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

Abstract

Through its pathological and genetic association with Parkinson disease (PD), a-synuclein (a-syn) remains a favorable therapeutic target that is being investigated using various modalities, including many passive immunotherapy approaches clinically targeting different forms of a-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 a-syn with low-affinity binding to monomeric forms. The avidity-driven binding, low drug concentration, and the very low a-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 BIIB054ea-syn complexes and then quantified the crosslinked complexes using a Meso Scale Discovery electrochemiluminescence assay. CSF samples from healthy volunteers (HVs, n ¼ 46) and individuals with PD (PD, n ¼ 18) from study 228HV101 (phase 1 clinical trial of BIIB054) demonstrated dose- and time-dependent binding of cinpanemab to a-syn with measurable complexes detected at doses ≥15 mg/kg. Complex formation displayed a direct positive correlation to drug concentration (Spearman rank correlation ¼ 0.8295 [HV], 0.8032 [PD] P < .0001 [HV, PD]). The observed binding of cinpanemab to a-syn in CSF is consistent with its low intrinsic affinity for a-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 cinpanemabea-synuclein (a-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 a-syn and provided confidence the drug had engaged its target at the desired site of action. This is the first demonstration of a-syn binding by an antibody in clinical samples from the central nervous system.