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
- 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
- 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.