Neurodegenerative diseases, including Alzheimer's, Parkinson's, Huntington's diseases, amyotrophic lateral sclerosis (ALS) and multiple sclerosis, afflict a large portion of the population. Many of these conditions also have been very resistant to therapeutic intervention primarily due to difficulties identifying pharmacologically modifiable molecular targets that provide disease altering effects. However, recent studies have identified mutations in the leucine-rich repeat kinase2 gene (LRRK2) in the most common familial forms and some sporadic forms of Parkinson's disease (PD). LRRK2 is a large protein comprised of multiple domains, including kinase and GTPase domains. Studies have also demonstrated that LRRK2's kinase activity is elevated in the most common PD-associated mutation G2019S and some other mutations. Furthermore, the increased kinase activity correlates with elevated neuronal toxicity, suggesting a gain-of-function mechanism of LRRK2. The purpose of this proposal is to develop hits identified from high throughput screening into more useful probes to elucidate the role of LRRK2. The specific aims of this proposal are to conduct structure- activity relationship (SAR) studies and optimization of three distinct structure series of LRRK2 kinase inhibitors, to conduct enzymatic characterization of the LRRK2 kinase inhibitors and to develop functional cell-based assays to confirm cell-based activity of LRRK2 kinase inhibitors. Even though subsets of PD patients carry mutations in LRRK2, it is found in both sporadic and familial cases and good probe compounds could lead to significant breakthroughs in understanding the function of the protein and the pathology of the disease.