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Difficulty: HardGenetic and Molecular Mechanisms

Match each clinical pharmacogenomic scenario with its precise underlying genetic and molecular mechanism.

  • Severe, life-threatening neutropenia and diarrhea following initiation of irinotecan for metastatic colorectal cancer.Reduced expression of a hepatic conjugation enzyme due to a dinucleotide repeat expansion in the promoter region.
  • Stent thrombosis shortly after percutaneous coronary intervention despite confirmed adherence to a clopidogrel regimen.Presence of a loss-of-function allele impairing the hepatic cytochrome P450-mediated oxidation needed to generate an active thiol metabolite.
  • Unexpectedly severe mucositis and pancytopenia following standard-dose capecitabine therapy for breast cancer.Inherited deficiency of the rate-limiting hepatic enzyme responsible for the pyrimidine catabolic pathway.
  • Fatal respiratory depression in a breastfed newborn whose mother was prescribed codeine for postpartum analgesia.Gene duplication resulting in amplified hepatic cytochrome P450 activity, accelerating the O-demethylation of a prodrug.

Answer

Irinotecan toxicity matches with the UGT1A1 promoter repeat expansion. Clopidogrel failure matches with the CYP2C19 loss-of-function allele. Capecitabine toxicity matches with dihydropyrimidine dehydrogenase (pyrimidine catabolic) deficiency. Codeine toxicity in the infant matches with CYP2D6 gene duplication (ultrarapid metabolizer).
Each clinical scenario precisely matches a well-characterized pharmacogenomic mechanism. UGT1A1*28 (a promoter repeat expansion) decreases glucuronidation of irinotecan's active metabolite. CYP2C19 loss-of-function alleles prevent the phase I oxidation necessary to activate clopidogrel. DPYD mutations severely impair the pyrimidine catabolism required to clear capecitabine/5-FU. Finally, CYP2D6 gene duplications create an ultrarapid metabolizer phenotype that dangerously accelerates codeine's conversion to morphine.

Step-by-Step Solution

1
Determine if the primary drug in the clinical scenario is administered as an active drug or a prodrug.
Irinotecan and capecitabine act via active metabolites that must be cleared, whereas clopidogrel and codeine are prodrugs that must be activated.
Understanding the pharmacologic state of the drug dictates whether toxicity is caused by impaired clearance or hyper-activation.
2
Identify the specific metabolic pathway and enzyme associated with each drug.
Irinotecan uses UGT1A1 (conjugation); Clopidogrel uses CYP2C19 (oxidation); Capecitabine/5-FU uses DPD (pyrimidine catabolism); Codeine uses CYP2D6 (O-demethylation).
Matching the drug to its precise molecular clearance or activation pathway isolates the correct genetic defect.
3
Link the clinical outcome (toxicity vs. therapeutic failure) to the molecular effect of the genetic variant.
Failure to activate clopidogrel leads to stent thrombosis (loss-of-function allele). Over-activation of codeine leads to infant respiratory depression (gene duplication). Failure to clear SN-38 (promoter repeat expansion) or 5-FU (catabolic enzyme deficiency) leads to direct chemotherapy toxicity.
This finalizes the mechanistic pairs based on pharmacodynamic consequences.

Key Concept

Pharmacogenomics: Genetic variations in drug-metabolizing enzymes dictate clinical outcomes, where loss-of-function mutations cause toxicity for active drugs but therapeutic failure for prodrugs, and gain-of-function mutations cause toxicity for prodrugs.
Estimated Time:1m 30s
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Topics

Genetic and Molecular Mechanisms

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