Development of fluorine-18 labeled peptidic PET tracers for imaging active tissue transglutaminase

Introduction: The protein–protein crosslinking activity of the enzyme tissue transglutaminase (TG2; EC 2.3.2.13) is associated with the pathogenesis of various diseases, including celiac disease, lung-, liver- and kidney fibrosis, cancer and neurodegenerative diseases. This study aims at developing a TG2 PET tracer based on the peptidic irreversible TG2 inhibitor Z006. Methods: Initially, the carbon-11 labeling of Z006 at the diazoketone position was explored. Subsequently, a set of analogues that allow for fluorine-18 labeling was synthesized. Two potent analogues, 6f and 6g, were radiolabeled with fluorine-18 and biodistribution and metabolite analysis in Wistar rats was performed. The identity of the main metabolite of [¹⁸F]6g was elucidated using LC–MS/MS. In vitro binding to isolated TG2 and in vitro autoradiography on MDA-MB-231 breast cancer tissue using [¹⁸F]6g was performed. Results: [¹⁸F]6f and [¹⁸F]6g were obtained in 20 and 9% yields, respectively. Following administration to healthy Wistar rats, rapid metabolism of both tracers was observed. Remarkably, full conversion to just one single metabolite was observed for one of the tracers, [¹⁸F]6g. By LC–MS/MS analysis this metabolite was identified as C-terminally saponified [¹⁸F]6g. This metabolite was also found to be a potent TG2 inhibitor in vitro. In vitro binding to isolated TG2 and in vitro autoradiography on MDA-MB-231 tumor sections using [¹⁸F]6g demonstrated high specific and selective binding of [¹⁸F]6g to active TG2. Conclusions: Whereas based on the intensive metabolism [¹⁸F]6f seems unsuitable as a TG2 PET tracer, the results warrant further evaluation of [¹⁸F]6gin vivo.