Drug interactions Flashcards
What is clearance?
rate of drug elimination/ drug plasma conc
(efficiency of irreversible elimination of a drug from systemic circulation)
What is a drug interaction?
when a substance alters the expected performance of a drug
- substance: drug, food, toxin
What is a pharmacodynamic drug interaction?
Occur when drugs have an effect on the same target or physiological system
What is a pharmacokinetic drug interaction?
Occur when a drug affects the pharmacokinetics (absorption, distribution, metabolism or excretion) of another drug
What are the 2 types of pharmacodynamic drug interactions?
Synergistic or antagonistic
What is summation?
different drugs used together to have the same effect as a single drug would (1+1=1)
What is potentiation?
enhancement of one drug by another so that the combined effect is greater than the sum of each one alone (1+1=1+1.5)
What are some risk factors for drug interactions?
● Narrow therapeutic index
● Steep dose/response curve
● Saturable metabolism
What is synergy?
interaction of drugs such that the total effect is greater than the sum of the individual effects (work together)
What is antagonism?
an antagonist is a substance that acts against and blocks an action (2
drugs opposed to each other)
How can a drugs absorption be affected?
- Drugs which alter pH of GI tract
- Formation of insoluble drug complexes
- P-glycoprotein induction/inhibition
- Motility
Describe how motility can affect drug absorption
if the gut has slowed digestion, the drugs won’t work as well (oral
contraceptive pill and antibiotics is the most common interaction)
Describe how pH affects absorption of drugs
- Changes in pH will alter the proportion of ionised and unionised drug
- Changes in gastric pH
How do is absorption affected by drug interactions?
One drug affects the rate or extent of absorption of another drug
Describe how formation of insoluble drug complexes affect drug absorption
Insoluble drug complexes will not be absorbed and will be retained in the GIT
Describe how P-glycoprotein induction/inhibition affects drug absorption
- P-glycoproteins (P-gp) are drug transporter proteins widely distributed in body
- Excretory function to remove toxic substances (incl drugs) out of cells
- Activity can be altered by Pgp inhibitors and inducers leading to increased or decreased level of Pgp substrates
(more Pgp can remove some drugs)
How can distribution affect a drugs absorption?
- Interactions can occur when drugs compete for protein binding
- If you give 2 highly protein bound drugs, they will make each other strong and increase their effect so you always make sure you know what drugs the patient has taken before giving them new drugs
What are CYP450?
Haemoproteins
What can inhibitions of CYP450 do?
Drug A blocks metabolism of drug B, leaving more free drug B in the plasma so it has an increased effect
What can induction of CYP450 do?
Drug C induced CYP450 isoenzyme leading to increased metabolism of drug D so it has a decreased effect
What does renal excretion depend on?
pH
What conditions are weak bases cleared faster in?
Acidic urine
What conditions are weak acids cleared faster in?
Alkali urine
Which drug has a lot of drug interactions?
Warfarin
