Pharmacokinetics Flashcards
Give some examples of site of administration of drugs.
Focal (eye, skin, inhalation)
Systemic - enteral (sublingual, oral, rectal)
- parenteral (subcutaneous, intramuscular, IV)
Why is the site of administration important when prescribing drugs?
Ease, practicality, localisation
Allows less systemic absorption and less off-target effects, as well as increasing the liklihood of proper use by the patient.
What is oral bioavailability
The proportion of a dose which reaches the systemic circulation in an unchanged form.
What does the amount of drug available after oral administration depend on?
Gut absorption, altered by food/disease.
First pass metabolism
What does the rate of availability of a drug after oral administration depend on?
Pharmaceutical factors and gut absorption.
How do you calculate bioavailability (%)?
AUC oral/AUC injected
AUC is the area underneath a plasma concentration curve.
What is the therapeutic ratio for a drug?
The maximum tolerated dose over the minimum effective dose.
What is first pass metabolism
The blood from the gut passes through the portal system so is delivered to the liver before systemic circulation, greatly reducing the concentration of the drug reaching the body.
How can first pass metabolism be avoided?
Parenteral route (e.g. subcutaneous/IV) Sublingual or rectal route
What is volume of distribution for a drug?
The theoretical volume into which a drug has been distributed assuming this has occurred instantaneously.
amount given/plasma concentration at 0s
The free level of a drug is what exerts an effect. When is this especially important for a drug? (i.e. a drug with specific properties)
If it is highly bound to albumin, has a small volume of distribution, has a low therapeutic index.
Describe the difference between an object drug and a precipitant drug.
Object - class I. Used at a dose much lower than the number of albumin binding sites.
Precipitant - class II. Used at a dose much greater than the available binding sites.
Give an example of an object drug and it’s corresponding precipitant drug.
Warfarin - sulphonamides, aspirin, phenytoin
Tolbutamide - sulphonamides, aspirin
Phenytoin - valproate
What is first order kinetics when referring to a drug?
When the rate of decline of plasma drug level is proportional to the level of the drug. It’s half-life can be defined.
What is zero order kinetics when referring to a drug?
When the rate of decline of a plasma drug level is constant.
Where does metabolism and excretion of drugs take place?
Liver - metabolism
Kidney - excretion
Describe phase I of liver metabolism.
Carried out by mixed function oxidases to oxidise, reduce or hydrolyse drugs.
- NADPH cytochrome P450 reductase
- cytochrome P450 in liver microsomes
They have low substrate specificity and have an affinity for lipid-soluble drugs.
The enzymes are inducible and inhabitable (competitive, non-competitive).
Give an example of an enzyme inducer in the liver and the drug that it affects.
Phenobarbitone - warfarin, phenytoin
Rifampin - oral contraceptive
Cigarettes - theophylline
Give an example of an enzyme inhibitor in the liver and the drug that it affects.
Cimetidine - warfarin, diazepam
Give a drug which interacts with warfarin and how it does so.
Alcohol - inhibits metabolism
Aspirin/sulphonamides/phenytoin - displace from plasma proteins
Broad spectrum antibiotics - decrease vitamin K synthesis by bacteria in the gut
Aspirin - reduces platelet function
Barbituates/rifampicin - induce liver metabolism enzyme
In liver disease, what drugs must you be careful administering?
Those with a low therapeutic ratio
What feature of urine will affect how much of a drug is excreted?
pH
For a weakly acidic drug, what effect will increasing alkalinity of the urine have on its excretion?
Decreases excretion because it ionises the drug, decreasing tubular absorption as the charged drug stays in the lumen of the blood vessel.
For a weakly basic drug, what effect will increasing acidity of the urine have on its excretion?
Inceases excretion
In renal disease, what effect does this have on the maintenance dose and loading dose of a drug?
Maintenance drug must be reduced
Loading dose remains unchanged unless the volume of distribution changes.
