Overexpression of the translocator protein, TSPO (18 kDa), formerly known as the peripheral benzodiazepine receptor, is a hallmark of activation of cells of monocytic lineage (microglia and macrophages) during neuroinflammation. Radiolabeling of TSPO ligands enables the detection of neuroinflammatory lesions by PET. Two new radioligands, ¹¹C-labeled N,N-diethyl-2-[2-(4-methoxyphenyl)-5,7-dimethylpyrazolo[1,5-α]pyrimidin-3-yl]acetamide (DPA-713) and ¹⁸F-labeled N,N-diethyl-2-(2-(4-(2-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-α]pyrimidin-3-yl)acetamide (DPA-714), both belonging to the pyrazolopyrimidine class, were compared in vivo and in vitro using a rodent model of neuroinflammation. Methods: ¹¹C-DPA-713 and ¹⁸F-DPA-714, as well as the classic radioligand ¹¹C-labeled (R)-N-methyl-N-(1-methylpropyl)-1-(2-chlorophenyl)isoquinoline-3-carboxamide (PK11195), were used in the same rat model, in which intrastriatal injection of (R,S)-α-amino-3-hydroxy-5-methyl-4-isoxazolopropionique gave rise to a strong neuroinflammatory response. Comparative endpoints included in vitro autoradiography and in vivo imaging on a dedicated small-animal PET scanner under identical conditions. Results: ¹¹C-DPA-713 and ¹⁸F-DPA-714 could specifically localize the neuroinflammatory site with a similar signal-to-noise ratio in vitro. In vivo, ¹⁸F-DPA-714 performed better than ¹¹C-DPA-713 and ¹¹C-PK11195, with the highest ratio of ipsilateral to contralateral uptake and the highest binding potential. Conclusion: ¹⁸F-DPA-714 appears to be an attractive alternative to ¹¹C-PK11195 because of its increased bioavailability in brain tissue and its reduced nonspecific binding. Moreover, its labeling with ¹⁸F, the preferred PET isotope for radiopharmaceutical chemistry, favors its dissemination and wide clinical use. ¹⁸F-DPA-714 will be further evaluated in longitudinal studies of neuroinflammatory conditions such as are encountered in stroke or neurodegenerative diseases.

Focal cerebral ischemia leads to an inflammatory reaction involving an overexpression of the peripheral benzodiazepine receptor (PBR)/18-kDa translocator protein (TSPO) in the cerebral monocytic lineage (microglia and monocyte) and in astrocytes. Imaging of PBR/TSPO by positron emission tomography (PET) using radiolabeled ligands can document inflammatory processes induced by cerebral ischemia. We performed in vivo PET imaging with [¹⁸F]DPA-714 to determine the time course of PBR/TSPO expression over several days after induction of cerebral ischemia in rats. In vivo PET imaging showed significant increase in DPA ( N,N-diethyl-2-(2-(4-(2-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide) uptake on the injured side compared with that in the contralateral area on days 7, 11, 15, and 21 after ischemia; the maximal binding value was reached 11 days after ischemia. In vitro autoradiography confirmed these in vivo results. In vivo and in vitro [¹⁸F]DPA-714 binding was displaced from the lesion by PK11195 and DPA-714. Immunohistochemistry showed increased PBR/TSPO expression, peaking at day 11 in cells expressing microglia/macrophage antigens in the ischemic area. At later times, a centripetal migration of astrocytes toward the lesion was observed, promoting the formation of an astrocytic scar. These results show that [¹⁸F]DPA-714 provides accurate quantitative information of the time course of PBR/TSPO expression in experimental stroke.

Although brain neuroinflammation may play an instrumental role in the pathophysiology of Alzheimer’s disease, its actual impact on disease progression remains controversial, being reported as either detrimental or protective. This work aimed at investigating the temporal relationship between microglial activation and clinical progression of Alzheimer’s disease. First, in a large cohort of patients with Alzheimer’s disease we analysed the predictive value of microglial activation assessed by ¹⁸F-DPA-714 PET imaging on functional, cognitive and MRI biomarkers outcomes after a 2-year follow-up. Second, we analysed the longitudinal progression of ¹⁸F-DPA-714 binding in patients with Alzheimer’s disease by comparison with controls, and assessed its influence on clinical progression. At baseline, all participants underwent a clinical assessment, brain MRI, ¹¹C-PiB, ¹⁸F-DPA-714 PET imaging and TSPO genotyping. Participants were followed-up annually for 2 years. At the end of the study, subjects were asked to repeat a second ¹⁸F-DPA-714-PET imaging. Initial ¹⁸F-DPA-714 binding was higher in prodromal (n = 33) and in demented patients with Alzheimer’s disease (n = 19) compared to controls (n = 17). After classifying patients into slow and fast decliners according to functional (Clinical Dementia Rating change) or cognitive (Mini-Mental State Examination score decline) outcomes, we found a higher initial ¹⁸F-DPA-714 binding in slow than fast decliners. Negative correlations were observed between initial ¹⁸F-DPA-714 binding and the Clinical Dementia Rating Sum of Boxes score increase, the MMSE score loss and the progression of hippocampal atrophy. This suggests that higher initial ¹⁸F-DPA-714 binding is associated with better clinical prognosis. Twenty-four patients with Alzheimer’s disease and 15 control subjects performed a second DPA-PET. We observed an increase of ¹⁸F-DPA-714 in patients with Alzheimer’s disease as compared with controls (mean 13.2% per year versus 4.2%) both at the prodromal (15.8%) and at the demented stages (8.3%). The positive correlations between change in ¹⁸F-DPA-714 binding over time and the three clinical outcome measures (Clinical Dementia Rating, Mini-Mental State Examination, hippocampal atrophy) suggested a detrimental effect on clinical Alzheimer’s disease progression of increased neuroinflammation after the initial PET examination, without correlation with PiB-PET uptake at baseline. High initial ¹⁸F-DPA-714 binding was correlated with a low subsequent increase of microglial activation and favourable clinical evolution, whereas the opposite profile was observed when initial ¹⁸F-DPA-714 binding was low, independently of disease severity at baseline. Taken together, our results support a pathophysiological model involving two distinct profiles of microglial activation signatures with different dynamics, which differentially impact on disease progression and may vary depending on patients rather than disease stages.