Is Viibryd (vilazodone) considered a selective serotonin reuptake inhibitor (SSRI)?

Is Viibryd (Vilazodone) Considered an SSRI?

Yes, vilazodone (Viibryd) is considered a selective serotonin reuptake inhibitor (SSRI) with the additional mechanism of 5-HT1A receptor partial agonism, making it a unique dual-action antidepressant. 1

Pharmacological Classification and Mechanism

Vilazodone has a distinct pharmacological profile that combines two mechanisms of action:

1. Primary SSRI Activity:

   • Selectively inhibits serotonin reuptake with high affinity (Ki = 0.1 nM) 1
   • Potently and selectively inhibits reuptake of serotonin (IC50 = 1.6 nM) 1
   • Does not significantly bind to norepinephrine (Ki = 56 nM) or dopamine (Ki = 37 nM) reuptake sites 1

2. Additional 5-HT1A Receptor Activity:

   • Functions as a partial agonist at serotonergic 5-HT1A receptors (IC50 = 2.1 nM) 1
   • This additional mechanism differentiates it from traditional SSRIs 2

Official Classification

The FDA label and clinical guidelines consistently classify vilazodone as an SSRI:

• The American Academy of Child and Adolescent Psychiatry (AACAP) explicitly lists vilazodone among medications in the SSRI class currently marketed in the United States, alongside citalopram, escitalopram, fluoxetine, fluvoxamine, paroxetine, and sertraline 3

• The FDA-approved drug label describes vilazodone as "a selective serotonin reuptake inhibitor and a 5HT1A receptor partial agonist" 1

Clinical Implications of Dual Mechanism

The dual mechanism of vilazodone has several clinical implications:

• Serotonin Syndrome Risk: Like other SSRIs, vilazodone carries a risk of serotonin syndrome, particularly in overdose or when combined with other serotonergic medications 4, 5

• Contraindications: Similar to other SSRIs, vilazodone is contraindicated with MAOIs due to risk of serotonin syndrome 1

• Bleeding Risk: Like other SSRIs, vilazodone increases the risk of bleeding events 1

• Discontinuation Syndrome: Gradual tapering is recommended when discontinuing vilazodone to avoid withdrawal symptoms, similar to other SSRIs 1

Unique Characteristics

Despite being classified as an SSRI, vilazodone has some distinguishing features:

• Novel Mechanism: It was specifically designed to combine SSRI activity with 5-HT1A partial agonism to potentially enhance therapeutic efficacy 6

• Dosing Requirements: Unlike some other SSRIs, vilazodone must be administered with food to optimize bioavailability 2

• Metabolism: Vilazodone is extensively metabolized through CYP pathways (primarily CYP3A4) and non-CYP pathways 1

Clinical Considerations

When prescribing vilazodone, clinicians should be aware that:

• It requires titration starting at 10 mg daily for 7 days, followed by 20 mg daily for 7 days, before reaching the target dose of 40 mg daily 1

• It should be taken with food to ensure adequate absorption 1

• Like other SSRIs, it carries a boxed warning regarding increased risk of suicidal thoughts and behaviors in pediatric and young adult patients 1

In summary, while vilazodone has a unique dual mechanism that distinguishes it from traditional SSRIs, it is officially classified as an SSRI with additional 5-HT1A receptor partial agonist properties in both FDA labeling and clinical guidelines.