Pharmacology Flashcards
(32 cards)
Define Volume of Distribution
A proportionality factor that relates the amount of drug in the body to the concentration of drug measured in the plasma. Vd = Amount of drug in body/Concentration of drug in plasma
What is Pharmacokinetics?
The mathematics of the time course of absorption, distribution, metabolism and excretion of drugs in the body. This determines the plasma concentration of a drug.
Why do we calculate PK parameters?
Factors such as age, drug interactions, disease state and pharmacogenomic profile affect the rates of metabolism and excretion. The study of PK enables clinicians to individualise a dosing regiment for a patient
what is Kel
The fraction of drug eliminated per unit of time
What is a first order kinetics concentration-time curve like?
Rate of metabolism is proportional to the plasma drug concentration
Half life is constant
Importance of zero-order kinetics
Rate of metabolism is independent of plasma drug concentration.
rate of metabolism is constant but half life is not
drugs can build up in the body and cause toxic side affects
Why is Volume of distribution important?
It is used to calculate drug dose
Plasma concentration is affected by the Vd of a drug
High Vd means drug accumulates in peripheral tissue and there is low plasma concentration
Low Vd means a high plasma concentration of drug
What is C0?
The initial concentration of drug in the plasma at time=0 when the dose is administered as an IV injection
equation linking Vd, dose and C0?
Vd= Dose/C0
How to calculate C0 from a concentration time graph?
Extrapolate the line and read the value for when T=0 (crosses the Y axis)
What is the half life of a drug?
The time taken for the plasma concentration of the drug to half
What is the clearance of a drug?
The volume of plasma cleared of a drug per unit time
The clearance of drugs is the PK parameter which most significantly limits the time course of action of a drug
Equation for clearance
Cl= (0.693/half life) X Vd
What is repeated dosing and steady state?
Often multiple doses are needed to provide a relative constant plasma concentration of drug
Steady-state is the equilibrium point where the amount of drug administered exactly replaces the amount of drug excreted.
What is an optimal therapeutic dosing regimen?
One which seeks to maintain steady-state plasma drug concentration within the therapeutic window for that drug
Equation for steady-state (Css)
Css= (Bioavailability X dose)/(interval dosing X clearance)
Link between half life and steady-state
The shorter the half-life the more rapidly steady state is reached
steady-state of drugs with a long-half life
For drugs with a long half life the drug will accumulate and chronic dosing is designed to achieve the therapeutic range at steady state
What is Chronic dosing ?
Long interval over long time period
What is loading dose?
Vd X desired Css (steady state plasma concentration)
What is polymorphic expression and how does to influence response to a drug?
A slight genetic difference between individuals which alter how well the drug metabolising enzymes functions
this can result in increased/decreased enzymes activity and levels of metabolism
What are fast metabolisers?
possess wild type enzyme which has normal activity
there are lower plasma concentration of parent drug and higher concentrations of metabolite
usually a normal therapeutic response
What are slow metabolisers?
possess a defective enzyme which has reduced/no activity
high plasma concentration of parent drug and low concentrations of the metabolite
can lead to exaggerated therapeutic response and so a reduction in drug dose may be necessary
what occurs in a person who has a liver disease?
capacity to uptake and metabolism lipid soluble drugs is reduced
first pass metabolism is reduced and so the bioavailability may be much greater
therefore the drug dose may need to be reduced
