Drug Pharmacokinetics

Pharmacokinetics is the study of the time courses of drug absorption, distribution, metabolism and excretion in the intract animal or in humans.

Drug Absorption

Drug absorption is the process of movement of a drug from the site of application toward the systemic circulation. Cell membranes act as semipermeable biologic barriers that selectively permit the passage of certain solutes or drugs molecules. Cell membranes are composed primarily of proteins, cholesterol and phospholipids. The membranes protein may be involved in transport processes and may function as reseptors for cellular regulatory mechanisms, Membrane lipid confers both hydrophilic and hydrophobic properties providing stability to the membrane and determining its permeability characteristics. Compounds are transported accros this biologic barrier by passive diffusion, facilitated difussion, active transport and pinocytosis.

Passive diffusion : Transport across the cell membrane in which the driving force for movement is the concentration gradient of the solute.

Facilitated diffusion : is a passive transport type that allows drug molecules to across membranes with the assistance of special transport proteins.

Active transport : Active transport appear to be limited to agents with close structural similarities to normal body constituents. Active transport have been identified for various ions, vitamins, and amino acids.

Pinocytosis : is general term used to describe the active movement of substances via membrane-bound packages from the extra cellular fluid to the cytoplasm or vice versa.

Drug Distribution

After the drug enters the general circulation, it is distributed first within the blood. Distribution is affected by extent of protein binding, pH, and the permeability of variuos membranes. The rate of drugs entry to the tissues dependent to the rate blood flow to the tissue, the mass of the tissue and the partition characteristics of the drug between blood and tissue.

Metabolism

Metabolism is the process of the chemical alteration of drugs in the body. The major site for drug metabolism is liver and the two major types of metabolic reactions are called nonsynthetic and synthetic. nonsynthetic reaction include oxidation, reduction, hydrolysis and combination of this processes. in the synthetic reactions (conjugation) the parent drugs formed by nonsynthetic reactions, combine with amino acid or glucuronic acid, to yield addition or conjugation products. The most important synthetic reactions are glucuronidation, amino acid conjugation, acetylation, sulfate conjugation, and methylation.

Excretion

The kidney is the major organ of excretion and is responsible for eliminating most nonvolatile and water-soluble substances. Although drugs may additionally be eliminated via saliva, intestines, sweat, breast milk, and lungs, but the overall contribution is generally small, except excretion of volatile anesthetics, whose major route of elimination is via the lung.