1. Now let’s consider delivery from a solid drug in a polymeric matrix.
The drug is initially well above its solubility limit CS in the matrix. At
later times, dissolution of the drug occurs into the exterior of the
matrix, and drug is then replenished by diffusion from the
supersaturated core. The supersaturated core region becomes thinner
with time hence we have a moving interface. There is however, no
concentration gradient within the core. In the exterior region of thickness
x there is a concentration gradient. See the figure below. (solution discussed
on the PDF file)

2. 1st order Drug Absorption and Elimination

4. These routes give absorption then elimination

5. Also see http://en.wikipedia.org/wiki/Inhalable_insulin

6. Absorption limiting
Dissolution limiting

7. hysteresis

8. 1st Order Drug Absorption and Elimination
Need mass balance on the drug
at the site of absorption and in the
body.
8.68
8.68
See pages

9. Here is the Matlab solution for this example 8
Here is the Matlab solution for this example 8.7, the code is on the blog:
fD/Vapp, mg/L =
ka, 1/min =
kte, 1/min =
Vplasma, L =

10. Two Compartment Models for IV and 1st Order Absorption
See pages

11. Note the biphasic
elimination with
two different
elimination rate
constants represented
by the slopes of these
lines.

12. Superposition Principle
States that each drug dose behaves independently of all other drug
doses that have been administered. Hence, we can write
Ctotal = CD1 + CD2 + CD3 + …
Be careful though in your computer models since the single dose
model equations for a given dose are in terms of the time since that
particular dose was administered which can be called for example
t’, so if doses D1, D2, D3 etc are given at t1, t2, t3 respectively, then
t’1 = t – t1, t’2 = t – t2, t’3 = t – t3, and t is the time when the first dose was
taken.
Superposition works as long as all elimination processes and absorption
processes are first order. It also works for multiple drug doses given by
two or more different types of drug delivery methods.

13. Superposition Principle