Pharmacodynamics Flashcards
What is pharmacodynamics?
when a drug binds to a receptor causing intracellular activity or enters the receptor altering gene expression
What are agonists?
drugs that bind to receptors and cause a biological response; they have intrinsic activity
What do drug-receptor interactions involve?
usually weak (covalent) bonds constant binding and dissociation
What is drug affinity?
the rate of dissociation
high affinity = low rate of dissociation
What are the principles of drug-receptor interactions?
magnitude of drug effect is proportional to the number of receptors occupied
up to a point, more drug = more effect
What is the law of mass action?
a drug produces its maximal effect when all receptors are occupied
What is the effective dose 50?
the dose that produces a response in 50% of subjects
What is the toxic dose 50?
the dose that produces a toxic effect in 50% of subjects
What is the lethal dose 50?
the dose that kills 50% of subjects
What is the therapeutic index?
TD50/ED50
What is the safety margin?
LD50 - ED50
What is drug potency?
how much drug is needed to produce an effect
Why does drug potency differ?
drug access to the brain
drug affinity; how efficiently it binds to a receptor
What is drug efficacy?
the maximum effect of a drug
Why does drug efficacy differ?
the mechanism by which they act at a receptor
amount of intrinsic activity at the receptor once bound
What is an antagonist?
drugs which bind to receptors and have no intrinsic activity, block the effect of agonists or endogenous ligands
What is a competitive antagonist?
a drug that competes with an agonist for receptor sites
maximum effect remains the same but potency is reduced
increased amounts of agonist are required for the same effect
What is a non-competitive antagonist?
reduces the effect of an agonist but does not compete for binding sites
binds to an inactive part of the receptor and interrupts intracellular processes
maximum effect and potency are reduced
What is an irreversible antagonist?
a drug which forms long-lasting bonds with a receptor
recovery is only possible by synthesis of new receptors
What is a partial agonist?
act as an agonist or an antagonist depending on the local concentration of other agonists
maximum effect remains the same but potency is reduced
What is tolerance?
when a drug effect because smaller after administration
How does overdose relate to tolerance?
tolerance build up means that the effective dose increases, making the margin of safety smaller, this dose is too close to the toxic/fatal dose
What is acute tolerance?
tachyphylaxis; drug effect decreases rapidly, within a single session
What is protracted tolerance?
drug effect decreases with repeated administration e.g. alcoholism
What is cross-tolerance?
when the effect of a drug decreases after administration of another drug e.g. regular alcohol drinkers are less effected by barbiturates
How does tolerance influence homeostasis?
tolerance reflects homeostatic adaptations to drugs; can be pharmacokinetic or metabolic e.g. chronic drinkers have increased levels of alcohol dehydrogenase
What are the mechanisms of tolerance?
homeostatic adaptations
behavioural learning
conditioning
receptor signalling
What is dependence?
when tolerance has developed, ceasing drug use will cause withdrawal effects opposite to the action of the drug abused
What is sensitisation?
when a drug effect gets bigger after administration
When can sensitisation occur?
in a chronic drinker, alcohol can cause cirrhosis of the liver, meaning alcohol is not metabolised as efficiently so more reaches the brain
How does affinity affect a drugs action?
high affinity = longer binding time
does not determine the maximum effect of a drug
