Pharmokinetics Flashcards
A drug is…
A single chemical entity that may be one of the constituents of a medicine
A medicine…
May include one or more active drugs together with additives that facilitate administration
An excipient is…
The ‘vehicle’ via which a drug can be administered
Therapeutic index is…
The margin between an effective dose of a medication, and a dose that will cause harm
2 types of administration route
Enteral - via gut, oral or rectal
Parenteral - avoiding gut, on skin, under tongue, via lungs
Absorption
Lipid-soluble drugs can dissolve through membranes
What are the GI factors influencing absorption? (4)
Oesophageal passage
Gastric motility (peristalsis)
Presence of food and drink (200ml water is ideal)
pH
How does pH affect absorption?
Lots of drugs are made basic so they’re water-soluble in the stomach, then carried in solute to the intestine where acidity reduces and the drug becomes fat-soluble
The small intestine is the largest area of absorption because…
It has a large surface area
Drug efflux proteins…
Sit in the cell membrane and reject drugs trying to pass through, and vacuum drugs that have entered the cell and reject them
Drug efflux proteins… over time because… which explains…
Increase
The body makes more and more
Why drug resistance builds up
Distribution is…
The amount of drug in body tissues, fluids or spaces. The movement of a drug between the blood plasma and tissues continues until the drug equilibrates.
The more fat-soluble the drug…
The faster it will leave the bloodstream - leaves and stores in fat
…affects blood flow in the tissue
Heat
Plasma protein binding
The more fat-soluble a drug is, the more it will bind to proteins. Drugs compete for protein-binding sites.
Metabolism
Enzymes metabolise drugs to make them water-soluble, so they stay in the bloodstream and are filtered out by liver rather than going to target cells
Phase I metabolism
Drug is broken up into metabolites by enzymes (women have half of these enzymes compared to men)
Phase II metabolism
Metabolites (or original drug) are conjoined with another molecule, making it more water-soluble
… are phase 1 metabolic enzymes
Cytochrome P450
… is the enzyme respondible for metabolising >60% of all therapeutic drugs
Cyp 3A4
… is an enzyme responsible for metabolising most antidepressants
Cyp 2D6
First pass effect
The extent of metabolism experienced by a drug after it passes through the liver/intestine wall.
If a drug has a high first pass effect, there is not much of the drug left in the bloodstream for effect - parenteral routes avoid this
… is an example of a drug that is effective after it is metabolised
Morphine
3 methods of eliminating drugs from plasma
- Redistribution
- Liver metabolism
- Renal excretion
