12 - substrates PBPK

Often, PBPK models for toxicokinetics application require special considerations (e.g., volatile toxicants may incur tissue-air partition coefficients and alveolar elimination rates). Partition coefficients are generally obtained by measurement in the laboratory, tissue volume/blood flow data are mostly available from the scientific literature (with allometric scaling from species to species), and biotransformation data are usually obtained from in vivo and in vitro kinetic studies. Biochemical constants for metabolic pathways are captured using the maximum rate of reaction, or Vmax5 and the binding affinity of the particular substrate for the metabolizing enzyme. 

While rate equations can be solved and metabolic rates can be estimated and determined easily in vitro because of the control of variables (specifically, substrate concentration), the situation is much more difficult in vivo. Here the advent of physiologically based pharmacokinetic (PBPK) models has offered a powerful tool for examining metabolic interactions that occur following exposure to chemical mixtures (see Chapter 3). 

Krishnan and colleagues developed an approach to the PBPK analysis of complex mixtures in which the toxicologic interactions of binary mixtures are first combined in the affected and modeled target tissue [30]. An example for a mixture of volatile organic chemicals (m-xylene, toluene, ethylbenzene, dichlo-romethane, and benzene) that interact via competitive inhibition was devel-oped.The approach requires thatPBPKmodels be available for each component models are interconnected by Kt values and, naturally, by substrate concentrations—here, in the liver. It is necessary that all binary interactions be characterized. For a mixture of n components, the number of binary interaction experiments performed to determine K, values will be n(n - 1)12. While not indicated, it seems that some reduction of resources would be... 

Similar to the techniques used for calculation of chemical disposition parameters, in vivo biotransformation kinetic parameters of a substrate can be estimated from in vitro systems such as microsomes, freshly isolated hepatocytes, fiver slices, and isolated perfused livers (24). Intrinsic clearance or Michaelis-Menten parameters for the whole liver can also be obtained by scaling in vitro parameters based on the cytochrome P450 enzyme content (25-27). These parameters can also be estimated from in vitro data obtained from recombinant human CYP systems, and also through allometric scaling of clearance estimates from animal PBPK models. 

In an attempt to circumvent the avaUabihty of analytical standards, several CYP450 studies were carried out using the substrate depletion method. This approach does not provide information on the products formed downstream, and may be of limited use in developing human enviromnental exposure PBPK/PD models that require extensive urinary metabolite data. Hydrolytic standards (i.e., alcohols and acids) were available to investigators who studied the carboxylesterase-catalyzed hydrolysis of several pyrethroid insecticides. The data generated in these studies are suitable for use in developing human exposure PBPK/PD models. 

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