To use rodent models effectively in in-vivo investigations on oral drug and vaccine delivery, the conditions in the gastrointestinal tract must be understood. Some fundamental information is currently unavailable or incomplete. We have investigated the pH, water content and lymphoid tissue distribution along the gastrointestinal tract, as well as the stomach volume, as these were critical to our investigations on pH-responsive drug delivery and colonic vaccination. The observed values were compared with those in man as an indication of the validity of the rodent model. The mouse stomach pH was 3.0 (fed) and 4.0 (fasted), and the corresponding values in the rat were 3.2 (fed) and 3.9 (fasted). The mean intestinal pH was lower than that in man (

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Laboratory animals are often used in drug delivery and research. However, basic information about their gastrointestinal pH, fluid volume, and lymphoid tissue is not completely known. We have investigated these in, post-mortem, healthy guinea pigs, rabbits, and the pigs to assess their suitability for pre-clinical studies by comparing the results with reported human literature. The mean gastric pH (fed ad libitum) was 3.1 and 4.4 in guinea pig and pig respectively. In contrast, a very low pH (1.5) was recorded in the fed rabbits. The small intestinal pH was found in the range of 6.4 to 7.4 in the guinea pigs and rabbits, whereas lower pH (6.1-6.7) was recorded in the pig, which may have consequences for ionisable or pH responsive systems when tested in pig. A relatively lower pH than in the small intestine was found in the caecum (6.0-6.4) and colon (6.1-6.6) of the guinea pig, rabbit and the pig. The water content in the gastrointestinal tract of guinea pig, rabbit and pig was 51 g, 153 g and 1,546 g respectively. When normalized to the body weight, the guinea pig, rabbit and pig had larger amounts of water compared to man (guinea pig > rabbit > pig > man); in contrast, a reverse order was found when normalized to per unit length of the gut (guinea pig < rabbit < pig < man). The lymphoid tissue distribution (lymphoid follicles, Peyer's patches and long strips) along the length of the gut in these animals is presented; in particular, an abundance of lymphoid tissue was found in pig's stomach, small intestine and caecum, and rabbit's appendix. Their ample presence indicated the potential utility of these animal species in oral and colonic vaccination. These differences in the gastrointestinal parameters of the guinea pig, rabbit and pig in comparison to man reiterates the crucial importance of correctly selecting animal models for pre-clinical studies.

In vivo performance of "pH-responsive" dosage forms is complex and influenced by a multitude of factors other than just in situ pH.

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