Lecture 14 - Drug distribution Flashcards
Clearance (4)
Volume of plasma cleared of drug per unit time.
Tells you how quickly body can get rid off drug.
CL = Rate of elimination / [drug plasma]
High clearance = Small half-life
First order (4)
Constant fraction of drug is removed.
Dose determines time taken for drug to leave.
No SATURATION kinetics - able to cope with more drug in system, capacity to clear drug.
Large clearance = Concentration decreases quicker = Small half life.
Zero order (4)
SATURATION kinetics - enzymes/transporters can be saturated to work at max rate - so a constant amount of drug is removed.
HL and clearance can fluctuate wiht a drug/ not constant.
HL/Clearance - amount of drug removed at that particular moment.
Diff saturation points of diff enzymes in peoples body, thnerefore difficult to predict, so AVOID it.
Zero order - examples (2)
o Alcohol- drug concentration would reduce, sub threshold and below saturation point when this happens then 1st order kinetics resumes.
o Phenytoin – useful cardiac drug, can show zero order kinetics, saturation varies across the population. When this is administered usually drug concentration in blood is watched to find the kinetics for patient.
Volume of distribution (Vd) (6)
How much drug is in or out of the blood vessels.
Total amount of drug / [plasma] = apparent volume of distribution.
Blood volume is 4-5L.
Indicates extent of distribution for a drug.
Clinically important for adjusting dosage.
Influenced by lipid/water solubility, binding to plasma protein.
Half-life (t1/2)
The time taken for drug concentration to reduce by half.
Bioavailability (F)
How much is available in the circulation in the end. Extent of absorption (F- fraction of drug in circulation compared to dose), higher F value the larger the extent of absorption.
Elimination
Removal of drug/metabolites from the body.
