In vivo PET Imaging of Monocyte/Macrophage Inflammation in Non-Human Primates

Objectives: Current methods for non-invasive imaging of inflammation have several serious limitations, such as the lack of specificity to the type of inflammatory response (as seen in WBC-SPECT) or to metabolically active but non-inflamed tissues (as seen in FDG-PET). Therefore, methods for imaging specific aspects of the immune response are currently needed, both for the design of pro- or anti-inflammatory drugs and protocols, as well as monitoring treatment response on a per-subject basis. In previous work (Normandin et al, Angewante Chemie, 2015), we demonstrated in mice a method for imaging resident macrophages and monitoring monocyte trafficking by PET using a chelateless radiolabeling of Feraheme (FH), an FDA approved iron oxide nanoparticle drug. Here we apply that work to monitor the trafficking and localization of circulating monocytes and tissue resident macrophages in non-human primates (NHP). Healthy animals, as well as those suffering inflammation due either to an acute injury (open wound) or chronic disease (arthritis), were injected with radiotracer and imaged by PET/CT for up to 14 days.

Methods: ⁸⁹Zr-FH was produced by heat induced radiolabeling as previously described (Yuan et al, Nature Protocols, 2018). Rhesus macaques (~15 kg) were intravenously administered 1.5 mCi of ⁸⁹Zr-FH, with mass doses of either [10, 0.1, 0.01] mg/kg. Animals were imaged by PET/CT for up to 14 days after injection using the GE Discovery MI TOF PET/CT. Blood samples were drawn for the duration of the study to measure radioactivity. Primates were imaged under three distinct conditions: healthy, inflammation following acute injury (open wound at base of tail, unknown origin), and chronic inflammation (arthritis); in some sessions, the subject also had incidental mild to moderate dermatitis as evidenced by redness of the skin and/or hair plucking behaviors. A subset of these studies also included an administration of ¹⁸F-FDG for PET/CT imaging of metabolic activity at sites of inflammation.

Results: ⁸⁹Zr-FH images in healthy NHP were consistent with monocyte/macrophage distribution, with activity primarily localized to liver, spleen, bone marrow and lymph nodes. It was also found that increased mass dose of the radiotracer corresponded to a reduced plasma half-life of the tracer. This was hypothesized a priori as circulating monocytes have a limited capacity to take up ⁸⁹Zr-FH and excess compound is preferentially sequestered via reticuloendothelial clearance. Imaging of acute tissue injury showed inflammatory responses at the base of the tail, as well as in the lymph nodes draining from the sites of inflammation. The site of injury showed a consistent activity (SUV=5) over the course of the study, but the draining lymph nodes displayed increasing activity over the course of the study, reaching SUV=17 at T=168 hours after injection. Areas of dermatitis also showed some localization of ⁸⁹Zr-FH. Imaging of NHP with chronic arthritic inflammation showed increased ⁸⁹Zr-FH accumulation in many of the joints of the body. The areas of increased activity were largely consistent with areas of increased metabolic activity as seen by FDG imaging performed in the same animal, however there were some areas of inflammation clearly detectable by ⁸⁹Zr-FH that were not visible under FDG imaging. Conclusion: As expected, the distribution of ⁸⁹Zr-FH in healthy animals was consistent with in vivo distribution of monocytes and macrophages. Additionally, areas of acute inflammation and their draining lymph nodes were clearly visible up to 14 days post injection. Imaging in an NHP with a chronic inflammatory condition showed clear areas of accumulation throughout the body, including some areas not detected by standard ¹⁸F-FDG PET. Therefore ⁸⁹Zr-FH appears to be a promising tool for non-invasive imaging of the immune response with high specificity, in both chronic and acute injury. Research Support: R01EB017699, R01MH100350, P41EB022544, S10OD018035