Organ perfusion Kidney model

In the isolated perfused kidney model, the artery of the kidney is perfused and urinary samples as well as venous blood samples can be collected to determine the drug concentration. A serious drawback of the model is that isolation and artificial perfusion greatly affect the function of the organ as shown by a dramatic drop in the glomerular filtration rate. Another in-vitro model is the isolated tubule in which samples can be taken from both the luminal and basolateral sites of the tubule [140,141]. The disadvantage of this technique as well as of the isolated kidney model, is that they require specific equipment and expertise and therefore can only be performed in rather specialized laboratories. Experiments using freshly isolated or cultured cells are more simple to carry out [142,143]. Tubular cells can be grown in a po-... 

Some efforts have been made to determine the effect P-gp has on its substrates by use of in situ perfusion methods, including intestinal perfusion, liver perfusion, kidney perfusion, and brain perfusion. These experiments allow the researcher to study the transport of compounds in a physiologically relevant environment in which the integrity of the organ is preserved with regards to cell polarity and representation of all cell types seen in the organ. Furthermore, the reduction in complexity of in situ models versus in vivo studies facilitates the conduct of complex studies and allows more definitive conclusions to be made regarding the role P-gp may play in disposition. 

In addition to cell-based models, tissue-based models such as the Ussing chamber technique, the everted gut sac approach, and perfused isolated intestinal segments are also used, but only when it is important to understand the absorption processes in more detail. Unlike Caco-2, tissue-based models have the correct physiological levels of transporters and the presence of an apical mucus layer. Also, in situ and isolated organ perfusion methods exist for the gut, liver, lungs, kidneys, and brain and can provide data not directly obtainable in vitro. The isolated perfused liver is particularly useful since it allows an assessment of first-pass hepatic clearance, the quantitative distribution of metabolites in liver, blood, and bile, the effects of binding to plasma proteins and intracellular sites, and cellular uptake processes. 

Figure 22.1 A. Schema for a physiologically based pharmacokinetic model incorporating absorption in the stomach and intestines and distribntion to various tissues. B. Each organ or tissue type includes representation of perfusion (Q) and drug concentrations entering and leaving the tissue. Fluxes are computed by the product of an appropriate rate law, and permeable surface area accounts for the affinity (e.g., lipophilic drugs absorbing more readily into adipose tissue). Clearance is computed for each tissue based on physiology and is often assumed to be zero for tissues other than the gut, the liver, and the kidneys.

PBPK models have also been used to explain the rate of excretion of inhaled trichloroethylene and its major metabolites (Bogen 1988 Fisher et al. 1989, 1990, 1991 Ikeda et al. 1972 Ramsey and Anderson 1984 Sato et al. 1977). One model was based on the results of trichloroethylene inhalation studies using volunteers who inhaled 100 ppm trichloroethylene for 4 horns (Sato et al. 1977). The model used first-order kinetics to describe the major metabolic pathways for trichloroethylene in vessel-rich tissues (brain, liver, kidney), low perfused muscle tissue, and poorly perfused fat tissue and assumed that the compartments were at equilibrium. A value of 104 L/hour for whole-body metabolic clearance of trichloroethylene was predicted. Another PBPK model was developed to fit human metabolism data to urinary metabolites measured in chronically exposed workers (Bogen 1988). This model assumed that pulmonary uptake is continuous, so that the alveolar concentration is in equilibrium with that in the blood and all tissue compartments, and was an expansion of a model developed to predict the behavior of styrene (another volatile organic compound) in four tissue groups (Ramsey and Andersen 1984). 

The one-compartment model of distribution assumes that an administered drug is homogeneously distributed throughout the tissue fluids of the body. For instance, ethyl alcohol distributes uniformly throughout the body, and therefore any body fluid may be used to assess its concentration. The two-compartment model of distribution involves two or multiple central or peripheral compartments. The central compartment includes the blood and extracellular fluid volumes of the highly perfused organs (i.e., the brain, heart, liver, and kidney, which receive three fourths of the cardiac output) the peripheral compartment consists of relatively less perfused tissues such as muscle, skin, and fat deposits. When distributive equilibrium has occurred completely, the concentration of drug in the body will be uniform. 

After IV application, peptides and proteins usually follow a biexponential plasma concentration-time profile that can best be described by a two-compart-ment pharmacokinetic model [13]. The central compartment in this model represents primarily the vascular space and the interstitial space of well-perfused organs with permeable capillary walls, especially fiver and kidneys, while the peripheral compartment comprises the interstitial space of poorly perfused tissues such as skin and (inactive) muscle [4]. 

Isolated lung, liver, and kidney perfusions have been recognized for decades as important models for toxicology and pharmacology. Part of their acceptance relates to the ease of harvest because these organs are anatomically structured with closed vascular systems containing easily identifiable arterial inputs and venous outputs, both amenable to catheterizations with minimal expertise in surgery. In contrast, outside the possible exception of ears, skin does not possess such a closed vascular system. 

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