hair, or go into convulsions. These are side effects of the drug. Thus, it is important to
remember that the whole body must be taken into account when a drug is administered.
If the problem of the side effects cannot be resolved, the drug may not be released for
NOTE: Another areas
in the distribution
of drugs are those that crosses the
placental barrier. Drugs may actively or passively cross the placental barrier and enter
the fetal circulation. The enzyme systems of the developing fetus may not be able to
adequately metabolize the drug. Toxic effects can result. At this time, it is virtually
impossible to predict whether a drug will pass the placental barrier.
3-11. METABOLISM (BIOTRANSFORMATION)
The process of drug absorption and distribution is dynamic. That is, it is
continually changing. Even as the drug is being distributed, the individual cells of the
body begin to chemically change or alter the drug. This metabolic process of changing
the drug is called metabolism or biotransformation. While many cells of the body will be
involved in this process, the liver is the organ primarily responsible for this
biotransformation. The liver changes drugs to make them more water-soluble so that
they may be more easily excreted from the body. During the process of metabolism, a
drug may be rendered inactive, converted from an inactive form to an active one, or be
made more toxic. The liver may oxidize, reduce, hydrolyze, or conjugate (bind with a
protein) the drug. The kidney will also play an active role in conjugating drugs.
a. The term bioavailability was defined in the first portion of this subcourse. In
Figure 3-4, all 100 micrograms of the drug was available to the system, since it was
absorbed. The amount of drug originally administered to the patient was not stated.
That amount of administered drug could have ranged from 100 micrograms to 1000
milligrams or more. From the reading, you should have noted that absorption is not an
easy process, and that any number of things can interfere with the process.
b. The controversy concerning generic drugs deserves consideration at this
point. For example, switching a patient from one generic brand of ampicillin to another
brand of ampicillin could cause some problems for some patients. Thus, one
company's generic brand of a drug might not be able to be absorbed as quickly as
another company's generic brand of the same drug. With other drugs and some
patients the switch from one company's drug to another company's drug is of little
c. Drugs that present the most problems in terms of bioavailability are oral
solids. That is because oral solids must disintegrate, dissolve, be water soluble, be fat
soluble, unionize, and pass through the drastic pH changes from the stomach to the