All patients will follow an identical visit schedule. Patients aged 12 years or over randomized to receive OGT 918 will commence treatment at a dose of 200 mg (2 x 100 mg capsules) OGT 918 three times daily. As some patients may experience initial gastrointestinal intolerance this dose may be modified, however the aim will be to maintain patients at the highest tolerable dose in order to achieve the most clinical benefit. Data analysis is planned, comparing OGT 918 to the no treatment group, when all patients have completed 12 months of the study. The study will be extended, up to a total of 12 additional months, to collect safety and efficacy data. The length of the extension study will be dependent on the results of the final analysis. All patients completing the main study, including those initially randomized to the no treatment group, will be able to participate in the extension study unless there are safety issues to prohibit this. All patients in the extension study will receive OGT 918.