Human studies intestinal metabolism

Owing to the limitations of the methods available to study intestinal metabolism in a clinical setting (see Section 12.4.1), in vitro approaches with human systems give important, often the only information accessible on the intestinal metabolism of a drug or an NCE. Human in vitro systems suitable for the determination of gut... 

The Caco-2 cell line was isolated from a human colon carcinoma, and has been characterized as one of the best in vitro models of intestinal epithelium. Indeed, in contrast to other intestinal cell lines, Caco-2 cells are able to constitute a homogenous monolayer and to spontaneously differentiate into polarized cells, highly similar to human mature enterocytes, after approximately 2 weeks of culture. Furthermore, the Caco-2 cells present microvillosities at the apical side and have a high transmembrane resistivity, which confirms the fact that the cells are confluent and link to one another via gap junctions. Finally, they can absorb different compounds, express many enzymes involved in intestinal metabolic pathways (Pinto et al. 1983, Musto et al. 1995, Salvini et al. 2002), and give reproducible in vitro results consistent with results obtained in in vivo studies (Artursson and Karlsson 1991). 

Esterases form a wide family of enzymes that catalyze the hydrolysis of ester bonds. They are ubiquitously expressed in all tissues including the intestine, and are found in both microsomal and cytosolic fractions. Prueksaritonont et al. [6] have studied the metabolism of both p-nitrophenol acetate and acetylsalicylic acid by esterases from human intestinal microsomal and cytosolic systems, and the activity values were 2.76 pmol min-1 mg-1 and 0.96 nmol min-1 mg-1, respectively. Thus, the activity for the hydrolysis of p-nitrophenol acetate in human intestine approaches that in the liver. 

Evidence of the formation of mono-M-octylphthalatc and phthalate ester metabolites has been shown in in vitro studies. The appearance of mono-w-octylphthalatc was observed with preparations of human small intestine, rat liver and intestine, ferret liver and intestine, and baboon liver and intestine (Lake et al. 1977). However, the amount of phthalic acid and other metabolites in these preparations was either minimal or not detected. The study authors concluded that di-ra-octylphthalate is probably absorbed primarily as mono- -octylphthalate (Lake et al. 1977). An in vitro study reported the formation of five keto acids and two diols when metabolic oxidation of the alkyl groups of di-ra-octylphthalate was simulated abiotically (Brodsky et al. 1986). Therefore, the in vivo and in vitro data indicate that major oxidation may occur in the remaining alkyl chain after di-ra-octylphthalate has been hydrolyzed to the... 

Prueksaritanont, T., Gorham, L.M., Hochman, J.H., Tran, L.O., and Vyas, K.P., Comparative studies of drug-metabolizing enzymes in dog, monkey, and human small intestines, and in Caco-2 cells, Drug Metab. Dispos., 24, 634,1996. 

Systemic bioavailability is the product of fraction of dose absorbed (/a), fraction of dose escaping gut metabolism (/g), and fraction of dose escaping first-pass metabolism (F ). Permeability class is based upon /a, which may be estimated either in vivo or in vitro by direct measurement of mass transfer across human intestinal epithelium. In vivo methods include (i) mass balance studies using unlabeled, stable-isotope labeled, or a radiolabeled drug substance (ii) oral bioavailability using a reference intravenous dose or (iii) intestinal perfusion studies either in humans or an acceptable animal model. Suitable in vitro methods involve the use of either excised human/animal intestinal tissues or cultured epithelial monolayers. All of these methods are deemed appropriate for drugs whose absorption is controlled by passive mechanisms. 

Cells throughout the gastrointestinal tract release somatostatin. Somatostatin inhibits acid secretion in the stomach and it promotes absorption of sodium, chloride and water in the small intestine and colon (Krejs 1986). The somatostatin analogs octreotide and lanreotide have been shown to decrease intestinal secretion in animal models (Botella et al 1993) and in humans with specific metabolic intestinal secretory disorders however, these drugs are not used widely in human medicine. In one study in horses, octreotide was shown to decrease gastric acidity (Sojka et al 1992) but its effects on intestinal or colonic secretion in horses have not been reported. 

C-labelled lithocholic acid sulphates, 27, found in both human and animal bile30,31, have been synthesized in a one-step32 sulphonation procedure (equation 11) to study the metabolic processes caused by human intestinal microflora and to make possible the identification of new mutagenic/carcinogenic products. Fecal bile lithocholic acid enhances liver and colon tumorigenesis. 

Experiments carried out on laboratory animals have shown that (radioactive) strontium is rapidly deposited in the skeleton, from which it is only slowly removed in the normal process of metabolism. Once ingested, of the total amount in vertebrate animals or humans more than 99 % of the strontium is localized in bone and connective tissue, which in total is about 320 mg for a 70-kg adult. For herbivorous animals the bone strontium level is higher than that present in humans, probably due to the higher dietary intake of strontium from plant material. Retention of strontium in the human body after oral doses of 100-250 mg has amounted to 12-24% in a month (Harrison et al. 1955). More strontium is absorbed by persons on a low calcium intake than on a high calcium intake. Animal as well as human studies have demonstrated that the intestinal absorptions of strontium and calcium are comparable, which led to the hypothesis that these elements share a common carrier system in the intestinal wall (Reid etal. 1986). Stron-... 

According to information made available by the INCELL Corp., cell lines from human small intestine and colon have been isolated, characterized for organotypic and cell-specific markers, and grown under conditions that induce growth and/or differentiation. However, only limited information is currently available, and these cell lines have not yet been extensively applied thus it will be some time before they can be confidently applied to study intestinal absorption or metabolism. 

To determine potential CYP450 interactions, in vitro, animal, and human studies are used, each with varying levels of accuracy for predicting clinically relevant interactions. While most research to date has been in vitro, the results of in vitro studies are often quite different from animal and human studies. Such differences may be caused by chemicals added to in vitro studies to promote uptake of some compounds that can exaggerate experimental findings by changing solubility that would otherwise limit uptake. In vitro studies sometimes use phytochemical concentrations that exceed those achieved in humans or other animals, or focus on isolated compounds that do not reflect the phytochemical complexity typical of extracts that may contribute to the bioactivity. Many compounds and extracts also undergo extensive metabolism in the intestines or liver,... 

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