Switching from Lamotrigine Immediate-Release to Extended-Release
Yes, you can immediately switch a patient from lamotrigine 250mg immediate-release (IR) daily to 250mg extended-release (XR) daily without dose adjustment. 1
Evidence for Direct Conversion
The pharmacokinetic evidence strongly supports a direct 1:1 conversion from lamotrigine IR to XR:
A crossover bioavailability study in elderly epilepsy patients demonstrated that lamotrigine IR and XR formulations are bioequivalent with respect to:
- Area under the concentration-time curve (AUC)
- Average steady-state concentration
- Trough concentration 1
The absolute bioavailability for lamotrigine IR and XR formulations was found to be 73% and 92%, respectively, indicating that the XR formulation may actually provide slightly better overall drug exposure 1
Benefits of Switching to Extended-Release
Converting to lamotrigine XR offers several advantages:
Reduced peak-related adverse effects: The XR formulation produces 15% lower peak concentrations compared to IR, which can reduce adverse effects like dizziness, blurred vision, and unsteadiness 2
Improved seizure control: A study of 55 patients converted from lamotrigine IR to XR showed a 46% median reduction in seizure frequency, with 47% of patients experiencing >50% reduction in seizures 2
Reduced concentration fluctuations: The XR formulation demonstrates 33% lower fluctuation in drug concentrations compared to IR, providing more stable levels throughout the day 1
Similar timing to peak concentration: While XR formulation delays time to peak concentration (3.0 vs 1.3 hours for IR), this doesn't affect the overall therapeutic effect 1
Important Considerations
When switching formulations, keep these points in mind:
Monitoring: While no dose adjustment is needed, it's prudent to monitor for any changes in efficacy or side effects in the weeks following the switch
Patient education: Inform the patient that the XR formulation must be swallowed whole and not chewed, crushed, or divided
Timing: Lamotrigine XR should be taken once daily, whereas the IR formulation may have been taken in divided doses
Missed doses: If a patient misses a dose of XR lamotrigine, they should take it as soon as they remember, but if it's close to the next scheduled dose, they should skip the missed dose and resume their regular schedule 3
Conclusion
The pharmacokinetic evidence clearly supports a direct 1:1 conversion from lamotrigine IR 250mg daily to lamotrigine XR 250mg daily. This switch may provide benefits of improved seizure control and reduced adverse effects due to more stable drug concentrations throughout the day.