INTRODUCTION: Neuroprotective therapy to slow Parkinson's disease (PD) progression is a critical unmet need. Neuroinflammation likely represents an important pathophysiologic mechanism for disease progression. Medications that target this inflammation, such as immunosuppressants, represent potential disease-modifying therapies for PD. The relation between these medications and PD risk might inform candidate selection. METHODS: We conducted a population-based case-control study using Medicare data from the United States. The study included 207,532 incident PD cases and 975,177 controls from 2016 to 2018, age 67-110. We examined the association between PD risk and immunosuppressant use before PD diagnosis/control selection. We considered 37 immunosuppressants, representing >
10 medication classes, in Part D prescription claims. We used logistic regression to estimate the relative risk (RR) and 95 % confidence interval (CI) between each medication and PD, while accounting for age, sex, race/ethnicity, smoking, and healthcare utilization. In sensitivity analyses we applied exposure lagging, restricted to immunosuppressant users, and corrected for multiple comparisons. RESULTS: Medicare beneficiaries using the calcineurin inhibitor tacrolimus (RR 0.49, CI 0.40-0.60) and mTOR inhibitors everolimus (RR 0.38, CI 0.26-0.56) and sirolimus (RR 0.59, CI 0.37-0.93) had a lower risk of PD compared to those not taking the medication. The TNF inhibitor certolizumab was also associated with lower PD risk (RR 0.54, CI 0.34-0.84). Tacrolimus and everolimus remained significant after Bonferroni correction. Sensitivity analyses otherwise confirmed results for all four medications. CONCLUSION: Calcineurin or mTOR inhibition might reduce PD risk. Future studies should examine whether these medications or structurally similar agents might have potential as disease-modifying therapies for PD.