Body Compartments And Drug Movement B

Question 1

What are the factors that affect the ability of a drug to diffuse across lipid membranes?

Answer 1

Temperature
Surface area 
Concentration gradient 
Particle size 
Medium

Question 2

What is the permeability coefficient?

Answer 2

A measure of the rate at which a molecule can cross a membrane such as a lipid bilayer.

Question 3

What are the factors that affect the permeability coefficient?

Answer 3

Lipid solubility; (partition coefficient between membrane and aqueous environment)

Diffusivity; (mobility of the molecules within the lipid, diffusion coefficient depends on particle size)

Question 4

What does the diffusion of a drugs across a membrane depend on?

Answer 4

The degree of ionisation.

Question 5

What is pKA?

Answer 5

Acid-base dissociation constant

Question 6

What does the pKA describe?

Answer 6

The ionic form a molecule will take at a range of pH values.

Question 7

What is the pKA value of a drug?

Answer 7

Is the hydrogen ion concentration at which 50% of the drug is ionised and 50% is unionised

Question 8

What is the effect of ionisation on diffusion?

Answer 8

Unionised drug is lipid soluble so CAN diffuse across the membrane.

Ionised drugs are lipid INsoluble so CANNOT diffuse across the membrane.

Question 9

What drug reaches an equal concentration in each compartment?

Answer 9

UNIionised drugs reach an equal concentration in each compartment but ionised drugs do not.

Question 10

How does the pH of the body affect drug ionisation?

Answer 10

The body has different pHs in different body compartments which changes the ionisation of the drug.
Where ionisation is increased - this ‘traps’ the drug.

Question 11

What affects the ionisation of a drug?

Answer 11

The different pH’s in body compartments.

Question 12

Approximately what proportion of aspirin will be ionised at a pH of 3.5?

Answer 12

pKA of 3.5

Question 13

What is the effect of changing pH of ionisation of alkaline drugs?

Answer 13

The ionisation will be the greatest at an acidic pH.

Question 14

What is the effect of changing the pH of ionisation of acidic drugs?

Answer 14

Ionisation is greatest at an alkali pH.

Question 15

What is the last barrier that the antibiotic must cross to get into the pleural cavity? (drug is given via IV)

Answer 15

Serous membranes
They line body cavities e.g. the pleural space, and overlie organs - so this is the last barrier the drug will cross on the way into the pleural space

If given orally the drug will have been absorbed across gut epithelium into interstitial fluid,