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

The study design is a randomized, double-blind, placebo-controlled trial. The trial consists of 3 stages followed by a washout period. * Screening and antiparkinsonian treatment optimization: \--- Potential participants will be screened for eligibility and consented for participation. Treatment of Parkinson's disease with dopaminergic drugs will be initiated/adjusted until an optimal, stable effect of treatment is established. This treatment regimen will be maintained throughout the first 32 weeks of the intervention stage, after which changes will be allowed. If an optimal, stable dose is not achieved during screening, these participants will not proceed further and will not be included in the study. * Intervention stage: \--- Participants will undergo randomization and will be assigned to receive either placebo or D-serine during different portions of the intervention phase. Participants will receive study drug (placebo or D-serine) for a total of 58 weeks. Thirty-two weeks after starting study drug, participants may have their dopaminergic drugs adjusted, if necessary. * Washout stage: * Upon completion of the intervention period, participants will discontinue study drug and will be followed for an additional 12 weeks. A final study visit will occur 12 weeks after discontinuation of study drug.