Colon, absorption from

Absorption from the stomach and colon is minor compared with that from the small intestine. 

Chapter 3 Drug Absorption from the Colon In Situ 79... 

Thus, the apparent membrane permeability characteristics of hydrophilic compounds listed in Table 3.4 indicate that colonic epithelium is different from small intestinal epithelium in selectivity, or size or density distribution of the paracellular pathway. This is further complicated because of the possible involvement of unidentified carriers or channels for some compounds, as suggested for glycerol and D-xylose. However, the colon-to-SI ratios of the apparent membrane permeability are generally comparable with (or lower than) those calculated considering the morphological surface area, suggesting that such factors are not in favor for colonic absorption in most cases. Matching... 

The following factors influence absorption from the colon ... 

Log D accounts for where the "dose" is, so to speak. The distribution depends on the pKa as well as the pH of the medium. The advantage of using log D is that it incorporates these factors, so, for simple processes such as absorption, distribution coefficients "ej lain" the whole process. An example from our earlier work ( ) is the colonic absorption of acids ranging from phenols to strong carboxylic acids. The absorption rate is given by an equation involving only log D terms. 

Tozaki, H., et al. 1997. Chitosan capsules for colon-specific drug delivery improvement of insulin absorption from the rat colon. J Pharm Sci 86 1016. 

Colonic targeting has gained increasing interest over the past years. A considerable number of publications dealing with colon targeting, colon-specific drug delivery, and absorption from the cecum and other colon sections indicates a growing focus of research activities in this area. 

The scientific rationale of drug absorption from the colon was developed some years ago as exemplified by a series of publications [4-8], The extent of colonic absorption of drugs has been controversial in the literature, although this only parallels the controversies regarding the bioavailabilities of such drugs in general [10]. 

For some drugs that, for physiological and anatomical reasons, mainly follow a passive absorption mechanism, a satisfactory colonic absorption was demonstrated. Similar absorption rates from the small and large intestine were found for oxprenolol, metoprolol, isosorbide-5-mononitrate, and glibenclamide [9], It has also been known for many years, that some lipophilic vitamins, as well as bile salts and some steroids, that undergo enterohepatic circulation show satisfactory colonic absorption [2],... 

Obviously, satisfactory colonic absorption of the drug to be delivered is a sine qua non for the successful development of a colonic drug delivery system. However, situations exist when even reduced absorption from the large intestine (compared with the small intestine) would still justify colonic delivery, for instance in the case of a peptide drug that would otherwise be efficiently digested in the small intestine. 

A controversial problem is still the selection of suitable enhancing agents to overcome the often limited absorption from the colon. In a survey a number of commonly used enhancers, bile salts, surfactants, fatty acids, glycerides, etc. were also reported as effective in the colon [86],... 

H. Tozaki, T. Fujita, A. Yamamoto, S. Muranishi, T. Sugiyama, A. Terabe, T. Mat-sumoto, and T. Suzuki, Chitosan capsules for colon-specific drug delivery Improvement of insulin absorption from the rate colon, Proceed. Intern. Symp. Control. Rel. Bioact. Mater. 23 551-552 (1996). 

Below or at the CMC In this case it was assumed that the paracellular absorption rate constant, ko2, was the same as k.2 in Eq. (2), whereas the membrane absorption rate constant, koi, was calculated as was kj in colon tests from Eq. (4). Accordingly, the global ka value should be ... 

Dodecylmaltoside increased the small intestinal and colonic absorption of phenol red in rats without causing membrane protein and phospholipid release (Yamamoto et al. 1996 Uchiyama et al. 1996). These results, however, are not in agreement with an in vitro study showing a significant protein and phospholipid release from rat jejunum and colon when dodecylmaltoside is applied in the same concentration (Yamamoto et al. 1997). 

Kreydiyyeh et al. (2001) provided scientific evidence to confirm the laxative property of parsley, as claimed in folk medicine, and explained its mechanism of action. A perfusion technique was used to measure net fluid absorption from rat colon. The addition of an aqueous extract of parsley seeds to the perfusion buffer, and the omission of sodium, both significantly reduced net water absorption from the colon, as compared with the control. Parsley, added to a sodium-free buffer, did not lead to any further significant change in water absorption as compared with parsley alone suggesting that with parsley, sodium absorption was already inhibited. Since K+ and Cl secretion depends on the activity of the NaKCl2 transporter, the latter was inhibited with furosemide, which increased net water absorption significantly. When parsley and furosemide were... 

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