A radioligand for in vitro autoradiography of CSF1R in post-mortem CNS tissues

Background Reactive microglia and recruited peripheral macrophages contribute to the pathogenesis of Alzheimer’s dementia (AD). Monocytes, macrophages and microglia all express the marker colony-stimulating factor 1 receptor (CSF1R). 4-Cyano-N-(4-(4-methylpiperazin-1-yl)-2-(4-methylpiperidin-1-yl)phenyl)-1H-pyrrole-2-carboxamide ( 1 ) is a high-affinity antagonist for CSF1R. We report the radiosynthesis of both [ 3 H] 1 and [ 11 C] 1 . The PET imaging properties of [ 11 C] 1 in mice and baboon were investigated. [ 3 H] 1 was studied in B max measurement in post-mortem autoradiography in the frontal cortex, inferior parietal cortex and hippocampus from donors diagnosed with AD and age-matched controls. In vitro binding affinity of 1 was measured commercially. Nor-methyl- 1 precursor was radiolabeled with [ 11 C]iodomethane or [ 3 H]iodomethane to produce [ 11 C] 1 and [ 3 H] 1 , respectively. Ex vivo brain biodistribution of [ 11 C] 1 was compared in normal mice versus lipopolysaccharide-administered (LPS) murine model of neuroinflammation. Dynamic PET imaging was performed in a healthy male Papio anubis baboon. Post-mortem autoradiography with [ 3 H] 1 was performed in frozen sections using a standard saturation binding technique. Results Compound 1 exhibits a high in vitro CSF1R binding affinity (0.59 nM). [ 11 C] 1 was synthesized with high yield. [ 3 H] 1 was synthesized similarly (commercially). Biodistribution of [ 11 C] 1 in healthy mice demonstrated moderate brain uptake. In LPS-treated mice the brain uptake of [ 11 C] 1 was ~ 50% specific for CSF1R. PET/CT [ 11 C] 1 study in baboon revealed low brain uptake (0.36 SUV) of [ 11 C] 1 . Autoradiography with [ 3 H] 1 gave significantly elevated B max values in AD frontal cortex versus control (47.78 ± 26.80 fmol/mg vs. 12.80 ± 5.30 fmol/mg, respectively, P = 0.023) and elevated, but not significantly different binding in AD hippocampus grey matter and inferior parietal cortex (IPC) white matter. Conclusions Compound 1 exhibits a high in vitro CSF1R binding affinity. [ 11 C] 1 specifically labels CSF1R in the mouse neuroinflammation, but lacks the ability to efficiently cross the blood–brain barrier in baboon PET. [ 3 H] 1 specifically labels CSF1R in post-mortem human brain. The binding of [ 3 H] 1 is significantly higher in the post-mortem frontal cortex of AD versus control subjects.