drug absorption

Question 1

Discuss the physico-chemical factors that affect the transfer of drugs across cell membranes.

Answer 1

Absorption: The process by which an UNCHANGED drug moves into the blood circulation
Bioaccumulation: The amount of drug which reaches circulation and is available
Physical: -Particle size

Chemical: -Ionisation: un-ionised particles are the only ones able to pass across the membrane. The ionisation depends on the PH of a solution. It can be calculated through the henderson-hasselback equation.
A weak acid will ionise less in a strong Ph solution
-lipid solubility: Lipid-water Partition Coefficient. Only lipid soluble particles in solution are able to pass to cross membranes. ( the brain, ovaries and testes contain a lot of lipid so lipid soluble molecules will easily be able to pass through these)

Question 2

Discuss the factors that affect absorption of a drug from the GI tract

Answer 2

• Gut motility: the rate of food absorption will affect how quickly drugs can be absorbed
• Food
• Illness: e.g. diarrhoea will increase the rate at which drugs are eliminated/ migraine decrease the rate of stomach emptying and therefore the rate of absorption of analgesic drugs

Question 3

Discuss the medical importance of rst pass metabolism.

Answer 3

-First pass metabolism: removes the drug from the system before it has reached circulation. Can be overcome by increasing dose

Question 4

Discuss the benefits of intravenous, topical and inhaled medication.

Answer 4

Intravenous:

• bypasses the first pass metabolism
• only small doses required
• Can be painful
• better for water soluble drugs
• depends on blood flow
• plasma levels cane be strictly controlled
• dermojet (mass inoculation) and pellet (solid under the skin)for subcutaneous injections

Inhaled:

• only5-10% drug is absorbed
• Rapid
• Good for volatile agents
• overcomes side effects of oral absorption- bypass first pass metabolism/nausea

Topical=transdermal:

• overcomes first bypass reaction
• very slow

Question 5

Where will an acidic drug be concentrated? what is this known as?
What is the Ph environment controlled by?

Answer 5

It will be trapped in a more a solution with a high Ph- Basic.
Ion trapping
Proton pumps inhibitors antacids

Question 6

What does increasing dose lead to?

Answer 6

An increase in peak concentration

It does not increase the time at which peak concentration is reached

Question 7

what does the conc/time graph show?

Answer 7

AUC- i.e. the amount of drug which reaches circulation

Question 8

Therapeutic range

Therapeutic index

Answer 8

Therapeutic range- range at which drug is effective- if lower= ineffective. If higher = toxic
Therapeutic index= range at which drug is safe and active.

Question 9

Active absorption

Answer 9

Drugs must resemble compounds in the body.

E.G. LEVADOPA IN THE BRAIN

Question 10

Filtration

Answer 10

small molecules- molecular weight of 100 or less
Through channels in cell membrane
Based on hydrostatic/osmotic pressure difference
Mainly with water soluble drugs- removal of drugs from CSF, entry into liver