Questions: Pharmacogenomics

Question 1

A patient is genotyped as a CYP2D6 ultrarapid metabolizer. They are prescribed codeine, a prodrug that CYP2D6 converts to morphine. What is the clinical concern?

• A. The patient will metabolize codeine too slowly, leading to no pain relief
• B. The patient will convert codeine to morphine much faster and in greater quantity than normal, risking morphine toxicity
• C. CYP2D6 does not affect codeine metabolism
• D. The patient will need a higher dose of codeine for adequate effect

Answer: (B)

Codeine is a prodrug that requires CYP2D6-mediated conversion to its active metabolite morphine. Ultrarapid metabolizers have multiple copies of functional CYP2D6 genes, producing excess enzyme activity. They convert codeine to morphine much faster and more completely than normal metabolizers, potentially achieving toxic morphine levels from standard doses. This has caused fatalities, particularly in children. CPIC guidelines recommend avoiding codeine in CYP2D6 ultrarapid metabolizers and using an alternative analgesic.

Question 2

Pharmacogenomic testing is only relevant for rare genetic disorders, not for commonly prescribed medications.

• T. True
• F. False

Answer: False

Pharmacogenomic variants are common in the general population. Roughly 7% of Caucasians are CYP2D6 poor metabolizers, 2-15% of various populations are CYP2C19 poor metabolizers, and nearly all individuals carry at least one actionable pharmacogenomic variant. CPIC provides guidelines for widely prescribed drugs including clopidogrel (antiplatelet), warfarin (anticoagulant), SSRIs (antidepressants), codeine (analgesic), and 5-fluorouracil (chemotherapy). Pharmacogenomics affects everyday prescribing, not just rare diseases.

Question 3

Explain why the same pharmacogenomic variant can have opposite clinical effects depending on whether the drug is an active compound or a prodrug.

Model answer: A drug-metabolizing enzyme variant (e.g., CYP2D6 poor metabolizer) reduces the enzyme's activity. For an active drug that the enzyme inactivates, reduced metabolism means higher and longer drug exposure, increasing the risk of side effects and toxicity — the patient effectively gets a higher dose than intended. For a prodrug that requires the enzyme to convert it to the active form, reduced metabolism means less active metabolite is produced, leading to therapeutic failure — the patient effectively gets a lower dose. The same genotype (poor metabolizer) causes toxicity in one case and inefficacy in the other, which is why the drug's metabolic pathway must be understood to interpret the genotype correctly.

This bidirectional effect is one of the most important concepts in pharmacogenomics. CYP2D6 poor metabolizers, for example, are at risk of toxicity from nortriptyline (active drug metabolized by CYP2D6) but at risk of therapeutic failure from codeine (prodrug activated by CYP2D6). The genotype is the same; the clinical recommendation depends on the drug.