Water unstirred layers

Irrespective of the physical form of the carotenoid in the plant tissue it needs to be dissolved directly into the bulk lipid phase (emulsion) and then into the mixed micelles formed from the emulsion droplets by the action of lipases and bile. Alternatively it can dissolve directly into the mixed micelles. The micelles then diffuse through the unstirred water layer covering the brush border of the enterocytes and dissociate, and the components are then absorbed. Although lipid absorption at this point is essentially complete, bile salts and sterols (cholesterol) may not be fully absorbed and are not wholly recovered more distally, some being lost into the large intestine. It is not known whether carotenoids incorporated into mixed micelles are fully or only partially absorbed. 

Avdeef, A., Nielsen, P. E., Tsinman, O. PAM PA - a drug absorption in vitro model. 11. Matching the in vivo unstirred water layer thickness by individual-well stirring in microtitre plates. Eur. J. 

Mucus layer (glycoprotein gel, 90% oligosaccharide, implicated in pH microclimate) Unstirred water layer... 

The glycocalyx and the mucus layer make up the structure of the unstirred water layer (UWL) [73]. The thickness of the UWL is estimated to be 30-100 pm in vivo, consistent with very efficient stirring effects [74]. In isolated tissue (in the absence of stirring), the mucus layer is 300-700 pm thick [73]. The pH in the unstirred water layer is 5.2-6.2, and might be regulated independently of the luminal pH (Section 2.3). The mucus layer may play a role in regulating the epithelial cell surface pH [73]. 

Figure 7.8 Permeation of drugs through oil-soaked microfilters comparisons to Caco-2 permeabilities (dashed curves) [546] (a) oil-free (untreated hydrophilic) filters (b) unstirred water layer permeability versus log MW (c) octanol-soaked (impregnated) filters (d) isopropylmyristate-soaked filters.

Perhaps the simplest Fick s law permeation model consists of two aqueous compartments, separated by a very thin, pore-free, oily membrane, where the unstirred water layer may be disregarded and the solute is assumed to be negligibly retained in the membrane. At the start (t = 0 s), the sample of concentration CD 0), in mol/cm3 units, is placed into the donor compartment, containing a volume (Vo, in cm3 units) of a buffer solution. The membrane (area A, in cm2 units) separates the donor compartment from the acceptor compartment. The acceptor compartment also contains a volume of buffer (VA, in cm3 units). After a permeation time, t (in seconds), the experiment is stopped. The concentrations in the acceptor and donor compartments, CA(t) and C (t), respectively, are determined. 

Intrinsic Permeability, Permeability-pH Profiles, Unstirred Water Layers (UWL), and the pH Partition Hypothesis... 

Unstirred Water Layer Effect (Transport across Barriers in Series and in Parallel)... 

The UWL permeability is nearly the same for drugs of comparable size, and is characterized by the water diffusivity (Daq) of the drug divided by twice the thickness of the layer (ftaq), Pu = Aiq / (2 h.Aq), in a symmetric permeation cell [40], The unstirred water layer permeability can be determined experimentally in a number of ways based on pH dependency of effective permeability [25,509,535-538], stirring rate dependence [511-514,552,578], and transport across lipid-free microfilters [25,546],... 

Figure 7.34 Permeability-pH profiles of ketoprofen (a) log-log plot solid curve represents effective permeability, and the dashed curve is the membrane permeability, calculated by Eq. (7.53). The latter curve levels off at the intrinsic permeability, Pq. The effective curve levels off to approximately the unstirred water layer permeability, Pu. (b) Direct plot the inset curve for the fraction neutral substance levels of at 100% (scale not shown). [Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission from Bentham Science Publishers, Ltd.]...

TABLE 7.15 Intrinsic Permeabilities and Unstirred Water Layer Permeabilities Determined from Iso-pH Dependence of Effective Permeabilities 2% DOPC in Dodecane... 

The method preferred in our laboratory for determining the UWL permeability is based on the pH dependence of effective permeabilities of ionizable molecules [Eq. (7.52)]. Nonionizable molecules cannot be directly analyzed this way. However, an approximate method may be devised, based on the assumption that the UWL depends on the aqueous diffusivity of the molecule, and furthermore, that the diffusivity depends on the molecular weight of the molecule. The thickness of the unstirred water layer can be determined from ionizable molecules, and applied to nonionizable substances, using the (symmetric) relationship Pu = Daq/ 2/iaq. Fortunately, empirical methods for estimating values of Daq exist. From the Stokes-Einstein equation, applied to spherical molecules, diffusivity is expected to depend on the inverse square root of the molecular weight. A plot of log Daq versus log MW should be linear, with a slope of —0.5. Figure 7.37 shows such a log-log plot for 55 molecules, with measured diffusivities taken from several... 

Carbamazepine, antipyrine, terbutaline, and hydrochlorothiazide were treated as neutral molecules. Their effective permeabilities were corrected for the unstirred water layer using estimated unstirred water layer permeabilities, determined by the other molecules of similar lipophilicities and size. 

Permeability is a property closely tied to the environment of the epithelial cell surface. There is little point in measuring permeability at pH 1.7, if the microclimate barrier has pH >5 and <8, averaging 6. An in vitro permeability screen based on donor pH 5.0-7.4 and acceptor pH 7.4 seems about right. It will be useful to correct the data for the unstirred water layer effect, using computational methods. 

Ruell, J. A. Tsinman, K. L. Avdeef, A., PAMPA—a drug absorption in vitro model. 4. Unstirred water layer in iso-pH mapping assays in pKR flux-optimized design (pOD-PAMPA) (submitted). 

I Hidalgo, K Hillgreen, G Grass, R Borchardt. Characterization of the unstirred water layer in Caco-2 cell monolayers using a novel diffusion apparatus. Pharm Res 8 222, 1991. 

Unstirred water layer (2.4 mm thickness in PAMPA, when plates are... 

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