Pharmacokinetics I: Absorption and Distribution. Flashcards
What is pharmacokinetics?
The study of the movement and handling of drugs through the body, encompassing four stages: absorption, distribution, metabolism, and excretion.
Define absorption in pharmacokinetics.
Absorption is a drug’s movement from its administration site to the systemic circulation.
Name three cases where drugs do not undergo absorption.
- IV drugs (directly enter systemic circulation).
- Inhaled drugs (act locally in the lungs).
- Certain antibiotics in the GI tract (act on microorganisms locally).
What are the two main mechanisms for drug absorption?
- Diffusion through lipid membranes.
- Carrier-mediated transport (can become saturated).
What is bioavailability?
The proportion of the administered drug that reaches the systemic circulation. Example: Citalopram has 80% bioavailability.
How does pH affect drug absorption?
- Acidic drugs can start absorption in the stomach.
- Basic drugs are absorbed in the intestine.
What is drug distribution?
The movement of a drug throughout the body, including binding to plasma proteins and partitioning into tissues.
What role do plasma proteins play in drug distribution?
Drugs bind to plasma proteins (e.g., albumin).
Only the free, unbound drug exerts a pharmacological effect.
Why is plasma-protein binding important?
It determines the fraction of free (active) drug in the plasma, which can be as low as 1%.
Give examples of how drugs partition into tissues.
Body fat: Acts as a non-polar compartment for lipid-soluble drugs.
Calcium-containing tissues: Tetracyclines accumulate in bones and teeth, unsuitable for children.
What factors affect drug distribution?
- Plasma-protein binding levels.
- Partitioning into fat and other tissues.
- Physiological conditions like pH, age, or disease.
Why is tetracycline not used in children?
It binds to calcium and accumulates in bones and teeth, potentially causing permanent discolouration.
What is the primary plasma protein for drug binding?
Albumin.
Name two resources for detailed pharmacological information.
BNF/BNFC (British National Formulary).
DrugBank (detailed database).
