Pharmacokinetics I Flashcards
What is the effect of a drug related to?
The blood concentration
Changes in blood concentration influence the effect of the drug
What is the one compartment model?
The body acts a single compartment
Drug rapidly equilibrates between plasma and tissue relative to the elimination rate
How are plasma concentration and tissue concentration related in the one compartment model?
Changes in [plasma] reflect a porportional change in [tissue]
Plasma and tissue concentrations may be different
The drug does not inherently distribute to all tissues
What is the shape of a semi-log plot of plasma concentration in a one compartment model when the drug follows first order kinetics?
Straight line over time
What is a two compartment model?
There are two compartments for distribution: a central and peripheral compartment
A drug takes longer time period to establish equilibrium between plasma and tissue
Drug can move between the two compartments
Elimination occurs from the central compartment
What is the shape of a semi-log plot of plasma concentration in a two compartment model when the drug follows first order kinetics?
Two phases: initial distribution and elimination
Initial distribution phase is steeper and shorter than the elimination phase
What are first order elimination kinetics?
The rate of decrease in drug concentration is directly proportional to the amount of drug in the body
A constant fraction of the drug is eliminated, independent of dose
Why is a constant fraction of drug eliminated in first order kinetics despite the dosage?
The metabolizing enzymes are not saturated
What are zero order elimination kinetics?
A constant amount of drug is eliminated, independent of dose
The enzymes involved are saturated
What two agents follow zero order kinetics?
Alcohol and Aspirin
What is bioavailability?
Fraction of unchanged drug reaching the systemic circulation after drug administration
What is the bioavailability of a drug administered by the intravenous route?
1
All other routes are less than one
Why are bioavailability studies done?
Comparison of different products with the same active ingredients
Assess influence of food or drug on bioavailability
What is the four half lives rule?
A drug is considered essentially eliminated from the body after about four half lives
What is the volume of distribution?
The apparent volume that a drug would be distributed within the body once an equilibrium is established with plasma
