A transgenic 'knock-in' mouse model expressing a human UGT1 locus (Tg-UGT1) was recently developed and validated. Although these animals express mouse UGT1A proteins, UGT1A4 is a pseudo-gene in mice. Therefore, Tg-UGT1 mice serve as a 'humanized' UGT1A4 animal model. Lamotrigine (LTG) is primarily metabolized to its N-glucuronide (LTGG) by hUGT1A4. This investigation aimed at examining the impact of pregnane X receptor (PXR), constitutive androstane receptor (CAR) and peroxisome proliferator-activated receptor (PPAR) activators on LTG glucuronidation in vivo and in vitro. Tg-UGT1 mice were administered the inducers phenobarbital (CAR), pregnenolone-16α-carbonitrile (PXR), WY-14643 (PPAR-α), ciglitazone (PPAR-γ), or L-165041 (PPAR-β), once daily for 3 or 4 days. Thereafter, LTG was administered orally and blood samples were collected over 24h. LTG was measured in blood and formation of LTGG was measured in pooled microsomes made from the livers of treated animals. A three-fold increase in in vivo LTG clearance was seen after phenobarbital administration. In microsomes prepared from phenobarbital-treated Tg-UGT1 animals, 13-fold higher CLint (Vmax/Km) value was observed as compared with the untreated transgenic mice. A trend toward induction of catalytic activity in vitro and in vivo was also observed following pregnenolone-16α- carbonitrile and WY-14643 treatment. This study demonstrates the successful application of Tg-UGT1 mice as a novel tool to study the impact of induction and regulation on metabolism of UGT1A4 substrates.
Bibliographical noteFunding Information:
This study was jointly funded by a grant from the National Institute of Neurological Disorders and Stroke NIH (Grant Number NINDS P50 NS16308) and US Public Health Service Grants (Numbers GM49135 and ES10337). The authors thank Glaxo (GSK) for providing the lamotrigine-2N-glucuronide standard and Dr. M. Nigeshi for providing phenobarbital. The authors thank Ms. Falguni Gadkari for reviewing the manuscript. The assistance of Ms. MyHang Tran with Winnonlin is greatly appreciated.
- Transgenic mice
- Uridine glucuronosyl transferases (UGTs)