Lecture 32

Question 1

What are three important pharmacokinetic parameters?

Answer 1

1. volume of distribution
2. clearance
3. bioavailability

Question 2

What is volume of distribution and how is it measured?

Answer 2

Relates the amount of drug in the body to the concentration of drug in the plasma

Vd = amount of drug in body over plasma drug concentration

Question 3

What does Vd (volume of distribution) represent?

Answer 3

Vd is the volume that would be required to contain all of the drug in the body at the same concentration as it is in the blood.

Question 4

What does a low Vd mean?

Answer 4

high in the vascular and low in the extravascular

Question 5

what does a high Vd mean?

Answer 5

low in the vascular and high in the extravascular

Question 6

What is the main use of the volume of distribution?

Answer 6

The main use of the volume of distribution is to determine the loading dose to quickly achieve a target plasma concentration.

Question 7

How to determine Vd?

Answer 7

Vd = dose over C0

C0 = concentration at time zero

Question 8

What are the two phases of a Vd graph?

Answer 8

The rapid fall phase = distribution

The slower phase = elimination

Question 9

What is clearance defined as?

Answer 9

defined as the volume of blood cleared of drug per unit time

clearance predicts the rate of elimination of a drug in relation to the drug concentration

Question 10

What are the two major sites of drug elimination

Answer 10

kidneys and the liver

Question 11

What is first-order elimination?

Answer 11

For a drug eliminated with first-order kinetics, CL is a constant, ie, the ratio of the rate of elimination to the plasma concentration is the same regardless of plasma concentration

Question 12

why do most drugs follow first- order elimination?

Answer 12

This occurs because the physiological mechanisms of drug elimination (enzymes and transporters) are not
saturated

the plasma concentration is below the Km

Question 13

What is half-life?

Answer 13

Half-life determines the rate at which blood concentration rises during a constant infusion and falls after infusion is stopped

at steady state

Question 14

During a constant IV infusion of a drug (half-life)

Answer 14

50% of Css is reached after 1 half-life
• 75% after 2 half-lives
• 87.5% after 3 half-lives
• 93.75% after 4 half-lives

vise versa for elimination

Question 15

how to change the Css?

Answer 15

2x the infusion rate

Question 16

What factors impact half-life

Answer 16

obesity = increases

pathologic fluid = increases

aging = increased

cardiac, liver, renal failure = increase

CYP induction = decreases

CYP inhibition = increases

Question 17

Why do some drugs show saturation kinetics?

Answer 17

Drug metabolism and tubular secretion are
saturable processes.

When drug concentration exceeds Km, nonlinear
kinetics is observed

Question 18

Which drugs show saturation kinetics?

Answer 18

aspirin at high doses

ethanol

phenytoin

Question 19

properties of saturation kinetics

Answer 19

elimination is zero order

a constant amount of drug is eliminated per unit time

The rate of elimination is maximal and independent of drug concentration

Question 20

What does a saturation kinetics graph look like?

Answer 20

linear

Question 21

What is not applicable with saturation kinetics?

Answer 21

the concept of “4 half-lives to steady state” is NOT applicable for drugs with nonlinear elimination kinetics