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Pharmacokinetics (The study of the movement of drug molecules (what its…
Pharmacokinetics
The study of the movement of drug molecules
what its target tissues will be
how long will it take to get to target tissues
how much of it will be absorbed during a given period of time
how long it will stay at target tissue
how fast a dosage form will disintegrate and dissolve
when and how it will be excreted
Drug-classes of molecules
Absorption
molecule has to move through several layers of cells, each of which has a membranes that is mostly lipid
Transport across lipid membrane
Excretion
Influenced by
ionization
pH of urine has a tremendous effect on how much of the drug gets reabsorbed
pH is enormously effected by diet
high carbohydrate, lots of acidifying agents (oranges, vit C, etc.) will have acidic urine
antibiotics (acidic) do not ionize in acidic solutions; only non-ionized drugs cross membranes passively
patients with highly acidic urine are going to re-absorb most of the drug that has filtered through their glomeruli.
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high-protein diet, alkalizing agents, urine will be alkaline
supply of non-ionized drug will be depleted as most of the drug ionizes and becomes unavailable for re-absorption
Lipophilicity
protein binding
Urine
Most antibiotics are excreted in the urine
taken out of the blood by glomerular filtration
drug flows along through the capillaries and encounters the glomerulus
glomerulus: semi-permeable, lets small molecules pass through the holes, and leaves the large ones (ex- protein bound antibiotics) in the blood
Reabsorbed
Excreted
Active transport
lipid membrane of the descending loop, active transporters pump impurities out of the blood into the urine
ex- penicillin
for some antibiotics, more drug is excreted by active transport into the urine than glomerular filtration
can use a competitor for the active transporter to excrete less, ex - probenecid competes for the active transport system in the membrane, so penicillin will not be transported actively into urine and only be excreted through glomerular filtration
Feces
pH of lower bowel might get up to 8 or 9
at that pH essentially all the drug inside the gut is ionized
Donnan effect
Disintegration & Dissolution
most drugs disperse to tiny fragments in the stomach but don't actually dissolve until they reach the duodenum
in duodenum: one particle dissolves into a few million molecules
Passive diffusion
Dissolves into the membrane and moves through it to the other side
Actively transported across the membrane by enzymes
Rate of transport affected by:
Affinity for proteins
molecule bound to protein is temporarily tied up and not available for transport
Donnan effect
- reduce the number of molecules on one side of a membrane and keep the rate steady, pulling molecules over until steady state is reached again.
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Tendency to Ionize
Ionized molecules cannot cross lips membranes
Ionization is reversible
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Lipophilicity
chemically repelled by fat molecules cannot get into a lipid membrane
too highly attracted to fat will never get out of a lipid membrane
accumulate in lipids
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partition coefficient (KD)
-the ratio of drug concentration in water.
KD is the rate of molecular movement into lipid divided by the rate of movement into water
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Kinetics
: the study of rates
KA = rate of absorption
KE = rate of excretion
A rate is a % per unit time
never absorbed
Side effects
of incompletely absorbed antibiotics
candidiasis
superinfection
Other squall of the destruction of normal gut flora
Diarrhea
Distribution
Capillaries
venules
portal vein
liver
vena cava
R side heart
lung
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collects in bile ducts
gallbladder: biliary cycle
discharged back into duodenum
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[drug] in gallbladder may be high (50ug/mg)
only ~ 200ml of bile is produced each day
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Hilary Parsons D20300