Drug, diffusion

In the commonest form of microbiological assay used today, samples to be assayed are applied in some form of reservoir (porcelain cup, paper dise or well) to a thin layer of agar seeded with indicator organism. The drug diffuses into the medium and after incubation a zone of growth inhibition forms, in this case as a circle around the reservoir. All other factors being constant, the diameter of the zone of inhibition is, within limits, related to the concentration of antibiotic in the reservoir. 

Fig. 8 Representation of a bioerodible matrix system. Drug is dispersed in the matrix before release at time = 0. At time = t, partial release by drug diffusion or matrix erosion has occurred.

The rate of drug diffusing out of the resin is controlled by the area of diffusion, diffusional path length, and rigidity of the resin, which is a function of the amount of cross-linking agent used to prepare the resin. 

Note that in the component mass balance the kinetic rate laws relating reaction rate to species concentrations become important and must be specified. As with the total mass balance, the specific form of each term will vary from one mass transfer problem to the next. A complete description of the behavior of a system with n components includes a total mass balance and n - 1 component mass balances, since the total mass balance is the sum of the individual component mass balances. The solution of this set of equations provides relationships between the dependent variables (usually masses or concentrations) and the independent variables (usually time and/or spatial position) in the particular problem. Further manipulation of the results may also be necessary, since the natural dependent variable in the problem is not always of the greatest interest. For example, in describing drug diffusion in polymer membranes, the concentration of the drug within the membrane is the natural dependent variable, while the cumulative mass transported across the membrane is often of greater interest and can be derived from the concentration. 

Analysis of drug transport in a solid tumor compartment could be represented in spherical coordinates as well as cylindrical [19] as depicted in Eq. (56). In this case, and assuming that drug diffusion occurs only in the radial direction, Eq. (53) can be written as... 

E Garrett, P Chemburkar. Evaluation, control and prediction of drug diffusion through polymeric membranes I. Methods and reproducibility of steady-state diffusion studies. J Pharm Sci 57 944, 1968. 

Here, Pe takes into account the aqueous boundary layers (/W), transcellular diffusion with metabolism, the paracellular pathway (Pparaceu), and the filter support (PF). It is assumed that drug diffusing through the paracellular route escapes metabolism and contributes insignificantly to the appearance of intact drug in the receiver. 

In these materials, the drug diffusion coefficients and release rates will vary greatly with environmental pH. 

This chapter starts with a short introduction on the skin barrier s properties and the methods employed for analyzing experimental data. This is followed by an overview of several selected approaches to predict steady-state diffusion through the skin. Then a few approaches that approximate the structural complexity of the skin by predicting drug diffusion in biphasic or even multiphasic two-dimensional models will be presented. Finally, the chapter concludes with a short summary of the many variables possibly influencing drug permeation and penetration. 

For example, obesity affects Vi because lipid-soluble drugs diffuse into the adipose tissues of the obese person. Vi is a useful parameter for determining the loading dose for a drug to attain equilibrium after the drug is administered. 

The therapy of a chronic disease requires repeated drug dosing. In the case of a short biological half-life, the drug has to be administered up to several times daily within short intervals. To reduce the application frequency, sustained formulations have been developed. For this purpose liquid crystalline excipients are appropriate candidates, because in a liquid crystalline vehicle the drug diffusion is reduced by a factor of 10 to 1000 in comparison with a liquid vehicle such as a solution [35-37]. The factor depends on liquid crystal. 

Epithelium. The predominant barrier to drug diffusion resides approximately within the outermost one-third of the epithelium. This is true of both keratinized and nonkeratinized epithelia. Therefore, keratinization is unlikely to offer major resistance to bueeal permeation. 

Drug release from soluble polymers is accompanied by the gradual erosion-type dissolution of the polymer. Therefore, polymer dissolution and drug diffusion may be the overall hybrid mechanism of release. Drug release from nonsoluble hydrogels generally follows Fickian or non-Fickian diffusion kinetics [51]. The mechanism of... 

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