The Absorption Concept

Under construction

Overview

Drug absorption can be characterized by two pharmacokinetic parameters:

• Bioavailability (F) quantifies the extent of absorption.
• k_a quantifies the rate of absorption.

Definition

The oral bioavailability (F) of a drug is determined by three factors:

$$\begin{array}{}\text{(1)}& F=F_a\times F_g\times F_h,\end{array}$$

where
* F_a is the fraction of drug absorbed,
* F_g is the fraction escaping gut-wall metabolism, and
* F_h is the fraction escaping hepatic extraction.

See Equation 1 and Figure 1 for an illustration of these processes.

Figure 1: Oral absorption.

Figure 2 depicts enterohepatic recirculation, which can also influence bioavailability. If a drug exhibits enterohepatic recirculation, it is common to see two or more absorption “peaks”.

Figure 2: Enterohepatic recirculation.

References