Caco-2 (short for Cancer coli-2) is a line of human cells originally taken from a colon tumor that has become one of the most widely used laboratory tools for studying how drugs and nutrients cross the intestinal wall. These cells are remarkable because, despite coming from cancerous tissue, they naturally mature into cells that closely resemble the absorptive lining of the small intestine. That property makes them invaluable for predicting whether an oral drug will actually make it into your bloodstream.
Where Caco-2 Cells Come From
The cell line was established in the 1970s by researcher Jorgen Fogh at the Sloan-Kettering Cancer Research Institute. He derived it from a human colorectal adenocarcinoma, a type of cancer that forms in the glandular cells of the colon. During that era, several epithelial cell lines were being created from gastrointestinal tumors, but Caco-2 stood out because of an unusual trick: when grown to full density on a surface, the cells stop behaving like cancer cells and start differentiating into something that looks and functions like the absorptive enterocytes that line the small intestine.
How Caco-2 Cells Mimic the Gut
Once Caco-2 cells are seeded onto a permeable membrane in the lab, they take roughly 21 days to fully differentiate. During that time, they form a single organized layer (called a monolayer) with a distinct top side facing “the gut” and a bottom side facing “the blood.” This polarity is critical because it lets researchers measure how a substance moves from one side to the other, simulating the journey a drug takes from your intestinal lumen into your circulation.
The differentiated cells express many of the same transport proteins found in real intestinal tissue. On their upper surface, they produce key efflux transporters: proteins that actively pump certain molecules back out into the gut. The three most important are P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), and multidrug resistance-associated protein 2 (MRP2). These pumps are present across the human duodenum, jejunum, ileum, and colon, so their expression in Caco-2 cells adds biological realism to the model.
There is also overlap between P-gp and a liver enzyme called CYP3A that breaks down drugs. The interaction between these two proteins can significantly affect how much of an oral drug actually reaches your system, and Caco-2 cells can help flag that interaction early in drug development.
How Researchers Use Them
The primary application is the permeability assay. Researchers dissolve a test compound and place it on the upper side of a Caco-2 monolayer, then measure how much appears on the lower side over time. This produces a permeability coefficient, a number that correlates with how well the drug would be absorbed in a living person. The assay works in both directions: top-to-bottom simulates absorption, while bottom-to-top reveals whether efflux pumps are pushing the drug back out. If the ratio between these two directions is less than 2, the drug is likely absorbed passively rather than being actively expelled.
Beyond single-compound screening, Caco-2 assays help identify drug-drug interactions. If one drug inhibits P-gp, it can change how much of a second drug gets absorbed. Researchers can detect these interactions by watching for shifts in the metabolic profile of the cells when exposed to known transporter inhibitors.
Role in FDA Drug Approval
Caco-2 data carries real regulatory weight. The FDA recognizes these cells as a validated tool for classifying drugs under the Biopharmaceutics Classification System (BCS), a framework that sorts drugs by how well they dissolve and how well they’re absorbed. When a drug company can demonstrate high permeability through a Caco-2 assay, it may qualify for a “biowaiver,” meaning it can skip expensive human bioequivalence studies. This applies to formulation changes during development, post-approval manufacturing changes, and generic drug applications.
There are limits to this regulatory acceptance. The FDA restricts Caco-2 data as the sole evidence of high permeability to passively transported drugs only. Because the cells can have lower or inconsistent expression of certain efflux and uptake transporters compared to living tissue, drugs that rely on active transport need additional evidence. To validate the assay, companies must show that their Caco-2 system produces a rank-order relationship between lab permeability values and known human absorption data across drugs with zero, low (under 50%), moderate (50 to 84%), and high (85% or above) absorption.
Growing and Maintaining Caco-2 Cells
Standard culture uses Dulbecco’s Modified Eagle Medium (DMEM) or Eagle’s Minimum Essential Medium supplemented with 10 to 20% fetal bovine serum, along with non-essential amino acids and glutamine. Researchers typically seed the cells onto transwell inserts, small cups with a porous bottom that separates the upper and lower compartments. Over the 21-day differentiation window, they monitor the monolayer’s integrity by measuring transepithelial electrical resistance (TEER), which gauges how tightly the cells are sealed together. Fully mature Caco-2 monolayers generally produce TEER readings in the range of roughly 90 to 140 ohm·cm², though values vary depending on the specific clone and measurement equipment used.
Known Limitations
Caco-2 cells are a simplified version of the gut, and that simplification comes with trade-offs. The real intestinal lining contains multiple specialized cell types: absorptive enterocytes, mucus-secreting goblet cells, hormone-releasing enteroendocrine cells, and antimicrobial Paneth cells. A standard Caco-2 monolayer is made entirely of enterocyte-like cells, which means there is no mucus layer. In a living gut, mucus acts as a physical and chemical barrier that slows down or traps particles, bacteria, bile acids, and enzymes before they reach the cell surface. Without it, permeability measurements may overestimate how easily a substance crosses the intestinal wall.
The cells also lack certain metabolizing enzymes and transporters found in healthy small intestinal tissue. Because Caco-2 originates from colonic (large intestine) tissue, it doesn’t perfectly replicate small intestinal physiology even after differentiation. TEER readings from Caco-2 monolayers tend to run higher than those from actual small intestinal tissue, and there is substantial variation between labs due to differences in equipment, cell passage number, and culture protocols.
Co-Culture Models That Fill the Gaps
To address the missing mucus layer, researchers often grow Caco-2 cells alongside a second cell line called HT29-MTX, which is derived from intestinal goblet cells. These cells produce both membrane-bound and gel-forming mucins, creating a mucus coating on top of the monolayer that better reflects real gut conditions. The most physiologically relevant seeding ratios fall between 90:10 and 75:25 (Caco-2 to HT29-MTX). This range strikes the best balance between maintaining a strong absorptive barrier and generating a meaningful mucus layer. The co-culture system provides a model containing the two most common epithelial cell types in the normal intestine: absorptive cells and goblet cells.