Metabolism + Excretion Flashcards
How does enzyme saturation effect the rate of removal of a drug + what order is this
Makes it constant (a further increase in enzyme conc. does not effect rate of removal)
0
What drug follows 0 order kinetics
Ethanal if alcohol dehydrogenase is saturated
Define half-life
Time taken for a conc. to reduce by a half
What is half-life in the context of drug elimination
Elimination rate from plasma
Why dow e measure elimination rate from plasma
Easy to measure
Define bioavailability
Fraction of administered drug that reaches the systemic circulation un-altered
Why do IV drugs have bioavailability (F) of 1
100% of the drug reaches circulation
Why would oral drugs have F<1
Incompletely absorbed or undergo first pass metabolism
How does bioavailability effect prescriptions
If drug has F=0.1 than we need an oral dose of 10*0.1
How is bioavailability measured
AUC oral/ AUC IV
What factor effects rate of distribution of water-soluble drugs
Rate of passage across membranes
What factor affects rate of distribution of lipid soluble drugs
Blood flow to tissues that accumulate the drug
Why is extent of drug distribution important
Determines total amount of drug that has to be administered to produce a particular plasma conc.
How is extent of drug distribution measured
Volume of distribution (Vd)
Vd = total amount of drug in body/plasma conc.
What does a low Vd indicate
Drug is confined to circulatory volume
What does a high Vd indicate
Drug is mostly distributed in total body water
Define clearance
Volume of blood or plasma cleared of drug per unit time
What is the plasma conc. of a drug with a high Vd
Low
How is rate of elimination related to Vd
Rate of elimination is inversely proportional to Vd
Formula for Rate constant of elimination
K= CL/Vd
Consequence of a CL of 0
Drug would not be removes and plasma conc. would remain at equilibrium indefinitely
Formula for clearance for IV drugs
Dose/AUC for IV
Formula for clearance for oral drugs less than 1
Dose x F/AUC
Why do we use repeated drug doses
Maintain constant drug concentration in blood
