Gastrointestinal tract permeability measurements

The permeability class boundary is based indirectly on the extent of absorption (fraction of dose absorbed, not systemic BA) of a drug substance in humans and directly on measurements of the rate of mass transfer across human intestinal membrane. Alternatively, nonhuman systems capable of predicting the extent of drug absorption in humans can be used (e.g., in vitro epithelial cell culture methods). In the absence of evidence suggesting instability in the gastrointestinal tract, a drug substance is considered to be highly permeable when the extent of absorption in humans is determined to be 90% or more of an administered dose based on a mass balance determination or in comparison to an intravenous reference dose. 

Bajka BH, Gillespie CM, Steeb CB, Read LC, Howarth GS (2003) Applicability of the Ussing chamber technique to permeability determinations in functionally distinct regions of the gastrointestinal tract in the rat. Scand J Gastroenterol 38(7) 732-741 Bass P, Kennedy JA, Wiley JN (1972) Measurement of faecal output in rats. Dig Dis 17... 

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