Lecture 5: Pharmacokinetics Part 3

Question 1

The elimination of most drugs under normal dose ranges is a

Answer 1

First order rate process

Question 2

First order rate process

Answer 2

The rate at which a drug is excreted from the body is directly dependent on the amount of drug in the body.

Question 3

Single Compartment Model

Answer 3

-Consider the body to be one unit which a drug distributes through instantaneously and evenly when injected via an IV bolus. With this model, we don’t have to model absorption or distribution. We only model elimination of the drug over time.

Question 4

Kel

Answer 4

The elimination rate constant; an inherent property of the drug.

Question 5

Describe the first order rate process in relation to drugs

Answer 5

When the drug concentration in the body is high, the rate of elimination is high. As the drug concentration decreases, the rate of elimination also decreases.

Question 6

Equation used to measure initial drug concentration

Answer 6

Co=(q/Vd)

• Co: Initial drug concentration
• Q: Dosage
• Vd: Volume of distribution

Question 7

How do we calculate the concentration of a drug anywhere along a concentration x time graph?

Answer 7

By using the equation:
C=(Co)(e^kel*t)

C= plasma concentration
Co= initial concentration
Q= dose
Vd= volume of distribution
kel

Question 8

Clearance

Answer 8

The volume of plasma from which a drug is removed per unit time. It reflects elimination of drug through metabolism and various routes of excretion

Question 9

Drug half life is (directly/inversely) proportional to its clearance

Answer 9

Inversely. The more quickly a drug is cleared from the body, the shorter its half life. As Cl increases, t1/2 decreases.

Question 10

The half life of drugs with big volumes of distribution (Vd):

Answer 10

Is very long, because they are eliminated at very low rates.

Question 11

When a drug is administered via continuous IV infusion, the plasma concentration quickly rises until it reaches a plateau where:

Answer 11

The rate of administration= rate of elimination.

Thereafter, the drug achieves steady state= Css

Question 12

Multiple dosing to achieve steady state

Answer 12

If someone takes an orally administered drug, concentrations will quickly rise, hit a peak, and then the drug will start to be eliminated. If we give another dosage before the first one being completely eliminated, we can build upon the first dosage. We can keep doing that until we get oscillations between peaks (Cmax) and throughs (Cmin) in the plasma concentration which are around the mean steady state concentration. The [drug] stays within the therapeutic window.

Question 13

With continuous IV infusion or repeated doses, Css is usually reached after:

Answer 13

3 half lives of the drug, regardless of what the drug is. This is a property of a drug that follows first order kinetics.

Question 14

How do we achieve a therapeutic benefit upon first dosage with multiple dosing?

Answer 14

By giving a large initial dose (loading dose). This pushes the concentration up into the therapeutic window, and once it is there you can keep it there with smaller maintenance doses.

Question 15

What happens if you extending the dosing interval? (the patient takes every second dosage or the amount of drug taken at each interval is decreased)

Answer 15

Reducing the rate of administration can lead to a lower than intended steady state plasma concentration (Css) and absence of a therapeutic benefit

Question 16

Increasing the rate of administration could lead to:

Answer 16

Unacceptable toxicity

Question 17

The target concentrations strategy

Answer 17

• Based upon the fundamental linkage of pharmacokinetics and pharmacodynamics.
• > Assumes that for most drugs the magnitude of therapeutic effect and risk for adverse effects in an individual is predictable for a given target concentration (TC) in the blood.

BUT, the dose required to achieve TC will vary between individuals

Question 18

The major variable for achieving a target concentration shows that:

Answer 18

Pharmacokinetics is the major variable, not pharmacodynamics