Which antidepressants affect the CYP2C19 enzyme?

Antidepressants That Affect CYP2C19

The SSRIs that are metabolized by and/or inhibit CYP2C19 include fluoxetine, citalopram, escitalopram, and fluvoxamine, while among tricyclic antidepressants (TCAs), amitriptyline, clomipramine, doxepin, imipramine, and trimipramine are demethylated by CYP2C19 to active metabolites. 1

SSRIs and CYP2C19

Substrates (Metabolized by CYP2C19)

The following SSRIs are substrates of CYP2C19, meaning genetic polymorphisms in this enzyme directly affect their plasma concentrations:

• Citalopram and escitalopram: CYP2C19 poor metabolizers (PMs) have impaired elimination and higher drug levels 2
• Fluoxetine: Metabolized by CYP2C19, with PMs showing significantly reduced metabolic function and higher parent drug concentrations 3
• Fluvoxamine: Also a CYP2C19 substrate 3

Inhibitors of CYP2C19

Several SSRIs potently inhibit CYP2C19 activity, creating drug-drug interaction risks:

• Fluvoxamine: Potent CYP2C19 inhibitor despite not being metabolized by it 2
• Fluoxetine: Moderate to potent inhibitor, causing dose-dependent increases in mephenytoin S/R ratio (a CYP2C19 probe) 3
• Sertraline and paroxetine: Minimal CYP2C19 inhibition 3, 2

Tricyclic Antidepressants and CYP2C19

Five TCAs undergo CYP2C19-mediated demethylation to pharmacologically active metabolites 4:

1. Amitriptyline → nortriptyline
2. Clomipramine → desmethylclomipramine
3. Doxepin → desmethyldoxepin
4. Imipramine → desipramine
5. Trimipramine → desmethyltrimipramine

These drugs and their metabolites then undergo CYP2D6-mediated hydroxylation to less active compounds 4.

Clinical Implications by Metabolizer Status

For CYP2C19 Poor Metabolizers (PMs)

When prescribing fluoxetine, paroxetine, citalopram, escitalopram, or venlafaxine to PMs:

• Consider alternative medications metabolized by different enzymes rather than dose reduction 1
• Rationale: Easier switching than careful monitoring of lowered doses, and no clear efficacy differences among SSRIs for depression 1
• PMs experience higher parent drug levels, longer half-lives, and reduced clearance 5

For CYP2C19 Ultrarapid Metabolizers (UMs)

• May have subtherapeutic drug levels and reduced efficacy
• Consider higher doses or alternative agents 1

Important Drug Interaction Considerations

Amitriptyline and fluoxetine significantly inhibit CYP2C19 and CYP3A4, which can increase plasma concentrations of co-administered drugs by 44-77% 6. When combining these antidepressants with other CYP2C19 substrates, dose adjustments may be required.

Fluvoxamine is the most potent CYP2C19 inhibitor among SSRIs and causes clinically significant interactions with drugs like clozapine, olanzapine, theophylline, and propranolol 2.

Common Pitfalls to Avoid

• Don't assume all SSRIs have identical CYP2C19 interactions: Paroxetine and sertraline have minimal CYP2C19 effects compared to fluoxetine and fluvoxamine 3, 2
• Remember dual enzyme involvement for TCAs: Both CYP2C19 (demethylation) and CYP2D6 (hydroxylation) affect TCA metabolism 4
• Consider concurrent medications: Drug-drug interactions can phenoconvert normal metabolizers to functional PMs 1