Therapeutic Drug Monitoring of Lacosamide: Is 10 to 20 mg/L a Suitable Reference Range for Patients With Epilepsy?

The reference range for lacosamide (LCM) has been updated from 1 to 10 mg/L to 10 to 20 mg/L. Historically, LCM range was defined from trough-level measurements, but the newer ranges were obtained from peak-level measurements. The purpose of the study was to evaluate the relationship between LCM plasma levels higher than 10 mg/L and the incidence of adverse effects.This was a single-center, retrospective, observational study of adult outpatients with epilepsy who were prescribed LCM and had LCM serum concentrations (LCM-SCs)10 mg/L on drug-fasting samples, measured from June 2017 to December 2020.A total of 55 LCM-SC samples corresponding to 44 patients (25 women [57%]) were analyzed. The median age was 47 (39-61) years. The median LCM-SC was 13.4 (11.2-17.8) mg/L. Adverse effects were reported in 18 patients (41%). Forty-eight percent (21 of 44) of patients required an LCM dose reduction, with a mean LCM-SC of 16.0 (13.2-18.1) mg/L, whereas, in the remaining patients (23 of 44), LCM dose was not modified, with a mean LCM-SC of 12.2 (10.7-14.2) mg/L (P = 0.0244). Forty-one percent (18 of 44) of patients reported adverse effects related to LCM, with a mean LCM-SC of 15.6 (12.7-18.4) mg/L, whereas, in the remaining patients (26 of 44), adverse effects did not occur, with a mean LCM-SC of 12.6 (10.7-16.5) mg/L (P = 0.0495).The 10 to 20 mg/L reference range clearly increases toxicity in patients treated with LCM. Adjusting the reference range upper limit to 12 mg/L with a routine therapeutic drug monitoring program is suggested, to achieve a reasonable probability of efficacy and decrease toxicity.