ABSTRACT
The scope of this chaptera is limited largely to considering factors affecting the absorption
of drugs from solid oral delivery systems, as the basic principles of drug absorption have
been discussed in chapter 2 of this volume. The discussion is restricted to drugs absorbed
by passive diffusion that therefore can appear to obey first-or zero-order absorption
kinetics (depending on the drug concentration within the contents of the gastrointestinal
tract, GIT). Absorption of a drug after administration of an oral delivery system involves
at least four steps: first and ideally, delivery of the drug to the site from which it will be
optimally absorbed; second, the release and dissolution of the drug from the dose form;
third, passage of the molecularly dispersed drug through the barrier membranes of the
GIT; and finally, movement of the drug away from the site of absorption into the general
circulation. Each of these steps is considered in turn. The order of the first two steps is not
absolute; the drug may dissolve either before or after reaching its absorption site, the
difference clearly affecting outcomes. It is imperative that the drug be in solution before it
can be absorbed. The slowest of the four steps mentioned earlier determines the rate of
availability of the drug from an oral dosage form. Those factors related to the
physicochemical properties of the drug and the design and production of the dosage form,
the pharmaceutical variables, can be closely controlled. Those variables resulting from the
anatomical, physiological, and pathological characteristics of the patient, the biological or
patient variables, are almost by definition more difficult to minimize. However, dosage
forms should be designed with the physiological constraints in mind. This will be even
more important in the future with the increased emphasis on personalized medicines,
addressed briefly in the last chapter in volume 2.
