Dose Ranging Study of the Efficacy and Tolerability of Tonabersat in the Prophylaxis of Migraine Headache

Primary objective: To investigate the efficacy and tolerability of two doses of tonabersat compared to placebo in the prophylaxis of migraine headache and to evaluate the longer term tolerability of tonabersat in an open label extension. Secondary objective(s): To obtain further data on the efficacy and dose response of tonabersat; To extend the safety and tolerability database of tonabersat; To obtain data on the pharmacokinetics of tonabersat.