Quantification of cinpanemab (BIIB054) binding to α-synuclein in cerebrospinal fluid of phase 1 single ascending dose samples.

Through its pathological and genetic association with Parkinson disease (PD), α-synuclein (α-syn) remains a favorable therapeutic target that is being investigated using various modalities, including many passive immunotherapy approaches clinically targeting different forms of α-syn and epitopes. Although published studies from some immunotherapy trials have demonstrated engagement in plasma, none has shown direct drug-antigen interactions in the disease-relevant compartment, the central nervous system. Cinpanemab (BIIB054) selectively targets pathological aggregated α-syn with low-affinity binding to monomeric forms. The avidity-driven binding, low drug concentration, and the very low α-syn levels, plus its heterogeneous nature in cerebrospinal fluid (CSF), made it impossible to measure drug-target interactions by conventional assays. Here we overcame these challenges by using zero-length crosslinking to stabilize the BIIB054-α-syn complexes and then quantified the crosslinked complexes using a Meso Scale Discovery electrochemiluminescence assay. CSF samples from healthy volunteers (HVs, n = 46) and individuals with PD (PD, n = 18) from study 228HV101 (phase 1 clinical trial of BIIB054) demonstrated dose- and time-dependent binding of cinpanemab to α-syn with measurable complexes detected at doses ≥15 mg/kg. Complex formation displayed a direct positive correlation to drug concentration (Spearman rank correlation = 0.8295 [HV], 0.8032 [PD] P < .0001 [HV, PD]). The observed binding of cinpanemab to α-syn in CSF is consistent with its low intrinsic affinity for α-syn monomer and provides evidence that the drug is behaving with expected binding dynamics in the central nervous system compartment. SIGNIFICANCE STATEMENT: A zero-length crosslinking method with Meso Scale Discovery detection was developed to enable quantification of cinpanemab-α-synuclein (α-syn) complexes in clinical cerebrospinal fluid samples by preventing signal loss caused by their rapid dissociation. Observed dose- and time-dependent binding was consistent with cinpanemab's affinity for α-syn and provided confidence the drug had engaged its target at the desired site of action. This is the first demonstration of α-syn binding by an antibody in clinical samples from the central nervous system.