GBP4 Flashcards
(23 cards)
Give two examples of drugs that express zero order kinetics
phenytoin
ethanol
What does Vmax mean
Maximum rate of reaction
What does Km mean
Concentration of drug needed for 50% of maximum rate, measure of affinity
What could influence Vmax in different patients
Low quantity of enzyme
drug interactions
What does changes in absorption lead to
change in area under curve, in tmax, in bioavailability, and in half life ( if absorption greatly prolonged)
What does changes in distribution lead to
Changes in Volume and in half life
What does changes in Elimination lead to
Changes in area under curve, in clearance and in half life
Name five possible mechanisms of drug transfer across membranes
Pinocytosis Filtration Active transport Bulk flow of water Passive diffusion
How are most drugs moved across membranes
passive diffusion
What does rate of passive diffusion depend on
Concentration gradient
Surface area for diffusion
Thickness of membrane
Diffusion constant (dependant on molecular size and lipid solubility)
What is amount of drug transferred dependant on
rate of transfer
residence time at membrane
What is the pKa
pH at which a weak acid or base is 50:50 charged:uncharged
What is the likely percentage ionisation for acids with a pKa of 5 in
a) the stomach (ph1.5)
b) the intestine (ph5.3)
c) plasma (ph 7.4)
a) 0
b) 61
c) 99.5
What is the likely percentage ionisation for bases with a pKa of 5 in
a) the stomach (ph1.5)
b) the intestine (ph5.3)
c) plasma (ph 7.4)
a) 100
b) 39
c) 0.5
What might be the effect of a PPI on drugs ionisation
PPIs make the stomach less acidic meaning more of an acidic drug would be ionised and less of an basic drug would be ionised
Where are weak acids best absorbed
stomach and intestine
Why are acidic drugs unlikely to cross the BBB
because they are mostly ionised at blood ph
Where are basic drugs best absorbed
intestine
Why are basic drugs likely to cross the BBB
because they are unionized at blood ph
What effect does the presence of food in the stomach have on absorption of
a) acid stable drugs
b) acid labile drugs
a) slowed absorption
b) more readily broken down and made inactive
What advice would you give to a patient taking an acid labile drug such as PPI or Flufloxacillin
Take it before food
What is the meaning of first pass metabolism
Some drugs are metabolised before they reach the systemic circulation
Where other than the stomach and intestine can drugs be absorbed
buccally- rapid and avoid first pass metabolism
Percutaneously- patches for topical use, avoids first pass metabolism
pulmonary- passive diffusion, local effects
