Do I need to assess the serum level of Lacosamide (Vimpat) in this patient?

Therapeutic Drug Monitoring of Lacosamide

Routine serum level monitoring of lacosamide is not necessary for most patients, but should be considered in specific clinical situations including suspected non-adherence, lack of efficacy despite adequate dosing, renal dysfunction, dialysis, elderly patients, and those taking enzyme-inducing antiepileptic drugs.

When TDM is NOT Routinely Needed

Lacosamide has several pharmacokinetic properties that distinguish it from older antiepileptic drugs that require vigorous monitoring:

• Linear pharmacokinetics with predictable blood concentrations eliminate the need for routine monitoring seen with phenytoin or carbamazepine 1
• Minimal drug-drug interactions as a non-enzyme-inducing antiepileptic drug make it safer when combined with other medications 2
• Dose-linear relationship between lacosamide dose and plasma concentrations has been consistently demonstrated 1, 3

Specific Situations Where TDM Should Be Considered

1. Suspected Non-Adherence or Treatment Failure

• When combining clinical and TDM data, 37/38 responding patients had serum concentrations above 10 µmol/L (lower limit of reference range), while 16/17 patients with lower concentrations were non-responders 4
• This suggests a clear threshold effect where levels below 10 µmol/L are associated with treatment failure 4

2. Renal Dysfunction and Dialysis

• Lacosamide pharmacokinetics are significantly altered in severe renal dysfunction and in patients on dialysis 1
• TDM is particularly useful in these populations to ensure therapeutic levels 1

3. Drug Interactions with Enzyme-Inducing AEDs

• A significant drug interaction exists between lacosamide and enzyme-inducer AEDs (such as phenytoin, carbamazepine, phenobarbital) 1
• Patients taking these concomitant medications may require TDM to ensure adequate lacosamide levels 1

4. Extremes of Age

• Lacosamide pharmacokinetics may be altered in older adults, warranting consideration of TDM 1
• Mean serum concentration/dose ratios are generally consistent (0.06-0.07 µmol/L/mg) but individual variability exists 4

Reference Range Considerations

• No well-established reference range exists, with recommended ranges varying from 2.2 to 20 mg/L across different sources 1
• The most clinically relevant threshold appears to be >10 µmol/L for therapeutic response based on real-world data 4
• Dose of lacosamide is linearly correlated with both plasma concentrations and efficacy 1

Timing of Level Assessment

• Lacosamide should reach steady state after approximately 3 days of consistent dosing (based on its half-life)
• Levels can be drawn as trough levels (before next dose) for consistency, though the long half-life means timing is less critical than with some other AEDs

Common Pitfalls to Avoid

• Do not assume all patients require TDM - this is not a phenytoin-like drug requiring routine monitoring 1
• Do not ignore low levels in non-responders - levels below 10 µmol/L are strongly associated with treatment failure 4
• Do not overlook enzyme-inducing AED interactions - these significantly alter lacosamide pharmacokinetics and may necessitate dose adjustments 1
• Abrupt discontinuation based on levels alone can precipitate withdrawal seizures; clinical context is paramount 2, 5