Pharmacokinetics Flashcards
What is pharmacology?
Pharmacology is the study of drug action.
What is pharmacodynamics?
Pharmacodynamics is the study of what the drug does to the body.
What is pharmacokinetics?
Pharmacokinetics is the study of what the body does to the drug.
What is the half-life of a drug in pharmacokinetics?
t1/2
The half-life of the drug is the time it takes to remove half of the current concentration of drug from the body.
What is the MEC?
The minimum effective concentration - minimum conc. needed for therapeutic effect.
What is the MTC?
The minimum toxic concentration - anything above this level causes side effects and toxicity.
Drugs can have a narrow therapeutic window. Therapeutic drug monitoring is needed in this case. Describe what narrow therapeutic window means.
A drug with a narrow therapeutic window has a very small range between the concentration at which the drug exerts a therapeutic effect and the concentration at which toxicity may occur.
What is meant by the term plasma concentration?
The concentration of the drug in the blood.
Two or more drugs can interact and cause potentiation. What does that mean?
Potentiation means that the interaction can cause one drug to increase the effect or concentration of the other drug giving rise to toxicity and side effects.
Two or more drugs can interact and cause antagonism. What does that mean?
Antagonism means that the interaction can cause one drug to lower the effect or concentration of the other causing a sub-therapeutic effect (below MEC).
In general, how does a pharmacokinetic drug interaction occur?
These occur when one drug alters the absorption, distribution, excretion or metabolism of another drug which may result in an increase or decrease in the amount of the drug.
Describe first-order elimination.
This is when a constant fraction of the drug is eliminated per unit time.
(Linear kinetics)
CONSTANT FRACTION, PERCENTAGE, PROPORTION ELIMINATED PER UNIT TIME = ALWAYS FIRST ORDER.
Describe zero-order elimination.
This is when a constant amount of the drug is eliminated per unit time.
(Non-linear kinetics)
AMOUNT not proportion.
What does enteral route of administration mean and give examples?
Enteral routes of administration include those where the drug is absorbed from the GI tract.
Oral, rectal, buccal and sublingual.
What does parenteral route of administration mean and give examples?
Parenteral routes of administration is interpreted as injecting directly into the body.
Sub-cutaneous, intra-muscular and intravenous.
Why may bioavailability of drug be less than 100% when administering via parenteral route?
Because enzymes on the way to the bloodstream can breakdown the drug.
List the advantages of oral administration.
Most convenient route of access for systemic circulation.
Cheap and safe.
Preferred route by most patients.
Large surface area in the GI tract for absorption.
List the disadvantages of oral administration.
Can have a delayed/variable absorption (variable - each person has different absorption levels) First-pass effect may be significant. Can cause GI upset causing nausea etc. Not suitable for all patients. Unsuitable for acid-labile drugs.
Where are drugs mainly metabolised?
Liver
Why is an initial higher dose given at the start of a treatment course?
Given to achieve adequate therapeutic levels rapidly before reducing the dose to a maintenance dose.
Describe sublingual administration.
The drug is administered under the tongue and diffuses across the mucous membranes, into the systemic circulation.
List the advantages of sublingual administration.
Has a rapid onset of action.
Bypasses first pass effect as there is a rich blood supply underneath the tongue.
Avoids gastric acid - good for acid-labile drugs.
List the disadvantages of sublingual administration.
Inconvenient for long-term use.
Limited to certain types of drugs.
How is the drug given in buccal administration?
The drug is placed between the gum and cheek.
Buccal tablets are often harder than sublingual tablets.
These are designed to dissolve slowly.
Takes longer to disintegrate.
List the advantages of buccal administration.
Has a rapid onset of action.
Bypasses first pass effect as drug gets absorbed through oral mucosa and enters directly into the bloodstream.
Avoids gastric acid - good for acid-labile drugs.
List the disadvantages of buccal administration.
Inconvenient for long-term use.
Limited to certain types of drugs.
Prochlorperazine is a drug used in buccal administration and sold under the name Buccastem. What is it used for and what is the benefit of it being a buccal drug?
Used to treat nausea and vomiting associated with migraines. Since the tablet isn’t swallowed, it avoids the drug being brought up if vomiting occurs.
List the advantages of rectal administration.
PARTIALLY avoids first-pass metabolism.
Avoids destruction by gastric acid and digestive enzymes.
Useful for patients unable to take drugs orally - if unconscious or vomiting.
List the disadvantages of rectal administration.
May irritate the rectal mucosa.
Issues with patient acceptability.
Rectal drugs can be used for local and systemic effects. Give two examples.
Local effect - for haemorrhoids
Systemic effect - relieve pain - paracetamol suppository
Explain why the rectal route of administration has a reduced first pass effect?
The rectal route partially bypasses first pass metabolism because two-thirds of the veins draining the rectum lead directly into the systemic circulation whereas one-third would lead to the hepatic portal system.
Give three examples of rectal formulations.
Suppositories, enema, rectal foam.
Why does intravenous administration give 100% bioavailability?
Drug is directly injected into venous blood via the peripheral vein so no absorption stage required for a systemic effect.
Intramuscular and subcutaneous require an absorption stage.
List the advantages of IV administration.
NB: IV must be administered by healthcare professionals
No first pass effect so 100% bioavailability.
Avoids digestion by enzymes and gastric acid.
Easy to titrate the dose - can control how many millilitres of drug you are going to give.
List the disadvantages of IV administration.
Cost.
Patient acceptability - phobia of needles.
Risk of sepsis and embolism.
Subcutaneous
This is a parenteral route of administration.
Its injection under the dermis into the subcutis/fatty tissue.
Commonly for insulin - patients are encouraged to rotate the site they use for injections and using same site can cause irritation.
Patient can be trained by healthcare professional to inject themselves.
List the advantages of subcutaneous administration.
No first pass effect.
Avoids digestion by enzymes and gastric acid.
Easy to titrate the dose - can control how many millilitres of drug you are going to give.
List the disadvantages of subcutaneous administration.
Pain at site of injection.
Patient acceptability - phobia of needles.
List the advantages of intramuscular administration.
NB: There is no risk of embolism and sepsis as injection is not directly into a vein.
No first pass effect.
Avoids digestion by enzymes and gastric acid.
Easy to titrate the dose - can control how many millilitres of drug you are going to give.
List the disadvantages of intramuscular administration.
Pain at site of injection.
Patient acceptability - phobia of needles.
Intramuscular
Injection into the muscle.
Easier to administer than IV.
Typical route for vaccines, adrenaline, morphine and diazepam.
Site of injection will influence absorption.
Respiratory
Involves the delivery of drugs locally to the lungs and anesthetic gases (affects CNS).
Can cause local or systemic effects.
List the advantages of respiratory route of administration.
No first-pass effect.
Rapid absorption.
Lungs provide a large area for absorption.
List the disadvantages of respiratory route of administration.
Some inhalation devices are expensive.
Inhaler must be used correctly for effect = inhaler technique.
3 types of respiratory inhalers: inhaler, nebuliser, anesthetic gases. Describe these.
Inhaler - local effect in the lungs, causes bronchodilation, allows airways to open up allowing patient to breathe more easily.
Nebuliser - local effect in lungs
Anesthesia - given as gas, given via respiratory route but moves to CNS for systemic effect.