- The central
circulation is the reference point for all
bioavailability calculations. It is the only fluid that
has contact with the entire body.
- Not all drug given to an animal per os (or other non-IV
routes) is absorbed into the central circulation.
- Not all of an
administered dose is "available" to the sites
- Bioavailability is defined as the fraction of drug
administered that reaches the central circulation,
i.e., the circulating post-portal venous blood.
- By definition, bioavailability for a drug administered
intravenously is 100%. The "term" used is
"F". An F of 1.00 is equivalent to a
bioavailability of 100%.
- One can find the "Area-Under-the-Curve" (AUC)
for a drug as a measure of the exposure of the animal to
the drug. In its simplest form, one plots the drug
concentration versus time and then adds up the areas of
successive trapezoids formed under the curve. This is the
so-called "trapezoidal rule".
- A fun, simple way of determining the the relative drug
exposure would be to cut out the area under the plasma
concentration versus time plot and weigh it.
- Find the area (weight!?) for the "test-route",
e.g., PO and compare it to the area (weight!?) of an
intravenous injection of the same drug dosage.
depiction of determining F
- Some representative values of F in animals:
- Ketamine, cat, IM, F=0.92
- Ampicillin HCl, cat, SC, F=0.56
- Sulfadimethoxine suspension, dog, PO, F=0.56
- Kanamycin Sulfate, sheep, IM, F=1.00
- Kanamycin Sulfate, sheep, Intratracheally, F=0.38
- Some drugs with low "F" in humans (F < .5) Rowland&Tozer89,p120
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Last modified: 03 Sep 1996 22:55glc