11C-DPA-713 Versus 18F-GE-180: A Preclinical Comparison of Translocator Protein 18 kDa PET Tracers to Visualize Acute and Chronic Neuroinflammation in a Mouse Model of Ischemic Stroke

Abstract

Neuroinflammation plays a key role in neuronal injury after ischemic stroke. PET imaging of translocator protein 18 kDa (TSPO) permits longitudinal, noninvasive visualization of neuroinflammation in both preclinical and clinical settings. Many TSPO tracers have been developed, however, it is unclear which tracer is the most sensitive and accurate for monitoring the in vivo spatiotemporal dynamics of neuroinflammation across applications. Hence, there is a need for head-to-head comparisons of promising TSPO PET tracers across different disease states. Accordingly, the aim of this study was to directly compare 2 promising second-generation TSPO tracers, ¹¹C-DPA-713 and ¹⁸F-GE-180, for the first time at acute and chronic time points after ischemic stroke. Methods: After distal middle cerebral artery occlusion or sham surgery, mice underwent consecutive PET/CT imaging with ¹¹C-DPA-713 and ¹⁸F-GE-180 at 2, 6, and 28 d after stroke. T2-weighted MR images were acquired to enable delineation of ipsilateral (infarct) and contralateral brain regions of interest (ROIs). PET/CT images were analyzed by calculating percentage injected dose per gram in MR-guided ROIs. SUV ratios were determined using the contralateral thalamus (SUV_Th) as a pseudoreference region. Ex vivo autoradiography and immunohistochemistry were performed to verify in vivo findings. Results: Significantly increased tracer uptake was observed in the ipsilateral compared with contralateral ROI (SUV_Th, 50-60 min summed data) at acute and chronic time points using ¹¹C-DPA-713 and ¹⁸F-GE-180. Ex vivo autoradiography confirmed in vivo findings demonstrating increased TSPO tracer uptake in infarcted versus contralateral brain tissue. Importantly, a significant correlation was identified between microglial/macrophage activation (cluster of differentiation 68 immunostaining) and ¹¹C-DPA-713- PET signal, which was not evident with ¹⁸F-GE-180. No significant correlations were observed between TSPO PET and activated astrocytes (glial fibrillary acidic protein immunostaining). Conclusion:¹¹C-DPA-713 and ¹⁸F-GE-180 PET enable detection of neuroinflammation at acute and chronic time points after cerebral ischemia in mice. ¹¹C-DPA-713 PET reflects the extent of microglial activation in infarcted distal middle cerebral artery occlusion mouse brain tissue more accurately than ¹⁸F-GE-180 and appears to be slightly more sensitive. These results highlight the potential of ¹¹C-DPA-713 for tracking microglial activation in vivo after stroke and warrant further investigation in both preclinical and clinical settings.

Keywords: PET; TSPO; ischemic stroke; neuroinflammation.

FIGURE 1.

(A) Study design timeline adapted from Doyle et al. (23). (B) Time–activity curves depicting dynamic ¹¹C-DPA-713 and ¹⁸F-GE180 uptake (%ID/g) in ipsilateral and contralateral brain ROIs at 2 and 6 d after dMCAO surgery (mean ± SD). Wilcoxon matched paired test (*P < 0.05).

FIGURE 2.

Representative ¹¹C-DPA-713 and ¹⁸F-GE180 PET/CT coronal mouse brain images of dMCAO mice (SUV_Th).

FIGURE 3.

(A) PET image quantification of ¹¹C-DPA-713 and ¹⁸F-GE-180 uptake (SUV_Th) in ipsilateral and contralateral ROIs in dMCAO mice at 2, 6, and 28 d and sham mice at 6 d after surgery (mean ± SD). (B) Ipsilateral to contralateral uptake ratios in dMCAO and sham mice (mean ± SD). Two-way ANOVA, Sidak’s post hoc test (*P < 0.05, **P < 0.01, ****P < 0.0001).

FIGURE 4.

(A) Representative ex vivo coronal brain autoradiography images of ¹¹C-DPA-713 and ¹⁸F-GE-180 uptake at 2, 6, and 28 d after dMCAO surgery. (B) Ratios of mean pixel intensity in ipsilateral versus contralateral brain ROIs for ¹¹C-DPA-713 and ¹⁸F-GE-180 autoradiography (mean ± SD).

FIGURE 5.

(A) Representative images of CD68 immunostaining in ipsilateral infarct border versus contralateral brain tissue after dMCAO and sham surgery (20× magnification). (B) Representative 6-d dMCAO brain section and quantification of percentage area covered with CD68 staining (mean ± SD) in ipsilateral (ROI indicated by red box) and contralateral (ROI indicated by blue box) tissue of dMCAO and sham animals. Two-way ANOVA, Sidak’s post hoc tests (**P < 0.01, ****P < 0.0001).

FIGURE 6.

(A) Representative images of GFAP immunostaining in ipsilateral infarct border versus contralateral brain tissue after dMCAO and sham surgery (20× magnification). (B) Representative 6-d dMCAO brain section and quantitation of percentage area covered with GFAP staining (mean ± SD) in ipsilateral (ROI indicated by red box) and contralateral (ROI indicated by blue box) tissue of dMCAO and sham animals. Two-way ANOVA, Sidak’s post hoc tests (***P < 0.001, ****P < 0.0001).

FIGURE 7.

Correlation between in vivo ¹¹C-DPA-713 and ¹⁸F-GE-180 signal (SUV_Th) and ex vivo CD68 (microglia/macrophage) and GFAP (astrocyte) immunostaining across all groups (¹¹C-DPA-713: day 2, n = 3; day 6, n = 4; day 28, n = 4; sham, n = 3; and ¹⁸F-GE-180: day 2, n = 2; day 6 n = 4; day 28, n = 4; sham, n = 3). Pearson correlation.