Drug Absorption & Distribution

Question 1

ADME

Answer 1

Absorption, distribution, metabolism & elimination

Helps drug companies to improve drug success rate

Helps doctors to consider the risk/benefit of a drug for a patient

Question 2

Routes of absorption

Answer 2

There are many routes of absorption for drugs, each with advantages and disadvantages

• Oral
• Skin (topical)
• Injection
• Inhalation

Question 3

Cellular absorption

Answer 3

Drugs must get into cells to have their effect and must sometimes cross cells to enter the bloodstream

• Simple diffusion
• Channel-mediated facilitated diffusion
• Active transport
• Pinocytosis

Question 4

Dissolution

Answer 4

The rate at which a drug goes into solution

A drug must be in its molecular form (in solution) for absorption to take place because only unionised drug can cross the plasma membrane

Question 5

Effect of pH on dissolution

Answer 5

Weak acids and weak bases exist in both ionised and unionised forms in solution

The ratio of ionised drug to unionised drug is determined by the pKa of the drug and the pH of the body compartment

Question 6

Weak acids

Answer 6

Drugs that are weak acids (e.g. aspirin) have greatest ionisation in more alkaline body compartments

Therefore, urinary acidification limits the excretion of weak acids

Question 7

Weak bases

Answer 7

Drugs that are weak bases (e.g. pethidine) have greatest ionisation in more acidic body compartments

Therefore, urinary acidification accelerates the excretion of weak bases

Question 8

Active transport

Answer 8

Transporters are used to help polar chemicals cross the plasma membrane

These transporters are relatively non-specific and so there will be competition between the drug and endogenous substrates for its use

The transporters are bidirectional and usually inducible (exposure to substrates increases their expression in that cell)