Lacosamide Concentration in Various Biological Matrices (Plasma, Saliva, and Hair) Does Not Correlate with Clinical Outcomes in Patients with Epilepsy.

BACKGROUND: Comparing drug concentrations in blood, saliva, and hair may provide important insights into pharmacokinetics. Salivary drug levels may serve as a substitute for blood concentrations, while drug content in hair can reflect long-term exposure and allow reconstruction of treatment history. The study investigated the relationship between lacosamide concentrations in plasma, saliva, and hair, the number of seizures during the previous 3 months, and treatment adherence. METHODS: An observational, retrospective study was conducted between 2020 and 2022 to evaluate the correlation between plasma, saliva, and hair lacosamide concentrations, seizure frequency in the preceding 3 months, and patient adherence. Lacosamide concentrations in plasma, saliva, and hair were determined using liquid chromatography-tandem mass spectrometry in the positive ionization mode. The study group consisted of patients with epilepsy treated at the Institute of Psychiatry and Neurology in Warsaw, Poland. RESULTS: A total of 39 patients with epilepsy were included. Neither hair nor plasma lacosamide concentrations correlated with the number of seizures in the previous 3 months. Unexpectedly, patients who reported regular medication intake had significantly lower lacosamide concentrations in hair (8.1 ± 2.90 ng/mg) compared with those who did not adhere to fixed dosing times (43.00 ± 14.76 ng/mg). A strong correlation was observed between plasma and saliva lacosamide concentrations (r = 0.85, P < 0.05), indicating that saliva may serve as an effective alternative matrix for monitoring lacosamide levels. CONCLUSIONS: Lacosamide concentrations in blood, saliva, or hair do not correlate with seizure frequency and are therefore not reliable markers of adherence. However, salivary lacosamide measurement may offer a practical alternative to plasma monitoring.