Pharmacokinetics:
General principles of pharmacokinetics:
Absorption:
Most drugs are administered orally and
the gastrointestinal absorption of drugs is a function of its physio-chemical
properties (PKa, lipophilicity, molecular weight) and the Physiology of the
gastrointestinal tract (pH, gastric emptying).
Distribution:
Distribution is a function of
affinity of drug for tissues and
plasma blood flow rates to organs.
physiochemical properties of drug.
Metabolism:
The drug may be bio transformed by
metabolism mainly in the liver or other tissues.
Elimination:
Is a function of biotransformation in
delivered to metabolites , excretion of the drug by the kidney.
What is dose?
Dose:
It is a specific amount of a drug
given to produce a desired effect.
Types of Dose:
Therapeutic dose / effective dose (ED50):
The average dose required to produce
therapeutic effect in 50% of population treated is called therapeutic or
effective dose (ED 50).
Maximal tolerated dose:
The substantial dose of a drug that
can be taken securely is called maximal tolerated dose.
Initial dose:
Those of a drug given at the start of
the treatment is called initial dose.
Maintenance dose:
Dose administered to replenish the
amount of the drug which is eliminated in intervals between two doses, so that
our steady concentration of drug is maintained in the plasma.
Loading dose:
The dose of a drug administered to
rapidly achieve the desired concentration of drug in the body. In loading
those, the amount of drugs should be increased but not the rate of
administration.
Loading dose = Vd×Tc
Where VD = volume of distribution and Tc
= target concentration.
Important note:
Drugs which is initiated with a initial
loading dose include digoxin, teicoplanin, variconazole and procainamide.
Phenytoin is for acute status epilepticus is given with a initial loading dose,
and Co-administered with benzodiazepine.
Toxic dose (LD50):
Dose above the effective dose produced
death in 50% of population treated is called toxic dose or lethal dose (LD50).
What is therapeutic index?
Therapeutic index:
Therapeutic index is the ratio between
toxic dose (LD50) an effective dose (ED50).
Therapeutic index = LD50 / ED50
The therapeutic index is a measure of
the drug safety because a large value indicates that there is a wide margin
between doses that are effective and doses that are toxic.
Examples:
Warfarin (anticoagulant) Is a drug with a small
therapeutic index.
Penicillin (antibacterial) is a drug with a
large therapeutic index.
Some other drugs with low therapeutic
index include Theophylline, lithium,
digitalis, anti arrhythmic agents and antiepileptic drugs.
What is therapeutic window?
Therapeutic window:
It is a well defined range of a drugs
serum concentration at which a desired effect occurs, below which there is a
little effect and above which toxicity occur. it is the safe range between the
minimum therapeutic concentration to minimum toxic concentration of a drug.
Learn more
What is pharmacokinetics?
What is the general principles of
pharmacokinetics?
What is absorption In
pharmacokinetics?
What is distribution in
pharmacokinetics?
What is metabolism in
pharmacokinetics?
What is elimination in
pharmacokinetics?
What is dose?
Types of dose
Therapeutic dose or effective dose
What is therapeutic dose?
What is effective dose?
What id ED50?
What is maximal tolerated dose?
What is initial dose?
What is maintenance dose?
What is loading dose?
Formula for loading dose
What is toxic dose?
What is LD50?
What is therapeutic index?
Formula for therapeutic index
Examples of therapeutic index
Examples of drugs in therapeutic index
What is therapeutic window?
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