Relative Importance of Intestinal and Hepatic Glucuronidation—Impact on the Prediction of Drug Clearance
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To assess the extent of intestinal and hepatic glucuronidation in vitro and resulting implications on glucuronidation clearance prediction.
Alamethicin activated human intestinal (HIM) and hepatic (HLM) microsomes were used to obtain intrinsic glucuronidation clearance (CLint,UGT) for nine drugs using substrate depletion. The in vitro extent of glucuronidation (fmUGT) was determined using P450 and UGT cofactors. Utility of hepatic CLint for the prediction of in vivo clearance was assessed.
fmUGT (8–100%) was comparable between HLM and HIM with the exception of troglitazone, where a nine-fold difference was observed (8% and 74%, respectively). Scaled intestinal CLint,UGT (per g tissue) was six- and nine-fold higher than hepatic for raloxifene and troglitazone, respectively, and comparable to hepatic for naloxone. The remaining drugs had a higher hepatic than intestinal CLint,UGT (average five-fold). For all drugs with P450 clearance, hepatic CLint,CYP was higher than intestinal (average 15-fold). Hepatic CLint,UGT predicted on average 22% of observed in vivo CLint; with the exception of raloxifene and troglitazone, where the prediction was only 3%.
Intestinal glucuronidation should be incorporated into clearance prediction, especially for compounds metabolised by intestine specific UGTs. Alamethicin activated microsomes are useful for the assessment of intestinal glucuronidation and fmUGT in vitro.
KEY WORDSclearance prediction glucuronidation intestine
intrinsic clearance corrected for non-specific protein binding
intrinsic clearance by glucuronidation
intrinsic clearance by cytochrome P450 metabolism
fraction metabolised by glucuronidation
fraction unbound from protein in the incubation
fraction unbound in the blood
fraction unbound in the plasma
human intestinal microsomes
human liver microsomes
root mean squared error
blood to plasma concentration ratio
uridine diphosphate glucuronosyltransferase
The Authors would like to thank Sue Murby and Dr David Hallifax (University of Manchester) for valuable assistance with the LC-MS/MS.
The work was funded by a consortium of pharmaceutical companies (GlaxoSmithKline, Lilly, Novartis, Pfizer and Servier) within the Centre for Applied Pharmacokinetic Research at the University of Manchester. Part of this study was presented at the 10th ISSX Meeting, May 18–21, 2008, Vienna, Austria.
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