Reevaluation of the absorption of carbenoxolone using an in situ rat intestinal technique

Application of rat in situ single-pass intestinal perfusion in the evaluation of presystemic extraction of indinavir under different perfusion rates

BACKGROUND/PURPOSE

First-pass effect has been an important concern for oral pharmaceuticals. An in vivo system was developed for measuring different concentrations of pharmaceuticals in the portal vein and hepatic vein (via the inferior vena cava) for delineating presystemic metabolism under different perfusion rates by using indinavir as an exemplary agent.

METHODS

An in situ single-pass intestinal perfusion technique was modified from previous studies to concomitantly obtain portal and hepatic venous bloods. Portal and hepatic venous samples were simultaneously taken from rats at appropriate time points using the perfusion model of 1 mg/mL indinavir at flow rates of 0.05, 0.1, 0.5 and 1.0 mL/min. The indinavir concentrations were assayed by binary-gradient high-pressure liquid chromatography with UV detection.

RESULTS

The mean indinavir concentrations in portal vein concentration-time profiles at different perfusion times under various flow rates were all higher than those obtained for hepatic veins. At flow rates of 0.5 and 1.0 mL/min, in particular, the area under the curve (AUC) and maximal concentration (Cmax) of indinavir absorption were significantly different between portal veins and hepatic veins (p < 0.05), indicating considerable hepatic involvement in the presystemic extraction of indinavir. The system also has potential for use when estimating the hepatic extraction ratio (E(H)) and hepatic clearance (Cl(H)).

CONCLUSION

This in vivo approach could provide another useful tool for improving our basic understanding of the absorption kinetics and hepatic metabolism of pharmaceuticals under development and facilitating the clinical application of such.

High-methoxyl pectin has greater enhancing effect on glucose uptake in intestinal perfused rats

OBJECTIVE

Pectins have been known to decrease blood glucose levels. However, the mechanism of this effect is unclear. The direct action of various pectins (high- or low-methoxyl pectins) on the intestinal absorption of glucose was investigated in gut-perfused rats.

METHODS

After equilibrium, jejunal and ileal segments were simultaneously perfused with an isotonic electrolyte solution (pH 7.4) containing glucose (10 mM/L) and high- or low-methoxyl pectins (10 g/L). Each test or control solution was perfused in a random sequence, with perfusion times of 30 min. Changes in glucose concentration of perfusate solution reservoir were determined over the experimental period.

RESULTS

High- and low-methoxyl pectins in the perfusate significantly inhibited jejunal uptake of glucose compared with the control (P < 0.05). High-methoxyl pectins had greater inhibitive effect on intestinal absorption of glucose than low-methoxyl pectins. The observed changes in glucose and water absorptions caused by high- or low-methoxyl pectins were reversible by switching to a pectin-free perfusate. In addition, net water absorption changed to secretion after addition of high- or low-methoxyl pectins.

CONCLUSIONS

These results suggest that the decrease in intestinal absorption of glucose observed after perfusion of high- or low-methoxyl pectins may be caused by viscosity-related increases in mucosal unstirred layer thickness.

A human physiologically-based model for glycyrrhzic acid, a compound subject to presystemic metabolism and enterohepatic cycling

PURPOSE

To analyze the role of the kinetics of glycyrrhizic acid (GD) in its toxicity. A physiologically-based pharmacokinetic (PBPK) model that has been developed for humans.

METHODS

The kinetics of GD, which is absorbed as glycyrrhetic acid (GA), were described by a human PBPK model, which is based on a rat model. After rat to human extrapolation, the model was validated on plasma concentration data after ingestion of GA and GD solutions or licorice confectionery, and an additional data derived from the literature. Observed interindividual variability in kinetics was quantified by deriving an optimal set of parameters for each individual.

RESULTS

The a-priori defined model successfully forecasted GA kinetics in humans, which is characterized by a second absorption peak in the terminal elimination phase. This peak is subscribed to enterohepatic cycling of GA metabolites. The optimized model explained most of the interindividual variance, observed in the clinical study, and adequately described data from the literature.

CONCLUSIONS

Preclinical information on GD kinetics could be incorporated in the human PBPK model. Model simulations demonstrate that especially in subjects with prolonged gastrointestinal residence times, GA may accumulate after repeated licorice consumption, thus increasing the health risk of this specific subgroup of individuals.

Oleuropein, an antioxidant polyphenol from olive oil, is poorly absorbed from isolated perfused rat intestine

Epidemiological studies have shown that the incidence of heart disease and certain cancers is lower in the Mediterranean region. This has been attributed to the high consumption of olive oil in the Mediterranean diet, which contains polyphenolic compounds with antioxidant activity. Although many in vitro studies have been performed to elucidate mechanisms by which these compounds may act, there are virtually no data relating to their fate after ingestion. Therefore, we decided to investigate the intestinal absorption of one of the major olive oil polyphenolics, oleuropein. To do this, a novel in situ intestinal perfusion technique was developed, and the absorption of oleuropein was studied under both iso-osmotic and hypotonic luminal conditions. Oleuropein was absorbed, with an apparent permeability coefficient (P:(app)) of 1.47 +/- 0.13 x 10(-6) cm/s (+/-SE) observed under iso-osmotic conditions. The mechanism of absorption is unclear but may involve transcellular transport (SGLT1) or paracellular movement. Under hypotonic conditions, the permeability of oleuropein was significantly greater (5.92 +/- 0.49 x 10(-6) cm/s, P: < 0.001). This increase is thought to be due to an increase in paracellular movement facilitated by the opening of paracellular junctions in response to hypotonicity. Overall, we determined that the olive oil polyphenolic oleuropein can be absorbed, albeit poorly, from isolated perfused rat intestine. Therefore, it is possible that it or its metabolites may confer a positive health benefit after the consumption of olive oil, most likely via an antioxidant mechanism.

The water-soluble extract of chicory reduces glucose uptake from the perfused jejunum in rats

Among the components of dietary fiber, the soluble fibers have been found to impair glucose absorption. Little is known, however, about the mechanism of this effect. The direct action of soluble fibers (chicory water-soluble extract and inulin) on the intestinal absorption of glucose was investigated in gutperfused rats. After equilibrium, both jejunal and ileal segments were simultaneously perfused with an isotonic electrolyte solution (pH 7.4) containing glucose (10 mmol/L) and chicory water-soluble extract (chicory extract) or inulin (10 g/L). Each test or control solution was perfused in random sequence, with perfusion times of 30 min. Chicory extract or inulin in the perfusate (10 g/L) inhibited the absorption of glucose from jejunum (P < 0.05). The observed changes in glucose and water absorption caused by chicory extract or inulin were reversible after switching to a fiberfree perfusate. Additionally, net water absorption changed to secretion upon addition of chicory extract or inulin. These results suggest that the reduction in intestinal absorption of glucose observed after perfusion of chicory extract or inulin may be caused by viscosity-related increases in mucosal unstirred layer thickness.

Effect of fat substitutes, sucrose polyester and tricarballylate triester, on digitoxin absorption in the rat

The effect of non-absorbable fat substitutes (sucrose polyester (SPE) and tricarballylate triester (TCTE)) on [3H]digitoxin intestinal absorption was studied in the rat using a small intestine in-situ perfusion technique. The effect of SPE and TCTE was compared with that of sunflower oil, oleic acid, and saline. After 120 min perfusion, 5% SPE emulsion significantly reduced (P < 0.001) digitoxin absorption compared with all other treated groups. Five per cent TCTE emulsion had a less marked effect than SPE (P = 0.0002) and did not differ from sunflower oil. No difference was found between saline and 5% oleate emulsion, which did not reduce digitoxin absorption compared with other treated groups (P < 0.02). When taurocholic acid and lipase were added, results for the saline-, TCTE-, and SPE-treated groups were similar to those above, but the sunflower oil-treated group showed significantly enhanced (P < 0.01) digitoxin absorption. Thin-layer chromatography of the lipid phases showed hydrolysis of sunflower oil in the presence of taurocholic acid and lipase, but not of TCTE or SPE. The inhibitory effect of the non-absorbable fat substitutes on digitoxin absorption could be related to drug sequestration by the persistent oil phase constituted by the undigested and then unabsorbed fat substitutes. That part of digitoxin dissolved in the undigested oil phase is consequently unavailable for intestinal absorption.