89Zr-Labeled Versus 124I-Labeled αHER2 Fab with Optimized Plasma Half-Life for High-Contrast Tumor Imaging In Vivo

Immuno-PET imaging of the tumor antigen HER2/neu allows for the noninvasive detection and monitoring of oncogene expression; such detection and monitoring are of prognostic value in patients with breast cancer. Compared with the full-size antibody trastuzumab, smaller protein tracers with more rapid blood clearance permit higher imaging contrast at earlier time points. Antigen-binding fragments (Fabs) of antibodies with moderately prolonged circulation achieved through the genetic fusion with a long, conformationally disordered chain of the natural amino acids Pro, Ala, and Ser (PASylation)-a biologic alternative to chemical conjugation with polyethylene glycol, PEG-offer a promising tracer format with improved pharmacokinetics for in vivo imaging. Recently, the transition metal radionuclide Zr-89 has attracted increasing interest for immuno-PET studies, complementing the conventional halogen radionuclide I-124. Methods: To allow direct comparison of these 2 radioactive labels for the same protein tracer, the recombinant alpha HER2 Fab fused with 200 Pro, Ala, and Ser (PAS(200)) residues was either conjugated with I-124 via an iodination reagent or coupled with deferoxamine (Df) and complexed with Zr-89. After confirmation of the stability of both radioconjugates and quality control in vitro, immuno-PET and biodistribution studies were performed with CD1-Foxn1(nu) mice bearing HER2-positive human tumor xenografts. Results: Zr-89.Df-Fab-PAS(200) and I-124-Fab-PAS(200) showed specific tumor uptake of 11 and 2.3 percentage injected dose per gram 24 h after injection, respectively; both led to high tumor-to-blood (3.6 and 4.4, respectively) and tumor-to-muscle (20 and 43, respectively) ratios. With regard to off-target accumulation, overt I-124 activity was seen in the thyroid, as expected, whereas high kidney uptake was evident for Zr-89; the latter was probably due to glomerular filtration and reabsorption of the protein tracer in proximal tubular cells. Conclusion: Both Zr-89- and I-124-labeled versions of alpha HER2 Fab-PAS(200) allowed PET tumor imaging with high contrast. With its residualizing radiometal, the tracer Zr-89.Df-Fab-PAS(200) showed better in vivo stability and higher tumor uptake.