[Ga]Ga-THP-tetrazine for bioorthogonal click radiolabelling: pretargeted PET imaging of liposomal nanomedicines.

Pretargeted PET imaging using bioorthogonal chemistry is a leading strategy for the tracking of long-circulating agents such as antibodies and nanoparticle-drug delivery systems with short-lived isotopes. Here, we report the synthesis, characterisation and / evaluation of a new Ga-based radiotracer [Ga]Ga-THP-Tetrazine ([Ga]Ga-THP-Tz) for bioorthogonal click radiochemistry and labelling of agents with slow pharmacokinetics. THP-tetrazine (THP-Tz) can be radiolabelled to give [Ga]Ga-THP-Tz at room temperature in less than 15 minutes with excellent radiochemical stability and . [Ga]Ga-THP-Tz was tested and for pretargeted imaging of stealth PEGylated liposomes, chosen as a leading clinically-approved platform of nanoparticle-based drug delivery, and for their known long-circulating properties. To achieve this, PEGylated liposomes were functionalised with a synthesised transcyclooctene (TCO) modified phospholipid. Radiolabelling of TCO-PEG-liposomes with [Ga]Ga-THP-Tz was demonstrated in human serum, and using both healthy mice and in a syngeneic cancer murine model (WEHI-164 fibrosarcoma). Interestingly data revealed that [Ga]Ga-THP-Tz was able to radiolabel liposomes present in the liver and spleen, and not those in the blood pool or in the tumour. Overall, these results demonstrate the potential of [Ga]Ga-THP-Tz for pretargeted imaging/therapy but also some unexpected limitations of this system.