Drug absorption

Question 1

State the physio-chemical factors that affect the transfer of drugs across cell membranes.

Answer 1

• ionisation
• Lipid solubility
• structure

Question 2

describe how Ionisation is one of the physio-chemical factors that affect the transfer of drugs across cell membranes.

Answer 2

-Most drugs do not completely ionise in solution. Since most drugs are weak acids or bases, the DEGREE OF DISSOCIATION depends on the PH of the environment.

• ionised drugs do not cross the membrane, but unionised drugs can cross the membrane until distribution is reached.
• therefore, small changes in pH can drastically affect the ability of the body to absorb a drug

Question 3

describe how Lipid Solubility is one of the physio-chemical factors that affect the transfer of drugs across cell membranes.

Answer 3

To diffuse across the phospholipid bilayer of the cell membrane, the drug must be lipid soluble.
A drug that is highly lipid soluble ill readily diffuse across but drugs like Gentamicin which aren’t lipid soluble may not be absorbed at all

Question 4

describe how Structure is one of the physio-chemical factors that affect the transfer of drugs across cell membranes.

Answer 4

For drugs to be absorbed through active transport, they put resemble naturally occurring substances or else they won’t fit through the pumps

Question 5

State the factors that affect the absorption of a drug from the GI tract.

Answer 5

Motility
Other drugs
Food
Illness

Question 6

Describe how Motility is a factor that affects the absorption of a drug from the GI tract.

Answer 6

The speed of gastric activity will affect the speed at which the drug will reach the site of absorption.

Question 7

Describe how Food is a factor that affects the absorption of a drug from the GI tract.

Answer 7

The absorption of some drugs can be impaired or enhanced

Question 8

Describe how Ilnness is a factor that affects the absorption of a drug from the GI tract.

Answer 8

MALABSORPTION (eg Coeliac’s disease-glucose intolerance) can impair or enhance the absorption rate of certain drugs.
MIGRAINES REDUCE the rate of stomach emptying and by proxy, the rate of absorption of analgesics.

Question 9

Describe the medical importance of first pass metabolism.

Answer 9

First pass metabolism refers to the metabolism of the drug prior to it reaching its site of absorption.
This can be avoided through different delivery methods, however some drugs can only be delivered in specific ways.

Question 10

Where do the drugs that follow first pass metabolism mainly occur?

Answer 10

• Gut lumen (acid, enzymes)
• Gut wall (metabolic enzymes)
• Liver (hepatic extraction ratio)

Question 11

Describe the benefits of IV medication.

Answer 11

• 100% bioavailability

- Avoids first pass metabolism

Question 12

Describe the benefits of Topical medication

Answer 12

• Can achieve local or systemic effects
• achieve controlled, sustained doses of the drug
• Avoids first pass metabolism

Question 13

Describe the benefits of inhaled medication.

Answer 13

• Drug is directly delivered to the site of action
• Has a rapid effect
• Small doses are used
• Little systemic absorption
• Reduced adversed effects

Question 14

Define bioavailability

Answer 14

The bioavailability of a drug is the amount of drug which reaches the circulation and is available for action.

Question 15

What are factors that affect the bioavailability of a drug?

Answer 15

• Formulation
• Drug’s ability to pass physiological barriers
• Gastrointestinal effects
• First pass metabolism

Question 16

Describe how drugs ability to pass physiological barriers can determine the bioavailability of a drug.

Answer 16

The drugs…

• particle size
• lipid solubility
• pH and ionisation

can affects its ability to pass physiological barriers

Question 17

Describe how gastrointestinal effects can determine the bioavailability of a drug.

Answer 17

Gut motility
Food
Illness