Development and evaluation of tau PET ligands

The worldwide prevalence of AD is estimated at 35 million, a number expected to quadruple by 2050, due to the increasing lifespan of the world population. Strict definite diagnosis can still only be made post-mortem, on the basis of two pathological hallmarks: SPs and NFTs. More than 90% of clinical trials aiming to intervene at the causative pathological elements have failed to produce disease altering effects. A major concern hereby is the lack of objective biomarkers assisting in the evaluation of inclusion and outcome criteria of participating patients, as many participants turned out to be misdiagnosed, particularly in the early AD stages. Both SPs and NFTs have been targeted by PET ligands that can serve as diagnostic biomarkers, as this is a sensitive in vivo method to visualize and quantify AD specific pathological changes in the living brain. Much effort has been invested in the development of PET tracers that specifically bind to Aβ. While a negative scan substantially limits the chance for development of AD and greatly influences clinic decision making, a positive amyloid PET scan is, by itself, not sufficient for a positive diagnosis of AD. In contrast, even at late stages of AD, the burden of NFTs is closely related to the clinical symptoms of AD. This likely implicates that a PET tracer that selectively targets tau aggregates could be a more relevant biomarker for AD conversion and for neurodegenerative tauopathies. Two recently published fluorine-18 labeled compounds, T808 and T807, were used as benchmark compounds in this manuscript, since they showed reasonable affinity (KD of, respectively, 22 and 15 nM) and specificity values (respectively 27 and 25-fold higher binding affinity for NFTs over Aβ) in autoradiography KD-determination studies on human AD brain tissue sections. Early clinical evaluation of both imaging agents seems to confirm preclinical findings. With favorable pharmacokinetics and a binding pattern consistent with the Braak staging, they are currently among the most promising PET candidates for tau imaging. Substantial defluorination has, however, been observed with [18F]T808 and reports of off-target binding with [18F]T807 were recently published. By performing parallel biodistribution studies, autoradiography studies, radiometabolite studies and µPET studies, we were able to provide a straightforward comparison of both [18F]T808 and [18F]T807, but also of newly developed radiolabeled tau PET tracers. Furthermore, it allowed us to identify structural requirements for selective interaction with tau. The biodistribution studies with [18F]T808 and [18F]T807 in NMRI mice revealed high initial brain uptake with rapid washout after 10 min. In contrast to the biological properties of [18F]T807, pronounced accumulation of radioactivity in bone was, however, observed for [18F]T808, likely due to defluorination in vivo. Rapid metabolism was seen in plasma for both compounds in NMRI mice, but only with [18F]T808 polar brain radiometabolites were detected as well. The in vitro autoradiography and immunohistochemistry studies with human AD brain sections showed preferential binding of [18F]T808 and [18F]T807 to tau fibrils in the cortical layers of the visual cortex, where large amounts of NFTs and neuropil thread deposits are to be expected, based on literature. Finally, in vivo evaluation was performed by doing µPET studies with [18F]T807 on a male rhesus monkey and three Wistar rats. Homogenous distribution of [18F]T807 in all observed brain regions was recorded for the monkey and the rats, with no substantial increased uptake in the corpus callosum. TACs of the baseline scan of [18F]T807 in the brain showed, comparable to the µPET scans on Wistar rats, a slow initial brain uptake and wash-out. Unlike the findings in Wistar rats, however, TACs of the skull of the rhesus monkey demonstrate that there was no [18F]fluoride bone uptake observed 120 min p.i. After establishing [18F]T808 and [18F]T807 as benchmark compounds, eight tau PET ligands, containing scaffolds with presumed affinity for tau fibrils such as benzothiazoles (TAU1-3), carbazoles (TAU6-8) and phenylthiazolylhydrazides (TAU4-5) were synthesized and evaluated in semi-quantitative autoradiography studies in comparison with T808 and T807. Five out of eight compounds tested (TAU1 and TAU5-8) were able to inhibit the binding of [18F]T808 with more than 70%, three of which (TAU6-8) even displaced it more than 90%. The efficacy of the latter three compounds was expected as they are structurally related to T807 and T808. The high potency of TAU6-8 proved that the pyridine moiety of T807 allows diverse substitution without compromising its affinity for PHFs. A distinct difference was also observed between TAU4 (47% block) and TAU5 (73% block), demonstrating that a side chain with a pyridine ring might thus be beneficial to interact with NFTs at similar sites as T807 and T808. This structural necessity was also confirmed with the development of several highly selective tau compounds with a pyridine moiety by Janssen Research and Development. Three of the latter highly selective tau compounds ([18F]JNJ059, [18F]JNJ934 and [18F]JNJ947) were further evaluated in vitro and in vivo by performing semi-quantitative autoradiography studies, biodistribution studies and µPET studies. These three compounds showed high specific binding to tau-rich regions on human AD brain slices, that was up to 80% displaceable by the cold reference compound T808. Biodistribution studies in NMRI mice revealed very high initial brain uptake (up to 4.8 %ID), but slow wash-out as compared to the biodistribution study of [18F]T808. Similar to the biodistribution studies, the µPET studies in female Wistar rats demonstrated high initial brain uptake for all three compounds, but also slow wash-out as compared to the µPET studies performed with [18F]T807. Moreover, high bone uptake was observed for [18F]JNJ947 in these rats, with possible spill-in from the skull in the cortex. In light of this overall inferior pharmacokinetic profile of [18F]JNJ059, [18F]JNJ934 and [18F]JNJ947, in comparison with the benchmark compounds [18F]T808 and [18F]T807, new scaffolds were explored by Janssen Research and Development. This second screening process identified highly selective and tau specific compounds in which the fluorine atom is bound to an aromatic ring and led eventually to the development of one compound in particular: [18F]JNJ311. Preclinical findings with this latter compound demonstrated high initial brain uptake and rapid wash-out in the biodistribution studies in NMRI mice and the µPET studies in Wistar rats and a rhesus monkey. The µPET studies also revealed a superior pharmacokinetic profile as compared to [18F]T807. Although [18F]JNJ311 showed substantial bone uptake in rats, no bone uptake was observed in the rhesus monkey. No radiometabolites were observed in brain after injection of [18F]JNJ311 in mice. In the semi-quantitative autoradiography studies, [18F]JNJ311 revealed high and selective binding to tau deposits in post-mortem brain slices of an AD patient in the latest Braak stage. Since [18F]JNJ311 is a highly potent and selective tau binder and demonstrated an optimal pharmacokinetic profile as compared to [18F]T808 and [18F]T807, this tau PET tracer was eventually patented in February of 2016. First-in-human studies will now reveal the true potential of this tau PET candidate in comparison with the current benchmark ligands.

Jaar van publicatie:2016

Toegankelijkheid:Closed