Caco-2 models

Taylor, E.W., Giboons, J.A., and Baeck-man, R.A. Intestinal absorption screening of mixtures from combinatorial libraries in the Caco-2 model. Pharm. Res. 1997, 14, 572-577. 

The pION double-sink GIT model, with donor pH 5, predicts the human jejunal permeabilities as well as the best reported Caco-2 model (Artursson s), and a lot better than the rest of the reported Caco-2 models, as shown in Fig. 7.63. 

Caco-2 Models for Prediction of Human Intestinal Absorption (HIA)... 

More than a decade ago, Caco-2 cells grown on permeable supports were introduced as an experimental tool for mechanistic studies of intestinal drug transport [1-4]. At the same time it was suggested that the Caco-2 model was suitable for screening intestinal drug permeability and predicting the oral absorption potential... 

Delie, F. and W. Rubas. A human colonic cell line sharing similarities with enterocytes as a model to examine oral absorption advantages and limitations of the Caco-2 model, Crit. Rev. Ther. Drug Carrier Syst. 1997, 34, 221-286... 

These suggested factors for standardization of the Caco-2 model might also be reasons for the variability in published permeability values obtained from different laboratories [86]. Clearly, data from different publications or laboratories should not be mixed without prior harmonization of the experimental protocols. 

As the Caco-2 cell model contains most of the important transporters, it can be used for the study not only of passive transport mechanisms but also of mechanisms involving transporters. This is a major advantage of the Caco-2 model when... 

Another limitation of the Caco-2 monolayers is their colonic origin and tight paracellular pathway, which tend to lead to underestimations in permeability to paracellularly transported compounds [97]. This is likely to be correct for small compounds (MW < 150) - i.e., compounds smaller than normal drugs - but it remains to be seen to what extent the Caco-2 model gives false-negative predictions of the fraction absorbed for polar drugs of normal size in humans where para-... 

Both active and passive transport occur simultaneously, and their quantitative roles differ at different concentration gradients. At low substrate concentrations, active transport plays a major role, whilst above the concentration of saturation passive diffusion is the major transport process. This very simple rule can be studied in an experimental system using cell culture-based models, and the concentration dependency of the transport of a compound as well as asymmetric transport over the membrane are two factors used to evaluate the presence and influence of transporters. Previous data have indicated that the permeability of actively absorbed compounds may be underestimated in the Caco-2 model due to a lack of (or low) expression of some uptake transporters. However, many data which show a lack of influence of transporters are usually derived from experiments... 

In addition to oxidative and conjugative metabolism, other enzymes are also present in the Caco-2 model, though at lower levels than in the human enterocyte in vivo. These are mainly apical cell surface peptidases, such as aminopeptidases... 

Gan, L.-S. L., Thakker, D. R., Applications of the Caco-2 model in the design and development of orally active drugs elucidation of biochemical and physical barriers posed by the intestinal epithelium, Adv. Drug Deliv. Rev. 1997, 23, 77-98. 

Walgren, R. A., Walls, T., The influence of plasma binding on absorption/exsorption in the Caco-2 model of human intestinal absorption, J. Pharm. Pharmacol. 1999, 52, 1037— 1040. 

Heylen, P., van den Mooter, G., Kinget, R., Drug absorption studies of prodrugs esters using the Caco-2 model evaluation of ester hydrolysis and transepithelial transport, Int. J. Pharm. 1998, 166, 45-53. 

Fig. 17.2. PLS score plot for the VolSurf Caco-2 model. Light open circles represent penetrating compounds dark open circles represent nonpenetrating compounds. Filled circles represent the projection (prediction) of compounds in Table 17.1 in the Caco-2 model.

Cefadroxyl and cefaclor are beta-lactam antibiotics which show high affinity for the PepTl carrier system, whereas the other two beta-lactams, cephalotin and cef-metazole, are not recognized by PepTl protein and are not actively transported in the intestine. However, as the VolSurf Caco-2 model predicts that all the beta-lactams are nonpenetrating compounds, it is very probable that, as they rely only the diffusion mechanism, cefadroxyl and cefaclor will not cross the cell monolayer. 

It is important to recognize that the in vitro permeability obtained in cell mono-layers (such as Caco-2 models) should be considered as a qualitative rather than quantitative value. Especially poor are predictions of fraction dose absorbed for carrier-mediated drugs with low Caco-2 permeability and predictions of high fraction dose absorbed in humans [7, 20, 42, 48, 51]. However, it is possible to establish a reasonably good IVIVC correlation when passive diffusion is the dominating absorption mechanism. 

Other Limitations The small intestine is a very dynamic environment the pH of the medium changes, the intestinal content is propelled by muscular contractions, food and xenobiotics are being digested by different enzymes and after absorption of compounds by the enterocytes, these compounds reach the blood vessels to be transported throughout the body. In contrast to the in vivo situation, the Caco-2 model is a static model consisting of one single cell type which is unable to secrete mucus. The unstirred water layer is thicker than the one in vivo and for some compounds it is difficult to work under sink conditions. 

Another drawback is the lack of correlation for paracellularly transported compounds. For example, some low molecular weight hydrophilic compounds (e.g., ranitidine, atenolol, furosemide, and metformin) showed poor permeability in the Caco-2 model despite an absorption larger than 50% in humans [168], This is due to the smaller size of the paracellular channels (controlled by tight junctions) in the Caco-2 model compared to human small intestine [146],... 

Physiological Buffer Systems Recently, a lot of efforts have been made on how to increase the biorelevance of the Caco-2 model [63, 47, 64, 65,105], Historically, the media used for Caco-2 experiments were buffered at pH 7.4 on both sides of the monolayer. The pH in the cellular interstice and blood compartment is known to be 7.4. However, the pH in the upper GI tract under fasted conditions ranges from 5.0 to 6.5, with an acidic microclimate existing just above the epithelial cell layer estimated to be between 5.8 and 6.3 [90], The pH of the apical medium can have a critical effect on the transport of drugs, especially for drugs with a pKa close to 7, or when pH-dependent transporters are involved. 

Similar to the PAMPA and the Caco-2 models, the experimental pH of the buffer solution can be changed in the Ussing chambers model. However, it seems that the impact of changing the pH of the mucosal (= apical) buffer solution is lower than for the other two systems [82], This is probably due to the presence of the mucus layer retaining the microclimate pH regardless of the luminal pH using the Ussing chambers technique [82],... 

He X, Kadomura S, Takekuma Y, Sugawara M, Miyazaki K (2004) A new system for the prediction of drug absorption using a pH-controlled Caco-2 model Evaluation of pH-dependent soluble drug absorption and pH-related changes in absorption. J Pharm Sci 93 71-77. 

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