FoM:L4 - Pharmacokinetics 3, excretion and interactions Flashcards
What are the main routes of drug excretion?
- pulmonary (volatile drugs)
- renal (water-soluble)
- faecal (large, lipid-soluble)
Outline glomerular filtration
- plasma filtered passively
- large molecules not filtered
- small molecules filtered into tubules
Outline passive reabsorption and secretion
- lipophilic drugs diffuse
- reabsorption of wanted substances
- active secretion of drug metabolites conjugated with glucuronide
What is drug elimination?
- metabolism and excretion
- helps maintain dose and achieve steady state plasma concentration
Why might multiple drugs be used?
- pharmacodynamic effects are complementary
- better clinical outcome
What is meant by the term ‘drug interactions’?
- unhelpful and unwanted
consequences - multiple drugs, or drugs and food
- unforeseen pharmacokinetic interactions.
What is an additive pharmacodynamic drug interaction?
0verall effect of the drug combination is the sum
of the effect of each drug
What is an antagonistic pharmacodynamic drug interaction?
overall effect of the drug combination is less than
the additive sum of the effect of each drug alone
What is a synergistic pharmacodynamic drug interaction?
overall effect of the drug combination is greater
than additive
What are pharmacokinetic drug interactions?
- interference by one drug with the pharmacokinetics of another
drug - plasma concentrations of
the other drug are altered
Outline CYP Induction
- drug leads to increased expression of CYP gene
- so drug is metabolised faster
Outline CYP Inhibition
- multiple drugs acted on by one CYP enzyme
- so rate of metabolism slowed
When are drug interactions most critical?
- drug has serious side effects, narrow therapeutic index and no other elimination pathways
- unavoidable or unforeseen
- patient is vulnerable
What are adverse drug interactions?
unwanted or harmful reaction that occurs after administering the drug (expected to be caused by the drug)
