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'''Caco-2''' refers to a cell monolayer absorption model. Cell-based functional [[assay]]s, such as the Caco-2 drug transport model for assessing intestinal transport, are extremely valuable for screening [[lead compound]]s in [[drug discovery]]. 

The Caco-2 cell line was developed by the Sloan-Kettering Institute for Cancer Research through research conducted by Dr. Jorgen Fogh.  

The Caco-2 cell line is widely used with in vitro assays to predict the absorption rate of candidate drug compounds across the intestinal epithelial cell barrier. The assay requires that drug absorption rates be determined 21 days after Caco-2 cell seeding to allow for monolayer formation and cell differentiation. 

The '''Caco-2''' cell line is a continuous line of heterogeneous human [[Epithelium|epithelial]] [[Colon (anatomy)|colorectal]] [[adenocarcinoma]] cells, developed by the [[Memorial_Sloan-Kettering_Cancer_Center|Sloan-Kettering Institute for Cancer Research]] through research conducted by Dr. Jorgen Fogh<ref>Fogh J and Trempe G in Human Tumor Cells In Vitro (J. Fogh, ed.), Plenum, 1975, 115-141</ref>.  

Although derived from a colon (large intestine) carcinoma, when cultured under specific conditions the cells become differentiated and polarized such that their phenotype, morphologically and functionally, resembles the enterocytes lining the small intestine<ref>Pinto M et al., Enterocyte-like differentiation and polarization of the human colon carcinoma cell line Caco-2 in culture. Biol Cell 1983;47:323-30</ref><ref>Hidalgo IJ et al., Characterization of the human colon carcinoma cell line (Caco-2) as a model system for intestinal epithelial permeability. Gastroenterology 1989;96:736-49</ref>.  Caco-2 cells express tight junctions, microvilli, and a number of enzymes and transporters that are characteristic of such enterocytes: peptidases, esterases, P-glycoprotein, uptake transporters for amino acids, bile acids carboxylic acids, etc.  They are commercially available through the American Type Culture Collection (ATCC; Manassas, VA, USA). 

When looking at Caco-2 cell cultures microscopically, it is evident even by visual inspection that the cells are heterogeneous.  As a result, over the years the characteristics of the cells used in different laboratories around the world have diverged significantly, which makes it difficult to compare results across labs<ref>Sambuy Y et al., The Caco-2 cell line as a model of the intestinal barrier: influence of cell and culture-related factors on Caco-2 cell functional characteristics. Cell Biol Toxicol 2005;21:1-26 </ref>.

Caco-2 cells are most commonly used not as individual cells, but as a confluent monolayer on a cell culture insert filter (e.g., Transwell®).  When cultured in this format, the cells differentiate to form a polarized epithelial cell monolayer that provides a physical and biochemical barrier to the passage of ions and small molecules<ref>Hidalgo IJ et al., Characterization of the human colon carcinoma cell line (Caco-2) as a model system for intestinal epithelial permeability. Gastroenterology 1989;96:736-49</ref>
<ref>Artursson P, Epithelial transport of drug in cell culture. I: A model for studying the passive diffusion of drugs over intestinal absorbtive (Caco-2) cells. J Pharm Sci 1990;79:476-82</ref>.  The Caco-2 monolayer is widely used across the pharmaceutical industry as an in vitro model of the human small intestinal mucosa to predict the absorption of orally administered drugs.  The correlation between the in vitro apparent permeability (P¬app) across Caco-2 monolayers and the in vivo fraction absorbed (fa) is well established<ref>Artursson P and Karlsson J, Correlation between oral drug absorption in humans and apparent drug permeability coefficients in human intestinal epithelial (Caco-2) cells. Biochem Biophys Res Comm 1991;175:880-5</ref>.[http://media.wiley.com/wires/WNAN/WNAN53/mfig003.jpg Transwell diagram]

This application of Caco-2 cells was pioneered in the late 1980s by Ismael Hidalgo, working in the laboratory of Ron Borchardt at the University of Kansas, and Tom Raub, who was at the Upjohn Company at the time.  Following stints at SmithKline Beecham and Rhone-Poulenc Rorer, Hidalgo went on to co-found a company, [http://www.absorption.com/Site/ Absorption Systems], in 1996, where he remains as Chief Scientist.  

The considerable impact of the Caco-2 cell monolayer model can be measured in at least two ways.  First, considering that poor pharmacokinetic properties accounted for ~40% of drug failures in development in the early 1990s and only ~10% by 2009, an interval in which Caco-2 monolayers were widely used throughout the pharmaceutical industry to predict absorption, it is not unreasonable to attribute some of that shift to this simple yet powerful model.  Second, the 1989 Gastroenterology paper that demonstrated the utility of the model for this application has been cited more than 1000 times since its publication.

The versatility of Caco-2 cells is demonstrated by the fact that, even to this day, they are serving as the basis for the creation of innovative new models that are contributing to our understanding of drug efflux transporters such as P-glycoprotein (ABCB1) and BCRP (ABCG2).  RNA interference has been used to silence the expression of individual efflux transporters, either transiently<ref>Watanabe T et al., Construction of a functional transporter analysis system using MDR1 knockdown Caco-2 cells. Pharm Res 2005;22:1287-93</ref> or long-term<ref>Zhang W et al., Silencing the breast cancer resistance protein expression and function in Caco-2 cells using lentiviral vector-based short hairpin RNA. Drug Metab Disp 2009;37:737-44</ref><ref>Darnell M et al., Investigation of the involvement of P-gp and MRP2 in the efflux of ximelagatran and its metabolites by using short hairpin RNA knockdown in Caco-2 cells. Drug Metab Disp dmd.109.029967; published ahead of print December 18, 2009</ref>.

== See also ==
*[[Cell culture]]
*[[Drug development]]
*[[Pre-clinical development]]


==References==
*Artursson P, Palm K, Luthman K., Caco-2 monolayers in experimental and theoretical predictions of drug transport, Adv Drug Deliv Rev. 2001 Mar 1;46(1-3):27-43.
*Shah P, Jogani V, Bagchi T, Misra A., Role of Caco-2 cell monolayers in prediction of intestinal drug absorption, Biotechnol Prog. 2006 Jan-Feb;22(1):186-98.

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[[Category:Clinical research]]
[[Category:Pharmaceutical industry]]
[[Category:Cell lines]]