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Function and modulation of physical and biological barriers to drug absorption in the rat intestinal mucosa
Mudra, Daniel R.
Mudra, Daniel R.
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Abstract
Preclinical drug development is dependent upon human and laboratory-animal derived experimental models of absorption, metabolism and elimination requiring a thorough understanding of drug absorption barriers. In the present study, a model of drug absorption, the single-pass in situ perfused rat intestine, was utilized to investigate the function and modulation of physical absorption barriers including epithelial tight-junctions and biological absorption barriers including cytochromes P450 (CYP) and P-glycoprotein (P-gp). Sample preparation and analytical methods utilizing high performance-liquid chromatography coupled with tandem mass spectrometry were developed to precisely and accurately measure levels of paracellular and transcellular drug permeants and oxidative metabolites in both intestinal perfusate and rat plasma. Isolated in situ segments of rat intestine were perfused with atenolol and verapamil (pH 6.5), with simultaneous monitoring of parent drug and norverapamil levels in perfusate and plasma collected from the mesenteric vein. These studies demonstrated a suitable experimental model to simultaneously monitor the function and modulation of passive diffusion barriers as well as CYP3A-mediated metabolism and P-gp-mediated secretory efflux in the rat intestinal mucosa where the presence of CYP3A or P-gp inhibitors resulted in the selective modulation of these biological barriers. A clearance-based pharmacokinetic model was developed to quantify the simultaneous linear absorption and saturable metabolism and efflux processes limiting the absorption of atenolol and verapamil, respectively. This multi-compartmental model appropriately identified several known chemical inhibitors as modulators of passive diffusion barriers (EDTA), CYP3A-mediated metabolism (ketcoconazole) or P-gp-mediated efflux (ketoconazole, PSC-833 and GF-120918). Finally, this combined experimental approach was used to investigate the effects of commonly used polyethoxylated solubilizing agents demonstrating that such preclinical excipients are capable of altering drug absorption across the rat intestinal mucosa by distinct mechanisms of action including modulation of passive absorption barriers (polysorbate 80), CYP3A-mediated metabolism (polyethylene glycol-400) or P-gp-mediated secretory efflux (d-α-tocopheryl polyethylene glycol-1000 succinate and Cremophor EL). Collectively, these studies demonstrate a novel utility and application of the in situ perfused rat intestine to simultaneously monitor absorption, metabolism and efflux processes and contribute extensively to the knowledge of rat intestinal drug absorption barriers and their perturbation in the presence of concomitantly administered drugs or preclinical excipients.
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Dissertation (Ph.D.)--University of Kansas, Pharmaceutical Chemistry, 2007.
Date
2007-12-31
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University of Kansas
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Keywords
Pure sciences, Atenolol, Diffusion barriers, Drug absorption, Intestinal mucosa, Morverapamil, Verapamil