CYP2C19 Interactions

## CYP2C19 Overview

CYP2C19 metabolizes approximately 10% of commonly prescribed drugs, including proton pump inhibitors (PPIs), the antiplatelet prodrug clopidogrel, several antidepressants (escitalopram, citalopram, sertraline), the antifungal voriconazole, and the antiepileptic phenytoin. Like CYP2D6, CYP2C19 exhibits clinically important genetic polymorphisms.

## Genetic Variation

The CYP2C19*2 and *3 loss-of-function alleles are common, with striking ethnic variation:

- **Poor metabolizers**: 2-5% of Caucasians, 12-23% of East Asians
- **Intermediate metabolizers**: carry one loss-of-function allele
- **Rapid and ultrarapid metabolizers**: CYP2C19*17 gain-of-function allele increases transcription; 20-30% of Caucasians carry at least one copy

## The Clopidogrel Problem

Clopidogrel is a prodrug requiring CYP2C19-mediated bioactivation to its active thiol metabolite. Poor metabolizers generate 25-30% less active metabolite, resulting in higher rates of cardiovascular events including stent thrombosis. The FDA issued a boxed warning in 2010 recommending consideration of alternative antiplatelet agents in CYP2C19 poor metabolizers.

CPIC guidelines recommend prasugrel or ticagrelor instead of clopidogrel for poor and intermediate metabolizers undergoing percutaneous coronary intervention. Many institutions now perform pre-procedure CYP2C19 genotyping.

## PPI Metabolism

All PPIs are CYP2C19 substrates, though to varying degrees. Omeprazole and lansoprazole are most affected. Poor metabolizers achieve 3-5 fold higher PPI plasma levels and better acid suppression, while ultrarapid metabolizers may have inadequate acid control at standard doses. For H. pylori eradication, CYP2C19 poor metabolizers achieve higher eradication rates, while ultrarapid metabolizers may need higher PPI doses.

## Key Inhibitors and Inducers

**Inhibitors:**
- **Omeprazole and esomeprazole** — moderate CYP2C19 inhibitors; the omeprazole-clopidogrel interaction was the subject of an FDA safety communication, though clinical significance remains debated
- **Fluoxetine and fluvoxamine** — strong CYP2C19 inhibitors
- **Fluconazole** — potent inhibition at standard doses

**Inducers:**
- **Rifampin** — strong inducer via PXR activation
- **Carbamazepine** and **St. John's wort** — moderate inducers

## Clinical Scenarios

Combining fluvoxamine with clopidogrel simultaneously inhibits CYP2C19 and reduces antiplatelet effect. Voriconazole dosing requires adjustment based on CYP2C19 genotype; poor metabolizers achieve 4-fold higher exposure and increased hepatotoxicity risk.

## Key Takeaways

- CYP2C19 poor metabolizer frequency varies from 2-5% (Caucasians) to 12-23% (East Asians)
- Clopidogrel requires CYP2C19 activation; poor metabolizers face increased stent thrombosis risk
- The FDA recommends alternative antiplatelets for CYP2C19 poor metabolizers
- Omeprazole inhibits CYP2C19 and may reduce clopidogrel efficacy
- Pharmacogenomic testing guides clopidogrel, voriconazole, and certain antidepressant therapy