Pharmacokinetics 2 Flashcards
What are 3 things pharmacokinetic models allow?
- Summarization
- Comparison
- Prediction
Name 2 pharmacokinetic models
- One-compartment
2. Two-compartment
What is the key point of 1st order kinetic drugs?
Half-life of the drug is constant
What 2 assumptions are made in a one-compartment model?
Once an injection or absorption has occurred:
- Drug is distributed instantaneously throughout all tissues into which it will distribute
- [Drug} in distributed tissues are not identical but rates of changes are same during elimination
Draw the graph of one-compartment mode [plasma] v time and mark the point of equilibrium
See Pharmacokinetics 2 Lecture - Slide 9
Why is the oral drug concentration line always lower than i.e. line?
First pass effect
What is the difference between one and two compartment models with respect to distribution?
Two-compartment model accounts for non-instant distribution
Draw the graph of two-compartment mode [plasma] v time and mark the point of equilibrium
See Pharmacokinetics 2 Lecture - Slide 10
Give the main characteristic of the graph of two-compartment mode [plasma] v time
- Rapid decline seen after absorption or I.V. injection as distribution rate quicker than elimination
- Once equilibrium reached rate of decline of drug reduces as rate of elimination is identical in all tissues
What is the abbreviation for half-life?
T½
What is the relationship between half life and clearance?
Inversely proportional
When is a drug essentially eliminated?
After 5 t½s have passed
Name 3 things which affect half life
- Physiological or pathological states
- Altered by smoking
- Subject to genetic variation
Name 4 physiological or pathological states that can alter half life
- Age
- Cardiovascular disease
- Renal disease
- Hepatic disease
How does smoking generally affect the half-life of a drug?
Usually decreases the half life of a drug
With regards to elimination rate, what is the difference between first order and zero order kinetics?
In first order kinetics, elimination rate always changes but it is constant in zero order kinetics
Why can zero order processes occur?
Elimination processes become saturated in therapeutic concentration range
What is the major example of a drug whose elimination pathways become saturated at low concentrations?
Ethanol (Alcohol)
What 2 factors influence the amount of drug infused in an I.V. infusion?
- Flow rate
2. [Drug in Solution]
What is a steady state level?
When the rate of elimination is equal to the rate of infusion
How is bioavailability calculated from a [plasma] v time graph?
- Calculate area under I.V. graph
- Calculate area under oral graph
- BA = Oral area / I.V. area
What does the area under a line in a [plasma] v time graph represent?
Volume of drug which is absorbed
What is the main aim of dosage determination?
Achieve a concentration of drug at its site of action in order to produce an effective therapeutic effect
What is a loading dose of a drug?
A rapid achievement of target concentration is required (usually administered by I.V. infusion)
How is the target concentration of a drug calculated?
Vd = Dose / TC
What is a maintenance dose of a drug?
The dose calculated to achieve a steady state level replacing the drug eliminated
What information is necessary to work out the loading dose in a human?
Weight of the person receiving the dose
How is a loading dose calculated?
Dose = Vd x TC
How is Vd calculated if a patient’s weight is known?
Vd = Weight x Drug distribution
How is the dosing rate calculated?
Dosing Rate = Clearance x TC
How is the maintenance dose calculated?
Maintenance dose = (Dosing Rate x Dosing interval) / Bioavailability
What must be taken into account if switching from I.V. and oral administration of a drug?
Bioavailability will be different so dosage must be changed
Why may vomiting affect the effects of a drug
Vomiting can lead to dehydration
