Drug absorption

Question 1

Drug absorption

Answer 1

The process of movement of unchanged drug from the site of administration to the systemic circulation.

Question 2

Factors effecting bioavailability

Answer 2

1. Formulation and mode of administration i.e. SR/MR preparations.
2. Ability of a drug to pass physiological barriers i.e. particle size, lipid solubility, pH and ionisation.
3. GI effects i.e. gut motility, food, illness
4. First pass metabolism

Question 3

Lipid-water partition co-efficient

Answer 3

ability for a drug to diffuse across lipid barrier. Ratio of amount of drug which dissolves in the lipid and water phase when in contact.

Question 4

Active transport of drugs

Answer 4

Is rare. The drug must resemble naturally occurring compounds and the drug reversibly bound to a carrier system.

Question 5

Passive transport of drugs

Answer 5

Very common, occurs down a concentration gradient. The drug must be non selective and non saturable. It requires no energy and no carrier.

Question 6

Facilitated diffusion

Answer 6

Occurs down a concentration gradient but requires carriers. The drug must be saturable and structure specific. Requires no energy.

Question 7

Filtration/bulk flow/pore transport

Answer 7

Normally occurs through channels in cell membrane. Drug must be of low molecular size with a molecular weight less than 100. It is driven by hydrostatic or osmotic pressure difference.

Question 8

Other methods of drug absorption

Answer 8

Ion pair transport. Endocytosis.

Question 9

Factors affecting absorption of a drug from the GI tract

Answer 9

1. Motility; Speed of absorption will affect speed at which drug reaches site of absorption, often the small intestine, and can be affected by other drugs, food, illness and drink.
2. Food/drink; can enhance or impair the rate of absorption.
3. Illness; malabsorption can increase or decrease rate. Migraine reduces gastric emptying time and therefore rate of absorption of analgesic drugs.

Question 10

First pass metabolism

Answer 10

The metabolism of a drug prior to reaching systemic circulation. May occur in the gut lumen (acids, enzymes), gut wall (metabolic enzymes), or the liver (hepatic enzymes). This can be a limit on oral route for some drugs e.g. insulin.

Question 11

Benefits of inhaled, IV and topical meds.

Answer 11

Bypass first metabolism, and has a direct entry and action.

Question 12

Other methods of drug administration

Answer 12

Subcut/IM, sublingual, rectal, transdermal.

Question 13

Considerations for route of administration

Answer 13

Purpose, site of action, disease effects, patients ability, speed of action, reliability of absorption