BackgroundLithium has a wide range of neuroprotective actions, has been effective in Parkinson’s disease (PD) animal models and may account for the decreased risk of PD in smokers.MethodsThis open-label pilot clinical trial randomized 16 PD patients to “high-dose” (n= 5, lithium carbonate titrated to achieve serum level of 0.4–0.5 mmol/L), “medium-dose” (n= 6, 45 mg/day lithium aspartate) or “low-dose” (n= 5, 15 mg/day lithium aspartate) lithium therapy for 24-weeks. Peripheral blood mononuclear cell (PBMC) mRNA expression of nuclear receptor-related-1 (Nurr1) and superoxide dismutase-1 (SOD1) were assessed by qPCR in addition to other PD therapeutic targets. Two patients from each group received multi-shell diffusion MRI scans to assess for free water (FW) changes in the dorsomedial nucleus of the thalamus and nucleus basalis of Meynert, which reflect cognitive decline in PD, and the posterior substantia nigra, which reflects motor decline in PD.ResultsTwo of the six patients receiving medium-dose lithium therapy withdrew due to side effects. Medium-dose lithium therapy was associated with the greatest numerical increases in PBMC Nurr1 and SOD1 expression (679% and 127%, respectively). Also, medium-dose lithium therapy was the only dosage associated with mean numerical decreases in brain FW in all three regions of interest, which is the opposite of the known longitudinal FW changes in PD.ConclusionMedium-dose lithium aspartate therapy was associated with engagement of blood-based therapeutic targets and improvements in MRI disease-progression biomarkers but was poorly tolerated in 33% of patients. Further PD clinical research is merited examining lithium’s tolerability, effects on biomarkers and potential disease-modifying effects.