Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Background: Transglutaminase 2 (TGase 2) is a multifunctional protein and has a prominent role in various physiological and pathophysiological processes. In particular, its transamidase activity, which is rather latent under physiological conditions, gains importance in malignant cells and supports tumor development and progression. Thus, there is a great need of theranostic probes for targeting tumor-associated TGase 2, and targeted covalent inhibitors appear particularly attractive as vector molecules in this regard. Such an inhibitor, equipped with a radionuclide suitable for noninvasive imaging, would be supportive for answering the general question on the possibility for functional characterization of tumor-associated TGase 2 in vivo. For this purpose, the recently developed ¹⁸F-labeled Nε-acryloyllysine piperazide [¹⁸F]7b, which is a potent and selective irreversible inhibitor of TGase 2, was subject to a detailed radiopharmacological characterization herein, including ex vivo biodistribution, metabolism and tumor uptake.
Results: An alternative radiosynthesis of [¹⁸F]7b under basic conditions is presented, which demands less than 300 µg of the respective trimethylammonio precursor per synthesis and provides [¹⁸F]7b in good radiochemical yields (17±7%) and high (radio)chemical purities (≥99%). Ex vivo biodistribution in healthy mice at 5 and 60 min p.i. revealed no permanent enrichment of ¹⁸F-activity in tissues with the exception of the bone tissue. In vivo pretreatment with ketoconazole and in vitro murine liver microsome (MLM) studies complemented by UPLC-MS/MS analysis demonstrated that bone uptake originates from metabolically released [¹⁸F]fluoride. Further metabolic transformations of [¹⁸F]7b include mono-hydroxylation and glucuronidation. Based on blood sampling data and MLM experiments, pharmacokinetic parameters such as plasma and intrinsic clearance were derived, which substantiated the apparently rapid distribution of [¹⁸F]7b in and elimination from the organisms. A TGase 2-mediated uptake of [¹⁸F]7b in different tumor cell lines could not be proven. Moreover, evaluation of [¹⁸F]7b in melanoma tumor xenograft models based on A375-hS100A4 (TGase 2 +) and MeWo (TGase 2 -) cells by ex vivo biodistribution and PET imaging were not indicative for a TGase 2-specific targeting.
Conclusion:
[¹⁸F]7b is a valuable radiometric tool to study TGase 2 in vitro under various conditions. However, its suitability for targeting tumor-associated TGase 2 is strongly limited due its unfavorable pharmacokinetic properties including a pronounced metabolization. Consequently, from a radiochemical perspective [18F]7b requires structural modifications to overcome these limitations.