3.1. Definitions

We will start this chapter with some definitions of terms that are used frequently in pharmacokinetics.

Pharmacokinetics involves the study of the manner in which the body handles a drug. It looks at the rate at which drug concentrations change in the body, and is involved with analyzing the kinetics of absorption, distribution, metabolism and excretion of a drug.
The study of pharmacodynamics involves the study of the effect that a drug has on the body. This will include looking at what effect the amount of drug has, called a dose response, and will also look at the site of action of the drug, such as the kinetics of how a drug interacts with a receptor, known as receptor dynamic.
Pharmacology is the study of how a drug changes the functioning of an organ.
This is a branch of pharmacology that deals with undesirable effects of molecules on the body. Before a drug company can file an NDA (New Drug Approval) with the FDA, extensive toxicological studies must be performed in at least three species.
All these fields are related, and play an important role in the observed effects of a drug on the body.
The adjective systemic means -involving the entire body. For example one can say that drug delivered by injection will have a systemic effect while one that is applied to a certain area on the skin has a local effect.
The bioavailability of a drug is the amount of drug that reaches the systemic (entire body) circulation in an active form. The process of breaking down a drug starts as soon as it enters the body. Some routes of administration are therefore better than others, depending on the type of drug. Drugs that are broken down by the pH or enzymes in the GI tract have a very low bioavailability via that route.
First pass effect
Another cause for low bioavailability is a phenomenon called the first pass effect. All the blood vessel that feed most of the GI tract drain into the portal vein which goes to the liver. That means that drugs absorbed from the gut go directly to the very organ in the body that is responsible for detoxification. So the first pass of orally administered drugs around the body in the circulation takes them not directly to the area where they are needed, but first to the liver, where they could possibly be broken down.
Biotransformation is the conversion of a drug molecule to another, usually inactive form, in the body. Some drugs, however, are administered as a prodrug, which is an inactive form which the body transforms into an active form. For the Parkinson's drug, L-Dopa, the advantage is the inactive form can pass into the brain, whereas the active form can not.