Pharmacogenomics

Variation in drug response is often due to genetic polymorphisms in drug-metabolizing enzymes, transporters, or receptors. The most well-known family of enzymes is the Cytochrome P450 (CYP) system. Polymorphisms in genes like CYP2D6, CYP2C9, and CYP2C19 can categorize patients into four phenotypes: Poor Metabolizers (PM), Intermediate Metabolizers (IM), Normal Metabolizers (NM), and Ultrarapid Metabolizers (UM). For a 'prodrug' (a drug that needs activation, like Codeine), a Poor Metabolizer will get no pain relief because they cannot convert it to morphine, while an Ultrarapid Metabolizer may experience toxicity. Conversely, for an active drug metabolised to an inactive form, PMs are at risk of toxicity and UMs may find the drug ineffective. Beyond metabolism, pharmacogenomics also involves immune-mediated reactions. For example, the HLA-B*57:01 allele is strongly associated with a severe hypersensitivity reaction to the HIV drug Abacavir. Similarly, variations in the TPMT gene predict the risk of life-threatening bone marrow suppression when using Thiopurines (Azathioprine). Integrating genetic data into prescribing helps transition from a 'one-size-fits-all' approach to 'precision prescribing.'

The molecular basis typically involves Single Nucleotide Polymorphisms (SNPs) in genes coding for Phase I (oxidative) or Phase II (conjugative) metabolic enzymes. CYP2D6, for example, is highly polymorphic, with over 100 known variants. Copy number variations (CNVs) can also occur; individuals with multiple copies of the CYP2D6 gene are ultrarapid metabolizers. In the case of Warfarin, response is governed by both a metabolic gene (CYP2C9) and a target gene (VKORC1, which codes for the vitamin K epoxide reductase enzyme). Individuals with specific VKORC1 variants are more sensitive to Warfarin and require lower starting doses. This demonstrates that pharmacogenomics can involve both pharmacokinetics (how the body handles the drug) and pharmacodynamics (how the drug affects the body).

Pharmacogenomics is increasingly appearing in clinical guidelines. The most common MLA-relevant applications are: 1. TPMT testing for Azathioprine/Mercaptopurine (to prevent myelosuppression). 2. HLA-B*57:01 for Abacavir (to prevent anaphylaxis). 3. DPYD for 5-FU/Capecitabine (to prevent severe gastrointestinal and hematological toxicity). 4. CYP2C19 for Clopidogrel (to ensure it is being activated to prevent stents from clotting). Identifying 'poor metabolizers' for Codeine explains why some patients find it useless for pain. Clinicians must realize that genetic testing can save lives by preventing predictable adverse drug reactions.

1. Genotyping for specific alleles (e.g., HLA-B*57:01). 2. Enzyme activity assays (e.g., measuring TPMT activity in red blood cells). 3. Multi-gene pharmacogenomic panels (becoming more common). 4. Therapeutic Drug Monitoring (TDM) often complements pharmacogenomic data.

Management based on results includes: 1. Dose reduction (e.g., lower Azathioprine dose if TPMT activity is low). 2. Drug avoidance (e.g., do not use Abacavir if HLA-B*57:01 positive). 3. Alternative selection (e.g., using Ticagrelor instead of Clopidogrel in poor metabolizers). 4. Standard management if the patient is a 'normal metabolizer.'