Introduction
Neuroinflammation is associated with several neurological diseases, such as multiple sclerosis (MS), Alzheimer’s disease (AD), and stroke. Innate pathology triggers inflammation that induces an increase in the expression of mitochondrial 18-kDa translocator protein (TSPO) in the microglia. TSPO is the main target for the PET tracers currently used for imaging neuroinflammation
in vivo [
1‐
5].
Although [
11C]PK11195, one of the first TSPO PET tracers, is still routinely used in clinical imaging, it has several limitations, including poor signal-to-noise ratio, high lipophilicity, low blood–brain barrier penetration, and the short half-life of carbon-11 [
6,
7]. Consequently, numerous second-generation PET tracers have been developed, including [
11C]PBR28, [
18F]GE180, [
18F]DPA-714, and [
18F]F-DPA, to name a few [
8‐
12].
The half-life of fluorine-18 (t
1/2 = 109.8 min) makes it a more desirable radioisotope than carbon-11 (t
1/2 = 20.3 min) for the development of radiotracers because of the possibility of distribution to PET centers lacking an on-site cyclotron. Furthermore, fluorine-18 emits positrons with low energy (E
β+ max = 0.63 MeV). Consequently, the positrons have a short range in tissue and provide higher-resolution PET images than those afforded by carbon-11 radiotracers. For these reasons, several
18F-labeled tracers have been developed, notably among them [
18F]DPA-714, which presents good binding potential and bioavailability. According to several animal studies, [
18F]DPA-714 is better for PET imaging than [
11C]PK11195, due to its low nonspecific binding in the brain and the longer half-life of the labeling radionuclide [
13,
14]. Recent longitudinal studies using mouse and rat models of AD have shown an increase in [
18F]DPA-714 uptake with disease progression [
15‐
17]; however, the results of human [
18F]DPA-714 PET studies in AD patients have been contradictory [
18‐
20].
We have previously published the synthesis of [
18F]F-DPA, an analogue of [
18F]DPA-714, by an electrophilic
18F-labeling route and showed that the position of the label directly on the aromatic moiety imparts a higher
in vivo stability than that of [
18F]DPA-714 in Sprague–Dawley rats [
12]. More recent studies have demonstrated the specificity of [
18F]F-DPA towards TSPO and its usefulness in imaging glial activation in the APP-PS1/21 mouse model of AD [
11] and in a model of ischemic stroke [
21]. In addition, a study comparing electrophilic and nucleophilic syntheses of [
18F]F-DPA demonstrated that a 100-fold difference in injected mass of [
18F]F-DPA affected both the tracer kinetics and uptake in the APP-PS1/21 mouse model. The higher injected mass gave a faster washout and more rapid establishment of tracer equilibrium while still providing a significant uptake difference between age-matched TG and WT animals [
10].
In this study we chose to compare [18F]F-DPA synthesized by the electrophilic approach with the clinically used TSPO tracers [18F]DPA-714 and [11C]PBR28 in the same TG and WT mice. The relative ease of the electrophilic synthetic procedure in our hands, the favorable kinetics of the tracer, and the high abundance of the target in TG mice motivated this study.
Discussion
Several studies have been performed in different animal models of AD using first-generation TSPO radiotracers such as [
18F]FE-DAA1106 [
26,
27], [
11C]AC-5216 [
27], and [
11C]PK-11195 [
4,
28], with marked disparities in the results. More recent studies using novel TSPO tracers such as [
11C]PBR28 [
29], [
18F]GE-180 [
30,
31], and [
18F]DPA-714 [
15,
16] have shown more consistent results in detecting inflammation.
In humans, increased microglial activation has been shown in
post mortem brain samples of AD patients, although the role of this activation is still controversial [
32,
33]. The first human PET studies targeting neuroinflammation were performed using [
11C]PK11195, with contradictory results [
34‐
37]. The varied findings with [
11C]PK11195 could be explained by the variability in the studied populations, as well as the limitations of the radiotracer itself, including high non-specific binding, high lipophilicity, low blood–brain barrier penetration, and low binding potential [
6,
7,
38]. Studies using newer TSPO radiotracers have also shown heterogeneous results in different populations [
19,
39‐
41]. Among these novel tracers, [
18F]DPA-714 has a better signal-to-noise ratio and a greater affinity than [
11C]PK11195 [
13]. The first human studies with [
18F]DPA-714 concluded that this tracer cannot be used to distinguish individual AD patients from healthy subjects [
19]. In contrast, a more recent human prospective study looked into early and protective microglial activation in AD and concluded that [
18F]DPA-714 can be a good tool for assessing neuroinflammation in early and preclinical AD [
20].
In the current study, we have shown in WT mice that [
18F]F-DPA has better brain penetration and faster washout than [
18F]DPA-714 and [
11C]PBR28, as shown in Table
1 by the peak tracer uptake and the highly significant differences in peak uptake/60-min ratios. In addition, we compared the uptake in the brains of 9-month-old APP/PS1-21 mice. We have shown in two longitudinal studies that significant [
18F]F-DPA and [
18F]DPA-714 uptake can be measured in 9-month-old APP/PS1-21 model mice [
11,
16] and therefore chose animals of that age for the direct tracer comparison. In the current study, higher SUVR
CB values were achieved with [
18F]F-DPA compared with [
18F]DPA-714 or [
11C]PBR28, with significant differences between TG and WT mice in [
18F]F-DPA uptake as soon as 20 min after the injection. In addition, the voxel-wise analysis confirmed the differences between the tracers, showing that the high uptake and fast washout of [
18F]F-DPA allows the detection of differences in uptake between the WT and TG mice at earlier time frames compared with [
18F]DPA-714 and [
11C]PBR28. Interestingly the voxel-wise analysis showed an asymmetry for [
18F]DPA-714 and for [
11C]PBR28 in particular in the earlier time frames (10–20 and 20–40 min) compared with [
18F]F-DPA in the same time frames; these asymmetries are attenuated in the last time frame (40–60 min), when the cluster sizes are also more similar. Given that the same animals were scanned with the three radiotracers, the most likely explanation is that this is due to the differences in uptake and washout speeds between the three radiotracers. This is in agreement with the results observed in Fig.
3, where in the last time frame (40–60 min) the smallest differences in SUVR
CB between WT and TG mice were observed for the three radiotracers.
The observed uptake differences between the closely related radiotracers [
18F]F-DPA and [
18F]DPA-714 could be due to the higher
in vivo metabolic stability of [
18F]F-DPA reported in rats and mice [
11,
12]. In those studies, we showed that [
18F]F-DPA is more metabolically stable than [
18F]DPA-714 in the brain; non-metabolized [
18F]F-DPA accounted for more than 90% of the remaining radioactivity even 90 min after injection, whereas for [
18F]DPA-714, only about 50% of the brain activity was the parent compound. This can be explained by the direct
18F-labeling of the aromatic ring in [
18F]F-DPA imparting a higher stability than the metabolically unstable alkoxy-linked
18F-label on the aromatic ring of [
18F]DPA-714. Although the injected mass of [
18F]F-DPA labeled by electrophilic
18F-fluorination was over 50-fold higher than the injected mass of [
18F]DPA-714 in this study, [
18F]F-DPA was better than [
18F]DPA-714 for differentiating between TG and WT animals. Recently we have demonstrated that a 100-fold difference in injected mass of [
18F]F-DPA in the same AD mouse model affected both the tracer kinetics and the tracer uptake. A higher injected mass resulted in a faster washout, with more rapid establishment of tracer equilibrium, but only an approximately 30% lower specific uptake [
10]. [
18F]DPA-714 and other TSPO binding radiotracers such as [
11C]PK11195, [
11C]DPA-713, [
18F]GE-180, [
18F]Fluoromethyl-PBR28, and [
18F]CB251 have been used for head-to-head comparisons in ischemic stroke or experimental autoimmune myocarditis [
42‐
45].
In this study, we used the CB as the reference region for analyses of the PET images. The use of the CB as a reference region is well established for amyloid quantification with [
11C]PIB, but the choice of the CB is more controversial for analyzing the binding of TSPO tracers. We previously observed an age-dependent increase in tracer accumulation in the cerebellum; however, this increase resulted in a significant difference between TG and WT animals only from the age of 12 months onwards, with no significant difference observed at 9 months. The reference region (hypothalamus) employed in the previous study [
10] is unsuitable as an
in vivo reference region due to its proximity to the pituitary gland. Our use of the CB as the reference region for [
18F]F-DPA, [
18F]DPA-714, and [
11C]PBR28 was based on our PET imaging data showing no differences in tracer uptake between TG and WT mice, as shown by the voxel-wise analysis in Fig.
4, and the cerebellar time-activity curves in Suppl. Figure
1. In addition, in previous studies, the CB has proven to be a reliable reference region, and its use as such also decreases group variability [
11,
16,
46]. From our study we can conclude that the novel TSPO radiotracer [
18F]F-DPA shows higher initial brain uptake, faster clearance, and better target-to-background ratios than [
18F]DPA-714 and [
11C]PBR28 when the comparisons are made with the same AD and WT animals. Furthermore, due to the washout kinetics, higher SUVR
CB values are measured with [
18F]F-DPA and at earlier time points after injection. With all of these characteristics, the novel tracer [
18F]F-DPA could prove very useful for the detection of low levels of microglial activation inflammation and, because of its fast clearance, would permit shorter dynamic scans.
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