Cytochrome P450 Polymorphisms - Enzyme Superfamily
- Heme-containing monooxygenases.
- Location: Primarily Endoplasmic Reticulum (microsomal); some in Mitochondria.
- Function: Key enzymes in Phase I drug metabolism.
- Catalyze reactions: Oxidation (most common), reduction, hydrolysis.
- Nomenclature: e.g., CYP2D6
- CYP: Cytochrome P450
- 2: Family (≥40% amino acid sequence homology)
- D: Subfamily (≥55% homology)
- 6: Individual gene.
⭐ CYP3A4 is the most abundant CYP enzyme in the human liver and metabolizes approximately 50% of clinically used drugs.
Cytochrome P450 Polymorphisms - Code Tweaks
Genetic variations in CYP genes alter drug metabolism. Key types:
- Single Nucleotide Polymorphisms (SNPs): Single base changes affecting enzyme activity.
- Copy Number Variations (CNVs): Gene duplications (↑ activity) or deletions (↓ activity).
Allele Nomenclature:
- *1: Wild-type (normal function).
- *2, *3, etc.: Variant alleles (altered function).
Metabolizer Phenotypes & Implications:
| Phenotype | Activity | Clinical Implication |
|---|---|---|
| Ultra-rapid (UM) | ↑↑ | Risk of therapeutic failure |
| Extensive (EM) | Normal | Expected drug response |
| Intermediate (IM) | ↓ | Risk of side effects, ↓ clearance |
| Poor (PM) | ↓↓/None | High risk of toxicity, minimal clearance |
⭐ Copy Number Variations (CNVs) of the CYP2D6 gene are a major cause of the ultra-rapid metabolizer (UM) phenotype.
Cytochrome P450 Polymorphisms - CYP All-Stars
Genetic variations in CYP enzymes significantly alter drug metabolism, affecting efficacy and toxicity.
| Enzyme | Key Variants | Common Substrates | Clinical Relevance of Polymorphisms |
|---|---|---|---|
| CYP2D6 | Non-functional: * extit{3},* extit{4},* extit{5} Reduced function: * extit{10},* extit{17} Increased function: Duplications | Codeine, Tamoxifen, β-blockers (metoprolol, carvedilol), Antidepressants (SSRIs, TCAs), Antipsychotics | Poor metabolizers (PMs): ↑ toxicity (e.g., TCAs) or ↓ efficacy (e.g., codeine, tamoxifen). Ultrarapid metabolizers (UMs): ↓ efficacy or ↑ active metabolite toxicity. |
| CYP2C19 | Non-functional: * extit{2},* extit{3} Increased function: * extit{17} | Clopidogrel, PPIs (omeprazole), Diazepam, Voriconazole | PMs: ↓ clopidogrel efficacy (↑ risk of stent thrombosis), ↑ PPI/diazepam exposure. UMs (*17/*17): ↑ clopidogrel active metabolite, ↓ PPI efficacy. |
| CYP2C9 | Reduced function: * extit{2},* extit{3} | Warfarin, Phenytoin, NSAIDs (ibuprofen, celecoxib), Losartan | PMs: ↑ warfarin/phenytoin exposure (↑ bleeding/toxicity risk). Requires lower doses. |
📌 Mnemonic for CYP2D6 substrates: Can Tom Beat All Antidepressants? (Codeine, Tamoxifen, Beta-blockers, Antipsychotics, Antidepressants).
Cytochrome P450 Polymorphisms - Dosing Dilemmas
- Genetic variations in CYP enzymes (e.g., CYP2D6, CYP2C19, CYP2C9) alter drug metabolism.
- Clinical Implications:
- Prodrug Activation: ↓ effect with Poor Metabolizers (PMs).
- Codeine (CYP2D6) → Morphine.
- Clopidogrel (CYP2C19) → Active form.
- Drug Toxicity: ↑ risk in PMs.
- Warfarin (CYP2C9).
- TCAs (CYP2D6).
- Therapeutic Failure: ↓ efficacy in PMs.
- Tamoxifen (CYP2D6) → Endoxifen.
- Prodrug Activation: ↓ effect with Poor Metabolizers (PMs).
- Dosing: Adjust based on genotype; Clinical Pharmacogenetics Implementation Consortium (CPIC) guidelines are key. Ethnic variations in allele frequencies impact population risk.
⭐ Patients with CYP2C19 loss-of-function alleles (e.g., *2, *3) exhibit diminished response to clopidogrel, leading to higher risk of major adverse cardiovascular events like stent thrombosis.
High‑Yield Points - ⚡ Biggest Takeaways
- CYP2D6 metabolizes many psychiatric drugs & opioids (codeine); polymorphisms affect efficacy/toxicity.
- CYP2C19 activates clopidogrel; poor metabolizers have ↑ stent thrombosis risk.
- CYP2C9 variants require ↓ warfarin dose due to ↑ bleeding risk.
- CYP3A4/5 metabolize ~50% of drugs; polymorphisms generally less critical.
- Inducers (rifampicin) ↑ CYP activity, ↓ drug levels; Inhibitors (ketoconazole) ↓ activity, ↑ drug levels.
- Genetic testing aids drug selection/dosing for narrow therapeutic index drugs.
- Ethnic variations in CYP alleles impact population drug responses.
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