Intestinal absorption
After ingestion of a drug, the active ingredient must first be released. This process is called release (liberation), and it is a prerequisite for subsequent absorption. Absorption (formerly: resorption) is the passage of an active pharmaceutical ingredient from the digestive pulp into the bloodstream in the stomach and intestines. Absorption occurs primarily in the small intestine. The essential step is the passage of the active ingredient across the unicellular layer of intestinal cells (enterocytes) and absorption into the underlying blood vessels. The following mechanisms are involved:
- Transcellular passive diffusion across cell membranes.
- Uptake via transporters and channels (facilitated diffusion, active transport consuming ATP).
- Transcytosis with vesicles
- Paracellular passive diffusion (intercellular spaces).
Efflux transporters such as P-glycoprotein counteract absorption. They transport the substrates back into the intestinal lumen and reduce bioavailability. Because the drugs are permanently transported away with the blood, there is a concentration gradient for the passive processes.
Influencing factors
Absorption depends largely on the physicochemical properties of the drug. Other influencing factors include:
- Release (see there).
- Gastrointestinal environment: digestive juice, bile, bile salts, pH.
- Intake with or without food
- Transit time
- Intestinal blood flow
- Diseases, age
- Drug interactions
Already in the intestinal cells and subsequently during the first passage through the liver, the drug can be biotransformed. In the so-called first-pass metabolism, a relevant proportion of the active ingredient can be inactivated, so that the proportion that finally reaches the target site is reduced. This is also referred to as a metabolic barrier.
