Discovery and preclinical characterization of [18F]PI-2620, a next-generation tau PET tracer for the assessment of tau pathology in Alzheimer's disease and other tauopathies

Abstract

Purpose: Tau deposition is a key pathological feature of Alzheimer's disease (AD) and other neurodegenerative disorders. The spreading of tau neurofibrillary tangles across defined brain regions corresponds to the observed level of cognitive decline in AD. Positron-emission tomography (PET) has proved to be an important tool for the detection of amyloid-beta (Aβ) aggregates in the brain, and is currently being explored for detection of pathological misfolded tau in AD and other non-AD tauopathies. Several PET tracers targeting tau deposits have been discovered and tested in humans. Limitations have been reported, especially regarding their selectivity.

Methods: In our screening campaign we identified pyrrolo[2,3-b:4,5-c']dipyridine core structures with high affinity for aggregated tau. Further characterization showed that compounds containing this moiety had significantly reduced monoamine oxidase A (MAO-A) binding compared to pyrido[4,3-b]indole derivatives such as AV-1451.

Results: Here we present preclinical data of all ten fluoropyridine regioisomers attached to the pyrrolo[2,3-b:4,5-c']dipyridine scaffold, revealing compounds 4 and 7 with superior properties. The lead candidate [¹⁸F]PI-2620 (compound 7) displayed high affinity for tau deposits in AD brain homogenate competition assays. Specific binding to pathological misfolded tau was further demonstrated by autoradiography on AD brain sections (Braak I-VI), Pick's disease and progressive supranuclear palsy (PSP) pathology, whereas no specific tracer binding was detected on brain slices from non-demented donors. In addition to its high affinity binding to tau aggregates, the compound showed excellent selectivity with no off-target binding to Aβ or MAO-A/B. Good brain uptake and fast washout were observed in healthy mice and non-human primates.

Conclusions: Therefore, [¹⁸F]PI-2620 was selected for clinical validation.

Keywords: AD; Alzheimer’s disease; Fluorine-18; PET; PET tracer; PI-2620; Positron-emission tomography; Tau; Tauopathies.

Fig. 1

Structures of ¹⁸F-labeled tau PET tracers

Fig. 2

Design of fluoropyridine regioisomers. Data from our screening campaign revealed that both pyrrolo[2,3-b:4,5-c’]dipyridine and pyrido[4,3-b]indole core structures exhibit high affinity for tau, but significantly reduced MAO-A binding was measured for pyrrolo[2,3-b:4,5-c’]dipyridine core structures only, providing the rationale to investigate all ten fluoropyridine regioisomers in more detail. Compound numbering: 1 (AV-1451), 2, 3 (RO6958948), 4, 5, 6, 7 (PI-2620), 8, 9, 10, 11 and 12

Fig. 3

Time activity curves (SUV) for [¹⁸F]PI-2620 ([¹⁸F]7) in brain regions at baseline in a rhesus macaque (a). SUV images at three different time intervals p.i. (b)

Fig. 4

Assessment of specific binding of PI-2620 (7) to tau aggregates in human AD brain sections. a [¹⁸F]PI-2620 was tested in ARG on AD patient-derived brain slices staged Braak I, III, and V as well as a non-demented control (NDC). Non-specific binding (NSB) was determined adding 5 μM unlabeled PI-2620. Specific ARG signal correlates with IHC staining on adjacent slices using AT8 antibody. b Micro-ARG in entorhinal cortex brain sections from an AD donor with [³H]PI-2620 revealed accumulation of silver grains on NFT which is blocked by adding 5 μM unlabeled PI-2620. Thioflavin S staining on adjacent sections correlated with the specific micro-ARG signal. TOTB, total binding; NSB, non-specific binding

Fig. 5

Assessment of specific binding of PI-2620 (7) to tau aggregates in human PSP brain sections. a ARG of [¹⁸F]PI-2620 on PSP patient-derived brain slices. Specific ARG signal correlated with IHC staining on adjacent section using the 4R-specific ET3 antibody (kindly provided by Peter Davies, Northwell, USA). b Micro-ARG signal on PSP patient-derived brain sections correlated with IHC staining on the same sections using the MC1 antibody