Positron emission tomography (PET) is a sensitive and non-invasive molecular imaging modality with various applications, including drug discovery and the development of novel therapeutics evaluation at an early stage. Pretargeted PET offers a solution for studying processes with slow pharmacokinetics using shortlived radionuclides. Such an approach decreases the radiation burden for the subject and improves target-to-non-target contrast. Spherical nucleic acids (SNA) represent the emerging alternative nanostructures of nucleic acid-based delivery agents of therapeutic oligonucleotides (ONs). SNAs have a range of benefits over linear ONs, however, the behavior of these structures in vivo requires thorough investigation.
Our aim was to develop radiolabeling methods for molecular SNA (MSNA) in both targeted and pretargeted PET imaging; this method was then used to study the structural effect on MSNA in vivo biodistribution in tumor-bearing mice. In the study, 2-[ 18F]fluoro-2-deoxy-D-glucose conjugated tetrazine ([ 18F]FDG-Tz) was first evaluated as a potential pretargeted agent in healthy mice using dynamic PET imaging, and then utilized for direct and pretargeted radiolabeling of MSNAs. This allowed the following to be studied: the influence of backbone chemistry, and the folate decoration and trans-cyclooctene (TCO)-load percentage on the biodistribution in tumor-beating mice with targeted and pretargeted PET. The results demonstrated suitability of [18F]FDG-Tz for direct and pretargeted labeling of MSNAs. MSNAs exhibited favorable biodistribution compared to linear, singlestranded ON.
In addition, the pretargeted approach showed the highest tumor uptake compared to directly labeled MSNA. In conclusion, [18F]FDG-Tz successfully enabled the radiolabeling of MSNA for both targeted and pretargeted approaches, facilitating the evaluation of structural effects on MSNA biodistribution.