# Tetrazine-trans-cyclooctene ligated lanthanide conjugates for biomedical imaging

## TL;DR

Researchers developed lanthanide-based imaging probes using a fast, catalyst-free reaction and tested their use in brain imaging with focused ultrasound.

## Contribution

A new method for creating lanthanide conjugates using tetrazine-TCO click chemistry for biomedical imaging is introduced.

## Key findings

- Tetrazine-TCO ligation enabled the creation of dual-modal lanthanide-rhodamine conjugates.
- The probes successfully crossed the blood-brain barrier and were taken up by neurons in mice.
- Gd(DO3A-tetrazine) showed T1 relaxivity comparable to a commercial contrast agent.

## Abstract

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 T1 relaxivities of Gd(DO3A-tetrazine) and Gd(DO3A-PEG4-rhodamine) were found to be similar to [Gd(DOTA)]− (Dotarem®). 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.

Tetrazine-functionalised lanthanide chelates and their catalyst-free ‘click’ reactions with a trans-cyclooctene-rhodamine fluorophore are reported. These probes were delivered across the blood–brain barrier using focused ultrasound and microbubbles.