The fibrillary aggregation of α-synuclein is a hallmark of Parkinson’s disease (PD) and a potential target for diagnostics and therapeutics. Although substantial effort has been devoted to the development of positron emission tomography (PET) probes for detecting α-synuclein aggregates, no clinically suitable tracer has been reported. The design and synthesis of 43 new N-(6-methoxypyridin-3-yl)quinolin-2-amine derivatives and an evaluation of their α-synuclein binding affinity is reported here. Compounds 7f, 7j, and 8i exhibited high affinity for α-synuclein and were selected for ¹¹C, ¹⁸F, ¹²⁵I, or ³H radiolabeling. A photoaffinity variant, TZ-CLX, structurally related to 7j and 8i, demonstrated preferential binding to the C-terminal region of α-synuclein fibrils. PET brain imaging studies using [¹¹C]7f, [¹⁸F]7j, and [¹¹C]8i in non-human primates indicated that these three α-synuclein PET tracers penetrated the blood–brain barrier. Both [¹¹C]7f and [¹⁸F]7j showed more favorable brain washout pharmacokinetics than [¹¹C]8i. In vitro binding assays showed that [¹²⁵I]8i is a very potent α-synuclein radioligand, with K_d values of 5 nM for both PD brain tissues and LBD-amplified fibrils; it is also selective for PD tissues versus AD or control tissues. These results strongly suggest that the PET probes based on the N-(6-methoxypyridin-3-yl)quinoline-2-amine scaffold have potential utility in detecting α-synuclein aggregates in vivo.

To identify structure activity relationships of ligands competing with the site-specific positron emission tomography probe [3H]BF-2846 in α-synuclein aggregates, we have synthesized and characterized the binding of over 300 compounds, exploring three distinct scaffolds and identifing 30 <10 nM affinity binders.