Pharmacokinetics I

Drug Concentration in the Body
as a Function of Time. First-Order
(Exponential) Rate Processes
Processes such as drug absorption and
elimination display exponential characteristics.
As regards the former, this follows
from the simple fact that the
amount of drug being moved per unit of
time depends on the concentration difference
(gradient) between two body
compartments (Fick’s Law). In drug absorption
from the alimentary tract, the
intestinal contents and blood would
represent the compartments containing
an initially high and low concentration,
respectively. In drug elimination via the
kidney, excretion often depends on glomerular
filtration, i.e., the filtered
amount of drug present in primary
urine. As the blood concentration falls,
the amount of drug filtered per unit of
time diminishes. The resulting exponential
decline is illustrated in . The
exponential time course implies constancy
of the interval during which the
concentration decreases by one-half.
This interval represents the half-life
(t1/2) and is related to the elimination
rate constant k by the equation t1/2 = ln
2/k. The two parameters, together with
the initial concentration co, describe a
first-order (exponential) rate process.