Purpose: To determine whether neurophysiological effects of levetiracetam (LEV) outlast its serum half-life of approximately 7 h. Demonstration of prolonged effects would help to explain the efficacy of LEV at conventional dosing intervals that are longer than the serum half-life.
Methods: Following an oral dose of LEV 3 g in 12 normal volunteers, we compared transcranial magnetic stimulation (TMS) measures of motor threshold (MT) and recruitment with LEV serum levels and subjective ratings of toxicity over 48 h. Subjects used a two-dimensional visual-analog scale to estimate the time course of any side effects.
Results: LEV serum levels and subjective toxicity both peaked around 1 h after oral administration. MT elevation was delayed in comparison to peak serum levels and subjective toxicity. MT was maximally elevated at 6-9 h, and recruitment maximally reduced at 0.6-9 h. Changes in both measures had recovered by approximately 50% at 24 h. Despite the time difference between toxicity and TMS changes, toxicity estimates correlated with the maximum increase in MT.
Conclusion: There is a substantial time lag between LEV serum levels and TMS measures of neuronal effects, and a similar temporal dissociation between subjective toxicity and maximum TMS change. The time course of neurophysiological effects, as measured by TMS, may help to explain the sustained clinical efficacy of LEV despite a short peripheral half-life.