What are the main types of drug binding?
From the bloodstream to various tissues of the body and finally to its site of action, the transport of a drug in body fluids is known as distribution. A drug can accumulate itself by binding to specific tissues in the body, this is allowed by the presence of drug reservoirs in it. The main types of drug binding are the plasma protein binding and tissue binding. The first one refers to the possibility of a drug, when entering the circulation, to become attached to proteins, which decreases the amount of free drug, restricting the quantity of drug that goes to the site of action. The last one indicates the lipid soluble drugs that have a high affinity for adipose tissue (stores energy in the form of fat), and the drugs that bind to calcium, these have an unusual affinity for bone. There are specialized structures made up of biological membranes that can serve as barriers to the passage of drugs at certain sites of the body. It can be a placental barrier, which is a membrane layer that separates the blood vessels of the mother and the fetus, not completely protecting the last because the barrier is permeable to lipid-soluble and insoluble drugs; and it can be a blood-brain barrier, which is a special anatomic arrangement that allows the distribution of only lipid soluble drugs into the brain and cerebrospinal fluid; compounds that are strongly ionized and poorly soluble in fat cannot enter the brain.
The metabolism is the chemical inactivation of a drug by converting it into a more water-soluble compound that can be excreted from the body. The liver is the main site of metabolism of drugs.
Finally, the elimination or excretion is the process where drugs are eliminated from the body. The excretion can be through the kidneys (most important route of drug elimination), intestine, lungs, sweat and salivary glands and mammary glands. The most important process of drug elimination that contributes to a good first step towards drug development is biliary excretion (the metabolite is secreted into the bile and eliminated with the feces after undergoing metabolism by the liver) According to Cayen, 2010, biliary excretion is one of the major components of the overall first-pass effect that could affect the bioavailability of orally administered drugs.