Comparative Kidney Uptake of Nanobody-Based PET Tracers Labeled with Various Fluorine-18-Labeled Prosthetic Groups

Nanobodies, or single-domain antibody fragments, are promising candidates for molecular imaging due to their small size, rapid tissue penetration, and high target specificity. However, a significant challenge in their use is high renal uptake and retention, which can limit the therapeutic efficacy and complicate image interpretation. This study compares five different fluorine-18-labeled prosthetic groups for nanobodies, aiming to optimize pharmacokinetics and minimize kidney retention while maintaining tumor targeting. Using an epidermal growth factor receptor (EGFR) targeting nanobody as a model, two labeling approaches were evaluated; direct labeling of RESCA (with and without polyethylene glycol (PEG))-conjugated nanobody using Al[18F]F and indirect labeling using ([18F]F-fluoropyridine ([18F]F-FPy)-based prosthetic groups (site-specific and nonsite-specific). Labeled nanobodies were characterized in vitro for binding affinity and cell uptake with in vivo behavior assessed in EGFR + A431 tumor-bearing mice using PET imaging and biodistribution studies. Labeling with Al[18F]F showed high renal retention, which was partially mitigated by PEGylation. However, PEGylation also led to a decreased tumor uptake, particularly with longer PEG chains. Labeling using [18F]F-FPy prosthetic groups exhibited the most favorable pharmacokinetics, with rapid renal clearance and minimal kidney retention while maintaining high tumor uptake. These constructs showed excellent tumor-to-background contrast as early as 1 h postinjection. The study confirms that the selection of the prosthetic group significantly impacts the in vivo behavior of nanobodies, particularly regarding kidney accumulation. [18F]F-FPy-based prosthetic groups show the most promising results, with high tumor and minimal kidney uptake. Robust production of [18F]F-FPy on Sep-Pak is adaptable to clinical translation. Moreover, the potential substitution of 18F with therapeutic radioisotopes such as 131I or 211At could expand the application of these nanobodies from diagnostics to targeted radionuclide therapy while maintaining a low kidney exposure. These findings have important implications for optimizing nanobody-based radiopharmaceuticals for molecular imaging and targeted radionuclide therapy.