Noninvasive Characterization of Tumor-Associated Macrophages via [ ¹⁸ F]F-Labeled M2-Peptide PET Tracer for Immunotherapy Guidance

M2 macrophages are pivotal in shaping the immunosuppressive microenvironment in the tumor, with their infiltration associated with tumor progression and poor prognosis. However, noninvasive imaging tools for quantitatively assessing tumor M2 macrophages with specificity remain limited. In this study, we developed novel positron emission tomography (PET) tracers based on M2 macrophage-binding peptide (M2pep) to identify optimal one for monitoring tumor immunotherapy. Two M2 macrophage-targeted tracers, [18F]SFB-M2pep and its polyethylene glycol (PEG)-modified [18F]SFB-PEG-M2pep, were designed. The targeting specificity of these tracers was assessed by PET scanning and blocking studies in tumor models. PET imaging demonstrated superior tumor uptake and the ratio of tumor to muscle for [18F]SFB-PEG-M2pep with 1 h postinjection compared to [18F]SFB-M2pep. [18F]SFB-PEG-M2pep was selected for further investigation to assess tumor response to colony-stimulating factor 1 receptor (CSF-1R) blockade. A significant reduction in tracer accumulation in tumors was observed in mice treated with the CSF-1R inhibitor, compared to the control group. This result was confirmed by both immunohistochemistry and flow cytometry. These findings suggest that the novel M2 macrophage-targeted PET tracers, particularly [18F]SFB-PEG-M2pep, have significant potential for precisely monitoring tumor immunotherapy and assessing therapeutic efficacy.