Abstract
Digoxin, a substrate of P-glycoprotein (Pgp) and cytochrome P450 3a (Cyp3a), was used to illustrate the inductive effects of pregnenolone-16α-carbonitrile (PCN), a ligand of the pregnane X receptor, on the absorption and disposition of [3H]digoxin in the vascularly perfused rat small intestine preparation. Although increased Cyp3a protein was observed with Western blotting analysis after PCN treatment, metabolism of digoxin to the digoxigenin bis-digitoxoside metabolite in the rat small intestine remained insignificant (<4% dose). PCN pretreatment significantly decreased blood perfusate [3H]digoxin concentrations for both systemic and intraluminal administrations of [3H]digoxin due to increased Pgp levels. The apical secretion by Pgp increased at 90 min with PCN treatment, from 11.2 ± 5.1% of dose to 20.1 ± 8.6% of dose after systemic administration of [3H]digoxin; this increase was, however, statistically insignificant (P = 0.13) because of the high variability among preparations. When the composite data for the control and PCN-treated preparations were fit to published physiologically based pharmacokinetic models: the traditional model and the segregated flow model, suboptimal parameters were obtained. The data were further fit to expanded models with a bilayer membrane compartment housing the Pgp adjacent to the apical membrane, or an unstirred water layer (UWL) external to the apical membrane. The models with the UWL yielded improved fits and reasonable parameters associated with digoxin absorption, suggesting that the UWL posed as a barrier for digoxin absorption. Similar results were obtained with the segmental models (the segmental traditional model and the segmental segregated flow model) using the UWL, when heterogeneous distributions of Pgp in the duodenum, jejunum, and ileum were considered.
Footnotes
-
This work was supported by the Canadian Institute for Health Research, Grant MOP64350.
-
Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
-
doi:10.1124/dmd.105.008227.
-
ABBREVIATIONS: Pgp, P-glycoprotein; PCN, pregnenolone-16α-carbonitrile; Dg3, digoxin; Dg2, digoxigenin bis-digitoxoside; Dg1, monodigitoxoside; Dg0, digoxigenin; HPLC, high-performance liquid chromatography; TM, traditional model; SFM, segregated flow model; STM, segmental traditional model; SSFM, segmental segregated flow model; UWL, unstirred water layer; S1–S8, small intestinal segments 1 to 8; PBS, phosphate-buffered saline; KHB, Krebs-Henseleit bicarbonate; CL, clearance; AUC, area under the curve; rbc, red blood cells; MSC, model selection criterion; WSS, sum of squared differences; SSR, sum of squared residuals; Hct, hematocrit.
- Received November 5, 2005.
- Accepted May 31, 2006.
- The American Society for Pharmacology and Experimental Therapeutics
DMD articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|