Pharmacokinetics passive diffusion

The CAT model estimates not only the extent of drug absorption, but also the rate of drug absorption that makes it possible to couple the CAT model to pharmacokinetic models to estimate plasma concentration profiles. The CAT model has been used to estimate the rate of absorption for saturable and region-depen-dent drugs, such as cefatrizine [67], In this case, the model simultaneously considers passive diffusion, saturable absorption, GI degradation, and transit. The mass balance equation, Eq. (51), needs to be rewritten to include all these processes ... 

Many drugs have been recognized to cross the intestinal epithelial cells via passive diffusion, thus their lipophilicity has been considered important. However, as described above, recent studies have demonstrated that a number of drug transporters including uptake and efflux systems determine the membrane transport process. In this chapter, we provide an overview of the basic characteristics of major drug transporters responsible not only for absorption but also for disposition and excretion in order to delineate the impact of drug transport proteins on pharmacokinetics. 

In contrast to mammalian cells, the membrane of bacterial cells is much more complex and, as in the case of Escherichia coli or mycobacteria, it is asymmetric (see Section 1.2.2). The reason for this is that these small cellular life forms depend on diffusion of nutrients and metabolites. All substrates going in and out of the cell must diffuse through their cell walls. This might be one reasons why the surface area to volume ratio is important for bacterial cell shapes. This ratio is determined by the structure of their outer cell wall. To cross such a barrier, mainly by passive diffusion, chemotherapeutics must have other properties in addition to those necessary for suitable pharmacokinetics in the host, as in most cases, the target of the chemotherapeutics is within the cytoplasm. 

Pharmacokinetics is now challenged by the growing importance of transporters, a relatively new and potentially major factor in drug absorption, distribution, metabolism and excretion (the ADME process). Several years ago, passive diffusion was the main advanced process by which xenobi-otics were believed to move through body membranes. The... 

Generally, beta-blockers are absorbed from the gastrointestinal tract via passive diffusion, so their absorption is not considered stereoselective. However, some beta-blockers such as talinolol may undergo an intestinal secretion process that may be modestly stereoselective, resulting in an apparent nonlinearity in the kinetics of the drug with increasing oral doses [14]. Nevertheless, despite the suggestion of an active intestinal secretion process, the overall pharmacokinetics of talinolol are not stereoselective [14]. 

The pharmacokinetic parameters-elimination half life, elimination rate constant, the apparent volume of distribution and the systemic, renal and metabolic clearances (Cls, Clr, and Clm, respectively) for a dmg are always independent of the dose administered as long as the drug follows a first-order elimination process and passive diffusion. 

In this question, plasma concentration versus time data is provided following the administration of two different doses of a drug (cinoxacin Cinobac). Because of the assumption of the first-order process and passive diffusion, one would expect the plasma concentration of a drug at any time to be directly proportional to the dose administered however, the fundamental pharmacokinetic parameters of a dmg will remain unaffected by the administered dose. We plotted... 

Linear pharmacokinetics applies that is, the rate process obeys passive diffusion and first-order elimination kinetics (please review first-order process). 

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