Gastrointestinal tract residence time

In the gastrointestinal tract, a mucoadhesive drug delivery system provides advantages in prolonging the residence time of devices. The use of pH-sensitive bioadhesive polymers has been proposed [26], An extensive review of pH-sensi-tive hydrogels is given by Brpndsted and Kopecek [27],... 

Colonic residence time as dictated by gastrointestinal tract motility... 

Table 1 Factors affecting drug absorption from the colon Physical characteristics of drug (pkTa> degree of ionization) Colonic residence time as dictated by gastrointestinal tract motility...

A formulation of ciclosporin (Sandimmun Neoral, Novartis) incorporates the drug in a preconcentrate which forms a microemulsion on dilution in aqueous fluids. This formulation offers an alternative to the oil formulation of ciclosporin (Sandimmun) which in vivo would be emulsified by bile salts and pancreatic enzymes. The residence time of ciclosporin in the gastrointestinal tract is shorter and the rate of absorption is faster with the microemulsion formulation (Fig. 7.19). 

There is no quantitative information on the absorption of gasoline following oral exposure in humans and animals. However, the absorption is believed to be relatively complete because of the high lipophilicity of the hydrocarbon compounds, the large surface area of the gastrointestinal tract, and the long resident time in the tract (NESCAUM 1989). 

P = effective permeability, r = intestinal radius, and = residence time in the small intestine [2]. The critical assumptions in this relationship are (1) that the drug is in solution and hence available for transport across the intestinal membrane and (2) the drug is chemically stable in the gastrointestinal tract and not a substrate for proteolytic or metabolic enzymes in the intestinal lumen or gut mucosa. These are highly simplifying assumptions. The inappropriate use of Eq. (2) to calculate absorption for compounds that are limited by solubility, instability, or metabolism can lead to seriously erroneous and misleading conclusions. 

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