Introduction: Biorthogonal pretargeted imaging using the inverse electron demand Diels Alder (IEDDA) reaction between tetrazine (Tz) and trans-cyclooctene (TCO) is one of the most attractive strategies in molecular imaging. It allows the use of short-lived radioisotopes such as fluorine-18 for imaging of long circulating vectors with improved imaging contrast and reduced radiation dose. Here we aim to develop a novel 18F-labeled trans-cyclooctene (TCO) with improved metabolic stability and assess its potential usefulness in a pretargeted PET imaging approach.
Methods: We have synthetized a new TCO-analogue containing a 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) chelator, allowing radiolabeling by chelation with aluminum fluoride (Al[18F]F). Stability and pharmacokinetic profile of Al[18F]F-NOTA-TCO ([18F]MICA-205) were evaluated in healthy animals at different timepoints after injection of the radiotracer. To assess the potential use of this new PET tracer for tumor targeting, in vivo pretargeted PET imaging was performed in LS174T tumor-bearing mice pre-treated with a tetrazine-modified anti-TAG-72 monoclonal antibody (CC49).
Results: The radiotracer was obtained with a radiochemical yield (RCY) of 12.8 ± 2.8% and a radiochemical purity (RCP) of ≥95%. It also showed a promising in vivo stability with 51.9 ± 5.16% of radiotracer remaining intact after 1 h. The biodistribution in healthy mice demonstrated mixed hepatobiliary and renal clearance, with a rapid blood clearance and low uptake in other tissues. The low bone uptake indicated lack of tracer defluorination. Interestingly, a pretargeted PET imaging experiment showed a significantly increased radiotracer uptake (0.67 ± 0.16%ID/g, p < 0.001) in the tumors of mice pre-treated with CC49-tetrazine compared to the CC49 alone (0.16 ± 0.08%ID/g).
Conclusions: [18F]MICA-205 represents a large improvement in in vivo metabolic stability compared to previous reported 18F-labeled TCOs, allowing a clear visualization of tumor tissue in a small-animal pretargeted PET imaging experiment. Despite the favorable in vivo stability and image contrast obtained with [18F]MICA-205, the development of next-generation derivatives with increased absolute tumor uptake is warranted for future pretargeting applications.
Keywords: Bioorthogonal chemistry; Positron emission tomography (PET); Pretargeted PET imaging; Trans-cyclooctene (TCO).
Copyright © 2019 The Authors. Published by Elsevier Inc. All rights reserved.