Drug absorption

Question 1

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

Answer 1

Lipid solubility
Charge of the drug
Size of the drug

Question 2

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


Answer 2

Motility - Speed of gastric absorption will affect speed at which drug reaches site of absorption (most drugs are absorbed in small intestine). Can be affected by other drugs, food/drink and illnesses (esp pain).

Food - Can enhance or impair rate of absorption.

Illness - Malabsorption (eg Coeliac disease) can increase or decrease rate of absorption. Migraine reduces rate of stomach emptying and therefore rate of absorption of analgesic drugs.

Question 3

Discuss the medical importance of first pass metabolism. 


Answer 3

If dose is too low, drug reaching systemic circulation may not reach therapeutic concentration in the blood.

Question 4

Discuss the benefits of intravenous medication. 


Answer 4

Bypasses first pass metabolism
100% bioavailability
Fast response

Question 5

Discuss the benefits of transdermal medication. 


Answer 5

• Avoids first pass metabolism.

* Can provide controlled release

Question 6

Discuss the benefits of inhaled medication. 


Answer 6

• Relatively rapid action.
• Better for volatile agents.
• Usually used for topical effect or to avoid problems of oral absorption (eg nausea).

Question 7

What is pharmacokinetics?

Answer 7

Pharmacokinetics sometimes abbreviated as PK, is a branch of pharmacology dedicated to determining the fate of substances administered to a living organism

Question 8

What does the study of pharmacokinetics allow us to do?

Answer 8

• Enables understanding of:
• Dosage
• Drug administration
• Drug handling
• Patient variability
• Potential for harm (toxicology)

Question 9

Define absorption

Answer 9

Absorption is defined as the process of movement of unchanged drug from the site of administration to the systemic circulation.

Question 10

Describe the different means of drug absorption

Answer 10

• Oral
• Intravenous
• Subcutaneous
• Intramuscular
• Other GI - Sublingual, rectal
• Inhalation
• Nasal
• Transdermal

Question 11

What does the AUC represent?

Answer 11

The area under the drug concentration-time curve (AUC) represents total drug exposure over time and the drugs bioavailability

Question 12

Define the therapeutic range of a drug

Answer 12

A drug is active and therapeutically effective over a range of concentrations
Below this there will be insufficient or no pharmacological action.
Above this toxicity occurs

Question 13

Define bioavailability of a drug

Answer 13

Bioavailability – the fraction of an administered dose of unchanged drug that reaches the systemic circulation (AUC)

Question 14

What is the bioavailibity of intravenous drugs?

Answer 14

100%

Question 15

What factors affect bioavailability of oral drugs?

Answer 15

• Formulation
• Ability of drug to pass physiological barriers
• Gastrointestinal effects e.g. motility, food, illness
• First pass metabolism

Question 16

What is ion trapping?

Answer 16

An acidic drug will be more concentrated in the compartment with high pH (‘ion trapping’)

Question 17

What does the Henderson-Hasselbalch equation describe?

Answer 17

In chemistry, the Henderson–Hasselbalch equation describes the derivation of pH as a measure of acidity (using pKa, the negative log of the acid dissociation constant) in biological and chemical systems.

Question 18

What factors can affect diffusion of a drug?

Answer 18

Ionization/change in pH

Lipid solubility

Question 19

Name a drug that is actively transported

Answer 19

Levadopa by brain for Parkinson’s.

Question 20

What is first pass metabolism?

Answer 20

Metabolism of drug prior to reaching systemic circulation involving the intestinal and hepatic degradation or alteration of a drug or substance taken by mouth, after absorption, removing some of the active substance from the blood before it enters the general circulation.

Question 21

What parts of the body contribute to first pass metabolism?

Answer 21

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

Question 22

What factors affect the viability of intramuscular and subcutaneous administration?

Answer 22

Depends on blood flow to site.

Question 23

What are the benefits of intramuscular and subcutaneous administration?

Answer 23

Bypasses first pass metabolism

Question 24

What are the benefits of sublingual/buccal administration?

Answer 24

Bypasses first pass metabolism

Question 25

What does the ability to inhale a drug rely on?

Answer 25

Depends on

• type of delivery system
• particle size
• patient technique

Question 26

How much of a drug is absorbed in inhaled?

Answer 26

5-10%

Question 27

Why is inhalation of drugs used?

Answer 27

Usually used for topical effect or to avoid problems of oral absorption (eg nausea).

Question 28

How can drugs cross membranes?

Answer 28

• Passive diffusion
• Filtration
• Bulk flow
• Active transport
• Facilitated diffusion
• Ion-pair transport
• Endocytosis

Question 29

What should be considered when picking how to administer a drug?

Answer 29

• Purpose and site of drug action
• Disease effects
• Patient’s ability to take medicine
• Speed of action
• Reliability of absorption