Topic 10 - Repeated Dosing Flashcards
Steady State Concentration
Css
Rate of Drug in = Rate of Drug out
When do we reach Steady State Concentration
When we administer drug at a constant rate a steady state concentration will be reached
Css and Elimination
First Order Elimination Curve mirrors the Infusion Curve
How long does it take to reach Steady State
When a drug has been constantly administered for 5 Half Lives (97% of drug has been eliminated)
Increasing Dosing Rate effects on Steady State
We do not reach steady state any faster
Time to reach Steady State is always around 5 half lives
Consecutive Doses (IV vs Oral)
An oral dose is not as constant as an IV but, still follows 5 half lives rule
Oral dose has more smoothing with less fluctuations
Why are oral doses more smooth
Takes a while for drug to be absorbed and eliminated
Causes lower peaks and slower elimination
If drug is more sustained released then peaks are even smoother
Steady State Concentration of Oral Drugs vs IV
The average Css of oral drugs is actually the same as an IV
Unless oral drug is less bioavailable
Oral Drug Bioavailability effects on Css
50% bioavailable drug would reduce Css by around 50% as well
Therapeutic Window
Range of plasma concentration that gives a therapeutic response without any side effects
Target Css should be within window
MTC
Maximum Tolerance Concentration
Max amount before you start seeing side effects
MEC
Minimum Effect Concentration
Amount needed to see any therapeutic effects
Why can’t we just decrease dosing rate to stay within therapeutic window
If dosing rate is reduced then Css will also be reduced and may fall below MEC
How do we stay within therapeutic window?
Maintain Dose Rate
By taking a lower concentration of dose but more often
15mg/1hr = 360mg/day
180mg/12hrs = 360mg/day
The 15mg an hour would have a curve with less fluctuations
Why use sustained release drugs
Can stay within therapeutic window due to less fluctuations in Css
