The action of a drug is believed to be due to the interaction of that drug with endogenous and exogenous substrate molecules found in the body. When one or more active drug molecules bind to the target endogenous and exogenous molecules, they cause a change or inhibit the biological activity of these molecules.
The effectiveness of a drug in bringing about these changes normally depends on the stability of the drug–substrate complex, whereas the medical success of the drug intervention usually depends on whether enough drug molecules bind to sufficient substrate molecules to have a marked effect on the course of the disease state.
The degree of drug activity is directly related to the concentration of the drug in the aqueous medium in contact with the substrate molecules. The factors affecting this concentration in a biological system can be classified into the pharmacokinetic phase and the pharmacodynamic phase of drug action.
The Pharmacokinetic Phase
The pharmacokinetic phase of drug action includes the Absorption, Distribution, Metabolism and Elimination (ADME) of the drug. Many of the factors that influence drug action apply to all aspects of the pharmacokinetic phase. Solubility, for example, is an important factor in the absorption, distribution and elimination of a drug.
Furthermore, the rate of drug dissolution, that is, the rate at which a solid drug dissolves in the aqueous medium, controls its activity when a solid drug is by enteral routes as a solid or suspension.
The bioavailability of a drug influenced by such factors as ADME. Which is defined as the fraction of the dose of a drug that is found in general circulation. It is not constant but varies with the body’s physiological condition.
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The Pharmacodynamic Phase
Pharmacodynamics is concerned with the result of the interaction of drug and body at its site of action, that is, what the drug does to the body. It is now known that a drug is most effective when its shape and electron distribution, that is, its stereoelectronic structure, is complementary to the steroelectronic structure of the active site or receptor.