Pharmacodynamics

Question 1

What is pharmacodynamics?

Answer 1

Pharmacodynamics is the study of drug concentration to pharmacological effects. I.e. what the drug does to the body.

Question 2

Explain what is meant by first order kinetics.3

Answer 2

1. First order kinetics is when a constant fraction of drug is eliminated per unit time.
2. The rate of elimination is proportional the amount of drug in the body.
3. The majority of drugs are eliminated this way.

Question 3

Explain the term half-life 3

Answer 3

1. Half-life (T1/2) is the time taken for plasma concentration of the drug to reduce by half.
2. After 4 half live, elimination is 94% complete.
3. Half life is constant and is independent of drug concentration.

Question 4

Explain the term Kel.

Answer 4

Kel is the fraction of the drug in the body eliminated per unit time. When represented on a graph it is the slope of the log of plasma concentration versus time. Just like T1/2, Kel is constant and independent in relation to drug concentration.

Question 5

What is zero order kinetics?

Answer 5

Zero order kinetics is when a constant amount of drug is eliminated per unit time. It is caused by the saturation or overload of the receptors by a drug. Examples include: alcohol, salicylic acid and phenytoin.

Question 6

Compare first order elimination versus zero order elimination. 10

Answer 6

First order elimination: 1. Drug decreases exponentially with time (constant fraction eliminated) 2. Rate of elimination is proportional to drug concentration 3. Plot of log drug/ ln drug vs time are linear 4. Half-life is constant regardless of drug concentration 5. Concentration vs time graph is not linear, decaying exponentially. Log concentration vs time is linear. Zero order elimination 1. Drug decreases linearly with time (constant amount eliminated) 2. Rate of elimination is constant 3. Rate of elimination is independent of drug concentration 4. No half-life 5. Concentration vs time graph is linear

Question 7

Explain what is meant by volume of distribution (Vd).

Answer 7

Volume of distribution (Vd) id the theoretical value that represent of volume the compartment a given drug would have to be uniformly distributed in order to give the observed plasma/blood if no elimination occurred. Vd= amount of drug in the body (D) divided by the concentration in the blood (C).

Question 8

What 3 factors affect Vd?

Answer 8

1. Lipid solubility 2. Plasma and tissue protein binding 3. Perfusion of tissues

Question 9

Explain what a lower value of Vd means

Answer 9

A decreased Vd would mean a lower lipid solubility with decreased tissue binding and increased plasma protein binding.

Question 10

Explain what a higher value of Vd means.

Answer 10

A higher value for Vd would mean increased lipid solubility with increased tissue binding and decreased plasma protein binding.

Question 11

What is clearance?

Answer 11

Clearance is the volume of blood in a defined region of the body that is cleared of a drug in a unit time. Total clearance is the sum of individual clearance

Question 12

Why is clearance more useful than t1/2?

Answer 12

Clearance is a more useful concept in reality than t1/2 as it takes into account blood flow rate. Furthermore, clearance varies with body weight and the degree of protein binding. NB clearance of a drug is independent of the dose and route of administration.

Question 13

What is meant by steady state?

Answer 13

At steady state the amount of drug administered is equal to the amount of drug eliminated between doses. At this state, optimum effect is observed.

Question 14

What does the term action mean in terms of drug action?

Answer 14

How and where the effect is produced is called the action.

Question 15

Where can drug actions occur?

Answer 15

1. Extra cellular 2. Cellular 3. Intracellular

Question 16

What does the term drug effect mean?

Answer 16

The type of response produced by the drug is termed the effect.

Question 17

What is a receptor?

Answer 17

A receptor is a molecule, parts of molecules or molecular complexes within an organism.

Question 18

What is intrinsic activity (alpha)?

Answer 18

Intrinsic activity (alpha) is the ability to give rise to a pharmacological effect

Question 19

What is affinity (pD2)?

Answer 19

Affinity pD2 is the ability yo bind to a receptor

Question 20

What is an agonist?

Answer 20

An agonist is a drug that possesses both affinity and intrinsic activity. I.e. it will bind and cause a pharmacological effect.

Question 21

What is an antagonist?

Answer 21

An antagonist is a drug that possesses affinity but not intrinsic activity. It will bind a receptor but cause no effect.

Question 22

What is the basis for drug action?

Answer 22

1. For a drug to work it must bind a receptor 2. The receptor can be a molecule within the cell or on the cellular membrane 3. The relationship between a drug and receptor is much like lock and key; very specific fit and any chemical change in drug or receptor can change. The interaction