Patients will be randomised to receive either placebo or the study drug for a period of 30 weeks, in addition to their standard Parkinsonian medications. During the first 8 weeks, the patient's levodopa doses may be adjusted if necessary by the investigator. For the remainder of the 22 weeks, all medications should be kept stable. Patients will be required to attend the clinic twice during the screening period and then a further 8 times during the treatment period. They will be required to complete home diaries where they will record their motor function. In addition, their doctor will assess their Parkinson's disease during the clinic visits. There will also be blood draws for safety and pharmacokinetic and pharmacogenomics evaluation. Following the treatment and assessment period, they will return to the clinic one month later for follow up.