Intestinal Perfusion Techniques

The CAT model considers passive absorption, saturable absorption, degradation, and transit in the human small intestine. However, the absorption and degradation kinetics are the only model parameters that need to be determined to estimate the fraction of dose absorbed and to simulate intestinal absorption kinetics. Degradation kinetics may be determined in vitro and absorption parameters can also be determined using human intestinal perfusion techniques [85] therefore, it may be feasible to predict intestinal absorption kinetics based on in vitro degradation and in vivo perfusion data. Nevertheless, considering the complexity of oral drug absorption, such a prediction is only an approximation. 

Methodological Aspects on in vivo Intestinal Perfusion Techniques... 

Methodological Aspects on In-vivo Intestinal Perfusion Techniques j 163... 

Keywords Oral absorption Bioavailability Models of small intestine Intestinal permeability Intestinal perfusion techniques Intestinal versus hepatic first-pass metabolism... 

In general, the intestinal perfusion technique has proved a powerful research tool, despite these shortcomings. Its primary application has been in the estimation of Peft the model also lends itself to comparing permeability differences from one site to another along the intestine, an essential prerequisite for accurately classifying controlled release products [59]. Cross-over experiments tend... 

Lennernas s group at Uppsala has performed extensive studies to confirm the validity of this in vivo experimental set-up at assessing the rate and the extent of drug absorption. Recovery of PEG 4000 (a non-absorbable marker) is more than 95%, which indicates that the absorption barrier is intact. In addition, maintenance of functional viability of the mucosa during perfusion has been demonstrated by the rapid transmucosal transport of D-glucose and L-leucine. Estimation of absorption half-lives from the measured Pefr agree well with half-lives derived from oral dose studies in humans (i.e. physiologically realistic half-lives). Human Peff estimates are well correlated with the fraction absorbed in humans, and served as the basis for BCS development, and hence the technique is ultimately the benchmark by which other in situ intestinal perfusion techniques are compared. The model has been extensively used to... 

To facilitate a standardisation of inter-laboratory results of permeability, it is now common practice to include a range of model drugs as internal standards in initial validation (i.e. method suitability) of intestinal perfusion techniques [116]. A list of 20 model drugs has been reported by the FDA for the standardisation of the in situ intestinal perfusion experiment, whereas six drugs are recommended for human studies. Once the method has been... 

Table 2.4 Compounds used to assess viability and/or modulate physiological functions in intestinal perfusion techniques.

Talinolol, a good P-gp substrate, is eliminated from the body mainly by intestinal and renal excretion with minimal metabolism in humans. In a clinical study, a P-gp-mediated interaction between talinolol and verapamil has been reported (45). The inhibitory effect of verapamil on the intestinal secretion of talinolol was determined in six healthy volunteers by using the intestinal perfusion technique. While perfusing the small intestine with a verapamil-free solution, the mean intestinal secretion rate of talinolol was 4.0 pg/min after an intravenous dose of talinolol. The intestinal secretion rate decreased to 2.0 pg/min when a verapamil-containing solution was perfused (45). Similar to the clinical data, talinolol-verapamil interaction was also observed in rats. Coadministration of verapamil (4 mg/kg, PO) resulted in a 2.5- and 2.2-fold increase in the plasma oral AUC for S- and //-talinolol, respectively, after an oral dose of racemic talinolol in rats. On the other hand, after an intravenous dose of racemic talinolol, the inhibitory effect of verapamil on talinolol was less significant and there was only a 40% and 30% increase in AUC, respectively, for S- and //-talinolol (46). These results suggest that the larger increase in AUC of talinolol ( 2- to 2.5-fold) after oral administration was likely due to the combination of an... 

Lennemas et al. have developed a method for measuring human effective permeability (H-Peff) using a regional intestinal perfusion technique. In this method, a perfusion apparatus consisting of a multichannel tube with two inflatable balloons (10 cm apart) is swallowed by the patient and eventually located in the proximal jejunum. Dilute solutions of the test drag are introduced at the inlet located at the center of the 10 cm section, and the loss of drag is determined from the concentration in the outlet intestinal perfusate. In such a fashion, the H-Peff for 22 carefully selected drug molecules has been determined and a theoretical model of H-Peff has been developed. " The small size of the published H-Peff database is most likely due to the expense of the human measurement. 

Clinical studies of Peff, secretion, and metabolism of various compounds such as drugs, environmental pollutants, and nutrients are rarely performed in vivo in humans even if experimental techniques are available (Figures 9.2-9.4) [3, 11, 13, 16, 17, 24—31]. Direct measurements of compound transport and metabolism in mesenteric and portal veins in humans are not possible for obvious reasons. Perfusion techniques, however, present great possibilities to measure intestinal processes. Over the past 70 years, different in vivo intestinal perfusion techniques have been developed and the importance of this work has been clearly demonstrated [3, 5, 6,11,13-16, 25-31]. The fundamental principle of an in vivo intestinal perfusion experiment is that Peff is calculated from the rate at which the compound disappears... 

Methodological Aspects of In Vitro Intestinal Perfusion Techniques 191... 

From studies using intestinal perfusion techniques with a recovery marker, it has been estimated that healthy subjects secrete about 4 to 6 g of bile acid in response to a liquid test meal (H18). This suggests that the enterohepatic cycle operates twice with each meal, so that for a three-meal day plus... 

Figure 4.14 Schematic drawing of the intestinal perfusion technique. The intestine of the animal is catheterised in both ends and a flow of buffer solution of 37°C is perfused using a pump. For in vitro studies in which only the absorption over the intestine is to be measured, the vascular support can be cannulated, and a separate buffer solution can be perfused through the intestine. If the influence of the liver on drug absorption is to be studied, an in situ system with an intact anaesthetised animal can be used by only perfusion of the intestine, keeping the blood flow of the vascular support intact.

Although a variety of models (subcellular fraction, cell monolayer model, isolated intestinal tissue, and intestinal perfusion) are available to predict the permeability of a drug, the cell monolayer model and rat intestinal perfusion techniques are the most commonly used techniques. 

The authors wish to thank Dr. Gary M. Gray and his laboratory for demonstrating the intestinal perfusion techniques, Amy Noma for many amino acid analyses and B. E. Powell and Lillie Davis for typing the manuscript. 

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