Jejunum, drug absorption

PSA was also shown to play an important role in explaining human in vivo jejunum permeability [18]. A model based on PSA and calculated log P for the prediction of drug absorption [19] was developed for 199 well-absorbed and 35... 

Fagerholm, U., Lennernas, H., Experimental estimation of the effective unstirred water layer thickness in the human jejunum, and its importance in oral drug absorption. Eur. J. Pharm. Sci. 1995, 3, 247-253. 

The layer of water adjacent to the absorptive membrane of the enterocyte is essentially unstirred. It can be visualized as a series of water lamellas, each progressively more stirred from the gut wall toward the lumen bulk. For BCS class 2 compounds the rate of permeation through the brush border is fast and the diffusion across the unstirred water layer (UWL) is the rate-limiting step in the permeation process. The thickness of the UWL in human jejunum was measured and found to be over 500 pm [3]. Owing to its thickness and hydrophilicity, the UWL may represent a major permeability barrier to the absorption of lipophilic compounds. The second mechanism by which the UWL functions act as a barrier to drug absorption is its effective surface area. The ratio of the surface area of the UWL to that of the underlying brush border membrane is at least 1 500 [4], i.e., this layer reduces the effective surface area available for the absorption of lipophilic compounds and hence impairs its bioavailability. 

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. 

Another important physicochemical parameter is the pKa, which describes the ionisation state of a compound at a given pH. The ionisation state of a compound in the different components of the gastro intestinal system (stomach, jejunum, ileum and colon) is crucial for the understanding of drug absorption (Dressman). Ionised compounds generally have better solubility, but passive permeation through the membrane is limited (Comer). 

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... 

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

Bile salts may also influence drug absorption either by affecting membrane permeability or by altering normal gastric emptying rates, the latter having been explored by Feldman et al [246,247]. Sodium taurocholate increases the absorption of sulphaguanidine from the stomach, jejunum and ileum, the effect... 

Absorption of angiotensin II antagonists in Ussing chambers, Caco-2, perfused jejunum loop and in vivo importance of drug ionization in the in vitro prediction of in vivo absorption. Eur. J. Pharm. Sci. 2000, 10, 215-224. 

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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