Key Inclusion Criteria: 1. Diagnosed with bilateral idiopathic PD 2. Males/females between 30 and 70 years at the time of surgery 3. Unified Parkinson's Disease Rating Scale (UPDRS) (Part III) score of between 30 and 60 in the "OFF" medication state 4. Presence of motor fluctuations and/or dyskinetic movement 5. Candidate for surgical intervention 6. Hoehn and Yahr (H\&Y) Stage 3 or 4 in the "OFF" medication state 7. Stable dosing of PD medication, including L-DOPA, for four weeks prior to screening with Levodopa equivalent daily dose (LEDD) of at least 900 mg Key Exclusion Criteria: 1. History of psychosis or current treatment with dopamine blocking agents and prior regular exposure to antipsychotic agents 2. History of stereotactic or other surgery for the treatment of PD, including Deep Brain Stimulation (DBS) 3. Participation in a prior cell or gene transfer therapy study 4. Contraindications to use of anaesthesia 5. Current or anticipated treatment with anticoagulant therapy or the use of anticoagulation therapy that cannot be temporarily stopped around the time of surgery 6. Diagnosis of multiple system atrophy 7. Abnormal MRI findings such as mega cisterna, septum pellucidum, signs of severe cortical or subcortical atrophy, brain tumours, vascular diseases, trauma or arteriovenous malformations 8. Presence of dementia