What are the guidelines for using lamotrigine and Depakote (valproate) together?

Guidelines for Using Lamotrigine and Valproate Together

The combination of lamotrigine and valproate requires significant dose adjustments due to their bidirectional pharmacokinetic interaction, with valproate increasing lamotrigine levels by 165% and lamotrigine decreasing valproate levels by approximately 25%. 1

Pharmacokinetic Interaction

Effect of Valproate on Lamotrigine

• Valproate markedly increases lamotrigine's half-life from 26 to 70 hours (165% increase) 1
• This occurs through inhibition of lamotrigine's glucuronidation 2
• Consequence: Lamotrigine doses must be substantially reduced when used with valproate

Effect of Lamotrigine on Valproate

• Lamotrigine decreases valproate plasma concentrations by approximately 25% 2
• Increases valproate oral clearance from 7.2 to 9.0 ml/hr/kg 2
• Importantly, lamotrigine does not increase formation of hepatotoxic valproate metabolites 2

Dosing Recommendations

Starting Lamotrigine with Existing Valproate

1. Use much lower lamotrigine starting doses (typically 25 mg every other day)
2. Titrate more slowly than usual (over several weeks)
3. Target a lower maintenance dose (typically 50-200 mg/day) compared to lamotrigine monotherapy 1, 3

Starting Valproate with Existing Lamotrigine

1. Reduce lamotrigine dose by 50% when adding valproate
2. Monitor lamotrigine levels and adjust as needed
3. Be prepared for potential toxicity as lamotrigine levels rise 1

Converting from Valproate to Lamotrigine

A four-step algorithm has been validated for this transition 3:

1. Add lamotrigine at low dose and slowly titrate to 200 mg daily while maintaining valproate
2. Begin gradual valproate withdrawal over 6 weeks
3. Increase lamotrigine to 500 mg daily as valproate is discontinued
4. Monitor lamotrigine levels throughout to maintain stable concentrations

Safety Considerations

Rash Risk

• Serious skin reactions (Stevens-Johnson Syndrome and toxic epidermal necrolysis) have been reported with concomitant use 1
• Starting with very low lamotrigine doses when combined with valproate reduces rash risk to levels comparable to other combinations 4
• Rash incidence is approximately 13-14% but discontinuation is required in only 7-9% of cases 4

Other Adverse Effects

• Monitor for:
  • Early effects: Fatigue (12%), gastrointestinal symptoms (9%), dizziness, headache, insomnia (3% each) 4
  • Late-onset effects: Ataxia, vertigo, headache, and rare movement disorders like tics may occur 9 months to 2 years after final dose adjustment 5
  • These late effects typically resolve with small dose reductions 5

Monitoring Recommendations

1. Check lamotrigine and valproate levels at baseline and after each significant dose change
2. Monitor more frequently during the first 8 weeks of combination therapy
3. Assess for clinical signs of toxicity at each visit (ataxia, dizziness, diplopia, headache)
4. Perform periodic liver function tests
5. Evaluate for skin reactions, especially during the first 8 weeks of treatment

Clinical Applications

This combination is particularly useful in:

• Bipolar disorder where valproate alone is insufficient 6
• Epilepsy patients with mixed seizure types 7
• Cases where monotherapy with either agent provides partial but incomplete seizure control

Pitfalls to Avoid

1. Starting lamotrigine at standard doses when adding to valproate (will lead to toxicity)
2. Failing to reduce lamotrigine dose when adding valproate to an existing regimen
3. Overlooking late-onset adverse effects that can occur months or years after stable dosing
4. Discontinuing the combination prematurely due to minor side effects that could be managed with dose adjustments
5. Abrupt withdrawal of either medication (lamotrigine should be tapered over at least 4 weeks)

By following these guidelines, the lamotrigine-valproate combination can be used effectively and safely in appropriate patients.