In vivo evaluation of 18F-labeled TCO for pre-targeted PET imaging in the brain

Introduction: The tetrazine-trans-cylooctene cycloaddition using radiolabeled tetrazine or radiolabeled transcyclooctene (TCO) has been reported to be a very fast, selective and bioorthogonal reaction that could be useful for in vivo radiolabeling of molecules. We wanted to evaluate the in vivo biodistribution profile and brain uptake of F-18-labeled TCO ([F-18]TCO) to assess its potential for pre-targeted imaging in the brain. Methods: We evaluated the in vivo behavior of[F-18]TCO via an ex vivo biodistribution study complemented by in vivo mu PET imaging at 5, 30, 60, 90, 120 and 240 mm post tracer injection. An in vivo metabolite study was performed at 5 min, 30 mm and 120 min post [F-18]TCO injection by RP-HPLC analysis of plasma and brain extracts. Incubation with human liver microsomes was performed to further evaluate the metabolite profile of the tracer. Results: mu PET imaging and ex-vivo biodistribution revealed an high initial brain uptake of [F-18]TCO (3.8%ID/g at 5 min pi) followed by a washout to 3.0%ID/g at 30 min pi. Subsequently the brain uptake increased again to 3.7%ID/g at 120 min pi followed by a slow washout until 240 min pi (2.9%ID/g). Autoradiography confirmed homogenous brain uptake. On the WET images bone uptake became gradually visible after 120 mm pi and was clearly visible at 240 mm pi. The metabolite study revealed a fast metabolization of [F-18]TCO in plasma and brain into three main polar radiometabolites. Conclusions: Although [F-18]TCO has previously been described to be a useful tracer for radiolabeling of tetrazine modified targeting molecules, our study indicates that its utility for in vivo chemistry and pretargeted imaging will be limited. Although [F-18]TCO clearly enters the brain, it is quickly metabolized with a non-specific accumulation of radioactivity in the brain and bone. (c) 2014 Elsevier Inc. All rights reserved.