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

Positron 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.

A 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 (K_i = 5 nM) against several kinases associated with inflammation. Radiation dosimetry in mice demonstrated that [¹⁸F]3 is safe for future translation to human subjects.

A 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.