Pharmacokinetics I & II

Question 1

Pharmacokinetics is

Answer 1

What the body does to a drug

Question 2

What four pharmacokinetic properties determine the speed of onset of a drug?

Answer 2

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

Question 3

Effective dose (ED) is

Answer 3

quantity of a drug that will produce the pharmacological effects

Question 4

Lethal dose is

Answer 4

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

Question 5

Fatal dose is

Answer 5

the dose that kills the 100% subject in whom it is given

Question 6

Median effective dose (ED50) is

Answer 6

a dose that produces the desired effect in 50 per cent of a population

Question 7

Median lethal dose (LD50) is

Answer 7

the quantity of an agent that will kill 50 per cent of the test subjects

Question 8

Median toxic dose (TD50) is

Answer 8

dose that produces a toxic effect in 50 per cent of the population

Question 9

what is therapeutic index

Answer 9

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 &raquo_space;> 1 (indicates the drug is safe)

The greater the index, the wider the safety margin for the drug

Question 10

Drug with different therapeutic index

Answer 10

High therapeutic index

• Benzodiazepines
• Certain antibiotics
• Paracetamol

Low therapeutic index

• Digoxin
• Theophylline
• Gentamicin
• Lithium carbonate

Question 11

Half-life is

Answer 11

time taken for its plasma concentration to be reduced to half of its original value

(t½ = 0.693 × Vd /CL)

Question 12

Clearance is

Answer 12

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)

Question 13

If drug follows the first-order kinetics, clearance and half-life of the drug are

Answer 13

constant

Question 14

Loading dose is

Answer 14

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)

Question 15

Steady-state (SS) is

Answer 15

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)

Question 16

Maintenance Dose (MD) is

Answer 16

repeated doses given at specific intervals after the attainment of target Css (concentration at steady state)

Maintenance dose = CL x CpSS

Question 17

Calculation for dose based on age

Answer 17

• Young’s rule
• Cowling’s rule
• Fried’s rule (infants)

Question 18

Young’s rule formula

Answer 18

dose of a child = age/ (age + 12) x adult dose

Question 19

Cowling’s rule formula

Answer 19

age at next birthday (years) x adult dose / 24

Question 20

Fried’s rule (infants) formula

Answer 20

dose for infant = age (months) x adult dose / 150

Question 21

Dose based on weight (most commonly used)

-Clark’s rule (child)

Answer 21

Clark’s rule formula

Dose for child = Weight (Ib) x Adult Dose / 150 (average weight of adult in Ib)

Question 22

Oral availability (F) is

Answer 22

the fraction of drug that reaches the systemic circulation after oral ingestion versus IV administration

F = AUC (oral) / AUC (iv)

Question 23

Oral availability determined by

Answer 23

absorption and first pass metabolism

Question 24

Absorption refers to

Answer 24

the ability of a drug to cross the gut wall into the portal vein

Question 25

First-pass metabolism describes

Answer 25

presystemic drug elimination and can occur in the gut wall, portal vein or liver. The liver is usually the most important contributor

Question 26

High first pass metabolism, ____ oral availability

Answer 26

low

Question 27

The total amount of drug in the systemic circulation is defined by

Answer 27

area under the concentration-time curve (AUC)

Question 28

When a medication is administered intravenously, its bioavailability is

Answer 28

100%

Question 29

Oral bioavailability cannot be 100% because

Answer 29

oral drugs have slower absorption and distribution than IV drugs and subjected to first pass hepatic metabolism

Question 30

Volume of distribution (Vd) determines

Answer 30

the distribution of drugs between blood and rest of the body

Question 31

Equation for volume of distribution

Answer 31

Vd = Ab / Cp

Question 32

Drugs with a small Vd have a ___ plasma concentration

Answer 32

high.

Thus,  drug with small Vd mainly stay in 
central compartment (bloodstream)

eg drugs: phenytoin, warfarin

Question 33

Drug with large Vd (high lipid soluble) have a ___ plasma concentration

Answer 33

low.

Thus, drug with large Vd will travel to ECF

eg drugs: propranolol, digoxin