Medical Pharmacology Question Bank

Chapter 1: General Pharmacology — Module 5: Drug Development and Regulation
Tier: Tier 2 — Conceptual Understanding

1. A woman with breast cancer is prescribed tamoxifen after surgical resection. Her oncologist orders CYP2D6 genotyping before initiating therapy. She is found to be a CYP2D6 poor metabolizer (carrying two loss-of-function alleles). Which of the following best explains why CYP2D6 status is clinically critical for tamoxifen efficacy, and what prescribing adjustment is most appropriate?

A) CYP2D6 poor metabolizers convert tamoxifen to a toxic reactive quinone metabolite at increased rates, producing hepatotoxicity and cardiotoxicity — genotyping identifies patients requiring dose reduction to avoid organ damage
B) Tamoxifen is a CYP2D6 substrate that is directly active as a selective estrogen receptor modulator; poor metabolizers accumulate tamoxifen to supratherapeutic plasma concentrations, producing exaggerated estrogenic blockade and paradoxically increasing the risk of endometrial hyperplasia and thromboembolic events
C) Tamoxifen is a prodrug that requires CYP2D6-mediated conversion to endoxifen — its primary active metabolite, which is 30–100 times more potent than tamoxifen as an estrogen receptor antagonist; CYP2D6 poor metabolizers generate substantially reduced endoxifen concentrations, resulting in inadequate estrogen receptor blockade and significantly worse breast cancer outcomes; an aromatase inhibitor (for postmenopausal women) or referral to a pharmacogenomics specialist for alternative endocrine therapy planning is the appropriate response
D) CYP2D6 poor metabolizers convert tamoxifen to 4-hydroxytamoxifen exclusively, which has equal potency to endoxifen — poor metabolizer status therefore has no clinically meaningful impact on tamoxifen efficacy and genotyping is not clinically useful
E) CYP2D6 poor metabolizer status increases tamoxifen renal clearance through upregulation of OAT3-mediated tubular secretion, reducing tamoxifen plasma concentrations and requiring dose escalation to maintain therapeutic anti-estrogenic effects

ANSWER: C

Rationale:

Tamoxifen pharmacogenomics represents one of the most clinically important and debated applications of CYP genotyping in oncology. Tamoxifen itself has modest estrogen receptor antagonist activity, but its pharmacological efficacy depends critically on sequential hepatic metabolism. CYP2D6 mediates conversion of tamoxifen to 4-hydroxytamoxifen (4-OHT) and, in combination with CYP3A4/3A5-mediated N-desmethylation, to N-desmethyltamoxifen, which CYP2D6 then converts to endoxifen (4-hydroxy-N-desmethyltamoxifen). Endoxifen is the primary pharmacologically active metabolite responsible for sustained estrogen receptor blockade in breast tissue — it is present at plasma concentrations approximately 5–10 times higher than 4-OHT in extensive metabolizers, and its sustained tissue concentration is the primary driver of clinical tamoxifen efficacy. Endoxifen's estrogen receptor binding affinity is 30–100 times greater than tamoxifen itself. In CYP2D6 poor metabolizers, endoxifen plasma concentrations are reduced by approximately 75–90% compared to extensive metabolizers. Multiple retrospective and prospective studies demonstrate that low endoxifen concentrations are associated with significantly worse relapse-free survival and overall survival in hormone receptor-positive breast cancer patients treated with tamoxifen. Furthermore, concurrent use of strong CYP2D6 inhibitors (fluoxetine, paroxetine) in tamoxifen-treated patients — producing drug-induced phenoconversion to poor metabolizer status — has been associated with increased breast cancer recurrence risk in pharmacoepidemiological studies. For postmenopausal CYP2D6 poor metabolizers, aromatase inhibitors (letrozole, anastrozole, exemestane) do not require CYP2D6 bioactivation and are the preferred endocrine therapy. For premenopausal patients, consultation with a clinical pharmacogenomics specialist for individualized therapy planning is appropriate. Option A is incorrect — CYP2D6 does not generate a hepatotoxic reactive quinone from tamoxifen; the relevant metabolic concern is inadequate endoxifen generation, not toxic metabolite accumulation. Option B is incorrect — tamoxifen's pharmacological activity is prodrug-dependent; poor metabolizers have reduced, not increased, pharmacodynamic effect. Option D is incorrect — 4-OHT plasma concentrations are substantially lower than endoxifen in EM individuals, and 4-OHT is not the primary driver of sustained tamoxifen efficacy; poor metabolizer status does clinically impact outcomes.

2. Option E is incorrect — CYP2D6 mediates hepatic metabolism, not renal excretion; OAT3 is irrelevant to tamoxifen pharmacokinetics. A 45-year-old woman of Han Chinese descent is newly diagnosed with trigeminal neuralgia and is being considered for carbamazepine therapy. Her neurologist is aware of a specific HLA-associated pharmacogenomic risk. Which of the following most accurately describes this risk, the appropriate pre-prescribing action, and the consequence of an alternative HLA allele in a different population?

A) HLA-A*3101 is associated with carbamazepine-induced SJS/TEN specifically in Han Chinese populations; HLA-B*1502 is associated with carbamazepine-induced maculopapular exanthema in Northern Europeans — both alleles must be tested before prescribing carbamazepine to any patient globally
B) HLA-B*1502 is strongly associated with carbamazepine-induced SJS and TEN in Han Chinese and Southeast Asian populations, with an odds ratio exceeding 1000 for SJS/TEN in HLA-B*1502 carriers; the FDA mandates HLA-B*1502 genotyping before initiating carbamazepine in patients of Asian ancestry; HLA-A*3101 is a distinct allele associated with carbamazepine hypersensitivity reactions (including SJS, DRESS, and maculopapular exanthema) in European, Japanese, and other non-Asian populations — these are different alleles with overlapping but distinct clinical phenotypes and population prevalences
C) HLA-B*5701 is the relevant allele for carbamazepine-induced SJS/TEN in all populations; genotyping should be performed using the same assay used for abacavir hypersensitivity screening, as the two alleles are in complete linkage disequilibrium
D) Carbamazepine-induced SJS/TEN in Han Chinese is associated with HLA-B*5801 rather than HLA-B*1502; HLA-B*5801 genotyping is already mandated before allopurinol prescribing and the same result can be used for carbamazepine screening in Asian patients
E) The HLA allele associated with carbamazepine SJS/TEN in Han Chinese has a prevalence of less than 0.1% in that population, making the absolute risk of SJS/TEN negligible — routine genotyping is therefore not cost-effective and is not recommended by any regulatory authority

ANSWER: B

Rationale:

Carbamazepine-associated severe cutaneous adverse reactions (SCAR) represent the most extensively studied HLA-drug interaction in clinical pharmacogenomics, and the genetics are population-specific — a critical nuance that must be understood for safe prescribing. In Han Chinese and Southeast Asian populations (Thai, Malaysian, Vietnamese, Filipino), HLA-B*1502 is the dominant pharmacogenomic risk allele for carbamazepine-induced SJS and TEN. The odds ratio for carbamazepine-induced SJS/TEN in HLA-B*1502 carriers compared to non-carriers in Han Chinese populations exceeds 1000 in the landmark study by Chung et al. (Nature 2004) — an extraordinarily strong genetic association. HLA-B*1502 prevalence is approximately 6–9% in Han Chinese, 10–15% in Thai, and higher in some Southeast Asian populations — making the population-attributable risk clinically substantial. The FDA issued a safety communication in 2007 requiring carbamazepine prescribing information to include a recommendation for HLA-B*1502 genotyping before initiating therapy in patients of Asian ancestry. In contrast, HLA-A*3101 is a different allele (an HLA-A allele, not HLA-B) that is more broadly distributed across European, Japanese, Korean, and other non-Asian populations. HLA-A*3101 is associated with carbamazepine hypersensitivity reactions across a spectrum of severity — SJS/TEN, DRESS, and maculopapular exanthema — but with lower odds ratios than HLA-B*1502 for the most severe reactions. CPIC (Level A evidence) provides guidelines for both alleles in their respective populations. These alleles are pharmacogenomically distinct — they cannot be used interchangeably, are detected by different genotyping assays, and have different clinical risk profiles. Option A reverses the population-allele associations (HLA-B*1502, not HLA-A*3101, is the Han Chinese risk allele for SJS/TEN) and incorrectly states that both must be tested in all patients globally. Option C is incorrect — HLA-B*5701 is the abacavir allele, not the carbamazepine allele; the two alleles are not in linkage disequilibrium. Option D is incorrect — HLA-B*5801 is the allopurinol SCAR allele; carbamazepine SCAR in Han Chinese is associated with HLA-B*1502, a different allele. Option E is incorrect — HLA-B*1502 prevalence of 6–9% in Han Chinese is not negligible; the FDA mandate for genotyping reflects regulatory recognition of the clinically meaningful absolute risk.

3. A clinical pharmacologist is reviewing the concept of phenoconversion in the context of a patient's CYP2D6 genotype and drug regimen. The patient's genotype predicts an extensive metabolizer (EM) phenotype, but her current medications include paroxetine — a potent, mechanism-based CYP2D6 inhibitor. She is being considered for codeine for postoperative pain. Which of the following best describes phenoconversion and its clinical implication in this patient?

A) Phenoconversion refers to the genetically determined switching of a patient's metabolizer phenotype from EM to UM status during puberty — paroxetine has no relationship to phenoconversion, which is an irreversible developmental pharmacogenomic event
B) Phenoconversion describes the situation where a patient's functional metabolizer phenotype differs from their genotype-predicted phenotype due to drug-mediated enzyme inhibition or induction — paroxetine, a potent CYP2D6 inhibitor, converts this genotypic EM patient to a functional poor metabolizer phenotype; if codeine is prescribed, the impaired CYP2D6 activity will reduce codeine conversion to morphine, producing analgesic failure; conversely, if paroxetine is discontinued in a patient previously stabilized on opioids, restored CYP2D6 activity will increase morphine generation and may cause opioid toxicity
C) Phenoconversion refers exclusively to CYP2D6 induction by rifampicin or carbamazepine, converting a poor metabolizer to a functional extensive metabolizer — paroxetine inhibition of CYP2D6 is classified separately as drug-induced metabolic suppression, not phenoconversion
D) Phenoconversion in this patient means that paroxetine has methylated the CYP2D6 gene promoter, producing epigenetic silencing of CYP2D6 expression — this is an irreversible change that persists after paroxetine discontinuation and permanently converts the patient to a poor metabolizer phenotype
E) Phenoconversion has no clinical relevance because pharmacogenomic testing always reports functional phenotype (accounting for current medications) rather than genotype-predicted phenotype — a genotypic EM patient on paroxetine would automatically be reclassified as a PM in all pharmacogenomic test reports

ANSWER: B

Rationale:

Phenoconversion is a clinically critical concept that highlights the gap between genotype-predicted and actual functional metabolizer phenotype — a gap that occurs when a patient's enzymatic activity is pharmacologically altered by concurrent medications, disease states, or other environmental factors, independent of their genotype. In this patient, CYP2D6 genotyping predicts an EM phenotype — implying normal CYP2D6 enzyme activity and normal codeine-to-morphine conversion. However, paroxetine is a mechanism-based (irreversible) CYP2D6 inhibitor: it forms a nitroso intermediate that covalently binds to and permanently inactivates CYP2D6 enzyme molecules. While new CYP2D6 protein is continuously synthesized (preventing permanent gene-level effects), steady-state CYP2D6 activity in a patient on therapeutic paroxetine doses is reduced to levels functionally equivalent to or approaching a PM phenotype — the patient has been phenoconverted from genotypic EM to functional PM. The clinical consequences in this patient if codeine is prescribed: reduced CYP2D6-mediated O-demethylation of codeine to morphine reduced analgesic effect at standard doses (analgesic failure). This is the opposite of the UM safety concern — phenoconversion to functional PM status causes underdosing rather than toxicity for codeine. Additionally, an important reverse scenario: if a CYP2D6 EM patient has been stabilized on an opioid regimen while receiving paroxetine (with dose titrated accounting for reduced CYP2D6 activity), and paroxetine is subsequently discontinued, CYP2D6 activity recovers over days to weeks as new enzyme is synthesized — the patient transitions back to EM phenotype, and the previously adequate opioid dose now generates more morphine, risking opioid toxicity. This bidirectional clinical risk makes paroxetine co-prescription with CYP2D6-dependent prodrugs and drugs particularly hazardous. Most pharmacogenomic test reports provide genotype-predicted phenotype and may or may not account for concurrent medications — clinicians must integrate medication history to determine the true functional phenotype. Option A is incorrect — phenoconversion is not a developmental event and is not irreversible; it is drug-induced and reversible upon drug discontinuation. Option C is incorrect — phenoconversion applies to both inhibition (functional PM) and induction (functional UM) and is a general concept not limited to specific inhibitors. Option D is incorrect — paroxetine causes reversible mechanism-based enzyme inactivation, not epigenetic methylation of the CYP2D6 gene promoter; the effect reverses as new enzyme is synthesized after paroxetine discontinuation. Option E is incorrect — most pharmacogenomic test reports reflect genotype and do not automatically account for current medications; the clinical integration of genotype with current drug regimen is the clinician's responsibility.

4. A 52-year-old man with gout is prescribed allopurinol. He is of Korean descent. His physician considers ordering HLA-B*5801 genotyping per CPIC guidelines. The test returns positive (carrier). Which of the following best describes the appropriate pharmacogenomic-guided management, and what alternative urate-lowering strategy should be considered?

A) A positive HLA-B*5801 result confirms that the patient has already developed allopurinol-induced SJS and requires immediate hospitalization and systemic corticosteroids — HLA-B*5801 genotyping is a diagnostic test for active drug reactions, not a pre-prescribing screening tool
B) A positive HLA-B*5801 result means the patient has a substantially elevated risk of allopurinol-induced SJS/TEN if allopurinol is initiated — allopurinol should not be prescribed; alternative urate-lowering therapies that do not carry HLA-B*5801-associated SCAR risk include febuxostat (a non-purine selective xanthine oxidase inhibitor), benzbromarone (a uricosuric agent), or probenecid (a uricosuric agent), selected based on patient-specific factors including renal function and gout severity
C) A positive HLA-B*5801 result indicates that the patient is an allopurinol ultrarapid metabolizer who will convert allopurinol to oxypurinol too rapidly, reducing its xanthine oxidase inhibitory efficacy — a dose increase to 600–800 mg/day is recommended to compensate for the accelerated metabolism
D) HLA-B*5801 genotyping is relevant only for patients of Han Chinese descent — the result in a Korean patient has no pharmacogenomic significance and allopurinol can be prescribed at standard doses without concern for SJS/TEN risk
E) A positive HLA-B*5801 result should prompt a desensitization protocol: allopurinol should be initiated at 50 mcg/day and titrated over 12 weeks to full therapeutic dose — this approach eliminates SJS/TEN risk in HLA-B*5801 carriers and is recommended by CPIC as the preferred management strategy

ANSWER: B

Rationale:

This question applies HLA-B*5801 pharmacogenomic knowledge to a specific clinical decision — one of the most straightforward and high-stakes applications of pre-prescribing genotyping in clinical practice. A positive HLA-B*5801 result in a patient being considered for allopurinol indicates a substantially elevated risk of allopurinol-induced SCAR (SJS/TEN). The association is well-established across multiple Asian populations beyond Han Chinese: it has been confirmed in Korean, Thai, Vietnamese, and other Asian populations. In Korean patients, HLA-B*5801 prevalence is approximately 12%, and the allele is associated with allopurinol SJS/TEN with an odds ratio exceeding 500 in Korean populations. CPIC guidelines (Level A evidence) recommend that allopurinol should not be used in HLA-B*5801 carriers — the risk of life-threatening SCAR is too high to justify use when effective alternatives exist. Alternative urate-lowering therapies: Febuxostat is a non-purine selective xanthine oxidase inhibitor with no structural similarity to allopurinol and no HLA-B*5801-associated SCAR risk — it is the preferred alternative in HLA-B*5801 carriers with adequate renal function. Uricosuric agents (probenecid, benzbromarone, lesinurad) that increase renal uric acid excretion rather than reducing its production are alternatives for patients with normal or mildly impaired renal function who are not uric acid overproducers. Selection among alternatives depends on renal function (febuxostat preferred in CKD), cardiovascular history (febuxostat has a cardiovascular safety signal warranting caution in patients with active cardiovascular disease), and availability (benzbromarone is not available in all countries). Option A is incorrect — HLA-B*5801 genotyping is a pre-prescribing predictive screening tool, not a diagnostic test for active reactions; a positive result indicates future risk, not current disease. Option C is incorrect — HLA-B*5801 is an immune hypersensitivity allele, not a metabolizer phenotype; allopurinol metabolism occurs via xanthine oxidase and aldehyde oxidase, not through HLA-related pathways. Option D is incorrect — HLA-B*5801 is pharmacogenomically relevant across multiple Asian populations including Korean, Thai, and Vietnamese — not exclusively in Han Chinese; the Korean-specific evidence is robust. Option E is incorrect — allopurinol desensitization protocols have been used historically in patients with mild prior reactions in whom no alternatives existed, but CPIC guidelines do not recommend desensitization in HLA-B*5801 carriers; the risk-benefit profile strongly favors avoidance and use of alternatives.

5. A clinical pharmacist is counseling a team of medical residents about the current state and limitations of clinical pharmacogenomics implementation. Which of the following most accurately reflects both the capabilities and the limitations of current pharmacogenomic testing and implementation?

A) Pharmacogenomic testing provides complete prediction of individual drug response because all pharmacologically relevant genetic variation has been catalogued — a comprehensive panel test eliminates uncertainty in drug prescribing for all medications across all patient populations
B) Pharmacogenomic testing captures genetically determined variation in drug metabolism, transport, and target sensitivity, and is increasingly supported by evidence-based CPIC guidelines — however, it does not capture environmental phenoconversion (drug-drug interactions altering functional metabolizer status), epigenetic regulation, disease-state effects on enzyme expression, or pharmacodynamic variability unrelated to known genetic loci; it covers only a fraction of clinically used drugs with sufficiently strong gene-drug evidence; and population-level allele frequencies mean that genotyping is most useful when the relevant allele prevalence and the severity of the associated adverse reaction together justify pre-prescribing screening
C) Pharmacogenomic testing is currently limited exclusively to CYP enzyme polymorphisms — HLA allele, transporter gene, and pharmacodynamic target gene testing are experimental and not validated for clinical use in any approved guideline
D) The primary limitation of pharmacogenomic testing is cost — once universal free testing is available, all drug prescribing decisions can be genotype-guided without clinical judgment, eliminating adverse drug reactions and therapeutic failure across all drug classes
E) Pharmacogenomic testing is most useful for common diseases affecting large populations and is not applicable to rare diseases, orphan drugs, or personalized cancer therapy — oncopharmacogenomics operates under a separate regulatory framework with no overlap with CPIC guidelines

ANSWER: B

Rationale:

This question addresses the appropriate epistemic humility required when applying pharmacogenomics in clinical practice — an understanding of both what current pharmacogenomic testing can and cannot tell us. The capabilities of current pharmacogenomics are substantial and growing: CPIC guidelines provide Level A evidence-based recommendations for numerous high-impact gene-drug pairs (CYP2D6-codeine/tamoxifen/opioids, CYP2C19-clopidogrel/PPIs/antidepressants, CYP2C9/VKORC1-warfarin, HLA-B*5701-abacavir, HLA-B*1502-carbamazepine, HLA-B*5801-allopurinol, SLCO1B1-statins, TPMT/NUDT15-thiopurines, DPYD-fluoropyrimidines, G6PD-rasburicase/primaquine). Pre-emptive genotyping panels testing dozens of variants simultaneously are increasingly clinically implemented, allowing results to be available at the point of prescribing. However, equally important limitations must be understood: (1) Phenoconversion — drug-induced metabolizer status changes are not captured by genotyping; a genotypic EM on paroxetine is a functional PM, but the genotype report will show EM. (2) Epigenetic regulation — DNA methylation and histone modification of drug metabolism genes vary by tissue, age, and disease state and are not captured by germline genotyping. (3) Disease-state effects — hepatic cirrhosis, inflammation (cytokines suppress CYP expression), and organ failure alter drug metabolism independently of genotype. (4) Incomplete coverage — for the majority of commonly prescribed drugs, the strength of pharmacogenomic evidence does not yet meet the threshold for CPIC actionability; genotyping cannot guide prescribing for these drugs. (5) Population allele frequency considerations — pre-prescribing screening is most cost-effective when the allele prevalence in the target population and the severity of the associated adverse reaction together justify the upfront testing investment. Option A is incorrect — not all pharmacologically relevant genetic variation has been catalogued, and even comprehensive panels have the limitations described above. Option C is incorrect — HLA, transporter, and pharmacodynamic target gene testing are clinically validated, guideline-supported, and routinely used. Option D is incorrect — cost is a practical barrier but not the sole or even primary limitation; the conceptual and evidence-based limitations described above would persist even with free universal testing. Option E is incorrect — pharmacogenomics is highly relevant to oncology (tumor and germline pharmacogenomics), rare diseases, and orphan drugs; CPIC guidelines explicitly include oncology pharmacogenomics (DPYD/fluoropyrimidines, TPMT/NUDT15/thiopurines, UGT1A1/irinotecan).