Development and In Vitro Characterization of [3H]GMC-058 as Radioligand for Imaging Parkinsonian-Related Proteinopathies

The molecular imaging of α-synuclein (α-syn) pathology in Parkinson's disease (PD) and related movement disorders is a clinically unmet need. The aim of this study was to discover and characterize in vitro a radioligand for imaging α-syn pathology. A library of 78 small molecules was developed and screened using recombinant α-syn fibrils and brain homogenates from Alzheimer's disease (AD) donors. The selection criteria were as follows: <i>K</i>i_α-syn < 30 nM, <i>K</i>i_tau and <i>K</i>i_A-β > 200 nM. Three compounds, GMC-073 (<i>K</i>_iα-syn: 8 nM), GMC-098 (<i>K</i>i_α-syn: 9.7 nM), and GMC-058 (<i>K</i>i_α-syn: 22.5 nM), fulfilled the criteria and were radiolabeled with ³H. [³H]GMC-058 was the only compound with negligible binding in controls, and was further evaluated using tissue microarrays, autoradiography on fresh-frozen brain slices, and in vitro saturation binding assay on brain homogenates. [³H]GMC-058 binding co-localized with α-syn inclusions in Parkinson's disease (PD) and multiple-system atrophy (MSA), with dense A-β plaques in cerebral amyloid angiopathy and AD and with p-tau inclusions in progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). Specific binding was highest in PSP and CBD. In vitro <i>K</i>_D was highest in AD (5.4 nM), followed by PSP (41 nM) and CBD (75 nM). The <i>K</i>_D in MSA, PD, and controls was >100 nM. [³H]GMC-058 is a novel radioligand displaying a low affinity for aggregated α-syn in tissue, with an in vitro profile also suitable for detecting tau pathology in 4R tauopathies.