Using Peff to Estimate the Extent of Absorption

The fraction of drug remaining in the intestine (unabsorbed) at steady state is defined as ratio of inlet to outlet concentrations and can be calculated by rearranging Eq. 2.18 as follows  

A key assumption of this steady-state model is that complete transfer of drug occurs from the lumen to the portal vein (i.e. mass of drug loss reflects mass of drug appearing in blood). Therefore, the fraction of drug absorbed (/a) can be thus defined as per Eqs. 2.20 and 2.21  

Using this dimensionless analysis approach, the fraction absorbed can be predicted from Eq. 2.21 on the basis of two dimensionless variables as follows  

The Graetz number of the human intestine can be estimated to be about 0.5 using an intestinal length of 500 cm, a flow rate of 0.5 ml/min and a diffusivity of 5x 10-6 cm2/s. Consequently, Eq. 2.24 becomes 

The fundamental relationship hypothesised by Eq. 2.25 was first validated by comparing the extent of absorption reported from humans and Peff calculated 

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