Definition
After a drug is ingested, it passes through the esophagus into the stomach and into the small intestine. There, the active ingredient must first be released from the dosage form. This is the prerequisite for it to be absorbed into the bloodstream via the cells of the mucosa. The dosage form thus exerts a decisive influence on pharmacokinetics and bioavailability, i.e., on the extent and speed with which the active ingredient enters the systemic circulation.
Decay
The shell of capsules is usually made of gelatin. It dissolves under the influence of heat and liquid to release its contents, for example, a fine powder. Tablets disintegrate into smaller particles after ingestion. To enable or accelerate this process, so-called disintegrants are added to them as excipients. These can be starches and celluloses, for example, which swell on contact with water. Disintegration increases the surface area and improves the dissolution of the active ingredients. In the case of tablets, a distinction is made between disintegration into coarse particles (granules) and the subsequent disaggreagation into small particles. Some active ingredients reach the stomach and intestines already in small particles or in solution. These include solutions, suspensions, syrups, meltable tablets, dispersible tablets, chewable tablets and dissolved effervescent tablets. The onset of action may be faster because a step has been anticipated.
Solution (dissolution)
The next step is for the active ingredient to go into solution. This is called dissolution. The solubility of active ingredients depends on several factors. These include:
- Physicochemical properties of the drug: lipophilicity, crystal form, drug salt (ionization), particle size, pKa.
- Dosage form, excipients
- Gastrointestinal environment: digestive juice, bile, bile salts, pH.
- Intake with or without food, filling state of the stomach.
- Transit time
- Drug interactions
Absorption of the active ingredient from the lumen decreases the concentration in the intestine. At the same time, the drug is continuously removed with the blood. This increases the solubility of the drug due to the concentration gradient. Many active ingredients are ionized in the drug, in the form of active ingredient salts, because this increases their solubility. It should be noted that a small amount of the active ingredient may go into solution directly from the tablet that has not yet disintegrated.
