Pharmacokinetics I & II Flashcards
(33 cards)
Pharmacokinetics is
What the body does to a drug
What four pharmacokinetic properties determine the speed of onset of a drug?
Absorption - drug absorption from the site of administration
Distribution - drug leave blood stream and enter ECF and ICF
Metabolism - biotransformation by liver or other tissues
Elimination - by urine, bile or feces
Effective dose (ED) is
quantity of a drug that will produce the pharmacological effects
Lethal dose is
quantity of an agent that will or may be sufficient to cause death in at least 1% of the animals or subjects given the drug
Fatal dose is
the dose that kills the 100% subject in whom it is given
Median effective dose (ED50) is
a dose that produces the desired effect in 50 per cent of a population
Median lethal dose (LD50) is
the quantity of an agent that will kill 50 per cent of the test subjects
Median toxic dose (TD50) is
dose that produces a toxic effect in 50 per cent of the population
what is therapeutic index
ratio of median lethal/toxic dose of a drug for 50% of the population (TD-50) divided by the effective dose for 50% of the population (ED50)
TI = TD 50/ ED 50 »_space;> 1 (indicates the drug is safe)
The greater the index, the wider the safety margin for the drug
Drug with different therapeutic index
High therapeutic index
- Benzodiazepines
- Certain antibiotics
- Paracetamol
Low therapeutic index
- Digoxin
- Theophylline
- Gentamicin
- Lithium carbonate
Half-life is
time taken for its plasma concentration to be reduced to half of its original value
(t½ = 0.693 × Vd /CL)
Clearance is
the rate at which a drug is removed from plasma(mg/min) divided by the concentration of that drug in the plasma (mg/mL)
Clearance = elimination rate (mg/min) / plasma drug concentration (mg/mL)
If drug follows the first-order kinetics, clearance and half-life of the drug are
constant
Loading dose is
a single or few quickly repeated doses given in the beginning of treatment to attain the great target concentration rapidly
loading dose = Cp (target) x Vd (times body weight)
Steady-state (SS) is
rate of drug administration equals the rate of elimination and plasma concentration
- Rate in = rate out = plasma concentration
- reached in 4-5 half-lives (linear kinetics)
Maintenance Dose (MD) is
repeated doses given at specific intervals after the attainment of target Css (concentration at steady state)
Maintenance dose = CL x CpSS
Calculation for dose based on age
- Young’s rule
- Cowling’s rule
- Fried’s rule (infants)
Young’s rule formula
dose of a child = age/ (age + 12) x adult dose
Cowling’s rule formula
age at next birthday (years) x adult dose / 24
Fried’s rule (infants) formula
dose for infant = age (months) x adult dose / 150
Dose based on weight (most commonly used)
-Clark’s rule (child)
Clark’s rule formula
Dose for child = Weight (Ib) x Adult Dose / 150 (average weight of adult in Ib)
Oral availability (F) is
the fraction of drug that reaches the systemic circulation after oral ingestion versus IV administration
F = AUC (oral) / AUC (iv)
Oral availability determined by
absorption and first pass metabolism
Absorption refers to
the ability of a drug to cross the gut wall into the portal vein
