Drug Absorption Flashcards
How do drugs achieve effect?
Must enter blood and be distributed to site of action
What are the factors that determine drug pharmacokinetics?
- Absorption
- Distribution
- Metabolism
- Elimnation
What is absorption of a drug defined as?
The process of movement of unchanged drug from the site of administration to the systemic circulation
Issues with absorption
- Amount of drug which enters systemic circulation
- Speed at which this happened (distribution)
What is the Tmax of absorption?
Time to reach peak concentration
What is the Cmax?
The peak concentration
What is the AUC?
Area under the drug conc. time curve representing the amount of drug which reaches systemic circulation
How does rate of absorption affect Tmax?
Rapid rate = earlier Cmax is reached
Affect of increasing dose on Tmax and Cmax?
Increase dose = no change to Tmax, but increases Cmax
What is the therapeutic range?
A measure of the range at which a drug is safe and active
What happens above and below therapeutic range?
- Above: toxicity
- Below: insufficient dose so no action
Define bioavailability
Estimate the amount of drug which reaches the circulation and is available for action
IV - 100% bioavailability
Factors affecting bioavailability
- Formulation
- Ability of drug to pass barriers
- Gastrointestinal effects
- First pass metabolism
How does formulation affect bioavailability?
Slow release drugs - effects speed at which drug is released
What are characteristics of drug that affects a drug’s ability to pass barriers?
- Particle size
- Lipid solubility
- pH and ionisation
Relationship between pH and ionisation of drugs?
Henderson-hasselbalch equation: change in pH affects ionisation of drug and therefor rate of absorption or diffusion
Can ionised drugs cross membranes?
No, only un-ionised drugs distribute across the membrane until equilibrium is reached
What is ion trapping?
An acidic drug is more concentrated in the compartment with high pH
What kind of drugs diffuse across a lipid layer?
Be in solution and lipid soluble
What is the lipid-water partition coefficient?
Ability of a drug to diffuse across a lipid barrier
What is required for absorption?
- Energy - occurs against concentration gradient
- Drug to resemble natural compounds
- Drug reversibly bound to a carrier
What is bulk flow?
Movement of drugs across a membrane due to hydrostatic or osmotic pressure differences
-water soluble drugs
What factors affect absorption of a drug from the GI tract?
- Motility
- Food
- Illness
Explain motility of a drug
Speed of gastric absorption affects speed that drug reaches site of absorption (small intestine)
Explain affect illness on absorption
- Malabsorption affects rate
- Migrained reduces rate of stomach emptying and therefor rate
What is First Pass Metabolism?
Metabolism of a drug before it reaches systemic circulation
How do you get over first pass metabolism?
- Increase dosage
- IM admin.
- Sublingual admin.
- Rectal admin. (slow)
- Transdermal admin.
What are the four primary system that affect first pass metabolism?
- Gut lumen enzymes
- Gut wall enzymes
- Hepatic (liver) enzymes
FPM effect of bioavailability
Decreases bioavailability of drug as metabolised before reaching systemic circulation
List the considerations for mode of administration
- Purpose & site of drug action - FPM
- Disease effects
- Ability to take meds
- Speed of action
- Reliability of absorption
