Background: The lack of facile methods for the specific characterization of malignant
thyroid nodules makes the diagnosis of
thyroid cancer (TC) challenging. Due to its restricted expression in such nodules, the cell-associated
lectin galectin-3 (Gal3) has emerged as a marker for TC with growing interest for in vivo imaging as well as targeted
radionuclide therapy. To accelerate translation into clinical application, we have developed a cognate chimeric human
antigen-binding
fragment (Fab) derived from the rat anti-Gal3
monoclonal antibody M3/38. Methods: The variable
immunoglobulin (Ig) light and heavy chain sequences were cloned from the hybridoma cell line, and the corresponding Fab carrying human
IgG1/κ constant genes was functionally produced in the periplasm of Escherichia coli and purified to homogeneity. To moderately prolong its plasma half-life and, thus, increase
tumor uptake, the recombinant Fab was fused with a long disordered
amino acid chain comprising in total 200 Pro, Ala, and Ser residues (PASylation). This novel tracer was subjected to in vitro characterization and in vivo validation by using two
thyroid cancer orthotopic murine models. To this end, the αGal3-Fab-PAS200 was conjugated with
deferoxamine (Dfo), labeled with 89Zr under mild conditions and tested for binding on TC cell lines. Athymic nude mice were inoculated either with FRO82-1 or with CAL62
tumor cells into the left thyroid lobe. After
intravenous injection with ∼3.0 MBq of 89Zr-Dfo-PAS200-Fab, these mice were subjected to positron emission tomography (PET)/computed tomography imaging followed by quantification of
tumor accumulation and immunohistochemical analysis. Results: The αGal3-Fab-PAS200 revealed high affinity toward the recombinant Gal3
antigen, with a dissociation constant ≤1 nM as measured via
enzyme-linked
immunosorbent assay, surface plasmon resonance spectroscopy, and radioactive cell binding assay. The in vivo Gal3-targeting by the 89Zr(IV)-labeled
protein tracer, as investigated by immuno-PET, demonstrated highly selective and fast accumulation in orthotopically implanted
tumors, with strong contrast images achieved 24 hours postinjection, and no uptake in the
tumor-free thyroid lobe, as also confirmed by biodistribution studies. Conclusions: The chimeric αGal3 89Zr-Dfo-PAS200-Fab tracer exhibits selective accumulation in the
tumor-bearing thyroid lobe of xenograft mice. Thus, this novel radioactive probe offers potential to change TC management, in addition to current diagnostic procedures, and to reduce unnecessary
thyroidectomies.