A leucine‐rich repeat kinase 2 (LRRK2) pathway biomarker characterization study in patients with Parkinson's disease with and without LRRK2 mutations and healthy controls

Increased leucine‐rich repeat kinase 2 (LRRK2) kinase activity is an established risk factor for Parkinson's disease (PD), and several LRRK2 kinase inhibitors are in clinical development as potential novel disease‐modifying therapeutics. This biomarker characterization study explored within‐ and between‐subject variability of multiple LRRK2 pathway biomarkers (total LRRK2 [tLRRK2], phosphorylation of the serine 935 (Ser935) residue on LRRK2 [pS935], phosphorylation of Rab10 [pRab10], and total Rab10 [tRab10]) in different biological sources (whole blood, peripheral blood mononuclear cells [PBMCs], neutrophils) as candidate human target engagement and pharmacodynamic biomarkers for implementation in phase I/II pharmacological studies of LRRK2 inhibitors. PD patients with a LRRK2 mutation (n = 6), idiopathic PD patients (n = 6), and healthy matched control subjects (n = 10) were recruited for repeated blood and cerebrospinal fluid (CSF) sampling split over 2 days. Within‐subject variability (geometric coefficient of variation [CV], %) of these biomarkers was lowest in whole blood and neutrophils (range: 12.64%–51.32%) and considerably higher in PBMCs (range: 34.81%–273.88%). Between‐subject variability displayed a similar pattern, with relatively lower variability in neutrophils (range: 61.30%–66.26%) and whole blood (range: 44.94%–123.11%), and considerably higher variability in PBMCs (range: 189.60%–415.19%). Group‐level differences were observed with elevated mean pRab10 levels in neutrophils and a reduced mean pS935/tLRRK2 ratio in PBMCs in PD LRRK2‐mutation carriers compared to healthy controls. These findings suggest that the evaluated biomarkers and assays could be used to verify pharmacological mechanisms of action and help explore the dose–response of LRRK2 inhibitors in early‐phase clinical studies. In addition, comparable α‐synuclein aggregation in CSF was observed in LRRK2‐mutation carriers compared to idiopathic PD patients.