MTM Pharmacotherapy Review & Drug Interactions 5 — Questions and Answers
Question 1: A patient on tacrolimus (an immunosuppressant) begins taking St. John's Wort. What outcome is expected?
- Tacrolimus levels increase due to CYP3A4 inhibition
- Tacrolimus levels decrease due to CYP3A4 induction, risking organ rejection (Correct answer)
- No interaction; herbal products do not affect CYP enzymes
- Tacrolimus absorption increases due to P-gp inhibition
Correct answer: Tacrolimus levels decrease due to CYP3A4 induction, risking organ rejection
St. John's Wort is a potent CYP3A4 and P-glycoprotein inducer that significantly reduces tacrolimus plasma levels, potentially leading to acute transplant rejection.
Question 2: Which pharmacodynamic interaction best describes the combination of an ACE inhibitor with a potassium-sparing diuretic?
- Synergistic antihypertensive effect without electrolyte concerns
- Additive hyperkalemia risk due to reduced aldosterone effect from both agents (Correct answer)
- Hypokalemia due to combined natriuresis
- Reduced ACE inhibitor efficacy due to diuretic-induced volume depletion
Correct answer: Additive hyperkalemia risk due to reduced aldosterone effect from both agents
Both ACE inhibitors (which reduce aldosterone) and potassium-sparing diuretics (which block aldosterone action) decrease potassium excretion, creating additive hyperkalemia risk.
Question 3: A patient on sustained-release theophylline begins ciprofloxacin. What pharmacokinetic change is anticipated?
- Decreased theophylline levels due to increased CYP1A2 induction
- Increased theophylline levels due to CYP1A2 inhibition (Correct answer)
- No change; theophylline is not metabolized by CYP enzymes
- Decreased theophylline absorption due to chelation
Correct answer: Increased theophylline levels due to CYP1A2 inhibition
Ciprofloxacin inhibits CYP1A2, the primary enzyme responsible for theophylline metabolism, leading to elevated theophylline levels and potential toxicity (seizures, arrhythmias).
Question 4: What is the most appropriate intervention when a patient's medication therapy problem involves an adverse drug reaction from a necessary medication?
- Discontinue the medication immediately without replacement
- Recommend a dose reduction, symptomatic management, or alternative therapy if available (Correct answer)
- Add another medication to counteract all side effects
- Ignore the ADR if the medication is considered essential
Correct answer: Recommend a dose reduction, symptomatic management, or alternative therapy if available
In MTM, adverse drug reactions are addressed by assessing severity, considering dose adjustments, adding supportive therapy, or switching to an alternative with a safer profile while maintaining therapeutic goals.
Question 5: A patient taking lithium is started on hydrochlorothiazide for hypertension. What is the mechanism of the resulting interaction?
- Thiazides enhance renal lithium excretion, causing subtherapeutic levels
- Thiazides cause sodium depletion, triggering compensatory renal lithium reabsorption and toxicity (Correct answer)
- Thiazides inhibit lithium's CNS distribution
- Thiazides compete with lithium for tubular secretion
Correct answer: Thiazides cause sodium depletion, triggering compensatory renal lithium reabsorption and toxicity
Thiazide-induced sodium depletion causes the kidney to compensatorily reabsorb both sodium and lithium in the proximal tubule, reducing lithium clearance and raising serum levels toward toxicity.
Question 6: Which parameter is used to calculate the maintenance dose needed to achieve a target steady-state concentration?
- Volume of distribution × half-life
- Clearance × target concentration (Correct answer)
- Bioavailability ÷ half-life
- Protein binding fraction × target concentration
Correct answer: Clearance × target concentration
Maintenance dose rate = Clearance × Target steady-state concentration; clearance determines how much drug must be administered per unit time to replace what is eliminated.
Question 7: A patient taking an SSRI is prescribed tramadol for pain. Which serious adverse effect is of highest concern?
- Additive CNS depression and respiratory failure
- Serotonin syndrome due to dual serotonergic activity (Correct answer)
- Hypertensive crisis from norepinephrine accumulation
- QT prolongation from combined cardiac effects
Correct answer: Serotonin syndrome due to dual serotonergic activity
Tramadol inhibits serotonin and norepinephrine reuptake in addition to its opioid activity; combined with an SSRI, this can precipitate serotonin syndrome.
MTM Pharmacotherapy Review & Drug Interactions 5
A patient on tacrolimus (an immunosuppressant) begins taking St.
John's Wort.
What outcome is expected?