Drug transfer across intact rat intestinal mucosa following surgical removal of serosa and muscularis externa

Assessing seromuscular layer and serosa removal on intestinal permeability measurements in weaned piglet everted sac segments

The integrity of the intestinal barrier is crucial for regulating the passage of pathogens and toxins, while facilitating nutrient absorption. The everted gut sac technique, an ex-vivo technique, can be used to study interventions on barrier function. This cost-effective approach utilizes relatively large gut segments to study specific intestinal regions. Typically, intact (non-stripped) intestinal segments are used, but their use may underestimate permeability due to the medial positioning of blood vessels relative to the seromuscular layer and serosa. However, removing these layers risks physical damage, resulting in an overestimation of intestinal permeability. Therefore, we investigated the impact of stripping jejunal segments on permeability to fluorescein isothiocyanate-dextran (FITC, 4 kDa) and tetramethylrhodamine isothiocyanate-dextran (TRITC, 40 kDa), and on the absorption of glucose, lysine, and methionine in jejunal segments from 80 piglets at 8 d postweaning. Piglets were subjected to either high or low sanitary housing conditions and diets provoking intestinal protein fermentation or not, expected to influence intestinal permeability. Stripping of the seromuscular layer and serosa increased the passage of 4 kDa FITC-dextran (stripped vs. non-stripped; 1.1 vs. 0.9 pmol/cm2/min, P < 0.001), glucose (40.0 vs. 19.1 pmol/cm2/min, P < 0.001), lysine (2.5 vs. 2.0 nmol/cm2/min, P < 0.001), and methionine (4.1 vs. 2.7 pmol/cm2/min, P < 0.001). As permeability increased, the differences in methionine passage between stripped and non-stripped intestinal segments also increased (slope = 1.30, P = 0.009). The coefficients of variation were comparable between stripped and non-stripped intestines (over all treatments, stripped vs. non-stripped 38% vs. 40%). Stripping, by isolating mucosal processes without introducing additional variation, is thus recommended for studies on intestinal permeability or absorption.

Challenges and Opportunities in the Oral Delivery of Recombinant Biologics

Recombinant biological molecules are at the cutting-edge of biomedical research thanks to the significant progress made in biotechnology and a better understanding of subcellular processes implicated in several diseases. Given their ability to induce a potent response, these molecules are becoming the drugs of choice for multiple pathologies. However, unlike conventional drugs which are mostly ingested, the majority of biologics are currently administered parenterally. Therefore, to improve their limited bioavailability when delivered orally, the scientific community has devoted tremendous efforts to develop accurate cell- and tissue-based models that allow for the determination of their capacity to cross the intestinal mucosa. Furthermore, several promising approaches have been imagined to enhance the intestinal permeability and stability of recombinant biological molecules. This review summarizes the main physiological barriers to the oral delivery of biologics. Several preclinical in vitro and ex vivo models currently used to assess permeability are also presented. Finally, the multiple strategies explored to address the challenges of administering biotherapeutics orally are described.

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

The absorption of the model drug carbenoxolone was reevaluated using an in situ rat intestinal perfusion technique in which disappearance from the intestinal lumen, binding to the perfused jejunal segment, and appearance in the mesenteric (jejunal) vein were measured. The effect of the degree of ionization on these processes was examined by employing perfusion solutions of pH 4.0, 4.4, 5.0, and 6.5. Tissue binding was observed to be independent of pH. There was a rank-order correlation of the transfer rate of carbenoxolone with the degree of ionization which indicated that carbenoxolone was absorbed faster in its ionized form. This observation is in direct opposition to the pH-partition hypothesis, a finding which appears to support the previous work of Bridges et al. Ion-pairing of carbenoxolone with sodium ion present in the pH 6.5 buffer is one possible explanation for the unusually high transfer rate seen at this pH. A more likely explanation is that at the low pH values, some carbenoxolone precipitated out of solution during the perfusion experiments, thereby reducing the driving force for diffusion across the intestinal wall.

Characterization of mitomycin C-induced gastrointestinal damage. II. In vitro everted sac experiment

The everted gut sac technique was employed to clarify the effects of preadministration of mitomycin C (MMC) on intestinal transport of various drugs. Loss of intestinal tissue weight and increase in mucosal-to-serosal fluxes of passively absorbed drugs were noted in the case of MMC pretreatment, although the extent of the latter effect varied according to the inherent absorbability of each drug. The maximal effect of MMC on intestinal tissue weight and transport of sulfanilamide, a model of a passively absorbed drug, was observed 48 hr after pretreatment. The increase in the transfer of sulfanilamide correlated well with the MMC-induced decrease in intestinal tissue weight. These phenomena may result from the shortened transfer distance from mucosal to serosal fluid and impaired barrier function of the intestinal mucosa. On the other hand, transport of actively absorbed 3-O-methyl-D-glucose was not influenced by MMC pretreatment, which could be explained by an increment in the passive permeation counterbalancing the decrement in the active permeation. The present study also suggested that the measurement of transport of drugs through everted gut sacs might be useful as a simple and qualitative index of gastrointestinal mucosal damage.

Effect of pH on theophylline transfer across the everted rat jejunum

The effect of pH on the cumulative transfer of theophylline across the everted rat jejunum in vitro was investigated. Intestinal integrity was assessed by light and scanning electron microscopy, while the biochemical viability of the intestine was evaluated using glucose transfer measurements. The initial (0-30 min) clearance of theophylline was directly proportional to the fraction un-ionized at pH 5.5, 7.4, 8.0, and 10.0. Plots of cumulative theophylline transfer versus time over 60 min were nonlinear, but could be subdivided into two linear segments of 30-min duration. Due to this nonlinearity, differences in theophylline transfer with pH were significant only over the first 30 min of the experiment. Intestinal tissue integrity and viability correlated with the time at which the clearance (slope) increased, while the magnitude of the increase in clearance was proportional to the degree of ionization of theophylline.

Intestinal absorption and metabolism of 6-mercaptopurine in the rat small intestine

The intestinal absorption of 6-mercaptopurine was examined in the rat by in vitro and in situ techniques for the purpose of establishing absorption characteristics which might explain the poor systemic oral availability of this drug. Experiments were designed to evaluate the significance of intestinal metabolism and active secretion processes. Since mucosal/serosal drug concentration gradients across in vitro segments were not significantly different (P greater than 0.05) for everted and noneverted preparations, with values of 1.21 +/- 0.27 and 0.91 +/- 0.12, respectively, it was concluded that active secretion or absorption mechanisms were absent. Varying the concentration of 6-mercaptopurine from 0.24 to 5.88 mM demonstrated saturability of the biotransformation of 6-mercaptopurine to 6-thiouric acid with a maximum rate of 1.6 X 10(-5) mmoles per min g for jejunal portions. Distal segments displayed 85% higher rates of biotransformation at concentrations of 1.47 mM. Inclusion of allpurinol (2 mM) completely inhibited biotransformation. With in situ loops perfused with 1.47 mM drug, collection of mesentery blood showed that absorption rates of 6-thiouric acid were 0.67 that of the parent drug.