Tracking macrophages by direct and indirect 89Zr PET imaging

Purpose. Tumour-associated macrophages are highly abundant in the tumour microenvironment, and their levels influence tumour progression and therapy response. Tracking macrophages by positron-emission tomography (PET) can yield critical information on the dynamics of macrophage infiltration, particularly in diseases such as breast cancer, in which tumour-associated macrophages are significant in the evolution of the tumour microenvironment.<br/>Methods. Two complementary PET imaging methods were used to track macrophages. The first method used a novel [89Zr]Zr-labelled antibody that targets the murine F4/80 receptor, which is located on the cell surface of in situ macrophages. [89Zr]Zr-DFO-anti-F4/80 and a non-specific [89Zr]Zr-labelled control antibody were developed and characterised for their specificity. The second method involved directly labelling mouse-derived macrophages with [89Zr]Zr-oxine, and the effect of [89Zr]Zr-radiolabelling on macrophage viability and phenotype was evaluated. Using PET imaging and ex vivo tissue counting, the in vivo biodistribution of [89Zr]Zr-DFO-anti-F4/80 and [89Zr]Zr-labelled murine macrophages was quantified in female BALB/c mice bearing syngeneic, orthotopic 4T1 breast cancer tumours.<br/>Results. [89Zr]Zr-DFO-anti-F4/80 showed target-specific uptake in vitro. At a dose of 100 g DFO-anti-F4/80 per mouse, [89Zr]Zr-DFO-anti-F4/80 enabled visualization of tumour-resident macrophages by PET/CT with high contrast. This was not observed for non-specific [89Zr]Zr-DFO-IgG2b. PET/CT imaging showed that intravenously administered [89Zr]Zr-labelled M0 macrophages migrated to the liver and spleen, with a proportion of macrophages trafficking to the tumour. [89Zr]Zr-oxine labelling did not affect macrophage viability nor phenotype, even at an absorbed radiation dose of 3.31 Gy. <br/>Conclusion. For the first time, we have applied and compared two complementary PET/CT imaging strategies to quantify the infiltration of macrophages in an orthotopic, syngeneic murine model of breast cancer. First, PET/CT imaging using an “indirect” cell tracking approach with a new [89Zr]Zr-DFO-anti-F4/80 antibody radiotracer showed that resident tumour-associated macrophages are abundant in this syngeneic breast cancer model. Second, PET/CT imaging using [89Zr]Zr-labelled M0 macrophages showed that macrophages in the blood pool migrate to the tumour, within 24 h. These tools have utility in quantifying macrophage distribution and migration in cancer, particularly in the context of new and existing therapies in which macrophage populations are perturbed.<br/>