Fluorine-18-labeled PET radiotracers for imaging the macrophage colony-stimulating factor 1 receptor (CSF1R)

BackgroundPositron emission tomography is a useful tool for quantitative imaging of neuroinflammation in human subjects; however, more suitable radiotracers are sought. One of the promising targets for imaging neuroinflammation is macrophage colony-stimulating factor 1 receptor (CSF1R) because of its expression on activated microglia. Several PET radiotracers for imaging CSF1R have been developed, however, a suitable CSF1R radiotracer radiolabeled with 18F is not yet available. We synthesized and performed preclinical evaluation of a new series of high-affinity, 18F-labeled PET radiotracers for imaging CSF1R.ResultsA new series of CSF1R ligands with nanomolar binding affinities and structural properties suitable for brain PET was synthesized. Five compounds of the series were radiolabeled with 18F to give the corresponding radiotracers [18F]1, [18F]3, [18F]4, [18F]5 and [18F]6. The radiotracers were tested in biodistribution experiments in control mice and demonstrated brain uptake above 1% injected dose per gram (%ID/g) of tissue, with [18F]3 exhibiting the highest uptake (12.5%ID/g) at 5 min after injection, followed by rapid washout. In the liposaccharide (LPS)-induced murine model of neuroinflammation all new radiotracers demonstrated a significant increase in brain uptake corrected for blood radioactivity vs. baseline controls, with [18F]3 showing the greatest increase (97%). The highest binding potential value in LPS-treated mice was seen for [18F]3 (BP = 1.05). In control baboon PET studies [18F]3 manifested high brain uptake (standardized uptake value, SUV = 5.2), reversible kinetics in baseline scan and moderate specific binding (up to 20%) in the blocking scans with two CSF1R inhibitors, CPPC and GW2580. In mice and baboon, [18F]3 undergoes metabolism with formation of a radiometabolite with insignificant brain penetration. The [18F]3 formulation in saline with 7% ethanol was stable for 2.5 h of storage. In vitro assay showed that 3 was selective for CSF1R (Ki = 5 nM) against several kinases associated with inflammation. Radiation dosimetry in mice demonstrated that [18F]3 is safe for future translation to human subjects.ConclusionsA series of new PET radiotracers for imaging CSF1R was synthesized and tested in vitro and in animals. One member of the series, 2-[18F]fluoro-N-(4-(4-methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl)pyrimidine-4-carboxamide ([18F]3), manifests PET imaging properties that may be suitable for further testing on non-human primate models of neuroinflammation and translation to human subjects.