Safinamide for Levodopa-induced Dyskinesia (PD-LID)

This will be a prospective, multi-center, randomized, double-blind, parallel group, placebo-controlled study, in participants with PD who are on a stable regimen of dopaminergic medication and have at least mild levodopa-induced dyskinesia. Eligible participants will be randomized to one of three treatment groups to receive adjunctive daily treatment with either safinamide 100 mg, safinamide 150 mg or placebo in a 1:1:1 ratio. Outcome will be assessed after 26 weeks of treatment.

Trial participation will be up to a maximum duration of 32 weeks and will comprise:

* Screening period (up to 4 weeks) for screening assessments;
* Two weeks titration period: participants randomized to the 100 mg study arm will receive 100 mg during Week 1 and throughout the rest of the study; participants randomized to the 150 mg study arm will receive 100 mg during Weeks 1 and 2 , and 150 mg from Week 3 and throughout the rest of the study; participants randomized to the placebo arm will receive identical placebo tablets.
* Twenty-four weeks maintenance period during which patients receive their randomized treatment as an adjunct to their standard anti-PD medications, which should remain unaltered. End of treatment evaluations will be performed at the end of Week 26 or at early discontinuation
* A telephone follow-up call will be performed 2 weeks after the end of treatment to assess adverse events and concomitant medications

Eligibility Criteria:
Inclusion Criteria:

1. Male and female participants aged 30 years and above at the time of signing the informed consent;
2. Female participants may participate if they are not of child bearing potential (post-menopausal with no periods for at least one year, or surgically sterilized);
3. Women of child bearing potential (WOCBP) may participate if they are not pregnant, not breastfeeding, and agree to follow the contraceptive guidance during the treatment period and for at least 30 days after the last dose of study intervention;
4. A diagnosis of Parkinson's disease consistent with the UK PD Brain Bank Society and MDS Clinical Diagnostic Criteria;
5. Levodopa immediate release and/or controlled release, given at least 3 times daily, on a stable regimen for at least 4 weeks prior to screening;
6. Participants may also be taking benserazide, catcechol O methyl transferease (COMT) inhibitors and dopamine agonists, the dose of which must be stable for at least 4 weeks prior to screening;
7. Predictable peak dose dyskinesia of at least mild severity and causing at least mild disability as defined as a score of ≥2 on the MDS UPDRS questions 4.1 and 4.2 at screening and at least two 30 minute periods of ON time with troublesome dyskinesia recorded in 24 hour PD home diaries performed in each of the two days prior to the randomization visit;
8. Participants with motor fluctuations with at least three 30 minute periods in OFF (for a total of 1.5 hours per waking day) in each of the two consecutive 24 hour home diaries completed at the end of the screening period in the 48 hours prior to Day 1
9. Provided written informed consent as described in Appendix 1 which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol, prior to the study screening procedures commencing.

Exclusion Criteria:

1. Participants with secondary or atypical parkinsonian syndrome
2. Participants with solely diphasic dyskinesia without peak dose dyskinesia
3. History of neurosurgical procedure for Parkinson's disease, including stereotactic surgery and Deep Brain Stimulation (DBS).
4. Treatment with monoamine oxidase inhibitors, amantadine pethidine, dextromethorphan, fluoxetine, fluvoxamine in the 8 weeks prior to the screening visit
5. Concomitant use of selective serotonin reuptake inhibitors (SSRIs), serotonin norepinephrine reuptake inhibitors (SNRIs), tricyclic/tetracyclic antidepressants may be permitted but used at the lowest doses.
6. Participants who are unable to complete the home diary and have 2 consecutive 24-hour periods with more than 4 missing periods per 24 hours in the diary, completed at the end of the screening period, in the 48 hours prior to Day 1
7. History of major psychiatric disease eg bipolar disorder, severe depression, schizophrenia or other psychosis.
8. Current history of Impulse Control Disorder
9. History of drug and/or alcohol abuse within 12 months prior to screening (DSM-V criteria)
10. History of dementia (DSM-V criteria) or cognitive impairment MMSE \< 24 at screening
11. Ophthalmologic history of the following conditions: albinism, uveitis, retinitis pigmentosa, retinal degeneration, active retinopathy, severe progressive diabetic retinopathy, inherited retinopathy or family history of hereditary retinal disease.
12. Moderate or severe hepatic impairment with transaminases \>2 times upper limit of normal (ULN) or bilirubin 1.5 times ULN
13. Any clinically significant or unstable medical or surgical condition that, in the opinion of the investigator, might preclude safe completion of the study or might affect the results of the study.
14. Use of any investigational drug within 30 days prior to screening or 5 half-lives, whichever is the longest.
15. Allergy/sensitivity or contraindications to the investigational medicinal product (IMP) or their excipients.