Lecture 10: medicine interactions

Question 1

4 types of interactions of drugs?

Answer 1

DESIRABLE

1. potentiation -additive effect
2. antagonism -antidotes

UNDESIRABLE

1. toxicity
2. loss of effect

Question 2

Types of drug interations?

Answer 2

PHARMACOKINETIC

• altering the concentration through absorption, distribution metabolism or elimination (harder to predict due to not being predictable from the clinical effects)

PHARMACODYNAMIC

• altering the effect through receptor mediated (B-blockers would inhibit/reduce the effect of bronchodilators) or non-receptor mediated-(Aspirin and warafin both inhibit blood clots)-(can be predicted based off known drug effects)

Question 3

example of pharmacokinetic interaction?

4 places of absorption blocking?

Answer 3

1. Calcium binds to Thyroxine and reduces its absorption
2. metoclopramide increases gastric emptying reducing digoxin absorption in stomach
3. changes in GI pH
4. drug binding in GI
5. changes in gut motility
6. malabsorption

Question 4

define: substrate, inducers, inhibitors?

Inducers and inhibors effect on substrate concentration? relative speed?

Answer 4

substrate: an agent that is metabolised by an enzyme into a metabolic end product nd eventually excreted

inducer: increases production of enzymes increasing metabolism and thus decreasing substrate concentration. Slow response as enzyme has to be prodced

inhibitor: interferes with the ability of the enzyme to metabolize the substrate resuting in increased concentration of the substrate. Fast response and determined by half life and time to SS of the inhibitor drug.

Question 5

Hepatic interactions involve?

Answer 5

A lot of them involve CYP450

eg. Warfarin uses CYP 2C9 that is inhibited by amiodarone and fluoxitine and induced by rafampicin and St Johns Wart

Not all use CYP450 though and can interact with other pathways

Question 6

4 sites for renal interaction?

Examples?

Answer 6

1. changed renal blood flow
2. competition for ionic cotransporters
3. competition for same transporters
4. change in urin pH

• Cemetidine (histamine agonist) competes with metformin (oral hypogylcaemic) for transport into renal cells and renal excretion
• Sodium bicarbonate alkalinises the urine and increases renal excretion of aspirin

Question 7

Sources of information about drug interactions?

Answer 7

Medsafe, New Zealand formulary

(alternatively: hospital medicine information service, online tables)