Novel hydroxyl dendrimer‐based PET tracer [¹⁸F]OP‐801 detects early‐stage neuroinflammation in 5XFAD mouse model with higher sensitivity than TSPO‐PET

Abstract

BackgroundChronic activation of macrophages/microglia plays a critical role in the onset and progression of neurological diseases, including Alzheimer’s. While PET imaging could enable non‐invasive visualization and quantification of activated macrophages/microglia in vivo, most available PET tracers are nonspecific for macrophages/microglia (Lambert, 2009). To address this need, we developed [18F]OP‐801, a synthetic hydroxyl dendrimer‐based PET tracer that is selectively (>95%) taken up by reactive macrophages/microglia across the blood‐brain barrier (Alnasser, 2018). Here, we evaluated the ability of [18F]OP‐801 to detect activated macrophages/microglia in the 5XFAD murine model of Alzheimer’s compared to an established neuroinflammation imaging approach (translocator protein 18 kDa [TSPO]‐PET, using [18F]GE180).Method[18F]OP‐801 (150‐250 μCi) was injected intravenously into female 5XFAD (TG) mice (n=12) and age/sex‐matched wild types (WT, n=7) at 3.75 and 5 months old. Sensitivity of [18F]OP‐801 was compared to [18F]GE180 by assessing brain signal‐to‐background ratios. [18F]GE180 (150‐250 μCi) was administered to a subset of TG (n=5) and WT (n=4) 3.75‐months‐old mice. Static 10‐minute PET/CT images were acquired at 50‐60 minutes post‐injection for both tracers. VivoQuant brain atlas was fit to CT images and fused to PET to quantify uptake in specific brain regions.ResultIn this model, macrophage/microglial activation is associated with amyloid plaque formation and can be detected using immunohistochemistry between 2‐4 months. Image quantification revealed 3‐fold higher PET signal in 3.75‐months‐old TG compared to WT mice using [18F]OP‐801, whereas [18F]GE180 signal provided no significant difference in brain regions known to contain activated microglia: cortex and hippocampus (Figure 1, Table 1). Significant differences in [18F]OP‐801 uptake were observed between 5‐months‐old TG and WT mice in cortex (p=0.005) (TG: 0.26±0.095%ID/g, WT: 0.11±0.041%ID/g), hippocampus (p=0.017) (TG: 0.18±0.065%ID/g, WT: 0.10±0.026%ID/g) and whole brain (p=0.004) (TG: 0.20±0.082%ID/g, WT: 0.10±0.039%ID/g). TG had almost 5‐fold higher [18F]OP‐801 signal compared to WT mice (Table 2).ConclusionThese results suggest that [18F]OP‐801 can detect early stage neuroinflammation with higher sensitivity than TSPO‐PET. We are currently replicating this study in a larger cohort of 5XFAD mice to correlate PET image findings with immunohistochemistry. [18F]OP‐801 is shows promise for visualizing the progression of neuroinflammation with high specificity and sensitivity, warranting further preclinical investigation.

Clinical Radiosynthesis and Translation of [18F]OP-801: A Novel Radiotracer for Imaging Reactive Microglia and Macrophages

“…Importantly, this reduces the likelihood of clinical doses failing quality control (QC) due to low SA and high non-radioactive mass. This starting mass of 100 μg is substantially less than the starting precursor mass for the most automated radiosynthetic methods ( i.e ., ≥1 mg), including preclinical synthesis of [ 18 F]­OP-801 ( i.e ., 3–5 mg), which in turn warranted significant methodology studies and synthesis optimization of both steps prior to translation.…”

“…Whole brain uptake of [ 18 F]­OP-801 (% ID/g) was plotted against the murine sepsis score for two sets of murine LPS imaging studies performed with the tracer synthesized via the preclinical high-mass and clinically validated low-mass protocols, respectively. Line is fit to all datapoints and shows high correlation between whole brain uptake and sepsis score, with no differences seen between synthetic routes or associated mass dose of the tracer …”

“…Accordingly, SA was significantly higher for optimized low mass syntheses (average SA = 10.18 GBq/mg, SD 2.78) relative to the preclinical high mass synthetic route (average SA = 1.87 GBq/mg, SD 1.48). We conducted PET imaging studies of mice 24 h after receiving an intraperitoneal injection of LPS (10 mg/kg) using our previously described methods, with tracer doses prepared via both methods. Importantly, whole brain uptake of [ 18 F]­OP-801 in mice with varying murine sepsis scores (levels of severity) was consistent across synthesis conditions: the slope did not differ significantly by adding low mass results ( n = 7 mice) to high mass results ( n = 15 mice), and linear correlation between score and whole brain uptake remained significant and positive (high mass synthesis only p < 0.0001, r = 0.854, combined p < 0.0001, r = 0.761; Figure ).…”

“…For all dosimetry studies, [ 18 F]­OP-801 was synthesized via the preclinical synthesis protocol described previously . Radiation-absorbed dose for human subjects was calculated using both in vivo imaging of mice in addition to ex vivo gamma counting of mouse tissues (see the Supporting Information for detail, Table 5).…”

“…We recently described the initial radiosynthesis and preclinical evaluation of a generation 4 PAMAM hydroxyl dendrimer PET tracer, [ 18 F]­OP-801, that is known to be selectively taken up (>95%) by activated microglia and macrophages, predominantly via fluid phase endocytosis. − This tracer has shown tremendous promise preclinically, displaying in vivo whole brain uptake that increases linearly with severity of sepsis-induced inflammation in a lipopolysaccharide (LPS) mouse model. Specifically, we found significantly elevated [ 18 F]­OP-801 signal in the cortex, medulla, olfactory bulb, and pons of LPS miceregions that correspond with the presence of reactive microglia and macrophages in this model . We also demonstrated the utility of 18 F-OP-801 for detecting neuroinflammation in the spinal cord in an experimental autoimmune encephalomyelitis (EAE) mouse model with high sensitivity and specificity and demonstrated that this tracer can monitor response to a novel CSF1R inhibitor in the EAE mouse model .…”