Thu7October1249PM 9

Drug A has a volume of distribution of 1 L/kg and an elimination rate constant (k) of 0.1.

Drug B has a volume of distribution of 2 L/kg and an elimination rate constant (k) of 0.2.

Which one of the following statements best describes the single compartment pharmacokinetics of drug A?

(Please select 1 option)

Drug A has a higher clearance than drug B

Drug A has a lower clearance than drug B This is the correct answer

Drug A has the same clearance as drug B Incorrect answer selected

Drug A has a shorter half life than drug B

Drug A has the same half life than drug B

Explanation

For a single compartment pharmacokinetic model, the fall in plasma concentration of a drug with time decreases exponentially (wash-out curve).

If the logarithm (ln) of plasma concentration of a drug is plotted against time a straight line is produced because a constant proportion of the drug is removed from the plasma per unit time. The gradient or slope of the line can be represented mathematically as k (the rate constant). The gradient can be used to predict the plasma concentration of a drug at any time and is related therefore to the half life (T1/2).

Mathmatically, clearance (CL), Volume of distribution (Vd) and the eleimination rate constant (k) are mathematically related given the the formula:

CL = Vd x k

For drug A, CL = 1 x 0.1 = 0.1units per minute

For drug B, Cl = 2 x 0.2 = 0.4 units per minute

Drug A has a lower clearance than drug B

Alternative calculation:

k = ln2/ T1/2 = 0.693/ T1/2

T1/2 for drug A = 0.693/0.1 = 6.93

T1/2 for drug B = 0.693/0.2 = 3.46

To find out the clearance (CL) of a drug, the T1/2, volume of distribution (Vd) and CL are related mathematically as follows:

T1/2 = ln2 Vd/CL; therefore T1/2 = 0.693 Vd/CL

CL= 0.693 Vd/ T1/2

CL for drug A = 0.693.1/6.93 = 0.1 units/minute

CL for drug B = 0.693.2/3.46 = 0.4 units/minute

More units of drug B are cleared per unit time than drug A.

Answer Statistics

1

8%

2

32%

3

39%

4

10%

5

13%

Times answered: 263