Saudi Prometric Pharmacist Pharmaceutical Chemistry and Pharmacology 2 — Questions and Answers
Question 1: The drug-receptor interaction for a full agonist is best described as:
- High affinity and high intrinsic efficacy — binds receptor and produces maximum response (Correct answer)
- High affinity but zero intrinsic efficacy
- Low affinity and partial response
- Binds but blocks response
Correct answer: High affinity and high intrinsic efficacy — binds receptor and produces maximum response
A full agonist has both high receptor affinity (binds the receptor well) and high intrinsic efficacy (produces the maximal pharmacological response upon binding). A partial agonist has affinity but submaximal efficacy. An antagonist has affinity but no efficacy.
Question 2: What is the therapeutic significance of the half-life (t½) of a drug?
- It determines the time to reach steady state (approximately 4–5 half-lives) and dosing frequency (Correct answer)
- It determines the maximum achievable plasma concentration
- It determines first-pass metabolism
- It predicts the drug's protein binding capacity
Correct answer: It determines the time to reach steady state (approximately 4–5 half-lives) and dosing frequency
The half-life determines the time to reach steady-state concentration (4–5 × t½), time to wash out of the body, and appropriate dosing interval. Short t½ drugs require frequent dosing or sustained-release formulations. Steady state is where rate in = rate out.
Question 3: Which type of drug interaction involves one drug inducing CYP450 enzymes and increasing the metabolism of another drug?
- Pharmacokinetic interaction — increased metabolism reducing drug levels (Correct answer)
- Pharmacodynamic interaction — additive effect
- Pharmaceutical incompatibility
- Receptor competition
Correct answer: Pharmacokinetic interaction — increased metabolism reducing drug levels
CYP450 enzyme induction (e.g., rifampin, carbamazepine, phenytoin) increases the rate of drug metabolism, reducing plasma levels of co-administered drugs that are CYP substrates. This can cause therapeutic failure. Pharmacodynamic interactions affect drug response at the receptor level.
Question 4: A pharmacist is checking a prescription for a patient with G6PD deficiency. Which drug class should be avoided due to risk of hemolytic anemia?
- Primaquine and sulfonamides (Correct answer)
- Beta-blockers
- ACE inhibitors
- Tetracyclines
Correct answer: Primaquine and sulfonamides
G6PD deficiency impairs the hexose monophosphate shunt, reducing the ability to regenerate NADPH and protect red blood cells from oxidative stress. Drugs like primaquine, dapsone, sulfonamides, and nitrofurantoin cause oxidative hemolysis in G6PD-deficient patients.
Question 5: Which pharmacological principle explains why a combination of two beta-lactam antibiotics does not produce additive antibacterial activity?
- Both drugs compete for the same PBP (penicillin-binding protein) targets — pharmacodynamic antagonism (Correct answer)
- Pharmaceutical incompatibility when mixed
- Pharmacokinetic interaction causing reduced absorption
- Each antibiotic has a completely different mechanism of action
Correct answer: Both drugs compete for the same PBP (penicillin-binding protein) targets — pharmacodynamic antagonism
Two beta-lactam antibiotics compete for the same penicillin-binding protein (PBP) targets. Combining them does not provide additional inhibition beyond what one drug achieves alone and may even antagonize effects. Combining drugs with different mechanisms (e.g., beta-lactam + aminoglycoside) provides synergy.
Question 6: A pharmacist reviews a prescription for warfarin. The patient has just started taking rifampin. What change in warfarin dose is expected?
- Warfarin dose needs to be increased — rifampin induces CYP2C9 and increases warfarin clearance (Correct answer)
- Warfarin dose should be decreased — rifampin inhibits warfarin metabolism
- No change needed — rifampin does not interact with warfarin
- Warfarin should be stopped while on rifampin
Correct answer: Warfarin dose needs to be increased — rifampin induces CYP2C9 and increases warfarin clearance
Rifampin is a potent CYP2C9 (and CYP3A4) inducer that dramatically increases warfarin metabolism, reducing warfarin plasma levels and anticoagulant effect. INR will fall, requiring warfarin dose increases. When rifampin is stopped, close monitoring is required to avoid over-anticoagulation.
Saudi Prometric Pharmacist Pharmaceutical Chemistry and Pharmacology 2
The drug-receptor interaction for a full agonist is best described as: