes. However, in this species, phenobarbital treatment enhanced lamotrigine glucuronidation slightly (0.017 nmol/min/mg). Among the drugs that undergo quaternary ammonium-linked glucuronidation, chlorpromazine, but not imipramine, amitriptyline and cyproheptadine, inhibited the glucuronidation of lamotrigine in vitro (IC50 of 5.0 x 10(-4) M).(ABSTRACT TRUNCATED AT 250 WORDS)