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

Inclusion Criteria: * A signed and dated written informed consent is obtained prior to participation. * Male or female patients between 18-65 years of age who are ambulatory and able to travel to the clinic; women of childbearing potential must be using a reliable form of contraception (contraceptive pill or double-barrier contraception - partner using condom and patient using spermicide, diaphragm, intra-uterine device or contraceptive sponge) for at least 2 months prior to enrolment and have a negative pregnancy test at screening. Women of childbearing potential must continue to practice birth control during and for at least two months after the study * Patients with an established history of migraine of at least 1 year with or without aura meeting the diagnostic criteria of the International Classification of Headache Disorders - Edition 2. * Patients should have experienced an average of at least 4 migraine attacks per month over the 3 months prior to entering the trial and at least 3 migraine attacks during the baseline period. Patients should report a maximum of 12 migraine headache days during the baseline period. Exclusion Criteria: * Patients with an onset of migraine according to the above criteria at age 50 years or more. * Patients who experience \> 12 headache days during the baseline period. * Patients who have failed to respond to adequate trials of 3 or more preventive medications. * Overuse of acute migraine treatments defined as ≥ 15 medication days per month of which no more than 9 days includes ergots or triptans. * Any woman who is pregnant, lactating or not using medically acceptable contraception. * Patients taking other medications used as prophylaxis for migraine including topiramate, methysergide, anti-spasticity agents (e.g. tizanidine) and new generation antipsychotics (e.g. olanzapine) currently or within 1 month prior to entry to the trial. * Patients taking any of the following medications: beta-blockers, tricyclic antidepressants, antiepileptic drugs, calcium channel blockers, or monoamine oxidase inhibitors during or within 1 month prior to the study; daily oral NSAIDs, daily paracetamol, high dose magnesium supplements (600 mg/day), daily multivitamin preparations containing more than 10 mg riboflavin, daily use of oral corticosteroids, herbal preparations (e.g. feverfew, butterwort and St John's Wort). Patients who have received parenteral administration of botulinum toxin within the previous 3 months will also be excluded. * Patients who, in the opinion of the Investigator, have significant cerebrovascular disease (e.g. transient ischemic attacks, stroke) or significant cardiovascular disease within 30 days prior to screening. * Patients suffering from any significant psychiatric disorder. * Patient has a concomitant disease or condition that, in the opinion of the Investigator, could interfere with the conduct of the study or could put the patient at unacceptable risk. * Patients with renal dysfunction, defined as a serum creatinine of greater then 2 mg/dL. * Patients with hepatic dysfunction defined as a liver function test (AST, ALT, alkaline phosphatase, bilirubin) of greater than twice the upper limit of normal for their age group. * Patients with known alcohol or other substance abuse. * Patient is a participating Investigator, sub-investigator, study coordinator, or employee of a participating Investigator, or is an immediate family member of the aforementioned. * Any factor, which in the opinion of the Investigator would jeopardize the evaluation or safety or be associated with poor adherence to the protocol. * The patient's primary care physician recommends the patient should not take part in the study.