Pharmacokinetics
Pharmacokinetics is the subject that deals with the rates of movement of a drug and/or its metabolite(s) in the body and forces acting on the process. Absorption, Distribution, Metabolism and Excretion of drugs are the processes in which a drug moves in the body at various rates. Relative rates of these "ADME processes" determine the time courseof the drug in the body, the most important of the receptor sites which are responsible for the pharmacological action of drugs. Pharmacokinetics will explain the rates of movement of drugs in the body with the help of a suitable mathematical model.
In pharmacokinetics, the concentration of a drug in plasma of any physiological fluid such as urine, saliva, milk etc., is determined with respect to time following its administration. The concentration of a drug in a physiological fluid versus time data is used to study the dynamics of th~ drug in the body with the help of mathematical equations, derived on the basis of a model and set of assumptions.
Biopharmaceutics
The therapeutic response of a drug is normally dependent on an adequate concentration of the drug being achieved and then maintained at the site or sites of action of the drug. In the case of systemically acting drugs it is generally accepted for clinical purposes that a dynamic equilibrium exists between the concentration of drug in blood and the concentration of drug at its site(s) of action. It means a linear relationship exists between the drug level in blood and drug concentration at the site of action. Therefore, drug concentration at the site of action can be predicted from blood concentration of drug. Strictly, the concentration of drug in plasma water (Le., protein free plasma) is a more accurate index of drug concentration at the site (s) of action than is the concentration of drug in whole plasma since a drug may often bind in a reversible manner to plasma protein. Only drug that is unbound (Le., dissolved in plasma water) can pass out of the plasma through the capillary endothelium and reach other body fluids and tissues and hence its site (s) of action.
However, to measure the concentration of an unbound drug in plasma water requires more complex and sensitive assay methods than to measure the total concentration of both unbound and bound drug in plasma.
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