Bioavailability

Bioavailability = AUCoral / AUCiv;
- The relative amount of drug that reaches the greater systemic circulation (i.e. following absorption & first-pass metabolism)
- It is the proportion of drug that gets into the plasma compared to if you injected the same amount IV.
In order to get to the plasma the drug goes through:
- Ingestion
- Transit to GI tract
- Absorption in small intestinal mucosa
- First pass metabolism in small intestinal mucosa and liver
Bioavailability is used to determine:
- The optimal route to achieve the therapeutic concentration.
- The amount of drug that needs to be given orally.

Pharmacokinetics - Absorption

Stomach: Little absorption takes place for most drugs here (thick gastric mucous layer prevents significant drug absorption).
Small intestine: Transit time 4-6h; 6-7 m long x 2.5cm internal diameter.
It has a larger surface area (30-35m^2) than the stomach (~1m^2), due to:
- Jejunal plicae circulare (circular infoldings) - increases surface area 1.6x
- Epithelial villi + microvilli: increases surface area 80-100x.
Additional factors aiding absorption:
- Peristalsis churns the intestinal contents, ensuring regular contact between drugs & epithelium.
- pH is 6-7 (weakly acidic) —> determines the ratio of drug molecules that exist in the ionised and unionised state.

4 methods:

Passive Diffusion

The commonest way for many drugs to cross lipid membranes (Esp. if small [<500 Da] & not strongly ionic).
Lipophilic drugs (e.g. steroids) easily pass through lipid bilayers.