A Clinical Trial of LY3962681 in Healthy Volunteers and in Patients With Parkinson's Disease

The purpose of this study is to evaluate the safety, tolerability, and pharmacokinetics/pharmacodynamics (PK/PD) of LY3962681 in healthy volunteers and patients with Parkinson's disease. The study consists of two parts, the Single Ascending Dose (SAD) study and the Multiple Ascending Dose (MAD) study. During the SAD portion of the study, healthy volunteers will receive a single dose of LY3962681 or placebo (artificial cerebrospinal fluid \[aCSF\]) administered intrathecally (into the spinal fluid). During the MAD portion of the study, patients with Parkinson's disease will receive two doses of either LY3962681 or placebo (aCSF) administered intrathecally (into the spinal fluid), 12 to 24 weeks apart. * The treatment period in the SAD study will be 1 day. The treatment period in the MAD study will be 2 dosing days, 12 to 24 weeks apart. * The follow-up period in the SAD study will be up to 52 weeks. The follow-up period in the MAD study will be up to 52 weeks after Dose 2.