Duodenum, drug absorption

Small intestine. The small intestine is the most important part for nutrient and drug absorption in the GI tract. The small intestine consists of the short, fixed duodenum and the relatively long, more mobile jejunum (the proximal half) and ileum (the distal half). The duodenum is the first part of the small intestine, connecting the stomach to the jejunum. The pancreatic and bile ducts open into the duodenum. The jejunum is broader and has a thicker wall than the ileum. The folds in the ileum are sparse and low in comparison with those in the jejunum. But the ileum has distinctive lymphoid tissue patches (Peyer s patches). The volume of the small intestine can vary from as little as 50 to 100 mL to several liters. The colon volume is also quite variable from 100 up to 2000 mL. 

The jejunum is the middle portion of the small intestine in between the duodenum and the ileum. Digestion of protein and carbohydrates continues after receiving pancreatic juice and bile in the duodenum, this portion of the small intestine generally has less contraction than the duodenum and is preferred for in vivo drug absorption studies... 

The drug dosage form may also be affected by food. For example, enteric-coated tablets may stay in the stomach for a longer period of time because food delays stomach emptying. If the enteric-coated tablet does not reach the duodenum rapidly, drug release and subsequent systemic drug absorption are delayed. In contrast, enteric-coated beads or microparticles disperse in the stomach, are less affected by food, and demonstrate more consistent drug absorption from the duodenum. 

Orally administered drugs are mainly absorbed in the small intestine (duodenum, jejunum, and ileum) and in the large intestine (colon) however, other regions, such as buccal cavity, stomach, and rectum, also can be considered potential sites for drug absorption. The various anatomical and physiological characteristics of each segment are briefly described in Table 1. 

The apical localized sodium-dependent bile add transporter (ASBT) is expressed in the human duodenum and ileum and is barely detectable in colon [16]. ASBT transports bile adds such as glycodeoxycholate and chenodeoxycholic add (XX) [49, 50]. Few examples exist where the bile acid scaffold has been used as a promoiety for a prodrug approach. ASBT has micromolar affinities for the natural substrates, and the studies on ASBT are too few to make a general statement on the potential and role of this transporter in drug absorption [49, 50]. 

The duodenum, jejunum and upper region of ileum provide the most efficient areas in the gastrointestinal tract for drug absorption. 

The location of the tip of the feeding tube is important when considering medication administration down a feeding tube. This is particularly true if the medication acts locally in the GI tract itself. For example, sucralfate and antacids act locally in the stomach. Therefore, administration of these medications through a duodenal or jejunal tube is not logical. Likewise, for medications such as itraconazole that require acid for best absorption, administration directly into the duodenum or jejunum would be expected to result in suboptimal absorption. Absorption of drugs when administered directly into the small bowel, especially the jejunum, rather than into the stomach is another area where more research would be useful. 

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