In vivo evaluation of (18)F-labeled TCO for pre-targeted PET imaging in the brain.
BACKGROUND
The tetrazine-trans-cylooctene cycloaddition using radiolabeled tetrazine or radiolabeled trans-cyclooctene (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 (18)F-labeled TCO ([(18)F]TCO) to assess its potential for pre-targeted imaging in the brain.


METHODS
We evaluated the in vivo behavior of [(18)F]TCO via an ex vivo biodistribution study complemented by in vivo μPET imaging at 5, 30, 60, 90, 120 and 240 min post tracer injection. An in vivo metabolite study was performed at 5 min, 30 min and 120 min post [(18)F]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
μPET imaging and ex-vivo biodistribution revealed an high initial brain uptake of [(18)F]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 μPET images bone uptake became gradually visible after 120 min pi and was clearly visible at 240 min pi. The metabolite study revealed a fast metabolization of [(18)F]TCO in plasma and brain into three main polar radiometabolites.


CONCLUSIONS
Although [(18)F]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 pre-targeted imaging will be limited. Although [(18)F]TCO clearly enters the brain, it is quickly metabolized with a non-specific accumulation of radioactivity in the brain and bone.
Molecular Imaging Center Antwerp, University of Antwerp, Antwerp, Belgium; University Hospital Antwerp, Department of Nuclear Medicine, Edegem, Belgium.
Leonie
L
Wyffels
Molecular Imaging Center Antwerp, University of Antwerp, Antwerp, Belgium; Laboratory of Medicinal Chemistry, University of Antwerp, Antwerp, Belgium.
David
D
Thomae
Molecular Imaging Center Antwerp, University of Antwerp, Antwerp, Belgium; Department of Translational Neurosciences, University of Antwerp, Belgium.
Ann-Marie
AM
Waldron
Molecular Imaging Center Antwerp, University of Antwerp, Antwerp, Belgium; Laboratory of Medicinal Chemistry, University of Antwerp, Antwerp, Belgium.
Jens
J
Fissers
Department of Translational Neurosciences, University of Antwerp, Belgium.
Stefanie
S
Dedeurwaerdere
Laboratory of Medicinal Chemistry, University of Antwerp, Antwerp, Belgium.
Pieter
P
Van der Veken
Laboratory of Medicinal Chemistry, University of Antwerp, Antwerp, Belgium.
Jurgen
J
Joossens
Molecular Imaging Center Antwerp, University of Antwerp, Antwerp, Belgium; University Hospital Antwerp, Department of Nuclear Medicine, Edegem, Belgium.
Sigrid
S
Stroobants
Laboratory of Medicinal Chemistry, University of Antwerp, Antwerp, Belgium.
Koen
K
Augustyns
Molecular Imaging Center Antwerp, University of Antwerp, Antwerp, Belgium. Electronic address: steven.staelens@uantwerpen.be.
Steven
S
Staelens
D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging*; D034242:Cyclooctanes / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics; D004355:Drug Stability; D005462:Fluorine Radioisotopes; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D049268:Positron-Emission Tomography / Q000379:methods*; D011854:Radiochemistry; D014018:Tissue Distribution
D034242:Cyclooctanes; D005462:Fluorine Radioisotopes
(18)F-trans-cyclooctene; PET; Pre-targeted imaging