Tetrazine- <i>trans</i> -cyclooctene 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 macrocyclic chelator 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 <i>T</i> ₁ relaxivities of Gd(DO3A-tetrazine) and Gd(DO3A-PEG₄-rhodamine) were found to be similar to [Gd(DOTA)]^- (Dotarem®). As a proof-of-concept <i>in vivo</i> test, the click conjugates were delivered to mice brains using the combination of focused ultrasound and microbubbles, with neuron uptake of the probes observed.