Pharmacogenomics and Drug Interactions

## From One-Size-Fits-All to Precision Dosing

Pharmacogenomics (PGx) examines how inherited genetic variation affects drug response. When a patient's genotype alters enzyme activity, the impact is functionally equivalent to a permanent drug interaction — a CYP2D6 poor metabolizer behaves as if they are taking a potent CYP2D6 inhibitor at all times.

## CPIC Guidelines

The Clinical Pharmacogenetics Implementation Consortium (CPIC) publishes evidence-based guidelines for genotype-guided prescribing. Key gene-drug pairs with strong evidence include:

| Gene | Drug(s) | Clinical Impact |
|------|---------|----------------|
| CYP2D6 | Codeine, tramadol, tamoxifen | Prodrug activation failure or toxicity |
| CYP2C19 | Clopidogrel, voriconazole, PPIs | Antiplatelet failure or antifungal toxicity |
| CYP2C9 + VKORC1 | Warfarin | Bleeding or inadequate anticoagulation |
| DPYD | Fluoropyrimidines (5-FU, capecitabine) | Fatal toxicity in deficient patients |
| TPMT/NUDT15 | Thiopurines (azathioprine, 6-MP) | Severe myelosuppression |
| HLA-B*57:01 | Abacavir | Hypersensitivity reaction |
| HLA-B*15:02 | Carbamazepine | Stevens-Johnson syndrome (SE Asian ancestry) |
| UGT1A1 | Irinotecan | Severe neutropenia and diarrhea |

## Gene-Drug-Drug Interactions

The most complex scenarios arise when genetic variation and drug interactions co-occur. A CYP2D6 intermediate metabolizer who takes fluoxetine (CYP2D6 inhibitor) becomes a phenotypic poor metabolizer. If this patient is also on tamoxifen, endoxifen formation is virtually eliminated.

### Clinical Example: Triple Interaction

Patient on codeine for pain, paroxetine for depression, genotype CYP2D6 *1/*4 (intermediate metabolizer):
- Genetic: reduced CYP2D6 function (IM)
- Drug inhibition: paroxetine further blocks residual CYP2D6
- Result: negligible morphine formation, complete analgesic failure

Without PGx testing, the prescriber might increase the codeine dose (futile) rather than switching to a non-CYP2D6-dependent analgesic.

## Pre-emptive vs. Reactive Testing

**Reactive testing**: ordered after an adverse event or treatment failure. Useful but delayed.

**Pre-emptive testing**: multi-gene panel (typically CYP2D6, CYP2C19, CYP2C9, VKORC1, DPYD, TPMT, HLA-B) performed before prescribing. Results stored in the EHR with clinical decision support alerts. Cost-effectiveness is supported by studies showing:
- 30% reduction in adverse drug reactions
- $4,000-6,000 savings per patient over 5 years in high-risk populations
- Particular value in psychiatry (antidepressant selection), cardiology (clopidogrel), and oncology (fluoropyrimidines)

## Ethnic Variation in Allele Frequencies

Allele frequencies vary dramatically by ancestry:

- **CYP2D6 PM**: 5-10% Caucasian, 1-2% East Asian, 0-5% African
- **CYP2D6 UM**: 1-2% Caucasian, 0.5% East Asian, 20-30% East African/Middle Eastern
- **CYP2C19 PM**: 2-5% Caucasian, 12-23% East Asian
- **HLA-B*15:02**: 2-8% Southeast Asian, rare in Caucasians

These differences explain population-level variation in drug response and interaction severity.

## Implementation Barriers

Despite strong evidence, PGx adoption faces challenges: variable insurance coverage, limited clinician training, lack of integrated CDS in many EHRs, and turnaround time for results. Initiatives like the IGNITE Network and CPIC are accelerating adoption.

## Key Takeaways

- A genetic poor metabolizer phenotype is functionally equivalent to taking a potent enzyme inhibitor permanently
- CPIC provides evidence-based guidelines for over 20 gene-drug pairs
- Gene-drug-drug interactions create compounded risk that PGx testing can predict
- Pre-emptive multi-gene panels are cost-effective in high-risk populations
- Allele frequencies vary significantly by ancestry; CYP2D6 UM is common in East African populations
- DPYD testing before fluoropyrimidine chemotherapy prevents potentially fatal toxicity