Jejunal Transport and Metabolism

Cytochrome P450 (EC 1.14.14.1) enzymes are well known for their ability to metabolize the majority of drugs, to detoxify environmental pollutants, and to activate some classes of carcinogens [93]. The most highly expressed subfamily is CYP 3A, which includes the isoforms CYP 3A4, CYP 3A5, CYP 3A7, and CYP 3A43 [93, 94]. The most abundant isoform is CYP 3A4, which corresponds to 30% of the total P450 content in the liver and about 70% of the total P450 content in the 

A direct in vivo assessment of the quantitative importance of gut wall metabolism and transport of drugs and metabolites in humans is difficult and consequently has been attempted only rarely [3, 6, 11, 12, 15, 16, 23, 25-32, 34, 35, 81]. The most direct in vivo approach to investigating these processes in drugs with variable and incomplete bioavailability was intestinal perfusion by single-pass per- 

A direct in vivo assessment was carried out with the single-pass perfusion approach in the human jejunum by using the Loc-I-Gut technique with R/S-verapamil (log D6 5 2.7, octanol/water pH 7.4 MW 455 Da) as the model compound for CYP 3A4 and P-gp-mediated local intestinal kinetics [2, 34, 35, 122] (see Figs. 7.7 and 7.9). The Peff for both enantiomers at each of the concentrations (4.0, 40, 120, and 400 mg L-1) was 2.5 x 10 4, 4.7 x 105.5 x 104 and 6.7 x 104 cm s-1, respectively (Fig. 7.15) [34, 35], A luminal concentration of 400 mg L 1 is expected to be achieved in the upper part of the small intestine after oral administration of a 100-mg dose of verapamil in an immediate-release dosage form [1, 34, 35], The three other perfusate concentrations represent fractions of the dose when 30%, 10%, and 1%, respectively are left to be absorbed [34, 35], The increased in vivo jejunal Peff of R/S-vcrapamil, along with its increased luminal perfusate concentration, is in accordance with a saturable efflux mechanism mediated by 

Investigations of regional differences in permeability and metabolism have been carried out using a variety of animal models [22, 29, 75, 102, 109, 112, 137]. Animal tissues (mainly from rats) are widely used in the Ussing chamber to investigate intestinal transport of drugs, and regional aspects [29, 75, 138], whereas few studies have been conducted with human tissues due to their limited availability. 

Overall, in this chapter we have attempted to emphasize the need for more in vivo studies to be conducted to clarify the dynamic interplay between mechanisms of drug transport and metabolism in the human intestine under in vivo conditions. There is also a need to develop additional in vivo techniques for direct measurements of these processes in regions along the GI tract in humans, and to relate the findings to various physiological/pathophysiological conditions. This would clearly increase our knowledge of the mechanisms involved, and provide in vivo data to help develop and validate rapid and reliable in vitro intestinal models. 

Ketoconazole, a well-known potent inhibitor of CYP3A4 metabolism and a less potent P-gp modulator, acutely inhibited CYP3A4 metabolism but did not affect the Peff of P/S-verapamil when they were coperfused through the human jejunal 

Glaeser et al. have shown that the majority of shed human enterocytes collected from an intestinal perfusion were still functionally active and did not show signs of apoptosis [38]. On the basis of a validation of the Loc-I-Gut system for the study of 

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ABSORPTION, DISTRIBUTION, AND ELIMINATION As with vitamin Bj, the diagnosis and management of folic acid deficiency depend on an understanding of the transport pathways and intracellular metabolism of the vitamin (Figure 53-10). Folates present in food are largely in the form of reduced polyglutamates, and absorption requires transport and the action of a pteroyl-glutamyl carboxypeptidase associated with mucosal cell membranes. The mucosae of the duodenum and upper part of the jejunum are rich in dihydrofolate reductase and can methylate most or aU of the reduced folate that is absorbed. Since most absorption occurs in the proximal small intestine, it is not unusual for folate deficiency to occur with jejunal disease. Both nontropical and tropical sprues are common causes of folate deficiency. 

A more recent example of this technique has been the study on human absorption characteristics of fexofenadine [109], Fexofenadine has been shown to be a substrate for P-gp in the in vitro cell lines its disposition is altered in knockout mice lacking the gene for MDRla, and co-administration of P-gp inhibitors (e.g. ketoconazole and verapamil) was shown to increase the oral bioavailability of fexofenadine [110-113], Hence, it is suggested that the pharmacokinetics of fexofenadine appears to be determined by P-gp activity. In the human model, the intestinal permeability estimated on the basis of disappearance kinetics from the jejunal segment is low, and the fraction absorbed is estimated to be 2% [114], Co-administration of verapamil/ketoconazole did not affect the intestinal permeability estimates however, an increased extent of absorption (determined by de-convolution) was demonstrated. The increased absorption of fexofenadine was not directly related to inhibition of P-gp-mediated efflux at the apical membrane of intestinal cells as intestinal Peff was unchanged. Furthermore, the effect cannot be explained by inhibition of intestinal based metabolism, as fexofenadine is not metabolised to any major extent. It was suggested that this may reflect modulation of efflux transporters in hepatocyte cells, thereby reducing hepatobiliary extraction of fexofenadine. 

The absorption of dietary zinc occurs over the duodenal and jejunal regions of the gastrointestinal tract, and mainly follows via a saturable carrier-mediated transport process (Zapsalis and Beck 1985, Lee et al. 1989). The mechanism and control of zinc absorption from the intestine has not yet been fully elucidated, although absorption of zinc is known to be regulated homeostatically, mainly under the control of pancreatic and intestinal secretion and fecal excretion. Homeostasis may involve metal-binding proteins such as metallothionein and cysteine-rich intestinal protein. Metallothionein plays an essential role in the regulation of zinc metabolism (Richard and Cousins 1975, Petering and Fowler 1986). Other unknown mechanisms may also exist, and the uptake from intestinal mucosa may involve both active and passive transport processes. 

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