PET imaging of colony-stimulating factor 1 receptor: A head-to-head comparison of a novel radioligand, 11C-GW2580, and 11C-CPPC, in mouse models of acute and chronic neuroinflammation and a rhesus monkey

Abstract

Colony-stimulating factor 1 receptor (CSF1R) is a specific biomarker for microglia. In this study, we developed a novel PET radioligand for CSF1R, ¹¹C-GW2580, and compared it to a reported CSF1R tracer, ¹¹C-CPPC, in mouse models of acute and chronic neuroinflammation and a rhesus monkey. Dynamic ¹¹C-GW2580- and ¹¹C-CPPC-PET images were quantified by reference tissue-based models and standardized uptake value ratio. Both tracers exhibited increased uptake in the lesioned striata of lipopolysaccharide-injected mice and in the forebrains of App^{NL-G-F/NL-G-F}-knock-in mice, spatially in agreement with an increased 18-kDa translocator protein radioligand retention. Moreover, ¹¹C-GW2580 captured changes in CSF1R availability more sensitively than ¹¹C-CPPC, with a larger dynamic range and a smaller inter-individual variability, in these model animals. PET imaging of CSF1R in a rhesus monkey displayed moderate-to-high tracer retention in the brain at baseline. Homologous blocker (i. e. unlabeled tracer) treatment reduced the uptake of ¹¹C-GW2580 by ∼30% in all examined brain regions except for centrum semi-ovale white matter, but did not affect the retention of ¹¹C-CPPC. In summary, our results demonstrated that ¹¹C-GW2580-PET captured inflammatory microgliosis in the mouse brain with higher sensitivity than a reported radioligand, and displayed saturable binding in the monkey brain, potentially providing an imaging-based quantitative biomarker for reactive microgliosis.

Keywords: 11C-GW2580; PET; colony-stimulating factor 1 receptor, positron emission tomography; microglia; neuroinflammation.

Figure 1.

Radiosyntheses of ¹¹C-GW2580 and ¹¹C-CPPC. RT, room temperature; DMF, N,N-dimethylformamide; DCM, dichloromethane.

Figure 2.

(a) Representative coronal ¹¹C-GW2580, ¹¹C-CPPC, and ¹¹C-AC5216 PET images of the mouse brain with lipopolysaccharide (LPS)-injection into the right striatum. The bottom row shows the PET images (presented in the top row) superimposed on an MRI template. The images were generated by averaging dynamic scan data at 15–90 min and are presented in neurological orientation. (b) Time-radioactivity curves of ¹¹C-GW2580 and ¹¹C-CPPC in the striata and cerebellum obtained from corresponding PET images from (a). The dotted circles delineate striata.

Figure 3.

(a) SUVRs for ¹¹C-GW2580 and ¹¹C-CPPC reached a steady state at 20 min in the lesioned striatum of a mouse at 12 days post LPS injection. (b) Time-course changes in average SUVR at 20–90 min (SUVR_20–90min) in the LPS-injected striata. Each data point represents an individual animal. (c) Correlation of the radiotracer binding [calculated as (SUVR_20–90min–1) × 100 (%)] between ¹¹C-GW2580 and ¹¹C-CPPC. The dashed line indicates linear regression (y = 1.8x +7). Each data point represents an individual animal.

Figure 4.

Correlation between SUVR and DVR obtained by RLogan. ¹¹C-GW2580 exhibited an excellent agreement between SUVR_60–90min and DVR (linear regression, y = 1.01 × −0.004; R² = 0.97). ¹¹C-CPPC showed less intimate correlation between SUVR and DVR than ¹¹C-GW2580; SUVR_30–60min was better correlated with DVR than SUVR_60–90min (linear regression for SUVR_30-60min, y = 0.96x +0.09; R² = 0.77). The dotted lines represent the lines of identity.

Figure 5.

Comparison of ¹¹C-GW2580 or ¹¹C-CPPC uptakes in the neocortex (CTX), hippocampus (Hippo), entire forebrain (FB), and striatum (STR) between WT and APP-KI mice. ¹¹C-GW2580 yielded greater inter-group difference and smaller inter-subject variability than ¹¹C-CPPC. *p < 0.05, **p < 0.01, ***p < 0.001, n.s., not significant. Error bars indicate standard deviation.

Figure 6.

(a) Representative transverse planes of ¹¹C-GW2580 and ¹¹C-CPPC SUV_60-120min images of a monkey brain superimposed on the monkey’s own MR images at baseline and with a homologous blocker treatment. (b) Time-radioactivity curves of ¹¹C-GW2580 and ¹¹C-CPPC in various brain regions obtained from corresponding PET images from (a). FCTX, frontal cortex; CS, centrium semi-ovale white matter.

Figure 7.

Immunohistochemistry with anti-IBA1 (red, a microglial marker) and anti-CSF1R (green) antibodies in LPS-injected FVB (a, b) and APP-KI (c) mouse brain sections. (a) Representative images of costaining of CSF1R and IBA1 in the non-lesioned striatum (arrows). (b) In the lesioned striatum, LPS locally induced microglial activation accompanied by CSF1R overexpression. (c) Co-localization and intensive immunoreactivity of IBA1 and CSF1R in FSB-positive amyloid-β plaque (gray inclusions)-associated reactive microglia in the neocortex of a 17-month-old APP-KI mouse (arrows). Arrowheads show examples of IBA1 and CSF1R immunofluorescence with spatial co-localization but mismatched intensities. Scale bars represent 50 μm.