In the current work, we applied the most recently developed tau tracer, 18F-APN1607, in a group of patients with probable AD and a group of NCs. Our study showed several advantages of this new tau tracer. First, this tracer revealed a clear background in the midbrain, basal ganglia and cerebral white matter regions in NCs. In patients with probable AD, the tracer demonstrated significantly increasing intensities in AD-associated cortical regions with medium to large effect sizes (mean effect size = 0.71 in the significant regions). Second, the regional SUVRs in AD-associated cortical regions showed significant correlations with the ADAS-cog scores and CDR-SB, suggesting that tau deposition correlated with clinical severity in vivo. Third, the pattern of sequentially increasing regional SUVRs from the 18F-APN1607 PET images corresponding well with disease severity and hence revealed the topographical progression of tau distribution in AD. Finally, the combined 18F-AV-45, 18F-APN1607 PET and regional atrophic ratio information from the same region could support the hypothesis that amyloid deposition would reach a plateau earlier than tau deposition before neuronal degeneration revealed. These results are in line with the pathological observations from different Braak stages of AD .
The characteristics of in vivo 18F-APN1607 PET imaging
In vivo imaging of the deposition of tau proteins faced several inherent obstacles, such as the intracellular deposition of tau aggregates, the six different isoforms of tau, the similarity of the β-sheet structure between tau and many other misfolded proteins, and the colocalization of tau with 5-20 times its concentration in β-amyloid protein in GM areas. Despite these challenges, several tau tracers have been synthesized in the past few years. The first-generation tracers (e.g., 18F-THK-5317, 18F-THK-5351, 18FAV-1451 and 11C-PBB3) have been extensively used in research studies. The second-generation compounds, namely, 18F-MK-6240, 18F-JNJ-64349311, 18F-PI-2620, 18F-GTP1, and 18F-APN1607, have started to be used for in vivo studies[14, 45]. The advantages of the second- generation compounds include a lack of off-target binding in the basal ganglia and thalamus and a relatively low affinity for the enzyme MAO-B[46-48]. A directly head-to-head comparison between the first-generation and second-generation tau tracers also revealed that different molecular binding targets existed in these tracers. In the current study, we showed that there is no significant uptake of the PET tracer 18F-APN1607 in the midbrain or the basal ganglia. Similar studies results have been found using 11C-PBB3 in healthy participants and using autoradiographic methods in human tissue[50, 51]. These tracers could be beneficial for research on various tau-related neurodegenerative diseases, such as progressive supranuclear palsy and corticobasal syndrome. Furthermore, the AD-associated cortical regions of subjects with probable AD showed significantly increased SUVRs in 18F-APN1607 PET images, with medium to large effect size, which could be used to easily distinguish the abnormal cortical regions in clinical practice. In the current work, the tracer 18F-APN1607 still had off-target binding in the choroid plexus in five of twelve NCs (42%) and in seven of ten participants with probable AD (70%). From a previous autoradiographic study using 18F-THK-5351 or 18F-AV-1451 in postmortem human brains, these first-generation tau tracers had strong binding properties in tissue with a high density of melanin-containing cells . The compound 18F-APN1607 may have similar characteristics. There are also other possible explanations that have been mentioned; for example, the epithelial cells of the choroid plexus contain tangle-like structures that could be labeled by 18F-AV-1451, or the choroid plexus could act as a gatekeeper for the accumulation of tau protein[52, 53].
Significant associations between regional SUVRs in 18F-APN1607 PET and clinical scores
In previous AD studies, cognitive decline and tau accumulation showed a close relationship [54-56]. Based on investigations using 18F-AV-1451, Aschenbrenner et al. suggested that increasing levels of tau most consistently relate to declines in cognition in patients with AD. In our results, SUVRs in 18F-APN1607 PET images from AD-associated regions showed significantly positive correlations with the ADAS-cog scores and CDR-SB scores (p < 0.01), which demonstrated that increasing tau burden correlated with decreasing cognition and increasing disease severity. The SUVRs inAD-associated regions shown on18F-AV-45 PET images also showed significant associations with the ADAS-cog scores and CDR-SB scores, which may be related to the small sample size in this study.
The sequential changes in regional SUVRs from 18F-APN1607 PET imaging
The pathological study showed that the spread of tau deposits started from the entorhinal cortex (Braak stages I/II), moving to the inferolateral temporal cortex and parts of the medial parietal lobe (stages III/IV), and eventually spreading throughout the association cortex (V/VI)[20, 58]. Our results using in vivo 18F-APN1607 PET images demonstrated a similar topographical pattern. At least three patterns of tau deposition could be found (Figure 3A). The first pattern was in the parahippocampal region; tau deposition rapidly increased and then reached a plateau (rapid saturation) as the ADAS-cog scores increased. The second pattern was in the posterior cingulate gyrus and the temporal, frontal and parietal regions, undergoing a slow progressive increase in tau deposits and then reaching to plateaus. The final pattern was in the occipital region, which showed a gradual increase of tau deposition without a plateau (Figure 3). The ADAS-cog scores at the inflection points of the sigmoidal curves showed the lowest value in the parahippocampus, followed by the precuneus, temporal lobe, posterior cingulate gyrus, frontal lobe, parietal lobe, anterior cingulated gyrus and occipital lobe. These findings were in agreement with the previous neuropathological evidence of neurofibrillary changes from transentorhinal stages to limbic stages and finally to neocortical stages.
The evolution of amyloid, tau and atrophic changes in different regions
In most of the ROIs, the evolution of SUVRs from 18F-AV-45 and 18F-APN1607 PET images and regional atrophic ratios showed that the amyloid burden usually rapidly increased to a plateau as the ADAS-cog scores increased, especially in the low ranges of ADAS-cog scores. The patterns of increasing tau deposition were regionally dependent. Finally, the regional atrophic ratios from MRI showed progressively increased values without plateaus (Figure 4).
When we combined the SUVRs from 18F-AV-45 and 18F-APN1607 PET images and regional atrophic ratio information in the same ROIs to explore the sequential changes, we found that the amyloid burden usually manifested earlier than tau deposition and tau deposition usually started earlier then regional atrophies in most regions (Figure 4). In the parahippocampal region, tau deposition and regional atrophic ratio rapidly increased in the low ADAS-cog scores range but the amyloid burden didn’t show a significant increase(Figure 4A). On the other hand, the occipital region showed a progressive increase of tau deposition and regional atrophy without a plateau phase(Figure 4B). These findings may indicate that cerebral amyloid deposition reached a saturation state more rapidly than tau deposition and neurodegeneration in most areas, but this sequential change also had the regional variability. Currently, our findings from the cross-sectional data could demonstrate the importance of amyloid-tau-neurodegeneration (ATN) sequential changes in regional base level, which were compatible with the widely hypothesized model of AD and ATN classification system in the AD research framework[59, 60].
Several limitations of the current work need to be addressed. First, the tau tracer 18F-APN1607 is a relatively new tracer, thus the pathological results are not yet available in our study. Up to now, only postmortem brain tissue had been studied with this tracer, and there have been no clinicopathological correlation studies using this tracer yet. Furthermore, the six isoforms of tau in the brain include 3R and 4R tau, whose misfolding is responsible for various neurodegenerative diseases, such as progressive supranuclear palsy, corticobasal syndrome and frontotemporal dementia. Whether the tau tracer 18F-APN1607 can differentiate among all isoforms is an open question that needs further investigation. In addition, direct application of MAO-B inhibitors in patients undergoing 18F-APN1607 PET imaging has not been performed, and it could be difficult to eliminate these concerns about the first-generation tau tracers. Second, our study had a small sample size, a significant age difference between AD patients and NCs, and no participants with amnestic mild cognitive impairment. We acknowledge the demographic differences between groups, and we used age and gender as covariates to study the correlations of regional SUVRs from 18F-APN1607 PET imaging with ADAS-cog and CDR-SB scores. Increasing the sample size and adding amnestic patients will help us explore the features of this tau tracer. Third, we used the ADAS-cog scores as the severity index for curve fitting with the regional SUVRs from 18F-AV-45 , 18F-APN1607 PET images and regional atrophy ratios. We acknowledge that any biomarker changes to be incorporated into the hypothetical model of AD should come from longitudinal studies rather than cross-sectional observations, and our findings must be interpreted conservatively. Future studies should focus on longitudinal changes in 18F-APN1607 PET imaging with the aid of other biomarkers, which will may provide further evidence for the AD hypothetical model.