Biotransformation individual variability

Because these different viability tests all reflect different aspects of cell viability, the choice of test depends on the aim of the study. For toxicity studies where biotransformation is an important bioactivation or detoxification step, metabolic function tests should be included to judge the validity of the method, whereas viability tests are needed to assess toxic effects. Both positive and negative controls should be included in such studies. When human liver is used, the characterization of metabolic activity is especially important because of the large inter-individual variability associated with this property [75]. 

For most drugs, their concentration is a function of dose, as well as of the individual s rates of metabolism and elimination, which are relatively stable unless a moderating variable intervenes. Examples of such moderating variables include diseases that affect organs important for metabolism or elimination (e.g., liver or heart), or if the patient is exposed to a concurrent agent that induces or inhibits the GYP enzyme responsible for the biotransformation of the drug of interest. 

Changes in the enzyme activities involved in biotransformation may be caused by various factors. These can be both non-variable and variable in nature with individually different modes of influence, (s. tab. 3.18) (s. fig. 3.11)... 

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