Inclusion Criteria:  Patient is 30 - 85 years old (inclusive) with a diagnosis of idiopathic PD according to the UK Parkinson's Disease Society Brain Bank Clinical Diagnosis criteria. PD patient is stably and optimally treated with L-DOPA; other anti-PD treatments are allowed if used for at least 4 weeks of previous continuous treatment. Patient agrees to be challenged with 150% of their normal L-DOPA dose (maximum L-DOPA dose 250 mg) 30 minutes prior to efficacy assessments at baseline (Visit 2) and at the 2 efficacy clinic visits (Visits 6 and 7). PD patient exhibits troublesome peak-dose LID, confirmed by a score of at least 1 on part IV, item 33 (disability) of the UPDRS at screening (Visit 1) and at Day 1 (baseline, Visit 2). At least two 30-minute time periods from 9 am to 4 pm and at least 90 minutes total of each 24-hour period are indicated as "ON with troublesome dyskinesia" (according to the PD Home Dyskinesia Diary) prior to Day 1 (baseline, Visit 2). Patient (and/or caregiver) demonstrates ability to accurately complete the PD Home Dyskinesia Diary entries during the screening visit. Patient can read well enough to understand the informed consent document and other subject materials. Female patients of child-bearing potential must have a negative urine pregnancy test at screening (Visit 1) and on Day 1 (Visit 2), must agree to avoid pregnancy during the study, and must practice abstinence (only allowed when this is the preferred and usual lifestyle of the subject) or must agree to use a highly effective method of contraception with a failure rate of < 1% to prevent pregnancy (combined [oestrogen and progestogen containing] hormonal contraception associated with inhibition of ovulation [oral, intravaginal, transdermal], progestogen-only hormonal contraception associated with inhibition of ovulation [oral, injectable, implantable], intrauterine device [IUD] or intrauterine hormone-releasing system [IUS]) starting from 4 weeks prior to administration of the study drug and continuing during the course of the study until 4 weeks after last after IMP administration. Female subjects must agree to refrain from donating eggs from the date of dosing until 3 months after dosing with the IMP. Their male partner must agree to use a condom during the same time frame if he has not undergone vasectomy.  Females of non-childbearing potential are defined as pre-menopausal females who are sterilised (tubal ligation or permanent bilateral occlusion of fallopian tubes); or females who have undergone hysterectomy or bilateral oophorectomy; or post-menopausal defined as 12 months of amenorrhea (in questionable cases a blood sample with detection of follicle stimulating hormone [FSH] 25-140 IE/L is confirmatory).  Male patients must be either vasectomised, consent to use condom or practice sexual abstinence to prevent pregnancy and drug exposure of a partner and refrain from donating sperm from the date of dosing until 3 months after dosing with the IMP. Their female partner of child-bearing potential must use highly effective contraceptive methods with a failure rate of < 1% to prevent pregnancy (see above) during the same period.  Exclusion Criteria:  Patient has severe PD with a Hoehn and Yahr stage = 5. Patient has unstable medical status, prior brain surgery (excluding deep brain stimulation [DBS], i.e., DBS patients will be allowed to be enrolled) or is scheduled to receive surgery during the trial period. Patient has orthostatic hypotension: a decrease in systolic blood pressure (at least 20 mm Hg) or diastolic blood pressure (at least 10 mm Hg) within 2 minutes of the patient standing up, compared to pressures obtained while in a supine position for at least 5 minutes. At screening and baseline visits (Visit 1 and Visit 2), vital signs to assess orthostatic hypotension will be conducted in triplicate, 15-20 minutes apart, with the average of the 3 assessments used for exclusion. Patient has dementia (MMSE <20). Patient has clinically significant renal or liver disorder. Patient currently exhibits generalized obsessive-compulsive disorder, panic disorder, bipolar disorder, post-traumatic stress syndrome (PTSD), clinically significant parasomnias or any other psychotic disorder as established by structured clinical interview for DSM disorders (SCID). Visual hallucinations are allowed. Any suicidal actions in the past 2 years (per investigator judgement i.e. actual attempt, interrupted attempt, aborted attempt, or preparatory acts or behavior). Any suicidal ideation of type 4 or 5 in the C-SSRS in the past 3 months (i.e. active suicidal thought with intent but without specific plan, or active suicidal thought with plan and intent). Patient has taken an anti-convulsant, an anti-psychotic (except quetiapine), pindolol, tertatolol or buspirone within 4 weeks of baseline (Day 1, Visit 2). Patient has taken any medication, within 4 weeks of baseline (Day 1, Visit 2) that inhibits or up-regulates CYP4503A4. Patient is concurrently participating in another investigational drug trial or has participated in another investigational drug trial within the past 3 months. Patient is at high risk of non-compliance in the Investigator's opinion.