VIMPAT (Lacosamide) Level of 6.9 mcg/mL: Therapeutic Range Assessment
A lacosamide level of 6.9 mcg/mL is below the established therapeutic range and is likely subtherapeutic for seizure control.
Established Therapeutic Range
The available evidence does not provide specific therapeutic blood level ranges for lacosamide in the guideline literature reviewed. However, the clinical context suggests this level is inadequate:
- Lacosamide is approved as adjunctive therapy for partial-onset seizures with typical maintenance dosing of 200-400 mg/day divided twice daily 1
- Loading doses studied include 300 mg oral or IV administration, though specific therapeutic levels were not established in the guideline evidence 1
- The drug requires titration to therapeutic effect rather than targeting specific blood levels, unlike older antiepileptic drugs such as phenytoin (therapeutic range 10-20 mg/L) 2
Clinical Implications of This Level
At 6.9 mcg/mL, this patient is at increased risk for breakthrough seizures:
- Studies demonstrate lacosamide efficacy at doses of 200-600 mg/day, with 400 mg/day showing optimal balance of efficacy and tolerability 3, 4
- Seizure control requires adequate dosing maintained over at least 4 weeks to assess therapeutic response 1
- A level of 6.9 mcg/mL likely reflects either inadequate dosing, poor adherence, or recent initiation of therapy 3
Recommended Actions
Assess adherence first, as non-adherence is the most common cause of subtherapeutic levels:
- Verify the patient has taken ≥80% of prescribed doses through pill counts, dispensing records, and patient/caregiver report 1
- Consider that blood levels should be drawn without advance notice to avoid artificially elevated results from temporary improved adherence 1
If adherence is confirmed, dose escalation is warranted:
- Increase lacosamide dose incrementally toward the target maintenance range of 200-400 mg/day (divided twice daily) 1, 3
- Titrate based on clinical seizure control and tolerability rather than targeting a specific blood level 4
- Allow at least 4 weeks at each therapeutic dose to assess efficacy before further adjustments 1
Important Caveats
Cardiac monitoring is essential during dose escalation:
- Lacosamide can cause PR interval prolongation, atrioventricular blocks, bradyarrhythmias, and rarely cardiac arrest 1, 5
- Obtain baseline ECG before initiating therapy and monitor regularly during titration, especially in patients with cardiac history or those on other sodium channel blockers 5
- Contraindicated in patients with pre-existing atrioventricular block or significant cardiac conduction abnormalities 5
Drug interactions require attention:
- Concomitant use with other sodium channel blockers (carbamazepine, eslicarbazepine, phenytoin) increases risk of cardiac conduction abnormalities 5
- Abrupt discontinuation can precipitate withdrawal seizures 1
Common adverse effects to monitor include: