Apparent permeability coefficient

Another relatively new lipophilicity scale proposed for use in ADME studies is based on MEKC [106]. A further variant is called BMC and uses mobile phases of Brij35 [polyoxyethylene(23)lauryl ether] [129]. Similarly, the retention factors of 16 P-blockers obtained with micellar chromatography with sodium dodecyl sulfate as micelle-forming agent correlates well with permeability coefficients in Caco-2 monolayers and apparent permeability coefficients in rat intestinal segments [130]. 

Norris, F. A. Powell, G. L., The apparent permeability coefficient forproton flux through phosphatidylcholine vesicles is dependent on the direction of flux, Biochim. Biophys. Acta 1030, 165-171 (1990). 

Tab. 5.1. List of marker compounds used to establish relationship between oral dose absorbed in humans (Fa) and apparent permeability coefficient in Caco-2 monolayers.

In our own studies to establish an in-house correlation between Caco-2 permeability and extent of drug absorption in humans, a set of 25 model drugs was used (Table 5.1). The importance of concentration and pH conditions were investigated and transport was studied both in apical to basolateral (absorptive) and basolateral to apical (secretory) directions. The apparent permeability coefficients were determined at concentrations of 10, 50, and 500 pM, and at two different settings of apical/basolateral pH values 6.5/7.4 and 7.4/7.4. The marker compounds represented a good diversity in molecular structure and transport properties and covered a range of low (<20%), moderate (20-80%) and high (>80%) extent of absorption in humans (Tab. 5.1). 

Fig. 5.4. Relationship between the oral fraction absorbed in humans and apparent permeability coefficients obtained in Caco-2 cell monolayers. Papp bands for predicted fraction absorbed of <20% (low), 20-80% (moderate) and >80% (high) are indicated.

It was postulated that the aqueous pores are available to all molecular species, both ionic and non-ionic, while the lipoidal pathway is accessible only to un-ionised species. In addition, Ho and co-workers introduced the concept of the aqueous boundary layer (ABL) [9, 10], The ABL is considered a stagnant water layer adjacent to the apical membrane surface that is created by incomplete mixing of luminal contents near the intestinal cell surface. The influence of drug structure on permeability in these domains will be different for example ABL permeability (Paq) is inversely related to solute size, whereas membrane permeability (Pm) is dependent on both size and charge. Using this model, the apparent permeability coefficient (Papp) through the biomembrane may therefore be expressed as a function of the resistance of the ABL and... 

Table 5.2 Apparent permeability coefficients (Papp) of low and high molecular weight compounds across the nasal mucosa of tissues excised from different animal species.

Table 9.1 Apparent permeability coefficients (Papp) of 14C-mannitol and budesonide across the passage-cultured human nasal epithelial cell layer grown under AIC or LCC conditions for 7 days.

Figure 10.2 Apparent permeability coefficients (Papp) observed in vitro across Calu-3 ( ) [62] and 16HBE14o- (o) [72] cell layers vs. in vivo rate constants (Ka) determined for absorption from the rat lung after intratracheal delivery of various molecules.

Table 11.2 Molecular Weights and Apparent Permeability Coefficients (Papp) of Various Proteins and Peptides Across Primary Human Versus Rat AEC Monolayers.

Based on the 96-well format, OCT-PAMPA was proposed and has proved its ability to determine (indirectly) log Poet [87]. PAM PA is a method, first developed for permeability measurements, where a filter supports an artificial membrane (an organic solvent or phospholipids) [88, 89]. With this method, the apparent permeability coefficient (log P ) of the neutral form of tested compounds is derived from the measurement of the diffusion between two aqueous phases separated by 1-octanol layer (immobilized on a filter). A bilinear correlation was found between log Pa and log Poct> therefore log Poet of unknown compounds can be determined from log Pa using a calibration curve. Depending on the detection method used a range oflog P within —2 to +5 (with UV detection) and within —2 to +8 (with LC-MS detection) was successfully explored. This method requires low compound amounts (300 pi of 0.04 mM test compound) and, as for the previous method, samples can be prepared in DM SO stock solutions. For these experiments, an incubation time of 4h was determined as the best compromise in term of discrimination. The limitation of the technique lies in the lower accuracy values... 

Fig. 4.9 (a) Logarithm of the apparent permeability coefficient, for perfusion experiments in rats (filled symbols) and Caco-2 (empty symbols) versus logarithm of the intrinsic permeability coefficients based on PAMPA. (b) The unstirred water layer effect in the Caco-2 data. 

The transport-enhancing effect of Zot was shown to be reversible and nontoxic (Fasano et al. 1991 Cox et al. 2002). More recently a smaller fragment of Zot in the size of 12 kDa referred to as /1G was identified (Di Pierro et al. 2001). AG displayed significant potential as permeation enhancer. In vitro studies showed that it is capable of significantly increasing the apparent permeability coefficients for a wide variety of drugs across Caco-2 monolayer (Salama et al. 2003, 2004). In the presence of peptidase inhibitors AG improved the bioavailability of mannitol, inulin and PEG 4000 after intraduodenal administration to rats (Salama et al. 2003, 2004). In another in vivo study the oral bioavailability of cyclosporin A was increased up to 50-fold due to the co-administration of AG when metabolic protection was provided (Salama et al. 2005). Results of this study are illustrated in Fig. 5.2. 

The majority of permeability screening is based on cellular assays mimicking intestinal adsorptive cells CACO-2 cells (human colon carcinoma), and the CACO-2 clone TC7, HT-29, T84, IL-6 or MDCK cells (Madine Darby Canine Kidney). In this widely used and accepted cellular assays compounds are classified into 3 classes of permeability high, medium, low. Moreover, the absorption potential of compounds within a chemical series can be compared concerning their apparent permeability coefficients (Artursson 1991 Hillgren et al. 1995 Artursson and Borchardt 1997). 

Permeability of compounds across a cell layer is measured in order to determine the absorption potential of a compound or a chemical series and select compounds for in vivo studies. Apparent permeability coefficients can be used to compare compounds within a series for ranking. Between different laboratories comparison of compounds should be done only on the basis of classification (high, medium, low) since permeability coefficients can differ between the labs (Artursson et al. 2001 see Critical Assessment of the Method). 

Apparent permeability coefficients (Papp in cm/sec) are calculated according to the following equation... 

FIGURE 4.7 Relationship for a series of 20 compoLinds betwreen apparent permeability coefficients in a Caco-2 cell model and the extent of absorption after oral administration to humans. (Reproduced with permission from Artursson P, Karlsson J. Biochem Biophys Res Commun 1991 175 880-5.)... 

The apparent permeability coefficient Papp for vaginal membrane permeation is defined by... 

For the vaginal absorption of ionizable compounds, such as the homologous series of n-alkanoic acids, the apparent permeability coefficient Papp becomes pH-dependent and is defined by Eq. (4) ... 

The apparent permeability coefficient Papp is related to the first-order rate constant for the disappearance of drug from vaginal lumen (ky) as follows ... 

Fig. 8 Cyclic variation in the apparent permeability coefficient, Tapp of methanol and n-octanol in the female rhesus monkey in response to its estrus cycle. The bars indicate the time of observed menstruation. (From Ref. l)...

Fig. 9 Comparison of apparent permeability coefficients, Papp. for the vaginal absorption of straight-chain alkanols in non-cyclic rabbits and cyclic rhesus monkeys. (From Refs. . )...

Fig. 12 The variation of apparent permeability coefficient with boundary fractal dimension. (From Ref. " f)...

When the permeabilities of the aqueous boundary layer and membrane are relatively large, reduced blood flow becomes the rate-limiting step. For example, the apparent permeability coefficient is approximately 85% controlled by blood flow when Pgq and Pmembrane are each approxi-mately 10 times greater than Pbiood-... 

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