[11C]MODAG-001-towards a PET tracer targeting α-synuclein aggregates

Purpose Deposition of misfolded alpha-synuclein (alpha SYN) aggregates in the human brain is one of the major hallmarks of synucleinopathies. However, a target-specific tracer to detect pathological aggregates of alpha SYN remains lacking. Here, we report the development of a positron emission tomography (PET) tracer based on anle138b, a compound shown to have therapeutic activity in animal models of neurodegenerative diseases. Methods Specificity and selectivity of [H-3]MODAG-001 were tested in in vitro binding assays using recombinant fibrils. After carbon-11 radiolabeling, the pharmacokinetic and metabolic profile was determined in mice. Specific binding was quantified in rats, inoculated with alpha SYN fibrils and using in vitro autoradiography in human brain sections of Lewy body dementia (LBD) cases provided by the Neurobiobank Munich (NBM). Results [H-3]MODAG-001 revealed a very high affinity towards pure alpha SYN fibrils (K-d = 0.6 +/- 0.1 nM) and only a moderate affinity to hTau46 fibrils (K-d = 19 +/- 6.4 nM) as well as amyloid-beta(1-42) fibrils (K-d = 20 +/- 10 nM). [C-11]MODAG-001 showed an excellent ability to penetrate the mouse brain. Metabolic degradation was present, but the stability of the parent compound improved after selective deuteration of the precursor. (d(3))-[C-11]MODAG-001 binding was confirmed in fibril-inoculated rat striata using in vivo PET imaging. In vitro autoradiography showed no detectable binding to aggregated alpha SYN in human brain sections of LBD cases, most likely, because of the low abundance of aggregated alpha SYN against background protein. Conclusion MODAG-001 provides a promising lead structure for future compound development as it combines a high affinity and good selectivity in fibril-binding assays with suitable pharmacokinetics and biodistribution properties.

Automated summary

The newly developed MODAG-001 PET tracer based on anle138b showed high affinity in vitro and good penetration and stability in the mouse brain. However, no detectable binding to aggregated alpha SYN was found in human brain sections of LBD cases, possibly due to low abundance of aggregated alpha SYN against background protein.