pharmacodynamics and pharmacokinetics Flashcards
examples of ways drugs can reach their target (3)
passively diffuse through the phospholipid bilayer of cells (lipid soluble); drug mimics natural protein and is actively transported (e.g. levodopa); via ion channels (small polar molecules e.g. Li+)
how are most drugs generally transported in the body
carrier-mediated transport
what can affect the ionisation of a drug (and thus its diffusability)
Ka of the drug: pH of the body fluid (favours ionisation?)
what is given to treat an aspirin overdose
NaHCO3 - increases the pH resulting in ionisation of aspirin and thus it entering the urine and being excreted
for what kind of drugs is an oral dose almost as good as IV and why
lipid soluble - they are non-polar molecules and so can easily pass through cell membranes to their target rather than relying on
why is heparin safe in pregnancy
a very polar molecule and so cannot cross the placenta
what is the purpose of the blood brain barrier (4)
protect the brain from harmful substances; allow nutrient through; tight capillary endothelia junctions; enzyme barrier
ways drugs can be eliminated (5)
urine; faeces; milk; sweat; expired air
what does zero order kinetics of a drug indicate
there is a limited capacity of the body to process the drug; as the dose rises above a certain level, metabolic processes cannot cope; dose increase results in sudden toxicity
what happens if the breakdown enzyme is saturated
drug cannot be broken down any fast - increasing toxicity (e.g. w alcohol, will get drunk faster after enzyme saturation)
what is bioavailability
the proportion of the drug that passes into the systemic circulation after administration
what can affect the bioavailability of oral drugs
absorption in GI; loss in faeces; breakdown by gastric acid; liver metabolism
what is used to calculate oral bioavailability
amount in circulation after oral dose; amount in circulation after IV dose
how can bioavailability be improved (3 + examples)
change in formulation - enteric coated capsules to stop gastric acid degradation;
route - sublingual GTN avoids liver metabolism;
inhibitors - peripheral dopa decarboxylase inhibitor stops levodopa being broken down before it can reach the brain
what is apparent volume distribution and what does it indicate
calculation of amount of drug in plasma after being taken up; high V indicated the drug mast have gone elsewhere e.g. fat; low V indicates drug is strongly bound to plasma proteins and not tissues
what is clearance
elimination of drug from plasma - the amount of plasma totally cleared of drug
what is half-life
the time taken for drug concentration to fall by half its initial value
2 main half life determinants
clearance; volume of distribution (i.e. amount of drug available in plasma for kidney/liver to excrete)
what is steady state
when a drug is given at a constant rate allowing the plasma conc to plateau; usually after 5 half lives
what should be done if a drug has a short half life (to increase - 4)
give a larger dose frequently; inhibit clearance; reduce movement of to plasma (e.g. using adrenaline which promotes vasoconstriction so drug remains local); modify formulation (e.g. sustained release tablets)
what is the most important factor in determining dosage regimen?
half life
why might stopping a drug not stop its adverse affects
it has a long half life and high volume distribution meaning that it is stored in the tissue even after it has stopped being given
