Drug Elimination

Question 0

First order elimination kinetics

Answer 0

Rate is a function of concentration
Elimination gets slower and slower over time
Biological half life
Say the half lifer is equal to 2 days
After 2 days the drug concentration will of dropped to 0.5
After another 2 days to 0.25, then 0.125
Regular plot will be exponential
Semi-log plot will be linear

Question 1

Zero-order elimination kinetics

Answer 1

Limited by metabolism
Eliminate at a constant rate
Elimination is saturated

Question 2

Half life rule of thumb

Answer 2

4-5 half lives for drug to be 93-97% eliminated
4-5 half lives for drug to accumulate to steady state, Css
4-5 half lives for drug to accumulate to a new Css when changing dose at Css

Question 3

Drug dosing

Answer 3

Give an individual a test dose and see what it does to its plasma concentration
See a change and then each day a certain amount is eliminated
Give a second dose, goes up again and rate eliminated is higher as more concentration
Reach a point where the amount eliminated equals the amount put in—steady state

Question 4

IV infusion to maintain steady state

Answer 4

For IV there is no dosing interval
Put in a certain amount of drug per time
Rate of administration is R0, mg/hr
R0 = Cl_tb Css

Question 5

Oral administration to maintain steady state

Answer 5

Take bioavailability into account
How often you take the pill
Tau = dosing interval 
Once a day is 24 hr
B.i.d is 12 hr
T.i.d is 8 hr

M.D_oral = (tau)(Cl_tb)(Css) / Foral