Absorption (pharmacology)
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Absorption is the journey of a drug travelling from the site of administration to the site of action.[1][2]
The drug travels by some
Absorption is a primary focus in drug development and medicinal chemistry, since a drug must be absorbed before any medicinal effects can take place. Moreover, the drug's pharmacokinetic profile can be easily and significantly changed by adjusting factors that affect absorption.
Dissolution
Oral ingestion is the most common route of administration of pharmaceuticals.
The rate of dissolution is a key target for controlling the duration of a drug's effect, and as such, several dosage forms that contain the same active ingredient may be available, differing only in the rate of dissolution. If a drug is supplied in a form that is not readily dissolved, it may be released gradually and act for longer. Having a longer duration of action may improve
The rate of dissolution is described by the
Where:
- is the rate of dissolution.
- A is the surface area of the solid.
- C is the concentration of the solid in the bulk dissolution medium.
- is the concentration of the solid in the diffusion layer surrounding the solid.
- D is the diffusion coefficient.
- L is the diffusion layer thickness.
As can be inferred from the Noyes–Whitney equation, the rate of dissolution may be modified primarily by altering the surface area of the solid by altering the particle size (e.g., with micronization). For many drugs, reducing the particle size reduces the dose needed to achieve the same therapeutic effect. The particle size reduction increases the specific surface area and the dissolution rate and does not affect solubility.
The rate of dissolution may also be altered by choosing a suitable
Coatings on a tablet or pellet may act as barriers to reducing the dissolution rate. Coatings may also be used to control where dissolution takes place. For example,
Drugs held in solution do not need to be dissolved before being absorbed.
Lipid-soluble drugs are absorbed more rapidly than water-soluble drugs.[9]
Ionization
The
Ions cannot passively diffuse through the gastrointestinal tract because the epithelial cell membrane is made up of a
The
However, the reverse is true in the basic environment of the intestines—weak bases (such as caffeine) will diffuse more readily since they will be non-ionic.
This aspect of absorption has been targeted by medicinal chemists. For example, they may choose an
The solubility and permeability of a drug candidate are important physicochemical properties the scientist wants to know as early as possible.[10]
Other factors
Absorption also varies depending on bioactivity, resonance, the inductive effect, isosterism, bio-isosterism, and consideration, amongst others.
Types
Types of absorption in pharmacokinetics include the following:[11]
- Instantaneous absorption: absorption is nearly immediate. A common example is bolus intravenous injection.
- Zero-order absorption: rate of absorption is constant. A common example is continuous intravenous infusion.
- First-order absorption: rate of absorption is proportional to the amount of drug remaining to be absorbed. Representative examples include typical cases of subcutaneous injection, and intramuscular injection.
See also
- Gastrointestinal transit time
- Flip–flop kinetics
References
- ISBN 978-0-12-386882-4.
- ISBN 978-0-12-802447-8.
- ^ LE.JENNIFER (2020-03-27). "Drug Absorption - Clinical Pharmacology". MSD Manual Professional Edition. Retrieved 2020-03-28.
- ^ Kaplan Pharmacology 2010, page 6, Absorption
- ^ Kaplan Pharmacology 2010, Video Lectures, Absorption chapter
- ^ Shimizu, Shinya. "Routes of administration" (PDF). The Laboratory Mouse. 1: 527–543.
- ^ Jean, Kim; Orlando, Jesus. "Medication Routes of Administration". StatPearls Publishing. 1: 121–141.
- S2CID 22893348.
- S2CID 79073985.
- . Retrieved 21 July 2021.
- ISBN 978-0-08-091922-5.
Further reading
- Avdeef, Alex (2003). Absorption and Drug Development. Hoboken, N.J: Wiley-Interscience/J. Wiley. ISBN 0-471-42365-3.