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.

Preclinical Evaluation of Novel PET Probes for Dementia

The development of novel PET imaging agents that selectively bind specific dementia-related targets can contribute significantly to accurate, differential and early diagnosis of dementia causing diseases and support the development of therapeutic agents. Consequently, in recent years there has been a growing body of literature describing the development and evaluation of potential new promising PET tracers for dementia. This review article provides a comprehensive overview of novel dementia PET probes under development, classified by their target, and pinpoints their preclinical evaluation pathway, typically involving in silico, in vitro and ex/in vivo evaluation. Specific target-associated challenges and pitfalls, requiring extensive and well-designed preclinical experimental evaluation assays to enable successful clinical translation and avoid shortcomings observed for previously developed 'well-established' dementia PET tracers are highlighted in this review.

Pilot imaging of the colony stimulating factor 1 receptor in the brains of virally-suppressed individuals with HIV

OBJECTIVE

Neuroimmune activation is a putative driver of cognitive impairment in people with HIV (PWH), even in the age of modern antiretroviral therapy. Nevertheless, imaging of the microglial marker, the 18 kDa translocator protein (TSPO), with positron emission tomography (PET) in treated PWH has yielded inconclusive findings. One potential reason for the varied TSPO results is a lack of cell-type specificity of the TSPO target.

DESIGN

[ 11 C]CPPC, 5-cyano- N -(4-(4-[ 11 C]methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl) furan-2-carboxaminde, is a radiotracer for use with PET to image the colony stimulating factor 1 receptor (CSF1R). The CSF1R is expressed on microglia and central nervous system macrophages, with little expression on other cell types. We used [ 11 C]CPPC PET in virally-suppressed- (VS)-PWH and HIV-uninfected individuals to estimate the effect sizes of higher CSF1R in the brains of VS-PWH.

METHODS

Sixteen VS-PWH and 15 HIV-uninfected individuals completed [ 11 C]CPPC PET. [ 11 C]CPPC binding (V T ) in nine regions was estimated using a one-tissue compartmental model with a metabolite-corrected arterial input function, and compared between groups.

RESULTS

Regional [ 11 C]CPPC V T did not significantly differ between groups after age- and sex- adjustment [unstandardized beta coefficient ( B ) = 1.84, standard error (SE) = 1.18, P  = 0.13]. The effect size was moderate [Cohen's d  = 0.56, 95% confidence interval (CI) -0.16, 1.28), with strongest trend of higher V T in VS-PWH in striatum and parietal cortex (each P  = 0.04; Cohen's d  = 0.71 and 0.72, respectively).

CONCLUSIONS

A group difference in [ 11 C]CPPC V T was not observed between VS-PWH and HIV-uninfected individuals in this pilot, although the observed effect sizes suggest the study was underpowered to detect regional group differences in binding.

Discovery of a High-Affinity Fluoromethyl Analog of [11C]5-Cyano-N-(4-(4-methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl)furan-2-carboxamide ([11C]CPPC) and Their Comparison in Mouse and Monkey as Colony-Stimulating Factor 1 Receptor Positron Emission Tomography Radioligands

[11C]CPPC has been advocated as a radioligand for colony-stimulating factor 1 receptor (CSF1R) with the potential for imaging neuroinflammation in human subjects with positron emission tomography (PET). This study sought to prepare fluoro analogs of CPPC with higher affinity to provide the potential for labeling with longer-lived fluorine-18 (t 1/2 = 109.8 min) and for delivery of higher CSF1R-specific PET signal in vivo. Seven fluorine-containing analogs of CPPC were prepared and four were found to have high inhibitory potency (IC50 in low to sub-nM range) and selectivity at CSF1R comparable with CPPC itself. One of these, a 4-fluoromethyl analog (Psa374), was investigated more deeply by labeling with carbon-11 (t 1/2 = 20.4 min) for PET studies in mouse and monkey. [11C]Psa374 showed high peak uptake in monkey brain but not in mouse brain. Pharmacological challenges revealed no CSF1R-specific binding in either species at baseline. [11C]CPPC also failed to show specific binding at baseline. Moreover, both [11C]Psa374 and [11C]CPPC showed brain efflux transporter substrate behavior in both species in vivo, although Psa374 did not show liability toward human efflux transporters in vitro. Further development of [11C]Psa374 in non-human primate models of neuroinflammation with demonstration of CSF1R-specific binding would be required to warrant the fluorine-18 labeling of Psa374 with a view to possible application in human subjects.

PET/CT imaging of CSF1R in a mouse model of tuberculosis

PURPOSE

Macrophages represent an essential means of sequestration and immune evasion for Mycobacterium tuberculosis. Pulmonary tuberculosis (TB) is characterized by dense collections of tissue-specific and recruited macrophages, both of which abundantly express CSF1R on their outer surface. 4-Cyano-N-(5-(1-(dimethylglycyl)piperidin-4-yl)-2',3',4',5'-tetrahydro-[1,1'-biphenyl]-2-yl)-1H-imidazole-2-carboxamide (JNJ-28312141) is a reported high affinity, CSF1R-selective antagonist. We report the radiosynthesis of 4-cyano-N-(5-(1-(N-methyl-N-([11C]methyl)glycyl)piperidin-4-yl)-2',3',4',5'-tetrahydro-[1,1'-biphenyl]-2-yl)-1H-imidazole-2-carboxamide ([11C]JNJ-28312141) and non-invasive detection of granulomatous and diffuse lesions in a mouse model of TB using positron emission tomography (PET).

METHODS

Nor-methyl-JNJ-28312141 precursor was radiolabeled with [11C]iodomethane to produce [11C]JNJ-28312141. PET/CT imaging was performed in the C3HeB/FeJ murine model of chronic pulmonary TB to co-localize radiotracer uptake with granulomatous lesions observed on CT. Additionally, CSF1R, Iba1 fluorescence immunohistochemistry was performed to co-localize CSF1R target with reactive macrophages in infected and healthy mice.

RESULTS

Radiosynthesis of [11C]JNJ-28312141 averaged a non-decay-corrected yield of 18.7 ± 2.1%, radiochemical purity of 99%, and specific activity averaging 658 ± 141 GBq/µmol at the end-of-synthesis. PET/CT imaging in healthy mice showed hepatobiliary [13.39-25.34% ID/g, percentage of injected dose per gram of tissue (ID/g)] and kidney uptake (12.35% ID/g) at 40-50 min post-injection. Infected mice showed focal pulmonary lesion uptake (5.58-12.49% ID/g), hepatobiliary uptake (15.30-40.50% ID/g), cervical node uptake, and renal uptake (11.66-29.33% ID/g). The ratio of infected lesioned lung/healthy lung uptake is 5.91:1, while the ratio of lesion uptake to adjacent infected radiolucent lung is 2.8:1. Pre-administration of 1 mg/kg of unlabeled JNJ-28312141 with [11C]JNJ-28312141 in infected animals resulted in substantial blockade. Fluorescence microscopy of infected and uninfected whole lung sections exclusively co-localized CSF1R staining with abundant Iba1 + macrophages. Healthy lung exhibited no CSF1R staining and very few Iba1 + macrophages.

CONCLUSION

[11C]JNJ-28312141 binds specifically to CSF1R + macrophages and delineates granulomatous foci of disease in a murine model of pulmonary TB.

First-in-human use of 11C-CPPC with positron emission tomography for imaging the macrophage colony-stimulating factor 1 receptor

PURPOSE

Study of the contribution of microglia to onset and course of several neuropsychiatric conditions is challenged by the fact that these resident immune cells often take on different phenotypes and functions outside the living brain. Imaging microglia with radiotracers developed for use with positron emission tomography (PET) allows researchers to study these cells in their native tissue microenvironment. However, many relevant microglial imaging targets such as the 18 kDa translocator protein are also expressed on non-microglial cells, which can complicate the interpretation of PET findings. 11C-CPPC was developed to image the macrophage colony-stimulating factor 1 receptor, a target that is expressed largely by microglia relative to other cell types in the brain. Our prior work with 11C-CPPC demonstrated its high, specific uptake in brains of rodents and nonhuman primates with neuroinflammation, which supports the current first-in-human evaluation of its pharmacokinetic behavior in the brains of healthy individuals.

METHODS

Eight healthy nonsmoker adults completed a 90-min dynamic PET scan that began with bolus injection of 11C-CPPC. Arterial blood sampling was collected in order to generate a metabolite-corrected arterial input function. Tissue time-activity curves (TACs) were generated using regions of interest identified from co-registered magnetic resonance imaging data. One- and two-tissue compartmental models (1TCM and 2TCM) as well as Logan graphical analysis were compared.

RESULTS

Cortical and subcortical tissue TACs peaked by 37.5 min post-injection of 11C-CPPC and then declined. The 1TCM was preferred. Total distribution volume (VT) values computed from 1TCM aligned well with those from Logan graphical analysis (t* = 30), with VT values relatively high in thalamus, striatum, and most cortical regions, and with relatively lower VT in hippocampus, total white matter, and cerebellar cortex.

CONCLUSION

Our results extend support for the use of 11C-CPPC with PET to study microglia in the human brain.

Synthesis and Evaluation of a 18F-Labeled Ligand for PET Imaging of Colony-Stimulating Factor 1 Receptor

Neuroinflammation involves activation of glial cells in the brain, and activated microglia play a particularly important role in neurodegenerative diseases such as Alzheimer’s disease (AD). In this study, we developed 5-cyano-N-(4-(4-(2-[¹⁸F]fluoroethyl)piperazin-1-yl)-2-(piperidin-1-yl)phenyl)furan-2-carboxamide ([¹⁸F]1) for PET imaging of colony-stimulating factor 1 receptor (CSF1R), an emerging target for neuroinflammation imaging. Non-radioactive ligand 1 exhibited binding affinity comparable to that of a known CSF1R inhibitor, 5-cyano-N-(4-(4-methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl)furan-2-carboxamide (CPPC). Therefore, we synthesized radioligand [¹⁸F]1 by radiofluorination of chlorine-substituted precursor 7 in 13–15% decay-corrected radiochemical yield. Dynamic PET/CT images showed higher uptake in the lipopolysaccharide (LPS)-treated mouse brain than in control mouse brain. Ex vivo biodistribution study conducted at 45 min after radioligand injection showed that the brain uptake in LPS mice increased by 78% compared to that of control mice and was inhibited by 22% in LPS mice pretreated with CPPC, indicating specificity of [¹⁸F]1 for CSF1R. A metabolism study demonstrated that the radioligand underwent little metabolism in the mouse brain. Taken together, these results suggest that [¹⁸F]1 may hold promise as a radioligand for CSF1R imaging.

In Vitro Evaluation of [3H]CPPC as a Tool Radioligand for CSF-1R

Microglia play a role in several central nervous system (CNS) diseases and are a highly sought target for positron emission tomography (PET) imaging and therapeutic intervention. 5-Cyano-N-(4-(4-[¹¹C]methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl)furan-2-carboxamide ([¹¹C]CPPC) is a radiopharmaceutical designed to selectively target microglia via macrophage colony stimulating factor-1 receptor (CSF-1R) in the CNS. Herein, we report the first preclinical evaluation of [³H]CPPC using radioligand binding methods for the evaluation of putative CSF-1R inhibitors in rodent models of neuroinflammation. The distribution of [³H]CPPC by autoradiography did not align with 18 kDa translocator protein (TSPO) distribution using [³H]PBR28 and IBA-1 staining for microglia. In the CNS, [³H]CPPC had considerable nonspecific binding, as indicated by a low displacement of the tritiated ligand by unlabeled CPPC and the known CSF1R inhibitors BLZ-945 and PLX3397. Spleen was identified as a tissue that provided an adequate signal-to-noise ratio to enable screening with [³H]CPPC and a library of 20 novel PLX3397 derivatives. However, unlabeled CPPC lacked selectivity and showed off-target binding to a substantial number of kinase targets (204 out of 403 tested) at a concentration relevant to in vitro radioligand binding assays (10 μM). These findings suggest that, while [³H]CPPC may have utility as a radioligand tool for the evaluation of peripheral targets and screening of CSF-1R inhibitors, it may have limited utility as an in vivo CNS imaging probe on the basis of the current evaluation.