Preclinical Characterization of the Tau PET Tracer [18F]SNFT-1: Comparison of Tau PET Tracers

Tau PET tracers are expected to be sufficiently sensitive to track the progression of age-related tau pathology in the medial temporal cortex. The tau PET tracer N-(4-[F-18]fluoro-5-methylpyridin-2-yl)-7-aminoimi-dazo[1,2-a]pyridine ([F-18]SNFT-1) has been successfully developed by optimizing imidazo[1,2-a]pyridine derivatives. We characterized the binding properties of [F-18]SNFT-1 using a head-to-head comparison with other reported F-18-labeled tau tracers. Methods: The binding affinity of SNFT-1 to tau, amyloid, and monoamine oxidase A and B was compared with that of the second-generation tau tracers MK-6240, PM-PBB3, PI-2620, RO6958948, JNJ-64326067, and flortaucipir. In vitro binding properties of F-18-labeled tau tracers were evaluated through the autoradiography of frozen human brain tissues from patients with diverse neurodegenerative disease spectra. Pharmacokinetics, metabolism, and radiation dosimetry were assessed in normal mice after intra-venous administration of [F-18]SNFT-1. Results: In vitro binding assays demonstrated that [F-18]SNFT-1 possesses high selectivity and high affinity for tau aggregates in Alzheimer disease (AD) brains. Autoradiographic analysis of tau deposits in medial temporal brain sections from patients with AD showed a higher signal-to-background ratio for [F-18]SNFT-1 than for the other tau PET tracers and no significant binding with non-AD tau, a-synuclein, transactiviation response DNA-binding protein-43, and transmembrane protein 106B aggregates in human brain sections. Furthermore, [F-18]SNFT-1 did not bind significantly to various receptors, ion channels, or transporters. [F-18]SNFT-1 showed a high initial brain uptake and rapid washout from the brains of normal mice without radiolabeled metabolites. Conclusion: These preclinical data suggest that [F-18]SNFT-1 is a promising and selective tau radio -tracer candidate that allows the quantitative monitoring of age-related accumulation of tau aggregates in the human brain.