Pharmacokinetics

Question 1

What is ion trapping?

Answer 1

When a weak acid enters the blood (pH 7.4) will become ionised and unable to cross cell membranes.
Due to dynamic equilibrium, a small amount of the unionised form will be generated and can cross so it will be slow release.

Question 2

What factors influence drug distribution?

Answer 2

Regional blood flow (this will be different depending on exercise)
Extracellular binding
Capillary permeability (ionised forms can pass in the aqueous pores)
Localisation in tissues (in fatty tissue)

Question 3

What are the two major routes of drug excretion?

Answer 3

Kidneys: polar molecules are excreted here mostly
Liver: Concentrates drugs using bile as it makes them polar and traps them so they are excreted by the GI tract (mostly large molecular weight conjugates)

Question 4

Why is active secretion necessary in the kidney?

Answer 4

80% of drugs are in the unionised form, acids and bases are actively secreted in the proximal tubule.
Lipid soluble drugs are reabsorbed into the blood in the proximal and distal tubules.

Question 5

What is meant by enterohepatic circulation?

Answer 5

Leads to drug persistence; the drug is excreted into the gut and then reabsorbed and returned to the liver and repeats

Question 6

Difference between first and zero order kinetics?

Answer 6

Zero order is a constant amount per unit time, it is saturable as it involves and enzymes and does not depend on the conc. of the drug.

Question 7

Which oxidation reactions are not carried out by cytochrome P450 enzymes?

Answer 7

N oxidation

Alcohol oxidation

Question 8

What is the most common phase II reaction?

Answer 8

Conjugation with glucoronidation