Drug Absorption Flashcards
State the physico-chemical factors that affect the transfer of drugs across cell membranes.
- Ionisation
- Lipid solubility
- Structure
Describe the physico-chemical factors that affect the transfer of drugs across cell membranes:
Ionisation
Ionisation:
- Most drugs do not completely ionise in solution since most are weak acids/bases.
- Degree of dissociation depends on environmental pH
- Ionised drugs cannot cross the membrane, while the unionised drug can distribute across the membrane until equilibrium is reached.
- Hence, small changes in pH can drastically affect the ability of the body to absorb the drug
Describe the physico-chemical factors that affect the transfer of drugs across cell membranes:
Lipid solubility
A drug that is highly lipid soluble will readily diffuse across the phospholipid bilayer.
Describe the physico-chemical factors that affect the transfer of drugs across cell membranes:
Structure
Drugs need to resemble naturally occurring substances in order to fit through the pumps and be absorbed through active transport
State the factors that affect the absorption of a drug from the GI tract.
- Motility
- Illness
- Food
Describe the factors that affect the absorption of a drug from the GI tract:
Motility
The speed of gastric activity will affect the speed at which the drug will reach the site of absorption.
Describe the factors that affect the absorption of a drug from the GI tract:
Illness
- Malabsorption (e.g Coelic disease) can increase or decrease the rate of absorption
- Migraine reduces the rate of stomach emptying and therefore the rate of absorption of analgesic drugs.
State the factors that affect the absorption of a drug from the GI tract:
Food
The rate of absorption of some drugs can be impaired or enhanced due to food
State what is meant by the term ‘first pass metabolism’
The metabolism of the drug prior to it reaching the site of ABSORPTION
Describe the medical importance of first pass metabolism
- First pass metabolism refers to the metabolism of the drug prior to it reaching the site of absorption.
- This should be avoided, can be done through different delivery methods
State where first pass metabolism mainly occurs.
- Liver
- Gut lumen
- Gut wall
(these factors can all be affected by drugs and illness)
Discuss the benefits of intravenous medication (2)
- 100% bioavailability
- Avoids first pass metabolism
Discuss the benefits of tropical medication (3)
- Avoids first past metabolism
- Can achieve local or systemic effects
- Able to achieve controlled, sustained doses of drug
Discuss the benefits of inhaled medication (5)
- Rapid effect
- Drug delivered directly to the site of action
- Small doses used
- Little systemic absorption
- Reduced adverse effects
State what is meant by the term ‘bioavailability’
The amount of drug which reaches the circulation and is available for action
