¹¹¹In-anti-F4/80-A3-1 antibody: a novel tracer to image macrophages

Here, the expression of F4/80 on the cell surface of murine macrophages was exploited to develop a novel imaging tracer that could visualize macrophages in vivo.

The immunoreactive fraction and IC₅₀ of anti-F4/80-A3-1, conjugated with diethylenetriaminepentaacetic acid (DTPA) and radiolabelled with ¹¹¹In, were determined in vitro using murine bone marrow-derived macrophages. In vivo biodistribution studies were performed with ¹¹¹In-anti-F4/80-A3-1 and isotype-matched control antibody ¹¹¹In-rat IgG2b at 24 and 72 h post-injection (p.i.) in SCID/Beige mice bearing orthotopic MDA-MB-231 xenografts. In some studies mice were also treated with liposomal clodronate. Macrophage content in tissues was determined immunohistochemically. Micro-single photon emission computed tomography (SPECT)/CT images were also acquired.

In vitro binding assays showed that ¹¹¹In-anti-F4/80-A3-1 specifically binds F4/80 receptor-positive macrophages. The immunoreactivity of anti-F4/80-A3-1 was 75 % and IC₅₀ was 0.58 nM. In vivo, injection of 10 or 100 μg ¹¹¹In-anti-F4/80-A3-1 resulted in splenic uptake of 78 %ID/g and 31 %ID/g, respectively, and tumour uptake of 1.38 %ID/g and 4.08 %ID/g, respectively (72 h p.i.). Liposomal clodronate treatment reduced splenic uptake of 10 μg ¹¹¹In-anti-F4/80-A3-1 from 248 %ID/g to 114 %ID/g and reduced ¹¹¹In-anti-F4/80-A3-1 uptake in the liver and femur (24 h p.i.). Tracer retention in the blood and tumour uptake increased (24 h p.i.). Tumour uptake of ¹¹¹In-anti-F4/80-A3-1 was visualized by microSPECT/CT. Macrophage density in the spleen and liver decreased in mice treated with liposomal clodronate. Uptake of ¹¹¹In-rat IgG2b was lower in the spleen, liver and femur when compared to ¹¹¹In-anti-F4/80-A3-1.

Radiolabelled anti-F4/80-A3-1 antibodies specifically localize in tissues infiltrated by macrophages in mice and can be used to visualize tumours. The liver and spleen act as antigen sink organs for macrophage-specific tracers.