Elsevier

Drug Discovery Today

Volume 8, Issue 21, 1 November 2003, Pages 997-1003
Drug Discovery Today

Review
In vitro trans-monolayer permeability calculations: often forgotten assumptions

https://doi.org/10.1016/S1359-6446(03)02873-3Get rights and content

Abstract

In designing effective therapeutic strategies, novel drugs must exhibit favorable pharmacokinetic properties. The physicochemical characteristics of a drug, such as pKa, molecular weight, solubility and lipophilicity, will influence the way the drug partitions from the aqueous phase into membranes, and thus, will influence its ability to cross cellular barriers, such as the lining of the gastrointestinal tract and the blood–brain barrier. Physicochemical characteristics also influence the degree to which a drug is able to cross a barrier layer, and the route by which it does this; whether transcellular (across the cells)-by diffusion, carrier-mediated transport or transcytosis-or paracellular-by diffusing through the tight junctions between the cells. The in vitro model systems that are currently employed to screen the permeation characteristics of a drug often represent a compromise between high throughput with low predictive potential and low throughput with high predictive potential. Here, we will examine the way in which in vitro cellular permeability assays are often performed and the assumptions that are implied but sometimes forgotten, and we will make simple suggestions for improving the methodological techniques and mathematical equations used to determine drug permeability.

Section snippets

pH

Many ionizable molecules coexist in both their charged and uncharged forms in solution. At physiological pH 7.4, most drug molecules are predominantly ionized. The pH partition hypothesis assumes that the uncharged form of a molecule might diffuse across lipid membranes, but that the charged form is impermeable. Lumenal pH values differ throughout the GI tract, therefore, the extent of permeation depends partly on where absorption takes place. The relationship between Papp and pH is an

Recommendations

Study of the literature suggests that the ideal intestinal in vitro permeability assay would have pH 6.0 and 7.4 in the donor wells, with pH 7.4 in the receiver wells (a two-pH combination could differentiate acids from bases and non-ionizables by the differences between the two Papp values) [2]. Furthermore, the receiver side would have 3% wt/vol BSA (or some sink-forming equivalent) to maintain a sink condition. The donor side may benefit from having a bile acid (i.e. taurocholic or

Conclusion

The choice of a test system always represents a compromise between high throughput with low predictive potential and low throughput with high predictive potential. Table 1 highlights mathematical and experimental techniques that should be taken into account when performing HTS and/or mechanistic studies. The introduction of radical changes to permeability studies is not the intention here; the intention is to make researchers, working within the industrial sector or academia, aware of how their

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