Preclinical Positron Emission Tomography Imaging of B7–H3 Expression Using Affibody Molecules Labeled with Gallium-68

Affibody molecules,
nonimmunoglobulin scaffold proteins, have a
high potential as probes for molecular imaging of different molecular
targets. One of the molecular targets for radionuclide diagnosis and
therapy is B7–H3 (known as CD276), which is overexpressed in
various cancers, whereas its expression is low in most normal organs
and tissues. The visualization of expression levels of B7–H3
has been performed using Affibody molecules labeled with Tc-99m. However,
radionuclide molecular imaging using PET offers several advantages
such as superior sensitivity, quantitation accuracy, and better spatial
resolution compared to SPECT. In this study, we aimed to introduce
a radiotracer for PET imaging of B7–H3. To design imaging agents
for labeling with the generator-produced positron-emitting radionuclide 68Ga, the macrocyclic triaza chelator (2-[4,7-bis(carboxymethyl)-1,4,7-triazonan-1-yl]acetic
acid) (NOTA) was site-specifically coupled to the C-terminal cysteine
of the anti–B7-H3 Affibody molecules. Four different variants
of Affibody molecules, ZB7–H3_2, ZB7–H3_3, ZB7–H3_4, and ZAC12 (as control), were produced, characterized,
and successfully labeled with 68Ga. 68Ga-labeled
conjugates bound specifically to B7–H3-expressing cells in vitro and in vivo. Biodistribution showed
that [68Ga]Ga-ZB7–H3_2 had
the highest tumor accumulation only 2 h after administration, which
was 2.8-fold higher than that for the control ZAC12. There
was a tendency for higher tumor-to-organ ratios compared to the other
variants, resulting in higher imaging contrast using [68Ga]Ga-ZB7–H3_2 for preclinical PET imaging
of B7–H3-expressing tumors. Thus, [68Ga]Ga-ZB7–H3_2 could be a promising candidate for
further development aimed at clinical PET in the future.