Drug Disposition III Flashcards
First Rate Drug Elimination (K>E)
There is a constant fraction of drug elimination per unit time, creating a exponential curve
It is a fractional rate of drug elimination
Biological Half Life (T>1/2)
The time required to reduce the plasma drug concentration by 50%
t>1/2 = 0.693/K>E, which is to say that half life is inversely proportional to the first order rate constant
Importance of Half Life
Determines duration of action. Takes 5-6 half lives for a dose to be effectively eliminated. Can help determine dosage intervals.
Rate of obsorption
Affected by:
Route of administration
Blood flow
Slow-release preparations
Slow absorption means low peak concentrations and longer duration of action
Plateau Principle
Steady state plasma concentrations reached under zero order drug administration and first order drug elimination.
Multiple Drug Dosing
Given multiple dosings at fixed intervals, the rate of elimination will increase as the concentration of drug increases in the plasma. The rate of elimination will eventually reach the rate equal to the amount of a single dose between doses. (steady state)
Plasma Clearence
Defined as the volume of plasma from which a drug is fully removed per unit time (ml/min)
Plasma clearance is equal to the sum of all organs
Rate of elimination = Plasma Clearance x Plasma Conc.
Steady State (Css)
Rate of elimination = Rate of administration
Rate of Administration
= steady state x plasma clearance
Choice of interval depends on
Drug Half Life
Size of therapeutic window
Frequency/severity of adverse effects
Convenience and patient compliance
Calculating Loading Dose
Loading dose is roughly equal to twice the maintenance dose.
Zero Order Kinetics (Drug Elimination)
Constant absolute amount of drug is eliminated per unit time regardless of the concentration of the drug in the plasma.
