Ethanol biotransformation processes

The concept of clearance is useful in pharmacokinetics because clearance is usually constant over a wide range of concentrations, provided that ehmination processes are not saturated. Saturation of biotransformation and excretory processes may occur in overdose and toxic okinetic effects should be considered. If a constant fraction of drug is eliminated per unit time, the elimination follows first-order kinetics. However, if a constant amount of drug is eliminated per unit time, the elimination is described by zero-order kinetics. Some drugs, for example, ethanol, exhibit zero-order kinetics at normal or non-intoxicating concentrations. However, for any drug that exhibits first-order kinetics at therapeutic or nontoxic concentrations, once the mechanisms for elimination become saturated, the kinetics become zero order and clearance becomes variable.3... 

There are biological processes, however, that involve saturable carrier or enzymatic systems, with a finite capacity for transport or catalysis. For instance, processes like active uptake at absorption sites, renal tubular secretion, or hepatic biotransformation of xenobiotics may become saturated at high exposure levels, yielding rates of disposition that are constant and independent of the concentration in blood. This is characteristic of zero-order kinetics. Biotransformations of ethanol in the liver and active tubular renal secretion of penicillin in urine are examples of biological processes that obey zero-order kinetics. Figure 7 illustrates the blood concentration... 

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