Abstract
The hepatic disposition of estradiol 17β D-glucuronide (E217G), a substrate of the Oatp's (Oatp1a1, Oatp1a4, Oatp1b2), was investigated in Wistar and TR-(Mrp2-mutant) rats to elucidate how absence of Mrp2, the major excretory transporter for both E217G and its 3-sulfate metabolite (E23S17G), affected the net sulfation. With absence of Mrp2, lower microsomal desulfation activity and higher Mrp3 but unchanged, immunoreactive protein expression of other transporters (Oatp's and Mrp4) and estrogen sulfotransferase (Sult1e1) were found in TR- rats. In recirculating, perfused liver preparations, the rapid decay of E217G and sluggish appearance of low levels of E23S17G in perfusate for Wistar livers were replaced by a protracted, bi-exponential decay of E217G and greater accumulation of E23S17G, whose levels reached plateaus upon the almost complete obliteration of biliary excretion of E217G and E23S17G in the TR- liver. Much higher amounts of E217G (28x) and E23S17G (11x) in liver and reduced net sulfation (40%±6% from 77%±6% dose, P <0.05) were observed at 2 h for the TR- vs. the Wistar rats. With use of a PBPK model, analytical solutions for the area under the curves (AUCs) for the precursor and metabolite were obtained to reveal how enzyme- and transporter-mediated processes affected the hepatic disposition of the precursor and metabolite in futile cycling. The analytical solutions were useful to explain transporter-enzyme interplay in futile cycling and predicted that a shutdown of Mrp2 function led to decreased net sulfation of E217G by raising the intracellular concentration of the metabolite, E23S17G, which readily refurnished E217G via desulfation.
- ABC transporters
- drug transport
- hepatobiliary transport
- isolated perfused liver
- mathematical modeling
- membrane transport
- metabolite kinetics
- pharmacokinetic modeling
- phase II drug metabolism
- physiologically-based modeling
Footnotes
- Received August 26, 2009.
- Accepted February 2, 2010.
- The American Society for Pharmacology and Experimental Therapeutics