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Figure 7.14 Relative concentrations of accetor and donor compartments as a function of time for the thin-membrane model.

We define this permeability as ''apparent,'' to emphasize that there are important but hidden assumptions made in its derivation. This equation is popularly (if not nearly exclusively) used in culture cell in vitro models, such as Caco-2. The sink condition is maintained by periodically moving a detachable donor well to successive acceptor wells over time. At the end of the total permeation time t, the mass of solute is determined in each of the acceptor wells, and the mole sum mA (t) is used in Eq. (7.10). Another variant of this analysis is based on evaluating the slope in the early part of the appearance curve (e.g., solid curves in Fig. 7.14):

It is important to remember that Eqs. (7.10) and (7.11) are both based on assumptions that (1) sink conditions are maintained, (2) data are taken early in the transport process (to further assure sink condition), and (3) there is no membrane retention of solute. In discovery settings where Caco-2 assays are used, the validity of assumption 3 is often untested.

The more general solutions (but still neglecting membrane retention, hence still ''apparent'') are given by ''two-way flux'' in Eqs. 7.12 (disappearance kinetics) and (7.13) (appearance kinetics).

2.303 Vd At

2.303 VD At

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