Exenatide Treatment in Parkinson's Disease

Inclusion Criteria: * Diagnosis of clinically probable Parkinson's disease according to the MDS clinical diagnostic criteria for PD * Males or Females * Hoehn and Yahr stage ≤ 2 in the ON medication state. This implies that all patients will be mobile without assistance during their best "ON" medication periods. * Patients are on levodopa treatment. * No need for extended treatment adjustment, no significant motor fluctuations during the last year. * All patients will be ≥25 and ≤80 years of age * Ability to self-administer, or to arrange carer administration of the trial drug. * Signed informed consent to participate in the trial. Exclusion Criteria: * Atypical or other causes of parkinsonism. Patients with suspected Multiple System Atrophy, Progressive Supranuclear Palsy, drug-induced parkinsonism, vascular parkinsonism, dystonic or essential tremor will not be included in the trial. * Prior intra-cerebral surgical intervention for Parkinson's disease. Patients who have previously undergone Deep Brain Stimulation, intra-cerebral administration of growth factors, gene therapy or cell therapies will not be eligible. * Already actively participating in a trial of a device, drug or surgical treatment for Parkinson's disease. * Previous exposure to Exenatide. * Known abnormality on CT or MRI brain imaging considered likely to compromise compliance with trial protocol/FDG-PET acquisition. * Patients with body mass index below 18.5. Exenatide causes weight loss, and individuals with already low BMI will not be eligible. * Patients with diabetes mellitus type 1. * Patients with prediabetes (HbA1c at screening 42-47 mmol/mol), or T2DM (known diagnosis, ongoing antidiabetic treatment or HbA1c \> 47 mmol-mol and fasting plasma glucose \> 7.0 mmol/L at screening). * History of pancreatitis. Baseline serum amylase value should be within the laboratory normal range +/- 20 percent. * Severe gastrointestinal disease including gastroparesis. * History of alcoholism. * History of severe cardiac disease. * History of pancreas cancer. * History or suspicion of thyroid cancer. Undiagnosed neck lump, hoarse voice, or difficulty swallowing not attributable to PD. * Personal or family history of medullary thyroid cancer. * Patients with Multiple Endocrine Neoplasia 2 (MEN2) syndrome. * End stage renal disease or creatinine clearance \< 50 ml/min. * Hyperlipidaemia. A lipid profile will be tested at the screening visit. Cholesterol or Triglyceride levels greater than 2 x the upper limit of normal will raise suspicion of a familial or acquired hyperlipidaemia and will prompt referral to a relevant specialist for investigation and treatment. * Concurrent treatment with warfarin. * Concurrent severe depression, defined as MADRS score 16. * Concurrent dementia, defined as Mini-Mental State Examination (MMSE) \< 22. * Pregnancy and Breastfeeding. * There are no safety data regarding Exenatide use in pregnancy. Women of childbearing potential should only be included after a confirmed menstrual period and a negative highly sensitive urine or serum pregnancy test, and will be asked at each visit to confirm regular use of an effective method of contraception. Those who are unwilling or unable to use an acceptable method to avoid pregnancy for the entire study period and until the end of the relevant systemic exposure period (i.e. 10 weeks after the last dose of study drug) will not be eligible. The following birth control methods are considered to be acceptable: combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation (oral, intravaginal or transdermal), progestogen-only hormonal contraception associated with inhibition of ovulation (oral, intravaginal or transdermal), intrauterine device (IUD), intrauterine hormone-releasing system (IUS), bilateral tubal occlusion, vasectomised partner, and sexual abstinence. * Known hypersensitivity or allergy or intolerance to GLP-1. * Known hypersensitivity to Exenatide or any of its excipients. * Potential participants who lack the capacity to give informed consent * Any medical, psychiatric or other condition which in the investigator's opinion compromises the potential participant's ability to participate in the trial.