18F-Labeled o‑aminopyridyl alkynyl radioligands targeting colony-stimulating factor 1 receptor for neuroinflammation imaging

Evaluation of a Positron Emission Tomography Tracer Targeting Colony-Stimulating Factor 1 Receptor for Detecting Pulmonary Inflammation

Colony-stimulating factor 1 receptor (CSF1R) is a type III receptor tyrosine kinase that is crucial for immune cell activation, survival, proliferation, and differentiation. Its expression significantly increases in macrophages during inflammation, playing a crucial role in regulating inflammation resolution and termination. Consequently, CSF1R has emerged as a critical target for both therapeutic intervention and imaging of inflammatory diseases. Herein, we have developed a radiotracer, 1-[4-((7-(dimethylamino)quinazolin-4-yl)oxy)phenyl]-3-(4-[18F]fluorophenyl)urea ([18F]17), for in vivo positron emission tomography (PET) imaging of CSF1R. Compound 17 exhibits a comparable inhibitory potency against CSF1R as the well-known CSF1R inhibitor PLX647. The radiosynthesis of [18F]17 was successfully performed by radiofluorination of aryltrimethyltin precursor with a yield of approximately 12% at the end of synthesis, maintaining a purity exceeding 98%. In vivo stability and biodistribution studies demonstrate that [18F]17 remains >90% intact at 30 min postinjection, with no defluorination observed even at 60 min postinjection. The PET/CT imaging study in lipopolysaccharide-induced pulmonary inflammation mice indicates that [18F]17 offers a more sensitive characterization of pulmonary inflammation compared to traditional [18F]FDG. Notably, [18F]17 shows a higher discrepancy in uptake ratio between mice with pulmonary inflammation and the sham group. Furthermore, the variations in [18F]17 uptake ratio observed on day 7 and day 14 correspond to lung density changes observed in CT imaging. Moreover, the expression levels of CSF1R on day 7 and day 14 follow a trend similar to the uptake pattern of [18F]17, indicating its potential for accurately characterizing CSF1R expression levels and effectively monitoring the pulmonary inflammation progression. These results strongly suggest that [18F]17 has promising prospects as a CSF1R PET tracer, providing diagnostic opportunities for pulmonary inflammatory diseases.

PET imaging of neuroinflammation: any credible alternatives to TSPO yet?

Over the last decades, the role of neuroinflammation in neuropsychiatric conditions has attracted an exponentially growing interest. A key driver for this trend was the ability to image brain inflammation in vivo using PET radioligands targeting the Translocator Protein 18 kDa (TSPO), which is known to be expressed in activated microglia and astrocytes upon inflammatory events as well as constitutively in endothelial cells. TSPO is a mitochondrial protein that is expressed mostly by microglial cells upon activation but is also expressed by astrocytes in some conditions and constitutively by endothelial cells. Therefore, our current understanding of neuroinflammation dynamics is hampered by the lack of alternative targets available for PET imaging. We performed a systematic search and review on radiotracers developed for neuroinflammation PET imaging apart from TSPO. The following targets of interest were identified through literature screening (including previous narrative reviews): P2Y12R, P2X7R, CSF1R, COX (microglial targets), MAO-B, I2BS (astrocytic targets), CB2R & S1PRs (not specific of a single cell type). We determined the level of development and provided a scoping review for each target. Strikingly, astrocytic biomarker MAO-B has progressed in clinical investigations the furthest, while few radiotracers (notably targeting S1P1Rs, CSF1R) are being implemented in clinical investigations. Other targets such as CB2R and P2X7R have proven disappointing in clinical studies (e.g. poor signal, lack of changes in disease conditions, etc.). While astrocytic targets are promising, development of new biomarkers and tracers specific for microglial activation has proven challenging.

Colony Stimulating Factor-1 Receptor: An emerging target for neuroinflammation PET imaging and AD therapy

Although neuroinflammation is a significant pathogenic feature of many neurologic disorders, its precise function in-vivo is still not completely known. PET imaging enables the longitudinal examination, quantification, and tracking of different neuroinflammation biomarkers in living subjects. Particularly, PET imaging of Microglia, specialised dynamic immune cells crucial for maintaining brain homeostasis in central nervous system (CNS), is crucial for staging the neuroinflammation. Colony Stimulating Factor- 1 Receptor (CSF-1R) PET imaging is a novel method for the quantification of neuroinflammation. CSF-1R is mainly expressed on microglia, and neurodegenerative disorders greatly up-regulate its expression. The present review primarily focuses on the development, pros and cons of all the CSF-1R PET tracers reported for neuroinflammation imaging. Apart from neuroinflammation imaging, CSF-1R inhibitors are also reported for the therapy of neurodegenerative diseases such as Alzheimer's disease (AD). AD is a prevalent, advancing, and fatal neurodegenerative condition that have the characteristic feature of persistent neuroinflammation and primarily affects the elderly. The aetiology of AD is profoundly influenced by amyloid-beta (Aβ) plaques, intracellular neurofibrillary tangles, and microglial dysfunction. Increasing evidence suggests that CSF-1R inhibitors (CSF-1Ri) can be helpful in preclinical models of neurodegenerative diseases. This review article also summarises the most recent developments of CSF-1Ri-based therapy for AD.

Candidate Tracers for Imaging Colony-Stimulating Factor 1 Receptor in Neuroinflammation with Positron Emission Tomography: Issues and Progress

The tyrosine kinase, colony-stimulating factor 1 receptor (CSF1R), has attracted attention as a potential biomarker of neuroinflammation for imaging studies with positron emission tomography (PET), especially because of its location on microglia and its role in microglia proliferation. The development of an effective radiotracer for specifically imaging and quantifying brain CSF1R is highly challenging. Here we review the progress that has been made on PET tracer development and discuss issues that have arisen and which remain to be addressed and resolved.