Tetrazine-TCO ligated lanthanide conjugates for biomedical imaging

Lanthanide chelates and copper-free click chemistry have important uses for targeted molecular imaging and therapeutic strategies. Herein, we report the complexation of lanthanides to a tetrazine-functionalised DO3A macrocycle and the tetrazine-TCO ligation between the lanthanide-DO3A-tetrazine complexes and a TCO-PEG4-functionalised rhodamine as a model agent. The luminescent and magnetic properties of the resultant dual-modal conjugates are described. The tetrazine moiety was found to sensitise terbium luminescence, resulting in a ‘turn-off’ effect upon its transformation to the dihydropyridazine linker, with the rhodamine moiety then dominating the fluorescence emissions. The T1 relaxivities of Gd-DO3A-tetrazine and Gd-DO3A-PEG4-rhodamine were found to be similar to Gd(DOTA). As a proof-of-concept in vivo test, the click conjugates were delivered to mice brains using the combination of focused ultrasound and microbubbles, with neuron uptake of the probes observed.