Table 7.18 lists the interpolated apparent and membrane permeabilities, along with the membrane retentions, of the probe molecules used in the gradient pH study, at pH values 5.0, 5.5, 6.0, 6.5, and 7.4. Collander Relationship between 2% DOPC and 20% Soy Intrinsic Permeabilities

The 20% soy lecithin (Table 7.17) and the 2% DOPC (Table 7.15) intrinsic permeabilities may be compared in a Collander equation, as shown in Fig. 7.44. The slope of the regression line, soy versus DOPC, is greater than unity. This indicates that the soy membrane is more lipophilic than the DOPC membrane. Intrinsic permeabilities are generally higher in the soy system. Three molecules were significant outliers in the regression: metoprolol, quinine, and piroxicam. Metoprolol and quinine are less permeable in the DOPC system than expected, based on their apparent relative lipophilicities and in vivo absorptions [593]. In contrast, piroxicam is more permeable in DOPC than expected based on its relative lipophilicity. With these outliers removed from the regression calculation, the statistics were impressive at r2 0.97.

7.7.7 Evidence of Transport of Charged Species

In Section 4.8 the topic of charged-species absorption (''fact or fiction'') was first considered. The partitioning properties of some lipophilic charged molecules in the

TABLE 7.17 Intrinsic Permeabilities and the Unstirred Water Layer Permeabilities Determined from Gradient-pH Dependence of Effective Permeabilities: 20% Soy Lecithin in Dodecane, Sink in Acceptor
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