PK 3

Question 1

What is the benefit of PBPK?

Answer 1

Very useful for predicting PK across species

Question 2

Using PBPK how can Vd be predicted?

Answer 2

By a knowledge of drug plasma protein and tissue binding

Question 3

Using PBPK how can CL be predicted?

Answer 3

Using allometry or in vitro enzymes kinetics

Question 4

What is Kleibers law?

Answer 4

Mass specific metabolic rate needed to sustain an organism decreases with body weight to the 1/4 power
i.e. Mass specific metabolic rate = constant x BW^0.25

Question 5

What impact does allometry have on clearance?

Answer 5

• The smaller the animal the greater the drug clearance
• Larger an animal is the lower the blood flow and as plasma clearance is dependent on blood flow clearance is also less in larger animals.
• BUT application of this theory less reliable on drug metabolism clearance due to enzymes- different species have different enzymes

Question 6

What is intrinsic clearance?

Answer 6

The enzyme mediated clearance that would occur without physiological limitations

Question 7

How do you calculate CLintrinsic?

Answer 7

Vmax/ Km

Question 8

What limits the clearance of high clearance drugs?

Answer 8

Blood flow

Question 9

How do you calculate the clearance for low clearance drugs?

Answer 9

fraction unbound x intrinsic clearance of the liver

Question 10

What are the reasons for variation in PPB?

Answer 10

• Elderly/ neonates have less proteins
• Disease states can affeect concentration
• Drugs may be displaced from plasma proteins by other drugs that have a higher affinity

Question 11

What will happen to CL and Vd if PPB increases?

Answer 11

They will decrease

Question 12

What is free drug concentration?

Answer 12

It is the effective concentration->it drives the effects. So if PPB decreased causing an increase in CL and Vd then free drug conc would stay the same.

Question 13

From a clinical point of view is a change in PPB an issue?

Answer 13

Normally no
3 exceptions:
- Rapid IV bolus
- Parenteral administration of displaced drug with high organ extraction ratio
- Therapeutic drug monitoring and drug displacement from plasma binding site

Question 14

What is the problem with rapid IV bolus?

Answer 14

• The rapid admin of the displacement agent could cause free conc to increase due to rapid displacement of displaced drug before redistribution occured.
• It will equilibriate and then not an issue
• e.g. sulfamide + bilirubin= Kernicterus

Question 15

Explain therapeutic drug monitoring and drug binding displacement from plasma binding

Answer 15

- e.g. 
Phenytoin alone(Ctotal=20)
Phenytoin + Valproic acid (Ctotal= 15)
- No dosage adjustment needed though because although Ctot decreases, fu Increases so C free doesn't change

Question 16

Describe the advantages controlled release drug therapy

Answer 16

• Advantages:
  Similar kinetics to IV infusion (0 order)
  Reduced risk of breaching therapeutic window
  Drug delivery period can be over several months from 1 administration

Question 17

What is the depot effect?

Answer 17

• Drug diffusion/ release= K x (drug conc at depot surface)
• K = the drug release constant
• Works like a bar of soap, outside film dissolves at a constant rate so specific amount of drug administered and the rest is sequestered inside.

Question 18

What is the Paratect bolus morantel system>

Answer 18

• Delivers morantel tartare for up to 90 days

- Controled release occurs through discs at the ends and the rest is sequestered inside.

Question 19

How do spot on anthelmintics work as controlled release drugs?

Answer 19

-They are oil based as so the drug is sequestered inside sebaceous glands and released with sebum at a constant rate

Question 20

How does the osmotic ruminal therapeutic system work?

Answer 20

Contact with water swells capsule via osmosis which pushes flow moderator releasing controlled, constant amount of drug