Quantitative assessment of intestinal first-pass metabolism of oral drugs using portal-vein cannulated rats.

PURPOSE: To evaluate the impact of intestinal first-pass metabolism (Fg) by cytochrome P4503A (CYP3A) and uridine 5'-diphosphate-glucuronosyltransferases (UGT) on in vivo oral absorption of their substrate drugs.
METHODS: CYP3A and UGT substrates were orally administered to portal-vein cannulated (PV) rats to evaluate their intestinal availability (Fa · Fg). In the case of CYP3A substrates, vehicle or 1-aminobenzotriazole (ABT), a potent inhibitor of CYP enzymes, was pretreated to assess Fg separately from Fa (Enzyme-inhibition method). On the other hand, since potent inhibitors of UGT have not been identified, Fg of UGT substrate was calculated from total amount of metabolites generated in enterocytes (Metabolite-distribution method).
RESULTS: After oral administration of CYP3A substrates in ABT-pretreated rats, the portal and systemic plasma concentrations of the metabolite were nearly the same, indicating almost complete inhibition of intestinal CYP3A-mediated metabolism. Using Enzyme-inhibition method, Fg of midazolam (1 mg/kg) was calculated as 0.71. Additionally, total amount of raloxifene-6-glucuronide generated in enterocytes after oral administration of raloxifene was estimated using Metabolite-distribution method and Fg of raloxifene (0.98 μmol/kg) was calculated as 0.21.
CONCLUSIONS: PV rats enabled in vivo quantitative assessment of intestinal first-pass metabolism by CYP3A and UGT. This method is useful for clarifying the cause of low bioavailability.