Utility of [(99m)Tc]Tc-tilmanocept, an immunosuppressive macrophage functional imaging agent in melanoma patients receiving checkpoint inhibitor treatment: a feasibility study.

BACKGROUND: Immunotherapy is a mainstay in the treatment of patients with advanced melanoma. Yet, resistance mechanisms exist, and tumour-associated macrophages (TAMs), particularly the M2-like phenotype, are associated with poorer outcomes, with CD206 serving as their specific marker. We present the first human SPECT/CT study to visualize CD206 + TAMs in patients undergoing immunotherapy and compare the findings to clinical outcomes (NCT04663126). MATERIAL AND METHODS: This prospective diagnostic open-label, non-randomized, feasibility study aimed to visualize CD206 + cells including M2-like TAM in target lesions (T-Lesion) of melanoma patients treated with immunotherapy using [(99m)Tc]Tc-Tilmanocept imaging. Patients had dynamic, whole-body planar and SPECT/CT acquisitions at 1- and 3-h after injection of 350 MBq ± 10% [(99m)Tc]Tc-Tilmanocept. SUV(max/peak/mean), MTV and TLA were measured on SPECT/CT imaging in T-Lesion with ratios to healthy tissues to compare with baseline [(18)F]FDG PET/CT imaging, multispectral immunofluorescence staining findings on lesions' biopsies, tumour response at three months and follow-up. RESULTS: Five patients were recruited. T-Lesion uptake on [(99m)Tc]Tc-Tilmanocept imaging remained stable at 1- and 3-h post-injection with strong and significant correlations with baseline [(18)F]FDG PET/CT. SUV(max) T-Lesion/ SUV(mean) fat-tissue ratio on [(99m)Tc]Tc-Tilmanocept SPECT/CT at 1-h was significantly associated with tumour response at three months (p = 0.005), total cells densities for macrophages and CD8 + cells on multispectral immunofluorescence staining and poorer outcomes during the follow-up (p = 0.026). CONCLUSION: These preliminary pilot data provide the first-in-human proof of concept that CD206-based functional imaging showed measurable signal in tumour lesions in patients with advanced melanoma. If validated it might be useful in reflecting tumour immune status, hence help predicting tumour response to ICI.