Preparation of tetrazine-containing [2 + 1] complexes of ^99mTc and in vivo targeting using bioorthogonal inverse electron demand Diels–Alder chemistry

Abstract

The aim of this work was to synthesize and evaluate [2 + 1] Tc(i) polypyridine complexes containing tetrazines, which along with the corresponding Re(i) complexes, represent a new class of isostructural nuclear and turn-on luminescent probes that can be derivatized and targeted using bioorthogonal chemistry. To this end, [2 + 1] complexes ofTc(i) of the type [Tc(CO)(N^N)(L)] (N^N = bathophenanthroline disulfonate (BPS) or 2,2'-bipyridine (bipy)), where the monodentate ligand (L) was a tetrazine linked to the metal through an imidazole derivative, were prepared. The desired products were obtained in nearly quantitative radiochemical yield by adding [Tc(CO)(N^N)(OH)] to the imidazole-tetrazine ligand and heating at 60 °C for 30 min. Measurement of the reaction kinetics between the tetrazine and (E)-cyclooct-4-enol revealed a second-order rate constant of 8.6 × 10 M s at 37 °C, which is suitable for in vivo applications that require rapid coupling. Stability studies showed that the metal complexes were resistant to ligand challenge and exhibited reasonable protein binding in vitro. Biodistribution studies of the more water-soluble BPS derivative in normal mice, one hour after administration of a bisphosphonate derivative of trans-cyclooctene (TCO-BP), revealed high activity concentrations in the knee (9.3 ± 0.3 %ID g) and shoulder (5.3 ± 0.7 %ID g). Using the same pretargeting approach, SPECT/CT imaging showed that the [2 + 1] tetrazine complex localized to implanted skeletal tumors. This is the first report of the preparation of Tc complexes of BPS and demonstration that their tetrazine derivatives can be used to prepare targeted imaging probes by employing bioorthogonal chemistry.

An Efficient Method for Labeling Single Domain Antibody Fragments with 18F Using Tetrazine-Trans-Cyclooctene Ligation and a Renal Brush Border Enzyme-Cleavable Linker

“…Similar patterns were seen in the normal mice biodistribution studies performed with both [ 18 F]­AlF-NOTA-Tz-TCO-GK-2Rs15d and [ 18 F]­AlF-NOTA-Tz-TCO-2Rs15d (Table S1 and Table S2). Higher intestinal uptake of activity from proteins including sdAbs radiolabeled utilizing the TCO-Tz IEDDAR has been reported previously. ,− Because intact radiolabeled sdAbs are expected to be predominantly eliminated via the kidneys, it is likely that the high activity levels observed in the intestines must be due to 18 F-labeled TCO-Tz-containing low molecular weight catabolites. Although both renal and fecal elimination pathways have been reported for 18 F-labeled tetrazine derivatives similar in structure to [ 18 F] 11 , , unless modified with a hydrophilic moiety, the intrinsic hydrophobic character of tetrazine should favor hepatobiliary excretion .…”

Tetrazine-Derived Near-Infrared Dye as a Facile Reagent for Developing Targeted Photoacoustic Imaging Agents

“…This allows a good PA agent to be used across a range of cancers and other diseases. This approach leverages well-established prior reports that have demonstrated straightforward methods to create TCO-derived targeting molecules from small molecules and antibody conjugates. − …”

Clickable Radiocomplexes With Trivalent Radiometals for Cancer Theranostics: In vitro and in vivo Studies

“…However, most of these processes are nowadays known and several derivatives with improved chemical stability have been developed (12,13). The targeting biomolecule is injected first and, when a satisfactory target accumulation is achieved, is followed by the injection of the radiolabelled small molecule to produce the desired on-site and in vivo conjugation (14)(15)(16)(17). This approach has proven to be especially useful in radioimmunotherapy (RIT), since the slow pharmacokinetics of antibodies requires several days for their accumulation in the tumor and delivers high radiation doses to healthy tissues (18,19).…”