D-SPARK: A Clinical Trial of D-Serine for Modifying Parkinson's Disease Progression

Inclusion Criteria: * A clinical diagnosis of PD\* according to the clinically established MDS clinical diagnostic criteria for Parkinson's disease within 5 years. * \[¹²³I\]FP-CIT single photon emission CT (DaTscan) confirming dopaminergic nigrostriatal denervation. * Hoehn and Yahr score \< 3 at enrollment. * Optimal symptomatic PD treatment, not requiring adjustments, for at least 2 weeks. * Age ≥40 and ≤ 80 years at time of enrollment. Exclusion Criteria: * Dementia or neurodegenerative disorder other than PD at baseline visit. * Atypical parkinsonism (PSP, MSA, CBD vascular parkinsonism, or drug induced parkinsonism). * Any known monogenic cause of PD (GBA1 variation is accepted). * Any psychiatric disorder that would interfere with compliance in the study. * Any severe somatic illness that would make the individual unable to comply and participate in the study. * Use of D-serine supplementation within 90 days of enrolment. * Metabolic, neoplastic, or other physically or mentally debilitating disorder at baseline visit. * Active of planned pregnancy during trial period. * Cognitive impairment as measured by the Mini Mental Status Exam MMSE) \< 20. * Weight \< 45 kg. * Urinary albumin/creatinine ratio ≥ 20 mg/mmol at time of enrollment. * Participants will be excluded if they have CKD stage 3 or higher, defined as: * Estimated golumerular filtration rate (eGFR) \< 60 mL/min/1.73min\^2 at screening, calculated using the CKD-EPI 2021 creatinine equation.