Characterization of Structurally Diverse 18F-Labeled d-TCO Derivatives as a PET Probe for Bioorthogonal Pretargeted Imaging

Karuna Adhikari; Jonatan Dewulf; Christel Vangestel; Pieter Van der Veken; Sigrid Stroobants; Filipe Elvas; Koen Augustyns

doi:10.1021/acsomega.3c04597

Characterization of Structurally Diverse ¹⁸F-Labeled d-TCO Derivatives as a PET Probe for Bioorthogonal Pretargeted Imaging

ACS Omega. 2023 Oct 8;8(41):38252-38262. doi: 10.1021/acsomega.3c04597. eCollection 2023 Oct 17.

Authors

Karuna Adhikari¹, Jonatan Dewulf², Christel Vangestel^{3

2}, Pieter Van der Veken¹, Sigrid Stroobants^{3

2}, Filipe Elvas^{3

2}, Koen Augustyns¹

Affiliations

¹ Laboratory of Medicinal Chemistry, University of Antwerp, Antwerp 2610, Belgium.
² Molecular Imaging Center Antwerp, University of Antwerp, Antwerp 2610, Belgium.
³ Department of Nuclear Medicine, Antwerp University Hospital, Edegem 2650, Belgium.

Abstract

Background: The pretargeted imaging strategy using inverse electron demand Diels-Alder (IEDDA) cycloaddition between a trans-cyclooctene (TCO) and tetrazine (Tz) has emerged and rapidly grown as a promising concept to improve radionuclide imaging and therapy in oncology. This strategy has mostly relied on the use of radiolabeled Tz together with TCO-modified targeting vectors leading to a rapid growth of the number of available radiolabeled tetrazines, while only a few radiolabeled TCOs are currently reported. Here, we aim to develop novel and structurally diverse ¹⁸F-labeled cis-dioxolane-fused TCO (d-TCO) derivatives to further expand the bioorthogonal toolbox for in vivo ligation and evaluate their potential for positron emission tomography (PET) pretargeted imaging. Results: A small series of d-TCO derivatives were synthesized and tested for their reactivity against tetrazines, with all compounds showing fast reaction kinetics with tetrazines. A fluorescence-based pretargeted blocking study was developed to investigate the in vivo ligation of these compounds without labor-intensive prior radiochemical development. Two compounds showed excellent in vivo ligation results with blocking efficiencies of 95 and 97%. Two novel ¹⁸F-labeled d-TCO radiotracers were developed, from which [¹⁸F]MICA-214 showed good in vitro stability, favorable pharmacokinetics, and moderate in vivo stability. Micro-PET pretargeted imaging with [¹⁸F]MICA-214 in mice bearing LS174T tumors treated with tetrazine-modified CC49 monoclonal antibody (mAb) (CC49-Tz) showed significantly higher uptake in tumor tissue in the pretargeted group (CC49-Tz 2.16 ± 0.08% ID/mL) when compared to the control group with nonmodified mAb (CC49 1.34 ± 0.07% ID/mL). Conclusions: A diverse series of fast-reacting fluorinated d-TCOs were synthesized. A pretargeted blocking approach in tumor-bearing mice allowed the choice of a lead compound with fast reaction kinetics with Tz. A novel ¹⁸F-labeled d-TCO tracer was developed and used in a pretargeted PET imaging approach, allowing specific tumor visualization in a mouse model of colorectal cancer. Although further optimization of the radiotracer is needed to enhance the tumor-to-background ratios for pretargeted imaging, we anticipate that the ¹⁸F-labeled d-TCO will find use in studies where increased hydrophilicity and fast bioconjugation are required.