Concentration-Effect and Concentration-Toxicity Relations with Lamotrigine: A Prospective Study

Summary: This prospective study was designed to ascertain whether measurement of lamotrigine (LTG) concentrations in the epilepsy clinic could be used to predict the onset of complete seizure control or the emergence of adverse effects. LTG was initiated in doses of 25 or 50 mg daily in 69 patients with newly diagnosed or poorly controlled epilepsy and was increased monthly in 50–mg increments until the patient became seizure-free for at least 6 months or developed adverse effects that abated after a reduction in dosage. LTG and other antiepileptic drug (AED) concentrations were measured at each clinic visit but were not supplied to the investigator examining the patients. Overall, 19 patients either withdrew due to lack of efficacy or defaulted from the clinic. Of the remaining 50 patients, 32 (19 monotherapy, 13 polytherapy) became seizure-free at widely varying daily LTG doses (median 200 mg, range 25–850 mg) and concentrations (median 3.8 mg/L, range 1.4–18.7 mg/L). Likewise, the 18 patients (5 monotherapy, 13 polytherapy) who experienced intolerable side effects showed substantial variations in daily LTG doses (median 300 mg, range 100–900 mg) and concentrations (median 4.0 mg/L, range 0.4–18.5 mg/L). No useful concentration-effect or concentration-toxicity relation with LTG could be demonstrated in this study; therefore, we believe that routine therapeutic drug monitoring with this new AED is not currently indicated.