From the FDA Drug Label
Felbamate Cmax and AUC are proportionate to dose after single and multiple doses over a range of 100 to 800 mg single doses and 1,200 to 3,600 mg daily doses. Cmin (trough) blood levels are also dose proportional.
Felbamate trough levels should be checked because Cmin (trough) blood levels are dose proportional, which allows for monitoring of the drug's plasma concentrations and adjustment of the dose as needed. 1
From the Research
Felbamate levels should be checked at trough, which is immediately before the next scheduled dose, to ensure the concentration remains within the therapeutic window and to minimize potential adverse effects. Trough sampling provides the most consistent and reliable measurement of felbamate concentration in the blood, allowing for proper dose adjustments and therapeutic monitoring. The typical therapeutic range for felbamate is not clearly established, but it is known that there is a concentration-efficacy and concentration-toxicity relationship 2. When collecting a trough level, the blood sample should be drawn just before the patient's next scheduled dose, after steady state has been achieved (typically after 5-7 days of consistent dosing). This timing is important because felbamate has a half-life of approximately 20-23 hours, and levels can fluctuate throughout the dosing interval. Monitoring trough levels helps ensure that the concentration remains within a safe and effective range, maximizing seizure control while minimizing potential adverse effects, such as aplastic anemia and hepatic failure, which are dose-independent but can be fatal 2.
Some key points to consider when monitoring felbamate levels include:
- Felbamate is well absorbed and has linear kinetics, with C(max) and AUC increasing linearly with dose 2
- The metabolism of felbamate takes place in the liver, with metabolites representing 40 to 60% of excretion and eliminated via the urine 2
- Clearance of felbamate is dependent on renal function, which should be taken into account when adjusting doses 2
- The therapeutic range for felbamate is not clearly established, but it is known that there is a concentration-efficacy and concentration-toxicity relationship 2
Overall, monitoring felbamate levels at trough is crucial to ensure the safe and effective use of this medication, particularly given its potential for serious side effects 3, 4, 5, 6.