Use of Amplified Lewy Body Dementia Fibrils and Autoradiography to Characterize Binding of Radioligand Tg-1-90B to Alpha-Synuclein Fibrils in Postmortem Brain Tissue

Parkinson's disease (PD) and Lewy Body Dementia (LBD) are defined by accumulation of alpha-synuclein (Asyn) fibrils within Lewy bodies (LBs) and Lewy neurites (LNs). The development of a Positron Emission Tomography (PET) tracer for quantifying Asyn fibrils would improve diagnostic accuracy and provide a biomarker for disease progression. We previously described radioligand [³H]Tg-1-90B, which binds to in vitro Asyn fibrils (PDB 2N0A) via interactions with residues Y39, S42 and K44. Here, we performed molecular docking studies with Tg1-90B and PD/LBD Asyn fibrils (PDB 8A9L), which predicts interactions with residues Y39 and K43 in a structurally distinct binding site. In radioligand binding assays, Tg-1-90B has moderate to high affinity (K_d 17.5 nM) for amplified LBD fibrils (PDB 8FPT), whose protofilament fold is highly similar to PD/LBD fibrils (PBD 8A9L). Autoradiography confirmed binding of [³H]Tg-1-90B to LBs in PD brain tissue. However, Tg-1-90B also binds to amyloid-beta fibrils in Alzheimer's disease (AD) tissue, indicating insufficient selectivity for Asyn fibrils. These results indicate that Tg-1-90B binds to Asyn fibrils in PD tissue but needs further structural optimization. Binding assays with amplified LBD fibrils and autoradiography with postmortem PD tissue can guide further development of Asyn fibril PET ligands for PD/LBD.