drug absorption Flashcards
there is always a correlation between plasma concentration of a drug and
the therapeutic response
The time to peak concentration
Tmax
The peak concentration
Cmax
The area under the drug concentration curve
AUC
Tmax
The more rapid the rate of absorption, the earlier the drug concentration peak
Cmax
Increasing the dose doesn’t affect the time at which peak concentration is reached but it does increase the peak concentration
AUC
The area under the drug-concentration time curve represents the amount of drug which reaches the systemic circulation
Therapeutic range
A drug that is active over a range of concentrations
Below therapeutic range
no pharmacological action
Above therapeutic range
toxicity occurs
AUC
allows for estimating the amount of drug which reaches the circulation and which is available for action
BIOAVAILABILITY
Factors effecting bioavailability
Formulation (slow release)
Ability of drug to pass physiological barriers
Gastrointestinal effects
First pass metabolism
Most drugs do not completely ionise in
water
The degree of ionisation depends on the
pH of the environment
Henderson-hasselbach equation
relationship between local pH and the degree of ionisation
results: small changes in pH may significantly influence the ionisation of a drug and therefore the rate of absorption or diffusion
Lipid solubility
ability of a drug to pass a lipid layer
Lipid-water partition coefficient
the ability of a drug to diffuse across a lipid barrier
ratio of the amount of drug which dissolved in the lipid and water phase when they are in contact
Passive Diffusion
Very common
occurs along a concentration gradient
Active absorption
unusual
occurs AGAINST a concentration gradient
requires carrier and energy
Facilitative diffusion
occurs along the concentration gradient
requires carriers
NO ENERGY needed
Gastrointestinal Factors
Motility (speed of gastric absorption will effect speed at which drug reaches site of absorption)
Food
Illness (malabsorption - eg celiac disease)
First Pass Metabolism
metabolism of drug prior to reaching systemic circulation
